Mastering concrete terminology

Every industry has its own technical terms and jargon, and the concrete industry is no exception. To test your knowledge, we have pulled together some of the most commonly used and confused terms

Although concrete specialists are usually employed to place concrete, it’s worthwhile for builders to understand what the terms they use in conversation and on plans really mean.

These definitions are presented in lay terms; for a technically precise definition, refer to the American Concrete Institute’s ACI 116 – Cement and Concrete Terminology.

Admixture vs. Additive

An admixture is a material other than water, aggregate, cement or fibres that is added to batches of concrete.

Typical examples are air-entraining, water-reducing, accelerating or retarding admixtures.

The term additive is normally reserved for materials used by cement manufacturers to modify the properties of cement, such as limestone or gypsum.

Entrained air vs. Entrapped air

Entrained air is made up of microscopic air bubbles that are intentionally incorporated into the concrete, normally to improve its freeze/thaw resistance.

Entrapped air is made up of bubbles, of 1mm diameter or larger, which are accidentally incorporated into the concrete during the production process. They should be removed via consolidation.

The presence of air reduces the strength of concrete. However, because entrained air is deliberately incorporated into the mix, the designer will have compensated for its presence. This is not the case with entrapped air.

Consolidation vs. Vibration

Consolidation (or compaction) is the process of inducing a closer spacing arrangement between the solid particles when placing concrete. It can be achieved by various methods, including:

• Vibration.
• Rodding.
• Tamping.
• Centrifugation.

Therefore, vibration is just one method of consolidating concrete.

Floating vs. Trowelling

The purpose of floating a concrete surface is to create a relatively even, but open texture.

Floating:

• Embeds large aggregates beneath the surface.
• Removes slight imperfections to produce a surface closer to the true plane.
• Compacts the surface.
• Closes minor surface cracks that may appear as the surface dries.

A bull float is a large float on a long handle used prior to the appearance of the bleed water. Hand floats are typically made of wood or magnesium. They do not have sharp edges, because these would close the surface of the concrete.

Power floating occurs after the bleed water has evaporated from the surface. It’s carried out using blades with turned-up edges to prevent them closing the surface.

Trowelling occurs some time after floating and creates a hard, dense, closed finish to the concrete surface.

Steel blades are used for hand trowelling. The blades on a power trowelling machine are thinner than a float blade and have sharp edges to close the concrete surface.

Placing vs. Pouring

Placing involves the deposition, distribution and consolidation of concrete in the place where it’s supposed to harden. This process is often inappropriately referred to as pouring. However, pouring only refers to the deposition phase. Always refer to placing rather than pouring concrete.

Prestress vs. Post-tensioned vs. Pre-tensioned Concrete

Prestressed concrete is hardened concrete that is in a state of compression prior to the application of the service loads. Subsets of this generic term are post-tensioned and pre-tensioned concrete, which indicate how the compressive force is applied to the concrete.

With pre-tensioning, the tendons are tensioned before the concrete is placed and hardened.

An example of this would be most of the preparatory flooring systems, such as Hollowcore slabs.

In post-tensioning, ducts are typically provided in the concrete and the tendons are tensioned after the concrete has hardened, such as post-tensioned slabs-on-grade.

Concrete vs. Cement

It may appear obvious, but some confusion does exist. Cement is the fine, normally grey powder that, when mixed with water, reacts and in time goes hard.

Concrete is a mixture of cement, aggregates, water and admixtures, which typically comes out the back of a ready mix truck.

Reinforcing Grade vs. Class

Grade refers to the strength of the reinforcing bar, typically 300 or 500MPa.

Class refers to the ductility (‘stretchability’) of the bar. The only accepted bar in New Zealand is Class E (Earthquake).