Building? Why no water should be added to the concrete on site
There is no user manual for all aspects of homeownership, from moving, to taking occupation and to maintaining and understanding common and uncommon defects. Albert van Wyk has more than 38 years’ worth of building experience and has put all he has learned into a concise, easy-to-use reference book entitled, The Proud Home Owner. He has granted HomeTimes exclusive access to republish portions of his book to help homeowners make better decisions around buying and selling, as well as maintaining their properties.
The purpose of the foundation is to transmit the LOAD of the building to the soil in such a way that the soil is not overstressed, or the building subjected to excessive movements.
The pressure of the foundation on the soil of an average single-storey house is about 35kPa to 40kPa and 60kPa to 65kPa of a double-storey (kPa stands for kilo Pascals and is a pressure measuring unit).
It is therefore important for the engineer to know what the bearing capacity of the soil is when the footings are designed.
Usually the strength of the soil increases with depth, but the expense of this choice could be high and other measures must be taken.
Types of foundations
Trenches are dug in the ground where every wall of the building will be built and concrete is placed in the trenches. The walls of the plinth will then be built on top of it to the floor level of the house.
The volume between the outside walls will be filled and then a concrete surface bed is cast on top. This will form the floor of the house.
The type of soil on your stand is recorded by the engineer on the NHBRC registration form. This information is available from various geological data centres.
Ask the engineer to inspect the trenches before the concrete is poured. Ask him to confirm that the soil is the same as the original classification according to NHBRC guidelines. The soil on your stand could be different from other stands in the township and may need different designed footings.
An engineer must design any specially required foundations, and will specify what the strength and the thickness of the concrete must be.
This is a reinforced concrete slab which is constructed on top of the natural ground and covers the entire area of the house. It is a combination of the footings and the floor slab all in one.
The slab is thickened under the internal and load-bearing walls.
This is now the preferred method to build on soil with a low bearing capacity with suspected subsidence possibilities such as clay or sandy soil.
For domestic projects 15MPa, 20MPa or 25MPa strength concrete is used for raft foundations. MPa (Mega Pascals) is the term which is used to measure the resistance of the concrete before it breaks under pressure. This is the strength which the concrete will reach 28 days after it was poured.
Take time off to be present when the first concrete is poured. This will be an indication to the contractor that you are serious about supervision and that you want to ensure that everything is correctly done. If you cannot be present appoint somebody to do it for you. Look out for the following:
Concrete which has arrived more than an hour after it has left the mixing plant could have started to set and is getting dry. You do not have to accept it. Confirm this with the mixing plant first.
No water should be added to the concrete on site. This will weaken the mix and the concrete supplier will not provide you with a guarantee of the strength.
- Make sure that the workers have some way of measuring the thickness of the concrete in the trenches. Normally 250mm or 300mm thick or whatever was specified by the engineer.
- The top of the concrete must be level and the steps must be square when it is done on a slope.
- Special precautions must be taken when you are building on rocks. The concrete may be placed in shuttering on top of solid rock formation, with the permission of the engineer. It might not be necessary to blast the rock.
The concrete should be 50mm around all the sides of a reinforced steel cage in the trench, and all structural beams.
The engineer should have inspected the trenches by now after they were dug and should have issued the relative instructions on how to proceed. It might be necessary for steel cages or additional compaction before he gives his approval for the concrete to be poured.
Albert says: “It’s not always possible to receive the correct volume of concrete that is needed. Have some other trenches like screen walls or boundary walls ready where extra concrete can be placed instead of being wasted”
Pegs can be installed in the bottom or sides of the trenches to ensure that the concrete is the correct thickness and level.
Wet the trenches before the concrete is poured. The reason for this is to slow down the drying and shrinking process of the concrete. This procedure also limits shrinkage cracks.
It is ideal when a poker vibrator can be used during the levelling process in order to remove all the air from the concrete. A vibrator must be used when concrete is poured around reinforcing.
The concrete should reach its required strength after 28 days, and by keeping it continuously wet for at least seven days that process will be enhanced. Request a report from the concrete supplier regarding the strength of the concrete which they have delivered to your site.
Next time: Why you’ll be glad damp proof course is correctly installed three years from now