waterproof guarantee basement construction build manage
  waterproof guarantee basement construction build manage

Page for Structural Engineers about to design a basement that we might build and guarantee.

Basements are usually waterproofed to BS 8102, which requires 2 defences against ingress of water. So although we are confident that our work will be completely waterproof, you only need to be confident that our work will substantially reduce the amount of water that a later defence will have to deal with.

Your reputation will be completely safe if you only rely on concrete to reduce the ingress of water and internal or external drainage, as appropriate, to complete the job.

First my ear-bending about waterproof concrete. Sorry, but you will find it educational.

Not a single BBA certified admixture has made public how it waterproofs concrete*. However every BBA certified admixture for concrete claiming to make concrete less permeable states on the certificate that concrete, proven as follows to be impermeable - even before their admixture is added, must be used.

I strongly suspect, and a working group of the UK's Concrete Society in 2013* made up of consultant engineers, academics and other experts, also strongly suspect, that it is the concrete that will be waterproof, not the admixture.

For years Basement Expert Ltd has always used the concrete that is proven to always be completely impermeable. Without any proprietary admixture not proven to do anything but cost a lot of money.

Here is a photographed extract from my copy of "Advanced Concrete Technology", the volume called "Processes", the Chapter called "Concrete construction for liquid-retaining structures" by Tony Threlfall, 2003, Butterworth-Heinemann. Page 16.2.

It says that any permeability better than 50mm is good. You will see on the certificate for impermeability that our results in this case were 1, 1 and 3mm. We have dozens of similar results going back years, so we tend not to pay for this test (£600 plus VAT) any more. But you can specify tests if you wish.
  Click on this image to be able to read the text from the whole page.   from Processes, water retaining BS EN 12390 8
Without any proprietary additive a BBA product certificate is not appropriate. In the text you will see that the appropriate test is to BS EN 12390 part 8. We have had many successful tests for water permeability under pressure.

Many, many large sites routinely test concrete for strength to BS EN 12390 part 3. We had concrete tested to BS EN 12390 part 8 as well. Depth of penetration of water under pressure on concrete. The pressure is equivalent to a depth under water of 30m and the pressure is maintained for 96 hours.

Click on either image to the left to see two original test certificates.
  concrete compressive strength report to BS EN 12390 3 concrete permeability report to BS EN 12390 8

* In 2013, the UK's Concrete Society published a report by a working group of 18 members, experts in their fields, that states: " water/cement ratio .... primary measure of water penetration and hence the durability of the concrete."
From: The influence of integral water-resisting admixtures on the durability of concrete. P36. Concrete Society. 2013.

We prefer to use a beefed-up version of BS 8007 C35A.

Slightly more OPC and quite a lot less water.

This means that the concrete gets hotter.

So we would like to share with you our experience of crack width restriction.

waterproof concrete basement In this photograph you can see that the bottom half of the wall is already cast. The concrete is in perfect condition and the formwork has already been struck to inspect it. This wall is going to get an upstand at the top and you can just make out the bottom of a window opening.

We have a page for specifiers going into far more detail about the concrete we like to use here.
You might specify:

C35A watertight concrete with a maximum of 0.50 water to cement and a minimum of 350kgs pure OPC per cubic metre.

One set of 5 cubes to be taken from the first pour and tested to BS EN 12390-8-2009. Maximum permissible average penetration 5mm.

Concreting workmanship to be to a high standard, at least BS 8007, and all concrete works to be effectively supervised throughout.

The major ways we build differently are:
  1. Improved C35A concrete, we take cubes to prove that the concrete used in every project is completely impermeable to BS EN 12390-8:2009.

  2. No kickers because kickers leak.

  3. No threaded bar holes because it is too easy to miss one or two when filling them.

  4. Half height wall formwork so that we can get the concrete to the bottom properly, and poker it properly, throughout.

  5. Our concreting is supervised.

  6. We want an upstand to above outside ground level as in the sketch at the top.

The Importance of an Upstand.

waterproof basement construction upstand

waterproof basement construction floor ledge
  It is always essential that a retaining wall extends to above outside finished ground level.

It is usually necessary for face brickwork to start below ground.

Unlike an ordinary house on footings, we need to prevent a horizontal ingress of water beneath the ground floor.

Please see the sketch at the top of the page and the bigger sketch below for ideas.

However, the upstand is not normally retaining so it just needs crack protection.

Room to properly place and compact concrete.

Further down the page you will see typical examples of engineers' designs that prevented good workmanship.

Below these articles you will see our diagram showing a selection of steel reinforcement that enables good workmanship.

The Basis Of Design in para 2.1.4 of BS8110-1:1997 states: "Design, including design for durability, construction and use in service should be considered as a whole." Your design has to allow, not prevent, the required standard of workmanship. there must be room to get concrete to the bottom and compact it properly.

Pouring concrete in walls.

Filling a wall bottom to top in one day results in poor concrete at the bottom. Therefore we form and pour less than 2m through a wide opening at the top and the remainder through the upstand another day.

Crack Protection.

This is typical of a crack where there is not enough distribution steel and the ends of the walls are restrained.

A 300mm thick wall with A393 and 12mm starter bars on 200mm centres in both faces will crack like this and may not fully heal, whereas the same steel in a 200mm thick wall seems to always heal completely.

If the starter bars completely restrain the bottom of the wall, the crack will not reach the bottom.

We want any crack to heal before we backfill, to heal before water could flow through and prevent the healing process.

More distribution bars and thicker starter bars seem to provide the solutions.

Note that we have never seen horizontal cracks, no doubt because the wall height can contract unrestrained on cooling.

waterproof concrete crack width
We see very little cracking in slabs and almost none that does not self heal with only two layers of A393. Probably because slabs lose heat much more easily than walls still in formwork.

Some recommend more bars rather than thicker bars for crack control.

Therefore specifying 12mm bars on 200mm centres may not be the best way of increasing distribution steel.

Two faces of A393 has always proved to be sufficient vertically (even though it is only 0.25%) but not horizontally in a 300mm thick wall. Two rows of 12mm starter bars are not always enough either.

We have found that it is better to fix extra 10mm distribution bars midway between the horizontal welded A393 bars, this means that distribution bars are on 100mm centres.

The A393 bars probably satisfy your structural needs and you will specify corner bars and bar laps as you see fit.

However these extra bars for cracking simply need to be in any crack and not slip, so we feel that at corners these bars just need to cross by 10mm and straight lengths lap by the minimum recommended in BS8110 which is 300mm.

Starter Bars and crack control at the joint.

16mm bars to the outside face and 12mm starter bars to the inside face on 200mm ccs is an acceptable minimum. 16mm bars to both faces slightly more reassuring.

Slab wall and other joints.

We prefer to scabble and clean joints to BS 8007 and have nothing in a joint to prevent us keeping it clean.

If you feel you must specify something in the joint, we favour metal waterstop from Max Frank.

It is fixed to the slab steel and strong enough not to fold when the concrete is poured on top.

A first strip is removed at the last moment to reveal the bituminous coating that is in the slab concrete and a second strip is similarly removed to reveal the coating that will be in the wall concrete.

  metal waterstop
Cantilever, Not Propped Cantilever.

An apparent advantage of a concrete floor over rather than engineered timber would be the support a concrete floor gives to the top of the retaining walls by propping the walls one side against the walls the other side and the backfill beyond.

But this would mean that the floor had to be in place and complete before the basement was backfilled and that causes safety problems.

It might also mean that internal basement walls had to be complete before the floor over could be installed.

The upstand to the waterproofing is essential. An engineered timber floor works with it far better than pre cast concrete.

The extra steel required in waterproof concrete is sufficient for the wall to just be cantilevered anyway.

Narrowing the wall and propping it might save concrete but it is false economy. It reduces the space to fill with wall concrete properly, resulting in more voids that require repair to stop leaks, as well as forcing the client to spend more on the concrete deck than he would spend buying engineered timber.

Also the client would need a service void under a concrete floor so the whole excavation needs to be deeper and the walls taller, costing more as well.

We always use threaded FRP rod and nuts instead of the usual steel divi because the FRP is cast in and waterproof.

waterproof formwork rod

It costs more for the rod but there are no holes to pay to be filled.

The Floor Over Habitable Basement Space.

An engineered timber floor joist can be any width and any height and stronger than concrete (for the same overall floor construction depth). It can be supported with a web, lattice sides or plywood sides. Services can go through it. It can be top chord supported on a timber plate or sit in joist supports.

Clearly, engineered timber joists give more choices than concrete and they are flat on top. With services through and not under, and without the need for a screed to overcome the curvature, timber floor construction depth is usually less than concrete.

Any Corbel.

There is more about corbels on the architect page. You can have anything.

But a corbel is only usually necessary with a concrete floor over the top because, by the time there is an upstand to contain the propping effect the brickwork needs a ledge outside to start below ground.

Please, therefore, look at the details above and below.

If you can specify engineered timber floor joists and a cantilever retaining wall you will save your client a lot of money versus a propped cantilever wall, a concrete floor, floor screed, a service void requiring the basement to be dug and constructed deeper, and so on.

Thickness of Wall Concrete.

At least 200mm is needed for the concrete to be waterproof but working room is tight so 225mm is better and 250mm better still. 300mm is the most common because of the width of construction above.

No Fibre Reinforcement.

Fibre reinforcement dries the concrete so properly placing and properly compacting it is very difficult without adding water. But extra water cannot be waterproof, so properly waterproof concrete cannot contain fibres.

In addition, we need more protection from cracking than steel fibres provide.

Steel fibres are dreadfully expensive as well as inefficient having most steel in a random direction and not the direction of tension.

What makes concrete waterproof?

According to the UK's Concrete Society report in 2013 not proprietary admixtures with BBA certificates.

They proved that they make much less difference than better concrete.

Extra sand, extra cement, correct cement, reduced water.

Fly ash in most concrete is barely reactive. It doesn't use water.

The important ratio is water to Ordinary Portland Cement. CEM 1.

Or water to a blend of Ordinary Portland Cement and Slag or Micro silica. CEM 3.

More Here
The usual mix required is
min P350
60mm slump
over sanded
plus PCE plasticiser

If the concrete delivery ticket says all these things and the concrete looks cohesive and consistent, it is waterproof.
It will be so dense that capillaries will become discontinuous.
Capping Bars.

Some engineers like to see capping bars all along the top of a wall and, sometimes, even a central distribution bar.

Concrete cannot be poured through a tube to the bottom or properly compacted with a vibrating poker with this amount of obstruction.

basement construction   basement construction

In contrast, we like to have the same bar Shape 21 on 1.2m centres both horizontally and vertically throughout the wall (1m from the slab and 2.2m from the slab) to control the space between the steel and keep it all perfectly upright throughout, not just along the top which would leave the middle to curl and wave without control.

basement construction

Neat steel gives everyone confidence.


Different Standards have slightly different requirements.

We would say 40mm outside and a minimum of 20mm inside.

Table 5.1 in BS 8110-2 says that in mild conditions the nominal cover for a concrete with either 350kgs of cement or a wcr of only 0.45 needs just 20mm cover; whereas those figures increase to 40mm in severe conditions.

BS 8110-1, table 3.2, describes Severe as exposed to severe rain, alternate wetting and drying or occasional freezing or severe condensation.

BS 8007 (at 2.7.6) states that the minimum cover for all steel should not be less than 40mm. But it is probably reasonable that BS 8007, concerned as it is with structures to retain water, has to allow for a vessel to be filled and emptied repeatedly.

But a waterproof concrete basement is always in mild conditions inside even if the outside might be considered more severe as water tables rise and fall.

Corner bars.

Half of you claim corners are a weakness and specify corner bars in 3s on 200mm centres.

basement construction

The other half claim corners are the strongest part and don't specify any corner bars. I never had one crack but we should have full crack protection. Corner bars and the extra distribution bars.

Bars in threes make concreting with a pipe a bit harder and threes should only be needed on reverse corners. Most basement corners should be fine with only 2 corner bars.

basement construction


You can see a list of the steel that best suits our high standard of workmanship below.

The space between faces of wall steel is carefully maintained by U bars so that no steel obstructs the concrete and poker getting down to the bottom properly. Perhaps you would like to specify these U bars.

The first mesh is tied to the starter bars so it is 2.4m high. Then half height formwork and the first wall pour no more than 2m high through a wide opening with easy access for concrete and pokers to the bottom. The top half of the wall is a separate operation.

basement construction  
Everything about a retaining wall with an upstand to create a horizontal, waterproof barrier at outside ground level is easier and cheaper with an engineered timber floor.

Our preferred steel is:
• A393 top and bottom in the slab.
• 16mm starter bars to both wall faces.
• A393 to both wall faces 2.4m high.
Additional horizontal 10mm bars added to both faces of wall mesh so that distribution bars are on 100mm centres.
• Rather than using mesh to the top wasting lots of it, the mesh can be continued up both faces with straight vertical 10mm bars tied to uprights and distribution bars on 100mm centres to the to the top, which might be at a different level in front and back faces.
• U bars controlling wall steel on 1.2m centres vertically and horizontally starting 1m off the slab.
• All steel on 200mm centres except the extra distribution bars.

Add U bars for a small heel or long, strong links in place of U bars if you need to spread the load further beneath the wall.

We have a number of other pages with information.

waterproof basement construction   You Must Have a Soil Report Before Design Starts   Please Ask Your Architect To Read This Page   Please Ask Your Structural Engineer To Read This Page   construction contract

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