Figure 6-3.-Field-constructed rig for washing aggregate.
aggregate. For work in which aggregate strength and
durability are of vital importance, such as paving
concrete, aggregate must be laboratory tested.
outlet. If the slope is less than 50°, segregation will
occur as material is discharged out of the bin.
Handling and Storage
A mass of aggregate containing particles of
different sizes has a natural tendency toward
segregation. Segregation refers to particles of the
same size tending to gather together when the
material is being loaded, transported, or otherwise
disturbed. Aggregate should always be handled and
stored by a method that minimizes segregation.
Stockpiles should not be built up in cone shapes,
formed by dropping successive loads at the same spot.
This procedure causes segregation. A pile should be
built up in layers of uniform thickness, each made by
dumping successive loads alongside each other.
If aggregate is dropped from a clamshell, bucket,
or conveyor, some of the fine material may be blown
aside, causing a segregation of fines on the lee side
(that is, the side away from the wind) of the pile.
Conveyors, clamshells, and buckets should be
discharged in contact with the pile.
When a bin is being charged (filled), the material
should be dropped from a point directly over the
outlet. Material chuted in at an angle or material
discharged against the side of a bin will segregate.
Since a long drop will cause both segregation and the
breakage of aggregate particles, the length of a drop
into a bin should be minimized by keeping the bin as
full as possible at all times. The bottom of a storage
bin should always slope at least 50° toward the central
The two principal functions of water in a concrete
mix are to effect hydration and improve workability.
For example, a mix to be poured in forms must
contain more water than is required for complete
hydration of the cement. Too much water, however,
causes a loss of strength by upsetting the wqter-
It also causes water-gain on the
surface-a condition that leaves a surface layer of
weak material, called laitance.
mentioned, an excess of water also impairs the
watertightness of the concrete.
Water used in mixing concrete must be clean and
free from acids, alkalis, oils, and organic materials.
Most specifications recommend that the water used in
mixing concrete be suitable for drinking, should such
water be available.
Seawater can be used for mixing unreinforced
concrete if there is a limited supply of fresh water.
Tests show that the compressive strength of concrete
made with seawater is 10 to 30 percent less than that
obtained using fresh water. Seawater is not suitable
for use in making steel-reinforced concrete because of
the risk of corrosion of the reinforcement, particularly
in warm and humid environments.
Admixtures include all materials added to a mix
other than portland cement, water, and aggregates.