Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.37
For concrete slabs, careless placing methods result in horizontal segregation,
with desired properties in the wrong location, the top consisting of excess water
and fines with low abrasion and weather resistance, and high shrinkage. For a good
surface in a one-course slab, low-slump concrete and a minimum of vibration and
finishing are desirable. Immediate screeding with a power-vibrated screed is helpful
in distributing low-slump, high-quality concrete. No further finishing should be
undertaken until free water, if any, disappears. A powered, rotary tamping float can
smooth very-low-slump concrete at this stage. Final troweling should be delayed,
if necessary, until the surface can support the weight of the finisher.
When concrete is placed for deep beams that are monolithic with a slab, the
beam should be filled first. Then, a short delay for settlement should ensue before
slab concrete is cast. Vibration through the top slab should penetrate the beam
concrete sufficiently to ensure thorough consolidation.
When a slab is cast, successive batches of concrete should be placed on the
edge of previous batches, to maintain progressive filling without segregation. For
slabs with sloping surfaces, concrete placing should usually begin at the lower edge.
For thin shells in steeply sloping areas, placing should proceed downslope.
Slump should be adjusted and finishing coordinated to prevent restraint by horizontal
reinforcing bars from causing plastic cracking in the fresh concrete.
9.34 BONDING TO HARDENED CONCRETE
The surface of hardened concrete should be rough and clean where it is to be
bonded with fresh concrete.
Vertical surfaces of planned joints may be prepared easily by wire brushing
them, before complete curing, to expose the coarse aggregate. (The timing can be
extended, if desired, by using a surface retarder on the bulkhead form.) For surfaces
fully cured without earlier preparation, sandblasting, bush hammering, or acid
washes (thoroughly rinsed off) are effective means of preparation for bonding new
concrete. (See also Art. 9.33.)
Horizontal surfaces of previously cast concrete, for example, of walls, are similarly
prepared. Care should be taken to remove all laitance and to expose sound
concrete and coarse aggregate. (See also Art. 9.32. For two-course floors, see Art.
9.35.)
9.35 HEAVY-DUTY FLOOR FINISHES
Floor surfaces highly resistant to abrasion and impact are required for many industrial
and commercial uses. Such surfaces are usually built as two-course construction,
with a base or structural slab topped by a wearing surface. The two
courses may be cast integrally or with the heavy-duty surface applied as a separate
topping.
In the first process, which is less costly, ordinary structural concrete is placed
and screeded to nearly the full depth of the floor. The wearing surface concrete,
made with special abrasion-resistant aggregate, emery, iron fillings, etc., then is
mixed, spread to the desired depth, and troweled before final set of the concrete
below.