Crushed Limestone Concrete Aggregate

Concrete, along with steel and wood products’ are the basic building materials of our modern society. Concrete is made by mixing sand, course aggregate, water and cement. Many people confuse concrete with cement. Cement is the gray powder which forms the bonding agent when sand and course rock is mixed with water, the result is concrete. This mixture will set up into a “rock hard” mass which will weigh about 135lbs per cubic foot and have a compressive strength of about 3000 pounds per square inch. Concrete is used for many things, but probably the most extensive application is concrete road paving and slabs for structures such as homes and commercial buildings.

Crushed limestone course aggregate for concrete is typically sold in competition with natural river gravel. River gravel is usually dug from deposits found in the bends of major rivers which have flowed out of rocky terrain. These stones are usually very hard and typically round because they have been broken away from the hills and then water currents literally “polish” them as they roll down the river. This gravel is found in deposits that also contain sand most of the time. Usually there is more sand than gravel. Many gravel deposits contain stones that are about the correct size to make concrete, so the only prep work necessary is to separate the sand from the gravel and remove any clay, wood or other foreign material. Because it is relatively easy to produce river gravel as compared to crushed limestone, usually the first source of aggregates developed for a community is gravel. As the gravel deposits are depleted and people learn the advantages of crushed aggregate, the use of crushed limestone in concrete increases.

Crushed limestone has several important advantages over river gravel as an aggregate for concrete. First, for a given amount of cement in the mix, crushed limestone concrete will have an approximately 10% strength advantage. This strength advantage comes from the fact that cement bonds tighter to limestone than to a slick/smooth gravel and the strength that is derived from the sharp angular faces of the crushed aggregate. Second, because crushed limestone has a lighter unit weight than gravel, it will require about 12% less crushed limestone than gravel to mix a cubic yard of concrete. Additionally, crushed limestone concrete is easier to saw through than gravel concrete. This is because most gravel is silica. Silica is almost impossible to cut with a steel saw.

Crushed limestone concrete also has a lower thermal coefficient of expansion than gravel concrete. This means that slabs poured with limestone concrete will expand and contract less than gravel concrete for a given change in temperature. Studies have shown that this thermal stability coupled with limestone’s superior curing properties results in greater crack spacing, and less crack width in limestone concrete slabs.

Most concrete is made by mixing a ¼” to 200 mesh (200 wires per inch) sand with aggregate graded from 1 ½” to 3/8” or aggregate graded from 1 ¼” to 3/8”. Occasionally aggregate with a top size of ¾” will be used. The most popular size is the 1 ¼” size. Both the 1” and 1 ½” size are relatively easy to make in the Texas Crushed Stone quarry.