Browsing by Author "Mrema, Alex L."

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    Assessment of Strength Compliance with Standards for Tanzania Eucalyptus Wood Poles Treated with Copper-Chromium-Arsenic Compounds
    (2007-07) Mrema, Alex L.
    Samples of copper-chromium arsenic compounds (CCA) treated Eucalyptus poles for power transmission were sampled from a lot following Militmy Standard MlL-STD 105D, Single sampling, Tightened Inspection, Acceptable Quality Level (A QL) of 4 as provided for in the South African Standard SABS 754: 1994 from lots containing 151-500 poles. Samples were randomly selected from a lot. Maximum fiber stresses were evaluated taking into account the actual taper in each pole. The cantilever loading test was peiformed on the samples following SABS 754:1994. It was found out that the average taper for the poles was smaller than that assumed in the standard due to the dijJerent pole growth characteristics and environment in Tanzania and that the average modulus of elasticity obtained for the poles was lower than the average assumed in SABS 754: 1994. The poles also showed excessive deflections at working loads. It is recommended to the Tanzania Bureau of Standards that although SABS 754: 1994 ;s meant to be used for eucalyptus poles grown ill Southern Africa south of the Sahara that are treated with creosote or CCA there is a need to review it to take into aCCOl/nt the actual characteristics of the poles grown in Tanzania where they are normally grown in highland areas with higher rainfall and colder climates.
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    Behavior of Wood in Transverse Compression
    (1994) Pellicane, P. J.; Bodig, Jozsef; Mrema, Alex L.
    A finite element analysis program exists to evaluate the stress distribution in wood members subjected to perpendicular-to-grain (transverse) compression. In this study, the program was used to perform parameter studies to evaluate the effects of key variables on the distribution of stresses and the maximum stress concentrations in wood subjected to transverse compression. These variables included: specimen geometry (length/depth), loading geometry (loaded length/specimen length), and material properties (ratio of moduli of elasticity of the wood in the direction of loading/perpendicular to loading). The results showed that a complex state of stress exists in members even when the load is distributed over the entire specimen surface. In particular, numerically-determined stresses nearly 3.5 times the nominal stress were found for certain combinations of input parameters. In addition, an empirically-derived equation is presented that estimates the magnitude of maximum stress concentration as a function of the three parameters investigated. The equation was developed with the use of multiple regression techniques and had a correlation coefficient of 0.958.
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    Cement Bonded Wood Wool Boards from Podocarpus Spp. For Low Cost Housing
    (2006) Mrema, Alex L.
    This paper gives results of a preliminary study on the properties of cement bonded woodwool boards manufactured as composites by using wood wool (excelsior) from podocarpus spp. wood species and ordinary Portland cement. Two main parameters were varied during the investigation and these were the width of the wood wool and the cement to wood wool ratio. Results show that the optimum mix proportion is three parts of cement to two parts of wood wool by weight and that a smaller excelsior width gives higher board strengths. The results from tests on flexural strength, compressive strength and tensile strength far exceeded the requirements of the German Standard DIN 1101:1989. Results of water absorption and swelling were, however, higher than those specified in the standard but this was attributed to insufficient pressure on the boards during manufacture an anomaly which can be corrected. The potential uses of the boards are in basements, floor units, permanent shuttering, partitioning, sound insulation in walls, ceilings and floors, roofs, sound barriers and thermal insulation. Further research is required to investigate the use of different wood species and different mineral binders.
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    Comparison of the Properties of Portland Cement and Portland Limestone Cement
    (2010) Mrema, Alex L.
    A study was made in one of the cement factories in Dar es Salaam, Tanzania, where Ordinary Portland Ce-ment (CEM I 42.5N) and Portland limestone cement (PLC) (CEM II/A-L/32.5R) are produced and conform-ing to the Tanzania Standard TZS 727 (Part1): 2002, which is equivalent to EN 197 published by the com-mittee for European normalization (CEN). A comparison was made between the two types of cements in terms of physical, chemical and mechanical properties. It was found out that they all complied with the standards, that there was no significant difference in their setting times and that the Portland cement had higher strengths than the PLC. Also it was observed that there was a slightly lower water demand for the same consistency when compared to OPC and hence there is an improvement of the cohesiveness of a con-crete mix when PLC is used. It was concluded, however, that the two cements are different and that using the two cements interchangeably as is done in Tanzania is wrong because they do not have equivalent strengths and therefore equivalent performance since the PLC is not optimized.
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    Earth Building in Tanzania – Use of Soil Stabilized Bricks and Blocks
    (2005-06) Mrema, Alex L.
    This paper presents experimental results of investigations done on the use of stabilized earth (soil) for the manufacture of bricks and blocks. The soil used was clayey sand of low plasticity. Two types of stabilizers were used to make stabilized bricks and these were cement and a combination of cement and lime. Different stabilizer proportions were employed. Results of tests carried on these bricks at 28 days indicate an optimum mix proportion of 7% cement if cement is used alone and an optimum mix proportion of 5% each if a combination of lime and cement is used as a stabilizer. With these stabilizer contents we obtain bricks of strengths of at least 2.5N/mm2 with acceptable functional capabilities in a wall for low cost housing. To make bricks comparable in strength with those made of cement and sand with a minimum strength of at least 3.5N/mm2 for load bearing walls as suggested in the Tanzania Standard, TZS 283:1986, the optimum mix proportion was found to be 8.5% cement and 7% each if cement and lime is used in combination. It is suggested that further research on the effectiveness of the various other stabilizers like bitumen, gypsum, ash/sand, and cow dung is required for the different types of soils commonly found in Tanzania.
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    Feasibility of Lightweight Aggregate Concrete for Structural and Non-Structural Works in Tanzania
    (2013-09) Mrema, Alex L.
    There has been serious land degradation in Tanzania due to extensive exploitation of granite aggregates even in areas where there is plenty of natural lightweight aggregates. Many Engineers have resorted to using normal weight aggregates in construction not only due to the feeling that lightweight aggregates (LWA) are too weak for structural purposes but mainly due to lack of knowledge of the design procedures for lightweight aggregate concrete (LWAC). Structures made of normal weight aggregates have higher dead loads which demand reduced span lengths, deeper structural members, more reinforcement and increased foundation area when compared to those made of LWAC. In Tanzania there are major deposits of LWA in areas of previous volcanic eruptions namely Kilimanjaro, Mbeya and Arusha and these remain largely un-exploited. Research was done to investigate the feasibility of using scoria and pumice aggregates in structural lightweight concrete. The aggregates were obtained from Kilimanjaro and Mbeya respectively. A mix design for grade 20 concrete was performed for both aggregates. It was found out that it was possible to use the scoria aggregates for structural concrete but not the pumice without mineral additives or chemical admixtures. Using pumice aggregates without any additives, the strengths obtained were less than 17 MPa which is the minimum strength acceptable for structural LWAC. It was concluded that the use of scoria aggregates for structural LWAC was feasible and that the pumice aggregates could be used for non-structural LWAC. Further research is required to find alternative deposits of pumice with better strengths for use in structural LWAC as the strengths of these aggregates vary from place to place and even within the same locality. Structural and Construction Engineers in Tanzania are now in a position to design and use LWAC as the design procedures have now been elaborated. The use of light weight aggregates especially in areas where these deposits are present will reduce the current costs of transport of normal weight aggregates from long distances and this will also protect the environment.
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    Modeling Wood in Transverse Compression
    (1994) Pellicane, P. J.; Bodig, Jozsef; Mrema, Alex L.
    A plane-stress, finite element model has been developed to predict the stress distribution in wood members subject to perpendicular-to-grain compression. This model exploits linear-strain, isoparametric triangular elements used in sufficient number to achieve a convergent solution. Model verification was achieved through comparison of numerically obtained deformation predictions with corresponding experimental data obtained from actual test specimens. Twenty-seven specimens were instrumented to determine their deformations at numerous locations. Test materials were sampled from three logs (two engelmann spruce, one western hemlock). Specimens were fabricated with three widely different orthotropic ratios, three geometries (length/depth ratios), and three loading geometries (uniformly distributed load across the entire length, one-half length, and one-quarter length). In total, 377 experimental measurements on 27 specimens were compared to finite element predictions. The results showed that on average the model predicted local deformation to within 5%.
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    Properties of Boards made from Rice Husk and Portland Cement as Binders and reinforced with Sisal Fibers
    (2013) Mrema, Alex L.
    A research study demonstrated that rice husks can be used to manufacture boards which can be used in low cost housing if bound by Portland limestone cement (CEM II/A-L/ 32.5R) and reinforced with sisal fibers to cater for their brittle behavior and that such boards can provide a substitute for wood panels. Boards made of rice husks and cement in the ratios of 1/3.3, 1/ 3.6 and 1/4 and with sisal fiber reinforcement contents of 0%, 0.4%, 0.5% and 0.8% were prepared and tested for their physical and mechanical properties. The results were compared with standard requirements for boards made with wood based fiber and particle panel materials according to DIN 1101 and ISO 2696 and BS EN 634-2. Results indicate that water absorption and thickness swelling decreased with an increase in fiber content and that there is an optimum rice husk/ cement ratio and fiber content that gives maximum flexural strength and that such boards could be used in low cost housing construction. The use of such boards could be seen as partially solving the problem of global warming and the problem of pollution associated with the disposal of rice husks in developing countries.
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    Properties of Concrete Made With Portland Limestone Cement Partly Replaced by Rice Husk Ash
    (2011-12) Mrema, Alex L.
    Research was conducted in Tanzania to investigate the properties of a concrete made by a replacement of 0%, 6%, 12% and 18% with RHA in Portland limestone Cement. What was investigated was the influence of the ash on the workability, strength development and water permeability of the concrete. The water/binder ratio was kept constant. The results obtained indicated that workability of the concrete decreases with an increase in ash content and suggests that there is an optimum replacement content which give maximum concrete strength. Beyond this optimum ash content, the strength of the concrete decreases. It was also found out that the permeability of the concrete decreased with an increase in RHA content and this confirmed earlier findings by some researchers that the introduction of RHA in concrete makes the concrete more durable. Other benefits of using RHA in concrete are increased resistance to chemical attack, reduced effects of alkali-silica reactivity, reduced shrinkage rates, increased insulation properties of concrete, reduced amount of plasticizers, and reduced potential for efflorescence. The other obvious advantage of using RHA apart from reduced environmental pollution is the reduction of carbon emission which is associated with the production of Portland cement. Any partial replacement with RHA in Portland cement implies reduced carbon emission which is the cause of global warming and which the world is fighting against.