Date of Award

6-30-2016

Document Type

Open Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Fabio Matta

Abstract

Stabilized earthen masonry (SEM), which is built with compressed stabilized earthen blocks (CSEB) and mortar, is emerging as a construction system for affordable, sustainable and high-quality dwellings. Compelling features of earthen masonry are local availability of suitable soils, thermal insulation properties, humidity control within healthy ranges, and relatively low carbon footprint. SEM is also appealing for the construction of affordable houses in rural areas and farmlands, where underrepresented and underprivileged groups often live. In fact, the lack of affordable housing is an issue across the United States where low-income families are most affected, often committing more than half of their income on housing. In addition, the low-income population tends to be more vulnerable to hazard events. For example, in the United States, high wind events, such as tornadoes and hurricanes, cause casualties and property damage. This is often due to the lack of adequate construction systems and practices. Although SEM has been used around the world for the construction of modest as well as high-end housing, the non-engineered nature of SEM materials and structures limits the use of SEM as a mainstream construction system, even more so for hazard-resistant dwellings. Filling the knowledge gap related to the structural behavior of the SEM, with an emphasis on extreme loads, is necessary to increase the confidence in and facilitate the acceptance of this affordable and sustainable masonry system.

Recent literature reported on the feasibility of using SEM in low-rise dwellings to withstand high wind loads. In addition, the lack of data on the mechanical and structural properties of stabilized earthen masonry, and the need for an experimentally characterized masonry prototype where strength and deformability properties and failure modes are verified, have been highlighted.

In this research, theoretically determined target strength ranges are used to guide the engineering and prototyping of an affordable and “green” SEM prototype. First, stabilized earthen blocks are prototyped using a silty loam soil, which is locally available in South Carolina as well as throughout the US “Tornado Alley”. The blocks are stabilized with ordinary Portland cement (OPC) in amounts of 0%, 6% and 9% in weight of soil (wt%). Stabilization is required to ensure sufficient erosion resistance and, in general, durability. In addition, randomly distributed non-biodegradable and recyclable plastic fibers are incorporated in the soil matrix as a sustainable means to radically enhance the toughness of the earthen blocks, thereby making them attractive for toughening against the localized impact of wind-borne debris. The amount of fiber reinforcement was kept to a value of 0.5 wt%. It was shown that the addition of plastic fiber reinforcement radically changes the damage tolerance of the compressed and stabilized earth blocks (CSEBs). In fact, unreinforced CSEBs were transformed into earthen blocks with significant post-cracking deformability and residual strength due to the crack-bridging effect of well-distributed and embedded plastic fibers.

Once the block strength requirements were met, the designing of a compatible mortar that provides sufficient bond strength at the block-mortar interface was carried out. Then, two SEM prototypes that are engineered for high-wind resistance were developed. The salient mechanical properties, obtained by load-testing masonry subassemblies, were used for the prototyping and mechanical characterization of SEM. First, a prototype built with unreinforced compressed earthen blocks and mortar that meets the strength requirements for high wind pressures is presented. The unreinforced SEM specimens were physically tested to characterize their structural response under compression, flexure and shear loading. Compressed earthen blocks with OPC in amounts of 0 wt%, 6 wt% and 9 wt% were combined with the selected mortar for the compression characterization of prisms and wallettes. The results from the compression tests were used to determine the combination of earthen blocks and mortar that provides the required strength to withstand high wind loads while minimizing the amount of OPC. Next, the selected combination of CSEB and mortar was characterized in flexure and shear. As a proof of concept, the strength values obtained for the unreinforced SEM are used for the analysis of a realistic EF3 tornado-resistant dwelling structure.

The second prototype, which is made with the plastic fiber-reinforced CSEBs paired with a mortar reinforced with low-cost plastic microfibers, aims to radically enhance the SEM damage tolerance and resist the impact of wind-borne debris. Subassemblies of fiber reinforced and stabilized earthen masonry (SREM) were used to evaluate the effect of fiber reinforcement on the in-plane and out-of-plane behavior of the masonry. The plastic fibers radically enhanced the tensile and shear strength of the block mortar interface, and the overall toughness of the earthen masonry. Finally, the results of two high-velocity flying-debris impact tests are presented to offer preliminary evidence of the impact resistance of the SREM prototype vis-à-vis its SEM counterpart.

Rights

© 2016, Mabel Cristina Cuellar-Azcarate

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