13-11-2012, 01:53 PM
Aggregates for Concrete
ACI Education Bulletin E1-07.PDF (Size: 665.24 KB / Downloads: 170)
INTRODUCTION
Hydraulic cement concrete is a cement and water paste in
which aggregate particles are embedded. Aggregate is granular
material such as sand, gravel, crushed stone, blast-furnace
slag, and lightweight aggregates that usually occupies approximately
60 to 75% of the volume of concrete. Aggregate
properties significantly affect the workability of plastic
concrete and also the durability, strength, thermal properties,
and density of hardened concrete.
This Bulletin describes types of aggregates normally used
in concrete, aggregate properties affecting performance of
the concrete, tests used to measure aggregate properties, and
methods used to obtain test samples. Normalweight as well
as lightweight aggregates are discussed.
The measurement system used in this Bulletin is the International
System of Units, or SI Units. Accordingly, readers
should make particular note that the term “weight” has been
replaced with “mass,” and “unit weight” has been replaced
with “density” when used in reference to the absolute volume
aggregates occupy in concrete, and with “bulk density”
when used in reference to aggregates, such as the mass per
unit volume of a collection of graded aggregate particles as
compacted in a volumetric bucket or the relation of mass to
volume of aggregates in a stockpile or bin. As a convenience,
most of the examples provided in the Bulletin are in both SI
and U.S. customary (in.-lb) units.
AGGREGATE PROPERTIES AND TEST METHODS Grading
Definition and test method—Grading refers to the
distribution of particle sizes present in an aggregate. The
grading is determined in accordance with ASTM C 136, “Sieve
or Screen Analysis of Fine and Coarse Aggregates.” A sample
of the aggregate is shaken through a series of wire-cloth
sieves with square openings, nested one above the other in
order of size, with the sieve having the largest openings on top,
the one having the smallest openings at the bottom, and a pan
underneath to catch material passing the finest sieve (Fig. 1).
Sieve sizes commonly used for concrete aggregates are detailed
in Table 1, and various physical properties of normalweight
aggregates, with typical range values, are shown in Table 2.
Absorption and surface moisture
Mixing water and water-cementitious material ratio—
The various moisture states in which an aggregate may exist
have been described previously. Two of these—oven-dry
and saturated surface-dry—are used as the basis for calculations
of specific gravity. Aggregates stockpiled on the job are seldom
in either of these states. They usually carry some free or surface
moisture that becomes part of the mixing water. Freshly
washed coarse aggregates contain free water, but because
they dry quickly, they are sometimes in an air-dry state when
used, and they absorb some of the mixing water.
At this point, it is necessary to define the terms “mixing
water” and “w/cm.” The mixing water in a batch of concrete
is all the water present in the concrete, with the exception of
absorbed water within aggregate particles. Mixing water is the
sum of the masses of free or surface moisture on the fine and
coarse aggregate and the mass of water added separately,
such as through a water meter or weigh batcher at the plant
or through a truck mixer water system or added to the mixer in
some other way. Mixing water is the water in freshly mixed
sand-cement grout, mortar, or concrete exclusive of any
previously absorbed by the aggregate.