Delamination in Concrete Floor 

What are Delaminations?

Delamination of Concrete Floor

Delaminations are separations of thin layers of the slab surface from the base concrete. The thickness of this layer can vary from 3 mm to 8mm thick, depending on the mechanism involved. The size of the delaminated area can vary from a couple of square centimeters to several square meters. For Industrial or Warehouse-type projects, Delamination is most commonly seen in hard troweled floor slabs/Power finished floors. Delamiantions are difficult to detect during finishing but become evident after the concrete surface is hardened (Normally, one and two weeks after the placing and finishing process).

Causes of Delamination:

Delamination is mainly caused due to bleeding  & the air content of concrete. Bleeding is the upward flow of mixing water in plastic concrete as solid settle. All concrete bleeds to some degree. It is necessary to wait for some time after placing the concrete before the start of the finishing process to allow water and air to migrate to the surface. This waiting period is the variable that can cause even the most experienced flooring contractor to be caught out and the reason is that this period could be influenced by several factors: 

1. The ambient environmental conditions - Direct sunlight or wind dries out the surface too quickly and tricks the finisher into believing the concrete has finished bleeding 

2. Concrete mix design - 

• High fines content: High fines content can delay the migration of water( Bleed water). The faster the mix can bleed the less likely the concrete will delaminate. It is advised that the concrete supplier is informed if a dry-shake topping will be applied.
• Air Entrained Concrete: To Minimize Delaminations Don’t use air-entrained concrete for slabs that will be hard troweled. Delay troweling as long as possible, and keep float blades as flat as possible to keep the surface open. Delay troweling as long as possible when the subgrade is cold, fly ash is used in the concrete, or the slab is thick. Be especially careful when concrete is placed directly on a vapor retarder. All of the bleed water has to escape through the top concrete surface, so the bleeding period will take more time, and troweling must be delayed. Air contents over 3% should be avoided. ( Generally less than 1% preferable).
• Fast Setting of concretes: air temperatures and delayed concrete discharge can cause fast setting, it can also be caused by the materials used in the concrete. Prevention is easier than fixing problems with fast-setting concretes. Avoid placing concrete that may be set too fast if there are any clues that it is likely to happen.
• Slow Setting of concrete: Some truckloads of concrete set more slowly than the others, causing localized soft spots in the placement that can cause finishing problems. An accidental overdose of water-reducing admixture or retarder can cause a slower setting, as can loads that are wetter than the rest (higher slump). When this happens, finishers must usually work around the slower setting areas to make sure they don’t seal the surface before the concrete has finished bleeding. This is likely to result in decreased floor flatness (lower F-numbers), but it is preferable to deal with possible delamination or blistering of the surface.
• Sticky Concrete: Over cohesive Concrete is more difficult to finish. The surface of sticky mixtures is more likely to tear during finishing, especially when wood floats are used. Stickiness can be caused by entrained air, high cement contents, or over-sanded mixtures and may lead to crusting, blistering, or delamination. To adjust sticky concrete that crusts, eliminate entrained air unless it’s needed for durability, reduce the amount of sand by 60to 120 kg /m3, and ensure proper vibration. Insufficient vibration of sticky mixtures doesn’t adequately release entrapped air, and over-vibration brings excessive fines to the surface. Either result invites crusting or blistering.

3. Admixtures - Admixtures can slow down the bleed rate and make concrete sticky which can trick finishers into believing the bleed has finished ( Already stated in an upper paragraph)

4. Vibration of the concrete - This should be sufficient and uniform 

5. Consistency of the concrete - slump between loads should be consistent to prevent differential set rates between loads 

6. Concrete supply rate - Should be consistent 

7. Training and experience - Not using an experienced finisher and lifting the blade angle too early will densify the surface too quickly 

8. Equipment use - Leaving power trowels sat on the surface for prolonged periods between finishing stages will affect the surface locally and should be avoided where possible.

9. Application of dry-shake toppings( Floor Hardener) - With the dry-shake application, a certain amount of bleed water is necessary to ensure complete hydration. However, the timing of the application of the topping is critical; too much bleed water can cause delamination especially if the finishing operation is delayed. On the other hand, delamination can occur if the concrete has reached its initial set before the dry shake has fully hydrated.

How to detect Delaminations?

At the start of a project, delaminations aren’t easy to identify. The skin looks consistent with the surrounding slab. Delamination can be detected by listening to Hollow sound –

• Dragging a chain across the surface  or
• tapping with a steel rod or
• tapping with a hammer 

Other non-destructive testing methods used include:

• Optical time domain reflectometer testing
• Ultrasound
• Radiographic imaging
• Infrared imaging

My favourite method for testing is to use approx 2m -long section of steel or metal bar ( easily available at construction site & hardware store ) so you can stand on it and tap the surface to determine an affected area. This allows you to identify smaller delaminated areas where you can identify the distinctive cracks

 Prevention of Delamination:

Greater care in lift placement and consolidation, allowing more time for bleed water to dissipate before closing the surface, proper curing and sealing measures to prevent moisture ingress, and checks to ensure minimum concrete cover over steel are provided. Given below are a few tips to avoid delamination-

•        The simplest way to prevent delamination is to start the final finishing of the slab after the bleeding process has run its course.
•        Avoid closing the slab surface too early.
•        Take precautions when finishing and check if the concrete is air-entrained.
•       Avoid finishing slabs placed on impervious surfaces too early.
•        Avoid placing concrete on cold subgrades when ground temperatures are below 40°F.
•        Warm the concrete or use small doses of a set accelerator to promote a more uniform setting of the mix.
•        Take extra care when ambient evaporative conditions are high and exceed concrete bleeding rates.
•        Take measures to counteract rapid evaporative conditions on the Jobsite.

•  Be aware that sticky mixes with higher cementitious or sand contents tend to bleed more slowly.

Repair of Delamination:

Delaminated surfaces can be repaired by removing the affected surface in areas bounded by shallow saw cuts and then filling with a cementitious- or resin-based proprietary mortar system, for example 

• Small patches. Cut a rectangle around the area and prepare (scabble or similar) the parent concrete to about 2–10mm. Infill with a suitable repair material. 
•  Larger areas. Cut a rectangle around the area and prepare to a depth of 20–30mm. Infill with cementitious mortar incorporating the dry-shake topping where applicable to match the existing finish.