Hi
I need to fill a strip foundation.This part of the strip foudation forms one section of the total strip. Hopefully I can apply the solution to the remainding parts.
The foundation will have to have 3 different levels. 2 for differring bridge over drain levels and the final building level.The bridging drain levels will have to be 1m back from from the drains to accomodate the respective steels/lintols. I am looking at a foundation with stepped levels. The only way I can think of doing this is with 3 pours. 1 for the 1st level and so on. The only problem with this is that the last our would only about 200mm thick from a toatal depth of 1.1m. I am thinking of using reinforcing bars through the total pour.
Is there a better and quicker way as I assume each pour of concrete would have to dry befor the next lot can be poured onto. Btw I am not using a ready-mixed.
Filling foundation trench
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Tony McC
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It's not too difficult to build 'steps' into a strip footing, as long as each step is not more than around 150mm. We normally form them in 75mm or 150mm units to match the coursing of the eventual brickwork, and thereby save the brickies from having to use a course of 'pig' to even out the level.
If, for example, we had to step 450mm, this would be formed as 3 steps @ 150mm, or 6 steps @ 75mm. Once you try to lift fresh (plastic) concrete more than 150mm, it tends to slump and so wipe out the step, but, as soon as it starts to stiffen up, it's a simple task to rebuild the step, as long as the levels to either side are relatively accurate.
The 'steps' are formed using concrete commons or trench blocks, set into the concrete to form a pretty shoddy riser/retainer. Remember that the concrete is hardening all the time, so it's usually just a matter of it staying in place for half-an-hour or so. Concrete is placed into the trench to the required lower level, then the bricks/blocks to form the step are set in position on top of the pour, and additional concrete is placed 'behind' the step to elevate the level. It's not uncommon to leave the 'back' of the step empty for 20 mins or so, until the concrete stiffens sufficiently to carry the weight of additional concrete without slumping and destroying the step. So, there might be 450-600mm of a gap at the 'back' of the step when first constructed, but this will be filled within the hour (assumming decent weather) to ensure correct levels and no 'day joint' in the foundation.
Do you have a longitudinal cross-section for this footing?
If, for example, we had to step 450mm, this would be formed as 3 steps @ 150mm, or 6 steps @ 75mm. Once you try to lift fresh (plastic) concrete more than 150mm, it tends to slump and so wipe out the step, but, as soon as it starts to stiffen up, it's a simple task to rebuild the step, as long as the levels to either side are relatively accurate.
The 'steps' are formed using concrete commons or trench blocks, set into the concrete to form a pretty shoddy riser/retainer. Remember that the concrete is hardening all the time, so it's usually just a matter of it staying in place for half-an-hour or so. Concrete is placed into the trench to the required lower level, then the bricks/blocks to form the step are set in position on top of the pour, and additional concrete is placed 'behind' the step to elevate the level. It's not uncommon to leave the 'back' of the step empty for 20 mins or so, until the concrete stiffens sufficiently to carry the weight of additional concrete without slumping and destroying the step. So, there might be 450-600mm of a gap at the 'back' of the step when first constructed, but this will be filled within the hour (assumming decent weather) to ensure correct levels and no 'day joint' in the foundation.
Do you have a longitudinal cross-section for this footing?
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Tony McC
- Site Admin
- Posts: 8346
- Joined: Mon Jul 05, 2004 7:27 pm
- Location: Warrington, People's Republic of South Lancashire
- Contact: