Tim Walker, Managing Director of CoGri Limited (New Zealand), sheds light on the problem of concrete floor delamination in warehouse settings. He looks into the issues that contribute to delamination and outlines proactive measures to manage associated risks.
Tim also examines the complexities surrounding the causes of flooring delamination, particularly in the context of increasing Low-Carbon Concrete usage.
Delamination describes an issue where the top surface layer of a concrete floor becomes separated, resulting in weakened zones beneath the surface. The surface can then break away – either on its own or after trafficking the slab. This significantly impacts the performance, durability, and appearance of a concrete floor. The increasing use of Low Carbon Concrete (LCC) has heightened the prevalence of delamination in recent years.
Delamination can be a contentious topic.
There is an old saying that “Power Trowelling is the root cause of delamination, but not all power trowelled slabs delaminate”.
The conundrum is – we need to power trowel slabs to get a dense and abrasion resistant surface that will stand the test of time – so, how do you achieve this and prevent or deal with the delamination menace?
This article summarizes the key causes of delamination in industrial concrete floors, steps that can be taken to prevent delamination, and some common repair options.
Delamination is defined as a separation or weak zone within a concrete slab, occurring parallel to the surface layer.
This defect can vary in size, depth, and severity based on material, environmental, and procedural factors during the construction process.
While delamination can occur in various concrete structures, it is most often seen in floor slabs, where it compromises the strength and durability of the surface.
Delamination is primarily caused by the entrapment of bleed water and air voids beneath the concrete surface during the finishing process. Understanding the root causes of this problem is crucial for mitigating the risk of surface defects.
These causes can be categorized into the following groups:
When the surface of the concrete is densified before the bleeding has stopped, water becomes trapped under the cement paste. This leads to shallow delaminations, usually within the top 3 mm of the surface.
Misjudging the right time to start finishing can occur due to external factors such as:
On the opposite end of the spectrum, finishing concrete too late can be just as problematic.
Delayed finishing can cause “over layering” where paste is removed from high spots to fill hollows, but because the concrete is already set the paste fails to bond in place. This situation is most likely to occur on large pours (such as post-tensioned slabs) where there is more of a risk that finishing will take longer than expected, and/or in high temperatures where set time can accelerate.
Excess air in the concrete mix can lead to the formation of air pockets and bubbles that distort during trowelling. This typically results in weak zones at a depth of 5-10 mm below the surface. These air voids not only reduce the durability of the concrete but also exacerbate the problem of delamination, as they create planes of weakness within the slab. It is well established that concrete with air-entrainment should not be power trowel finished because of the likelihood of delamination.
As well as issues with air-entrainment, accidental air entrapment can occur – for instance due to an issue with an admixture added to the concrete.
Concrete mixes containing Supplementary Cementitious Materials (SCMs) like Flyash and GBBS, can exhibit unusual bleeding or set time patterns, increasing the risk of delamination. Examples include:
Differential set between loads of concrete in a floor can also lead to a higher risk of delamination by making the timing of finishing difficult.
Admixtures are often the culprit here – for instance, using a different dosage of a set retarding admixture at different points during a pour will mean that the setting time / finishing window will change.
It is also possible for a concrete mix design to have too fast a set time to be properly finished – most often with higher strength concretes or accelerators being used.
In either case, the mechanism of delamination is typically the same as premature finishing with bleed water being trapped underneath the surface.
Once any amount of delamination has been identified in a floor we recommend a comprehensive investigation.
This is because if one area of the floor is obviously delaminated, there is a high likelihood of other weak areas existing in other locations.
We typically recommend the following hierarchy for investigating delamination:
Some delamination will be extremely obvious with the surface having dried prematurely and broken away. Excessive “crazing” and discolouration of an area are also a sign of delamination. A visual inspection and measurement of the thickness of delaminated sections can also help to hone in on the likely mechanism such as premature finishing versus excess air content in the mix.
Using a chain drag survey and hammer, areas of delamination can be detected by the hollow sound produced when tapping on the surface. Typically, this is the most efficient way to map out the delamination in the floor.
These advanced technologies can help detect delamination beneath the surface but are typically reserved when simpler techniques like sounding/chain dragging aren’t effective.
Extracting cores from suspected areas and analysing them microscopically can provide definitive evidence of delamination and its causes. We would typically reserve this technique for when the cause of delamination isn’t clear based on the evidence that can be gathered from concrete batch records, procedural records and the like.
As well as the above testing, reviewing environmental conditions, concrete test results, batching records, and any evidence around the timing of finishing can help to diagnose the mechanism and likely cause of the delamination.
Preventing delamination in warehouses requires careful control of environmental, material, and finishing process during floor construction.
Environmental conditions such as temperature, wind, and humidity play a significant role in the risk of delamination.
Concrete surfaces exposed to fluctuating environmental conditions can experience uneven setting, leading to delamination.
To mitigate this, it is recommended to pour the warehouse floor in a weathertight and windproof building. If there are gaps in cladding or door openings these can be covered with tarps.
In large buildings where the floor construction needs to begin before the roof/cladding are finished, windbreaks can be installed near the face of the pours to help to de-risk.
Concrete mix designs should maintain a balance that ensures sufficient bleed water to protect the surface without leading to excessive bleeding. One of the challenges with modern concrete mixtures in countries like New Zealand and Australia is the use of high-range water reducers that tend to reduce bleeding, making it important for contractors to monitor and adjust mixes accordingly.
Monitoring bleed water closely on-site and being mindful of the impact of environmental conditions like temperature and humidity on evaporation rate can significantly reduce the risk of delamination.
Air content in concrete should be regularly monitored throughout a concrete pour, especially when using admixtures such as PCE water reducers that rely on defoaming agents to avoid air entrapment. We recommend that site-based testing can complement plant-based testing for real-time adjustments.
The timing of finishing operations is crucial for avoiding delamination.
Concrete contractors should carefully document the timing and method of each finishing phase, including screeding, floating, and trowelling. Better documentation can reduce disputes and help prevent delamination.
Surface treatments like dry-shake powders can enhance the hardness and wear resistance of a concrete surface but require careful application to avoid delamination. We generally recommend they be avoided as they are unnecessary in a typical warehouse environment and increase the risk of delamination.
Choosing the correct repair method for a delaminated concrete floor depends on the depth and severity of the delamination. The building owner’s opinion on the appearance of the floor and appetite for future maintenance will also play a role. Common repair methods include:
Grinding & Polishing: For shallow but widespread delamination (less than 3-4 mm depth), grinding the surface back to a competent layer of concrete and polishing/densifying to reinstate abrasion resistance can be a good option for fixing the issue. The downside to this method is that the floor will require re-polishing from time to time if the owner wants to maintain a consistent appearance or look.
Patch repairs: For isolated areas of delamination, the dilapidated surface can be ground/broken away and reinstated with a repair mortar (epoxy or cementitious based). If the repair is carried out correctly the new patches will be very robust, however, there is an aesthetic impact of having a floor that looks like a “patchwork quilt” that some owners are reluctant to accept.
Epoxy Injection: For small, isolated areas of delamination that present as “hollow sounding” but have yet to break away, it is sometimes possible to remediate the issue by injecting epoxy into the void between the slab surface and the underlying slab to re-bond the surface.
The surface is then typically ground/polished to try and improve the appearance.
Re-surfacing the whole floor: For wholesale delamination and situations where the building owner will not accept patch repairs, the entire floor can be re-surfaced with a pump screed or epoxy coating.
Bonded concrete toppings can also be used to re-surface an entire floor, although typically a pump screed type product will be more cost-effective and not create the added issues of a bonded topping which will lift the finished floor level.
Concrete delamination is a complex issue involving concrete properties, environmental conditions, and finishing processes.
While modern technologies have reduced some of the risks inherent in concrete construction, delamination remains a significant challenge for large industrial slabs. By understanding the causes of delamination and implementing effective mitigation strategies, contractors and engineers can minimize the occurrence of surface defects, ensuring that concrete floors perform as expected over the long term.
If your warehouse floors are showing signs of delamination, call the flooring specialists.
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