Hi. I am Doctor Q and I am here to answer all your queries relating to Concrete and its usage. Post the questions on the DoctorQ section of the Website and
I'll be happy to help.

Doctor Q Answers

Query : Doctor Q there are buildings built using 3D concrete printing technique in China where High strength concrete is used, without using formworks. Also, curing is not done. But still, multi-storeyed buildings are built. How is it possible???

Anju Johny, MITS

Doctor Q : Any material made with cement requires curing, while the materials used for 3d printing are usually metals and plastic. If 3d printing is done with cement concrete, it also requires curing. In China as per reports, Houses are fabricated using a specially formulated and patented "ink"(The "Ink" include construction waste such as concrete, fibreglass, sand and special hardening agent) and not cement concrete; which does not require curing. (source: https://3dprint.com/38144/3d-printed-apartment-building/)

Query : Curing plays a key role in the life of concrete but the interlocking concrete pavers of the present day are not cured at all. The manufacturers say curing is not required. The only admixture added to it, to my knowledge, is a lignin-based compound. What could be the reason? Does the absence of curing undermine the strength of the pavers?

Kevin Mathews Kurian, Saintgits College of Engineering, Kottayam

Doctor Q : Even though in case of interlocking concrete block pavement, the concrete used for the manufacture is of zero slumps and is compacted and vibrated forcefully into the mould which produces the pavement blocks of high strength, still requires curing for long-term durability of the pavement blocks.

Query : Doctor Q What are the various considerations generally required to develop the procedure for mix proportioning of High Performance Concrete ?

Kevin S Fernandez, NICMAR, Pune

Doctor Q : First of all we need to define the required Performance criteria Like 01. What is the peformance required with respect to Permeability( DIN 1048, ISAT test) 02. Similarly performance levels required in terms of RCPT value, Sulphate resistance etc 03. If the HPC is for thick sections, we need to consider heat build up and how to insulate the structure during initial curing Period 04. We need to also consider any special requirements like higher flexural strength, etc 05. Do we have special requirement to reduce cracks in plastic state? 06. Exposure and Service life situations Based on which we will select A. water cement ratio B. Type of Binders( Like different SCMs along with type of Cement) C. Total Binder Content D. Coarse and fine aggregates (and are they stable and have passed Alakali aggregate reactivity tests?) E. Whether the coarse and fine aggregates are devoid of chlorides and any other impurities etc F. If reinforcement congestion then we need to consider smaller size aggregates like 12.5mm or 10mm down G.If we are going for SCC ? then we should select good amount of powder content

Query : Doctor Q, my question is, we can achieve characteristic strength in laboratory condition but can we achieve the same in practical condition though we are suface curing it for certain time period for like 28 days or even less some times , is there any factor of safety that we need to consider in Mpa's when it comes to analysing or designing a structure??

Koushik, Mangalore

Doctor Q : We achieve a certain strength when we do trials in controlled lab conditions. Generally design concrete and do trials in lab to achieve a certain strength which is referred to as " Target mean strength". As per mix design principles ,Target mean strength= Characteristic strength+ Margin= Characteristic strength+ 1.65*standard deviation Once we achieve Target mean strength in lab trials , which we can also consider as Potential strength of concrete ( which we can achieve only if the concrete is produced ,transported,placed and cured exactly as we have done in lab trials, following all best practices). But unfortunately we see differences from lab trial cube results and Field cube strengths due to many variations that occur in materials,production and post production activities . As a thumb rule 6 to 8 Mpa is the margin on characteristic strength to be considered in design of concrete mixes upto M40. Above M40, it is better to consider 10 to 12 Mpa as margin on Characteristic strength for mix design, as the SD increases. To judge the correctness of higher grades of concrete the field cube strengths should achieve almost 95% to 100% of characteristic strength in 7days "For design purposes the strength of concrete in structure shall be assumed to be 0.67 times the characteristic strength ". But structural designers use their own methods and experience to specify grade and use concrete . But once specified the concrete is supposed to be as per the acceptance criteria specified in IS 456.

Query : How we can differentiate a shrinkage crack and a structural crack?

BIJU., Kadavantra

Doctor Q : Plastic shrinkage and plastic settlement cracks take place during the very initial time after placing of concrete . These cracks occur mostly due to external factors especially when the rate of evaporation is more than rate of bleeding. Temperature, humidity and wind velocity control the rate of evaporation and these factors play big role in changing rates of evaporation and hence cracks formation.They are said to be non structural in nature and does not affect structural integrity . Plastic shrinkage cracks generally appear almost like diagonal lines on a slab or floor Cracks may form a random pattern or may appear parallel to one another. They range in length from 25 mm to 2 meters, but are usually 300 to 600 mm long. They are spaced from a few centimeters to as much as 3m apart. Plastic shrinkage cracks begin as shallow cracks, but can become deeper if not attended in time. They need to be closed as quickly as they are formed by just filling up with polymer + cement slurry filling or injection grouting if cracks are deeper or any other suitable methods of crack filling. Plastic settlement cracks occur generally due to settlement of different ingredients of concrete and subsequent evaporation of water Plastic settlement occurs as a result of restraining the settlement of concrete. Plastic settlement cracks are so-called because it is formed while the concrete is still plastic. Plastic settlement cracks are different from shrinkage cracks by its distinguished pattern. Plastic settlement cracks tend to mirror the restraining elements, such as reinforcing steel. Plastic settlement cracks may become noticeable at an early stage. However, the cracks often noticed after a few hours of concrete placing. The cracks occur while the concrete is plastic, and the bleeding water is rising. These cracks can seriously affect reinforced concrete that exposed to harsh environments. These cracks will facilitate the ingress of aggressive chemicals such as chloride and sulfate. The ingress of chloride will result in corrosion of steel reinforcement, which may reduce the service life of the structure. REPAIR OF PLASTIC SETTLEMENT CRACKS In pre-hardened concrete The most effective repair is to close the cracks shortly after formation by re-vibration and reworking the surface while the concrete is still plastic. Careful timing is essential to ensure the concrete re-liquefies under the action of the vibrator so that the cracks are fully closed. Re-vibrate too soon and cracks may reform; too late and the bond to the reinforcement may be damaged. Mechanical re-trowelling of the surface may be sufficient to close the cracks and compact the concrete around the reinforcement provided the cover is not too great, but the best result is where this is combined with some form of vibration A variety of issues can cause structural cracks, like poor compaction ,poor soil bearing, overloading, swollen soil, and poor practices construction sites, some structural design problems . Structural cracks are accompanied by problems on the inside of your home. Examples include sloping floors and moving objects like doors and windows that could be difficult to close. Structural cracks are usually easy to identify. They can take on many shapes and sizes, can be horizontal, vertical, diagonal, or appear like a staircase in foundation blocks. May be some times big cracks in beams, retaining walls ,slabs ,cantilevers etc.

Query : Doctor Q, My question is Why there is big difference between Concrete cube strength during lab Trial and concrete cube strength of Supplied concrete ?

Nuthan kumar, Nitte meenakshi Institute of technology, Bangalore

Doctor Q : When we do lab trials we do all activities fully under controlled conditions and we get cube strengths which can be closer to our Design strength ,which we name as Target mean strength . This can also be called as Potential strength of concrete mix with a certain set of ingredients for a particular WC ratio But what actually happens during production is 01. The properties of aggregates may change( Compared to the original aggregate properties ,taken during mix design/lab trial) 02. The performance of Binders like Cement and SCMs may differ compared earlier during lab trial 03.During production if the moisture and absorptions are not considered and mixes are not corrected on the day of production,this may lead to change in WC ratio ,which is the KEY to strength of concrete. some times even if everything was done properly and concrete was produced properly as per mix design /lab trial with best QA/QC ,later 04. During transportation or after transportation before placing WC may be tampered(increased) 05. Placing of concrete may not take place within 2-3 hours,and longer time taken to place concrete can affect both fresh and hardened properties 06.Compaction of concrete if not done properly ,affects the strength leading to probably lower strengths 07. Improper curing may also affect the strength while above can influence ACTUAL CONCRETE strength in field cubes v/s the lab trial cube strength But on the other hand even when everything is done properly as per best practices ,the cube strengths can change from actual strength and lab strength due to 08. Improper sampling 09. Improper mixing of sample before casting 10. Bad casting ,not as per standard 11. Unstandard cube moulds 12.Not applying mould release agents correctly 13. Quality of mould release agent itself may be bad 14. Intermittent curing and not continuous curing 15. Curing temperature not as per standard 16. Faulty testing machine 17. NOT Calibrated CTM 18. Improper testing like wrong rate of loading , Not keeping cube in Centre of platten

Query : Dear doctor Q, Is there a chance for cracks in an always air-conditioned building due to change in temperature in outside and inside surfaces of a slab.? The internal temperature might be 16 degrees to 20 degrees the exterior can go up to 40 degrees.

Joseph george, Kottayam

Doctor Q : Any material expands and contracts as the temperature changes. Concrete also expands and contracts due to variations in temperatures Here if the room temperature is continuously maintained at a reasonably constant temperature the cracks will not occur especially if the expansion joints etc are provided and concrete has good strength. But if the concrete itself is subjected to very low temperature on one side or portion and another side/portion at higher temperature it may crack due to difference in temperatures which cause different expansion ,contractions. Also if the concrete slab /ground slab is there,let us say the surface temperature is low and at bottom of slab let us say the temperature is higher ( Temperature differential) the expansion contraction can cause cracks ,if the concrete is not provided with proper expansion joints. The concrete mixes for such concrete floors can be M35 to M40 ,with proper reinforcement and additional steel fibers and other fibers to increase tensile strength etc. Such concretes need to be designed taking into consideration service life exposure to different temperatures also many other performance requirements .

Query : When we get cube strengths at 7 days more than the characteristic strength of concrete, what is the reason behind that?

Neelamma, Bangalore

Doctor Q : As we are when we are doing mix design ,we take into consideration that some variations can occur during production and post production activities. when we do ,Target Mean Strength= Fck+ Margin , we assume even when something goes wrong, we should reach that Fck value during large scale production. Since such assumptions are made during concrete mix design , if the QA/QC systems are best and controls in all stages of production are exercised well many times ,we may get 7days strength either close to or just above characteristic strength. Many times as we have too much variables like unexpected delays in reaching sites,poor compaction, curing etc, variations in properties of ingradients , even though such mix designs appear to be OVER DESIGNED MIXES or Conservative mixes such mixes are used by many producers. Such mixes are in fact required when we use OPC mixes where the strength gain is faster in early days and many times more than 85% to 90% of strengths are achieved in 7days itself and strength gain is less after 7 days. So such mixes are better to be adopted in case of OPC mixes especially in high grade concretes above M40. When we use OPC+GGBS or OPC+PFA mixes are used as later age strengths are better, we need not have to design mixes which can reach characteristic strength in 7days itself . If consistently the performance of such a mix is showing low SD ,such mixes can be re designed.

Query : Doctor Q, it is said that efflorescence is caused in geopolymer concrete, my question is what's the reason for it and will it affect the strength of concrete.

Arthik Rai, Mangalore

Doctor Q : We know that efflorescence is the process of formation of whitish salt deposits on or near the surface of concrete. Generally when this happens in OPC concrete ,it is said to be harmless. Efflorescence causes discolouration and many times especially in precast and important exposed concrete elements is seen as an asthetic aberration. But as alternative technologies like Geopolymer concrete are gaining popularity for many scientists ,efflorescence in Geopolymer concrete has become a challenge. We all know that alkai reactors are used in good proportions in Geoploymer concrete. Many times excess concentration of such alkalies or excess quantities of such alkalies may leave unreacted alkalies in the body of geopolymer concrete,which leaches out through pores and eventually reacts with CO2 to leave whitish salts on the surfaces. Efflorescence in Geopolymer concrete is said to have a negative impact on strength . But still the phenomenon is being studied by many Geopolymer experts.

Query : Doctor Q, My question is How does Cement properties influence Workability ?

Aishwarya Kurandwad, Bangalore

Doctor Q : Cement chemistry properties have very profound effect on setting,gain of strength and Workability ,other properties. 01. physical property of cement like fineness if increased beyond an optimum limit will increase water demand and hence reduces workabiliy. Increasing cement fineness from 2700 to 4000cm2/gm, is said to have a positive effect on workability. Further workability is also dependent on particle size distribution of cement particles . But a general observation and simple way to explain is as clinker is ground to higher fineness,since the area of interaction with water increases the rate of hydration increases such cements may reduce workability. sometimes coarser cements can lead to higher bleeding rates and can lead to different set of problems like reduced pumpability. 02. Many times if cements directly reach to concrete producer in a short time,cements may be observed to be at higher temperatures and hence can reduce workability/slump retention 03.It is also said that water demand may increase with higher Al2O3 or C2S contents 04.Higher C3A more may cause lesser workability. 05. The kind of Gypsum found in cements can also affect the workability. The best is said to be Gypsum bihydrate ( CaSO4 .2H2O) . But sometimes if the cements contain Gypsum Hemihydrate ( CaSO4 0.5 H2O) such cements will give lower workability. The worst case is cements having Anhydrous Gypsum (CaSO4). We need to understand that variations in Cement compositions will lead to huge impact on workability specially when used with various kinds of chemical admixtures

Query : Dear doctor Q Is there any admixture comparability issue if water proofing admixtures are added with normal PCE based superplasticizers.

HARI, Kottayam

Doctor Q : If lignosulphonate based waterproofing compounds are used they are compatible with PCE based Superplasticizers ,so they can be used. But in general since many commercially sold waterproofing compounds may generally trigger set delay, we need to minimize dosage of waterproofing compound to be used along with a certain dosage of PC admixture ,trials be run as a precautionary measure to ascertain any misbehavior of the combination on concrete. We need to note here,SNF based admixtures are mostly incompatible with PC based admixtures. So in short trials is a must before use of any waterproofing compound in conjunction with any other Superplasticiser( Irrespective of whether it is PC based or SNF based)

Query : How low density Concrete can be achieved.Is there a risk of reduced durability for low density Concrete

AJITH.T.V, Thaipparambil house,Thycattuserry P.O,Cherthala,Alappuzha

Doctor Q : Low density concrete can be achieved in many ways 01. By using Air entraining admixtures in concrete 02. By using Polystyrene beads ,in which case the density of low density mortars/screeds and low density concretes with Polystyrene beads. 03. By using naturally occuring light weight aggregates like Perilite, pumice. 04. By using artificially made light weight aggregates 05. By using foaming agents to make light weight mortar In many of the above cases the light weight concrete or mortars are used as NON structural filling material or covering Screed etc. In such cases the durability is not considered as performance criterion. Here we need to note strength is also dependent on density and when very low densities are achieved strengths are compromised and such concrete/mortars may not be considered as durable. But when some specific light weight concretes are designed to be used in structural elements in such cases durable concretes can be achieved with required compressive strengths not taking down densities too much lower. For example structural light weight concretes can be made with lower W/C ratio,using Microsilica as strength enhancer, with HRWRA and polymer emulsion taking EPS beads as light weight aggregates. In such cases we can have durable moderately lightweight structural light weight concrete.

Query : Doctor Q, How can we modify concrete to prevent corrosion in reinforcement.Does this effect strength and durability of concrete?

Aswin chandran, Mavelikkara

Doctor Q : We can make mix designs to increase corrosion resistance by A. Including Supplementary Cementitious materials like Flyash,GGBS,Microsilica ,Metakaolin or combinations of them with Optimum cement content B. Reducing W/C ratio as much as possible,just adequate enough to provide required slump/flow using High Performance Chemical Admixture C. Selecting least Binder content considering exposure conditions and minimum Cementitious contents D.Incorporating specially made corrosion resisting admixtures either based on Calcium Nitrite or specially made Migrating corrosion inhibitors Such materials as mentioned above along with incorporation in E. Self comapcting concrete Followed by proper practices in all stages post production like G. Transporting and placing within specified time H. Proper compacting if SCC can not be done I. Proper finishing and curing can all increase significantly the resistance to corrosion. In other countries to reduce human intervention in curing ,internal curing compounds are also being used. All the above specially can be used judiciuosly ,perfectly to enhance corrosion resistance and overall Durability. Incorporation and use of such materials in mixes does not effect strength on the contrary all durability parameters are enhanced.

Query : Is it possible for a lower grade concrete to have more durability than a higher grade concrete, what are the tests available to find out the durability of concrete ?

Abin T Varghese, Kottayam

Doctor Q : When we use Supplementary Cementitious materials as we go on replacing more OPC ,with higher % of SCM, we may end up with lower strengths. For example,let us say we have a Cement content of say OPC 400 kg as Control or REFERENCE mix. Let us say we make the following mixes with Flyash Mix 1 fly......360 OPC+ 40 flyash(10% replacement) Mix 2 fly.......320 OPC+80 flyash(20% replacement) Mix 3 fly.......280 OPC+120 flyash(30% replacement) Mix 4 fly.......240 OPC+160 flyash(40% replacement) When we do trials as above ,comparing all mixes say at a constant WC ratio( Not reduced WC ratio,even though we can reduce as we get better workability with flyash mixes , as we need to understand behaviour at same WC ratio), the Flyash mixes ,most likely will end up with lower 7 and 28day strengths compared to REFERENCE mix(Control mix without Flyash). But these mixes will show either equal or better durability , as 01. Flyash or any other SCM , will participate in secondary hydration and make denser concrete with better gel pores 02. The Heat of hydration of such mixes are lesser Overall durability will be either comparable or better . Some properties like Chloride ion resistance ,Sulphate ion resistance etc are remarkably improved even at same water cement ratios. The tests to be done to assess the durability are 01. Permeability as per DIN 1048 02. ISAT test as per BS1881 03. Sorptivity test as per ASTM C1585 04. RCPT as per ASTM C1202 05. Resistance to sulphates 06. Carbonation test 07. Drying shrinkage The standards and references can vary from country to country. But the above tests are most popular to ascertain durability of concrete and used also as basis for HPC mix validation .

Query : Hi DoctorQ, my question is, we know that creep is a property of hardened concrete. How can we measure creep of concrete ? How the aggregates in concrete influences the creep of concrete? and what are the measures that can be adopted to avoid creep of concrete?

Hareesh K V, Karikkethu (H) Pampakuda P.O Pampakuda, Pin: 686667

Doctor Q : When hardened concrete is loaded it deforms.The non elastic time dependent deformation is called Creep. Creep is usually determined by measuring the change with time in strain of specimen subjected to constant stress and stored under appropriate condition. The reference standard is ASTMC 512. Among the various causes of creep, the following may be mentioned: 1.Closure of the void-structures of concrete on the application of load; 2.Squeezing of water from within the cement gel when the concrete is loaded, 3.The flow of cement-water paste, in a viscous state, causing a permanent shrinkage under load; 4.Readjustment of aggregates under load. It is very clear that aggregates have a great role to reduce creep. To reduce Creep 01. Reduce W/C ratio 02. Creep is inversely proportional to the strength ,and hence make higher strength concrete 03. Reduce paste content/use optimum paste content 04. Use bigger size aggregates 05. Make mix design with high aggregate content 06. Light weight aggregates and aggregates with high porosity ,can increase creep,hence use good quality aggregates with least absorption(Porosity) 07. Use stronger aggregates 08. Longer curing time with all precautions excercised during initial curing . 09. Concretes with flyash many times have given lower creep,so flyash additions is better.

Query : Doctor Q, Is there any strength variation will occur due to adding of water proofing compound ? Also what are changes may occur in the concrete due to mix up of two chemical compound (HRWRA and Water proofing admixtures) together ?

George Francis, Madathilkunnel house mundakayam East P.O, 34 th mile

Doctor Q : A. There are many types of waterproofing compounds available in market and most popular of them are based on Lignosulphonates plus some amount of air entraining admixture. Such waterproofing admixtures increase workability......1 increase air content.......2 increase cohesiveness......3 increase impermeability.....4 slightly can increase set time.....5 Becuase of (2)....they tend to reduce strength . We need to note here that every 1 % of air entrainment will lead to about 5% of strength. Hence when using such waterproofing compounds we need to use minimum dosage and try to make proper adjustments in mix design including water reduction. If we use such waterproofing compound along with regular Superplasticiser cum retarder(Type G ) which are generally known as HRWRA ,they may prolong setting time also they can reduce strength too. We normally recommend trials to be done before using such compounds either independently or in conjunction with HRWRA ,by reducing W/C ratio and keeping the dosages of both of them to minimum required levels ,to observe ,record both fresh and hardened properties before actual use in production situations. There are other kind of waterproofing admixtures like Crystalline waterproofing admixtures ,which are also used extensively. These admitures when used along with HRWRA, reduce slump causing pumpability issues and sometimes may cause set time delays. Hence the above are recommended to be added at the placing point in truck ,mixed well and pumped. Again in case of these compounds ,trials to be done prior to large scale use.

Query : Doctor Q, my question is, How we will ensure the Quality of Readymix Concrete before it send from our production plant?

Ashna Hussain, Kottayam

Doctor Q : At batching plant the following points need to be taken care Stage 1: 01. Selection of materials to meet requirements of relevant Indian standards 02. Making of an approved material list with sources,vendors 03. Reciept of materials 04. After receiving materials visual testing of materials like aggregates,sand etc and check conformity to relavent standards 05. Check MTCs of Cement, Admixture etc for the supplied consignment and check conformity to relavent standards 06. Water, cement,admixture ,aggregates to be sent to 3rd party NABL acredited lab for various tests 07. Mix design and check the fresh properties(, strength etc ,validation of mix design stage 2 08. Check proper storage of different aggregates and sand without intermixing, proper drainage 09. Check proper storage of cement and cementitious materials in weather proof silos 10. Check the cleanliness of Central mixer and Truck mixers. 11. Check calibration of batching plant 12. Calibration of all lab equipments, weigh bridge etc stage 3 13. Before production check all materials again. Understand customer requirements like required grade of concrete etc. Select a correct mix from batch computer 14. Take moisture on different aggregates and apply to respective materials to get corrected mix. 15. Start production, check slump at plant if any corrections are to be done let that be done at plant. 16. Observe fresh properties of concrete 17. Check batch record and details like W/C ratio correctness? is it as per design WC ratio? 18. Make Yield test including weighing concrete truck with and without concrete on weigh bridge 19. Ensure drum of mixer is rotating at lowest speed and there is no leakage from truck water tank onto the concrete. All these steps need to be followed before sending truck out of the production facility. After sending the truck ,at site Stage 4: 20. Check slump at site 21. Report slump and any other observation immediately to plant 22. Take sample as per proper sampling procedure 23. Mix sample and cast as per standard procedure 24. Protect concrete cubes against direct sunlight or rain 25. Transport cubes back after setting to central lab for curing 26. Test cubes and make report 27. Make statistical analysis 28. If the cube strengths are unacceptable mix design need to be reviewed All four stages form the basis of QA/QC in readymix concrete company

Query : Why are we not deducting the quantity of steel while calculating the quantity of slab concrete ?

Naveen Kumar Shetty, Bengaluru

Doctor Q : Through out the world there are some set norms in Quantity surveying ,estimation and measurements followed in construction industry. As per these generally and mostly ,while calculating the required concrete for a structure ,concrete volume is directly taken and not deducted. This is done because 01. The steel volume in general is mostly 1 to 3% of concrete volume, which is very less or negligible 02. When we produce concrete,transport and pump concrete or place concrete ,some wastages do occur Hence considering above the practice is steel volume is not taken into consideration.

Query : Doctor Q. How early age strength can be achieved in concrete. In some cases for example in prestressed structures early achieving of strengths is desirable. Can this be achieved.

Christy p thonas, Pulluvelil (h)kannur

Doctor Q : Now a days in precast and Prestressed concrete industry have almost moved completely to High early strength concrete. They want this as 01. They want to produce more and send out their products quickly to increase productivity 02. They have limited space and want and can not store too many concrete products for too long 03. They will have limited number of moulds and they want to produce more with less number of moulds. 04. A certain strength is required to lift the concrete element off the mould and subsequently transport and place in their intended final position. 05. A certain strength is required at the time of prestressing 06. In winters the precast manufacturers can not wait long for strength gain ,hence they need early strength gain. 07. In some construction sites they want to remove shuttering and re use the shuttering material to build further to complete structure faster. 08. In some emergency repair situations or constructions ,we may need faster gain of strength to use the structure The strength requirements are specified as 24 Hour or 3 days OR 5 DAY strength requirements. In many cases the specifications can be 12 hour or 16 hour strength =16 Mpa or 18 Mpa Such high early strength concretes can be made by A. Using OPC with Accelarating admixtures B. Using OPC with PC based HRWRA , with a water reducing ability of 30% to 40%. C. Taking W/C down to say 0.28 to 0.32 D. Even sustainable greener High Early strength Concretes are made possible now a days with High performance PCE based Superplasticiser cum accelarator ,where high early strengths are achieved within 16 to 24 hours. E. Such mixes can be even incorporated with synthetic fibers (Polypropylene/steel etc) to suit some specific requirements.

Query : Can you please explain the procedure of using Packcrete ??

Anjali, Tripunithura

Doctor Q : Packcrete is ready to use ,premixed Concrete produced by QCRETE Readymix(India) Pvt Ltd. Here the concrete ingredients are packed in a bag and just we need to add water at site ,which is pre determined and given in specs of QCRETE. Generally PACKCRETE is made to a grade of M25. Just take a bucket,take required quantity of water ,put about 80% to 90% the water into the bucket then after opening PACKCRETE BAG slowly go on adding the ingradients and mix using a trowel or small shovel ,after making the mix wet,we can add remaining water to get right workability. Or spread ingradients on a metal sheet(non absorbant surface) ,add slowly water and mix with a shovel. Alternatively if the concrete quantity is more ,then 2 or 3 bags can be poured into a small mixer and required water can be added to mix it to a uniform consistency Generally the PACKCRETE is prepared to give an intial slump of about 150mm and should be placed within 30 to 45 minutes to after mixing with water,so all preparations should be done before mixing. QCRETE can give variants of the PACKCRETE, to meet different customer requirements. QCRETE can also supply Wet PACKCRETE, which comes completely with Premixed concrete in buckets,in which case, customer need to just pour the concrete to the structure. The advantages of PACKCRETE are 01. Factory produced concrete in bags/buckets 02.Quality assured concrete 03. Need not have to bother about quality of ingradients ,as it comes with QCRETE Quality certificate. 04. Smaller concrete works like small repairs or smaller structural elements can be easily installed. 05. Some times if the works are small,no need to call labour, we can do ourselves,just open PACKCRETE.....mix and pour.

Query : How does water cement ratio influence strength of the concrete ??

Reenu k joseh, Bangalore

Doctor Q : Theoritically a w/c ratio of about 0.22 is required for Cement to undergo Hydration. But in general we add more water to concrete ,just to make it more fluid so that we can easily place and finish the concrete. This extra water goes on evaporating leading to more porosity in concrete. So as we increase W/C ratio, the porosity increases leading to lesser strength. Further the interface strength(Bond strength) between Coarse aggregate and cement paste/cement sand paste is also governed by W/C ratio Hence 2 laws have been made Strength is inversly proportional to W/C ratio and Permeability is directly proportional to W/C ratio So higher strengths and durability can be achieved with Lower W/C ratio. In short W/C ratio is said to be the KEY to all Engineering Properties of concrete

Query : Dear Doctor Q, my question is, What is the relationship between concrete strength measured using cubes & in cylinders.? And why concrete cube strengths are preffered for strength assessment than cylinder cubes?

Dilip Kumar R, Bangalore

Doctor Q : IS 516-1959 is code for Method of testing of compressive strength of concrete cubes Where as ASTM C39 defines method of testing of Cylindrical specimens EN codespecify concrete like C20/25, means if we test cylinders we should get 20Mpa and 25 Mpa as cube strength. The strength classes are specified as C16/20,C20/25,C30/37 etc. Generally the cube strengths are higher than cylinder strengths . To get cylinder strength we need to multiply cube strength by 0.8,which is an approximate widely accepted factor. Reasons for concrete cube to have greater compressive strength as compared to cylinder is contact area of a standard cube with the upper platen in the testing machine is more which results in more confinement . As the cube have a four confined ends , evidently concrete cube takes more load as compared to cylinder having no confined ends liable for easy failure. Cylinder results are said to be closer to field strengths and hence many prefer to use the cylinder strengths. But casting cylinders ,and preparation of cylinder specimens before testing by making sulphur capping etc is slightly longer process. Capping needs to be done carefully & perfectly and all these have to be done by skilled trained technicians. Comparatively cube casting and testing is easier and hence we can use the cube strengths. We can later convert the cube strength to cylinder strength. Most importantly in India,Indian standards talk only of cubes and hence we use them.

Query : Hello, Other than OPC what cementitous material and admixture can be used for high strength concrete i e for M60 Grade. I had referred few websites but didn't find the appropriate answer. Please help me on this.

Ishwar Singh, Mangalore

Doctor Q : In general all High strength concretes above M50 contain SCM like Microsilica There can be different options OPC+ Microsilia + PCE based admixture OPC+ Alcofine +PCE based admixture OPC+ Metakaolin+PCE based admixture In case of good pumpability required we can use OPC+Flyash+ Microsilica+ PCE based admixture OPC+ GGBS+ Microsilica+ PCE based admixture Note: 1.Microsilica can be replaced by Metakaolin OR Alcofine 2. General dosage of these materials are 6% to 7% for grade M60( Full dosage range 5% to 12%) 3. PCE based admixture is a HRWRA with about water reduction of 30% to 40% 4. W/C ratio of M60 is about 0.28 to 0.32, depending upon the quality of ingradients One of the mixes can be suggested as follows as an example OPC=420Kg GGBS=100 Kg Microsilica=25 kg W/C ratio=0.29

Query : Doctor Q, If we give less or excess vibration during compacting of concrete, How it impacts on concrete?

Lathashree U, Mangalore

Doctor Q : If concrete is not vibrated well( less than required), it may leave entapped air in concrete. It is said that every 1% air (left behind ,due to incomplete vibration or under vibration) will reduce strength by 5% to 7%. Hence such concretes can not reach their potential strength Under compaction can also lead to honey combing,which can in turn lead to durability issues . On the other hand ,when over vibration takes place a. It may lead to separation of slurry from concrete , which results completely non uniformity of properties across different sections of concrete member b. Many times complete settlement of aggregates and slurry moving to top section , which may lead to durability problems and problems like all bleeding water coming to top and collecting like layer with cement particles, leading to cracks on surface weak top surface .some times delayed setting in top portion of member low abrasion resistance and dusting etc.

Query : Hi, There are certain instances where Sulphate Resistant Concrete is required. So my question is wether this can be achieved using GGBFS, Fly Ash, some other cementitious material or liquid admixture ? If yes how does it influence sulphate resistance ?

Sumith Kumar, Mangalore

Doctor Q : It has been observed throughout the world overall durability of concrete is increased by using Flyash or GGBS ,when these additions are used in a judicious way. With respect to resistance ti sulphates A. Flyash and GGBS ,when used in concrete consume ca(OH)2 in their pozzolonic reaction. This makes lesser ca(OH)2 available for reaction with sulphate ions to attack concrete B.Secondly ,C-S-H gels formed by the pozzolonic reaction of GGBS,Flyash fills the pore structure of concrete paste ,making the concrete microstructure denser ,better to resist sulphate attack. Class F ,Flyash used in concrete exposed to sulphates in soils and water ,can reduce sulphate attack to a great extent. Class F flyash is recommended to be used between 20% to 30% to improve resistance to sulphates. GGBS need to be between 50% to 70% for good resistance against sulphates. It has been noted that 70% GGBS addition to concrete with OPC is said to be one of the best and time tested way to increase both resistance to sulphates and chlorides. Hence such a big addition is common in concrete where high sulphate and chloride resistance is required ,especially like marine structures which are directly in touch with sea water Further these concretes are also designed with low to very low W/C ratios like 0.3 to 0.4 ,incorporating good HRWRA.

Query : Doctor Q, My Question was after slab concreting work we done initial curing,but cracks are developing day by day,then why cracks are developing?

Jeeva Balan, Vellore

Doctor Q : Initially before concrete hardens loss of water due to quick evaporation leads to plastic shrinkage and plastic settlement cracks. These are said to be non structural cracks and said to be un harmful as they do not make an impact structural integrity. However as these cracks can allow deleterious materials to enter RCC, which may cause corrosion and other durability problems.hence they need to be closed. The other cracks are said to be Drying shrinkage cracks which start later and can occur much later. The cracks which you may be referring to are something different probably caused by restraint or improper supports (scaffolding) or some other external factors.

Query : Doctor Q What are the advantages obtained in fresh concrete if fine and ultrafine ground granulated blast furnace slag is used?

Tinu, NICMAR, Pune

Doctor Q : Ultrafine GGBFS, is highly ground slag to a fineness of about approximately 12000cm2/gm , which means almost 4 times the fineness of OPC. In fresh concrete this is said to improve 01. pumpability 02.slump retention But that is not the purpose of using such a material. The basic use of Ultrafine GGBS(One popular material in this class is ALCOFINE) to improve 03. Overall Durability of Concrete 04. To improve strengths to higher levels ,in mixes where we need High strength(Both for early High strength and 28day strength). 05.The increased fineness and unique Particle size distribution will help the packing of cementitious materials in a better way. In India such materials are used as an equivalent to Microsilica (in higher grades of concrete especially above M50 ),even though they are totally different class of materials.

Query : Hi team How much water proof add to concrete for per m3 can it differs to grade wise

Sharanu galave,

Doctor Q : The dosage of waterproofing compounds are given as a % of total cementitious content in concrete mix. So even if we use same % , total kg/m3 will change as per grade. For example,let us say a waterproofing compound is used at 0.2% of cementitious as per manufacturer recommendation. If the Cement content for M20, is 300 kg/m3......this will be 0.2%* 300= 0.6kg/m3 Same when it goes to M35, for a mix having 380 kg/m3.......this will be 0.2%*380=0.76kg/m3

Query : 1. What is the relation between coarse aggregate and fine aggregate in the concrete preparation

Prineesh prakash, Kochi

Doctor Q : When we do mix design,one of the principles used is to fill up gaps of bigger aggregates (let us say 20mm) by smaller aggregates(say 12.5mm) and smaller aggregates gaps to be filled up by sand particles(which are 4.75mm below) and finally cement particles fill up the gaps between sand particles. Now this kind of filling up using different sizes of materials is referred to as mix proportioning to achieve Optimim packing or optimum packing. So based on this model, there are certain grading requirements for each aggregate size as per our Indian standard IS 383-1970 Also once we have individual gradings done,we also need to meet certain Combined grading requirement as per IS383-1970, which is guideline envelope in which a certain % of fine aggregate has to be combined with a certain % of coarse aggregates. We also need to find ratio of % of 20mm: % of 12.5mm ,before we move to combined grading or ALL IN AGGREGATE grading. So this is how a relation or proportion of COARSE AGGREGATE to Fine aggregate is arrived at. For example normally this can be 60% Coarse aggregate and 40% Fine aggregate or 55: 45 based on properties of individual materials and pumpability requirements. Both Coarse and fine aggregates are fillers to be bound by Cementitious system. The fine aggregate becomes especially 75 microns below and some portion of 150 microns below will become part of PASTE content ,which is CEMENT+ VERY FINE PARTICLES OF FINE AGGREGATES, which help in increasing pumpability. In SCC mixes the Coarse to fine aggregate ratio may reverse,when compared to normal concrete. For example it can be 45% coarse aggregate: 55% fine aggregates.

Query : Why is concrete gray? Can we get any other colours?

Fahar, Tvpm

Doctor Q : In Cement there is Tetracalcium Aluminoferrite C4AF in about 5% to 10% . This makes the colour of cement (concrete) grey or dark grey. But this compound does not contribute to strength of concrete. When this portion is reduced by reducing Iron oxides and other oxides also by using higher amounts of lime stone,china clay , white coloured cements are manufactured. White cements will have higher fineness and hence will be expensive. Concretes of different colours can be manufactured by adding different colour pigments to basic OPC or white cement.

Query : What are the Common Concrete Problems and how to solve it's?

Faharudeen, Tvpm

Doctor Q : The question can not be answered in a page but needs a book to be answered in detail. However, I try to make this as short as possible and present here below. Problems in Fresh state 01.Slump Loss ; Use compatible admixture to a selected cement. Do sufficient number of trials before use. Maintain proper QA/QC on all ingradients. Use right moisture in batch plant. 02. Bleeding and segregation: The mix design has to be done with good Zone 2 sand with reasonable amount of fine fines( It should have good amount of fines below 150microns ). The fine to coarse aggregate ratio need to be perfect. Concrete should show good pumpability during trials and this need to be checked. Right W/C ratio.....more water more bleeding Right Admixture dosage Avoid gap graded aggregates Avoid flaky and elongated aggregates Compatibility of Admixture to Cement Do not change aggregate,sand and cement sources once finalised after trials 03. Honey combing Good amount of sand and a cohesive mix can help to reduce honey combing Too large aggregates try to separate ,hence use smaller aggregates most importantly give proper vibration during compaction Use good shuttering material to avoid leakage of slurry which may lead to honey combing. Use SCC to avoid honey combing 04. Bug holes These are holes left due to either too much vibration ,hence an optimum vibration level to be maintained Use optimum cementitious contents too much lean mixes can cause bug holes Even too much high amount of sands can cause the bug holes, be aware of this. 05. Cracks A. Plastic shrinkage and Plastic settlement cracks This is a result of quick evqporation of water . Do proper early curing mostly starting after about 2 to 3 hours after casting before the concrete completely sets. cover concrete with polythene sheet to avoid quick evqporation of water In case if we find cracks in plastic state , re tamping has to be done to close cracks Use low W/C ratio concrete pouring ,compaction and finishing need to be better done in evenings when temperatures are low Using polypropylene ,steel or any other fibers of acceptable quality can help to reduce plastic shrinkage cracks continue to cure concrete to get best results B. Drying shrinkage cracks Use optimum amount of Cementitious material Use low w/c ratio Use higher aggregate contents Use also Shrinkage reducing Admixtures C. Bigger cracks Can be avoided by providing Expansion joints,construction joints D. Thermal cracks Can be avoided by reducing heat built up within concrete element by using SCMs and hence controlling heat of hydration Use lesser cementitious content 6. Variation in concrete quantity check yield as per IS Standards daily Use for production same source aggregates which were use during trials. 7. Low strength Use proper mix design Use best practces of QA/QC before ,during and after production Use right W/C ratio either equal to or lesser than what is design Water cement raio Proper sampling,casting,curing and testing as per standards are required essential 8.Durability issues Can be avoided by lower W/C ratio, A/C ratio . Corroion of embedded metal can be reduced by using Corrosion inhibitors Resistance to chlorides,sulphates can be increased by using SCMs in concrete with low W/C ratio

Query : What are factors causes of Delay setting time of concrete and is there any strength reduction in delay setteled concrete


Doctor Q : Here we just list the factors which can cause the delay in setting time. 01. Low temperatures can cause delay in set time 02. Cement composition like higher Gypsum content or lower C3A etc can also cause retardation in concrete 03. when we use an admixture and if that is simple WRA or HRWRA without any retarder, it is ok but when we use a ASTM C 494,Type G admixture which is Superplasticiser cum retarder ,if the dosage is more than specified by manufacturer ,it can cause delay in set time 04. Higher replacements of OPC with either Flyash or GGBS can cause set time delay ,we need to be carefull about this 05. Some times excessive bleeding also can also can cause delayed setting time. 06. Incompatibility between cement and admixture also is a cause for set time delay. 07. Older cements ,which might have already absorbed some moisture can also cause set delay. 08. Presence of too much of organic impurities in sand or aggregates or water , can also cause delay in set time 09. If diesel or grease or oils by mistake are spilled on to aggregates/sand they can disturb reaction between cement and water and hence cause set delay. 10. Use of some water proofing compounds ,can cause set delay. Hence we need to conduct trials before we do production. Set delayed concrete may not necessarily reduce strength. There are many instances where superplasticised and retarded concretes have slowly gained strengths ,many times better than normally set concrete

Query : Dr q, can we divert the lower grade concrete to higher grade concrete by adding cementetious materials ???

Gopal kanwale, Bangalore

Doctor Q : We are aware of the fact that Strength of concrete is inversely proportional to W/C ratio. Let us say we have produced a concrete with a Cement content of 300kg/m3, having a w/c ratio of 0.53. So here the Water content in concrete is 159nlitres. This gives say M20 in 28days. Now we need to convert this to M25, for this we need to have a w/c ratio of 0.49 , then we Should have a cement content of 159/0.49= 324.48 kg/m3 to get M25. Let us round it off to 325 kg/m3. Now to convert a lower grade concrete to higher grade concrete ,M20 to M25 , we need to add 25kg/m3 . Here any such change of grade need to be done A. As early as possible.not after 2 HOURS at all. B. We need to know we are changing the hydration process by altering it after some time it has to be done very quickly, I would say within an hour after water comes in touch with cement. C. We need to know ,when we have same water and we have increased cement content, slump drops . So at this point we can only boost slump using chemical admixture and NOT water. Care must be taken to use proper dosage or otherwise the concrete may show delayed setting. In short converting a lower grade concrete to higher grade is Not easy and VERY risky activity. It is easier to down grade a concrete ,divert and use. Even this has to be done as early as possible.

Query : Dear Doctor Q, Is 70:30(GGBFS:OPC) recommended for slabs. If No, Why?

Prateek R shetty, 3/33/2916-2, kadri kambla

Doctor Q : When we add GGBFS : OPC at 70%: 30% OPC, it gives a highly durable concrete. When such high amounts of GGBFS is used with lower W/C ratio, the concrete is going to give improved performance in a. Long term strength Gain better than OPC mixes b. Increased cohesion and pumpability c. Lower Heat of Hydration d. Increased resitance to Chlorides e. Increased resistance to sulphates f. Increased Impermeability Such a concrete is specified for marine structures,especially when they are in touch with sea water or soils with high sulphates. 1 such concretes need to be used with low W/C ratios to harness potential of higher GGBS ,which demands slightly higher dosages of Admixtures! 2.Here since the early strengths of such High GGBS content mixes are generally lower than normal OPC or low GGBS content mixes, we need to have patience and understand that RATE of strength gain is not very fast but Gradual. WE CAN NOT DESHUTTER as usual. 3. We need to cure such mixes for slighly more time than normal concretes. From the above as we do not like to wait( we are in a rush to finish concrete/construction) such mixes are not used for general residential roof slab. But we know that the benifits of using such concretes far outweigh small inconviniences we are tuned to observed!!!!

Query : Doctor Q what is the lifespan of the concrete?

Tabis Ahamad, AT-Bisenikhurd, post-Biseni kala, ps- nasrigang, state-( Bihar)

Doctor Q : Normally Concrete is expected to have a life of about 100 years ,but it can last only 30 years some times. When we talk of life span of concrete ,generally it is life span of RCC structure. The service life of concrete depends upon 01. W/C ratio, which is the govering factor of its durability. 02. Type of Cement used. It is an accepted fact that the when Concrete is made with Flyash or GGBS or such supplementary materials, the overall durability will improve considerably. 03. Type of exposure. Concrete durability will depend upon whether it is exposed to MILD,MODERATE ,SEVERE and EXTREME exposure conditions. 04. Overall QA/QC systems implemented before,during and after Production. 05. Construction Practices implemented like Compaction,curing ,finishing (Degree of QA/QC ,supervision at site). 06. Quality of shuttering material( does it allow leakage of slurry?),quality of mould release agent? etc 07. Expansion joints /construction joints provided or not? Further we need to know left to itself, a protected Plain cement concrete can last longer ,longer than 100 years. But the whole problem of durability starts when concrete is embedded with structural steel to make RCC structures.

Query : What is the difference between ppc concrete and OPC concrete?


Doctor Q : OPC is simplest form of Cement ,which is predominantly used in many countries. PPC is.......OPC+Flyash......Here the OPC clinker is mixed or interground with Flyash. PPC is also called a blended cement. Concretes with PPC are more durable compared to concretes with OPC.

Query : What is the quality test for concrete on wet condition (except slump test) ?

D. Ashok, Vellore

Doctor Q : Slump Test is the most popular and widely used test in Quality control test of concrete . Other than this there is Compaction Factor test ( which is used for low workability mixes) and there is also Vee Bee Consistometer tests. But for workability mostly people use Slump test. Other than this 01. Air content test ....to find out quantity of air content in concrete. 02. Unit weight test.....to find Yield of concrete. 03. Set time test ......to find out whether concrete sets normally or not 04. Bleed test.............to check whether concrete bleeds heavily? Other than above . a simple observation is also made to check cohesion of concrete/pumpability etc .This is a subjective test. We also have for SCC concrete 05. flow test 06. U box test 07. L box test 08. V funnel test which are popular when using SCC mixes.

Query : How is concrete measured?

Rahul Kaibarta, Gram - bakra po badia thana musabani east singhbhum jharkhand

Doctor Q : We understand your question as How is Concrete quantity measured? Concrete quantity is generally measured in India in Cubic Metres( m3). In other countries it is some times measured in cubic foot(CFT) OR cubic Yard.

Query : What is the differance between cement and concrete?

Arun kumar singh, At- marajgang (up)

Doctor Q : Cement is a factory made product by burning crushed lime stone ,clay , iron ore etc at high temperature like 1450 degree Centigrade. When Coarse aggregates, sand and water is added to cement ,mixed properly ,the mixture becomes Concrete. Concrete is second in the list of highly used materials by human beings ,after WATER.

Query : Dear sir, How long does it take for concrete to cure in hot weather

Srikanth, Mangalore

Doctor Q : The best is to cure concrete for 28days . Concrete is known to attain maximum strength when Coninuosly cured for 28days. Concrete can reach its potential strength or maximum Desired/design strength after 28days of proper hydration ,when cured continuosly for 28days. However considering practical difficulties ,it is recommended to cure for 7days.......OPC concrete minimum 10 days ,when concrete is subjected to dry weather. minimum 14 days,when concrete is used with SCMs like Flyash or GGBS . IS 456 clearly states about the curing period.

Query : Doctor Q, How do we prepare to resume our works, for we know, we haven't been able to cure concrete amid lockdown.

Anirudh Rao, katapadianirudh@gmail.com

Doctor Q : Concrete needs to be cured for minimum periods as recommended in IS 456.

Query : Doctor Q, What is the compatibility of chemical admixture with cement? How can we check the admixture-cememt compatibility?

Mahi M, Trivandrum

Doctor Q : Compatibility of a chemical admixture with a particular brand of cement means whether the two in conjunction give a normal concrete accetable both in fresh and hardened state. Compatibility of Admixture with cement is a big issue ,which is many times faced by Concrete Professionals. When we say an admixture has some compatibility issue with a particular brand/type of cement, this means when we make a mix with the admixture+ particular cement and make concrete ,it may lead to 01. Rapid slump loss.....making it impossible for a concrete producer to use such a combination 02. In some extreme cases ......initial slump itself may be very low 03. In some cases lead to very quick stiffening 04. Very heavy bleeding and segregation. 05. Low strength gain 06. Total delay in setting sometimes concrete may not set at all!!! 07. Totally increased water demand Best way to check these is make small batches of concretes and check during trials and even before large scale production This is also checked in lab by using a MARSH cone . The test is called a Marsh cone test. Becuase of this ,we recommend each batch of admixture need to be checked with cement recieved in the corresponding week. We also say normally to avoid such problems keep the sources same.

Query : Dear sir, My question is- How water-cement ratio plays a vital role in in concrete strength gaining? & how it affects the different grades of concrete?

KIRAN KUMAR R, Chitradurga

Doctor Q : Theoritically a w/c ratio of about 0.22 is required for Cement to undergo Hydration. But in general we add more water to concrete ,just to make it more fluid so that we can easily place and finish the concrete. This extra water goes on evaporating leading to more porosity in concrete. So as we increase W/C ratio, the porosity increases leading to lesser strength. Further the interface strength(Bond strength) between Coarse aggregate and cement paste/cement sand paste is also governed by W/C ratio Hence 2 laws have been made Strength is inversly proportional to W/C ratio and Permeability is directly proportional to W/C ratio So higher strengths and durability can be achieved with Lower W/C ratio. In short W/C ratio is said to be the KEY to all Engineering Properties of concrete As we want to get higher strengths ,we need to reduce water to lower and lower levels As examples ,we can say in southern India , with best aggregate sands that we get , we can list the W/C for different grades as follows M20........0.52 to 0.55 M25.........0.48 to 0.5 M30.........0.45 to 0.47 M35........0.4 to 0.44 M40.......0.37 to 0.39 M45.......0.34 to 0.36 M50.......0.33 to 0.34 M55.......0.30 to 0.32 M60......0.28 to 0.30 M70.......0.26 to 0.28 M80........0.24 to 0.26

Query : Hello sir, I'm Sandeep my question is Why seggreagtion of concrete occurs in a cohesive design mix & how it can be prevented?

SANDEEP, Chitradurga

Doctor Q : Seggregation is opposite of cohesion and this means different ingradients of Concrete are separated ,not together in concrete. As we all know concrete is a mixture of many ingradients ,with different specific gravities . Like Cement=3.15. aggregates and sand with 2.6 to 2.8 .water=1 and admixture=1.05 to 1.25 If we make concrete with SCMs the system may become more complex. So the mix proportioning has to be done in a scientific way based on experience/data available on similar or same materials . Segregation can happen if 01. Grading of aggregates is not good and they are gap graded So select good aggregates which fits the requirements as per IS383 02. Coarse sands with least fines( below 150 microns and 75 microns) can also cause bleeding and segregation So use well graded sands preferably in zone 2 and with good amount of fines below 150 microns 03. More water can cause BLEEDING and Segregation Use right W/C ratio 04. Cement admixture compatibility,if a bad uncompatibility can result in heavy bleeding,seggregation . We need to check Cement admixture compatibility before use 05. High dosage of admixture can cause seggregation Trials need to be done and then only we need to use 06. Total fine aggregate content if it is less( coarse aggregates more) then it may cause segregation. A good mix proportion is basic requirement. Proper selection of coarse to fine aggregates ratio, proper selection of 20mm: 12.5mm is also important. 07. Flaky ,elongated aggregates can lead to segregation Good aggregate source selection and also good QA/QC with reciept of aggregates etc are important. 08. Some times the some particular Cement and SCMs may cause bleeding and segregation It is essential to do proper trials before we start large scale production We should note that in readymix industry when we are looking for 2 to 3 hours slump retention ,some amount of initial segregation and bleeding is required otherwise slump retention may become difficult. We need to be only interested in knowing only whether the mix is easily pumpable(cohesive) at the point of pouring or not. We should also note that segregated mix , can not be pumped easily ,also it may lead to honey combing ,un uniform hardened properties and durability issues.

Query : Hello Dr Q, if the customer asked us for a design and that design is not available with our batching unit what will we do?

Shamnad, Trivandrum

Doctor Q : I understand this as if a customer asks for a specific mix,if we do not have then first 01. We will collect and understand fully his specs+ requirements 02. Do trials at lab ,check fresh and hardened properties. If they are acceptable , then mix design will be ready to be submitted to customer.

Query : 1. We know & as explained by you using of SCM’s like GGBS leads us to lower initial strengths & high dry shrinkage – How much impact does it really make & is there any alternative to overcome it ??


Doctor Q : whenever we use SCMs the initial strengths are lower. Lower means what? Only if we compare the strengths of SCMs like OPC+GGBS or OPC+FLYASH at same W/C ratio,same slump to ANOTHER OPV mix of same/similar grade. If we dont compare strengths are not lower but sufficient/adequate for noram construction requirements. To give you an example let us take OPC mix 300 kg ,w/c ratio=0.51 also take 300 kg OPC+GGBS mix 150OPC+150 GGBS at w/c ratio=0.51, here the 7days will be 22Mpa and 18Mpa for OPC,GGBS mixes and 28days can be 31Mpa and 28Mpa respectively for OPC and GGBS mixes, This is a general trend. A slight difference is there in 7day strength. But we need to note both mixes almost pass the required grade. However we need to note that there are extremely good admixtures using which such differences can be almost nullified! if that is a requirement of customer. Secondly on Drying shrinkage. I made a statement that with SCMs ,all durability factors are either significantly improved or some properties are at samilar level compared to OPC mixes. Improvements can be achieved in Drying shrinkage properties of OPC+GGBS mixes with proper mix design, lowering W/C ratio and using good aggregates. Drying shrinkage can be managed well as mostly whenever we use GGBS and Flyash mixes we adopt lower W/C ratios than Control Concretes. Hence all Durability properties will improve if SCM mixes are compacted well and cured for longer times(longer than OPC mixes) Cost was not part of webinar . Direct costs of producing OPC+GGBS or OPC+PFA mixes are lesser than OPC mixes. But benifits of OPC+GGBS and OPC+PFA mixes are greater than OPC mixes and hence, they need to be sold at Higher selling prices than OPC mixes

Query : What is the impact of using GGBS in concrete? (apart from cost effective)


Doctor Q : GGBS improves the properties of concrete as follows In fresh state A. When we use GGBS in good proportions the mixes will yeild lower W/C ratio,which leads to improvement in hardened properties B. Better Slump retention compared to OPC C. Slightly more set times which allow concrete producers to transport,pump and place the concrete for longer times D. When the mixes are designed well good cohesive ,less bleeding mixes. In hardened state E. Better Long term compressive,flexural and tensile strengths than OPC mixes F. Better impermeability G. Higher Chloride ion resistance H. Higher Sulphate resistance G. Improves Drying Shrinkage,Creep resistance or performs equally to OPC H. Similar carbonationation resistance in long term I. Lower heat of hydration All the above lead to overall Durability improvement On shrinkage cracks Concrete cracks due to many reasons mainly due to quick evaporation of water in the initial stages of hardening . The cracks that occur until concrete completely sets ,occur due to quick loss of water due to evaporation , The evaporation rates are dependent on Temperature,wind velocity and humidity. So when rate of evaporation is greater than rate of bleeding plastic shrinkage cracks develops. Even plastic settlement cracks are connected to quick evaporational rates. So we need to keep the concrete cooler and make the evaporational losses minimum to contain plastic shrinkage and plastic settlement cracks. Apart from this 01. High slump mixes with higher water contents ,which when vibrated bring slurry to top and may show cracks. 02. Very low slump mixes with least bleeding can also lead to cracks if the rate of evaporation exceeds rate of bleeding 03. If the construction is happening at very big heights ,not surrounded by other buildings high speed winds can take out water quickly and cause cracks so good initial sprinkling of water after placing concrete in hot weather conditions is essential. Please note the mixes with SCMs need to be cured for longer times than normal mixes to attain their potential strengths!!!! In your specific case you can incorporate Fibres( Polypropylene fibers) to minimise the plastic shrinkage and plastic settlement cracks. Some companies have specially made Shrinkage Reducing admixtures ,which when mixed with concrete can greatly help to reduce cracks. Chennai has higher temperatures especially in summers, we need to do concrete when temperatures are least, so concreting has to be done at night(concrete has to be protected with plastic sheet once laid) and over that sprinkle water to keep concrete as cool as possible to reduce evaoprational rates. One more very important thing to be practiced. Once you compact concrete ,if cracks are seen still when the concrete placement is going on other portions of slab , REVIBRATE or RETAMP ,to close the cracks. Your other question on achieving strengths on 30 days seems to be a problem with your casting,curing etc. Curing Temperature should be as per IS ,27+-2 DEGREE . Concrete cubes need to be cured in well maintained curing tanks. Please also check most importantly whether W/C rato is properly maintained?

Query : What are the precautions to be taken while doing a cold joint

P Kanatt,

Doctor Q : The best way to prepare for cold joint is to gently remove all the mortar laitants from the set layer with water jet of light chipping.Then treat the surface with mortar of same w/c ratio as that of your concrete and then place the the subsequent concrete layer on top. Vibrate the top layer properly. Rest was covered in the lecture.

Query : What are the common concrete defects in high rise buildings


Doctor Q : I have covered almost every issue in my lecture so I am not repeating it again. However CURING I had not covered in my lecture at length and also the aspect of mechanized concreting, cracks etc. These will be covered in the subsequent lectures . Thanks for attending our Webinar.

Query : What should be the plastic thickness used to cover the concrete for curing & what is the best time to place the cover after concrete ?


Doctor Q : Any thickness will do.Thicker the plastic the better it will last the rough handling at site. It has to cover the surface of the concrete fully and not fly away due to wind.

Query : Why not do the work at night shifts rather than have to put the plastic sheets to avoid plastic shrinkage cracks?


Doctor Q : In the night too due to wind and low humidity plastic shrinkage cracks can occur if the plastic sheet is avoided.

Query : What would you suggest as technology of concrete for new beginner as a contractor, whether RMC or Site mix.


Doctor Q : When you take up contracts , we recommend to consider RMC. 01. The quality assurance and Quality Control will be of high standards and will be in line with IS4926. 02. The ingradients will be selected ,checked ,tested to be as per relavant standards( For Ex. Sand, coarse aggregate,water and also of course Cementitious materials like CEMENT,FLYASH or GGBS) 03. The RMC plant will be managed by Professional Engineers and trained personnel ,who make best efforts to ensure right Concrete is delivered to you. 04.RMC comes with a stamp of quality and guaranteed properties (both fresh and hardened properties) 05. As a end customer or client or contractor you need not have to invest your valuable time in ordering different ingradients also need not have to worry about different vendors etc for each ingradient 06. You will be peaceful as the headache of storing different ingradients to make concrete ,spilling aggregates or sand on road or in your yard ,also no necessary to allocate your valueble space for storing the ingradients for concrete. 07. W/C ratio is the Key to all Engineering properties of concrete and that can be successfuly applied as per Mix Design ,through Computerised batch plant ..which will never happen to that accuracy in site mix. 08. You will get 28 day strengths of the concrete through cube strength reports. 09. Faster placing of concrete at your site which can lead to speedy constructions 10. From a menu of different mixes ,you can order a concrete mix as per your requirement The above are some advantages which can not be overlooked at all. Having said above ,if your site is far away ,in a remote place where there is no chance of getting readymix Concrete ,you will nave to opt for Site mix. In any case, we would suggest you to go through all basics of Concrete Technology,understand dos and Donts to start your contracts successfully with satisfaction

Query : 1. For curing do we need to check the water properties like pH etc.

Haroon Ali,

Doctor Q : In construction sites ,for making concrete and curing water has to meet the requirements of IS456 and relevant parts of IS3025 . Random sampling and testing can be done to assses the quality of water. In most of the project sites they dont change source of water to be used in making concrete and also for curing once selected,so that once these are checked ,certified by 3rd party lab they need not have to too much worry on this. Many project sites they check at every fortnight as per the meet the IS 456 .

Query : 1. For workability which is lesser evil ,more water or additives?


Doctor Q : To get more workability ,we can use more dosage of Chemical admixture(additive) and get desired workability +strength. But if we increase water to increase workability ,W/C becomes more and it leads to lower strength,lower durability. Too much water is too bad and it will spoil the concrete. So it can be evil or even devil for concrete On the other hand additives also if used more than Prescription(by manufacturer ) can cause problems like delayed set but fortunately they dont disturb WC ratio . In short more additives is lesser evil(only if it is within certain limits) than more water(as it shifts quickly concrete to lower grade)

Query : 1. Are there additives to reducing reinforcement corrosion ?


Doctor Q : Yes there are additives available for reducing reinforcement corrosion. Corrosion can be reduced by 01. By using mineral admixtures like GGBS,FLYASH ,Microsilica etc ,reducing Water /cement ratio ,which leads to better chloride ion resistance,improved impermeability and hence complete improvement in Corrosion resistance/durability. 02. By using Nitrite based corrosion inhibitors........which have a good history 03. By using migrating corrosion inhibitors 04. Surface applied materials which penetrate into the concrete Each of the above(02,03,04) have several varieties within their class ,and can be used effectively to enhance corrosion protection to steel bars.

Query : If hard water is only available for construction then what admixture to use?

Tej Shah,

Doctor Q : Water for Concrete Construction and curing has to meet the requirements of IS456. The standard stipulates many the limits for impurities which can affect not only fresh ,hardened properties but durability too. It is mandatory to use water which complies to standards. Now hard water contains generally some calcium and magnesium salts and some minerals. It is considered not too bad for concrete. However if they are in excess they may affect strength, set time and if water contains high amounts of alkali carbonates and bicarbonates they may trigger Alkali Silica Reaction in some extreme situations. So we need to test concrete with water in Question against normally used acceptable water as per IS456 ,ALSO with distilled water. The concrete with water in question(in your case hard water) should not give less than 90% of Control or reference concrete. If we are trying to use such water , any non potable water with NON CONFORMANCE to IS 456 limits , we need durability tests too on such concretes before using in RCC structures. The treatment of such waters may be costly and can change on case to case basis (as presence of different minerals may affect different properties of concrete). There is no such specific treatment of hard water to be used in large scale construction projects. We may have to ion exchange or such treatments . The best can be to use OPC+ GGBS or OPC+FLYASH , with very low W/C ratio ( using a High performance Chemical admixture)and also incorporate a Corrosion inhibitor and asses all durability tests ,before we use for large scale production.

Query : We are casting Hexapods which are at 16m3 and 24m3 and we are facing lot of problems of cracks at joints ,the structure is PCC

Devang Shah,

Doctor Q : The volume of Concrete in each Hexapod is big. If OPC mixes are there there will be heat build up which can cause some cracks. Preferably it can be done with OPC+GGBS or OPC+Flyash mixes with low W/C rato. Such Combinations of Binders can reduce heat of hydration which in turn help to minimise thermal cracks Low W/C ratio is a key to Durability,strength and also help in minimising plastic shrinkage cracks. The temperature of concrete and Ambient temperatures play major role on rate of evaporation of water from the Hexapods after casting. So intial curing is very essential. Once the formwork is off,the Hexapods need to be immediately put into continuos curing Regime. If this is not done cracks may appear There are also REENTRANT CORNERS ,Re-entrant corners are defined as any inside corner that forms an angle of 180° or less. In a solid object that is subjected to internal or external loads, re-entrant corners create high stress concentrations. If that solid object is concrete, which is strong in compression but weak in tension, then it will inevitably lead to a crack that will propagate at approximately 135° from the corner. we need to consider this aspect too. We can not judge all these sitting at a different place away from your site. My suggestions are 01. Do concrete with SCMs( Avoid pure OPC mixes) 02. Do proper initial curing and later continuos curing 03.Do concrete in low temperatures of the day( like evenings or night) 04.Slump should not be too high . 05. Mix should have low W/C ratio (as low wc ratio as possible). For which a good HRWRA can be used. 06. Incorporate fibers (Polypropylene fibers) 07. Incorporate Shrinkage reducing admixtures 08. Use good mould release agent otherwise during demoulding excess force can weaken or crack corners. While lifting and transporting also we must be careful to handle smoothly so asto avoid any chipping off or cracks at corners 09.Compaction should be done as per best practice guidelines otherwise honey combing or cracks can appear 10. Concreting when temperatures are very high and when winds are blowing fast can take away lot of water due to evaporation and hence that needs to be avoided. Concretes should not be done when rate of evaporation is greater than or equal to 1kg/m2/hr 11.You need to review mix design too. If the mixes are having too much sand , and aggregates are less,that may lead to higher drying shrinkage also. 12. You need to sprinkle water on aggregates to keep them cool otherwise that will lead to slump loss and higher water demand.

Query : If OPC and PPC doesn't qualify then what we use?


Doctor Q : We believe the question pertains to cement for plaster. It was told by Dordi Sir that a cement to be used to make plaster has to have certain amount of Air entertainment and water retentivity. For this OPC and PPC does not qualify and masonry cements are made to meet these requirements. Now since Cement Companies do not manufacture such cements , we can take OPC or PPC cement and add Air entraining ADMIXTURE available in market to incorporate some amount of air and increase water retentivity of plaster mixes.

Query : how much admixture dosage correction is allowed or can be done ... owing to temperature change... seasonal as well as in a day


Doctor Q : A. GENERALLY THE DOSAGE OF ADMIXTURE SHOULD NEVER EXCEED THE SPECIFIED MAXIMUM LIMIT STATED BY THE ADMIXTURE MANUFACTURER . THE QUANTITY INCLUDES THAT WHICH IS ADDED AT THE PLANT. HOWEVER IF YOU ARE WELL WITHIN THE MAXIMUM LIMIT THEN YOU MAY ADD THE ADMIXTURE TO GET THE REQUIRED SLUMP .Precaution should be taken to mix concrete giving the transit mixer max revolutions for at least 8 minutes. B. The correction dosage due to temperature. Let us say in winter/rainy season the dosage of Superplasticiser is about 0.4 % ,in summer owing to higher temperatures the dosage can go upto 0.6% ,about 0.2 to 0.3% higher. And this can be vice Versa in winter or rainy season. But practically each season along with temperatures the other changes also happen with ingradients of concrete and that may have a positive or negative impact on dosage. Concrete Producer also adds a small dosage on site to boost the slump,in case of slump loss during hot weather conditions or when the truck takes more time to travel. This correction dosage is 0.05% to 0.1% to boost the slump on site. The Cement and fines of sand &aggregates ,many times have more profound effect on slump retention than small changes in temperatures.

Query : Could you please explain the hydration process of alkali activated concrete and strength imparting hydration productions.are those hydration products superior to CSH


Doctor Q : It is true that AAM durability is much more compared to normal concretes . All properties are improved and now scientists can design these concretes/cements to give great advantages against existing concretes. Alkali activated concretes/materials, the mechanisms are not completely understood ,very complex and world wide lot of research is going on in this field. 1. Dissolution of alumina and silica present in alumino silicate materials at high pH created by the activator components. This dissolution is strictly governed by the co-ordination number of alumina present in the fly ash or slag. 2. The dissolved alumina and silica will then re orient to form a cross linked gels 3. Final process is the crystallization of the gel to form a hardened structure. The above points talk about basics of reaction mechanisms (hydration process)of AAMs. Your second question about Self healing Concrete Even though lot of scientists are working in this field ,as of now only limited number of large scale commercial applications are done. Hence we suggest lot of work is to be done in this area to improve/standardise specifications . But this will be a very promising area in the concrete field.

Query : 1.As you said, use of higher ggbs in mixes tends to increase albedo value and subsequently increase reflectivity of structures and keeps indoor temperature at low but, what about outdoor temperature as it will get increased or not? Kindly suggest. 2.How concrete with higher albedo value is possible for PQC as the light reflection tends to sight irritation to road users, can we control it or not?

Hrushikesh Prayag,

Doctor Q : When light gets reflected ,the reflected light is not contained and it spreads in atmosphere( which is open) and hence heat build up will not take place. Let us understand that Heat build up and Heat island effect happens in urban areas ,where lot of heat is locked due to absorption of heat (which heat up those surfaces ) and due to surfaces which are darker( towards black or grey colour). second question is, in PQC when we use GGBS based concrete , the reflectance will be more (Albedo number will be more) which means you need least light to move in dark nights. It does not irritate eyes.(Irritation comes when light from opposite vehicles directly fall on your eyes).

Query : I have some query about the SNF & PCE based admixtures. Pls explain which admixture is better and why ??

Vikram, Qcrete

Doctor Q : Dear Mr Vikram, SNF based admixtures have a long history and their use started in 1960/1970. They are used world wide at dosages between 0.4% to 1.5% some times even at higher dosages. Their water reducing ability is 12% to 25% ,based on dosage,formulation etc. They are very popular even now in many countries including India. But in 1990s the PCE based admixtures came into the construction world. Here the PCEs give huge water reductions between 25% to 40% ,based on formulations. So advantages of PCE based admixtures 01. Very High water reductions leading to high grade concretes 02. Now we can produce M25 To M60 with Single admixture 03. Very high Early strength Concretes specially for Precast can be made using special PCE based admixtures for example 12 hr,16 Mpa 04. Very high slump retentions,we can have PCE based admixtures which can give 4 to 5 HOUR Slump retention ,with special formulations,which is very difficult with SNF based admixtures 05. Suitability to use with Msand,CSS etc 06. By using PCE based admixtures we can produce SCC(which can not be done using SNFbased admixtures). 07. M80 etc can be done easily using PCE based admixtures withOPC+scm COMBINATIONS. Disadvantages 07.Some PCE based admixtures can make concrete very sticky and unpumpable 08. Some PCE based admixtures can be very very sensitive to moisture and certain brands of cements ,leading to heavy bleeding with change in moisture etc.Hence that type of PCE based admixtures will become problematic and needs extreme QAQC supervision at all points. The chemistry is totally different(from SNF) and in PCE they have long main chain and side chain molecules which deposit more negative charges on the cement particles and higher Dispersion,higher workability and great WATER REDUCTIONS.

Query : my question is.. If Calcium hydroxide is consumed then how does alkalanity of concrete is maintained and how is steel protected.

Shraddha Anawkar,

Doctor Q : In my view Ca(OH)2 is responsible for maintaining the alkalinity near the steel so that a passive film of Iron oxides can be formed. In concrete containing pozzolana the Ca(OH)2 produced by the hydration of cement will not be consumed immediately by the pozzolan. Hence there will be a sufficient time for the passivation film to be formed on the surface of the steel. This passivation film will become unstable only when you create an environment of pH <10. we also need to know that Alkalinity(pH) is not directly Proportional to the amount of Calcium Hydroxide. Even Though it is true that Ca(OH)2 is consumed by the pozzolan, not all the Ca(OH)2 will be consumed. Please remember that for every 1 gram of C3S hydration 0.4 grams of Ca(OH)2 will be produced. (These numbers are arrived through modelling). For a concrete containing 40% fly ash still there will be a lot of Ca(OH)2 left after the pozzolanic reaction. Secondly, in concrete containing pozzolan the corrosion is prevented by creating a denser matrix so that water and other corrosive ions take a longer period of time to reach the steel surface.

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