Page 15 - Concrete-Technology-Interacted_Book_Prof-MIK_Neat

Concrete properties: setting and hardening 4/3
Figure 4.2 shows a comparison of different methods under uniaxial compression.

Acoustic emission

Pulse
velocity

Energy method

Change in initial
modules

0 1000 2000 3000 4000 5000
Strain × 10–6

Figure 4.2 Methods of detecting damage (Spooner and Dougall, 1975).

Research by Spooner and Dougill (1975) revealed that the failure of concrete was
progressive. During loading microcracking occurs. When the load is released and re-
applied, the modulus of elasticity has reduced if cracking has taken place and no further
damage occurs until the peak load attained previously is reached. From this point onwards
further microcracking occurs. It was noted that the envelope drawn around curves for the
cyclic loading fell within the envelope obtained from a specimen subject to monotonically
increasing strain, i.e. a single load cycle (Figure 4.3). This research tended to disprove
previous theories that postulated that concrete behaved in a linear-elastic way up to some
discontinuity stress level (about one third the ultimate load). However, it should be noted
that in the Spooner and Dougill model, the amount of damage (microcracking) that
occurs when the loading is restricted to about one third the ultimate load is very small.
Therefore loading to about half the ultimate load provides a safe basis for design for most
structures. At higher levels of loading, many loading cycles will tend to increase the
amount of microcracking and eventually may result in failure by fatigue.

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