Civil Engineering Reference
In-Depth Information
When aggregates are not delivered by truck, accept-
able and inexpensive results can be obtained by forming
the stockpile in layers with a clamshell bucket (cast-and-
spread method); in the case of aggregates not subject to
degradation, spreading the aggregates with a rubber-tire
dozer and reclaiming with a front-end loader can be used.
By spreading the material in thin layers, segregation is
minimized. Whether aggregates are handled by truck,
bucket loader, clamshell, or conveyor belt, stockpiles
should not be built up in high, cone-shaped piles since this
results in segregation. However, if circumstances necessi-
tate construction of a conical pile, or if a stockpile has
segregated, gradation variations can be minimized when
the pile is reclaimed; in such cases aggregates should be
loaded by continually moving around the circumference
of the pile to blend sizes rather than by starting on one
side and working straight through the pile.
Crushed aggregates segregate less than rounded
(gravel) aggregates and larger-size aggregates segregate
more than smaller sizes. To avoid segregation of coarse
aggregates, size fractions can be stockpiled and batched
separately. Proper stockpiling procedures, however,
should eliminate the need for this. Specifications provide
a range in the amount of material permitted in any size
fraction partly because of segregation in stockpiling and
batching operations.
Washed aggregates should be stockpiled in sufficient
time before use so that they can drain to a uniform mois-
ture content. Damp fine material has less tendency to
segregate than dry material. When dry fine aggregate is
dropped from buckets or conveyors, wind can blow away
the fines; this should be avoided if possible.
Bulkheads or dividers should be used to avoid
contamination of aggregate stockpiles. Partitions between
stockpiles should be high enough to prevent intermingling
of materials. Storage bins should be circular or nearly
square. Their bottoms should slope not less than 50
degrees from the horizontal on all sides to a center outlet.
When loading the bin, the material should fall vertically
over the outlet into the bin. Chuting the material into a bin
at an angle and against the bin sides will cause segregation.
Baffle plates or dividers will help minimize segregation.
Bins should be kept as full as possible since this reduces
breakage of aggregate particles and the tendency to segre-
gate. Recommended methods of handling and storing
aggregates are discussed at length in
Matthews (1965 to
1967)
,
NCHRP (1967)
, and
Bureau of Reclamation (1981)
.
Seashells may be present in the aggregate source.
These shells are a hard material that can produce good
quality concrete, however, a higher cement content may
be required. Also, due to the angularity of the shells, addi-
tional cement paste is required to obtain the desired work-
ability. Aggregate containing complete shells (uncrushed)
should be avoided as their presence may result in voids in
the concrete and lower the compressive strength.
Marine-dredged aggregates often contain salt from the
seawater. The primary salts are sodium chloride and
magnesium sulfate and the amount of salt on the aggregate
is often not more than about 1% of the mass of the mixing
water. The highest salt content occurs in sands located just
above the high-tide level. Use of these aggregates with
drinkable mix water often contributes less salt to the
mixture than the use of seawater (as mix water) with salt-
free aggregates.
Marine aggregates can be an appreciable source of
chlorides. The presence of these chlorides may affect the
concrete by (1) altering the time of set, (2) increasing drying
shrinkage, (3) significantly increasing the risk of corrosion
of steel reinforcement, and (4) causing efflorescence.
Generally, marine aggregates containing large amounts of
chloride should not be used in reinforced concrete.
Marine-dredged aggregates can be washed with fresh
water to reduce the salt content. There is no maximum
limit on the salt content of coarse or fine aggregate;
however, the chloride limits presented in Chapter 9
should be followed.
RECYCLED-CONCRETE AGGREGATE
In recent years, the concept of using old concrete pave-
ments, buildings, and other structures as a source of aggre-
gate has been demonstrated on several projects, resulting
in both material and energy savings (
ECCO 1999
). The
procedure involves (1) breaking up and removing the old
concrete, (2) crushing in primary and secondary crushers
(Fig. 5-25), (3) removing reinforcing steel and other embed-
MARINE-DREDGED AGGREGATE
Marine-dredged aggregate from tidal estuaries and sand
and gravel from the seashore can be used with caution in
some concrete applications when other aggregate sources
are not available. Aggregates obtained from seabeds have
two problems: (1) seashells and (2) salt.
Fig. 5-25. Heavily reinforced concrete is crushed with a
beamcrusher. (69779)
