Effect of fiber reinforcement and distribution on unconfined compressive strength of fiber-reinforced cemented sand

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 fiber-reinforced 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.     

Effect of fiber-reinforcement on the strength of cemented soils

This study aims to verify the differences in the strength of an artificially cemented sandy soil with and without fiber reinforcement. The controlling parameters evaluated were the amount of cement, porosity, moisture content, and voids/cement ratio. A series of unconfined compression tests and suction measures were carried out. The results show that fiber insertion in the cemented soil, for the whole range of cement studied, causes an increase in unconfined compression strength. The UCS increased linearly with the amount of cement and reduced with the increase in porosity (η) for both the fiber-reinforced and unreinforced specimens. A power function fits well as the relation between unconfined compressive strength (UCS) and porosity (η). Finally, it was shown that the voids/cement ratio is a good parameter in the evaluation of the unconfined compressive strength of the fiber-reinforced and unreinforced cemented soil studied.     

Fiber reinforcement effects on sand considering a wide cementation range

This paper describes laboratory drained standard triaxial tests conducted on artificially cemented Osorio sand specimens reinforced with randomly oriented discrete extensible polypropylene fibers. Cemented specimens were prepared with cement contents varying from 0% to 10% by weight of dry sand and cured for seven days. Fiber length and diameter were 24 mm and 0.023 mm, respectively, in the contents of 0% and 0.5% by weight of dry sand–cement mixture. Test results indicated that the addition of cement to sand increases stiffness, peak strength and brittleness. Both cement and fiber insertions affect dramatically the stress–dilatancy behavior of the sand. The fiber reinforcement increases peak strength just up to a certain cement content (up to about 5% in the present study), increases ultimate strength, decreases stiffness and changes the cemented sand brittle behavior to a more ductile one. The triaxial peak strength increase due to fiber inclusion is more effective for smaller amounts of cement, while the increase in ultimate strength is more efficacious when fiber is added to sand improved with higher cement contents. Peak strength envelopes indicate that the friction angle is about 46° for fiber-reinforced specimens containing up to 7% cement content, reaching 51.5° for higher cement contents. Cohesion intercept is drastically affected due to fiber addition to all cement contents, increasing for cement contents up to 4% and reducing for higher cement contents. It is important to make clear that the trends observed herein are relevant for the soil, cement and fiber type used in the present research and that further studies are necessary to generalize such findings.     

Evaluation of natural and artificial fiber reinforcements on the mechanical properties of cement-stabilized dredged sediment

The combination of chemical stabilization and fiber reinforcement can simultaneously improve the strength and ductility of dredged sediment. The polypropylene fiber (PF) and straw fiber (SF) were respectively used as artificial and natural reinforcements of cement-stabilized dredged sediment (CDS). A series of unconfined compressive strength (UCS) tests were conducted to investigate the effects of cement content, fiber content, fiber length and water content on the mechanical properties of PF-reinforced CDS (CPFDS) and SF-reinforced CDS (CSFDS). Furthermore, the cementation-reinforcement mechanism was explored and analyzed via macro failure characteristics and micro interfacial morphologies inside typical CPFDS and CSFDS samples. The results showed that increasing cement content or decreasing water content significantly improved the UCS and aggravated the brittleness of CPFDS and CSFDS. The suitable addition of PF can effectively improve the UCS of CDS, while incorporating SF exhibited the opposite role. The maximum 7d-, 28d-, 60d- and 90d-UCS of CPFDS were respectively 17.7%, 43.6%, 10.7% and 9.7% higher than that of CDS. The optimum length of PF inside CPFDS and SF inside CSFDS was 3 mm and 5–10 mm, respectively. Both incorporating PF or SF can effectively improve the ductility of CDS. Based on the proposed parameters of total-water/cement ratio and fiber cementation factor, the effective strength development models of CPFDS and CSFDS considering cement content, fiber content, water content and curing time were empirically established. The fiber “bridge” effect and interfacial friction between fiber and cemented soil particles were mainly responsible for the strength evolution and ductility improvement of CPFDS and CSFDS.     

Effects of freeze-thaw cycles on the unconfined compressive strength of straw fiber-reinforced soil

Although natural fibers can improve the strength behavior of frozen-thawed soil, the reinforcing mechanism is still not fully understood. To investigate the effects of freeze-thaw cycles on the strength of natural fiber-reinforced soil, unconfined compression tests, single-fiber pull-out tests and scanning electron microscopy (SEM) tests under 0, 3, 5, 10, 15, and 20 freeze-thaw cycles were conducted on cotton straw fiber-reinforced soil. It was found that the unconfined compressive strength (UCS) of fiber-reinforced soil decreases exponentially with the number of freeze-thaw cycles. In addition, fiber reinforcement weakens the softening degree of frozen-thawed soil under unconfined states. The UCS reduction in fiber-reinforced soil under freeze-thaw conditions is smaller than the strength reduction at the fiber-soil interface because fiber reinforcement is mainly governed not only by the fiber-soil interface but also by the spatial stress network established by discrete fibers. The complex spatial stress network, which improves the reinforcement of the fibers, is monitored by SEM after freeze-thaw cycles.     

Fibre-reinforced cemented soils compressive and tensile strength assessment as a function of filament length

This study aims to develop a dosage methodology based on tensile and compressive strength for artificially cemented fibre reinforced soils considering filament length. The controlling parameters evaluated were the fibre length (l), the cement content (C), the porosity (η) and the porosity/cement ratio (η/C_iv). A number of unconfined compression and split tensile tests were carried out in the present work. The results show that fibre insertion in the cemented soil, for the whole range of cement content studied, and the increase of reinforcement length improve unconfined compressive and split tensile strengths. It was shown that the porosity/cement ratio, in which volumetric cementitious material content is adjusted by an exponent (0.28 for all the fibre-reinforced and non-reinforced cemented soil mixtures) to end in unique correlations for each mixture, is a good parameter in the evaluation of the unconfined compressive and split tensile strength of the fibre-reinforced and non-reinforced cemented soil studied. Analysis of variance (ANOVA) performed on the results of a factorial experiment considering the effect of adjusted cement content, fibre length and porosity showed that all of these factors are significant in affecting the measured changes in split tensile and unconfined compressive strength values. Finally, unique dosage relationships could be achieved linking the unconfined compressive strength (q_u) and the split tensile strength (q_t) of the sandy soil studied with porosity/cement ratio (η/C_iv) and fibre length (l).     

Strength characterization of cohesionless soil treated with cement and polyvinyl alcohol

Lime, cement, and bitumen are well-known traditional binders for improving the bearing capacity of soils. However, the production of these binders results in a massive impact on the environment due to the emission of greenhouse gases, such as carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). In this study, a novel cement–polyvinyl alcohol (PVA) mixture is proposed to fabricate strong composite geomaterials. The advantage of the proposed materials is that they can increase the unconfined compressive strength (UCS) and, combined with cement hydration, producing PVA glue, can be used to fill up the soil pores. Laboratory tests indicate a threefold increase in UCS with the cement–PVA-combined mixture compared to a cement-stabilized one. The results of scanning electron microscope (SEM) observations suggest that the cement–PVA composite can ameliorate the pore structure that is more solid than the cement-stabilized one. Moreover, by curing at 80 °C, the strength of the cement–PVA stabilized soil decreases by threefold, plateauing at the same strength as the non-PVA stabilized soil. In addition, the results of cyclic thermal exposure tests suggest that, with the increase in the number of heating/cooling cycles, the UCS gradually decreases compared to the initial one. However, the loss of UCS is less than 25 % under the three cycles of exposure. Thus, these composites have the potential to promote urban renewal projects in an ecofriendly manner.     

Behavior of strip footing on fiber-reinforced cemented sand adjacent to sheet pile wall

In urban areas, shallow foundations are often placed along the ground surface above a sheet pile wall. In this research, the potential benefits of reinforcing the active zone behind a model sheet pile wall by using polypropylene fiber and cement kiln dust have been investigated experimentally and numerically. Tests were conducted by varying parameters including fiber ratio (R_F), cement kiln dust (CKD) ratio, thickness of reinforced layer, footing location relative to the sheet pile wall and curing time of reinforced layer. Finite element computer code PLAXIS 2D foundation was used for numerical modeling. Close agreement between the experimental and numerical results was observed (maximum difference 14%). Experimental and numerical results clearly show that fiber insertion into the cemented soil causes an increase in ultimate bearing capacity of footing and significant reduction in the lateral deflection of the sheet pile wall. At higher fiber ratios (R_F ≥ 0.75%), the bearing capacity ratio (BCR) increased by about 42% and the effect of CKD ratio on BCR is more pronounced. The addition of fibers changed the brittle behavior of cemented sand to a more ductile one. Critical values of reinforcing parameters for maximum reinforcing effects are established.     

编织袋袋装水泥红土的力学性能试验研究

通过室内抗压试验,研究了在不同模型袋类型、水泥掺入比、养护龄期以及不同含水量下编织袋袋装水泥红土的力学性能,并获得了两种编织袋袋装水泥红土材料的应力—应变全过程曲线。在对试验结果进行回归分析后,给出了不同水泥掺入比、不同龄期下的复合编织袋袋装水泥红土之间的强度推算公式。为了深入研究,同时进行了相同条件下水泥红土的平行对比试验。     

Using fiber and liquid polymer to improve the behaviour of cement-stabilized soft clay

This work studies the effect of using two types of polymer (fibers and liquid) to enhance the strength of cemented soft clay. Four polymer contents were used (0, 0.1, 0.2, 0.5 and 1% by dry weight of the soil) to investigate the unconfined compressive strength, q_un of soft clay mixed with three cement contents (5, 10 and 15%). For wide understanding to the polymer/cemented soil behaviour several factors were considered in this study such as curing time, dry unit weight, the mixture workability and the behaviour after disturbance. This investigation revealed that both fibers and liquid polymers can improve the cemented soft clay strength, however the fiber mechanism in improving the mixture is totally different than the liquid. Increasing the fiber content shall increase q_un till a peak point at fiber content of 0.5%, where the strength started to reduce after. The mixture workability has been improved with increasing the liquid polymer content, and reduced with increasing the fiber content. Fibers can be used to raise up the strength of disturbed cemented soft clay up to 240% by using fiber content of 0.5%. Both fibers and liquid polymers showed a remarkable mechanically, economically and environmentally dominance to be used as additive to cement in improving the soft clay.     

纤维橡胶砂浆的力学性能与砌体轴压试验

为提高橡胶砂浆的力学性能和适用性,通过单掺聚乙烯醇(PVA)纤维或钢纤维、复掺PVA-钢纤维对橡胶砂浆进行改性,探讨其对超声波传播速度、抗折强度和抗压强度的影响。将普通水泥砂浆、20%橡胶砂浆、复掺PVA-钢纤维橡胶砂浆分别与环保压缩砖结合制作砌体,通过试验研究轴心抗压强度和弹性模量。结果表明:掺入PVA纤维或复掺PVA-钢纤维降低了超声波在橡胶砂浆中的传播速度; 掺入PVA纤维或钢纤维提高了橡胶砂浆的抗折强度; 复掺一定量的PVA纤维和钢纤维能有效提高橡胶砂浆的抗压强度; 20%橡胶砂浆的抗压强度比普通水泥砂浆降低了46%,对应砌体的破坏荷载降低了12.8%; 复掺PVA-钢纤维橡胶砂浆的抗压强度比20%橡胶砂浆提高了14.9%,对应砌体的开裂荷载和破坏荷载分别提高了63.4%和4.5%; 掺入适量PVA纤维和钢纤维能改善橡胶砂浆的强度明显低于普通水泥砂浆的缺点,将复掺PVA-钢纤维橡胶砂浆应用于砌体工程能改善结构的隔音性,延缓试件的开裂,提高橡胶砂浆砌体的抗压强度。     

Evaluation of compressibility and small strain stiffness characteristics of sand reinforced with discrete synthetic fibers

This experimental investigation evaluates the compressibility and small strain stiffness of sand reinforced with discrete synthetic fibers. Varying fiber contents (FC), fiber aspect ratios (AR), and void ratios were selected as testing variables in this study, and the modified oedometer tests were conducted to measure the compression index (C_c) and maximum shear modulus (G_max) of fiber-reinforced sand. The results of this study demonstrate that the C_c of the tested fiber-reinforced sand increases with an increase in FC because the packing of sand grains in the fiber-reinforced sand is very loose due to a disruption of direct contact between the sand grains due to the presence of long discrete fibers. Additionally, this disruption of direct contact between sand grains due to the fibers results in a reduction of interparticle contact and coordination number between sand grains. Therefore, the G_max of tested fiber-reinforced sand decreases with an increase in FC. Most notably, the G_max of the tested fiber-reinforced sand with varying FC and AR can be expressed as a single function of the void ratio at a given applied stress, which implies that the inclusion of fibers just alters the packing state of sand grains, and the interparticle contact stiffness is mainly determined by the contacts between sand grains.     

A constitutive model for evaluation of mechanical behavior of fiber-reinforced cemented sand

The aim of this study is to present a constitutive model for prediction of the mechanical behavior of fiber-reinforced cemented sand. For this purpose, a generalized plasticity constitutive model of sandy soil is selected and the parameters of the model are determined for three types of sandy soils using the results of triaxial tests. Next, the proposed model is developed using the existing models based on the physico-mechanical characteristics of fiber-reinforced cemented sand. The elastic parameters, flow rule and hardening law of the base model are modified for fiber-reinforced cemented sand. To verify the proposed model, the predicted results are compared with those of triaxial tests performed on fiber-reinforced cemented sand. Finally, the efficiency of the proposed model is studied at different confining pressures, and cement and fiber contents.     

Modelling tensile/compressive strength ratio of artificially cemented clean sand

The present work proposes a new theoretical model for predicting both the splitting tensile strength (q_t) and the compressive strength (q_u) of artificially cemented sand and assesses their ratio for a given material. The proposed model is based on the concept of the superposition of the failure strength contributions of the sand and cement phases. The sand matrix obeys the concept of critical state soil mechanics, while the strength of the cemented phase can be described using the Drucker-Prager failure criterion. The analytical solutions are compared against the results of tests on three different types of cemented clean sand cured for different time periods. While the analytical relation fits the experimental data well, it also provides a theoretical basis for the explanation of some features related to the experimentally derived strength relationships for cemented clean sand. The value of the power relationship between the strength and the porosity/cement ratio index seems to be governed by the soil matrix properties, while the interdependency of the strength and the curing time can also be captured. For a given cemented sand, the model equally confirms the existence of a unique tensile/compressive strength ratio (q_t/q_u), independent of the curing time and primarily governed by the compressive to tensile strength ratio (or the friction properties) of the cement. It is also confirmed that the q_t/q_u ratio changes within a narrow range for different frictional properties of the cementing phase.     

A numerical investigation into the response of free end tubular composite poles subjected to axial and lateral loads

This paper presents a numerical investigation of cantilevered glass fiber-reinforced polymer (GFRP) tubular poles subjected to lateral and axial loads. A 3D finite element analysis was conducted to establish the lateral load–deflection responses under different axial loads and the axial load–bending moment interaction curves at ultimate. The model accounts for geometric nonlinearities and the composite laminate structure. Failure modes were established based on either material failure according to the Tsai-Wu failure criterion, or stability failure. The model was validated by using experimental results. A parametric study was then carried out on poles with various angle-ply and cross-ply laminates as well as different diameter-to-thickness (D/t) and length-to-diameter (L/D) ratios. The study showed that the reduction in axial strength as (L/D) ratio increases becomes more severe as (D/t) ratio is reduced. The GFRP laminate structure has a considerable effect on axial and flexural strengths of the poles for certain (D/t) ratios. It was also shown that axial load–moment interaction curves are generally linear. Increasing the fraction of longitudinal fibers in cross-ply laminates or reducing the fiber angle with the longitudinal direction in angle-ply laminates results in a larger interaction curve. A simplified design approach for the poles has been proposed.     

水泥土搅拌桩止水帷幕工程特性研究

水泥土搅拌桩作为基坑止水帷幕已经得到了广泛应用,为了更深入的理解作为止水帷幕的水泥土的工程特性,通过对不同水泥掺入量的水泥土无侧限抗压强度和渗透系数的室内试验研究,利用CBR-1承载比试验仪和TSS-2柔性壁三轴渗透仪对水泥土进行了无侧限抗压强度和渗透试验,分析了养护龄期及水泥掺入量对水泥土的无侧限抗压强度和渗透系数的影响。试验结果表明,水泥土的无侧限抗压强度随养护龄期和水泥掺入量的增大而增大,并通过曲线的拟合,得出了无侧限抗压强度的预测公式;渗透系数随养护龄期和水泥掺入量的增大而减少,通过数据对比得出28天之后水泥土渗透系数主要是受水泥掺入量的影响。     

Verification of unified effective stress theory based on the effect of moisture on mechanical properties of fiber reinforced unsaturated soil

The unified effective stress theory based on suction stress (SSCC theory) enables the characterization of soils under both saturated and unsaturated conditions with one closed-form relationship. This study provides experimental verification of this theory through the unconfined compressive strength test (UCS) and indirect tensile test strength (ITS) on silty clay soil stabilized with fiber. A series of matric suction, ITS, and UCS tests were conducted to validate the SSCC theory through the representation of the results of ITS and UCS tests in terms of mean total stress (p) versus deviatoric stress (q) and mean effective stress (p`) versus deviatoric stress (q). The results of the validation procedures showed that the SSCC theory is applicable and valid at a range of 6%–16% of water content on the silty clay and the silty clay fiber-reinforced soils. There is a small fluctuation in the increase of ITS and UCS values with increasing fiber content due to randomly oriented distribution of the fiber. The addition of glass fiber does not significantly affect the capacity of water retention of the soil. It improves the condition of the mechanical soil properties at the end of construction more than of the effective stress condition.     

Strength properties of soft clay treated with mixture of nano-SiO_2 and recycled polyester fiber

This paper investigates the effect of recycled polyester fiber, produced from polyethylene(PET) bottles, in combination with nano-Si O2 as a new stabilizer to improve the mechanical properties of soils. We intend to study the effect of adding nano-Si O2 and recycled polyester fiber on soil engineering properties,especially the shear strength and unconfined compressive strength(UCS), using clayey soil with low liquid limit. Three different combinations of fiber-soil ratios ranging between 0.1% and 0.5% as well as three different combinations of nano-soil ratios ranging between 0.5% and 1% are used. The shear strength and UCS of treated specimens are obtained from direct shear test and unconfined compression test, respectively. Results of this study show that the addition of recycled polyester fiber and nano-Si O2 increases the strength of soil specimens. Both the shear strength and UCS are improved by increasing the contents of recycled polyester fiber and nano-Si O2 in the soil mixture. The increase in the nano-Si O2 content leads to a reduction in failure strain, but the increase in the content of recycled polyester fiber leads to an increase in failure strain. The increase in the contents of recycled polyester fiber and nanoSi O2 leads to an increase in elastic modulus of soils. Based on the test results, the addition of recycled polyester fiber improves the mechanical properties of soils stabilized with nano-Si O2 as well as the recycled polyester fiber has a positive effect on soil behaviors.     

Ca(OH)2对低掺量水泥土的强度影响

用低掺量水泥加固3种不同的土进行室内试验研究,测试了不同Ca(OH)2掺量及不同龄期下3种水泥土的无侧限抗压强度。分析了随Ca(OH)2掺量的增加,不同龄期的3种水泥土无侧限抗压强度变化规律及原因。试验结果表明：水泥红粘土强度随Ca(OH)2掺量的增加提高最为明显,粉质粘土次之,砂土最弱。分析原因是由于土体的细度对水泥土强度影响较大。土体越细,土体中粘土矿物越多, Ca(OH)2掺量的增加促进了更多的离子交换作用和火山灰作用的发生,从而提高了水泥土强度。试验所用的3种土中红粘土最细,所以水泥红粘土强度随Ca(OH)2掺量的增加提高最为明显。     

Soil improvement using plant-derived urease-induced calcium carbonate precipitation

This study addresses a soil improvement technique using plant-derived urease-induced calcium carbonate (CC) precipitation (PDUICCP) as an alternative to microbially induced carbonate precipitation (MICP). A crude extract of crushed watermelon (Citrullus lanatus) seeds was used as the urease source along with calcium chloride (CaCl₂) and urea (CO (NH₂)₂) for CC precipitation. Test specimens (φ?=?2.3?cm, h?=?7.1?cm) made from commercially available Mikawa sand (mean diameter, D₅₀?=?870?µm) were cemented, and estimated unconfined compressive strength (UCS) of several kPa to MPa was obtained by changing the concentration of CaCl₂- urea, urease activity, curing time, and temperature. The increase of curing time and that of the CaCl₂-urea concentration from 0.3?M to 0.7?M caused an increase in estimated UCS value. The average estimated UCS obtained after 14?days’ curing time for 0.7?M CaCl₂-urea and 3.912 U/mL urease was around 3.0?MPa and for 0.3 and 0.5?M CaCl₂-urea and 0.877 U/mL urease, it was around 1.5–2.0?MPa at 25?°C. By changing each of the abovementioned parameters, it may be possible to apply this method for strength improvement of loose sand, to mitigate the liquefaction, protection and restoration of limestone monuments and statuaries, and artificial soft rock formations. Crude urease from crushed watermelon seeds has the potential to replace commercially available urease for carbonate precipitation and for use as a low environmental impact type soil improvement method.