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    Early High Strength Concrete Advantages and Challenges

    High Strength Concrete

    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.
    The reasons for these demands are many, but as engineers, we need to think about the durability aspects of the structures using these materials. The consumers in these cases have been catered for their requirement through their insitu batching plants. With long term durability aspects kept aside we have been able to fulfill the needs. The concrete of these properties will have a peculiar Rheological behavior. Some observations made during the trials for Early High Strength Concrete (EHSC) are discussed in this paper along with some of the durability issues. The need to understand the rheological parameters in connection with the durability aspects need to be given a careful attention. The technology of EHSC is being used more in the infrastructural projects and pre-cast industry in some cases, it is being used indiscriminately and in few cases it is being adopted without proper technical backup. Cement and admixtures are selected and rejected based on economic criterion, the very sensitive issues like the cement admixture compatibility and micro plastic-cracks are not addressed in the right way. The concept of EHSC is a boon for the pre-cast industry but needs to be nurtured in a better way so that it will not get in to a trap like what happened to the use of flyash in concrete, which even today has not got 100% confidence with the technical fraternity.

    Introduction

    Concrete 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 Achieved

    The 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 the ingredients used in the HSC are:

    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
    C3S > 50%
    C2S > 24%
    C4AF > 15%
    LOI < 2%

    Properties of Flyash

    PFA 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.

    Chemical Requirements

    SiO2 > 60%
    SiO2 + AI2O3 + Fe2O3 = 85%
    LOI 2% Maximum
    Fineness Max. 10% (retained on 45 Micron)

    Aggregates

    Fine Aggregates: Should fall in Zone II

    Chemical Admixtures

    The 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 Demand

    The 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:

    CASE 1

    Infrastructure Project

    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
    Mix Grade M-50 M-50 M-50
    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
    Crushed Sand 8.448 8.448 9.12
    Total Water (Incl. Abs) 1.974 1.98 1.99
    Crushed Sand 8.448 8.448 9.12
    Total Water (Incl. Abs) 1.974 1.98 1.99
    Admixture M-715 M-715 M-715
    Dosage (in %) @0.6 @0.6 @0.7
    Slump / Flow (in mm)      
    0 min 210 200 200
    30 min 150 150 165
    Wet Density (kg/Cu. M) 2462 2405 2482
    Comp. Strength (N/Sq. mm)      
    01 Day 28.04 30.04 31.32
    02 Days 40.12 42.44 46.20
    07 Days 47.80 48.20 50.00
    28 Days      
    Batch Size / Cu. m 0.012 0.012 0.012

    CASE 2

    Infrastructure project
    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?

    CASE 3

    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.

    Workability
    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
    60 110 Nil
    90 50 Nil
    120 20 Nil

    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
    Mix Grade M-60 M-60 M-60
    Cement 53 grade OPC Brand X Brand Y Brand Z
    Cement 470 470 470
    PFA 130 130 130
    W/C 0.26 0.26 0.26
    20 mm 454 454 454
    12.5 mm 454 454 454
    Natural Sand 750 750 750
    Free Water 154 154 154
    Total water - - -
    Admixture G30(S3) G30(S3) G30(S3)
    Dosage(%)      
    Comp. Str. (Mpa) average of all cube results
    12 Hours 14.15 5.38* 4.5*
    14 Hours 14.33 13.33* 12.85*

    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 Technology

    The 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
    Conventional 2900 30 - -
    Self -compacting 3100 36 6.70% 20.00%
    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.

    Conclusion

    The 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.

    Reference

    • 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.

    NBMCW March 2009

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