Tensile strength behavior of cement-stabilized dredged sediment reinforced by polypropylene fiber

Abstract

This study evaluated the feasibility of using polypropylene fiber (PF) as reinforcement in improving tensile strength behavior of cement-stabilized dredged sediment (CDS). The effects of cement content, water content, PF content and length on the tensile strength and stress–strain behavioral evolutions were evaluated by conducting splitting tensile strength tests. Furthermore, the micro-mechanisms characterizing the tensile strength behavior inside PF-reinforced CDS (CPFDS) were clarified via analyzing macro failure and microstructure images. The results indicate that the highest tensile strengths of 7, 28, 60, and 90 d CPFDS were reached at PF contents of 0.6%, 1.0%, 1.0%, and 1.0%, exhibiting values 5.96%, 65.16%, 34.10%, and 35.83% higher than those of CDS, respectively. Short, 3 mm, PF of showed the best reinforcement efficiency. The CPFDS exhibited obvious tensile strain-hardening characteristic, and also had better ductility than CDS. The mix factor (C_C^a/C_w^b) and time parameter (q_t0(t)) of CDS, and the reinforcement index (k_t-PF) of CPFDS were used to establish the tensile strength prediction models of CDS and CPFDS, considering multiple factors. The PF “bridge effect” and associated cementation-reinforcement coupling actions inside CPFDS were mainly responsible for tensile strength behavior improvement. The key findings contribute to the use of CPFDS as recycled engineering soils.

Graphical abstract

Keywords

dredged sediment / chemical stabilization / fiber reinforcement / splitting tensile strength / micro-mechanisms

1 Introduction

Sediments are a key part of ecosystems [1]. They are commonly dredged from rivers, lakes or sea, and are also the cause of some water pollution and channel shallowing. Cleaning and disposal of underwater sediments has become a key to river and lake management, water protection, and waterway maintenance. Dredging is a widely accepted method for sediment cleaning, maintaining smooth channel and water quality [2]. Worldwide, the annual output of dredged sediment (DS) reaches millions of tons [3], and landfill, dumping into the sea or storing in open spaces have been the major traditional disposal approaches for decades [4,5]. The DS with high moisture content and rich organic matter has no bearing capacity, making it unsuitable for direct use as recycled soil for geotechnical applications [6]. Chemical stabilization can effectively improve the mechanical properties of DS by feeding binders [7]. Portland cement (PC) is still used in stabilizing DS because of the availability of raw materials and its good strength gain [8,9], despite the environmental costs associated with production and use. Therefore, recycling PC-stabilized DS (CDS) as road filling materials, such as base course, can achieve the double benefit of reusing waste DS and saving natural soil-rock resources.

Tensile cracking often occurs in many geotechnical structures such as embankments, pavements, dams, especially when the tensile stress matches the tensile strength of soil [10,11]. Cement-stabilized soil generally exhibits good compressive strength but poor tensile strength, and has a typical brittleness defect [12]. Tensile strength is an essential mechanical index for designing and evaluating the stability of geotechnical structures [13,14]. The methods of determining soil tensile strength are mainly divided into indirect and direct methods; the former includes hollow cylinder test, Brazilian tensile test, flexure beam test, whereas the direct methods are difficult to conduct because of sample preparation and test procedure issues, which require new test setups and technologies [15–17]. Incorporating fiber can effectively improve the tensile strength, and the fiber involved acts as a source of tensile resistance in cemented soil matrix [18,19]. Incorporating fiber has no effects on the chemical properties of soil and the ecological environment [20,21]. Furthermore, the fiber can be easily mixed with cemented soil mixture, just as it is used in concrete. The interfacial bonding and friction between fiber and cement-soil matrix mainly result in the tensile strength enhancement of fiber-reinforced and stabilized soils [22]. However, fibers with distinct physical and mechanical properties play different roles. Natural and artificial fibers are two commonly used fiber reinforcements for soil stabilization [23]. Natural fibers have the advantages of low cost, relative environmental neutrality, ready availability, and biodegradability, and have been widely studied and used as reinforcements in soil stabilization [23–25]. However, natural fibers also have obvious disadvantage of poor durability [26,27]. Compared with natural fibers, artificial fibers have high tension and corrosion resistance [21,28–32]. The utilization of waste DS as reclaimed soils for road filling engineering with the help of combining cement-stabilization and artificial fiber-reinforcement is a promising and sustainable practice.

The interfacial mechanical interaction and friction inside fiber-soil matrix lead to the pull-out resistance and associated tensile strength improvement of fiber-reinforced soil [11,13]. However, without the help of chemical stabilization, the fiber-soil matrix exhibits weak tensile strength gain. Using distributed fibers as reinforcement for stabilized soil has been widely investigated by several researchers. Divya et al. [10] found that incorporating fibers, especially long fibers, can effectively improve the tensile strength and prevent tensile cracking of stabilized soil. Correia et al. [12] reported that adding a low quantity of fibers weakened the stiffness, compressive and direct tensile strength. Cristelo et al. [33] conducted indirect tensile tests on cement-stabilized sandy-clay, and found that incorporating fiber affected the post-cracking behavior, and increasing the fiber content led to higher pre-peak strength, and higher peak stress. Xiao et al. [30] and Zhao et al. [34] combined various fibers with a microbially induced carbonate precipitation technique to stabilize sand, and found that increasing fiber content enhanced the tensile properties, while fiber type also played a significant role. Most existing studies focus on the qualitative and quantitative evaluations of the tensile strength characteristic evolutions of stabilized soils, but the corresponding prediction models are also important, especially in construction design and safety assessment. Xiao and Liu [35] proposed a tensile strength prediction model of cemented clay considering randomly orientated fibers and verified the predictive validity. Festugato et al. [36] proposed a tensile/compressive strength ratio prediction model based on density, cement content and fiber content. However, up to now, the tensile strength prediction of fiber reinforced CDS has been rarely reported.

Based on the above review and analysis, polypropylene fiber (PF) was adopted as reinforcement for CDS, and a range of splitting tensile strength (STS) tests were performed to evaluate the tensile strength and stress–strain characteristic evolutions, considering multiple factors. Based on observed STS behaviors laws, tensile strength prediction models of PF-reinforced CDS (CPFDS) containing multiple factors were established, and the corresponding predictive validities of the models were verified by comparing predicted and measured results. Furthermore, the PF “bridge effect” and associated interfacial friction coupling actions between PF and cement-soil matrix were revealed with the assistance of scanning electron microscopy (SEM) tests. The key findings elucidate the tensile strength patterns of behavior of CPFDS and contribute to its application for recycling as reclaimed engineering soils in geotechnical engineering applications.

2 Materials and methods

2.1 Materials

The DS was sourced from Minhang District, Shanghai, China. The natural water content of on-site stocked DS was reduced to 49.5% by a period of air-drying. The basic physical characteristics of DS were tested as per Chinese standard GB/T 50123-2019 [37], and the results are given in Tab.1. The microstructure, chemical composition, and crystalline phases were determined with the help of SEM, X-ray fluorescence, and X-ray diffraction (XRD) tests, respectively, as in Fig.1 and Tab.2. The DS was composed of common clay minerals, and irregular-shaped soil particles without cementation were clearly seen. The # 42.5 PC was used for DS stabilization, and Tab.2 lists its chemical compositions. Five lengths of PF were adopted as the reinforcement of CDS, as presented in Fig.2, and their basic properties are presented in Tab.3.

2.2 Samples preparation

The pre-treatments of DS, including oven-drying at 60 °C and ball mill grinding were conducted, and then it was sieved using a 2-mm sieve. The CPFDS samples were prepared by blending dry DS, PC, water and PF, as per the details of test scenarios shown in Tab.4. Both PC content, PF content and water content were calculated based on the dry weight of DS. The single factor controlled variable method was adopted to examine the effect of various factors on the tensile strength behavior of CPFDS, and the controlled conditions are also given in Tab.4. Furthermore, the same tensile mechanical properties of pure CDS were evaluated as comparison.

During the mixing process, the dry DS and PC were first blended at low speed and the required water was then added. Afterwards, the PF was gradually incorporated and mixing was continued at high speed until homogenization was achieved. The prepared CPFDS was poured into the split cylindrical molds with sizes of 50 mm in length and 50 mm in diameter. Meanwhile, the vibration compaction was conducted to reach the equal level of dry density. Finally, the samples along with molds were moved to the standard curing box at temperature of (23 ± 2) °C and humidity of 95% ± 2%. After 24 h, demolding was conducted. For each mix formulation, three parallel samples were prepared for ensuring effective result of subsequent STS tests.

2.3 Methods

The STS (q_t) tests were performed as per ASTM C496-11 [38]. The sample was placed radially and the plane face was vertical. Then, the load was applied at a constant displacement rate of 0.5 mm/min until achievement of tensile failure along the diameter direction, or until the maximum tensile strain of 15% was reached. q_t can be obtained from the following expression.

(1)q_{\mathrm{t}}=\frac{2P}{\text{π}hd},

where P is the tensile peak load, in N; d and h (50 nm) are diameter and height of the sample, respectively.

After the STS tests, the typical failure samples were collected for further microstructural analysis. 1 cm × 1 cm cut pieces were obtained from designed samples, and were kept in alcohol until testing. Before testing, the well-prepared samples were first gold-coated using a sputtering technique to obtain the conductivity and to eliminate charging effect, and then the microstructure images with different magnifications were achieved.

3 Results and discussion

3.1 Tensile strength evolution

Fig.3 presents the STS evolution of CPFDS taking account of various factors. Correspondingly, the STS of CDS with the same ranges of PC content and water content are given as comparison. As shown in Fig.3(a), increasing PC content significantly improved the STS of CPFDS and CDS. This was attributed to the action of PC hydration products in bonding soil particles together and improving the splitting tensile resistance. The more PC was involved, the stronger the bonding that was achieved. Specifically, the STS of CPFDS increased by a factor of 9.04 as PC content increased from 10% to 30%. With PC content of 10%–30%, the CPFDS exhibited obvious STS advantage over CDS, except in the case of 7 d of curing. This observation confirms that the inclusion of 0.6% PF effectively improved the STS of CDS.

Fig.3(b) gives the STS of CPFDS against PF content; the STS of CDS was reduced with addition of 0.2% PF, indicating that small amount inclusion of PF played the side actions. This was due to the insufficient PF addition resulting in limited reinforcement action. The maximum 7 d-STS was reached at PF content of 0.6%, while the 28, 60, and 90 d-STS reached the maximum at PF content of 1.0%. This indicated that the optimum PF content was increased with the extension of curing time. The continuous generation of hydration products inside CPFDS needed more PF to provide adhering zone for cemented soil particles, restraining the rolling of soil particles and pulling-out of PF. The highest 7, 28, 60, and 90 d-STS values of CPFDS were respectively 5.96%, 65.16%, 34.10%, and 35.83% higher than those of CDS, showing that incorporating suitable PF can effectively enhance the STS of CDS.

Fiber length also affects the reinforcement effectiveness in improving tensile behavior of soil [25]. Fig.3(c) presents the STS evolution of CPFDS with varying PF lengths; it can be seen that the 7d-STS remained nearly constant with increasing PF length, indicating that PF length variation had an insignificant effect on the early STS evolution of CPFDS. This was due to the fact that, within 7 d, the amount of hydration products was insufficient. After curing for 28 d, the 3-mm PF exhibited optimum reinforcement efficiency, indicating that the long PF exhibited a side effect on the STS gain of CPFDS. The long fiber easily led to local aggregation and folding, and weakened the uniformity of fiber distribution inside CPFDS [39]. Furthermore, the agglomerated long fibers obstructed the effective cementation between soil particles [40].

Fig.3(d) presents the STS of CPFDS versus water content, together with that of CDS as a comparison. Increasing water content obviously reduced the STS of CPFDS and CDS, indicating that high water content significantly weakened the STS gain of CPFDS. The CPFDS exhibited higher STS than CDS at various water contents except in the case of 7 d-STS, further confirming that PF exhibited positive reinforcement contribution on CDS. The side effects of increasing water content could be originated from the following reasons. The matrix suction and effective stress were reduced, and excessive free water existing at the soil-particles to fiber interface acted as lubricant layer and weakened the frictional coefficient of the cementation-reinforcement matrix. The cementation capacity of hydration products was also affected by the variation of water content. Furthermore, increasing water content enlarged the spacing among soil particles and also weakened the cementation between hydration products and soil particles.

3.2 Tensile stress–strain analysis

Fig.4 shows the tensile stress–strain evolutions of typical 28 d CPFDS versus PC content, PF content, PF length and water content under controlled conditions. It can be seen that the tensile stress increased uniformly with increasing strain until reaching the first peak tensile stress and strain, i.e., brittle tensile peak stress (σ_sbmax) and failure strain (ε_fb), then the tensile stress decreased suddenly without strain evolution. Afterwards, the residual tensile stress increased again with further strain increase after tensile cracking. Finally, the ultimate tensile peak stress (σ_sumax) and corresponding ultimate failure strain (ε_fu) were achieved after multiple increasing and decreasing alternations. These tensile stress–strain evolutions indicated that the residual bearing capacity still existed after failure, showing a “secondary bearing effect”, confirming that the PF incorporation contributed to the improvement in ductility of CDS. Furthermore, the ε_fb was in the range of 0%–3% regardless of other factors. Fig.4(a) shows that the sudden drop in σ_sbmax became more intense with increasing PC content, exhibiting increasingly obvious brittleness characteristics. Fig.4(b) indicates that the brittleness characteristics tended to become gentle with increasing PF content; in particular, the CPFDS with 1.0% PF almost exhibited tensile strain-hardening characteristic without brittle failure. The effect of PF length on the tensile stress–strain is shown in Fig.4(c), demonstrating that the brittleness was gradually weakened with increasing PF length. Specifically, the brittle failure of CPFDS incorporating 12 and 15 mm PF was almost absent. Fig.4(d) presents the tensile stress–strain curves of CPFDS and CDS against water content, indicating that the sudden drop in σ_sbmax became more intense with decreasing water content, suggesting that increasing and decreasing water content corresponded to the brittleness and ductility improvements, respectively.

3.3 Tensile failure strain and peak stress

The comparisons of σ_sbmax and σ_sumax, ε_fb and ε_fu can further illustrate the tensile stress–strain behaviors under various factors. Fig.5 shows the tensile failure strain and peak stress of CPFDS. The effect of PC content on tensile failure strain (ε_fb and ε_fu) and tensile peak stress (σ_sbmax and σ_sumax) is shown in Fig.5(a); the ε_fb of both CPFDS and CDS decreased gently with increasing PC content, further confirming that the brittleness characteristic of CPFDS and CDS increasingly apparent. For CPFDS with 10%–30% PC, the ε_fu was larger than ε_fb. The ε_fu of CDS with 10%–20% PC was equal to ε_fb. The ε_fu was larger than ε_fb when the PC content was 20%–30%. The tensile peak stress including σ_sbmax and σ_sumax enhanced as increasing PC content except that the σ_sbmax of CPFDS decreased with increasing PC content from 25% to 30%. Generally, the σ_sumax values of CPFDS were obviously higher than σ_sbmax values, exhibiting tensile strain-hardening characteristic. By comparison, both tensile failure strain and peak stress of CPFDS were respectively higher than those of CDS, suggesting a significant improvement in ductility caused by fiber reinforcement.

Fig.5(b) gives the tensile failure strain and peak stress of CPFDS against PF content, indicating that the ε_fb tended to increase with increasing PF content, and increased especially sharply as PF content increasing from 0.8% to 1.0%, while the ε_fu value did not exhibit a clear evolutionary pattern with increasing PF content. Correspondingly, both σ_sbmax and σ_sumax increased with the increase of PF content. Furthermore, the ε_fb was lower than ε_fu under the PF content of 0.2%–0.8%, and the two were equal when PF content was 1.0%. Similarly, the σ_sbmax value was lower than the σ_sumax value under the PF content of 0.2%–0.8%, and the two were equal when the PF content was 1.0%. The above observations suggest that both tensile load bearing capacity and ductility behavior of CPFDS were effectively improved by increasing PF content, exhibiting obvious tensile strain-hardening characteristics.

Fig.5(c) presents the tensile failure strain and peak stress versus PF length, showing that that both ε_fb and ε_fu decreased at first and then increased with increasing PF length, with a minimum at 6- and 12-mm PF, respectively. By comparison, the ε_fb was respectively less than and equal to ε_fu when PF length was less than and greater than 12 mm. The σ_sbmax and σ_sumax decreased with increasing PF length, indicating that long PF length weakened the tensile load bearing capacity of CPFDS. Furthermore, the σ_sumax was larger than σ_sbmax under PF length of 3–12 mm, and the two were equal when PF length was 12 and 15 mm. These observations indicate that increasing PF length contributes to the improvement in ductility, but weakens the tensile load bearing capacity.

The effect of water content on the tensile failure strain and peak stress is shown in Fig.5(d), where it can be observed that the ε_fu showed insignificant change law. The ε_fb changed slightly with the enhancement of water content. The tensile peak stresses, including σ_sbmax and σ_sumax of CPFDS and CDS, decreased with increasing moisture content, signifying a weakness of tensile load bearing capacity. By comparison, the tensile peak stresses (σ_sbmax and σ_sumax) of CPFDS were larger than those of CDS, suggesting that the tensile load bearing capacity of CDS was effectively improved by incorporating PF.

3.4 Tensile strength prediction

3.4.1 Tensile strength prediction of cement-stabilized dredged sediment

Fig.6 presents the STS behaviors of CDS with changing PC content (C_C), water content (C_w), and curing time (t), using fitting analysis, and the corresponding fitting equations are also given. As shown in Fig.6(a) and Fig.6(b), the power functions of q_t = aC_{\mathrm{C}}^b (b > 1) and q_t = aC_{\mathrm{w}}^b (b < −1.5), with high coefficient of correlation, can be determined to characterize the STS behavior versus C_C and C_w. The effects of t on the STS behavior of CDS under given C_C and C_w were fitted, as shown in Fig.7 and the specific fitting equations with high coefficient of correlations are also provided. It was found that the relation between q_t and t could be represented by logarithmic function (q_t = alnt + b) with t. Based on the above analysis, with control of two of the three factors of C_C, C_w, and t, then the STS of CDS could be effectively predicted via fitting equations presented in Fig.6 and Fig.7. However, in practical applications, the STS and its influencing factors, C_C, C_w, and t, often change as per the requirements of engineering. Therefore, it is necessary to establish a tensile strength prediction model for the effects of C_C, C_w, and t.

It follows from Fig.6 and Fig.7 that the STS of CDS increased, decreased and increased in the form of power function q_t = aC_{\mathrm{C}}^b (when b > 1), power function of q_t = aC_{\mathrm{w}}^b (when b < −1.5) and logarithmic function (q_t = alnt + b) with C_C, C_w and t, respectively. Therefore, the mix factor C_{\mathrm{C}}^a/C_{\mathrm{w}}^b and time parameter q_t0(t) can be used to evaluate the coupling effects of C_C and C_w, and t on the STS development of CDS. In other words, the mix factor C_{\mathrm{C}}^a/C_{\mathrm{w}}^b can be further regarded as reflecting the cementation action of soil particles, water and PC hydration products. Therefore, the STS prediction model of CDS considering C_C, C_w, and t was proposed, as seen in Eq. (2).

(2)q_{\mathrm{t}}=\frac{C_{\text{C}}^a}{C_{\text{w}}^b}{q}_{t0}(t),

where parameters a and b can be obtained by fitting the relationship between q_t and C_{\mathrm{C}}^a/C_{\mathrm{w}}^b. By multiple attempts, a and b are determined to be constant at 1.5 and 2.0, respectively, for optimum linear fitting between q_t and C_{\mathrm{C}}^a/C_{\mathrm{w}}^b, as shown in Fig.8 (a). The 7, 28, 60, and 90 d- q_t0(t) values are 470, 609, 750, and 838, respectively. The relationship between curing time and time parameter q_t0(t), as presented in Fig.8 (b), can be expressed as logarithmic function (q_t = alnt + b) via fitting analysis, as shown in Eq. (3).

(3)q_{\mathrm{t}0}\left(t\right)=141.4\text{ln}t+176.4, R^2=0.968.

Combining Eqs. (2) and (3), the STS prediction model of CDS can be obtained by comprehensively considering C_C, C_w, and t, as given in Eq. (4).

(4)q_{\mathrm{t}}=\frac{C_{\text{C}}^{1.5}}{C_{\text{w}}^{2.0}} \left(141.4\text{ln}t+176.4\right).

3.4.2 Tensile strength prediction of polypropylene fiber reinforced cement-stabilized dredged sediment

It can be observed from Fig.3(b) that when the curing time was 7 d, there was an optimal C_PF. After curing for more than 28 d, the STS increased uniformly until C_PF reached 1.0%. To quantify the STS development of CPFDS with C_PF, the interface friction coupling between PF and cement-soil matrix was regarded as the reinforcement of PF; a reinforcement index of CPFDS (k_t-PF) was proposed and was used to characterize the ratio of the STS of CPFDS with a certain PF incorporation (q_t-PF) to that of CDS without PF (q_t). The calculation expression for k_t-PF is as follows.

(5)k_{\text{t-PF}}=\frac{q_{\text{t-PF}}}{q_t},

where k_t-PF represents the reinforcement index of CPFDS, and is dimensionless; q_t-PF and q_t are the STS values of CPFDS and CDS, respectively, in kPa. Fig.9 shows the variation of k_t-PF with C_PF, which is in agreement with the behavior of STS with C_PF as shown in Fig.3(b). Based on the nonlinear fitting analysis, k_t-PF changed with C_PF in a quadratic function, with an opening downward, and the fitting equation can be expressed as follows.

(6)k_{\text{t-PF}}=-2.167C_{\text{PF}}^2+2.613C_{\text{PF}}+0.262, R^{\text{2}}=0.959.

For curing times exceeding 28 d, the variation of k_t-PF with C_PF is shown in Fig.9(b), together with the normalized fitting. It can be seen that the k_t-PF increased linearly with increasing C_PF, and exhibited high correlation. To explore the effect of curing time on the variation of k_t-PF with C_PF, the k_t-PF for three curing times (28, 60, and 90 d) was further averaged, and Fig.10 shows the fitting relation between average k_t-PF with C_PF, showing this to be the same as can be seen in Fig.9(b). This confirms the rationality of normalizing or averaging the k_t-PF to obtain an approximate relationship between k_t-PF with C_PF. Therefore, the dependence of k_t-PF on C_PF can be expressed as Eq. (7).

(7)k_{\text{t-PF}}=0.673C_{\text{PF}}+0.767, R^2=0.985.

Combining Eqs. (4)–(7), the strength prediction model of CPFDS, considering C_C, C_PF, C_w, and t, can be expressed as Eq. (8). It should be acknowledged that the applicability of Eq. (8) is limited to the STS evolution prediction of CPFDS with 10%–30% PC, 50%–90% water content and 0%–1.0% PF. The equation’s wide applicability will be further developed in the follow-up investigations.

(8)q_{\mathrm{t}\text{-PF}}=\{\begin{array}{rl}& C_{\text{C}}^{1.5}/C_{\text{w}}^{2.0}\left(141.4\text{ln}t+176.4\right),\\ & \left(-2.167C_{\text{PF}}^2+2.613C_{\text{PF}}+0.262\right),(t=7\text{d})\\ & C_{\text{C}}^{1.5}/C_{\text{w}}^{2.0}\left(141.4\text{ln}t+176.4\right),\\ & \left(0.673C_{\text{PF}}+0.767\right),\left(t=28,60,90\text{d}\right)\end{array}

3.4.3 Strength prediction model verification

The verification of prediction effectiveness of the STS prediction model of CPFDS (Eq. (8)) was evaluated, and the independent verification tests were designed and performed using the same sample preparation and test conditions as given in Section 2. The specific design of verification tests is provided in Tab.5, and a total of 16 independent tests were conducted and 48 samples were prepared. The measured STS values were achieved by the STS tests, and the predicted STS values were calculated based on the Eq. (8). The predicted and measured STS values were compared, and the relationship between the two was linearly fitted and analyzed, as shown in Fig.11. The correlation between the measured and predicted values were closely distributed near the 45° fitting line, and all fall within the 95% predicted band. Therefore, the STS prediction model (Eq. (8)) can be effectively used to predict the STS evolution of CPFDS.

3.5 Cementation-reinforcement mechanism

The macro failure and microstructure images, Fig.12 and Fig.13, were used to characterize the cementation-reinforcement coupling actions inside CPFDS under tensile load. The continuous increase in splitting tensile load caused several cracks on the surface of a sample until the formation of an obvious through-tensile-crack, as shown in Fig.12. The incorporated PF impeded the further extension of the through-tensile-crack, exhibiting the fiber “bridge effect” inside CPFDS. This fiber “bridge effect” was mainly responsible for the transition from brittle behavior to ductile behavior, and led to the emergence of strain-hardening characteristics. Fig.13(a) shows the typical microstructure of 28d-CPFDS with about 200x magnification, and the dashed portion in Fig.13(a) was enlarged to about 500×, as given in Fig.13(b). From Fig.13(a), it can be seen that the combination of PF and cement-soil led to the formation of cementation-reinforcement matrix. The PF surface adhered to cementitious gels, which not only promoted the cementation and aggregation of soil particles, but also increased the roughness of PF itself. These two actions successfully transferred the tensile load from the cemented soil matrix to PF, and the PF pull-out resistance was also enhanced. Furthermore, the hydration products also filled the micro-cracks and pores among PF and soil particles, producing the more compact microstructure inside CPFDS. The above observations and analyses confirm that the cementation-reinforcement coupling actions produced by PF and cemented soil particles were mainly responsible for the improvement of tensile strength behaviors.

4 Conclusions

This study investigated the feasibility of using PF as reinforcement in improving improve the tensile strength behavior of CDS. The tensile strength and stress–strain evolutions of CPFDS considering multiple factors were clarified. Furthermore, the cementation-reinforcement coupling actions inside CPFDS were clarified via analysis of macro failure and microstructure images. The key findings are as follows.

1) The optimum PF content inside CPFDS were affected by the curing time. The 7, 28, 60, and 90 d values of STS of CPFDS, containing optimum PF contents of 0.6%, 1.0%, 1.0% and 1.0%, were, respectively, 5.96%, 65.16%, 34.10%, and 35.83% higher than those of CDS. Short PF of 3 mm was more suitable than longer PF for improving the STS of CPFDS.

2) The tensile stress of CPFDS increased uniformly with the strain evolution until achieving the brittle tensile peak stress (σ_sbmax) and failure strain (ε_fb), and then decreased suddenly without increase of strain. The residual tensile stress increased again with continuous strain evolution until reaching the ultimate tensile peak stress (σ_sumax) and failure strain (ε_fu). The incorporation of PF contributed to a “secondary bearing effect” and tensile strain-hardening characteristics of CPFDS under tensile loading.

3) σ_sumax was larger than σ_sbmax for CPFDS, confirming the tensile strain-hardening characteristics. Generally, σ_sbmax and σ_sumax values of CPFDS were respectively higher than those of CDS, indicating that the tensile load bearing capacity and ductility of CDS were simultaneously enhanced by incorporating PF.

4) The mix factor (C_{\mathrm{C}}^a/C_{\mathrm{w}}^b) and time parameters q_t0(t) were proposed to establish the STS prediction model of CDS. A reinforcement index of CPFDS (k_t-PF), characterizing the ratio of the STS of CPFDS to that of CDS, was proposed to establish the STS prediction model for CPFDS, considering multiple factors. The prediction effectiveness of this STS prediction model was verified by comparing predicted values and measured values of independent tests.

5) The incorporated PF provided a fiber “bridge effect” inside CPFDS, contributing to the transition from brittle behavior, limiting the sustained development of tensile cracks, and promoting the formation of tensile strain-hardening characteristic. The cementation-reinforcement coupling actions produced by PF and cemented soil particles were mainly responsible for the tensile strength behavior improvement.

The outcomes of this study could provide an effective basis for improving tensile strength behavior of CDS, advancing the resource utilization of waste DS. However, the applicability of research results and established STS prediction model needs to be verified in conjunction with actual engineering, which will be systematically investigated in the future.

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