All About Different Kinds of Concrete Mix
There are many types of concrete mix design and ingredients – but one size certainly does not fit all….
This is probably the most typical concrete mix design of all and consists of cement & ballast (sand & aggregate). Mix ratios are usually between 1:3 and 1:5. Whilst this type of concrete mix can have incredible compressive strength it has very poor tensile strength and ductility. This weakness is of no concern when making small items such as blocks and paving etc. as they will never be put under any tensile stresses. When constructing larger items/structures reinforcement is required. The most common type of reinforcement is steel. With the combination of steel and concrete almost any size shape and form is possible.
The only drawback to using a wetcast concrete for worktops is the weight issue. The concrete needs to be 50mm thick to encase the steel. This means the overall size of worktops we can produce would be smaller. The only time we would consider using a wetcast concrete is if we need to cast in situ or when producing smaller pieces.
GRC / GFRC
This stands for glass reinforced concrete or glass fibre reinforced concrete. This type of concrete mix design was first conceived during the 1940’s in Russia. Although the first mixtures contained regular E-glass which proved to be useless due to the degrading effects of the high alkalinity present in the concrete. By the mid 70’s British scientists developed an alkaline resistant glass fibre known as AR glass. This new concrete mix design became popular with architects due to the fact it was light-weight, had excellent tensile strength and fire resistance. Façade panels, columns and architectural mouldings are all typical uses of GRC. Mix ratios are usually 1:1 cement/sand with the addition of 1-5% glass fibres. Polymers are also added for extra ductility and increased curing properties.
GFRC is an excellent choice for concrete countertops as it is possible to fabricate very large, lightweight and 3 dimensional surfaces. At Fluid we can produce and deliver anything up to 4.8 metres long by 1.5 metres wide. Sizes over this are possible but logistics can make projects very expensive.
Did we mention water?
Everybody knows that you just add water to concrete to make it fluid but not many people realise how important it is to get the proportions right. The water to cement ratio is probably the most important element of any concrete mix design. It is easily possible to turn the perfect concrete mix into a weak porous mess by adding too much water.
The water to cement ratio is directly proportional to its final strength, density and permeability. In order to obtain this we must ensure precise measurement of water and use of superplasticisers (water reducers). These admixtures are added to the mix to help gain fluidity whilst obtaining very low water to cement ratios. Through the use of superplasticisers and good curing practices typical design strengths in excess of 70mpa can be achieved in 28 days. This is over twice the strength of a regular general purpose structural concrete.
Pozzolans are usually by-products from other industries such as coal and steel. They are used to replace a portion of the cement used in the concrete mix not only to increase the ‘green’ credentials of the concrete but also to increase strength and durability.
Waste product from the coal industries such as power plants. This pozzolan will increase workability, reduce water demand, reduce segregation and bleeding, and also lower the heat of hydration. Fly-ash will increase the ultimate concrete strength, reduce permeability, increase sulphate resistance, and reduce the potential for alkali-aggregate reaction. Fly-ash reaches its maximum strength more slowly than concrete made with only portland cement.
This is the most powerful pozzolan which is used in high performance concretes designed for bridges, skyscrapers and civil engineering applications. It will greatly improve ultimate strength, density, reduce permaebility and alkali-aggregate reaction. The plastic properties of silica fume concrete are also affected with a largely increased water demand, reduced slump & workability. Silica fume concrete needs large doses of superplasiciser to compensate for the extreme loss of workability. There is no point in making extremely strong concrete if you can’t place it.
This pozzolan has similar plastic and ultimate qualities to silica fume concrete but with less water demand. It has a creamy non-segregating texture. This pozzolan is also white which gives a pleasant finish to pre-cast concrete worktops.
Ground granulated blast furnace slag (GGBFS)
This is also a white pozzolan which like fly-ash impoves workability and slows the curing times / heat of hydration. This pozzolan is increasingly being used for polished concrete floors due to the large cement replacement ratio. Upto 50% of the cement required can be replaced by slag cement. This is due to the fact that this is also the only pozzolan which has inherent hydraulic properties (will react with water).
We have spent countless hours testing various concrete mix designs using finely blended pozzolans and cement ratios. We promise you will have the most durable concrete worksurface available which will outperform many natural stone alternatives.