To avoid too much pressure on forms for large projects, the filling rate should not exceed 5 vertical feet per hour
except for columns. Coordinate the placing and compacting so that the concrete is not deposited faster than it can
be compacted properly. To avoid cracking during settlement, allow an interval of at least 4 hours, but preferably
24 hours, between placing columns and walls and placing the slabs, beams, and girders they support.
When constructing walls, beams, or girders, place the first batches of each layer at the ends of the section, then
proceed toward the center to prevent water from collecting at the form ends and corners. For walls, stop off the
inside form at the construction level. Overfill the form for about 2 inches and remove the excess just before the
concrete sets to ensure a rough, clean surface. Before placing the next lift of concrete, deposit a 1/2- to 1-inch-
thick layer of sand-cement mortar. Make the mortar with the same water content as the concrete and with a slump
of about 6 inches to prevent stone pockets and help produce a water girth joint. See view 1 of Figure 4-6. When
placing walls, be sure to remove the spreaders as you fill the forms.
When constructing slabs, place the concrete at the far end of the slab first, and then place subsequent batches
against previously-placed concrete, as shown in view 3 of Figure 4-6. Do not place the concrete in big piles and
then move it horizontally to its final position because these practices result in segregation.
Placing Concrete on Slopes
View 4 of Figure 4-6 shows how to place concrete on slopes. Always deposit the concrete at the bottom of the
slope first, then proceed up the slope placing each new batch against the previous one. When consolidated, the
weight of the new concrete increases the compacting of the previously placed concrete.
Except for concrete placed underwater, you must compact or consolidate all concrete after placement.
Consolidation eliminates rock pockets and air bubbles and brings enough fine material both to the surface and
against the forms to produce the desired finish. You can use such hand tools as spades, puddling sticks, or
tampers, but mechanical vibrators are best. Any compacting device must reach the bottom of the form and be
small enough to pass between reinforcing bars. The process involves carefully working around all reinforcing
steel with the compacting device to ensure proper embedding of reinforcing steel in the concrete. Be careful not
to displace the reinforcing steel because the strength of the concrete member depends on proper reinforcement
The best compacting tool is a mechanical vibrator, as shown in Figure 4-7 on page 4-16. Vibrators consolidate
concrete by pushing the coarse aggregate downward, away from the point of vibration. Vibrators allow placement of
mixtures that are to, stiff to place any other way, such as those having a 1- or 2-inch slump. Stiff mixtures are more
economical because they require less cement and present fewer segregation or excessive bleeding problems. However,