Concrete Workability - A Quick Guide

What is concrete workability?

Concrete workability basically refers to how easily freshly mixed concrete can be placed, consolidated and finished with minimal loss of homogeneity.

Generally the workability of concrete is determined by how fluid the mix is (i.e. the cement to water ratio). This is commonly referred to as "slump" (to find out more about slump read our concrete slump test guide).

Essentially - the more fluid the concrete, the higher the slump and whilst the slump is seen as a measure of water content, it is typically also used as a measure of concrete consistency.

What affect's the workability of concrete?

There are 3 primary factors that wil affect the workability of concrete, including:

Water to Cement Ratio

The ratio of water to cement. The higher proportion of cement typically means a stronger concrete mix. With the right amount of cement paste the coating of aggregates delivers a better consolidation and finish. If the mix isn't hydrated adequately you will find the strength development is rather poor. It is also a lot harder to place and finish. However, if too much water is used then this can lead to a negative impact on segregation and final strength so could be detrimential to your build. Typically most mixes look to get a ratio of around 0.45 to 0.6 to achieve workable concrete.

Aggregates

The size and shape of the aggregates (stones and sand) used in a concrete mix will affect its workability. As aggregate surface area increases, the more cement paste is needed to cover the entire surface of aggregates. So - concrete mixes with smaller aggregates will be typically less workable when compared to larger sizes. Crushed aggregate with decent proportions tend to bond best with the cement and deliver decent workability.

Admixtures

Chemical admixtures are used in concrete to improve things like strength (once set) and handling / workability of the concrete mix. A few examples include; plasticisers to help regulate consistency, air entrainers to improve freeze/thaw resistance and internal curing to help reduce damage such as cracks and strength loss.

Types Of Concrete Mix

There are essentially 3 types of concrete mix, these are:

1. Unworkable Concrete
2. Medium Workable Concrete
3. Highly Workable Concrete

But what does this mean?

Unworkable Concrete

Also known as "harsh concrete" - unworkable concrete is a mix that uses very little water. This kind of mix has a high segregation of aggregates as cement is not typically lubricated enough to stick to them. It can be very difficult to maintain the homogeneity of concrete mix and compaction needs a lot more effort. The water to cement ratio is typically below 0.4.

Medium Workable Concrete

This is concrete with medium workability. It is used in most construction works as it is easier to mix, transport, place and compact without much segregation and loss of homogeneity. Generally it is used in construction with light reinforcement. The water to cement ratio is typically between 0.4 to 0.55.

Highly Workable Concrete

Highly workable concrete is a more fluid mix that is very easy to mix, transport, place and compact. This kind of concrete is normally used where effective concrete compaction is not possible or in mass concrete. It flows easily and will settle with minimum effort. An example of this kind of mix is self-compacting concrete that contains a cement to water ratio of 0.55 or more.