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The Influence of Metakaolin from Saaba (Burkina Faso)

Burkina Faso

The paper evaluates the feasibility of reducing clinker in the Portland Cement production using local metakaolin in Burkina Faso. Standardized testing methods Burkina Faso have been used for this purpose, and experiments were performed on mortar prisms containing different amounts of metakaolin. Important results about the physical, mechanical and durability characterization of blended mortars were carried out in this study. The obtained results are discussed based on available literature data. These results have shown increased physical and durability properties for blended mortars. Although the mechanical strengths remained relatively low for higher MK incorporations, the latter grow to surpass these of PC mortars (the reference) at 28, 56 and 90 days of curing. The results in the paper, have confirmed the possibility of using metakaolin to partially substitute cement, a possibility to reduce the CO2 production by the cement industry in Burkina Faso.

The progressive increase of the energy prices over the past decades in Africa, has restrained the availability of the conventional building binder “cement” to the large majority of poor and middle-income countries populations. These same countries, display the highest demographic trends and hence, high decent housing demands. In addition to this cost-related hindrance, the recent environmental considerations of cement production industry (emission of one ton of CO2 per ton of cement) motivated the search of “alternatives”. The cement industry contributes for up to 5% – 8% of global CO2 emissions in 2014 [1] .

The potential of Burkina Faso in terms of clayey materials has recently drawn the attention of researchers from varied fields of engineering and technology. These include road construction materials stabilization [2] [3] , general building materials (bricks) [4] and water treatment and sanitation applications [5] . Although varied are these areas of research, none has addressed the exploitation of pozzolanic activity of thermally activated clays in hydraulic binders.

In fact, clays with high kaolinite content (kaolin clays) are potential pozzolanas and can be used as supplementary cementitious materials when activated (thermally or chemically). In the geological context of Burkina Faso, most of the kaolin clay quarries are found nearby the tuff-hillsides and are sought to have evolved from them [6] . These kaolin clays, when sintered at temperatures around 500˚C – 800˚C produce metakaolin [7] . This material with high content of amorphous silica (SiO2) and alumina (Al2O3) reacts with the cement hydration Portlandite (Ca(OH)2) to create secondary calcium silicate hydrates (C-S-H) and alumina containing C-A-H and/or C-A-S-H phases [8] . In this study, higher proportions were attempted for the optimization of CH consumption. In fact, Kostuch et al. [9] have found 20% MK incorporations necessary for full removal of CH in concrete at 28 days of curing.

The synthesis of MK by the dihydroxylation/amorphization of kaolin clay is a key step for optimum pozzolanic activity of the resulting material [10] . The influence of heat treatment parameters over the reactivity of MK have been explored extensively [11] [12] [13] [14] . Ambroise et al. [15] have suggested temperatures above 700˚C. Nevertheless, higher temperatures can lead to the recrystallization and formation of mullite and cristobalite non-reactive phases [16] [17] [18] . Raw clays differing in terms of purity, the most accurate heat treatment program should be deduced from individual clay thermal analyses using TG/DTA.

The incorporation of MK in cement production would have a substantial economic impact and environmental relief in Burkina Faso. In fact, there are no local industries that produce Portland Clinker for its production requires high technology and considerable amount of fuel (energy). Hence, the clinker is imported from the West and Asia, increasing its price. On the environmental point of view, for instance the production of one ton of flash metakaolin only produces 0.098 ton of equivalent CO2 versus 0.913 tons per one ton of PC of type CEM I [19] .

Considering the two motivational aspects, this study aims at evaluating the possibility of using the local calcined clay (metakaolin) at high proportions as a partial substitute of clinker in West Africa. Throughout this study, the effect an increasing substitution rate of cement by MK over the physical, mechanical and durability properties of the MK blended cement was explored. The used materials were characterized using the ICP-OES for chemical composition, the x-rays diffraction for mineralogy and TG/DTA for thermal analyses.

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Detection Tool for Unbalanced Bids

Detection Tool

Unbalanced bidding is a cash flow management Detection Tool strategy that is recognized as an illegal/disqualifying practice by public owners; and unethical practice by most private owners. This Detection Tool practice provides the awarded bidder with unjustified advantages at the expense of the owner. Unfortunately, limited tools and techniques are currently available to identify and detect unbalanced bids during the evaluation process. This paper presents an innovative detection tool to identify unbalanced bids in unit price contracts during the bid evaluation process. The proposed technique develops BMDI graphs to visualize total markup variation patterns during the project lifetime to detect unbalanced bids. The proposed method also uses Monte Carlo simulation to take in consideration the impact of cost uncertainties and risks. An illustrative example was presented to show the capabilities and features of the proposed method in determining the status of submitted bids during the evaluation process.

Cash flow management is a concern for contractors as payments from owners lag behind project expenditures. This time lag adversely affects the contractors’ cash flow and cause contractors to partially finance their projects. Unbalanced bidding, which is considered an illegal/unethical practice, may be used as a risky mitigation strategy [1] . Unbalanced bids can occur in fixed and unit price contracts. However, this study focuses on unbalanced bids in unit price contracts. In unit price contracts, unbalanced bids can be prepared by manipulating the item’s prices without affecting the total bid price [2] . Unfortunately, it is usually difficult for owners to determine the existence and/or extent of potential inflation/deflation of bid item prices [2] . The difficulty comes from the fact that the award decision depends on the total bid price. The variations in the items’ unit prices are usually not considered. They may be due to legitimate reasons such as variations in the bidders’ expertise. They may also be due to a deliberate manipulation of unit prices to hide certain advantages, construction means, or proprietary technologies, or due to an honest mistake or a bad business decision.

Cattell [2] classifies unbalanced bids into three groups, namely, Front-End loaded, Back-End loaded, and Quantity Error Exploitation. Front-End loaded bids inflate the prices of early stage activities to positively impact the contractor cash-in flow. They usually result in owner overpayments when the value of time is considered [3] . In other words, Front-End bids allow for an interest-free loan from the owner to the contractor. Equally important, the overpayments would weaken the owner’s position and reduce the contractor’s incentives to complete the project. Back-End loading consists of inflating the prices of late schedule items. This pricing strategy is not common in relatively short duration construction projects that located in low inflation rate countries such as the United States. In Quantity Error Exploitation scenarios, the contractor increases the unit price of items in which actual quantities are expected to exceed the ones stated in the bid documents [2] . For example, in one of the contracts awarded by the Florida Department of Transportation (FDOT), the bid winner, inadvertently or deliberately, offered a high unit price (i.e., $420/ft versus $171/ft) for trench support sheet. As the owner’s offered quantity was 30% of the actual one (i.e., 500 ft versus 1729 ft) the contractor was able to submit the lowest bid. Unfortunately, FDOT did not detect this problem, which resulted in additional unnecessary cost of $516,180 [4] . Quantity Error Exploitation bids, which are more difficult to detect than other types, may have severe consequences on the owner cost.

The US public and private sectors have different approaches to unbalanced bids. The federal code of regulation (48 C.F.R. § 15.404−1 (g)) forbids unbalanced pricing because of its adverse consequences on performance risk and payments. However, unbalanced bids are not forbidden in the US private construction industry sector. However, they are considered as unethical and risky acts. Moreover, the private sector strives to detect unbalanced bids in advance as a preventive action.

Significant research work has been conducted to address unbalanced bids. However, most of the published research work has focused on developing optimization models to help contractors maximize their profit while submitting the lowest possible bid price [5] [6] [7] . Moreover, limited tools and techniques have been published to help owners/clients detect and prevent unbalanced bids during the tender evaluation process. This paper presents an innovative detection tool for unit price projects that helps owners/clients to visualize the distribution of markup along the project timeline and detect unbalanced bids.

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Effects of Wood Ash and Waste Glass Powder on Properties

Wood Ash

The potential for using fly ash as a supplementary cementing Wood Ash material in concrete has been known almost since the beginning of the previous century. Fly ash was used Wood Ash as a supplementary cementing material (SCM) in the production of Portland cement concrete. A supplementary cementing material, when used in conjunction with Portland cement, contributes to the properties of the hardened concrete through hydraulic or pozzolanic activity, or both. In this study, the fly ash and waste glass powder were used in concrete blocks to study the improvement of concrete in terms of workability and strength. Therefore, an experimental study will be conducted to measure the engineering properties of cured concrete. In this research, local raw material from Jaresh area was used.

In recent decades, research has demonstrated that high dosage levels of fly ash (40% to 60%) can be used in structural application producing concrete with good mechanical properties and durability (Marceau 2002) [1] . The use of good quality fly ash with a high fineness and low carbon content reduces the water demand of concrete and, consequently, the use of fly ash should permit the concrete to be produced at lower water content when compared to a Portland cement concrete of the same workability. Although the exact amount of water reduction varies widely with the nature of the fly ash and other parameters of the mix. A well-proportioned fly ash concrete mixture will have improved workability when compared with a Portland cement concrete of the same slump. This means that, at a given slump, fly ash concrete flows and consolidates better than a conventional Portland cement concrete when vibrated. The use of fly ash also improves the cohesiveness and reduces segregation of concrete. The spherical particle shape lubricates the mix rendering it easier to pump and reducing wear on equipment. The main objective of this paper was to develop a mixture design of concrete with fly ash and waste glass powder to produce concrete with high compressive strength comparing to normal concrete samples. Another objective was to reduce some industrial waste material on the environment such as fly ash and waste glass, which causes environment pollution.

Baboo Rai et al. (2011) [2] used marble powder and its granules as partial replacement for the fine aggregate. Using these materials, cement mortar and concrete were prepared. Afterwards, the relative workability test, compressive strength test and flexural strength test were conducted. Based on the test results, they concluded that when the percentage of marble powder added in concrete increases, the compressive strength and flexural strength of concrete also increase.

Akshay C. Sankh et al. (2009) [3] stated that there is a need to find the new alternative material to replace the river sand, such that excess river erosion and harm to environment is prevented. Many researchers are finding different materials to replace sand and one of the major materials is quarry stone dust. Using different proportion of these quarry dust along with sand, the required concrete mix can be obtained. This paper presents a review of the different alternatives to natural sand in preparation of mortar and concrete. The paper emphasizes on the physical and mechanical properties and strength aspect on mortar and concrete.

Belachia M. et al. (2011) [4] used the recycled aggregates in the making of hydraulic concrete. Properties like density, workability, compressive strength and flexural strength of the hydraulic concrete were found and compared with the properties of conventional concrete. The optimum percentage of recycled aggregate found from the comparative study was 25% for the ultimate strength and 50% for the ultimate density.

Goliya, H.S. et al. (2008) and ACI Committee 232 [5] [6] conducted a research with the aim of identifying a suitable alternative for concrete ingredients. Cement is replaced by fly ash, stone dust, ground granulated blast-furnace slag etc. and sand is replaced by stone dust, fly ash etc., Ravina, D. and Mehta, P. K. [7] [8] . to find out the strength properties, such as the compressive strength and flexural strength of concrete, and this through a partial replacement of both cement and sand by glass powder as a pozzolana and by pond ash in concrete respectively.

Leema Rose. A et al. (2011) [9] showed that glass powder is obtained as a waste material after the extraction and processing of glass to form fine particles less than 4.75 mm. Glass powder has been used in large scale in highways as a surface finishing material and also used in the manufacture of hollow blocks and light weight concrete prefabricated elements.

Monica et al. (2013) and ACAA, 2005 Coal Combustion Product [10] [11] used various alternate materials like marble powder, quarry dust, wood ash and paper pulp in concrete as a replacement of cement in making concrete. In this paper, they concluded that, by using these materials in concrete, nearly 14% to 20% of cement was saved. Chemical properties of concrete like sulfate attack resistance and alkali aggregate resistance were increased while using these materials. By using these waste materials in concrete, the problems in disposal of these materials on lands get reduced and the environmental pollution is prevented.

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Quasi-Static Explicit Buckling Analysis for Thin-Walled Members

Walled Members

The quasi-static explicit finite element method (FEM) and element free Galerkin (EFG) method are applied to trace the post-buckling equilibrium path of thin-walled members in this paper. The factors that primarily control the explicit buckling solutions, such as the computation time, loading function and dynamic relaxation, are investigated and suggested for the buckling analysis of thin-walled members. Three examples of different buckling modes, namely snap-through, overall and local buckling, are studied based on the implicit FEM, quasi-static explicit FEM and EFG method via the commercial software LS-DYNA. The convergence rate and accuracy of the explicit methods are compared with the conventional implicit arc-length method. It is drawn that EFG quasi-static explicit buckling analysis presents the same accurate results as implicit finite element solution, but is without convergence problem and of less-consumption of computing time than FEM.

Thin-walled members of various shapes have been widely used in civil and mechanical engineering. Under many conditions, when these members are subjected to compressive loads, buckling and post-buckling behaviors usually dictate the design considerations. Analytical solutions of buckling of thin-walled members can only obtain for the simple cases of elastic linear/nonlinear buckling. Analysis of nonlinear buckling problems of plastic and large deformations primarily relies on the numerical methodology.

In the nonlinear buckling analysis of thin-walled structural member, the implicit arc-length algorithm is generally accepted as an effective numerical method for tracing the post-buckling path [1] [2] [3] . It is well known that the implicit method is established on the operation of stiffness matrix, where a large amount of computing resource is required for processing highly incremental iteration. For complex nonlinear problems, the disadvantage of non-conver- gence of this method is obvious and usually difficult to be overcome, for example, the singularity of the stiffness matrix near the critical point. Therefore it is necessary to study the explicit method for solving highly nonlinear post-buck- ling problems.

Quasi-static analysis is a simulation of static problem with motion analysis which restricts the load velocity so that the outcome of this analysis can only have a little inertia influence that can be neglected. As an explicit algorithm, the advantages of quasi-static buckling analysis lie in the lower computing cost and no convergence consideration. However, structural dynamic responses caused by loading speed and inertia force significantly influence the quasi-static explicit results. Usually very small loading step is needed to approach the static equilibrium state at each loading moment, which inversely decreases the convergence rate. The efficiency of quasi-static explicit method mainly depends on the problems being solved. The key factors that can reduce the dynamic responses, like computing time, loading function and damping relaxation, must be specified in order to keeping the ratio of dynamic energy to internal energy within a low level. Zhuang [4] presented the conventional method of quasi-static analysis and compared the differences between implicit and explicit methods. Ji [5] used the quasi-static analysis for solving stable problems of stiffened plates under axial pressure, and obtained the structure responses which static analysis could not reach with a little time cost. Fan [6] did research on the effect of the duration and functional form of the time histories of loads by the analysis of a linear spring mass oscillator. Li [7] interpreted the principle of static calculation by using dynamic relaxation method and put forward a new method on value adoption of mass damp and rigidity damp. Lee [8] applied the explicit arc-length method to trace the post-buckling equilibrium path of structures on the basis of dynamic relaxation method with kinetic damping.

Finite element method (FEM) is a stable and reliable computing method through meshing the continuum into discrete units. When structures undergo large deformations, the computing accuracy is significantly influenced by the distortion of discrete units. In explicit method, a stable time step must be very small if the distortion of discrete units occurs, which greatly adds up the computing cost. Element-free method (EFM) is studied by many researchers for avoiding the effects of discrete units on numerical consequence. Solution with EFM depends on the discrete nodes setting up within or at the edge of a domain. Shape function is constructed on local nodes, so there is no mesh-dependence problem. The primary advantage of this approach is that there is no singularity of stiffness matrix induced by distortion of discrete units in the solution of large deformation and discontinuity problems. Element free Galerkin (EFG) method is based on the global Galerkin weak forms and the integration of background grids. The displacement approximation functions are generated by using the least squares approximation constructed via nodes in local fields. The computational accuracy and convergence rate of EFG methods have been demonstrated to be the same as FEM. The stability of this method is not affected by the irregular nodes, and furthermore, it can be combined with FEM and BEM (boundary element method) to improve the computing efficiency.

EFG method has been well used in the buckling analysis of thin-walled members. Liu [9] developed an EFG formulation to calculate the buckling loads of symmetrically laminated composite plates based on the principle of minimum potential energy, and found that solving the eigenvalue problem is much more computationally efficient compared to the FEM. Chinnaboon [10] developed a BEM-based meshless method for buckling analysis of elastic plates with various boundary conditions that include elastic supports and restraints. Liew [11] used an EFG method to study the elastic buckling behavior of stiffened and un-stif- fened folded plates under partial in-plane edge loads. Tamijani [12] employed the EFG method for buckling and static analysis of plates with arbitrary curvilinear stiffeners. Peng [13] obtained the critical buckling load of ribbed plates using the mesh-free method based on the first-order shear deformation theory. Xiang [14] predicted buckling behavior of microtubules based on an atomistic- continuum model. Lu [15] developed an adaptive enrichment mesh-free method to capture wrinkling and post-buckling behavior in sheet metal forming. Li [16] used mesh-free method for numerical simulations of large deformation of thin shell structures, which showed simplicity in both formulation and implementation as compared to shell theory approach. Lin [17] used a non-linear dynamic explicit scheme for the post-buckling analysis of thin-walled structure based on the meshless shell formulation. Compared with the finite element method, the mesh-free method possesses the same accuracy and can save some computing time as well as work out the problems that can’t be solved by the traditional FEM.

The FEM and EFG quasi-static explicit methodologies are applied to trace the post-buckling path of thin-walled members in this paper. The key factors that control the convergence rate and dynamic responses, such as the computation time, loading function and damping relaxation, are discussed and suggested in the numerical buckling analysis. Three examples of thin-walled members occurred snap-through, overall and locally buckling are studied in detail by quasi-static explicit FEM and EFG method, and the efficiency and accuracy of the applied methods are demonstrated through the comparison with the conventional solution of implicit arc-length method.

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Practical Aspects of the Design and Construction of a Small Cable Roof Structure

Roof Structure

Cable roof structures have only become widespread in large span structures in the latter part of the twentieth century. However, they still represent a relatively new form of roof construction, especially as in the present case of a small span innovative structural solution. The contribution of this text to the structural engineering community lies in the increased interest in building simple cable roof structures. Since its completion in September 1996, this small cable roof structure has been recognized as an interesting architectural and structural example. The text describes aspects of the design and construction of a small cable roof that was designed as a roof for an open-air theater stage for the city of Sao Jose do Rio Pardo, Sao Paulo, Brazil. A cable network, in the shape of a hyperbolic paraboloid surface, is anchored in a reinforced concrete edge ring. The projection of the ring’s axis onto the ground plane is an ellipse. Workers with specialized training were employed in the various stages of the construction, which was completed in September 1996.

The cable roof network, initially in the form of a hyperbolic paraboloid surface, is anchored in a ring of reinforced concrete whose axis projects an ellipse in the horizontal plan. The larger and smaller axes of the ellipse measure 20.00 m and 13.00 m, respectively. The network is formed by an orthogonal mesh 10 by 6, which is parallel to the ellipse axes. Both end points of the larger axis are 1.75 m below the surface center, while both end points of the smaller axis are 1.00 m above the surface center. The center of the surface is 4.50 m above the ground. A wire rope with diameter of 1 inch (25.4 mm) and composed of galvanized steel wires of high resistance was specified for the cables. Cable clamps were used at the intersection of two cables and purlins were fixed over the cable clamps in the direction parallel to the ellipse’s smaller axis. A pre-painted steel sinusoidal sheet was used for roof cladding. The cross section of the edge ring is rectangular measuring 1.00 m wide by 0.45 m high. The edge ring axis follows the form of the hyperbolic paraboloid surface. The ring is sustained by four identical reinforced concrete columns with 3.71 m high and rectangular cross section measuring 0.25 m by 0.50 m. The axis of the smaller moment of inertia of the rectangle is tangent to the ellipse equation. The structure is shown in Figure 1. Notice the rotation of the cross section of the edge ring.