Concrete polishing contractors must help their clients
to understand how results are likely to vary when they are polishing suspended
slabs architects and owners choose concrete
polishing as an alternative to floor coverings, but sometimes the results
can be disappointing when polishing is specified for a slab that is elevated.
The concrete contractors have a responsibility to help engineers, owners, and
architects to understand how the final appearance of the suspended slab will be
different from slabs on the ground after polishing.
The final result after polishing suspended slabs can vary
from a cream surface to fine aggregate or coarse aggregate. Cracks that are on
suspended slab surface will be visible. The porosity of the surface varies when
concrete aggregate that is lightweight is polished.
Slabs on ground
The recommended 28 days compressive strengths are from 3500
to 4500psi. Polishing at later ages may increase the costs of polishing because
aggressive surface preparation methods are needed. Placing of the concrete for
polished slabs on the ground is often set near project completion to avoid the
higher strengths. This will also minimize the protection that is needed to
prevent other trades from damaging the concrete surface after and before
polishing.
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Normal weight concrete is used for slabs on the ground.
Contraction joints are recommended to minimize cracking and curling of the slab
by spacing the joints. The most common method of controlling curling and
cracking of slabs to be polished is by use of contraction joints. The mock-ups
of slab on the ground is easy to duplicate although it must include specified
concrete mix, levelness, contraction and flatness joints and also consider the
timing---- concrete’s age before polishing.
Suspended slabs
The specified compressive strength of suspended slabs varies
depending on its structure. Concrete that is placed on metal decks and
supported by steel beams is likely to have specified strength that is close to
that of slabs on the ground. The compressive strength of the concrete formed
and shored post-tensioned or concrete that is conventionally reinforced and
suspended slabs are typically driven by the schedule of three days for form
removal or post tension. At this age, the compressive strength of the concrete
is comparable to that of slab on the ground at 28 days which is 3500 to
4500psi. Due to the extended curing in concrete
polishing of suspended slabs, they have increased concrete strength.
Lightweight concrete is likely to be used on metal decks that
are supported by onshore steel beams, but normal weight concrete is used for
post conventionally reinforced shored and formed structures. The depth of
grinding and the final appearance of polished concrete are affected by the
hardness of the surface and the difference in density. The achievement on
normal weight concrete may not be achievable on the lightweight concrete.
The range of crack widths in the suspended slabs will depend
on the type of structure. Crack width and not crack location is controlled by
the reinforcement because contraction joints are not used in suspended slabs.
Post tension and reinforcing bars keep cracks tight in the suspended slabs.
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