Monolithic Pouring of Basements - a Tip
While concreting basements, the principal concern is Waterproofing related that, the water may not enter or cause seepage inside the basement. Off course there are other precautions but here we are only discussing the stages of pouring in a basement. The primary emphasis is to avoid honey combing and voids, especially in the lower most section of basement walls.
SEPARATE POURING OF RAFT AND WALLS
In the past, the basements were constructed such that the floor (raft) was poured in one attempt and in the next, the walls up to 9 feet were poured. Since the height of walls is 9 feet with double steel net in only 6 inch wall thickness space, proper pouring could not have been done and thus causing honey combing at the base of the wall. This is because it is literally impossible to apply vibrator up to nine feet deep. The space on each side of the steel net is hardly ‘one and a half inch’ and in the middle there are spacers and steel strips used for binding the shuttering. Thus it was literally impossible to lower down the vibrator and use it properly. The vibrator has solid steel rod in the first 2~3 feet, while the rest is rubber pipe.
Therefore a new method was evolved which is commonly known as the Monolithic concreting in which walls up to around 4 feet are also poured along with the raft avoiding a perpendicular joint at floor level. In next (second) pouring the walls are poured up to another 5~6 feet. To ensure that the structural joint does not leak, a new product called ‘water stopper bar’ (2mm x 2mm) is placed in the center of the wall between vertical steel rods. This bar absorbs water and expands irreversibly filling all the gaps and voids left during pouring. This bar is expensive and sometimes the home owners avoid its use.
It was thought that the new method will allow better vibrating opportunity and the concrete will come more compacted and free from honey combing. But unfortunately the results were not as expected. The reason was that the raft below is freshly poured and not hard enough therefore; it allows the concrete of the bottom of the wall to submerse into it while the concrete little above stays tied up with the steel net. Thus, voids are created in the lower section of the walls (near the floor) even in monolithic concreting which allows water infiltration and are not easy to fix later on. The theories that sound so good on paper sometimes backfires when practically put into operation.
The question is; ‘what can be done?’ One option is to pour in three stages instead of two. First the raft, then walls up to 4 feet and then in third attempt the walls will be completed up to 9 feet. Now this means that there will be three pouring with two structural joints. Even if we apply water stopper on both joints, it would be compromising on the strength of the wall because of two structural joints and also adding the cost of water stopper bar which will then be used twice.
NEW WAY IN MONOLITHIC POURING
The monolithic concreting should be done in such a way that first the raft is poured and left to become a bit hard for two to three hours. The labor can have lunch and rest during that time. Then before pouring, apply old to new concrete bond and leave it for enough time to become sticky. Thus; the concrete of raft will get hard enough to hold the weight of concrete of wall pouring. Now, the walls up to 4 ft. can be poured and vibrator be applied. No water stopper bar will be required at this stage. Since the raft concrete is not completely set, it will unite with the newly poured concrete and the old to new concrete bond will further help in binding the two concretes which are hardly two to three hours apart.
By giving time to the newly poured raft to get a bit hard, we can prevent the sinking of the lower section of the wall concrete. The vibrator can easily be used in the wall section without the fear that, it might cause sinking. It’s a small alteration to the procedure of Monolithic Pouring, but those who are acquainted with the problem, may understand its utility very well.