Materials for engineering, 3rd Edition - (Malestrom)

Glasses and ceramics 153
40
Compressive strength (MPa)
20
Crack
linkage
Isolated crack
growth
Linear elastic
regime
Shear bands form
0
0 0.25 0.50 0.75
Strain (%)
4.14 Stress–strain curve for cement or concrete in compression.
the strength of concrete. A mix containing large particles of aggregate will
have a higher strength than one containing small aggregate. Furthermore, a
coarse crushed rock aggregate will have greater strength than a smooth,
rounded aggregate, since, in the latter situation, shearing tends to take place
round the aggregate rather than through it. Similarly, the greater the proportion
of aggregate in the mixture, the more probable it is that shear occurs through
it. Unfortunately, cement with a low water/cement ratio and a high proportion
of coarse aggregate will be almost unworkable, although this is the condition
for maximum strength. The composition of a practical high-strength concrete
will thus be a compromise between these conflicting requirements.
4.4.4 High-strength cements
Pozzolanic reaction
Pozzolanas are materials which have constituents that will combine with
lime in the presence of water to form stable insoluble compounds possessing
cementing properties. This is, in effect, the pozzolanic reaction:
lime + pozzolana + water = C-S-H
C + S + H = C-S-H
Pozzolanas may be natural, generally volcanic ash containing silica-rich
glass or artificial, such as pulverized fuel ash and burnt clays. Another source
of relatively pure silica is ‘condensed silica fume’, which is a by-product of
the production of silicon by the reduction of quartz in an electric furnace.