Compressive strength effect water-reducing admixtures

The major physical attributes of concrete as a construction material are a high compressive strength and stiffness, an ability to protect and restrain steel and, most important of all, to retain these properties over a considerable period of time. The effects that water-reducing admixtures have on these properties can be considered from the point of view of design parameters, i.e. those properties of concrete at a relatively early age (usually 28 days) which are used for structural calculations, and longer-term aspects or durability. 

Air-entraining water-reducing admixtures require special consideration the presence of entrained air leads to a reduction in compressive strength, whilst the water reduction results in a compensatory increase in strength. The effect can be quantified, however, by considering the amount of entrained air in terms of an equivalent volume of water to calculate the (air and water)-cement ratio. This new factor can be used to estimate the expected strength from Fig. 1.37. 

At equal dosages, all water reducers are effective in producing concrete of equal or higher compressive strength than that of reference concrete. At equal cement content, air content, and slump, the water reducers increase the 28 day strength by about 10-20%. Table 9 shows the influence of lignosulfonate type admixture on the compressive strength of concrete.Water reduction varies between 5 and 8%. 

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