g PUSq 2 2
2 2 e
X Concrete gains strength with age. Initially strength developed is
more. However, the ratio of gain in strength diminishes with age. It is customary to assume the
28 days strength as full strength of concrete.
Age Age factor for low
(Months) strength concrete
7 days 0.65–0.7
1 1.00
3 1.10
6 1.15
12 1.20
2 2 i 2 e
X 28 day strength is known as characteristic strength of concrete.
Sometimes it is required to determine the concrete strength at 1 day or 7 day to accelerate the
progress of work. Following relation may be used
f 28 =k 2 (f 7 )k^1
Where f 7 , f 28 are strengths at 7 and 28 days, respectively
k 1 = 0.3–0.8 (for different cements)
k 2 = 3–4 (for different curing conditions)
g
6e
2
X Provided other factors are kept constant, cement-aggregate will
greatly influence concrete strength. With an increase in cement to aggregate ratio the ultimate
strength will increase to some extent as shown in Fig. 10.25.
p 2IH PS2s"22e Gg222'2 '22gi
2 2
Eg
2
X The effect of w/c ratio on the strength of concrete has been
discussed in Sec. 10.4 and is shown in Fig. 10.13. The aim is generally to use the lowest water-
cement ratio which will give a concrete sufficiently plastic to place in position. Approximate
relation of strength concrete with water-cement ratio is given by
S = k(^)
c^2
w ca