Civil Engineering Reference
In-Depth Information
abrasion than does weak concrete. Since compressive
strength depends on water-cement ratio and curing, a low
water-cement ratio and adequate curing are necessary for
abrasion resistance. The type of aggregate and surface
finish or treatment used also have a strong influence on
abrasion resistance. Hard aggregate is more wear resistant
than soft aggregate and a steel-troweled surface resists
abrasion better than a surface that had not been troweled.
Fig. 1-22 shows results of abrasion tests on concretes of
different compressive strengths and aggregate types. Fig.
1-23 illustrates the effect hard steel troweling and surface
treatments, such as metallic or mineral aggregate surface
hardeners, have on abrasion resistance. Abrasion tests can
be conducted by rotating steel balls, dressing wheels, or
disks under pressure over the surface (ASTM C 779). One
type of test apparatus is pictured in Fig. 1-24. Other types
of abrasion tests are also available (ASTM C 418 and C 944).
Volume Stability and Crack Control
Hardened concrete changes volume due to changes in
temperature, moisture, and stress. These volume or length
changes may range from about 0.01% to 0.08%. Thermal
volume changes of hardened concrete are about the same
as those for steel.
Concrete under stress will deform elastically. Sus-
tained stress will result in additional deformation called
creep. The rate of creep (deformation per unit of time)
decreases with time.
Concrete kept continually moist will expand slightly.
When permitted to dry, concrete will shrink. The primary
factor influencing the amount of drying shrinkage is the
water content of the freshly mixed concrete. Drying shrink-
age increases directly with increases in this water content.
The amount of shrinkage also depends upon several other
Compressive strength, 1000 psi
3
4
5
6
7
8
9
10
10
8
6
4
Aggregate type
Limestone
Quartzite
Traprock
Chert
2
0
20
30
40
50
60
70
Compressive strength, MPa
Fig. 1-22. Effect of compressive strength and aggregate type
on the abrasion resistance of concrete (ASTM C 1138). High-
strength concrete made with a hard aggregate is highly
resistant to abrasion (
Liu 1981
).
120
Traprock
topping
Traprock surface
treatment
100
Metallic aggregate
surface treatment
80
60
40
Monolithic single-
course slab
20
Dressing-wheel abrasion test
0
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
Fig. 1-24. Test apparatus for measuring abrasion resistance
of concrete. The machine can be adjusted to use either
revolving disks or dressing wheels. With a different
machine, steel balls under pressure are rolled over the
surface of the specimen. The tests are described in ASTM C
779. (44015)
Depth of abrasion, mm
Fig. 1-23. Effect of hard steel troweling and surface treat-
ments on the abrasion resistance of concrete (ASTM C 779).
Base slab compressive strength was 40 MPa (6000 psi) at 28
days. All slabs were steel troweled (
Brinkerhoff 1970
).
