Aggregate Strengths in Concrete

Composition

• The mineral composition of the aggregate can affect the concrete's strength. Aggregates made from tougher materials like granites and gravels will generally yield greater durability. Limestone, shale and marble yield poor hardness, although the strength of limestone aggregate is greater than that of sandstone. Cherts are especially undesirable. Basalt, granite, slate, serpentine and quartzite are superior in most respects. Hard igneous rocks generally yield products with better abrasion resistance than soft sedimentary rocks.

Size

• The shape is also very important. If the aggregate particles have a rough texture and are elongated or angular, more water is required to produce a workable mixture, so the cement content must be increased. Angular particles create more void space -- unfilled gaps between particles in the aggregate. The ideal particles are cubical or spherical in shape. Generally, the quantity of flat and elongated particles as a percentage of the total is limited to a maximum of 15 percent for best results.

Size

• Size is another consideration. Aggregates are often made from a mixture of coarse- and fine-grained material; a well-graded distribution of sizes is very important. Larger aggregate requires less mixing water but on the other hand may form weaker transition zones with more tiny cracks in them, so small variations in average aggregate size do not have significant effects on strength. For high-strength concrete it's best to hold the aggregate size down to a maximum of 19 millimeters or so, and the amount of fine aggregate is usually reduced slightly.

Considerations

• Strength is not the only important attribute of the concrete -- it's just one of several factors to be considered. Concrete must be workable, meaning construction crews must be able to pour and mold it readily. It should also be relatively inexpensive and shrink to a minimal extent during curing. Choosing the right aggregate can sometimes involve tradeoffs between multiple factors. Larger aggregate, for example, may reduce project costs at the expense of workability.