No abstract

No abstract

Fly ash and other pozzolans are increasingly used in concretes that must consistently achieve high compressive strength and other measures of performance. In contemporary design and construction practice, however, there is some concern as to whether or not use of commercial fly ash materials contribute to or reduce variations observed in field compressive strengths and properties of the fresh concrete. Regardless of its potential for improving the performance of the concrete, if the daily ongoing use of commercial fly ash will decrease the consistency of the concrete produced, the use of fly ash will be curtailed. In this paper, the experiences of geographically diverse, specific concrete suppliers using their respective sources of fly ash are summarized in detail. The nominal compressive strength of concretes, made with and without fly ash, are analyzed statistically. Special attention is given to moderate- and high-strength concretes. In conclusion, it is shown that concretes made with fly ash may not be more consistent than concretes with only portland cement.

Some favorable properties of lightweight aggregate concrete are shown to originate from its internal structure, which combines relatively flexible aggregate with more rigid cement paste matrix. Scanning electron microscopy (SEM) was used to analyze the cracking pattern which developed under cyclic loading. The cracks appear to originate in the hard brittle matrix and progress into the aggregate. At low load levels, some cracks seem to deflect and lose energy as they penetrate the aggregates, with many never emerging from the other side or emerging as thinner cracks. A process of healing of cracks through the formation of new products appears to restore integrity and enhance strength of a previously cracked concrete which was allowed to rest for a long period after cracking. Micrographs of the healed cracks are presented.

No abstract