Historically, aggregate gradation has been controlled by specifications that call out envelopes for individual fractions, typically the coarse aggregate and the fine aggregate. The shortcoming of this approach is that the gradation of the overall system is not addressed. While it is sensible to stockpile coarse and fine fractions separately to prevent segregation, it is the combined system that is critical in the final mixture. The combined grading of aggregates used in concrete mixtures for paving applications can have a direct impact on workability, and indirectly on mixture performance. The measurement of what comprises a good combined gradation is the topic of this tech brief. It covers: BACKGROUND and IMPLEMENTATION (1. Mixture Proportioning and 2. Construction).
Mixtures that must be heavily vibrated because the workability is poor run the risk of segregation and creation of low-durability vibrator trails due to the air void system being compromised (Taylor 2006). Too often, additional water is used to compensate for poor workability, thus increasing the water-to-cementitious materials ratio (w/cm) and compromising potential durability and strength.
Mixtures that have well-graded aggregate and are responsive to vibration can lead to significant savings during construction because less effort is required to consolidate and finish the slab. The slabs are likely to last longer because less abuse has been applied to the surface to create the required finish.
Another significant benefit of designing a well-graded aggregate system is that the paste content of the mixture can be managed and potentially reduced. Concrete mixtures containing excess paste (i.e., more than is needed for placing and finishing) exhibit numerous undesirable characteristics.
The Tech Brief, “Blended Aggregates for Concrete Mixture Optimization … Best Practices for Jointed Concrete Pavements”, by authors Peter Taylor, Director National Concrete Pavement Technology Center and Gary Fick, Trinity Construction Management Services, Inc., was developed under Federal Highway Administration (FHWA) contract DTFH16-14-D-0005.
To download the PDF of the Tech Brief, please click here.
