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The content of fines (particles smaller than 0.075 mm [No. 200] mesh) in aggregates used for the production of concrete is generally limited by most standards worldwide. In the current study, the effect of the addition of fines to normal-strength concrete at levels of up to 227 kg/m^sup 3^ (383 lb/yd^sup 3^) was studied in concrete mixtures prepared with constant workability.
It was found that as long as workability can be controlled by reasonable amounts of admixture, the addition of fines improves concrete strength by as much as 30%, somewhat reduces the carbonation rate, and slightly increases the volume changes of fresh and hardened concrete. When high dosages of admixture were required to maintain workability due to the presence of large amounts of ultra-fine particles (less than ~5 micron), properties of the concrete were seriously affected.
Keywords: aggregate; hardened concrete; workability.
INTRODUCTION
Fine aggregates (smaller than 4.75 mm, No. 4 mesh) play a very important role in controlling the properties of fresh concrete. They help to improve the cohesiveness of fresh concrete, improve its workability, and prevent segregation and bleeding.1,2 However, the presence of very fine particles (smaller than 0.075 mm or minus No. 200 mesh), known as "fines," is generally limited for three reasons:
1. Particles of this size may lead to reduced workability due to the large surface area that must be wetted. This leads in turn to an increase in the amount of water required to maintain proper workability, to an increase in cement content required to maintain strength, and, as a result, to higher shrinkage values and greater sensitivity to cracking;3
2. Very fine particles tend to adhere to the surface of larger particles and prevent proper bonding between the cement paste and the aggregate.4 The result is the formation of a weak aggregate-paste bond that promotes intensive cracking and weakens the concrete; and
3. Clay particles, which are smaller than a few microns, undergo significant volume changes when they absorb water and dry out thereafter. Wet clay particles, which expand in the fresh state of concrete, later shrink and, as a result, their presence may lead to large volume changes of the hardened concrete, increased sensitivity to cracking, ingress of deleterious substances, and reduced concrete strength.5
In view of...