Geotextiles and Geomembranes 27 (2009) 162–166 Contents lists available at ScienceDirect Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem Technical Note Effect of ﬁber reinforcement and distribution on unconﬁned compressive strength of ﬁber-reinforced cemented sand Sung-Sik Park* Division of Civil, Environmental and Urban Engineering, Wonkwang University, 344-2 Sinyongdong, Iksan, Jeonbuk 570-749, Republic of Korea a r t i c l e i n f o a b s t r a c t Article history: Received 31 March 2008 Received in revised form 8 September 2008 Accepted 9 September 2008 Available online 8 November 2008 A series of unconﬁned compression tests were carried out to examine the effect of ﬁber reinforcement and distribution on the strength of ﬁber-reinforced cemented sand (FRCS). Nakdong River sand, polyvinyl alcohol (PVA) ﬁber, cement and water were mixed and compacted into a cylindrical sample with ﬁve equal layers. PVA ﬁbers were randomly distributed at a predetermined layer among the ﬁve compacted layers. The strength of the FRCS increases as the number of ﬁber inclusion layers increases. A ﬁberreinforced specimen, where ﬁbers were evenly distributed throughout the ﬁve layers, was twice as strong as a non-ﬁber-reinforced specimen. Using the same amount of ﬁbers to reinforce two different specimens, a specimen with ﬁve ﬁber inclusion layers was 1.5 times stronger than a specimen with one ﬁber inclusion layer at the middle of the specimen. The ﬁber reinforcement and distribution throughout the entire specimen resulted in a signiﬁcant increase in the strength of the FRCS. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Fiber-reinforced sand Cemented sand Fiber distribution Unconﬁned compressive strength 1. Introduction Clay or granular soils are mixed with cement or gypsum to improve engineering properties such as cohesion and friction angle (Abdulla and Kiousis, 1997; Clough et al., 1981; Ismail et al., 2002; Kim et al., 2008; Liu et al., 2006; Saxena and Lastrico, 1978). Cemented soils can be used for the reinforcement of soft ground, the backﬁll materials of retaining walls, and for the sub-base of roads and railroads because cemented soils are less compressible and have an increased strength over that of natural soils. While such cemented soils can signiﬁcantly increase compressive strength with an increasing cement ratio, this causes a brittle or sudden failure without plastic deformation to occur. Cemented sand that shows a brittle failure pattern can be reinforced with ﬁber to prevent such brittle failure. Several researchers (Consoli et al., 1998; Consoli et al., 2003; Kaniraj and Havanagi, 2001; Maher and Ho, 1993; Tang et al., 2007) have attempted to study the combined effect of ﬁber and cement on granular or clayey soils. Such ﬁberreinforced cemented specimens were prepared for laboratory testing by manually mixing sand, cement and ﬁbers with water. For most of the experimental research that has been carried out on ﬁber-reinforced soils (Babu et al., 2008; Chauhan et al., 2008; Kaniraj and Gayathri, 2003), the ﬁbers were considered to be uniformly distributed throughout a small specimen. However, in practice the ﬁbers may be poorly distributed or may ﬂow into one * Tel.: þ82 63 850 6716; fax: þ82 63 850 7140. E-mail address: [email protected] 0266-1144/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2008.09.001 place when ﬁber-reinforced samples are prepared in the laboratory. While applying such ﬁber-reinforced materials in the ﬁeld, the ﬁber concentration or difﬁculties with ﬁber mixing could be signiﬁcant and the anticipated strength increase of ﬁber-reinforced materials may not be achieved. The objective of this study is to examine the effect of ﬁber distribution and concentration on the strength of ﬁber-reinforced cemented sand. A series of unconﬁned compression tests were performed on artiﬁcially cemented samples with layered ﬁber reinforcement. The unconﬁned compressive strength (UCS) of the samples was evaluated to quantify the uncertainties associated with ﬁber distribution or concentration in the strength of ﬁberreinforced soils. The strength of ﬁber-reinforced soils could be factored to consider the uncertainty induced by non-uniform ﬁber distribution, which can be applied to a reliable design (e.g., limit state design) of soil reinforcement. 2. Unconﬁned compression tests 2.1. Sand and cement Clean Nakdong River sand is used in this study and its gradation curve is shown in Fig. 1. As described in Table 1, an X-ray Fluorescence (XRF) analysis of Nakdong River sand shows that it consists of silica sand with more than 76% silica. A microphotograph of Nakdong River sand is shown in Fig. 2. The sand was sieved to a size ranging between 2 mm and 0.075 mm diameter and washed with a No. 200 sieve (0.075 mm in diameter) to remove ﬁnes. It was then oven-dried for testing. Ordinary Portland cement was used as S.-S. Park / Geotextiles and Geomembranes 27 (2009) 162–166 163 100 90 Percent finer (%) 80 70 60 50 40 30 20 10 0 10 1 0.1 0.01 Grain size (mm) Fig. 1. Grain size distribution of Nakdong River sand. a cementing agent for the preparation of artiﬁcially cemented samples. Fig. 2. Microphotograph of Nakdong River sand. 2.2. Fiber Polypropylene ﬁber is the most widely used inclusion in the laboratory testing of soil reinforcement (e.g., Khattak and Alrashidi, 2006; Santoni et al., 2001; Tang et al., 2007; Viswanadham et al., 2009; Yetimoglu and Salbas, 2003; Yetimoglu et al., 2005). This ﬁber is lighter than water, and weak in alkalis. Polyvinyl alcohol (PVA) ﬁber is a synthetic ﬁber that has recently been used in ﬁber-reinforced concrete, since its weather resistance, chemical resistance, and tensile strength are superior to that of polypropylene ﬁber. PVA ﬁber has a signiﬁcantly lower shrinkage from heat than nylon or polyester. It has a speciﬁc gravity of 1.3, a good adhesive property to cement, and high anti-alkali characteristics. For this reason, it is suitable for mixing with cement and is widely used in concrete and cement reinforcement. The type of PVA ﬁber used in this study is RECS100L (produced by Kuraray in Japan) with a tensile strength of 1078 N/mm2. The PVA ﬁber shown in Fig. 3 is 12 mm in length and 0.1 mm in diameter, and is thicker than PVA ﬁber normally used for reinforced cement. three ﬁber-reinforced specimens (Case 1, Case 2, and Case 3) were prepared for testing, as illustrated in Fig. 4. The number of ﬁber inclusion layers for each specimen varied: zero (no ﬁber), one (middle layer only), three (middle, top and bottom layers), and ﬁve (all layers). Depending on the number of ﬁber inclusion layers, the degree of ﬁber reinforcement or distribution was determined as 0%, 20%, 60%, and 100%. One of the difﬁculties in preparing ﬁber-reinforced specimens is the method used to mix ﬁbers into a non-plastic soil such as sand (Freitag, 1986). Cemented sand was prepared by mixing dry sand and cement with an optimum water content of 14%. The cemented sand was then divided into ﬁve portions and each portion was mixed by hand with a predetermined amount of ﬁber. Therefore, 2.3. Sample preparation The cement ratio is based on dry sand and deﬁned as follows: rc ¼ Wc 100ð%Þ Ws (1) where Wc is the weight of cement and Ws is the weight of dry sand. The cement ratio used in this study is 4%. The ﬁber ratio is based on the amount of ﬁber used in each ﬁber inclusion layer. It is deﬁned as follows: rf ¼ Wf 100ð%Þ Ws (2) where Wf is the weight of ﬁber used in each ﬁber inclusion layer, not in the entire specimen. One non-ﬁber-reinforced specimen and Table 1 X-ray Fluorescence analysis of Nakdong River sand. Classiﬁcation Content (%) SiO2 76.62 Al2O 12.37 CaO 1.65 K2O 4.19 Na2O 2.88 Fe2O3 1.98 MgO – P2O5 – TiO2 0.28 Fig. 3. PVA ﬁber RECS100L (diameter 0.1 mm). 164 S.-S. Park / Geotextiles and Geomembranes 27 (2009) 162–166 a b c 1000 140 mm Layer 4 Cemented sand Layer 3 Fiber-reinforced cemented sand Layer 2 Layer 1 70 mm Case 1 Case 2 Case 3 Fig. 4. Fiber-reinforced specimen. a uniform distribution of ﬁbers throughout the specimen was achieved for laboratory testing. However, the hand mixing method will not be applicable to large-scale work in the ﬁeld. After mixing, each portion was compacted directly into a small-sized mold by applying 55 blows per layer via a metal rammer that has a 65 mm diameter circular face with a weight of 22 N. The metal mold was separated and then the specimen was cured for 7 days. Each specimen was 70 mm in diameter and 140 mm in height. Unconﬁned compression tests were carried out at a shearing rate of 1%/ min (1.4 mm/min). 3. Results of unconﬁned compression tests Two series of unconﬁned compression tests were carried out to investigate the strength of ﬁber-reinforced cemented sand (FRCS). Each series consisted of three ﬁber-reinforced cases (Case 1, Case 2, and Case 3), as shown in Table 2. The aim of the ﬁrst series, the L series, was to evaluate the effect of ﬁber reinforcement on an UCS. In the L series, the ﬁber ratio of rf is constant at 0.2%. Fig. 5 shows the results of the L series where the number of ﬁber inclusion layers varied when using the same amount of ﬁbers. As the degree of ﬁber reinforcement increases, the UCS signiﬁcantly increases, as summarized in Table 3. The 100% (fully reinforced) ﬁber-reinforced specimen was twice as strong as the 0% ﬁber-reinforced specimen, as shown in Fig. 6 and Table 3. The aim of the second series, the T series, was to evaluate the effect of ﬁber distribution on an UCS. The effect of ﬁber distribution or concentration on the UCS was accessed by increasing the ﬁber inclusion layer and by using the same amount of ﬁbers in the entire specimen. In the T series, the rf of Case 1, Case 2, and Case 3 is 1%, 0.33%, and 0.2%, respectively. However, the total amount of ﬁbers used in the entire specimen is 1% of the weight of dry sand and constant in all cases. L-3 and T-3 are essentially the same conditions and their results show repeatability of test results. Fig. 7 shows the results of the T series. As the degree of ﬁber distribution increases, Table 2 Summary of unconﬁned compression tests. Specimen Reinforced layer Fiber ratio, rf Strength (kPa) Secant elastic modulus (MPa) No ﬁber No ﬁber No ﬁber 440 37 L series L-1 L-2 L-3 Layer 3 Layer 1, 3, 5 All layers 0.2% 0.2% 0.2% 530 696 857 47 35 41 T series T-1 T-2 T-3 Layer 3 Layer 1, 3, 5 All layers 1% 0.33% 0.2% 609 741 894 31 35 40 Unconfined compressive stress (kPa) Layer 5 800 600 400 No fiber 200 L-1 L-2 L-3 0 0 1 2 3 4 5 Axial strain (%) Fig. 5. Results of L series. the UCS signiﬁcantly increases, as summarized in Table 4. The 100% (fully distributed) ﬁber-distributed specimen was 1.5 times stronger than the 20% ﬁber-distributed specimen, as shown in Fig. 8 and Table 4. The same cases in each series (L-1 and T-1, L-2 and T-2, L-3 and T-3) have the same degree of ﬁber reinforcement and distribution of 20%, 60%, and 100%. With the exception of Case 3, the rf of the T series is greater than that of the L series. By making a comparison between the same cases in each series, it was conﬁrmed that, for the effect of the amount of ﬁber on the UCS, its strength increased with an increase in ﬁber content (Ang and Loehr, 2003; Kaniraj and Havanagi, 2001; Santoni et al., 2001; Tang et al., 2007). The secant elastic modulus of each specimen was calculated from one half of the axial strain at peak strength. As shown in Table 2, the comparison shows that it does not seem to be inﬂuenced by the ﬁber reinforcement and distribution. Park et al. (2008) conducted unconﬁned compression tests on fully reinforced or distributed (Case 3 in this study) cemented specimens using four different cement ratios. Test results demonstrated that a secant elastic modulus depended on a cement ratio. Fig. 9 shows photographs of the non-ﬁber-reinforced specimen and other specimens in the L series after failure. In the non-ﬁber-reinforced specimen, a vertical crack throughout the entire specimen appeared after failure, as shown in Fig. 9(a). In the ﬁber-reinforced specimens, cracks began to appear in the non-ﬁber inclusion layer and spread to the ﬁber inclusion layers, as shown in Fig. 9(b)–(d). The top plate slightly penetrated into the specimen at the end of the tests. This may not have signiﬁcantly inﬂuenced the UCS of the specimen because it occurred after peak stress. Such penetration of the plate may be Table 3 Results of L series. Specimen Degree of ﬁber reinforcement (%) Strength (kPa) Ratio of strength increase (%) No Fiber L-1 L-2 L-3 0 20 60 100 440 530 696 857 100 120 158 195 200 200 180 180 Strength Increase (%) Strength Increase (%) S.-S. Park / Geotextiles and Geomembranes 27 (2009) 162–166 160 140 120 100 160 140 120 0 20 40 60 80 100 100 0 20 40 60 Degree of Fiber Reinforcement (%) Degree of Fiber Distribution (%) Fig. 6. Effect of ﬁber reinforcement in L series. Fig. 8. Effect of ﬁber distribution in T series. 1000 Unconfined compressive stress (kPa) 165 80 100 concentrated ﬁbers. From a practical point of view, a ﬁber mixing method, which can control the uniformity of ﬁber distribution throughout the specimen, is very important especially when applying ﬁber-reinforced soils in the ﬁeld. 800 600 400 No fiber 200 T-1 T-2 T-3 0 0 1 2 3 4 5 Axial strain (%) Fig. 7. Results of T series. related to the ductile behavior of ﬁber-reinforced soils. It could be partially attributed to the non-conﬁnement of a specimen in an unconﬁned compression test. The effect of conﬁnement due to rough plates may be insigniﬁcant especially in unconﬁned compression tests because there is no conﬁnement in the horizontal direction. In summary, the strength of the FRCS depends on the degree of ﬁber reinforcement and distribution. The UCS of cemented sands reinforced with evenly distributed ﬁbers can be up to 1.5 times greater than those reinforced with poorly distributed or Table 4 Results of T series. Specimen Degree of ﬁber distribution (%) Strength (kPa) Ratio of strength increase (%) T-1 T-2 T-3 20 60 100 609 741 894 100 122 147 Fig. 9. Specimens after failure: (a) non-ﬁber-reinforced specimen; (b) L-1 specimen; (c) L-2 specimen; (d) L-3 specimen. 166 S.-S. Park / Geotextiles and Geomembranes 27 (2009) 162–166 4. Conclusions The effect of ﬁber reinforcement and distribution on the strength of the FRCS was investigated in terms of an UCS. PVA ﬁbers were randomly reinforced at a predetermined layer among ﬁve compacted layers. The UCS of ﬁber-reinforced cemented specimens gradually increases as the number of ﬁber inclusion layers increases. The specimen where ﬁbers were evenly distributed at ﬁve layers was twice as strong as the non-ﬁber-reinforced specimen. When the same amount of ﬁbers was reinforced in the entire specimen, the specimen with ﬁve ﬁber inclusion layers was 1.5 times stronger than the specimen with one layer at the middle. The strength ratio of 1.5 is a factor that can consider the strength reduction of the FRCS induced by non-uniform ﬁber distribution. The strength of ﬁber-reinforced cemented soils depends not only on the degree of ﬁber reinforcement and distribution but also on the amount of ﬁber used in the specimen. These results can be used to quantify the uncertainties associated with ﬁber distribution or concentration in the strength of ﬁber-reinforced soils. Using ﬁber-reinforced soils does not necessarily guarantee an anticipated strength increase or a safe design unless the uncertainties associated with the ﬁber distribution or concentration are adequately considered. Acknowledgement This paper was supported by Wonkwang University in 2008. References Abdulla, A.A., Kiousis, P.D., 1997. Behavior of cemented sands – I. Testing. International Journal for Numerical and Analytical Methods in Geomechanics 21 (8), 533–547. Ang, E.C., Loehr, J.E., 2003. Specimen size effects for ﬁber-reinforced silty clay in unconﬁned compression. Geotechnical Testing Journal 26 (2), 1–10. Babu, G.L.S., Vasudevan, A.K., Haldar, S., 2008. Numerical simulation of ﬁber-reinforced sand behavior. Geotextiles and Geomembranes 26 (2), 181–188. Chauhan, M.S., Mittal, S., Mohanty, B., 2008. Performance evaluation of silty sand subgrade reinforced with ﬂy ash and ﬁbre. Geotextiles and Geomembranes 26 (5), 429–435. Clough, G.W., Sitar, N., Bachus, R.C., Rad, N.S., 1981. Cemented sands under static loading. Journal of the Geotechnical Engineering Division 107 (GT6), 799–817. Consoli, N.C., Prietto, P.D.M., Ulbrich, L.A., 1998. Inﬂuence of ﬁber and cement addition on behavior of sandy soil. Journal of Geotechnical and Geoenvironmental Engineering 124 (12), 1211–1214. Consoli, N.C., Vendruscolo, M.A., Prietto, P.D.M., 2003. Behavior of plate load tests on soil layers improved with cement and ﬁber. Journal of Geotechnical and Geoenvironmental Engineering 129 (1), 96–101. Freitag, D.R., 1986. Soil randomly reinforced with ﬁbers. Journal of Geotechnical Engineering 112 (8), 823–826. Ismail, M.A., Joer, H.A., Sim, W.H., Randolph, M.F., 2002. Effect of cement type on shear behavior of cemented calcareous soil. Journal of Geotechnical and Geoenvironmental Engineering 128 (6), 520–529. Kaniraj, S.R., Gayathri, V., 2003. Geotechnical behavior of ﬂy ash mixed with randomly oriented ﬁber inclusions. Geotextiles and Geomembranes 21 (3), 123–149. Kaniraj, S.R., Havanagi, V.G., 2001. Behavior of cement-stabilized ﬁber-reinforced ﬂy ash–soil mixtures. Journal of Geotechnical and Geoenvironmental Engineering 127 (7), 574–584. Kim, Y.T., Kim, H.J., Lee, G.H., 2008. Mechanical behavior of lightweight soil reinforced with waste ﬁshing net. Geotextiles and Geomembranes 26 (6), 512–518. Khattak, M.J., Alrashidi, M., 2006. Durability and mechanistic characteristics of ﬁber reinforced soil–cement mixtures. International Journal of Pavement Engineering 7 (1), 53–62. Liu, H., Deng, A., Chu, J., 2006. Effect of different mixing ratios of polystyrene prepuff beads and cement on the mechanical behaviour of lightweight ﬁll. Geotextiles and Geomembranes 24 (6), 331–338. Maher, M.H., Ho, Y.C., 1993. Behavior of ﬁber-reinforced cemented sand under static and cyclic loads. Geotechnical Testing Journal 16 (3), 330–338. Park, S.-S., Kim, Y.-S., Choi, S.-G., Shin, S.-E., 2008. Unconﬁned compressive strength of cemented sand reinforced with short ﬁbers. Journal of the Korean Society of Civil Engineers 28 (4C), 213–220. Santoni, R.L., Tingle, J.S., Webster, S.L., 2001. Engineering properties of sand–ﬁber mixtures for road construction. Journal of Geotechnical and Geoenvironmental Engineering 127 (3), 258–268. Saxena, S.K., Lastrico, R.M., 1978. Static properties of lightly cemented sand. Journal of the Geotechnical Engineering Division 104 (GT12), 1449–1464. Tang, C., Shi, B., Gao, W., Chen, F., Cai, Y., 2007. Strength and mechanical behavior of short polypropylene ﬁber reinforced and cement stabilized clayey soil. Geotextiles and Geomembranes 25 (3), 194–202. Viswanadham, B.V.S., Phanikumar, B.R., Mukherjee, R.V., 2009. Swelling behaviour of a geoﬁber-reinforced expansive soil. Geotextiles and Geomembranes 27 (1), 73–76. Yetimoglu, T., Salbas, O., 2003. A study on shear strength of sands reinforced with randomly distributed discrete ﬁbers. Geotextiles and Geomembranes 21 (2), 103–110. Yetimoglu, T., Inanir, M., Inanir, O.E., 2005. A study on bearing capacity of randomly distributed ﬁber-reinforced sand ﬁlls overlying soft clay. Geotextiles and Geomembranes 23 (2), 174–183.