Civil Engineering Reference
In-Depth Information
Because of the slow pozzolanic reaction of some
supplementary cementing materials, continuous wet
curing and favorable curing temperatures may need to be
provided for longer periods than normally required.
However, concrete containing silica fume is less affected by
this and generally equals or exceeds the one-day strength
of a cement-only control mixture. Silica fume contributes to
strength development primarily between 3 and 28 days,
during which time a silica fume concrete exceeds the
strength of a cement-only control concrete. Silica fume also
aids the early strength gain of fly ash-cement concretes.
The strength development of concrete with fly ash,
ground slag, calcined clay, or calcined shale, is similar to
normal concrete when cured around 23°C (73°F). Fig. 3-15
illustrates that the rate of strength gain of concrete with fly
ash, relative to its 28-day strength, is similar to concrete
without fly ash. Concretes made with certain highly reac-
tive fly ashes (especially high-calcium Class C ashes) or
ground slags can equal or exceed the control strength in 1
to 28 days. Some fly ashes and natural pozzolans require
28 to 90 days to exceed a 28-day control strength, depend-
ing on the mixture proportions. Concretes containing Class
C ashes generally develop higher early-age strength than
concretes with Class F ashes.
Strength gain can be increased by: (1) increasing the
amount of cementitious material in the concrete; (2) adding
high-early strength cementitious materials; (3) decreasing
the water-cementing materials ratio; (4) increasing the cur-
ing temperature; or (5) using an accelerating admixture.
Fig. 3-16 illustrates the benefit of using fly ash as an addi-
tion instead of a cement replacement to improve strength
development in cold weather. Mass concrete design often
takes advantage of the delayed strength gain of pozzolans
as these structures are often not put into full service imme-
diately. Slow early strength gain resulting from the use of
some supplementary cementitious materials is an advan-
tage in hot weather construction as it allows more time to
place and finish the concrete. With appropriate mixture
adjustments, all supplementary cementitious materials can
be used in all seasons.
60
8500
Cold weather cure:
First 24 hrs at 23
°
C (73
°
F)
Remainder at 4
°
C (39
°
F)
55
7800
50
7100
45
6400
40
5700
35
5000
Control
Fly Ash A 20% S
Fly Ash A 20% P
Fly Ash A 20% A
30
4300
25
3600
20
2900
0 0 0 0 0 0 0 0 0 0
Age, days
Fig. 3-16. Compressive strengths for concretes cured at 23°C
(73°F) for the first 24 hours and 4°C (40°F) for the remaining
time. Control had a cement content of 332 kg/m
3
(560 lb/yd
3
)
and w/c of 0.45. The fly ash curves show substitution for
cement (S), partial (equal) substitution for cement and sand
(P), and addition of fly ash by mass of cement (A). The use of
partial cement substitution or addition of fly ash increases
strength development comparable to the cement-only
control, even in cold weather (
Detwiler 2000
).
Supplementary cementing materials are often essen-
tial to the production of high-strength concrete. Fly ash,
especially, has been used in production of concrete with
strengths up to 100 MPa (15,000 psi). With silica fume,
ready mix producers now have the ability to make concrete
with strengths up to 140 MPa (20,000 psi), when used with
high-range water reducers and appropriate aggregates
(
Burg and Ost 1994
).
Impact and Abrasion Resistance
The impact resistance and abrasion resistance of concrete
are related to compressive strength and aggregate type.
Supplementary cementing materials generally do not
affect these properties beyond their influence on strength.
Concretes containing fly ash are just as abrasion resistant
as portland cement concretes without fly ash (
Gebler and
Klieger 1986
). Fig. 3-17 illustrates that abrasion resistance
of fly ash concrete is related to strength.
180
160
140
28 days
120
100
80
60
Freeze-Thaw Resistance
It is imperative for development of resistance to deteriora-
tion from cycles of freezing and thawing that a concrete
have adequate strength and entrained air. For concrete
containing supplementary cementing materials to provide
the same resistance to freezing and thawing cycles as a
concrete made using only portland cement as a binder,
four conditions for both concretes must be met:
With fly ash
Without fly ash
40
20
0
1
10
100
1000
10000
Age, days
Fig. 3-15. Compressive strength gain as a percentage of
28-day strength of concretes with and without fly ash
(
Lange 1994
).
