waterproof concrete

Sika Watertight Concrete Powder explained.

A combined water-resisting and super plasticising mixture.

But there is no clue about it resisting water beyond the effect of the plasticiser, and evidence that the plasticiser is not particularly potent.

The Sika BBA Certificate.

My comments

The certificate states that the product is a blend but we cannot recognise any effect from a water-resisting component different to the effects of a plasticiser.

It is not hazardous so it does not contain portland cement.

only 'can be obtained'

This clause seems to exclude the Sika from the equation.

This exceeds C35A in BS 8007. So the base mix is already watertight.

S3. The Concrete Society has an explanation of slumps and consistence class here.
S3 is 100mm to 150mm.
These quotes in dark red are all from the Sika product certificate on the BBA web site here.

But we have changed the order in which they appear.

This Agrement Certificate Product Sheet relates to Sika Watertight Concrete Powder, a combined water-resisting and HRWR/superplasticising admixture used to provide Sika Watertight Concrete, suitable for basements .....

This certificate ... has to be read, considered and used as a whole document - it may be misleading and will be incomplete to be selective

1 Description
Sika Watertight Concrete Powder is a combined water-resisting and High Range Water Reducing (HRWR)/ superplasticising admixture, for incorporation into concrete mixes to enhance the water resistance and durability properties of the hardened concrete.

3.4 The product is not classified as 'hazardous'

4.1 Sika Watertight Concrete Powder is satisfactory for use in concrete mixes at an addition rate of 1.75 kg per m³ of concrete to provide watertight concrete for basements, swimming pools, tunnels and culverts, without the requirement for additional applied protection.

7.2 Concrete made with a high water/cement ratio can have a water vapour permeability greater than 3000 by 10–12 g/m (Ns)–1. The permeability of concrete is strongly dependent on the exact mix design and the figures given in Table 2 indicate the levels that can be obtained using the product.

For Grade 3 (where control of water vapour is required), it will be necessary to provide a mix with a sufficiently low vapour permeability in combination with an adequate section thickness (see sections 6.2 and 6.3).

20.2 The concrete must have a minimum cement content of 350 kg/m–3 and be batched with a maximum water/cement ratio of 0.45 and have a minimum consistence of S3.

The slump in the control and Sika concretes tested for certification was quickly much less than 100mm to 150mm. Did they test different concrete?

From Table 1 Effects of Sika Watertight Concrete Powder
on the properties of fresh wet concrete(1)
Property Control concrete Sika watertight Concrete Powder
Water / cement ratio 0.47 0.40

Slump (mm)

0 min
115 120

30 min

75 40
Setting time (min)

initial set
170 230

final set
240 290

(1) The specific effect of the product on these properties for a particular mix and site conditions should be evaluated through site trials prior to use.
  The certificate states that a S3 consistence is purchased.

This test concrete was S3 when mixed but apparently, by the time a truck arrives and is discharging, it will be S1. Are these figures a warning?

Customers must buy 0.45 water, why test at 0.40 which would produce better results?

If the slump has reduced after 30 minutes we don't know when it reduced, only that it was within 30 minutes. Usually before the concrete has arrived and is discharged.

A 40mm slump is of little use to anyone. Workers spreading it by hand would want water adding. A pump operator would insist that water was added. Once water is added the benefits of the Sika in making the concrete slightly more watertight would be lost.

There is no written warning on the certificate that Sika retards the concrete, yet these figures suggest it does.

The initial setting time of 170 minutes strongly suggests that this concrete contains flyash. Therefore the water/cement ratio relative to OPC is actually far higher. If a third of the cement is flyash the wcr to OPC would be 0.67.

From Table 2 Effects of Sika Watertight Concrete Powder
on the hardened properties of concrete(1)
Property Control concrete Sika watertight Concrete Powder
Water permeability [m/s-1] 9.32 x 10-13 4.75 x 10-13
Capillary absorption [% by mass of control]
7 day 100 48
90 day 100 57
Water vapour permeability [g/m (N/s)-1] 417 x 10-12 296 x 10-12

(1) The specific effect of the product on these properties for a particular mix and site conditions should be evaluated through site trials prior to use.
  The control is very close to typical structural concrete (Concrete Society, 2013). This does not correlate with the mix specified on the BBA certificate at all. Any P350 max wcr 0.45 pumpable concrete would be better.

So why have they used much worse concrete?

The test concrete with Sika in is half as permeable. So still permeable.

We have to wonder why these figures are here. Are they a warning?

The Sika concrete still sucks in about half the mass of water by capillary action.

We would anticipate that a plasticiser would make the capillary pores thinner. Apparently they are halved in size but still present and still sucking in water.

To the occupier of the completed structure we would say there will be no difference.

Water vapour permeability is reduced nearly half as well. Perhaps correlated with the narrower capillary network.

Over the years we have had a few phone calls from people who saw our web site but decided, or their specifier decided, to go with a famous brand or a BBA certificate. These people wanted to tell us how they regretted their choice and wished they had used us. Some were incandescent with anger at how different their experience had been compared to what they expected. We mention it here because they all seem to have involved Sika.

See the chart below.

BBA certificates have figures for water permeability.

The Concrete Society had the idea of plotting them all on a chart. We have done that as well. But we have also included some best guesses. For instance, only the Caltite certificate has no figures for compressive strength.

Some of the control concretes were better than concretes with admixture in them.

The Sika is in a very poor position.

waterproof concrete