Concrete and Screed

Concrete and screed and are formed of the same basic ingredients – cement, aggregates and water. But what makes them different is the size of aggregates, the cement grade, mix consistency, and their intended application or use.

Composition and Mix Proportion

Concrete in its simplest form is a rock like mass formed by the hardening of cement paste and aggregates mixed in a proportion of – 1 Cement: 2 Sand: 3 Coarse aggregates (ballast/gravel). But many variations are applied to this basic mix design to suit different uses and strength requirements.

Screed is a thin layer of cement paste and sand/ fine aggregates, laid onto a concrete floor base or under floor heating to act as a smooth flat levelling surface for taking on the final floor finish. Screeds generally follow a mix proportion of 1Cement: 3-5Sand (0-4mm washed sharp sand for levelling screeds).

For heavy duty screeds 25 percent of the 0-4mm sand is replaced by 6-10mm single sized aggregates, making the mix proportion 1:3:1 – cement:0-4mm sand:6-10mm single sized aggregates. This is done to facilitate trowelling and to increase the density of the screed.

Finish

Concrete has a wide range of uses as building material, as a wearing surface and in masonry. It can be given a range of finishes from coarse to extremely smooth, depending on the intended use of the concrete.

But screed is used to create a smooth level surface for the final finish. It is not intended to be used as a final wearing surface. But screeds should be able to provide sufficient support and strength to its intended use.

Consistency of mix

Looking at consistency, concrete is mixed to a wetter consistency while the screed mix should be of a semi-dry, non-crumbly consistency. For concrete, the slump rates depend on the application and mix design.

Being a wetter mix, concrete is usually applied using floats or piston pumps, whereas the semi dry screeds are installed using pneumatic pumps or forced-action mixers. However, certain screeds that use an anhydrite base instead of cement are mixed to a free-flowing consistency and are applied using grout pumps/worm pumps.

CVC ReadyMix, the perfect ready mix concrete.

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