Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

Concrete and asphalt are by far the most common construction materials used today. Concrete is a composite material consisting of cement, water, air, coarse aggregate, and fine aggregates. Fine aggregates are typically sands and coarse aggregates are natural or crushed rocks. Chemical admixtures to modify certain specific properties are also commonly used (i.e., superplasticizers to make the concrete fluid during casting). Asphaltic mixes consist primarily of asphalts, coarse aggregates, and fine aggregates, in addition to a number of emulsifiers and other additives used to improve viscosity during placement.

In both concrete and asphaltic mixes, aggregates make up a very significant portion of the mix volume, as economy requires that the amount of cement and asphalt be minimized. Two types of aggregates are commonly recognized: coarse aggregates, defined as particles larger than about 4.75mm (rocks), and fine aggregates, consisting of smaller particles (sands). Other important characteristics of aggregates are that they be rigid, durable, and chemically inert with respect to the concrete mortar or asphalt. Aggregates are intended to be fillers, but they are not intended to play a key role in the behavior of either material. However, the stiffness and strength of the aggregates needs to be higher than the concrete mortar or asphalt, so as not to be the controlling phase.

For effective performance, several characteristics of the aggregates, ranging from their mechanical and chemical properties to their size distribution, need to be taken into consideration in the aggregate mix design. Moreover, both concrete mixes undergo very different behavior when being placed, with the materials resembling a Newtonian fluid, and when in their hardened configuration, with the materials resembling an elastic solid. Additionally in the case of asphalt, the service temperature range is very important, as the properties of asphalts are temperature-dependent within the usual serviceability temperature range.

In this laboratory, we will examine the basic properties of aggregates that are needed to develop successful concrete mix designs. The properties needed for asphalts are very similar, but sometimes utilize different testing techniques. The primary characteristics that we will look at are the size distribution, specific gravity, absorption, moisture content, and bulk density, all of which will be described and measured in this laboratory exercise. Other important characteristics that will not be addressed in this module are the shape and angularity of the particles, abrasion and impact resistance, chemical stability, as well as the soundness and presence of harmful organics.