Deviations in Standard Aggregate Gradation and its Affects on the Properties of Portland Cement Concrete

Aggregate gradation is the particle size distribution of both the stone and sand present in the concrete matrix. It is an aggregate property that has been heavily researched for over a century but the effects of which on concrete properties is still somewhat misunderstood. Past research has revealed that aggregate gradation dictates the proportion of aggregate to cement paste in concrete, and can play a major variable that determines the overall durability of the construction material. Increasingly, aggregate in South Carolina are failing to meet the standard aggregate gradation for portland cement concrete, however, the effects of such failed aggregate gradations on concrete properties is poorly understood, in order to develop a justification for acceptance or rejection of a given concrete load. The principal objective of this investigation was to study the influence of variations in aggregate gradations on properties of concrete. The overall goal of this project is to provide SCDOT a rational method for guiding whether concrete containing coarse or fine aggregate that fails to meet gradation specifications should be accepted or rejected. In this investigation, the gradation of both fine and coarse aggregate was varied, independent of each other, purposefully engineering some gradations out of the accepted cumulative percent passing band for selected sieves. The total quantity of coarse and fine aggregate was fixed as well as the water cement ratio. Local aggregates with unique properties that are used by the SCDOT were implemented in this study. Multiple tests were conducted on both the fresh and hardened concrete in an attempt to develop a sound knowledge of the extent to which aggregate gradation can deviate from current specifications before selected concrete properties are negatively affected. Results indicated that it is more critical for the fine aggregate rather than the coarse aggregate to conform to accepted specifications. Results from this study also illustrated that some concrete properties such as compressive strength did not show much dependence on aggregate gradation while others such as split tensile strength were heavily affected from this aggregate characteristic. Rapid chloride ion permeability and drying shrinkage tests confirmed that gradation is a major variable in determining the concrete\u27s durability. Based on the findings from this study, the suitability of concrete containing a failed aggregate gradation should be based on the criticality of structure with respect to a specific property, for ex. for a concrete that requires high crack resistance and durability, failed aggregate gradations should be rejected, however, where it is not critical concrete with failed gradations within a reason can be accepted