A MATHEMATICAL MODEL FOR PREDICTION OF THE STRENGTH OF SANCRETE CEMENT BLOCK

[APPLICATION OF HENRY SCHEFFE’S AND OSADEBE’S CONCRETE OPTIMISATION THEORY]

Abstract

Sandcrete blocks are prismatic units manufactured from lean (sand-cement) mortar mixes and are the predominant walling materials used in buildings in Nigeria and other countries. Although the blocks are preferred to other similar walling material such as bricks and concrete blocks due to the availability of the dominant material (sand) in the country and the ease of manufacture, the material is known to have several drawbacks: it has a low compressive strength which could be a factor for structural failure in buildings. Apart from the compressive strength, little is known about the other strength and permeation properties of the material. Thirdly, little research has been conducted into the influence of water-cement ratio which is known to wield a strong influence on the strength and permeation properties of cementitious materials. On the basis of a thorough review of the literature, a detailed experimental programme was designed and carried out in Nigeria on the factors that influence the strength and permeation properties of the material using over 10 destructive test methods. Over 1500 sandcrete blocks were moulded and tested. The literature review also indicated that lack of quality control in the Nigeria sandcrete blocks manufacturing industry which accounts for the poor quality of the material in the country. The results of the experimental programme indicate that with the right constituent materials,   manufacturing   and   curing   processes,   it   is   possible   to   improve   the compressive strength of sandcrete blocks to over 7.0N/mm2 (the minimum specfied by the British Standards) with a lean mix of 1:12 cement/sand ratio. The compressive strength of the material was found to have a strong relationship with other strength properties of the material and could therefore be used as the main criterion for determining its quality. The inclusion of quarry dust and coarse aggregate (sharp sand) significantly improved the strength and permeation properties of the blocks. Compared with normal concrete, sandcrete; was found to have a relatively high permeability. Through this study, a data base on several strength and permeation properties has been developed which can assist engineers to estimate these properties. Finally recommendations were made on ways of improving the quality of sandcrete blocks in the country.

CHAPTER ONE

INTRODUCTION

1.1.            Research Background

Sandcrete blocks are produced from a lean mix of Portland cement, natural sand and water and are the main masonry unit used in the urban and peri-urban areas of Nigeria, West Africa and many parts of the developing world. Unlike other masonry units such as concrete blocks and burn bricks which are used in other civil engineering infrastructure, they are predominantly used as walling units in the construction of shelter: both as external cladding and for partitions. They have an appeal to the construction industry due mainly to its ease of manufacture and the widespread availability of the main constituent material (sand).

A number of studies including the project National de Recherche/Development (Sablocrete) (1994) carried out in France have designated sandcrete as a member of a family of concretes, which can be used when environmental and economic constrictions restrict the use of coarse aggregate. This project carried out in France (Sablocrete) revealed the economical and mechanical advantages of this material and showed typical applications in public works and building.

In researching into the properties of sandcrete blocks as defined by this study, one is faced with a delimma, little material exists in the technical literature on certain properties of sandcrete blocks such as its durability, tensile, transverse and flexural strength and the value of elastic constants. One way of overcoming this problem is to compare sandcrete with some of its closest relatives such as concrete and mortar. In fact as a material, sandcrete can be considered as a form of zero-slump concrete or mortar. This three materials share many common characteristics even though their properties diverge at a point. For this reason, extensive references are made in this study to concrete and mortar.

Sandcrete blocks are mostly produced in Nigeria by the small scale manufacturers who as business people have profit as their main motive and therefore often use sub-standard materials and improper procedures in the production of the blocks. This results in the output of poor quality blocks with poor strength properties. Whereas Nigerian burnt bricks are reported to have compressive strength in the range of 11.5N/mm2 to 26.0N/mm2, the corresponding range for sandcrete blocks was only found to be 1.45M/mm2 to 3.5N/mm2 (Graham, 1990; Danso, 1994). This problem tends to limit the application of the material and could also be a major factor in relation to the structural failure of buildings in the event of earthquakes.

As mentioned above, apart from the compressive strength of the unit, very little is known about the other properties of the block such as the Young’s Modulus, tensile, flexural and transverse strengths which are required by structural engineers and other scientists for design computations. The lack of information on these strength properties of sandcrete blocks leaves room for much speculation and approximation, which could be detrimental to the design of structures.

Virtually all the numerous research and publications on the material seem to focus on the strength properties of the material. Very little exist in literature on the durability of sandcrete blocks. In recent years however, reports from around the globe indicate that the premature deterioration of concrete and other cementitious materials is a major problem. For instance it has been reported that significant proportions of national budgets have been expended on the maintenance of concrete structures in the US and Europe. Thus the philosophy that a strong concrete is a durable concrete has failed as this is not always the case. It can therefore be argued that to avoid huge maintenance costs of structures constructed from concrete and for that matter sandcrete blocks, the basic masonry units must be durable as well as strong.

Closely linked with permeation (durability) and strength properties of cementitious materials is the water-cement ratio. Most researchers agree that water cement ratio has a profound impact on the durability and strength of these materials but few researchers of sandcrete blocks have examined the effect of water-cement ratio on the properties of the blocks.