Walls are the basic element in the construction of most buildings. They are often required to be load bearing, especially in low rise buildings (1-2 upper floors). Sandcrete blocks are the most commonly used unit in wall construction in modern Nigeria and, indeed, the entire West Africa. The use of laterite and other forms of walling units, for the construction of modern residential buildings have not made much progress when compared to the use of sandcrete blocks. The same can also be said of bricks. The major advantage of sandcrete blocks is the ease of production and laying of the blocks
The structural properties of blocks of interest include compressive strength, flexural strength, water absorption, modulus of elasticity, shear modulus and split tensile strength. Others are density, fire resistance, durability and thermal conductivity. These properties are dependent to a very large extent on the relativeproportions of the constituents and the method of production process.
Sandcrete blocks are traditionally made of cement, natural river sand and water. The constituents are mixed and placed in a mould which is removed immediately after compaction and leveling of the top. The newly produced blocks are self-supporting; hence they are often referred to as zero slump concrete. Individual blocks are joined together, after curing, to form walls using cement-sand mortar. It is often the practice to partially replace the sand portion with other materials such as laterite, coarse aggregate or quarry dust. Sand-quarry dust blocks are sandcrete blocks in which the sand portion has been partially replaced with quarry dust.
1.2 Statement of the problem
There has been a growing interest recently, especially in the developing countries, in the use of quarry dust (or quarry rock dust) in the production of concrete and masonry products. This is partly as a result of the unchecked depletion of the natural sources of fine aggregates (river sand) which has resulted to an increase in their price (Sekar et al, 2011, Ilangovana et al, 2008, Devi and Kannan, 2011) and environmental degradation (Khamput, 2006, Jayawardena and Dissanayake, 2006). Many sand mining sites have been closed because of the damage they cause to the environment. For example, a popular sand mining site within the campus of the Federal University of Technology, Owerri, Imo State of Nigeria was closed because of the environmental degradation resulting from the river sand mining operations. Another site close to a major culvert on the Otamiri River along Aba road, Owerri was also closed for the same reason.
The problem of scarcity of good fine aggregate in some areas has been exacerbated by the ever increasing demand for concrete and masonry products. Attempts have been made to either partially or completely replace sand with other materials in the production of concrete and masonry products. Such materials have included laterite and quarry dust. The problem of sand scarcity is of great concern that some equipment manufacturers have developed rock crushing equipment that produce “manufactured sand” whose gradation closely resemble that of natural river sand (Norman 2008, Hudson, 1997). Quarry dust should be differentiated from manufactured sand in that unlike quarry dust, manufactured sand is purposefully made to resemble the gradation of the sand through washing, sieving and blending of the quarry waste (Cement Concrete and Aggregates Australia, 2008).
The great infrastructural development in many countries, Nigeria inclusive, has led to the establishment of many rock quarry sites, resulting in the production of great volumes of quarry dust. Quarry dust is a by-product of the rock quarrying process whose size ranges from below 63 micrometers to 5 millimeters (< 63µm – 5mm). It generally has a large proportion of its particle being far less than 2.36mm in size. Only a little portion of quarry dust produced is utilized. Its major use is as a filler material in base and wearing courses in roads. Use must therefore be found for the greater portion remaining in order that it would not constitute another environmental hazard. It is this need to utilize the excess quarry dust, and to reduce the need for natural river sand that has led to it (quarry dust) being used in full or partial replacement for natural sand in concrete, sandcrete block and in soilcrete block production. The use of quarry dust is further encouraged when it is noted that some engineering properties of concrete and masonry products are improved when quarry dust is used as partial replacement for natural river sand Sekar et al, 2011, Nanda et al, 2010, Oyekan and Kamiyo, 2008, Falade, 1993).
One area of interest in which the use of quarry dust has been tried is in the production of sandcrete blocks where it has been used partially to replace sand. In this regard answers must be provided to such questions as:
(i) what is the effect of the inclusion of quarry dust on the structural properties of the blocks?
(ii) what percentage of quarry dust is optimal to achieve the desired value of a
(iii) what is the cost implication of incorporating quarry dust in sandcrete blocks?
The effect of the inclusion of quarry dust in sandcrete block production has been partially addressed by some researchers (Boeck et al. 2000, Oyekan and Kamiyo 2008, Olaniyan et al. 2012, Danso 2005). In their works, it was confirmed that the inclusion of quarry dust did improve the compressive strength, but at varying degrees. A replacement of not more than 40% of the sand was suggested by Danso (2005).
Works on quarry dust utilization in sandcrete block production has centered mainly on the determination of their compressive strengths and, occasionally, on the hygrometric characteristics. Not much has been done on other parameters such as Static modulus of elasticity, flexural strength, split tensile strength and water absorption which are also very important in structural designs. Such properties of blocks need also be investigated and documented.
Mix proportioning is of great concern as it is well known that the properties of concrete and its related products are greatly influenced by the proportions of the constituents. A desired property of the block can only be achieved through a proper mix design. Many mix design methods have been developed over the years for concrete but not much exist for sandcrete blocks. The use of empirical methods and the application of existing concrete design methods (for example, ACI: 211.3 (1987)) to sandcrete block mix proportioning is, to say the least, wasteful and uneconomical. This is even worse with the introduction of quarry dust in the mix. The use of existing research works are often not helpful as the information are sometimes not properly documented. Many researchers often do not specify unambiguously the amount of water or the water/cement ratios employed in their works. The use of such statement as: “Water was added as to make the mix good enough for block making” is very common. Such a statement grossly neglects the well-known effect of water/cement ratio in concrete works and often makes replication of the work difficult.
There are virtually no models that predict the structural characteristics of sandcrete blocks made with sand partial replacement with quarry dust. The need for such models cannot be overemphasized as they will be of great help in mix proportioning byreducing the time and cost of conducting trial mixes.
1.3 Statementof Objectives
The objectives of this work will be to:
- investigate the effect of partial replacement of sand with quarry dust on the structural characteristics of sand-quarry dust blocks.
- develop mathematical models that can predict the structural characteristics of sand-quarry dust blocks.
- establish mathematical relationships, if any, between the compressive strength and the other properties of sand-quarry dust blocks notably water absorption, flexural strength, split tensile strength and modulus of elasticity.
- develop models for predicting the production costs of various sand-quarry dustmixes and blocks.
- study component interactions of sand-quarry dust blocks through the various models developed
- present the result of the various models developed in such a way that they can be readily used in commercial sand-quarry dust block production.
1.4 Scope of Study
This work will be limited to the formulation of model equations for predicting the structural characteristics of sandcrete blocks with the sand partially replaced with quarry dust. The replacement will range from 10 to 40% with the cement/combined aggregate ratio ranging from 1: 6 to 1: 10. Three model mixture experiment forms will be considered notably:
- Scheffe’s simplex lattice model (pseudo component model),
- Scheffe’s simplex lattice model (component proportion model)
- Osadebe’s model.
The structural properties investigated will be limited to compressive strength, water absorption, flexural strength, split tensile strength, shear strength and static modulus of elasticity. All tests will be on hardened specimens.
1.5 Justification of Study
- The study will provide additionally needed data on use of quarry dust in block production.
- The mixes developed for block production will be of great help in assisting commercial block producers in meeting required strength targets.
- The models that will be developed will be of great assistance in mix design by forming the basis of trial mix designs, thus eliminating great experimental works and the high cost often associated with such ventures.
- The models that will be developed will be of great help in optimizing constituents of sand-quarry dust blocks to meet a desired property.
- The cost models will be of great use in optimizing the cost of sand-quarry dust blocks.
- The relationships formulated between the compressive strength and the other properties will be of great help in making estimates for these properties since, most times, the compressive strength is the only property tested for.
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