Casting Ground Beam for Raft foundation
When constructing your raft foundation, the ground beams are very important because there take away the loads of the walls and columns of the building and spread them over the foundation slab (raft) at the edges and under load bearing walls. Therefore, it is very critical because the foundation slab actually rests on them. Here are some tips to know when casting ground beams on the casting day (the day concrete is poured):
Step 1: At the position where the ground beams will be, a ridge or hole would be dug (not too deep) which would then be blinded with concrete. Then the wooden formwork is erected and the reinforcement bars placed into the formwork following the bending schedule given by the engineer. The height of the formwork would depend on the height of the ground beam to be constructed.
Step 2: There would be reinforcement bars or upbars that will protrude to about 5 feet above the formwork located at the corners or middle of the foundation. These protruding rods will be used to form the pillars or columns that will join block walls for the superstructure of the building.
Step 3: The plumber is also going to be around to make sure pipes for the wastes and drains are installed inside the formwork to avoid breaking the floor again after it has been constructed.
Step 4: The concrete to be poured in will then be mixed by mixers (people who mix the concrete). There is a given ratio for the concrete mix, which can be ratio 1:2:4; which means 1 head pan of cement, 2 head pan of sand and 4 head pan of granite will be mixed together in addition of water to form the concrete to be poured into the formwork containing the reinforcement bars.
If the concrete mixer which is to be used in mixing the concrete is bigger than usual, then the mixture would contain one bag of cement, 4 head pan of sand and 8 head pan of aggregate. The size of the granite usually used is the three quarter (¾) inch granite, but all the pieces of granite are not actually the same. At this point, the engineer/builder who is experienced in building would then decide if more granite would be used over less sand and vice-versa. When mixing the concrete, you have to realize that the type of water used is very important e.g., salty water should not be used for mixing concrete.
Step 5: After the concrete has been allowed to set, then the formwork is removed and the ground beam construction is complete.