Civil Engineering Reference
In-Depth Information
100
minimum. Aggregates are generally graded to produce a
void content of 35% to 40%. Fine aggregate used in the
grout is generally graded to a fineness modulus of
between 1.2 and 2.0, with nearly all of the material passing
a 1.25 mm (No. 16) sieve.
Although the preplaced aggregate method has been
used principally for restoration work and in the construc-
tion of reactor shields, bridge piers, and underwater struc-
tures, it has also been used in buildings to produce
unusual architectural effects. Since the forms are com-
pletely filled with coarse aggregate prior to grouting, a
dense and uniform exposed-aggregate facing is obtained
when the surface is sandblasted, tooled, or retarded and
wire-brushed at an early age.
Tests for preplaced aggregate concrete are given in
ASTM C 937 through C 943. Preplaced aggregate concrete
is discussed in more detail in
ACI 304-00
,
Guide for
Measuring, Transporting, and Placing Concrete.
50
Inside temperature
80
40
Surface
temperature
Protected
surface
cools slowly-
no cracking
t
>
20
C
60
30
(36
F)
40
20
Form
removal
Surface
cracking
10
20
Unprotected
surface cools fast
t
<
20
C
(36
F)
0
0
0
1
2
3
4
5
6
7
8
Days
Fig. 18-10. Potential for surface cracking after form re-
moval, assuming a critical temperature differential,
t
, of
20°C (36°F). No cracking should occur if concrete is cooled
slowly and
t
is less than 20°C (36°F) (
Fintel and Ghosh
1978
and
PCA 1987
).
The maximum temperature rise can be estimated by
approximation, if the concrete contains 300 to 600 kg of
cement per cubic meter (500 to 1000 lb of Type I/II cement
per cubic yard) and the least dimension of the member is
1.8 m (6 ft). This approximation (under normal, not adia-
batic conditions) would be 12°C for every 100 kg of
cement per cubic meter (12.8°F for every 100 lb of cement
per cubic yard). For example, the maximum temperature
of such an element made with concrete having 535 kg of
Type I/II cement per cubic meter (900 lb of cement per
cubic yard) and cast at 16°C (60°F) would be about
16°C + (12°C x 535/100) or 80°C
(60°F + [12.8°F x 900/100] or 175°F)
Temperatures and temperature differences in mass con-
crete can also be calculated by a method in
ACI 207 (1996)
.
The slow rate of heat exchange between concrete and
its surroundings is due to the concrete's heat capacity. Heat
escapes from concrete at a rate that is inversely propor-
tional to the square of its least dimension. A 150-mm (6-in.)
thick wall cooling from both sides will take approximately
1
1
⁄
2
hours to dissipate 95% of its developed heat. A 1.5-m
(5-ft) thick wall would take an entire week to dissipate the
same amount of heat (
ACI 207
). Inexpensive thermocou-
ples can be used to monitor concrete temperature.
NO-SLUMP CONCRETE
No-slump concrete is defined as concrete with a consis-
tency corresponding to a slump of 6 mm (
1
⁄
4
in.) or less.
Such concrete, while very dry, must be sufficiently work-
able to be placed and consolidated with the equipment to
be used on the job. The methods referred to here do not
necessarily apply to mixtures for concrete masonry units
or for compaction by spinning techniques.
Many of the basic laws governing the properties of
higher-slump concretes are applicable to no-slump con-
crete. For example, the properties of hardened concrete
depend principally on the ratio of water to cement, pro-
vided the mix is properly consolidated.
Measurement of the consistency of no-slump concrete
differs from that for higher-slump concrete because the
slump cone is impractical for use with the drier consisten-
cies.
ACI 211.3
,
Standard Practice for Selecting Proportions for
No-Slump Concrete,
describes three methods for measuring
the consistency of no-slump concrete: (1) the Vebe appa-
ratus; (2) the compacting-factor test; and (3) the Thaulow
drop table. In the absence of the above test equipment,
workability can be adequately judged by a trial mixture
that is placed and compacted with the equipment and
methods to be used on the job.
Intentionally entrained air is recommended for no-
slump concrete where durability is required. The amount
of air-entraining admixture usually recommended for
higher-slump concretes will not produce air contents in
no-slump concretes that are as high as those in the higher-
slump concretes. The lower volume of entrained air, how-
ever, generally provides adequate durability for no-slump
concretes; while the volume of entrained air is not there,
sufficient small air voids are present. This departure from
the usual methods of designing and controlling entrained
air is necessary for no-slump concretes.
For a discussion of water requirements and computa-
tion of trial mixtures, see
ACI 211.3
.
PREPLACED AGGREGATE CONCRETE
Preplaced aggregate concrete is produced by first placing
coarse aggregate in a form and later injecting a cement-
sand grout, usually with admixtures, to fill the voids.
Properties of the resulting concrete are similar to those of
comparable concrete placed by conventional methods;
however, considerably less thermal and drying shrinkage
can be expected because of the point-to-point contact of
aggregate particles.
Coarse aggregates should meet requirements of
ASTM C 33 (AASHTO M 80). In addition, most specifica-
tions limit both the maximum and minimum sizes; for
example, 75-mm (3-in.) maximum and 12.5-mm (
1
⁄
2
-in.)
