Concrete

All concrete mixes share the same basic ingredients, course aggregate, fine aggregate (sand), cement water and often additives to aid workability or curing time, however there are significant differences in performance between mixes and therefore the applications.

The usual nominal aggregate size for concrete is 20mm however 10mm may offer better workability in certain applications. 40mm aggregate is in some cases available however not readily specified

Strength 

The strength of concrete is measured by compressive strength in N/mm2 (Newtons per millimetre squared). The declared strength is the strength measured at 28 days. Simply, the higher the strength the higher the performance.

Consistency 

The consistency or workability of concrete is specified by ‘S’ class which is the amount the concrete slumps in a slump test. In essence it is how wet the concrete is. The higher the slump, the wetter the concrete.

A slump test is a cone filled with the concrete and when the cone is removed the amount the concrete falls is the slump measured in millimetres.

The specified slump is important as it directly effects how easy the concrete is to work with. A higher slump may be useful if laying foundations or a slab but not good at all if you are trying to lay kerbs!

S1 – S3 are the most common classes specified with S4 and S5 usually used for specialist applications with a flow test used in place of slump testing to test the consistency.

S4 and S5 consistency classes often contain additives to reduce segregation.

Suggested Consistency Classes can be found in the common uses table tab

In certain situations there are additional elements to considers such as aggressive environments or ground conditions.

This is where it gets more complicated, thankfully the majority of projects do not need to worry about these however they are very important if they do apply.

As a very brief overview British Standard 8500 defines classes for aggressive ground conditions or aggressive environmental exposure of the concrete.

The information on the ground conditions is obtained by testing the ground into which the concrete is to be placed.  British Standard 8500 details methods of specifying concrete for resistance to chemical attack, sulfate attack and to resist freeze thaw.

The classes of conditions are broken down into the following.

AC – Aggressive Chemical Environment is used to specify the resistance of the concrete to Sulfate attack

DC -Design Chemical Class is used to specify a concrete capable of resisting an aggressive chemical environment

DS – Design Sulfate Class is used to classify the sulfate content of the ground adjacent to the concrete.

This is a complex subject and does not often apply to a standard domestic project. Should you wish to learn more there are some excellent resources across the web such as the British Ready-Mix Concrete Association website www.brmca.org.uk