Concrete What is concrete Concrete is a building

Concrete:

What is concrete? • Concrete is a building material made from a mixture of broken stone or gravel, sand, cement, and water, which can be spread or poured into moulds and forms a stonelike mass on hardening.

Site preparation: • The most important step when working with concrete is planning. Always plan every step before any concrete is ordered and delivered. Proper planning avoids delays, wastage, segregation and problems, which develop from these. Proper concrete planning also eliminates problems of delay, segregation and wastage. • Workers on the site should always wear protective clothing, strong boots and, if required, helmets or eye protection. Always avoid direct contact with cement and never kneel in or touch the concrete mix directly. Wear gloves or use barrier creams. Ensure that everyone using heavy equipment, such as screeds or vibrators, has been properly trained.

Site preparation (2): • Measure and stake out the area to be concreted and consider how thick the slab must be. The thickness will depend on the weight the concrete must carry. For example, a driveway carries the weight of a car and needs to be thicker than a garden path. • Once thickness of concrete has been established, work out where the concrete will finish. For example, concrete cannot finish too high against steps or the external house wall and should not cover any part of weep holes in the wall. Thus the finishing level shows how much digging or excavation must be done. Pavements must slope away from buildings and boundaries.

Mix proportions for concrete: • A concrete mixture ratio of 1 part cement, 3 parts sand, and 3 parts aggregate will produce a concrete mix of approximately 3000 psi. • Mixing water with the cement, sand, and stone will form a paste that will bind the materials together until the mixture hardens. The strength property of the concrete is inversely proportional to the water/cement ratio.

Mixing large Batches of Concrete Mixes: Low Strength Concrete mix: • 15 Mpa This is a low-strength concrete mix and is suitable for house foundations that are not reinforced, and for boundary walls and freestanding retaining walls • To make 1 cubic metre of 15 Mpa concrete you will need to mix 5 1/2 bags of cement with 0, 75 cubic metres of sand 0, 75 cubic metres of stone.

Medium Strength Concrete Mix: • 25 Mpa This is a medium strength concrete and is suitable for reinforced foundations, light-duty house floors, patio slabs, footpaths, steps, driveways and garage floors. • To make 1 cubic metre of 25 Mpa concrete you will need to mix 7 bags of cement with 0, 70 cubic metres of sand 0, 70 cubic metres of stone.

Strong Concrete Mix: • 30 Mpa This is a high strength concrete and is suitable for suspended structural beams, pre-cast beams and flagstones, heavy-duty workshop floors and suspended reinforced floors. • To make 1 cubic metre of 30 Mpa concrete you will need to mix 10 bags of cement with 0, 65 cubic metres of sand 0, 65 cubic metres of stone.

Mixing small Batches of Concrete Mixes: • You can use containers such as buckets, drums or tins. It is important that the same size container is used for all materials in a batch.

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Questions: 1) What is concrete? 2) How would you prepare a site to mix concrete? 3) Give a ratio used for a low, medium and high strength mixture.

Answers: 1) Concrete is a building material made from a mixture of broken stone or gravel, sand, cement, and water, which can be spread or poured into moulds and forms a stonelike mass on hardening. 2) Do your planning, choose the correct area, clean the area, collect all material and tools, make sure that the workers are dressed correctly and qualified to do the job, mix material properly, transport concrete safely and clean area and tools.

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