Service Rock Products

Frequently Asked Questions

Here are some of the most common question we are asked about aggregates and ready mixed concrete:

What are aggregates?: Aggregates are rock and sand that is processed at a mining facility to a well graded product.

If I want a 1/2" rock, is that what I get?: All aggregate products are a blend of rocks with a maximum size aggregate graded down to a minimum size. For instance if you ordered a 1/2" rock, at least 10% by weight is retained on the 1/2", then 40% to 60% retained on a 3/8" screen, 5% to 10% retained on a 1/4" screen and so on. These grading are specified by the various agencies in the Southern California area and may have different variations. If you are looking for a true 1/2" rock, you may want to check places that specialize in decorative rock.

What makes a base product?: A base product is made up of a crushed or natural coarse rock and sand that is well graded to produce, when compacted, a firm stabilized sub-grade for streets, highways or even concrete slabs.

What makes for a good drainage material?: There are many products that will provide a good drainage material. The material to be used is dependent on how much of a water flow there is. If the water flow is limited, a 1/4" size rock can be used. If the water flow is great, a larger size rock with a high void ratio (the open space between compacted rock) is needed such as a 3/4" to 1" size rock.

What is a vapor barrier?: A vapor barrier is what goes under a concrete slab to prevent water or moisture from entering the slab. This can be a layer of sand and/or up to a 1" maximum size rock, at a 2" to 4" depth. If a source of water is present under a slab, without a vapor barrier, the concrete will absorb the moisture and can cause early deterioration of the concrete. By putting this layer of sand or rock it prevents the water from rising to the bottom of the slab.

What is concrete?: Concrete is a mixture of Portland cement, fine and coarse aggregates (rock and sand), and water. Mineral and chemical admixtures may be used to enhance the workability and durability of the mixture. When in a plastic state it can be molded or formed in a variety of shapes and sizes. Once hydration takes places, it is a strong and durable product.

What is cement?: Cement (known as Portland cement) is a combination of limestone, cement rock, clay and iron ore. These minerals are appropriately combined, grounded down to a raw material and processed through a kiln at temperatures of 2700° F that chemically changes into cement clinker. This is then mixed with gypsum and grounded down to a powered material.

What is a mineral admixture?: Mineral admixtures can be a pozzolan or slag material, when combined with Portland cement can have cementitious properties. A pozzolan (also known as fly ash) is a coal burning by-product or a variety of natural pozzolan such as shale’s, volcanic ashes or pumicites, and diatomaceous earth’s. If used properly, it can densify concrete to help increase it’s durability properties.

What is a chemical admixture?: Chemical admixtures are used to enhance the workability and durability of concrete. Some of the chemical admixtures include; water reduction, retarding or accelerating hardening set times, waterproofing, reducing shrinkage and corrosion inhibitor of steel rebar.

What can I do to prevent cracking?: There are many ways to help prevent cracking and would take up many pages here. Here are 3 major steps that you can take to help mitigate cracking. If possible try to keep the amount of water added to the mix as low as possible, proper curing of the concrete to help reduce shrinkage and proper use of control joints.

During the hydration process the excess water will leave the plastic concrete through absorption or evaporation. As the water leaves, the concrete starts shrinking to fill these voids left by the water. If the shrinkage is not controlled, the concrete will find the path of least resistance and a crack will form. By keeping the water as low as possible, it will reduce the amount of voids left behind. By curing the concrete (by chemical or water method) it controls how fast shrinkage will occur and do it evenly. The last important step is the use of control joints. Control joints are cuts in the concrete, at a certain length and depth, that forces any cracking that may occur into this cut, thereby relieving the greater stresses caused by the shrinkage. Concrete will crack unless you can take measures to control it.

If you have any question that you would like answered, e-mail us at: mrobert@servicerock.com

If you would like more information on aggregate and concrete we recommend these book titles:

"The Aggregate Handbook"

Published by:

National Stone Association * 1415 Elliot Place, Northwest * Washington D.C. 20007

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"Design and Control of Concrete Mixtures"

Published by:

Portland Cement Association * 5420 Old Orchard Road * Skokie, Ill 60077-1083