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The slab wall joint and any leaks.
Within BS 8007:1987, Code of practice for design of concrete structures for retaining aqueous liquids, can be found:
Waterstops should not be necessary in properly made construction joints.
Waterstops are never necessary in my construction joints. I always take sufficient care and I have a cheap and simple repair for peace of mind.
I will show on this page the folly of buying tapes or strips instead of supervising the concreting works.
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Yet tapes, strips or waterstops are almost always specified because joints are generally expected to leak. But joints still leak even with these expensive products inside.
It seems that if workmanship is poor that tapes, strips and stops do not overcome it.
The Standard expects the joint to be Properly Made.
What is properly made? More from BS 8007:1987:
The surface of the first pour should be roughened to increase bond strength and provide aggregate interlock
Care should be taken to ensure that the joint surface is clean
Particular care should be taken to ensure that the new concrete close to the joint has an adequate fines content and is fully compacted.
Roughened usually means scabbling. Scabbling removes any laitance (dried watery cement) that was on the top as well as creating a rough surface.
This is an example of laitance.
Most concrete would have excess water which rises to the surface, bringing cement with it, shortly after the concrete is compacted.
Laitance is weak, porous and liable to shrink and crack when it dries.
It would flake off if structural concrete was joined to it. |
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Laitance is also proof that there was too much water in the concrete for that concrete to be waterproof.
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This is scabbled waterproof concrete.
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This is waterproof concrete just left after the poker has passed.
No laitance because there
was no excess of water.
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Both are fairly equally rough and both would provide aggregate interlock. Plus the fact both have steel starter bars through the joint creating far more interlock than a roughened surface.
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Time after time I have found that perfect waterproof concrete perfectly placed and compacted and left to cure makes a joint that is perfectly waterproof with the next concrete poured against it. By The Thames, by the sea, anywhere, regardless of water pressure.
Let us remind ourselves of the 3 requirements in the BS.
first pour surface should be roughened; the joint surface needs to be clean; the new concrete close to the joint has an adequate fines content and is fully compacted.
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Inadequate fines.
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Not clean.
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You cannot see poor compaction.
Compaction must be supervised.
One certain source of poor compaction is excess water that will dry off leaving voids.
Another is poorly compacted concrete in a kicker.
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I supervise my concrete pours. We do not use kickers. We do not add water. We place concrete through a tube. We vibrate properly.
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You would not think that good supervision, good workmanship and avoiding kickers and adding water was difficult to achieve, yet every public building made with concrete has evidence of covering up voids in the concrete.
The bottom of a wall very often has large voids because the first concrete in the top hits reinforcing steel and sprays everywhere, the slurry sticks to the formwork and the stones fall to the bottom without fines.
| This is the Rose Bowl Cricket Ground in Southampton. |
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In any case, if workmanship is poor then a tape would not be fastened down well, not kept dry and clean and not protected from damage by people walking through the bars wearing boots and carrying heavy loads.
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It should now be quite clear that if workmanship is generally poor and the concreting not carried out well, that any tape or strip in the joint, specified solely to try to overcome poor workmanship, would be useless because of poor workmanship fixing it and protecting it plus the tape being surrounded by stones and any small amount of slurry that reaches the bottom getting under it so it ceases to be in the joint.
I find, time and again, that if the work is all done well (which means with supervision of ordering the concrete, placing the concrete, compacting the concrete and curing the concrete) that there is no laitance to remove and the joint seals itself. Never the less I usually repair the joint as a precaution.
Any waterproof joint in concrete must contain steel designed by your structural engineer. When the concrete in the wall sets, it heats up and expands. As it hardens so it cools down. It would contract except that the steel starter bars will hold it in tension. So the joint should not crack.
When I build waterproof basements, which I do several times every year, my permanent formwork starts off with a 60mm rip that I can take out to chisel a channel and refill it as a precautionary repair.
This is very cheap and quick and I reason that if the wall was going to crack then my chiselling will induce it to crack now, not later.
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Therefore, I start the polystyrene formwork with 60mm of foam.
After the wall is poured this thin strip is removed one side and the bottom of the wall concrete is exposed.
I use a small pneumatic chisel with a wide blade to go in about 30mm and about 20mm deep removing some concrete from both the wall and the slab.
If there was any doubt that the concrete I found was perfect then I would remove more.
AND I BLOW THE SITE CLEAN WITH THE AIR COMPRESSOR BEFORE I CONTINUE; SEE HOW CLEAN IN THE PHOTO BELOW. And the ability to seal any leaks in clean, dry conditions is what you get only from supervision.
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The concrete is usually found to be perfect when I supervise the work.
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I chisel out the joint like this (more if there is any defect).
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I fill first with Thoro Waterplug. It sets in about a minute and will stop a flow of water from behind if there is one. It is important to get the Waterplug to seal across the joint.
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I complete filling the chase with a sharp sand concrete. This might contain waterproofer or it might be drypack. Whatever is appropriate.
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100% planned. 100% effective and cost effective as well.
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