waterproof concrete

What might specifiers, architects and engineers, make of my research and opinions?

Certification, crack restriction, good practice.


On other pages I have explained and backed up the Concrete Society findings that BBA certification is low value at best.

As well as the evidence on other pages:
  1. All the BBA certificates for these products state that there must be site testing to determine the specific mix and thickness of concrete that will meet the design requirements for water resistance.

  2. And all the certificates state:
    This certificate: Has to be read, considered and used as a whole document - it may be misleading and will be incomplete to be selective.
So none of the certificates say just specify this product and it will work.

Instead specifiers might consider getting the concrete actually used on site tested for permeability. 100mm or 150mm cubes can be taken and tested to BS EN 12390-8:2009.

Crack control in waterproof concrete is essential.

Different parts of elements and elements of different size cool and shrink differently.

BS 8007 has formulae but only based upon CEM2 concrete which gets less hot and gains strength slowly.

Waterproof concrete made with pure OPC, CEM1, gets relatively hot but is hardened before it gets a chance to cool.

Other sources of crack width calculation are
CIRIA, early-age thermal crack control in concrete, C660, CIRIA, London 2007. THE CONCRETE CENTRE. Concrete basements Guidance on the design and construction of in-situ concrete basement structures to the Eurocodes. MPA, London, 2012.

I am not a chartered engineers so I am going to give you the benefit of my experience to add to your professional learning, not tell you how to do your job.

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

I 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, so they do not get as hot.

Caltite, I have been told, recommend more bars rather than thicker bars for crack control. It is not an authority but I have followed that advice with success.

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.

I think it is better to fix extra 10mm distribution bars midway between the horizontal welded A393 bars, this means that 10mm 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 crack restriction 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 in BS 8110 which is 300mm.

  waterproof concrete crack width

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.

In contrast, wI like to see the same bar Shape 21 on 1m centres both horizontally and vertically throughout the wall 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.

Good Practice.

I have a number of rules worth following.

  1. Use good quality, cohesive concrete.

  2. Deliver it to within 1250mm of the bottom of where you are placing that concrete.

  3. Vibrate it properly. In, out slowly, every 200mm.

  4. Do not have to lift the poker more than 2m to move it along. That probably restricts formwork height to 2m each pour.

  5. Do not loosen or remove the formwork for 3 days.
If your project has enough labour and a supervisor to watch these 5 rules are applied, no concrete ought to visibly leak.
  basement construction   basement construction
basement construction

To have a kicker at the base of a wall or not?

Kickers are usually the height of a piece of timber readily available on site. 50mm, 75mm or 100mm.   The kicker concrete sinks into the slab during compaction if cast at the same time, or else concrete added later is not fully compacted because there is not enough there to poker properly.
forming kicker tied to starter bars   forming kicker

Therefore, in my experience, kickers always leak and the concrete needs replacing later since general porosity cannot be waterproofed by resin injection. Neither can you coat inside with water coming in pushing your remedy off before it cures. Any repair outside will get damaged during backfilling, because protection board is no match for a digger bucket.

However the Concrete Society stated, (The Structural Engineer, Vol 93, issue 1, Jan 2015, p48) "In liquid-tight construction, it is recommended that a kicker with a height of 150-250mm is constructed monolithically with the base slab. While kickerless construction may be preferred by some contractors in the interest of speed of construction, the associated additional risk of water penetration is not warranted in liquid-tight construction."

Unfortunately they do not explain any of their thinking,
and I cannot either. None of it makes sense to me.

I did ask one structural engineer who thought the benefit of a kicker was to clamp the formwork on tight and reduce grout loss.

I use spray foam to reduce grout loss.
  preventing grout loss with spray foam

  1. Monolithically.
    This can normally mean at the same time or later with a scabbled joint.

    At the same time with the same quality of workmanship as the slab itself is largely impossible. The kicker must be filled and compacted when the slab has stiffened yet before the two concretes can no longer be mixed, else there could be laitance through the joint that will leak.

    Later is no different to forming the walls themselves later. The deliberate construction joint needs to be sealed.

  2. Why should kickerless construction be preferred in the interest of speed of construction?

    Not using kickers is definitely more time consuming for me, considerably more. I save by not having to carry out repairs.

The kickers in the first photo is the usual method. Bits of timber, some nails and wire. Forming a kicker is rarely time-consuming.
  1. I have pages about kickers and joints.

    The "risk" comes from having a kicker, not from not having one.
Perhaps someone responsible for the article in the Structural Engineer will let me know the arguments behind their advice.

On my Joints page, in the Warranty section, I discuss what I think is the best strip to have in a joint. If a kicker is demanded then this joint strip should be tall enough to start on the floor slab steel mesh and extend through the kicker into the wall proper.

To have a tape or strip in a joint or not?

The Concrete Society stated, (The Structural Engineer, Vol 93, issue 1, Jan 2015, p47) It is good practice to protect all joints by incorporating water bars.

The accompanying sketch in the journal is of a preformed PVC strip backstop water bar.

These products need proper care to form and weld them the correct dimensions and monolithically around a slab wall joint. Unfortunately, a few mm too long and they sag and flop and tend to get smothered by wall concrete and buried in the concrete instead of remaining up the side where they would work.The Fosroc Rearguard products are of this type.

Without supervision most workforces will see a joint strip as a good reason not to do their work properly.

My own Supervision concentrates on getting the concreting carried out properly ignoring the strip.

Such strips overcome small errors and have value in a robust design. But they cannot overcome wholesale bad workmanship or very little care.


Does your idea of a guarantee really exist anywhere? Are you hoodwinked?

The first question I have usually been asked by architects and engineers has usually been "How long is your guarantee?"

And they seem to mean, against leaks.

But concrete does not leak. It is dirt, cracks and voids that visibly leak and a capillary network that lets in damp. And no BBA certified water resisting admixture supplier seems to guarantee against those.

Caltite (that doesn't work), Pudlo (that is mostly a little microsilica) and Sika (that is mostly a plasticiser) have all, so we have heard, carried out some repairs (some quite expensive). But they did not sign any work off until they inspected and found no leaks first. In other words, they like leak repairs that are paid for. They don't pay for many themselves.

Here on the right is Kryton's Technical Data Sheet web page for Krystol Internal Membrane. If you look at the very bottom you will see Warranty, which I have reproduced here, large enough to read. waterproof concrete guarantee

And this Caltite guarantee was new in 2015.

The Concrete Society says that good concrete is inherently waterproof. So I wonder whether there is really anything that Cementaid still guarantee with Caltite. Bearing in mind it does not work beneath ground and there representative does not supervise. caltite cementaid
  waterproof concrete guarantee

Supervision is beneficial.

Is there really any point, and is it justifiable, to make your client pay for a useless additive so someone watches the concrete pour but absolves themselves of supervision or responsibility?

Perhaps specify a Clerk of Works instead?