Page 14 - Concrete-Technology-Interacted_Book_Prof-MIK_Neat

4/2 Concrete properties: setting and hardening

significant differences between these measures of compressive strength are due not to
differences in the response of the concrete but mainly to differences in the lateral restraint
provided by the machine. With cubes, the lateral restraint by the machine platens is a
significant factor in resisting failure and consequently the resulting ‘strength’ is higher
than, say, a 150 mmφ × 300 mm cylinder. The European concrete standard, EN 206-
1(2000), put this difference at about 20 per cent for normal weight concrete in their dual
classification of strength class. EN 206-1 uses the strength of a 150 mmφ × 300 mm
cylinder as the first (and reference) classification followed by the strength of 150 mm
cubes, e.g. C40/50. The strength class for lightweight concrete has a different relationship
between these two measures of strength, e.g. LC40/44.

Conditions where the machine platens provide no lateral restraint can be achieved by
a number of techniques, e.g. loading via two layers of plastic film which have between
them a thin layer of grease. These loading conditions give even lower strengths (Table 4.1)
and the specimens fail with tensile cracks parallel to the direction of loading (Figure 4.1).

Table 4.1 Compressive strength with and without lateral restraint
by the machine platens (data from Hughes and Bahramian, 1967)

Normally tested With platen restraint removed

Cubes1 Prisms2 Cubes1 Prisms2

59 41 37 37
100 68 68 66
41 34 34 31
90 66 64 61

Notes:
1 102 mm cubes.
2 244 × 102 mm prisms.

Cube Cylinder

Figure 4.1 Typical failure patterns for a cube and cylinder without lateral restraint from the machine platens.

Various techniques have been used to study the failure mechanism of concrete including:

• stress–strain curves (change in initial modulus)
• acoustic emission
• volumetric strain
• energy method
• pulse velocity.

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