Concrete comes in different grades or strengths. Normally the engineer specifies the design strength of the concrete, which may vary for different parts of the building. Using concrete which is under strength will lead to problems with the structure, which could cause cracking and in the worst case catastrophic failure of the building. Using concrete of a higher strength or grade than required is more expensive.
Concrete can be mixed on the building site, usually using a mechanical mixer. or it’s ordered from a ready mix concrete supplier which delivers the concrete in concrete spinner/ready-mix trucks with revolving drums which continually mix the concrete. When ordering concrete it’s important to specify the grade or strength and the amount of concrete required.
Concrete is a mixture of cement, water, sand and stone. The strength of concrete depends on the ratio of cement to water. The more cement the stronger the concrete, while the more water the weaker the concrete. More water usually also means that the concrete shrinks more as it dries, which usually causes shrinkage cracks that are unsightly and which allows moisture to penetrate the concrete.
Adding stone reduces the amount of water. But too much stone means that the concrete will be difficult to work with and there will be voids between the stones. It’s important to note that the stone should be clean and free of dust and dirt, since the dust and dirt will require more water, which will make for a weaker concrete unless more cement is added. Stone should be generally of a uniform size.
Sand helps create the cement paste that fills the voids between the stones. Sand should be clean and free of contaminants, such as roots, sticks, rubbish and impurities. Not all sand is suitable for concrete. The best sand is a river sand which is of even grading. Sand which is too fine will require more water to wet it, and as already discussed more water means more cement or a weaker concrete. When the sand is very course the concrete may have voids, or be difficult to compact. If the sand is very wet then less water should be added to the mixture. Where possible sand for concrete should be kept dry and covered when there’s lots of rain.
Sometimes, depending on the available sands and stone it may be necessary to blend two sands together.
Sand, water, cement and stone must be well mixed so that the cement is evenly spread through the mixture and there are no lumps of material. The mixture is now known as concrete.
Concrete usually begins to set within four hours of being mixed. The time will be shorter when it’s very hot. The setting time will be extended when it’s cold, or when retarders (certain chemicals) are added. In fact, add sugar and the concrete will never set. So concrete should be placed and compacted as soon as possible. When concreting bigger structures it’s important to ensure that the first concrete placed doesn’t dry out before the next load of fresh concrete is added next to it. So it’s good practice when pouring slabs and beams to start pouring from one side of the structure, first bringing the concrete to the top surface, then steadily advancing with each new load of concrete, ensuring that the fresh concrete knits with the older concrete. If the new concrete is placed against older concrete that has already started to set it won’t join properly and there’ll be a ‘cold joint’ (visible joint) which is unsightly and weakens the structure, even meaning that the structure is unsuitable and should be condemned. It’s therefore critical to schedule concrete deliveries at the correct frequencies, so that there isn’t a long gap between trucks and also so that trucks are not standing, waiting for previous trucks to be offloaded, meaning that the concrete is already starting to set. You must place the concrete at a fast enough rate so that the concrete doesn’t start setting before fresh concrete is added.
Concrete is best compacted by mechanical vibrators, or compactors, to ensure there’re no voids in the structure and to bring some of the concrete water to the surface. Concrete shouldn’t be over compacted.
The concrete mixture should always be workable, having sufficient water, cement and sand so that it can easily flow into voids, between and around reinforcing steel, and so it can be readily compacted and worked with. However, construction crews should never add additional water to the concrete mix to make it more workable. Remember the golden rule, more water equals weaker concrete! Only the mixing crew should add more water, plus the required additional cement.
As the concrete starts to set the top surface should be finished and smoothed off. The final finish will depend on what will come on top of the concrete. Floors that will have a carpet or vinyl floor finish will be required to have a smooth (‘steel float’) finish. Floors that will be covered with tiles may only require a rougher finish. However, in all cases it’s important to ensure that the top surface of the concrete is finished to the correct required levels without dips and humps. A smoother finish takes longer, so can be more expensive. A very smooth finish could be unsuitable when other layers are added, say when a screed is to be placed over the concrete.
Protecting Your Newly Laid Concrete
Concrete shouldn’t be allowed to dry out too quickly which will cause shrinkage cracks and slow down the strength gain. Fresh concrete should be cured for seven days so that it retains moisture. This curing should start as soon as it has been worked smooth and set. Curing of concrete can be done by:
- Painting the surface of the concrete with a curing compound.
- Wrapping the concrete in a plastic membrane.
- Wrapping the concrete in hessian or sacking which is then kept wet.
- Continuously wetting the concrete.
- Forming ponds or dams on slabs (building sand walls) and keeping these ponds filled with water.
Extra care needs to be taken when it’s very hot or windy as this will accelerate the drying process. The concrete will need to be worked smooth quickly. Cold joints will form more easily so the next load of concrete must be placed sooner.
Conclusion – Ensuring your Concrete Building Structure is Sound
It’s important that those working with concrete ensure that the correct grade of concrete is used. As soon as the concrete is mixed, or it’s delivered to the project, it should be poured or placed into the structure. Concrete that’s too dry or that has too much stone is difficult to work with and won’t always get into all the corners and around the reinforcing in the structure. Concrete that’s too wet will often have excessive cracks in it when it dries.
Care should be taken not to add additional water to the wet concrete since this weakens the concrete. Adding additional water usually means you should also add more cement.
Concrete must be thoroughly compacted and care must be taken to ensure it’s finished to the correct level and worked smooth.
Using the wrong grade of concrete, adding additional water, using unsuitable sands or contaminated materials, not compacting the concrete correctly, allowing the concrete to set before the next batch of concrete is added, or failing to cure the concrete correctly, could result in excessive cracking and a weakened structure which will cause problems for your new house.
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