Civil Engineering Reference
In-Depth Information
Grading
Gradation of aggregates significantly affects concrete mix-
ture proportioning and workability. Hence, gradation
tests are an important element in the assurance of concrete
quality. The grading or particle size distribution of an
aggregate is determined by a sieve analysis test in which
the particles are divided into their various sizes by stan-
dard sieves. The analysis should be made in accordance
with ASTM C 136 (AASHTO T 27).
Results of sieve analyses are used in three ways: (1) to
determine whether or not the materials meet specifica-
tions; (2) to select the most suitable material if several
aggregates are available; and (3) to detect variations in
grading that are sufficient to warrant blending selected
sizes or an adjustment of concrete mix proportions.
The grading requirements for concrete aggregate are
shown in Chapter 5 and ASTM C 33 (AASHTO M 6/M
80). Materials containing too much or too little of any one
size should be avoided. Some specifications require that
mixture proportions be adjusted if the average fineness
modulus of fine aggregate changes by more than 0.20.
Other specifications require an adjustment in mixture
proportions if the amount retained on any two consecu-
tive sieves changes by more than 10% by mass of the total
fine-aggregate sample. A small quantity of clean particles
that pass a 150 µm (No. 100) sieve but are retained on a 75
µm (No. 200) sieve is desirable for workability. For this
reason, most specifications permit up to 10% of this mate-
rial in fine aggregate.
Well-graded aggregates contain particles on each sieve
size. Well-graded aggregates enhance numerous factors
that result in greater workability and durability. The more
well-graded an aggregate is, the more it will pack together
efficiently, thus reducing the volume between aggregate
particles that must be filled by paste. On the other hand,
gap-graded aggregates—those having either a large quan-
tity or a deficiency of one or more sieve sizes—can result in
reduced workability during mixing, pumping, placing,
consolidation and finishing. Durability can suffer too as a
result of using more fine aggregate and water to produce a
workable mix. See
Chapter 5
and
Galloway (1994)
for addi-
tional information on aggregate grading.
Fig. 16-1. Sample splitter commonly used to reduce coarse
aggregate samples. (70012)
standard or standard color solution. If the color of the
solution containing the sample is darker than the standard
color solution or Organic Glass Plate No. 3, the fine aggre-
gate should not be used for important concrete work
without further investigation.
Some fine aggregates contain small quantities of coal
or lignite that give the liquid a dark color. The quantity
may be insufficient to reduce the strength of the concrete
appreciably. If surface appearance of the concrete is not
important, ASTM C 33 (AASHTO M 6) states that fine
aggregate is acceptable if the amount of coal and lignite
does not exceed 1.0% of the total fine aggregate mass. A
fine aggregate failing this ASTM C 33 (AASHTO M 6) limit
may be used if, when tested in accordance with ASTM C 87
(AASHTO T 71), the 7-day strengths of mortar cubes made
with the sand (ASTM C 109 or AASHTO T 106) are at least
95% of the 7-day strengths of mortar made with the same
sand, but washed in a 3% solution of sodium hydroxide
and then thoroughly rinsed in water. It should be realized
that appreciable quantities of coal or lignite in aggregates
can cause popouts and staining of the concrete and can
reduce durability when concrete is exposed to weathering.
Local experience is often the best indication of the durabil-
ity of concrete made with such aggregates.
Moisture Content of Aggregates
Objectionable Fine Material
Several methods are used for determining the amount of
moisture in aggregate samples. The total moisture con-
tent for fine or coarse aggregate can be tested in accor-
dance with ASTM C 566 (AASHTO T 255). In this
method a measured sample of damp aggregate is dried
either in a ventilated conventional oven, microwave
oven, or over an electric or gas hotplate. From the mass
before and after drying, the total moisture content can be
calculated as follows:
Large amounts of clay and silt in aggregates can adversely
affect durability, increase water requirements, and in-
crease shrinkage. Specifications usually limit the amount
of material passing the 75 µm (No. 200) sieve to 2% or 3%
in fine aggregate and to 1% or less in coarse aggregate.
Testing for material finer than the 75 µm (No. 200) sieve
should be done in accordance with ASTM C 117
(AASHTO T 11). Testing for clay lumps should be in
accordance with ASTM C 142 (AASHTO T 112).
