WHAT ARE FACTORS AFFECTING CONCRETE STRENGTH?

 Factors affecting concrete strength

Water/Cement Ratio

 In cement hydration, it has been pointed out that the density of hardened cement is governed by the water/cement ratio. With higher w/c ratio, the paste is more porous and hence the strength is lower.  Also, the strength continues to increase with decreasing w/c ratio only if the concrete can be fully compacted. For concrete with very low w/c ratio, if no water-reducing agent is employed, the workability can be so poor that a lot of air voids are entrapped in the hardened material. The strength can then be lower than that for concrete with higher w/c ratio. While w/c ratio is the most important parameter governing the strength of concrete, it is not the only parameter.

Age and Curing Condition

 Provided the concrete is properly cured, the strength increases with time due to the increased degree of hydration. The 7-day strength can range from 60 – 80% of the 28-day strength, with a higher percentage for a lower w/c ratio. After 28 days, the strength can continue to go up. Experimental data indicates that the strength after one year can be over 20% higher than the 28-day strength. The reliance on such strength increase in structural design needs to be done with caution, as the progress of cement hydration under real world conditions may vary greatly from site to site.

Aggregates

 For the same w/c ratio, mixes with larger aggregates give lower strength. This is due to the presence of a weak zone at the aggregate/paste interface, where cracking will first occur. With larger aggregates, larger cracks can form at the interface, and they can interact easier with paste cracks as well as other interfacial cracks. With the same mix proportion, rougher and more angular aggregates give higher strength than smooth and round aggregates.  However, with smooth aggregates, a lower w/c ratio can be employed to achieve the same workability. Therefore, it is possible to achieve similar strength with smooth and rough aggregates, by adopting slightly different w/c ratios.  For a fixed w/c ratio, the strength increases slightly with the aggregate/cement ratio. This is because aggregates are often denser than the cement paste. With less paste in the concrete, the overall density is increased.  For normal strength concrete, the aggregate strength is seldom a concern. However, in the development of high strength concrete, it is important to select aggregates with strength higher than that of the hardened paste.

Admixtures

Air-entraining agents decrease concrete strength by incorporation of bubbles. Set retarding and accelerating agents affect the early strength development but have little effect on ultimate strength. Incorporation of mineral admixtures increases ultimate strength through the pozzolanic reaction.