Small scale modelling of structures is normally employed as a means to study the structural behaviour of engineering structures in order to predict the real behaviour of such structures under various conditions of loading as well as under the influence of extreme weather events like flooding, earthquakes, severe wind storms etc. But in order to use small scale models to predict prototype behaviour, understanding is necessary of the effect of scale on such models. The fracture mechanics perspective of the effects of size on models is explored from available literature and its significance to reliable model studies is established in this review.

Concrete is a ubiquitous construction material used globally to build bridges, homes, hospitals, schools and sewage systems. Concrete used in sewage systems is exposed to an aggressive environment, like elevated temperature and humid conditions, in addition to aggressive sulphate. The combined effect of these conditions results in the premature deterioration of structures. This study was investigated the effect of replacing cement by fly ash (FA) in cement mortar by different ratios and a quarry dust as sand. The reference mortar mix grade was (35 MPa) and the other mixes were with various percentages of FA (5%, 10%, 15%, 20%, 25%, 30%, and 35%) by weight of cement. This study evaluated the effect upon the compressive strength, splitting tensile strength, fracture strength, and modulus of elasticity of mortar containing FA exposed to the magnesium sulphate (MgSO4) with concentration 30000 mg/l. Mixes were created using varying ratios of FA to identify the optimal concentration; these were compared against normal concrete. At replacement ratios of 20% at all ages, optimum compressive strength, splitting strength, fracture strength, and elastic modulus were obtained. Conversely, mixes with replacement ratios greater than 20% produced less strength than the control mix (without FA). The FA mortar's strength remained higher than that of regular mortar exposed to the same conditions of magnesium sulphate after 28 days, despite a reduction in compressive strength. As result, the replacing some of the cement in the concrete mix, particularly in the ratio of 20%, with FA, the concrete can be formed that is more resistant and more capable to withstand MgSO3 attack.

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