378
4000.038
Lydon, Francis D.
Thesis
Paper
Dept. of Engineering, University of London
1967
1-275
Y
Lightweight aggregate structural concrete has been used for many years, especially abroad. The porous nature of the aggregates has led to speculation as to the effects of this on water absorption and on the properties of the concrete.Most data available is incomplete and difficult to interpret; this investigation attempted to clarify the situation. A simple, but objective, test is suggested for assessing the surface-dry condition of lightweight aggregates moisture-conditioned in various ways. This was used in computing the free water/cement ratios of a wide range of mixes containing aggregates initially saturated, dry or a combined mixture of wet and dry materials.
Cube, equivalent cube, flexural and, to a lesser extent, cylinder-splitting strengths were examined together with modulus of elasticity and density. It was concluded that all, except density, were, for a given computed free water/cement ratio, independent of the initial moisture condition of the aggregates. Mixes containing initially saturated aggregates exhibited higher wet densities than similar mixes containing initially dry aggregates.
Compacting factor and Vebe tests (and a number of modifications of the latter) were examined statistically as control tests and recommendations made as to which were better to use. These and some other criteria of comparison were used in assessing the behaviour of similar mixes containing differently-conditioned aggregates and it was concluded that there were no consistent effects on the various strength and elastic properties examined due to the initial condition of the aggregates.
The effects of undercompaction on the compressive and flexural strengths of lightweight aggregate concrete are shown to be similar to those exhibited by normal concrete and to be independent of initial aggregate condition.