Cement, Energy and Environment

6 MORE kN’s WITH LESS CONCRETE The strategy of achieving more strength with less concrete brings us to the simultaneous application of SCMs and super-plasticizers but the actual results depend on how effectively one makes and places concrete with lower and lower water-to-cementitious materials (w/c-m) ratio. The super-plasticizers are capable of reducing the water requirement of a concrete mix, typically up to 30%. The critical w/c ratio for Portland cement is considered to be 0.36, at which the volume of hydrated material equals the volume of the original material. Under the normal circumstances the w/c ratio is maintained at 0.36 + the curing water. If the w/c ratio is increased beyond this level, the porosity and permeability of concrete increase very steeply as shown in Fig. 10. However, due to the presence of finely ground supplementary cementitious materials in blended cements, the water content in mortar and concrete tends to increase, thereby affecting the strength properties. It is, therefore, essential that the blended concrete mixes with high proportion of SCMs contain appropriate doses of compatible super-plasicizers so that the w/c-m ratio remains low. A typical example is given in Table 1 for super- plasticized high-volume slag concrete with different levels of w/c-m ratio. Fig. 9. Increasing trend of limestone filler use in cement and concrete Table-1 Strength properties of high-volume slag concrete with super- plasticizer at different w/c-m ratios Fig. 10. Rise in capillary porosity and permeability of concrete with increase in w.c ratio