2019 splitting tensile strength of fiber reinforced and biocemented sand choi

Splitting Tensile Strength of Fiber Reinforced and Biocemented Sand Technical Note Splitting Tensile Strength of Fiber Reinforced and Biocemented Sand Sun Gyu Choi, Ph D 1; Tung Hoang2; E James Alleman, Ph D 3; and Jian Chu, Ph D 4 Abstract This technical note examines the splitting tensile strength properties of natural sand treated with polyvinyl acetate (PVA) fiber in combination with biocementation using the microbially induced calcite precipitation (MICP) process Ottawa 20 30 sand was mixed.

Technical Note Splitting Tensile Strength of Fiber-Reinforced and Biocemented Sand Downloaded from ascelibrary.org by Nanyang Technological University- Library on 11/06/19 Copyright ASCE For personal use only; all rights reserved Sun-Gyu Choi, Ph.D 1; Tung Hoang 2; E James Alleman, Ph.D 3; and Jian Chu, Ph.D Abstract: This technical note examines the splitting tensile strength properties of natural sand treated with polyvinyl acetate (PVA) fiber in combination with biocementation using the microbially induced calcite precipitation (MICP) process Ottawa 20-30 sand was mixed with PVA fiber at five different fiber ratios (0.0%, 0.2%, 0.4%, 0.6%, and 0.8% by weight) and then stabilized using urease-producing bacteria plus urea and calcium chloride (CaCl2 ) solutions Splitting tensile strength was determined for the treated sand samples The results showed that the splitting tensile strength and splitting secant elastic modulus increased with increasing in either calcium carbonate content or fiber ratio The use of PVA fibers together with MICP treatment could also increase the failure strain and the postfailure splitting tensile strength DOI: 10.1061/(ASCE)MT.1943-5533.0002841 © 2019 American Society of Civil Engineers Author keywords: Microbially induced calcite precipitation (MICP); Splitting tensile strength; Reinforced cemented sand; Calcium carbonate content; Polyvinyl acetate (PVA) fiber Introduction Ordinary portland cement (OPC) has been used for decades for soil improvement or ground stabilization In recent years, an alternative material, biocement by microbial activity without traditional cement, has been developed and applied in lieu of cement for soil stabilization One of the biocementation processes is microbially induced calcite precipitation (MICP) (Dejong et al 2010, 2013; Burbank et al 2011; Chu et al 2012; O’Donnell and Kavazanjian 2015; Choi et al 2017b; Li et al 2018) However, sand treated by biocement will suffer from the same shortcomings as that treated by OPC, that is, OPC treated soil or biocement is brittle (Mortensen et al 2011) in splitting tensile strength (Choi et al 2016a) And the tensile strength of soil is an important mechanical parameter in the design of geosystems, i.e., slopes, dams, embankments, and hydraulic barriers (Li et al 2014) Multiple attempts have been made to use a variety of fiber additives to improve the mechanical properties of OPC One of such studies involved the use of metallic (typically steel) or fiber reinforcement additives (Apostolopoulos and Papadakis 2008; Xing et al 2008; Maher and Ho 1994; Song and Hwang 2004; Park 2009) Fiber reinforced, biocemented sand was also studied by some researchers Li et al (2016) similarly conducted experiments using biocementation with homopolymer polypropylene and multifilament fiber An increase in friction angle and effective Senior Researcher, Disaster Prevention Research Division, National Disaster Management Research Institute, Ulsan 44538, Republic of Korea Lecture, Faculty of Bridge and Road Construction Engineering Univ of Danang–Univ of Science and Technology, Danang 550000, Vietnam Professor, Dept of Civil, Construction, and Environmental Engineering, Iowa State Univ., Ames, IA 50014 Professor, School of Civil and Environmental Engineering, Nanyang Technological Univ., Blk N1, 50 Nanyang Ave., Singapore 639798 (corresponding author) ORCID: https://orcid.org/0000-0003-1404-1834 Email: CHCHU@ntu.edu.sg Note This manuscript was submitted on October 4, 2018; approved on March 28, 2019; published online on June 21, 2019 Discussion period open until November 21, 2019; separate discussions must be submitted for individual papers This technical note is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561 © ASCE cohesion, as compared to clean sand performance, was observed after the soil had been treated with 0.3% of fiber and MICPprocessing at a level that would reach 6.6%–8.0% calcium carbonate content (measured on an additive weight basis) Choi et al (2016a) also studied biocemented sand with polyvinyl acetate (PVA) fiber as reinforcement A 30% increase in unconfined compressive strength and 160% in splitting tensile strength was observed after the sand had been treated using a 0.8% fiber ratio plus sufficient MICP processing to secure a 6.8%–13.1% calcium carbonate buildup Given that clean sand has no splitting tensile strength, the use of fiber plus MICP will improve ductility and produce splitting tensile strength The splitting tensile strength properties of fiber- and MICP-treated soils are affected by several factors, including the type of fibers, fiber ratio, and calcium carbonate content However, studies on the effect of fibers and biocementation on the splitting tensile strength of biocemented sand are still uncommon In this technical note, a study on the splitting tensile strength of PVA fiber reinforced, MICP biocemented sand is presented Tests with different fiber ratios of 0.0%, 0.2%, 0.4%, 0.6%, and 0.8%, and different calcium carbonate content of 3%–5%, were conducted After treatment, the splitting tensile strength and calcium carbonate content of the treated samples were measured to establish a relationship between splitting tensile strength and processing factors such as fiber ratio and calcium carbonate content Experimental Work Materials The employed PVA fiber materials were 0.1 mm in diameter and 12 mm in length The basic technical properties of these PVA fibers were as follows (Park 2011): specific gravity ¼ 1.3, tensile strength ¼ 1,078 MPa, and Young’s Modulus ¼ 25,000 MPa Ottawa 20-30 sand was used for all tests, with a specific gravity of 2.65 The grain size for the involved sand material ranged from 0.6 mm (sieve #30) to 0.85 mm (sieve #20), with a mean grain size of 0.73 mm 06019007-1 J Mater Civ Eng., 2019, 31(9): 06019007 J Mater Civ Eng Downloaded from ascelibrary.org by Nanyang Technological University- Library on 11/06/19 Copyright ASCE For personal use only; all rights reserved The materials used for biocementation included: (1) ureaseproducing bacteria (UPB), S pasteurii (American Type Culture Collection, ATCC 11859), (2) urea, and (3) calcium chloride The growth medium for culturing S pasteurii was made of a yeast extract (20 g), (NH4 Þ2 SO4 (10 g), and 0.13 M tris buffer (pH ¼ 9.0) solution (Zhang et al 2014) UPB was injected in autoclaved medium for days The urea and calcium chloride were both applied at a concentration of 0.3 M (Choi et al 2016a) Specimen Preparation Test specimens were prepared with the following mix recipes: 32.4 g of Ottawa sand, PVA fiber addition at 0.0%, 0.2%, 0.4%, 0.6%, and 0.8% (by weight of sand, respectively), and 7% of distilled water (by weight) to easily mix the fiber uniformly in the sand column These materials were mixed together by hand and placed into clear acrylic, 5-cm diameter and 10-cm height (10 layers) cylinders having 1.65 g=cm3 of unit weight by rammer To promote a uniformly distribution of fiber in the sand column, each layer (1 cm) was also compacted to achieve a unit weight of 1.65 g=cm3 (Park 2011) At the bottom of the specimen, a layer of gravel and scrub sponge was used More details regarding these sample preparation methods can be found in Choi et al (2016a) After each test sand specimen was initially formed, UPB, urea, and calcium chloride were then injected using the following steps First, 10 mL of UPB solution (urease activity 3.7 mM urea=min) was circulated through the specimen using a peristaltic pump (1.0−1.5 mL=min) for h and then drained from the bottom Second, 500 mL of the 0.3-M urea and CaCl2 solutions were then circulated through the column by pump for 20 h After completing this 24-h processing sequence, the column specimens were then drained and repeated until a total of seven such cycles had been completed to precipitate about 3.8%–4.7% of calcite content Table Test results Test ID F01 F02 F03 Average F21 F22 F23 Average F41 F42 F43 Average F61 F62 F63 Average F81 F82 F83 Average Fiber (%) 0.0 0.2 0.4 0.6 0.8 Calcium carbonate content (%) Splitting tensile strength (kPa) Splitting secant elastic modulus (Es50 , MPa) 3.8 4.1 4.2 4.0 4.1 3.8 4.3 4.1 3.9 4.4 4.6 4.3 4.1 4.3 4.6 4.3 4.5 4.7 4.4 4.5 48.1 50.0 56.8 51.6 52.0 55.0 59.9 55.6 71.4 74.4 80.9 75.6 77.7 75.0 82.8 78.5 85.3 88.6 86.2 86.7 5.0 5.8 5.9 5.6 5.9 5.5 5.9 5.8 7.3 6.9 7.5 7.2 7.3 7.1 7.3 7.2 7.5 7.5 7.3 7.4 Testing A total of 15 column specimens were prepared using the aforementioned method and then used for splitting tensile strength tests according to ASTM C496 (ASTM 2011) by unconfined compressive strength equipment With each fiber ratio (0.0%, 0.2%, 0.4%, 0.6%, and 0.8%), three duplicate tests were carried out As expected, these specimens would typically fracture in a vertical fashion After each test, g of the sample were taken from the center portion of the tested specimen for calcium carbonate determination using a washing and elution method that has been previously documented (Zhao et al 2014; Montoya and DeJong 2015; Choi et al 2017a) Testing Results Engineering Properties Table summarizes the calcium carbonate content, splitting tensile strength, and secant elastic modulus results obtained from the splitting tensile strength tests on biocemented specimens with PVA fiber ratios of 0.2%, 0.4%, 0.6%, and 0.8%, respectively All the specimens showed biocement to produce splitting tensile strength ranging from 48.1 to 88.6 kPa at 3.8%–4.7% of calcium carbonate content in Table The stress-strain curves for specimens with/ without PVA addition are shown in Fig The zero-PVA specimens were very brittle These specimens failed at strain levels of 0.68%–0.76% Similar observations were also made by other researchers (Mortensen et al 2011; Choi et al 2016a) However, the other plots reveal a pattern in which the failure strain levels tended to increase in relation to higher fiber ratio levels Similarly, © ASCE Fig Strain-stress behaviors of samples versus different fiber ratio the splitting tensile stress levels within the postpeak region also increased in relation to fiber ratio (Park 2011; Choi et al 2016a), additionally, samples in higher fiber ratio tended to induce higher residual strength in this study Relationship of calcium carbonate contents and splitting tensile strengths were plotted in Fig It illustrates the effect of calcium carbonate content and fiber ratio on tensile strength It can be seen from Fig 2(a) that the splitting tensile strength increases with additions in calcium carbonate content for tests with five different fiber ratios In general, the splitting tensile strength also increases with the increase in fiber ratio The average splitting tensile strength increased with fiber ratio as shown in Fig 2(b) The relationships between splitting secant elastic modulus and calcium carbonate content and fiber content are also shown in Fig The splitting secant elastic modulus is calculated using the method proposed by Ye et al (2009) The splitting secant elastic 06019007-2 J Mater Civ Eng., 2019, 31(9): 06019007 J Mater Civ Eng Downloaded from ascelibrary.org by Nanyang Technological University- Library on 11/06/19 Copyright ASCE For personal use only; all rights reserved Fig Relationships of calcium carbonate content, fiber ratio, and splitting tensile strength: (a) individual data; and (b) average of tensile strength by different fiber ratio Fig Relationships of calcium carbonate content, fiber ratio, and splitting secant elastic modulus: (a) individual data; and (b) average of splitting secant elastic modulus by different fiber ratio modulus (Es50 ) was calculated It can be seen that the splitting scant elastic modulus in general also increases by increasing the calcium carbonate content and fiber ratio Comparison and Analysis In an early study (Choi et al 2016a), the splitting tensile strength was reported to be in the range of 53–592 kPa for sand improved with fiber and biocement having a higher calcium carbonate content ranging from 4.8% to 10.4% By combining the data from the two studies, the influence of calcium carbonate content and fiber ratio on the splitting tensile strength over a wider calcium carbonate content range can be established In general, the splitting tensile strength increases with calcium carbonate content The effect of fiber content on the splitting tensile strength is only pronounced when the calcium carbonate content is relatively high, or in other words, when the splitting tensile strength is relatively high Thus, the proportions of the contributions by calcium carbonate content and fiber content are different depending on the slitting tensile strength achieved For example, for fiber reinforced, biocemented specimens with low calcium carbonate (less than 4%), the splitting tensile stress in © ASCE Fig Relationships of calcium carbonate, splitting tensile strength, and fiber ratio 06019007-3 J Mater Civ Eng., 2019, 31(9): 06019007 J Mater Civ Eng Downloaded from ascelibrary.org by Nanyang Technological University- Library on 11/06/19 Copyright ASCE For personal use only; all rights reserved Fig SEM for specimen with 0.8% CaCO3 : (a) 20 times magnification; and (b) 120 times magnification strain-stress curves dropped quickly after failure as reported by Choi et al (2016a) In this case, fiber does not seem to contribute much to the improvement of a sudden failure On the other hand, for fiber-reinforced, biocemented sand with high calcium carbonate content (10.4% or higher), the splitting tensile stress does not decrease so drastically after failure and therefore, the fiber may have contributed to the prevention of a sudden decrease in the postpeak tensile stress Combining the effects of calcium carbonate content and fiber ratio, the dependence of splitting tensile strength on both calcium carbonate content and fiber ratio is shown in Fig For low calcium carbonate content (4% or lower), the influence of fiber ratio on splitting tensile strength is much lower compared with that for high calcium carbonate content Microstructure PVA Fiber, Biocemented Sand Scanning electron microscope (SEM) images were taken for selected samples after drying at 60°C The SEM images for a sample with 0.8% fiber ratio CaCO3 at 20 and 120 times magnification are shown in Fig It can be seen that both the sand grains and fibers are coated with calcium carbonate, which also bound the sand grains as well as the fibers together and the PVA fibers are embedded in the sand grains to act as reinforcement The PVA fibers are also bound together by calcium carbonate and bridge across different sand grains as seen in Fig 5(b) These binding effects contribute toward the increase in the shear strength The calcium carbonate crystals were observed to be around 10 μm in Fig 5(b) Similar observations were also reported in Zhao et al (2014), Choi et al (2016b), and Cui et al (2017) Conclusions The splitting tensile strength and the elastic modulus of Ottawa sand treated with biocementation plus PVA fibers were studied using experiments The following conclusions can be drawn: The splitting tensile strength of biocemented sand increases with increasing calcium carbonate content Inclusion of PVA fiber also increases the splitting tensile strength However, the effect of fiber content on the splitting tensile strength is only pronounced when the calcium carbonate content is higher than 6% Within a calcium carbonate content range of 3.8%–4.7%, a fiber ratio of 0.4% appears to be optimal for the tested sand The use of PVA fiber increases the ductility of biocemented soil as indicated by an increase in the value of failure strain by 130% and the splitting elastic modulus by 133% © ASCE Acknowledgments This study forms part of a collaboration between Nanyang Technological University, Singapore, and Iowa State University We would like to acknowledge that part of this study is supported by the project “Biogrouting 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The splitting tensile strength of biocemented sand increases with increasing calcium carbonate content Inclusion of PVA fiber also increases the splitting tensile strength However, the effect of. .. Relationships of calcium carbonate content, fiber ratio, and splitting tensile strength: (a) individual data; and (b) average of tensile strength by different fiber ratio Fig Relationships of calcium... that both the sand grains and fibers are coated with calcium carbonate, which also bound the sand grains as well as the fibers together and the PVA fibers are embedded in the sand grains to act