Challenges, Opportunities, and Solutions in Low Cost Building Envelopes: A Case Study of Low Strength Masonry Systems

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1 564 Challenges, Opportunities, and Solutions in Low Cost Building Envelopes: A Case Study of Low Strength Masonry Systems Esther Obonyo 1, Peter Donkor 2, Fabio Matta 3, and Ece Erdogmus 4 1 Associate Professor, M.E. Rinker Sr., School of Building Construction, University of Florida, P.O Box , Gainesville, FL 32611; PH: (352) ; Fax: (352) ; obonyo@ufl.edu 2 Doctoral Student, M.E. Rinker Sr., School of Building Construction, University of Florida, P.O Box , Gainesville, FL 32611; PH: (352) ; donpiero@ufl.edu 3 Assistant Professor, Department of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Room C210, Columbia, SC 29208; PH: (803) ; Fax: (803) ; fmatta@sc.edu 4 Associate Professor, Architectural Engineering Program, Durham School of Architectural Engineering and Construction, University of Nebraska-Lincoln (Omaha Campus), 205A PKI, 1110 S. 67th Street, Omaha, NE ; PH: (402) ; Fax: (402) ; eerdogmus@mail.unomaha.edu ABSTRACT In addition to addressing the basic need of shelter, building envelopes must address concerns with load transfer, energy efficiency, durability and resilience with respect to natural/man-made disasters among other structural and aesthetic concerns. This paper discusses the key barriers to the use of low tech compressed earth blocks in a resilient and sustainable manner. The paper also presents a process improvement approach through which low tech masonry approaches can be used to provide a context-appropriate, cost-effective, resilient solution. Typical compressive strength values of masonry can range from 7 MPa to 20 MPa for the commonly used CMU (concrete masonry units). Compressed earthen masonry usually peaks at somewhere between 3 and 5 MPa. When working with these lower numbers, general concerns that arise from masonry being an elastic-brittle material become more significant. This paper adopts a process improvement approach to propose a stabilization strategy that can be used to enhance the quality of blocks produced. Fibers are included in the blocks in this research to address susceptibility to local failure when the masonry is exposed to impact load from, for example, flying debris in a high wind region. The process improvement approach is used optimize the use of fiber in compressed earth blocks and also explore the feasibility of using a low strength soil-cement mortar to achieve strength compatibility with the optimized blocks. INTRODUCTION Building materials can be rated as best for the environment depending on whether or not they: (1) are made with salvaged, recycled, or agricultural waste content; (2) conserve natural resources; (3) avoid toxic or other emissions; (4) save energy or water; (5) contribute to a safe, healthy, built environment (Lazarus, 2009). There has been a specific concern over greenhouse gases (GHGs) that has resulted in

2 565 several efforts being directed towards reducing their release into the atmosphere. In Condition 2 of LP Hedelberg s movement, The Natural Step, there is a caution against the use of man-made materials that take a long time to decompose, with dioxins being explicitly identified as examples of compounds that will almost never be broken down by nature. Given that building materials are largely inert, key areas of concern from an ecological perspective are ensuring that the manufacturing is done with the least impact and also designing the products for easy disassembly and recycling (Kibert, 2005). An assessment based on an ecological perspective makes earth-based building materials good options to explore as a way of minimizing the adverse effects of buildings on the environment. However, it is important to bear in mind that ecological concerns comprise just one aspect of greening the built environment. The definition of what would constitute a sustainable building system is much broader. For the discussion in this paper, the authors have linked the performance goals to the efforts directed at delivering high performance building systems. A high performance building here is based on the Energy Policy Act of 2005 (2005) definition: a building that integrates and optimizes all major high performance building attributes, including energy efficiency, durability, life cycle performance, and occupant productivity. Given that natural and man-made hazards remain constant threats to communities and infrastructure systems in the U.S. and across the globe, efforts directed at realizing high performance building systems are increasingly coupling sustainability with resilience. The research discussed in this paper investigates the use of an ecological building material (earthen masonry) to deliver a resilient, high performance, building system. The deployment context requires resistance to wind loads from, for example, flying debris, a problem experienced by communities in several parts of the U.S., including Florida and Nebraska. Examples of stakeholders interested in implementing the findings of the research include a local communitybased non-profit in Alachua, Florida, who intend to use earthen masonry to showcase sustainable use of materials in their proposed Eco Village Training Center Project. Similar to other masonry systems, the resilient and sustainable walling system being investigated is a composite material comprised of earthen blocks set in a mortar matrix. Masonry is a heavy, brittle material with low tensile strength (Page, 1996). Because brick/block units and mortar have different properties, masonry exhibits distinct directional properties and contains potential planes of weakness created by the low tensile (bond) strength at each mortar-unit interface. Being a low strength material, these concerns become more critical when using earthen masonry. Typical compressive strength values of masonry can range from 7 MPa to 20 MPa for the commonly used CMU (concrete masonry units). Compressed earthen masonry usually peaks at somewhere between 3 and 5 MPa. When working with these lower numbers, general concerns arising from masonry being an elastic-brittle material become more significant. For low strength compressed earth blocks this number can sometimes fall to below 2 MPa,a concern that was raised in an NSF funded workshop (Obonyo et al, 2010, Obonyo et al 2011). With such low strength values, it is not surprising that compressed earthen blocks have not been widely deployed in a broad range of applications across the globe. This is largely because of concerns of structural adequacy (Prinster and

3 566 Warden, 2005). There are also significant variations in strength values from block to block (Obonyo et al, 2010). Further, there are no universally accepted building codes on the use of the materials. These concerns have greatly impeded the widespread use of compressed earth blocks in structural applications. Some of the problems can be attributed to the specific properties of locally available soil. It is generally accepted that good soil for compressed earth blocks should have a clay content of 20% (Auroville, ND). However, because soil is highly variable, it is common for the locally available soil to have either more or less than the recommended clay content. When the stabilization strategy is not customized to the specific properties of the locally available soil, the resulting blocks are either going to be over or under-stabilized. In some cases it may be possible to import good soil to replace or blend with the locally available marginal soil. However, on the basis of sustainability metrics, there is a limit to how far one can import good soil without the material costs being deemed uneconomical and therefore not sustainable. This paper adopts a process improvement approach to propose a stabilization strategy that can be used to enhance the quality of blocks produced. Fibers are included in the blocks in this research to address susceptibility to local failure when the masonry is exposed to impact load from, for example, flying debris in a high wind region. The process improvement approach (based on identifying inefficient and ineffective operations) is used to optimize the use of fiber in compressed earth blocks and also to explore the feasibility of using a low strength soil-cement mortar to achieve strength compatibility with the optimized blocks. EXPERIMENTAL PROGRAM Materials The materials used included local soil from Gainesville/Newberry, Florida (See Table 1), commonly available Type I Ordinary Portland cement (OPC) (ASTM C150), and commercially available MasterFiber MAC Matrix macro synthetic PP fibers obtained from BASF Corporation. Table 1. Physical Properties of Soil Property Liquid Limit (%) Plastic Limit (%) Plasticity Index (%) Sand (%) Clay (%) Silt (%) Optimum Moisture Content Maximum Dry Density Composition 33% % 12.2% 1.5% 9% 17.5 KN/m 3 Brick Fabrication The soil preparation and mixing followed the recommended compressed earth blocks procedures. Dry soil was run through a manual sifter with a 3.40 mm² mesh size to remove lumps. The sifted soil was kept in an oven set at 93 o C for 24 hours,

4 567 after which it was taken out for cooling before production commenced. The samples produced consisted of 4 plain and 16 PP fiber-reinforced locks. PP fibers were gradually introduced into the mix after the initial dry mix of the sand and OPC had been observed to be thorough. Mixing continued for 3 minutes, after which the matrix was visually considered to be a uniform, thoroughly mixed batch with well- dispersed fibers. The dry mix was watered gradually while continuously mixing the batch. The process lasted an additional 2 minutes, after which the mix was visually deemed to be homogeneous and also passed the drop test, which is performed by taking a fistful of wet matrix, shaping it into a ball, and dropping the ball from a height of one meter onto a hard surface. If the ball completely disintegrates, the mix is too dry; if it breaks up into four or five pieces, the moisture content is right; if it flattens out without breaking or breaks into 2 pieces, the mix is too wet (Rigassi, 1995). A target moisture content of 9% obtained from the proctor compaction test (see Table 1) was used as a guide when determining the required quantity of water. Parallel experimental work that will be reported in a different publication established that the PP fibers worked best at the commercially available length of 54mm. The effect of fiber content was investigated through varying the weight fraction from 0% to 0.2%, 0.4%, 0.6%, 0.8%, and 1%. The manual compression process was replaced by a motorized approach to minimize variations that can be attributed to the force being exerted. Soil-cement-fiber matrix was fed into the mold of a hydraulic-operated block-making machine. The press is designed to compress matrices from the bottom of the mold. It is set up such that the top plate is flush with the top of the mold. After the top plate is engaged to cover the top of the mold, the bottom plate is engaged for a minimum of 3 seconds to compress the matrix in the mold exerting a maximum pressure of 300psi. After compression, the blocks were ejected from the mold, carefully moved, and placed on pallets outdoors. The nominal dimensions of blocks produced were 190.5mm x 203.2mm x mm for the blocks for compressive strength testing and 228.6mm x 203.2mm x mm for blocks for flexural strength testing. After de-molding, the samples were left in situ for 24-hours prior to being stacked for curing. Testing was done after 28 days. Soil-Cement Mortar Testing Dry soil similar to what had been used for block production was run through a manual sifter with a 3.40 mm² (0.135 in. 2 ) mesh size to remove lumps. The sifted soil was kept in an oven set at 93 o C (200 o F) for 24 hours, after which it was taken out for cooling before production commenced. The mixing proportions have been provided in Table 2. Mixing was done using a concrete mixer starting with a dry mix of soil and OPC then for the reinforced samples, gradually introducing PP fibers and continuing to mix for an additional 3 minutes upon which the matrix was observed to be uniform, consistent with well dispersed fibers. The dry mix was watered gradually in a uniform manner. The mixing process was continued for an additional 2 minutes. At this point the wet mix was visually deemed to be homogeneous and workable. A total of 20, 101.6mm x 203.2mm cylinders were cast; 5 cylinders for each of the 4 mix designs. They were cured indoors under polythene sheets for 28 days.

5 568 Table 2. Properties of mortar (Cylinder size = 101.6mm x 203.2mm) Sample No. Mortar Proportion by Weight (C:S:F)* 1 1 : 11.5 : : 11.5 : : 11.5 : 0.05 (PP fibers) 2.7 *C= cement; S= soil; F= fibers Water Cement Ratio Prior to resting, steel retaining caps lined with compression pads were fitted over the ends of cylinders. This was done to distribute the test load uniformly to ensure consistent breaks. The cylinders were tested under uniaxial compression using a Forney FX 250/300 compression test machine with a maximum load capacity of 224 KN. The rate of compression was between 20 to 50 psi/sec until failure. The test set-up and procedure complied with ASTM C780. Compressive strength was computed using the following equation: Where f = compressive strength, [N/mm² (psi)]; P = maximum applied load indicated by the testing machine, [N (lbft)]; D = diameter of cylinder, [mm (in.)] Masonry Prism Testing An assemblage of block units and mortar was constructed to give an indication of the properties of masonry assemblages. Two 203.2mm x 203.2mm compressed earth blocks were placed one on top of the other and bonded using soilcement mortar. For the initial set of experiment, fiber was not included in the mix. The blocks used had 0.4% weight fraction PP fiber. Compressive strength tests were conducted 28 days after casting the prisms.. Figure 1. Masonry Prism with soil-cement mortar

6 569 RESULTS The PP fibers were used in their commercially available length of 54mm. Blocks reinforced with 0.2, 0.4, and 0.6% PP fibers recorded higher compressive strength values compared to the unreinforced blocks. 0.4% emerged as the optimal fiber content (see Table 3). Table 3. Compressive strength results Mix type Fiber content by weight/% Average - 5 Samples (MPa) Coefficient of Variation (%) PP PP PP PP PP PP The initial results presented in Table 4 on mortar testing suggest that the soilcement mortar designed as part of this experiment has strength compatibility with the fabricated blocks. The plain mortar with no fiber had the highest compressive strength values. Changing the water cement ratio from 1.0 to 2.7 resulted in 2.82 and 2.83 MPa. The samples with fiber had the lowest compressive strength values (1.93 MPa). Sample No. Table 4. Properties of mortar (Cylinder size = 101.6mm x 203.2mm) Mortar Proportion by Weight (C:S:F)* Water Cement Ratio Average - 5 samples (MPa) Std. Dev. Coefficient of Variation (%) 1 1 : 11.5 : : 11.5 : : 11.5 : 0.05 (PP fibers) *C= cement; S= soil; F= fibers Masonry prism test results obtained so far show that strength of the masonry prism is closer to that of the block that that of the mortar (see Table 5 and Figure 2). Block mortar joints did not fail, giving an indication of good bonding. Bond strength testing will be completed in future projects/work to further investigate block-mortar strength compatibility.

7 570 DISCUSSION AND FURTHER WORK As previously indicated, this research investigates the use of compressed earth blocks in a sustainable and resilient manner. The findings from the preceding section indicate that even where the locally available soil has less than the recommended 20% clay, it is possible to produce blocks that are of acceptable strength values. The successful realization of this goal can be process improvements in the production of the block. Following a detailed soil study, which included deploying SEM/EDM analysis, it was established that 8% OPC was necessary for chemical stabilization. The sandy nature of the soil triggered concerns over the effectiveness and efficiency of using a light, manual device. A heavy, motorized equipment was used to ensure that the sandy soil was fully compacted (see Figure 3). In addition to creating a dense, fully compacted medium, the use of this equipment also resulted in an additional benefit of the variation in strength properties for block to block being significantly reduced. The force exerted using motorized equipment remained constant throughout the block production process. Table 5. Prism compressive strength results Prism No. Compressive Strength (MPa) Mortar Figure 2. Typical failure mode of blocks (a) prism still held together by fibers even after failure (b) failure surface pulled apart to allow for observation

8 571 Figure 3. The motorized compressed earth brick equipment Although fiber inclusion has been known to have an adverse effect on the compressive strength of blocks, the empirical data from this research suggests that the PP fiber selected for the study have improved bonding with the soil. This is being further investigated through a microstructural analysis of the failure surface. The development of strength properties of soil-cement-fiber mixes primarily depends on the formation of fiber-matrix, matrix-matrix, and fiber-fiber bonds. These identified bonds can be affected by dimensions, surface conditions, and the quantity of fiber present (Khedari et al., 2005). The increase in fiber content therefore resulted in a decrease in fiber-matrix and matrix-matrix bond, which, when coupled with an increase in fiber-fiber bonds, lowered the blocks compressive strength. Greater amounts of PP fiber (more than 0.6%) may have also resulted in micro-fractures at fiber-soil interfaces resulting in a decrease in compressive strength (Namango, 2006). It is generally recommended that soil-cement mortar be used for compressed earth blocks (Guillaud & Joffroy, 1995; Walker, 1999; Reddy & Gupta, 2006; Morton, 2008). The ideal mortar for an earthen masonry system should have strength and material properties that are similar or compatible to those of the blocks. Mortar with strength values that are much lower cause the blocks to be highly susceptible to erosion and water infiltration, thus contributing to the deterioration of the blocks. High strength mortar bears the risk of water stagnating on the exposed surface (Guillaud & Joffroy, 1995). The stagnated water can erode the blocks and can also trigger cracking thereby lowering the masonry system s overall strength. The soilcement mortar compressive strength values for samples which had no fiber averaged 2.82 and 2.83 MPa, for a water-cement ratio of 1:9 and 2:7, respectively. Although these initial results suggest that there may be little value in using more cement for this particular soil, further testing of a larger sample size will be necessary to validate these initial findings. When PP fiber was introduced into the mix, the compressive strength value dropped significantly to 1.93 MPa. There were some workability issues with the fiber-reinforced mortar that could partly explain this. In further work, lime will be introduced into the mix to address this problem. Strength compatibility

9 572 of the block-mortar combination will be further investigated through testing masonry prisms based on a modification of the provisions of ASTM C The interest in earthen masonry in modern construction was initially driven by economic reasons in the late 1970s and 1980s. Over the last two decades or so, the green building movement gave the material second life through their campaigns for more ecological materials particularly ones limiting the use of cement and cementbased products. In more recent times, there is an additional performance target of hazard resilience. The design and construction of a context-appropriate masonry systems must address economic, ecological and hazard resilience needs. The discussion in the foregoing sections has demonstrated that it is feasible to engineer sustainable and resilient earthen masonry system using locally available soils through cement stabilization them. To ensure that the soil-cement mix was fully compacted, a motorized equipment was used to produce the blocks. This also minimized the variations in block properties that can be attributed to the use of a manually-operated press. Further experimental work was done to investigate the feasibility of using engineered fibers to enhance the blocks resistance to local failure when subjected to high wind loads. The findings have indicated that the proposed approach to designing and producing engineered, fiber-reinforced earthen blocks can potentially be used for sustainable and resilient housing in the Florida context. ACKNOWLEDGEMENTS The work presented in this paper was supported by National Science Foundation (NSF). NSF Project Award No : Collaborative Research: Resilient and Sustainable Engineered Fiber-Reinforced Earthen Masonry for High Wind Regions Investigators: E. Obonyo (UF), E. Erdogmus and A. Schwer (UNL), F. Matta (USC) REFERENCES Auroville Earth Institute (ND), Compressed Stabilised Earth Block < (Aug. 15, 2013) Bomberg. M.; Onysko, D. Energy Efficiency and Durability of Buildings at the Crossroads; < (July 1, 2009). Energy Policy Act of 2005 Public Law AUG. 8, 2009; EPA: Washington, DC, USA, 2005; < (July 1, 2009). Guillaud, H., and Joffroy, T. (1995). Compressed earth blocks volume II. Manual of production. Vieweg, Eschborn, Germany. Kibert, C. Sustainable construction. In Polymers in Construction; Akovali, G., Ed.; Rapra Technology Limited: Shawbury, UK, 2005; pp Khedari J., Watsanasathaporn P., and Hirunlabh J., (2005), Development of fibrebased soil cement block with low thermal conductivity, Cement & Concrete Composites, 27:

10 573 Lazarus, N. Potential for Reducing the Environmental Impact of Construction Materials; Commissioned by Bioregional Development Group: Surrey, UK, 2005; < >(June 4, 2009). Morton, T., (2008), Earth masonry: Design and construction guidelines, BRE Press, Bracknell, Berkshire, UK. Obonyo, E., Tate, D.,Sika, D. and Tia, M. (2010), Communication: Advancing the Structural Use of Earth-based Bricks: Addressing Key Challenges in the East African Context, Sustainability 2010, 2(11), Obonyo, E. (2011), Optimizing the physical, mechanical and hygrothermal performance of compressed earth bricks, Sustainability, 3(4): Prinster, M.and Warden, B. (2005) Understanding Stabilized Earth Construction: Building with Strength in Mind; School of Architecture and Urban Design, University of Kansas: Lawrence, KS, USA, < (Sept. 18, 2013). Page, AW (1996), Unreinforced Masonry Structures An Australian Overview, Bulletin of the New Zealand National Society for Earthquake Engineering, Vol. 29, No.4, December < Unreinforced-Masonry.pdf > (Sept. 14, 2013). Venkatarama Reddy, B. V. and Ajay Gupta, Strength and elastic properties of stabilised mud block masonry using cement soil mortars, Journal of Materials in Civil Engineering, ASCE, Vol. 18, No. 3, 2006, pp Walker P. J., (1995), Strength, durability and shrinkage characteristics of cement stabilised soil blocks, Cement & Concrete Composites, 17: