Shivram Bagade, Concrete Technologist – BASF Construction Chemicals India Pvt. Ltd, Bangalore Nagesh Puttaswamy Manager (TASC) UltraTech Cement Ltd., Hyderabad
Fast cars, fast travel schedules, fast track construction has become the order of the day. In many ways, these technological advancements have been an economic boon for the mankind but at what cost? Speeding cars are thrill on the race tracks but the risks are also a part of the system. The concept of ‘time saved is money saved’ induced the fast track work in construction industry, since then engineers and administrators are making a holistic approach by making every part of construction to contribute into the system in making the construction faster. High Strength concrete is also one of them. The history of high strength concrete is about 35 years old, in late 1960s the invention of water reducing admixtures lead to the high strength precast products and structural elements in beam were cast in situ using high strength concrete. Since then the technology has come of age and concrete of the order of M60 to M120 are commonly used. Concrete of the order of M200 and above are a possibility in the laboratory conditions.
With this as a confidence level the industry today has some very challenging demands for the cement manufacturers, admixture manufacturers. The demand for high strength in a very short duration has come up. There have been demands like,
- 40 MPa of M60 concrete in 3 days,
- 50% of target strength in 24 hours,
- 12 MPa in 12 hours,
- 12 MPa in 10 hours.
IntroductionConcrete based on the 28 day strength, is classified as high strength concrete so and so forth. Till about 1970s the concretes that could achieve strength of above 40Mpa were classified as high strength concrete. When concrete mixtures of about 60Mpa and above were commercially produced the bench mark for the high strength concrete was raised to 55Mpa or more.
The history of high strength concrete is about 35 years now, since the development of super plasticiser admixtures in the late sixties the Japanese with their high strength pre-cast products using ‘naphthalene sulfonate’ and the Germans with under water concrete using ‘melanine sulfonate’ were the pioneers of the technology.
Three to four decades ago despite the availability concrete as versatile construction material. Most of the highrise buildings all over the world have used steel elements as structural frame. The famous Twin Towers at Manhattan (World Trade Centre) had steel frames. The reason was that with the strength of concrete available in those days the members made of concrete would have been bulky and ugly.
With advent of high strength the bulkiness in the concrete members are gone and we are able to make slender sections in concrete too. Since then high strength concrete has come a long way and is running a race to reach the strength of steel. The concrete of the order of 200 Mpa has become a reality at least at the laboratory conditions and concrete of the order of M60 to M120 are commonly used at sites. The properties of the high strength concrete are well studied and understood by the engineers today, the use of very high strength concrete no longer raises eye brows. The drawbacks of the high strength concrete have been countered by the user.
How High Strength in Concrete AchievedThe higher strength in concrete could be achieved by using one of the following methods or a combination some or many of the following:
- Higher cement content
- Reducing water cement ratio
- Better workability and hence better compaction
|Some of the Codal requirement of the high strength concrete|
|Compressive strength||60 Mpa or more|
|Durability||Permeability < 5 mm as per DIN 1048|
|Workability||to be placed in areas of high congestion.|
The requirement of high strength concrete requires a higher cementitious material in the concrete mixture, which could be in the range of 400kg plus per M3. The cementitious content of higher value would lead to higher thermal shrinkage and drying shrinkage, and there would be a stage when any further addition of cementitious material will not affect the strength. For the durability aspects, the minimum and maximum cement content in concrete is governed by the codal provisions, reduction of water cement ratio has also its own limitations especially in the site conditions. The hunger for the higher strength leads to other materials to achieve the desired results, thus emerged the contribution of cementitious material for the strength of concrete.
- Addition of pozzulanic admixture like the Pozzulanic Flyash (PFA) or granulated blast furnace slag (GGBS) which helps in formation of secondary C-S-H gel there by improvement of strength.
The addition of pozzulanic admixture like the flyash used as admixture will reduce the strength gain for the first 3 to 7 days of concrete and will show gain beyond 7 days and give a higher strength on long term.
- Addition of mineral admixtures like the silica fumes or metakaolin or rice husk ash.
The highly reactive pozzulanic admixtures like the silica fume or metakaolin and Rice Husk Ash (RHS) will start contributing in about 3 days. The RHS has an advantage over the PFA because the RHS is more reactive.
- Use of chemical admixtures like the Super-plasticiser or Hyper-Plasticiser, set controlling admixtures will help in attaining the higher strength in concrete.
The research and the experience indicate that the admixtures based on the Poly Carboxylic Ethers (PCE) called the hyper plasticisers are the best suited for the job as they have a water reducing capacity of 18% to 40% in reference to the control or reference concrete.
- Combination of all of the above or some of the above to achieve the desired strength.
The combination of at least a few of these methods has now become invariable as the HSC came along with some complexities like higher shrinkage, higher heat of hydration etc., all these complexities need to be neutralised or controlled. Most of the problems were handled by a combination of PFA or GGBS and PCE admixture.
In order to accelerate the cement hydration steam curing methods are also adopted however this may not result in higher strength. The strength gain at early age can be achieved by replacing a part of the fine aggregate by flyash or blast furnace slag, without increase in the water requirement of the concrete mixture.
Properties of Cement Required
|Compressive strength||> 60 Mpa|
|C3A Content||< 6|
|Fineness||300 + 20 Sq. M per Kg|
|Total alkali content||Max. 6% expressed as Na2O|
Properties of FlyashPFA required enhancing the strength, impermeability and durability of concrete. Class F PFA has to be used.
It reduces segregation and bleeding in fresh concrete, creep in hardened concrete, it also lowers heat of hydration.
|SiO2 + AI2O3 + Fe2O3||= 85%|
|Fineness||Max. 10% (retained on 45 Micron)|
AggregatesFine Aggregates: Should fall in Zone II
Chemical AdmixturesThe High Range Water Reducing Admixture (HRWRA) have to be used, normally the PCE admixtures are formulated for the specific need. High Range Water Reducing Admixture (HRWRA): It is a known fact that for the durability of the structure permeability in concrete plays a major role, one of the important factors that govern the issue water–cement ratio during the manufacture of concrete, lower the water-cement ratio lower will be the capillary pores and hence lower permeability and enhanced durability.
Poly Carboxylic Ethers (PCE) based admixtures called hyper plasticizers invented in the 1990’s have fulfilled the conditions in an excellent way, the advantages of which are being exploited in the production of high strength and high performance concrete. The water reducing capacity of these admixtures is between 18–40% of the control or reference concrete. These admixtures assist in achieving higher slumps more than 180 mm at much lesser w/c ratios (less than 0.30). They impart better control over the rheology of the concrete and that’s one of the reason such admixtures are always used for producing for selfcompacting Concrete. The only disadvantage of these admixtures is that they do not have a longer retention beyond 45 minutes and are always used along with retarding agents, adding to the complexities of the mix. Situations of these complexities need to be handled very carefully, however the construction chemical industry has come out with combination of chemicals or let us say cocktail of chemicals to meet the demand.
Industry’s DemandThe construction industry turning towards pre-cast elements and requirement of post-tensioning has made the requirement of the high strength of concrete invariable and the engineers had to overcome these drawbacks, which to a great extent we have been able to do. The construction of modern days have become fast track where the economics on the investment on the form work are considered the use of high strength concrete is invariable and has become a must. The construction today is crucial when it comes to economics very fine aspects are considered to achieve savings in construction work. The speed of construction and its technology is measured in terms of the number of cycles of the use of formwork. This has now turned into one of the basic requirement of concreting process, the demands of the industry has become very complicated.
We had an opportunity to observe and work with some of these cases. Some of the examples are as follows:
This concrete requirement was for precast segmental girders to be post tensioned for a fast track infrastructural project. The time is the main objective of the contract between the consumer and contractors. The segments need to be hoisted into place within a record low time, and needs to be post-tensioned. The continuity in work, the construction area being very congested for work demands that the infrastructure for the casting moves further at the earliest.
|Grade of concrete required||M50|
|Special requirement specified|
|01||Minimum strength required in 24 hrs||25 Mpa.|
|02||Workability required: Initial collapse slump|
|03||Workability required: Pumpable concrete after 90 minutes|
|04||To get high early strength without stickyness|
|These large segmental girders for a elevated highway project|
|Trial 1||Trial 2||Trial 3|
|Cement OPC 53 grade||5.4||5.4||5.4|
|W/C, A/C, W/B|
|CA - I 20mm||6.228||5.292||5.472|
|CA - II 12.5mm||5.748||6.372||5.472|
|CA - III 6mm||2.4||2.712||2.76|
|Total Water (Incl. Abs)||1.974||1.98||1.99|
|Total Water (Incl. Abs)||1.974||1.98||1.99|
|Dosage (in %)||@email@example.comfirstname.lastname@example.org|
|Slump / Flow (in mm)|
|Wet Density (kg/Cu. M)||2462||2405||2482|
|Comp. Strength (N/Sq. mm)|
|Batch Size / Cu. m||0.012||0.012||0.012|
|Grade of concrete required||M60|
|Special requirement specified|
|01||Minimum strength required in 24 hrs||40 Mpa.|
|02||Workability required as in SCC|
|03||Quantity of cementitious material Pre-fixed - fly ash from a fixed source|
|04||Fine aggregate||100% crusher dust|
The concrete had to achieve more than 50% of its final strength in 24 hours as we have already discussed earlier the whole process to happen in about 14 hours of the final setting time of cement!
The project requirement was for the shell-plate roof elements to cast on ground and needs to be hoisted to place and was post-tensioned. The requirement of early high strength of concrete was for the fact that the members need to be shuttered early and moved to the curing yard and the number of cycles of formwork was critical economical criterion. The segments had special shapes and folds hence there were a requirement of the self-compacting property. The retention of workability in combination with the requirement of early high strength pulled the system in opposite directions.
The concrete in this case was made at the captive batching plant at site and the necessary conditions were achieved with a couple of alternative cement brands. The concrete was placed into the mould using transit mixers within the site had retain the workability criterion for nearly 45 min to 1 hour. The details of the mix design could not be provided here as all trials were done by the consumer and we could not get the permission to publish here. The admixture there had to be specially fine tuned for the specific requirement and it had to under further fine tuning with respect to change in the ingredient sources. Several thousand cubic meters of concrete has been consumed. Is it going to be the trend setting benchmark for other projects?
|For a Segmental Construction of Towers|
|Grade of concrete required||M60|
|Special requirement specified|
|01||Minimum strength required in 12 hrs||12 Mpa.|
|02||Quantity of Cement||Pre-fixed|
|03||Quantity of cementitious material||Pre-fixed - fly ash from a fixed source|
|04||High reactive mineral admixtures like Silica Fume||Not permitted for economic reasons|
|05||The minimum strength requirement was modified to 12 Mpa in 10hrs|
Initial slump : Collapse.
Slump after 30 min : Collapse and flow min 450 mm.
The requirement of this concrete was for the making pre-fabricated segments of towers cast at logistically suitable place. The circular segments with variable diameter from one end to other have to be fabricated in a yard and hoisted to the curing yard. The cost of the formwork and the restriction on the space available for the casting area demands the segments to be hoisted from the casting yard to the curing yard, the very early strength requirement is to cater for the hoisting load. The consumer is applying a protective coating on the precast segments for ensuring the durability. The segments are transported on trucks to destinations as far as 600-700 kilometers for actual use. The economic criterion for the consumer demanded that there should be at-least 2 cycles of casting for every 30 hours.
The following mix design was arrived at for getting the desired conditions the average of nearly 10 to 15 trials were made with different some selected brands of cements, the critical results between different brands of cement are listed in the Tables.
|Time in minutes||Slump in mm||Flow in mm||Remarks|
|0||Collapse||575 to 625||These parameters were kept constant for all brands of cement for a definite dosage of admixtures|
|30||Collapse||495 to 525|
It was observed that the loss of slump and flow was rapid after the 45-50 minute mark; the values indicated here are the average of several tests that were conducted. After 60 minutes the concrete exhibited unusual stiffness and elastic/spongy feel.
|Mix proportioning details|
|Cement 53 grade OPC||Brand X||Brand Y||Brand Z|
|Comp. Str. (Mpa) average of all cube results|
The results from the Brands Y and Z are very important to be noted the gain of strength between the 12thhour and the 14thhour are almost 3 times.
The Drawbacks of the TechnologyThe implication of such rapid strength gain is alarming and needs a serious attention. Considering the fact that the final setting time of most brands of cement in India is around 120 minutes, there will be a lot of questions answered regarding the durability aspect.
Irrespective of these question marks, technology has been able to satisfy the requirement of the customer and the work is in progress. As technologists, we need genaralise the requirement for the use as many more requirements may come and all may have to have captive batching plants to produce these special concrete.
The high strength concrete has great advantage in the modern construction scenario as many statistics show that it has not only delivered in the strength aspects but also in terms of economy. Studies show that an approximate increase of 5 times in the strength of concrete will have only about 3 to 3.25 folds increase in concrete. If the designer is able to exploit the conditions the over all costing of the project will definitely come down. Hence lots of projects today are adopting the high strength concrete of the order of M60 or above regularly.
The advancement of the formwork technology is making the erecting and removing of formwork system has become easier and simpler. The cycle time for formwork is reducing drastically one-week one-slab concepts are widely being adopted, in the developed countries this cycle period is about 4 days. In order to achieve this, we need the concrete to attain a minimum amount of strength in that time.
That means Early High Strength Concrete will become the order of the day.
The precast concrete element, manufacturers are already adopting the technology with or without proper technical know how. Why not when we have been able to incorporate the self-compacting property or the self-leveling property with a slight modification of cost. The consumer has been able to make the concrete element with a slightly cheaper cost. Now if the consumer can increase the production of the elements in terms more number of cycles per form work the project cost shall be benefited.
|Type of Concrete||Cost||No. of Cycles per month||Increase in cost||Increase in no. of cycle|
|SCC / EHSC||3200||45||10.35%||50.00%|
With all the parameters kept constant this is a very lucrative offer for any business proposition. The construction industry would be turning towards this technology called the EHSC in near future.
The technologists should deliberate about this concept/technology in a very critical way or else this concept could be abused by indiscriminate use. The deliberations are more important now because this technology is being used in the infrastructural project.
ConclusionThe EHSC is here to stay but we need to be careful that we will not forgo the durability aspects of the material. There are few products that are available to ensure durability in these concretes as coatings and secondary admixtures within the concrete mixture. A careful effort should be taken in to impart knowledge on the protective systems along with the technology of early high strength concrete.
With many of the construction sites and some of the precast units being manned by non-technical or untrained personal the technology should be allowed to have sustained growth not allowed to have premature problems.
- Concrete Microstructure properties and Materials P. Kumar Mehta, Paulo J.M. Monterio
- Workability of self-compacting concrete by Chiara F Ferraris, Lynn Brower, Joseph Daczko
- Concrete Admixture Handbook V.S. Ramachandran
- Properties of Concrete A.M. Neville
- Properties of Ultra High strength Cement Konstantin Sobolev, Svetlana Soboleva
- The article depicts the views of the authors based on the studies and observations made by them while working on specific cases and observations made there off. The inference of the information given in this article is left to the readers, it is only an effort made by the authors to open deliberations on the topic rather than conclude anything.