Chapter 6 Concrete 115
Obtaining Concrete
Large quantities of concrete are ordered from and delivered by a local
transit-mix company. When only a small quantity of concrete is needed for a
job, it can be mixed by hand or in a small, portable mixer. Premixed concrete
can be purchased in bags that weigh approximately 80 pounds. These bags
yield about two-thirds of a cubic foot of concrete.
Concrete can also be made by mixing one part Portland cement, two
parts sand, and three parts gravel. Mix the dry ingredients thoroughly. Then
mix enough water with the dry ingredients to make the batch workable. Add
water slowly to prevent a soggy mixture.
Placing
Placing concrete involves pouring concrete as near as possible to its final
location, Figure 6-12. Excess handling of fresh concrete can cause segrega-
tion of the aggregate, causing coarse and fine particles to separate. Concrete
can be moved to the appropriate location using conveyors, pumps, buckets,
chutes, or wheelbarrows.
Once the concrete is in the form, it is consolidated. Consolidation
compacts fresh concrete so it fits snugly inside the concrete form. This is
done most often using a gasoline or electric-powered vibrator. A puddling
stick, Figure 6-13, is a long, thin board that is worked up and down inside
wall forms to compact the concrete around reinforcing steel and into corners.
A jitterbug is a tamping tool that has holes to allow cement paste and
fine aggregate to rise to the top while pushing the large aggregate down,
Figure 6-14.
consolidation:
A method for
compacting fresh
concrete to fill all
voids and air pockets
inside the concrete
form.
puddling stick:
Long, thin boards
that are worked up
and down inside wall
forms to compact
the concrete around
reinforcing steel and
into corners.
Figure 6-12. This concrete is being moved using a chute. (Photo courtesy of Constructionphotographs.com)