High-Strength Concrete: Key Parameters and Challenges
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 which could distinguish HSC from NSC, the American Concrete Institute defines HSC with compressive strength of more than 6,000 psi. The main advantages of HSC are: (1) it reduces cross-section of structural elements and therefore increases available space, (ii) it improves aesthetics due to slimmer cross-section, (iii) it reduces self-weight of the structure, (iv) it increases modulus of elasticity of concrete and reduces creep (deformation under continuous loading) that controls short-term and long-term deflections, and (v) it improves long-term durability of structures which is a key concern toward sustainable use of construction materials. HSC 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 strength requirement is 10,000 psi or more. The bonding with cement paste around aggregate is improved for smaller-sized coarse aggregate. For HSC, the amount of cement is to be increased compared to NSC. The amount of cement in HSC varies from 420 to 650 kg per cubic metre 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 NSC. 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 NSC. Due to the use of more cement, heat of hydration and plastic shrinkage of fresh concrete will be increased, but can be controlled with the utilisation of mineral admixtures with clinker, such as fly ash, slag, etc. 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 HSC is water to cement ratio (W/C). The W/C for HSC 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 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 HSC. With the reduction of W/C, workability (flow ability) of concrete will be reduced. However, the flow ability of concrete can be improved significantly by using water-reducing chemical admixture.
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 production of HSC 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 utilised for making HSC. A mixture of concrete using Maddhapara hard rock with W/C of 0.30, CEM Type I cement of 460 kg per cubic metre, and silica fume of 40 kg per cubic metre produced compressive strength of 11,000 psi. The strength can be improved further by reducing maximum size of coarse aggregate and increasing cement content.
For production of HSC, it is recommended to make trial mixes before construction. 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 HSC. HSC is a durable concrete and, therefore, all construction works in marine exposure can be planned with HSC.
Prof. Dr. Md. Tarek Uddin, PEng. is Professor, Department of Civil and Environmental Engineering (CEE), Islamic University of Technology (IUT).