Page 261 - Civil Engineering Project Management, Fourth Edition

241
Site concreting and reinforcement
beams. However, a margin of safety must be allowed for possible differences
in compaction between the test beam and the in situ concrete.
To keep a watch on concrete quality during construction a record of the
type shown in Fig. 19.2 can be used on which are plotted 7- and 28-day con-
crete cube strength test results. The figure illustrates a decline in strength at
7 January, remedied later by reducing the water/cement ratio.

Fig. 19.1. Flexural test as BS 1881 on concrete beam for 20mm maximum aggregate size.
For simple site testing (see text) it may suffice to use a single central roller to apply the load

5

x A
x xx x x x x x x x x x x x x x x x x x x x x x x x x x x B 28 day
Compressive strength10 3 lb/in 2 4 3 x x x x E C 7 day

D

2

0.7
1
W/C
ratio
0 0.6
16 24 30 3 10 16 21 29 3 10 12 15 18 27 30 2 7 10 14 18 23
Nov Dec Jan Feb

Fig. 19.2. Graphical check on concrete test results. A, upper action line 4200lb/in 2
2
2
2
(29.0N/mm ); B, upper warning line 4000lb/in (27.6N/mm ); C, lower warning line
2
2
2
2
3200lb/in (22.0N/mm ); D, lower action line 3000lb/in (20.7N/mm ); E, 7 day warning
2
2
line 2000lb/in (13.8N/mm ). Each point mean value of five test results.