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Blemishes in power trowelled slabs

Power trowelled concrete floors occasionally exhibit surface blemishes believed to be caused by the proximity to the surface of coarse aggregate particles.

Aggregate transparency
Aggregate transparency is seen as mottled patches of colour on the surface. These correspond to the position of coarse aggre­gate particles just below the surface. This may be due to a localised variation in the density and water/cement ratio in the thin mor­tar layer above the coarse aggregate particles. This in turn may be induced by a ´pumping´ action when the float and/or trowel blades locally compress the mortar layer.

Black spots
This blemish - sometimes described as an ´orange peel´ or ´boiler plate´ effect, depending on the severity and frequency of the spots - is caused when trowel blades burnish the mortar directly above a coarse aggregate particle at the surface. The black spots can also be discerned as slight bumps on the surface. This slight level variation may be exacerbated by plastic settlement of the mortar frac­tion around the coarse aggregate particle.

Loss or mortar over coarse aggregate particles
In some instances when ‘black spots’ occur the thin layer of mortar over the coarse aggregate particle may be worn away or mechanically removed by traffic leaving a shallow depression exposing the aggregate particle. This effect is more likely where flatter pieces of aggregate are orientated parallel to the surface. Shrinkage of the mortar layer may show as a star pattern above the coarse aggregate particle, which can be worn away. Coalescence of a number of these blemishes where loss of surface occurs may appear as a delamination.

Rocking or loose aggregate particles
Rocking or loose aggregate particles are generally not noticeable when a floor is finished and only becomes apparent later after time and trafficking. The problem can often be associated with black spots and is first seen as a small roughly circular crack above the coarse aggregate particle that develops into an annular hole on the surface.

Typically only limited areas of the floor surface will be affected by such blemishes, their occurrence being randomly distributed in patches although very occasionally large areas of floor surface may also be affected.

In order to reduce the risk of such blemishes, the factors that lead to a high concentration of aggregate particles in the surface layer should be considered by the designers, specifiers and contractors before work begins on a power-trowelled floor.

Acknowledgement: The Concrete Society

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