Till the late 1970s, the use of concrete in excess of M40 in tall buildings was not known. Now, with the effective use of admixtures and other sophisticated applications of concrete technology, it is easy to achieve a strength of 50 MPa in 12 to 18 hours, and above 70 MPa at 28-days. It is feasible to produce concrete having a compressive strength of up to 150 MPa at 91-days.
Concretes in this strength range can be made using carefully selected cementitious materials, sand, 10 mm to 20 mm coarse aggregate and admixtures through the use of very low water-cement ratios (say 0.25-0.35) besides careful quality control in production.
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The fineness of cement, as well as the heat of hydration, can be used to advantage. The desired workability is achieved by high range water reducing admixtures (HRWRA) or superplasticizers, The selection of an admixture may need a considerable amount of research/trials.
The time taken to achieve a specific strength is an important economic and design parameter in high strength concrete construction. In fact, a high early strength may bring far more practical and economic benefits than a high strength at a later age, in terms of early removal of formwork, increased productivity of precast units, early transfer of pre-stress, and early application of service loads.
43 grade or 53 grade of ordinary Portland cement can be used in the production of high strength concrete. Their properties were shown below. It is not possible to achieve a compressive strength higher than about 60 MPa at 28 days with the use of such cement alone. In order to attain higher strengths, other options have to be explored such as the use of chemical admixtures or mineral admixtures or combinations thereof. Thus, various options are :
- use of 43 grade or 53 grade ordinary Portland cement
- use of cement + water-reducing admixtures
- use of cement + pulverized fuel ash (PFA)
- use of cement + condensed silica fume+ admixtures
The design of high strength concrete using pulverized fuel ash can be done in accordance with the recommendations of the American Concrete Institute (ACI).
Properties, Admixture, and advantage of High strength concrete
High-strength concrete (HSC) is a type of concrete with high compressive strength compared to normal-strength concrete (NSC). Although there is no exact limit of compressive strength that could distinguish HSC from NSC, the American Concrete Institute defines High strength concrete with a compressive strength of more than 6,000 psi.
Advantages of High strength concrete (HSC) are:
- It reduces the cross-section of structural elements and therefore increases available space,
- It improves aesthetics due to slimmer cross-section,
- It reduces self-weight of the structure,
- It increases the modulus of elasticity of concrete and reduces creep (deformation under continuous loading) that controls short-term and long-term deflections, and
- It improves the long-term durability of structures which is a key concern toward sustainable use of construction materials.
- High strength concrete is a useful material for high-rise buildings, long-span bridges, heavy-duty industrial floors, pre-stressed concrete, etc.
Concrete is a mixture of cement, water, coarse and fine aggregates with or without chemical and mineral admixtures. As aggregate covers 75 percent of the volume of concrete, for HSC, high-strength well-graded aggregate is essential.
The presence of different sizes of aggregate (well-graded) in appropriate proportions is important to reduce void. The maximum size of aggregate is also another important factor. Due to the internal bleeding of water in concrete, the bonding can be poor for large-sized aggregate.
20 mm downgraded aggregate can be used for making concrete of strength of 6,000 psi or higher. However, it can be reduced to 12 mm if the strength requirement is 10,000 psi or more.
The bonding with cement paste around aggregate is improved for smaller-sized coarse aggregate. For High strength concrete, the amount of cement is to be increased compared to normal strength concrete.
The amount of cement in High varies from 420 to 650 kg per cubic meter based on the strength requirement. As the amount of cement is increased, the size of the fine aggregate is also to be increased compared to normal strength concrete.
An increase in the sizes of fine aggregate will create adequate free space among aggregates for cement hydration products generated from a larger amount of cement compared to the normal strength concrete. Due to the use of more cement, the heat of hydration and plastic shrinkage of fresh concrete will be increased but can be controlled with the utilization of mineral admixtures with clinker, such as fly ash, slag, etc.
Admixture in High strength concrete
A part of cement (5 to 8 percent) can also be replaced by silica fume. As silica fume particles are very fine (less than one-hundredth of cement particles), they fill nano-scale voids in concrete. It can also convert calcium hydroxide generated in the hydration process of clinker to new strength-giving material, CSH gel.
Another important parameter of high strength concrete is water to cement ratio (W/C). The W/C for high strength concrete can be fixed within the range from 0.25 to 0.40 based on the strength requirement. For complete hydration of cement, 23 percent of water is required. The extra water will create a void in concrete and will eventually reduce strength. By lowering W/C, the amount of void in concrete is reduced, which is a mandatory requirement of high strength concrete. With the reduction of W/C, the workability (flowability) of concrete will be reduced. However, the flowability of concrete can be improved significantly by using water-reducing chemical admixtures.
Based on a laboratory investigation, it has been found that to produce strength over 5,000 psi, it is necessary to use stone chips as coarse aggregate. A further study has been conducted for the production of high strength concrete using Maddhapara hard rock mine of Dinajpur.
The stone chips produced at this quarry site are very strong (abrasion loss is less than 23 percent), and therefore can be utilized for making high strength concrete A mixture of concrete using Maddhapara hard rock with W/C of 0.30, CEM Type I cement of 460 kg per cubic meter, and silica fume of 40 kg per cubic meter produced compressive strength of 11,000 psi. The strength can be improved further by reducing the maximum size of coarse aggregate and increasing cement content.
For the production of high strength concrete, it is recommended to make trial mixes before construction. A pre-construction meeting with the contractor is also important. During construction extra care is necessary to prevent plastic shrinkage and thermal cracking.
Extra care is also necessary for quality control of materials, mixing, transportation, placing, compaction, and curing. As we are investing a huge amount for construction of several mega projects and the private sector has also come forward to invest in the construction of high-rise buildings, our civil engineers need to familiarise themselves with the parameters and challenges for making high strength concrete.
high strength concrete is a durable concrete and, therefore, all construction works in marine exposure can be planned with HSC.
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