Civil Engineering Reference
In-Depth Information
CHAPTER 11
Placing and Finishing Concrete
PREPARATION BEFORE PLACING
Preparation prior to placing concrete for pavements or
slabs on grade includes compacting, trimming, and mois-
tening the subgrade (Figs. 11-1, 11-2, and 11-3); erecting
the forms; and setting the reinforcing steel and other
embedded items securely in place. Moistening the sub-
grade is important, especially in hot, dry weather to keep
the dry subgrade from drawing too much water from the
concrete; it also increases the immediate air-moisture level
thereby decreasing the amount of evaporation from the
concrete surface. The strength or bearing capacity of the
subgrade should be adequate to support anticipated
structural loads.
In cold weather, concrete must not be placed on a
frozen subgrade. Snow, ice, and other debris must be
removed from within the forms before concrete is placed.
Where concrete is to be deposited on rock or hardened con-
crete, all loose material must be removed, and cut faces
should be nearly vertical or horizontal rather than sloping.
Recently placed concrete requiring an overlay is usu-
ally roughened shortly after hardening to produce a better
Fig. 11-2. Water trucks with spray-bars are used to moisten
subgrades and base course layers to achieve adequate
compaction and to reduce the amount of water drawn out of
concrete as it's placed. (69931)
Fig. 11-1. A base course foundation for concrete pavement
is shaped by an auto-trimmer to design grades, cross
section and alignment by automatic sensors that follow
string lines. (69935)
Fig. 11-3. (top) Adequate compaction of a base course
foundation for concrete pavement can be achieved by using
a vibratory roller. (bottom) Vibratory plate compactors are
also used to prepare subgrades under slabs. (69934, 69930)
