Technical Terms

Strength

The strength of concrete is measured by how much force is required to crush it. This compressive strength is measured in megapascals (MPa).

Factors affecting strength are the water-cement ratio and the extent to which the concrete is cured. The addition of too much water to concrete in the truck (i.e. beyond the intended mix design) will reduce the strength and durability of the concrete, even if it is properly placed, finished and cured.

Slump

Slump is a term used to describe concrete’s hardness and consistency before it is cured. A slump test (measured in millimetres) is a standardised test used to ensure a batch of concrete meets the required quality standards for the application. As a guide:

• Flowing concrete
  180mm-plus slump for ease of placement and labour savings
• Medium slump concrete
  100mm-140mm slump for exceptional pumpability
• Normal slump concrete
  80mm slump, giving very low shrinkages due to reduced water content