Wet Crack Injections in Basement Slabs Explained
Written By: Mr. James Lim
Date: 21 Apr 2022
Wet crack repair is becoming an integral part of concrete design to manage cracks that are prone to water seepage, especially in the lowest basement slab where water pressure is high.
In today’s construction design, the base slab or the raft foundation of a building is often designed with waterproofing admixture with the intention to waterproof the base slab against ingress of ground water. The use of waterproofing admixture is seemingly a trend now as structural engineers prefer admixture over the traditional membrane system. In order for it to work, the concrete has to be perfectly placed, vibrated properly so that a honeycomb and crack free concrete is produced at the end of day. However, the reality often gives a different outlook of this concept of admixture waterproofing system because it fails to address the natural occurrence of cracks in concrete structures. Whether it is based on pore blocking or formation of crystals that are only visible under the microscope, the truth is that the journey down this path is not so smooth sailing after all when cracks start to develop. All the so called self-heal mechanism of the admixture in the concrete will quickly evolve into a discussion of crack mapping and repair methodology not to mention who will be bearing the cost of the cracks repair.
When cracks start to develop, coupled with high water table of our ground condition, it does not take long for us to realise that the basement slab will soon become wet. Water ingress from the ground is almost inevitable in this scenario and there is only one way out from here. The objective has become clear that the project manager of the job site is now looking at the best way to seal those cracks so that the leaking cracks will not leak again. This might sound like a daunting task for a project manager as now he has to look for money to perform this task.
Main contractors will probably agree that injection grouting is the way to solve these problems relating to water ingress at the basement slab. Structural engineers prefer the cracks to be sealed with epoxy resin as it gives strength and good bond but majority of the epoxy resin in the market caters for dry crack condition. Epoxy resin is good for dry cracks normally found in intermediate floor slab. In the case of a base slab, epoxy resin will not be effective to seal against ingress of water from the ground. So a polyurethane (PU) based injection resin is preferred. Having said that, the majority of the polyurethane resin in our market consists mostly of the foaming type of PU resin (1K) and this normally come in a single pack water reacting resin. Water acts as the catalyst to start the reaction when the PU resin is injected into the wet cracks thus creating a foam that is filled with air bubbles. The foaming PU resin is normally formulated with a foaming factor between 20 to 40x. The higher the foaming factor, the lighter the foam as there are more air bubbles within the foam structure. As it is a foam, it does not come with strength and the foam can easily break or shrink over time. The German injection methodology treats this type of injection with the PU foam as temporary seal. So if you were to get a recurring leak after the injection, the PU foam has either shrunk and failed to resist the water pressure thus allowing water to escape through the cracks again. If someone says PU does not work, that basically means he has bad experienced with foaming PU resin.
What’s next if you have lost faith in PU injection? It is perhaps time to look at dual component PU technology. The formulation of a two-component (2K) PU resin allow the mixed resin to achieve a compressive strength of up to 80MPa which was previously not possible with 1K PU. A 2K PU resin will set after mixing the two separate components together without the presence of water. This is an important criteria of a good resin as not all parts of the wet cracks are wet and the PU resin will still set inside the crack if it finds a dry patch. The PU resin in the dry section will set just like an epoxy resin giving it good bond and compressive strength. Unlike foaming PU, the resin will not set in locations without the presence of water because water is required to catalyze the PU resin to start the foam reaction.
There are really no real advantages of using foaming PU resin to seal any wet cracks once you understand the chemistry of a foaming resin. The only reason why foaming resin is widely used is due to cost factor as the resin is much cheaper compared to a 2K resin. But does it really save money for the project manager?
The project manager shall look at the overall cost of the injection repair when it comes to wet crack repair rather than ask for a per point injection rate from the specialist contractor. Allowing your applicator to quote you based on a per point basis type of injection contract is like signing on a blank cheque and allowing your applicator to fill in the amount. Can you imagine the damages this concept can do to your project’s bottom line? In conclusion, concrete admixture alone is insufficient to prevent water ingress as a result of crack formation. Perhaps then, it is time to switch to a two-component injection resin system for a permanent fix.
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