EFFECT OF MIX PROPORTIONS AND AGGREGATE DUST UPON COMPRESSIVE STRENGTH OF CONCRETE

The compressive strength of concrete at a constant cement/water ratio is shown to decrease with increasing cement contents and therefore confirms earlier work in Europe. This decrease, however, is generally quite small especially for the uniaxial compressive strength. Two exceptions are possible, depending upon the sand content: at high sand contents a critical maximum sand/cement ratio is exceeded such that air voids become unavoidable and the strength decreases with decreasing cement content; at low sand contents a critical minimum sand/cement ratio is not attained and large losses in strength then occur with increasing cement contents. The critical minimum sand/cement ratio depends upon the type and grading of the aggregate and the mix proportions. It is due to the onset of very extensive ‘water gain’ and to the lack of adequate restraint to shrinkage of the cement paste. These effects can (and should) be avoided in normal concretes. The presence of dust in the fine aggregate can be beneficial for low-strength concretes probably because of its ability to modify the structure of the cement paste and reduce the water gain. It has little effect on high-strength concretes. © 1968, Thomas Telford Ltd. All rights reserved.