Concrete – What’s Supporting Your Home

The word concrete comes from the Latin word “concretus” (meaning compact or condensed), the perfect passive participle of “concrescere”, from “con-” (together) and “crescere” (to grow). Most information available about concrete is written for contractors, for those who design concrete mixes, and for those who perform invasive testing.

Concrete is a composite material composed of various constituent materials: a binder, usually cement, aggregates, and water. Different mixes = different products. Cement + water = cement paste; Cement paste + sand = mortar; Mortar + aggregate = concrete. Each of these constituent materials can vary in its chemical makeup and performance characteristics, depending on where it was quarried, and the manufacturing methods and conditions used to produce it.

Different factors can affect concrete and the problems that inspectors will see. How concrete hardens, strengthens and the qualities of its surface depend on a number of things, including the properties of its constituent materials. Although Portland cement is the most commonly used binder, pozzolans may be substituted. Pozzolans are materials that, in addition to undergoing primary hydration, undergo a secondary hydration, producing a gel that fills tiny voids between cement particles, making concrete less porous and less likely to absorb moisture or chemical solutions that can damage concrete or steel reinforcement.

Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick. Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.

Recently the use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a by-product of coal-fired power plants. This use reduces the amount of quarrying and landfill space required as the ash acts as a cement replacement thus reducing the amount of cement required. In modern times, researchers have experimented with the addition of other materials to create concrete with improved properties, such as higher strength or electrical conductivity. Marconite is one example.

Cracks that appear before the concrete has hardened are called plastic cracks. Plastic cracks are typically due to poor mix design, placement practices or curing methods, and may also be caused by settlement, construction movement, and excessively high rates of evaporation. Cracks that appear after concrete has hardened can have a variety of causes, and sometimes have more than one cause.

Plastic shrinkage is shrinkage caused by the loss of water to the atmosphere. Autogenous shrinkage is shrinkage that takes place with no loss of water to the atmosphere. Autogenous shrinkage is caused by internal drying, with water being absorbed by the constituent materials in the concrete. As the long-term chemical hydration process continues – and it can continue for many years — water in the pores within the cement paste is absorbed, and the pores are filled, to some degree, by materials produced during hydration. This process leads to decreased permeability and increased strength and durability of the cement paste. Absorption of water from the pores also causes shrinkage.

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