Concrete Delamination – how to avoid it !

Concrete delamination refers to the separation of the paste layer of concrete from the concrete surface. The paste layer of the concrete surface then becomes prone to all kinds of abraders in the atmosphere which consequently weakens the structure. This issue develops with troweled concrete, commonly during the early spring and late fall with concrete placement on a cool subgrade. When a composite structure is hit, the impact causes shockwaves that travel through the structure, and cause delamination. Delamination reduces the strength of the composite structure and may cause the structure to break at a fraction of the load it would have carried before the impact. Most causes of delamination relate to the finishing technique and its timing not being appropriate for the behavior of the concrete being used in the particular environment.

Delaminations are similar to blisters as in both occurs due to due to rapid heating of moist composites. The area gets affected due to bleed water and air being trapped below the prematurely closed mortar surface. The primary cause od delamination is finishing the surface before bleeding has occurred. Usually, it’s the top 1/8- to 1/4-inch which gets densified. This is mostly due to premature and improper finishing and a thin layer of air or water causing the separation. An affected area may range in size depending on the severity of the delamination, from several square inches to square feet. Cracking and color differences commonly develop on the surface when the thin surface dries too rapidly during the curing process.

Causes of concrete delamination

Delaminations are also more likely to occur when factors that extend the bleeding time of concrete are combined with factors that accelerate surface setting. It is predominantly related to the timing of the final trowel finishing operations. Conducting the trowelling operation before the initial setting time makes the bleeding water or air underneath to form blisters. When the surface is subjected to any sort of load action, these blisters would delaminate out. Going with the finishing processes before the bleeding is completed traps the water or air-voids below the finishing layer. These, as a result, form subsurface voids which make that concrete zone weak. As time passes, these subsurface voids detach with loads. Apart from the timing and finishing problem concrete delamination can occur if

1. An underlying concrete sets slow due to cool subgrade
2. The concrete temperature or mixture ingredients delay the concrete setting;
3. Entrained air is present in the concrete, or the overall air content is higher than wanted than the needed application;
4. Higher cementitious material or sand-fines content have created a sticky concrete mixture;
5. Environmental conditions have created a rapid drying situation, in turn causing a crust-like surface and the appearance that its ready to finish;
6. Excessive vibration draws too much mortar to the surface;
7. Using a dry shake, especially with air-entrained concrete;
8. A thick concrete slab
9. The slab placement is directly on top of a vapor retarder.

How to avoid concrete delamination

It is necessary to wait for a period of time after placing the concrete to allow air and water to escape from the concrete. The waiting period varies with the concrete mixture, mixing and placing procedures, and weather conditions. The main items which should be controlled to minimize the risk of delamination are: Uniformity of placement, bleed rate and setting time over the surface, evaporation rate and finishing.

Maintaining uniformity of placement, bleed rate and setting time

Consistent bleed rate and setting time between the loads are important to avoid varying finishing times over the area placed. Placing concrete progressively from one side to the other, while maintaining a workable concrete face on which fresh concrete is placed, also allows finishing to proceed uniformly in the direction of placement. The precautions taken in this stage are-

• Uniform mix design, controlled within tolerance slump and consistent delivery
• Supply from a single concrete batch plant for each pour
• Air-entraining admixtures should be avoided
• Set-retarding admixtures should be avoided
• Ensure that the sub-base is dense and saturated
• Achieve optimum flatness during initial placement and screeding
• Ensure uniform compaction as bleeding and settlement may increase in less compacted areas
• Reinforcement should be located at the correct height and supported on chairs at the recommended spacing
• Establish a uniform placement pattern so that concrete is always placed against a workable face and that there are no significant setting
• The finishing operation must follow this same pattern

Evaporation rate

If the rate of evaporation of water from the surface is greater than the rate at which the bleed water is rising to the surface, you can observe that the surface will appear dry and firm. You need to maintain an evaporation rate at this stage. The precautions taken in this stage are-

• Ensure that climatic factors and finishing times are uniform over the entire concrete surface
• To reduce the evaporation rate of bleed water from the surface, fog misting or an evaporation retarder can be used
• The product must be applied as a fine mist after bull floating and thereafter as necessary up until final finishing

Finishing

The finishing process should match the path of concrete placement. This should be undertaken at an appropriate time and rate so that concrete is finished at a consistent time after concrete placement. The precautions taken in this stage are-

• Ensure that all finishing processes are performed by competent personnel
• Bull-floating should be completed prior to any significant bleed water appearing on the concrete surface. It removes ridges of mortar formed by the screeding operation and improves the finish by closing minor holes in the surface.
• Final floating should not commence whilst bleed water is present. The first pass of final floating is to flatten the surface.
• Take time and ensure the surface is not closed
• Plan, test and float areas
• Use a walk-behind power floating machine for the initial pass. This will aid surface leveling before commencing with ride-on pan floats
• For troweling the first pass should be with blades as flat as possible to avoid moving the surface mortar around
•  Sufficient time should be allowed between passes for the water that has been squeezed to the surface to evaporate
• Water should not be applied to the surface to aid the movement of finishing equipment

Tips you need to follow to avoid concrete delamination

1. Don’t seal the concrete surface early.
2. The air and bleed water below need a sufficient amount of time to escape to the surface.
3. Avoid using dry shakes on air-entrained concrete.
4. Don’t finish slabs placed on impervious surfaces too early.
5. Don’t place concrete on cold subgrades when ground temperatures are below 40°F.
6. Never place concrete on top of a vapor retarder.
7. Take extra care when ambient evaporative conditions are high and exceed concrete bleeding rates.
8. Do take special precautions when ambient evaporative conditions are high and exceed concrete bleeding rates.

To achieve a high-quality surface, remove the defective concrete to a depth where only sound concrete remains. The proper removal of unsound concrete can be done by suitable methods-such as shot blasting, grinding or hydro-demolition.

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