How do you avoid durability problems with a waterproofing admixture?

And when it comes to concrete mix design, how do you avoid the problems that moisture causes in concrete?

Changes onsite

If the concrete is too stiff to pour, once on site, more water may be added to help workability. This is often the case if you are using an admixture that sucks up the moisture.

But adjusting the water level in this way, can have a negative effect on the durability. This is opposite to the result intended!

Extra water - lower durability

When the water ratio in the concrete is too high, the proportion of GP cement and other fines : volume is reduced. This results in the CSH (Calcium Silicate Hydrate) bands not forming fully. Consequently, compressive strength is lower, and the structure will be weaker.

The risk of shrinkage cracking is higher as well, contributing to the lower strength and lowering durability.

A further risk is the porosities that are formed as a result of bleed water. This occurs in every concrete pour, but more porosities form when there is more water.

What happens in the long-term, is that these porosities allow soluble contamination to travel into and around the structure via moisture movement (capillary action). This contamination may corrode reinforcing steel, or eat away at the concrete itself.

Having control on the mix design all the way through to placement will ensure that the service life of the structure is longer.

Back to the design

But how to avoid the risk of the concreting team mix when it doesn't pump as fast as desired? or isn't as workable?

Can you design the mix so that workability doesn't decrease when the concrete includes a waterproofing admixture?

Let's say you have a project where integral waterproofing is the best way to waterproof the concrete (or part of the solution - like this waterproofing system) - admixtures are key. The entirety of the concrete will be waterproof, no installation is required - and not only will the concrete not allow water in, but will prevent moisture from moving around within when the concrete has fully cured - this should increase concrete durability, all going to plan.

But a lot of admixtures based on pore-filling crystals (crystalline admixtures) actually make the mix harder to work with. The crystals react with the mix and use the mix water to grow. The reaction intended to take place in the setting concrete effectively happens beforehand as well. This dehydrates the mix and results in less flow, higher pump pressures, difficult-to-finish concrete, and shorter finishing time.

The concrete team feel the need to make the mix more workable, by adding water. And sometimes, this water addition isn't monitored.

Quoting a leading concrete producer: ...it doesn't take long to add 100 litres of water to a truck load say (5m3) of mixed concrete. The effects will be (a) an average increase of about 80mm of slump over the slump specified, (b) an average reduction in compressive strength of about 5 MPa and (c) uneven strength throughout the concrete mass comprising a number of truck loads with varying slumps.

What can be done at design stage to prevent this?

The waterproofing admixture has been included in the concrete mix design because it's necessary in the project long-term - but even if the design hasn't included a crystalline waterproofing admixture, there are other reasons why the workability (or concrete rheology) may be reduced when the truck turns up onsite.

Why not prepare in the design phase?

If you will be designing a waterproof mix, and decide to go with a waterproofing admixture - why not use a liquid waterproofing admixture, rather than crystalline?

Some liquid additives for concrete have been associated with risks such as noxious fumes - some concrete ready-mix plants refuse to dose truckloads with certain admixtures that require HAZMAT PPE to be worn.

But we're referring to Nil-VOC (no nasties), non-toxic admixtures - particularly, admixtures that are made up of colloidal silica.

Increase the Workability.

Colloidal silica admixtures, such as those in the CONQOR range, are water-based and add to the workability of the concrete. Rather than reducing the effect of mix-water, adding this liquid to the mix will amplify the water's effect on rheology, and make the mix flow smoother and faster without dramatically changing the water to cement ratio - the liquid has a very low dose-rate.

Waterproofing

Once the concrete is curing, the liquid reacts with the high-alkalinity water to create a hydrogel, turning all available mix water into a gel that blocks the porosities of concrete. Bleed water causes concrete to be very porous when viewed by a microscope, but colloidal silica that has become hydrogel actually forms more of the CSH strands that are destroyed when water bleeds to the surface. This increases the impermeability of concrete, and greatly reduces contamination which causes corrosion of reinforcing steel.

WH&S

Rather than requiring heavy PPE to be used when dosing a concrete truck with this liquid, colloidal silica admixtures like CONQOR B50 are non-toxic.

Being safe for sensitive environments such as marine concrete structures, a number of colloidal silica treatments have received potable-water certification, as well as the licence from Eco Choice Aotearoa. They are recognised by Green Building Council of Australia (GBCA) as satisfactory to be used in Green Star projects.

Approaching the design of concrete mixes with a systemised view will make it easier to make decisions.

Ask us about the CONQOR B50 sustainable waterproofing admixture.

You’ll be put in touch with a project consultant who, with the entire team, has decades of combined experience with colloidal silica treatments. This consultant will advise on the project as to whether the admixture will be a suitable fit. The same team will supply the product, and warrant works undertaken by our team.

Reach out if reducing risk of concrete failure onsite is one issue you'd like to solve quickly!

Contact here.