Durability of concrete and cement composites

Introduction Dimensional stability and cracking processes in concrete

To calculate the long-term deformation and deflection of structural concrete members for their design life, the relations between stress, strain, time and temperature are required. In common with other engineering materials, concrete deforms almost elastically when a load is first applied, but under sustained load, the deformation increases with time at a gradually reducing rate under normal environmental conditions. Timber behaves in a similar manner, whereas steel only creeps under very high stress at normal temperature or under low stress at very high temperature. With all engineering materials, dimensional instability can occur at high loads in the form of a time-dependent failure or creep rupture, but this can be avoided with concrete if stresses are kept below approximately 60% of the short-term strength. Elasticity and creep are considered in Sections 3.2.1 and 3.2.4, respectively. In addition to deformation caused by the applied stress, time-dependent volume changes due to shrinkage (or swelling) and temperature variation of concrete are of considerable importance because they can contribute signifi- cantly to elasticity and creep of concrete members in assessing the total time- dependent deformation. There are different types of shrinkage, the most com- mon being drying shrinkage, the rate of which gradually reduces with time. Like creep, drying shrinkage is associated with the movement of gel water within and from the hardened cement paste. Shrinkage and temperature movement are discussed in Sections 3.2.3 and 3.2.5, respectively. In other practical cases, movements are often partly or wholly restrained thereby inducing tensile stress, the level of which needs to be minimised to avoid cracking. Concrete is of course very weak in tension so that cracks must be avoided or controlled as they can impair durability as well as structural integrity, besides being aesthetically undesirable. The subject of non-structural or intrinsic cracking processes, as opposed to structural cracking associated with external loading, is dealt with in Section 3.3. First, the different types of cracks are described (3.3.1) then plastic shrinkage cracking (3.3.2), followed by early-age thermal cracking (3.3.3), types of restraint (3.3.4 and 3.3.5) and drying shrinkage cracking (3.3.6). Theoretical aspects of cracking are summarised under fracture mechanics in Section 3.3.7, which precedes Future trends and suggested further information (3.4), and finally References (3.5).

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