Physico-mechanical behaviour of sandcrete block masonry units

The popularity of sandcrete blocks and their extensive application as walling material in Nigeria and other developing countries cannot be overemphasised. Sandcrete blocks, when properly produced, meet BS 2028 (1968) recommendations for density and compressive strength of structural masonry.

Sandcrete blockwalls are usually not designed to support loads other than their own weight. However, one of the earliest warning signs of failure is often manifested by the formation of serious critical structural cracks (Figure 1) long before the actual event. Recent structural collapses in Nigeria and elsewhere have raised serious concerns for more in-depth and intensified study on the mechanism of resistance of all components of the structure, as reported by Wenapere (2008), Gajanan et al (1983), Adisa (1997) and Ogar (1997). However, while significant research effort has been devoted to steel and reinforced concrete, concrete framework, concrete masonry blocks and blockwalls, there have been few studies on sandcrete, and these have been limited to block units only. There are very few recorded tests on sandcrete blockwalls.

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Structural cracks may be associated with differential foundation movement or other movements leading to the transfer of load from frame to blockwall, among others (Figure 2). Thus, the blockwall may be expected to possess enough strength to support its own loads, the effects of additional environmental and accidental loads, and any loads transferred from the structural frame. In addition, the blockwall should provide some reasonable in-fill restraint to the structural frame.

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Current views on the strength deformations and failure mechanism of concrete masonry under static and dynamic loads are presented in the recent studies by Abrams (1996, 1997), Andam (2002a, 2002b), Page (1981), Paulson et al (1990) and Stroven (2002), among others. The problems of frame–in-fill interaction are addressed in the investigative studies by Ephraim et al (1990), Liauw et al (1983) and Madan et al (1997), among others. The few important studies that have attempted to address the connection of mixed composition and compactive effort on the strength and economy of sandcrete masonry include those by Wenapere (2003), Andam (2002a, 2002b), Chandhari and Gumel (2000) and Uzomaka (1977), among others.

The laboratory investigation of blockwall is highly limited by the sheer size and the loading equipment requirements, and due to this, a model study is perhaps the more realistic way forward, and is also provided for in Act 318 (1) and BS. 8110 (1997). In fact, most masonry models investigated have been in the form of scaled-down walls built of prototype concrete or brick block units. It is obvious that the use of reduced-scale blocks will decrease the laboratory space requirement and lighten the loading equipment required for testing. This would invariably increase the level of research, especially in situations where sophisticated and heavy equipment is not available.

Thus, the main objective of this research was to investigate and establish the adequacy of a one-quarter scale structured model of hollow-block sandcrete masonry in order to reproduce the physical and structural characteristics of the prototype masonry units and blockwall as a function of mixed proportion and applied compressive loading. This will become a tool for sandcrete building designers and engineers aiming to diagnose likely failure modes in the blockwalls.

This study is focused on the verification of the reproducibility of prototype engineering properties of sandcrete block by its ¼ scale model. The results are analysed and discussed in the relevant section that follows.

Comparisons are carried out in terms of density and compressive strength of prototype and model sandcrete blocks.

The comparative analysis of laboratory test results of the influence of mixed proportions, water–cement ratio and age on the strength of sandcrete blocks for prototype and model has been undertaken in this research with a view to confirm the applicability of code recommendation on modelling to sandcrete masonry structures. The results reveal the practical correspondence of test values and trends for major characteristics of strength and density, which provides concrete evidence of reproducibility of prototype sandcrete physico-mechanical behaviour under load by its ¼ scale model.

The above conclusions have substantiated the applicability of the Code of Practice recommendation in respect of use of models for analysis and design, with particular reference to the ¼ scale model. The unique contribution of this research is the extension of the codes recommendation to sandcrete masonry structures, thus opening up the potential, scope and opportunities for research in sandcrete masonry structures, especially in developing nations and elsewhere where sophisticated and heavy equipment are not available for prototype scale tests. To this end, model tests are recommended to study the strength, durability and failure mechanisms of the masonry structures in other stressed states including flexure, shear, dynamic loading and their combinations, such that the national concern over building collapses will ultimately receive a practical solution.