Crack Resistance and Mechanical Properties of Polyvinyl Alcohol Fiber-Reinforced Cement-Stabilized Macadam Base

Mountain Bridge and Materials Engineering Research Center of Ministry of Education, School of Materials Science and Engineering, Chongqing Jiaotong University, No. 66 Xuefu Road, Chongqing 400074, China School of Civil Engineering, Chongqing Jiaotong University, No. 66 Xuefu Road, Chongqing 400074, China Hebei Jiaotong Vocational & Technical College, No. 219 Zhujiang Road, Shijiazhuang 050091, China School of Economics and Management, Chongqing Jiaotong University, No. 66 Xuefu Road, Chongqing 400074, China


Introduction
Cement-stabilized macadam (CSM) is a kind of material stabilized with inorganic binders and exhibits many advantages (such as strong integrity, high strength, strong antiscouring ability, excellent frost resistance, and low cost) and is widely applied in base or subbase of the highway [1][2][3]. CSM is made of cement, sand, and graded macadam mixed with water. With the interlock of aggregate and the intensity property of cement itself, CSM effectively improves the loadbearing capacity of pavement [4,5]. Due to the characteristics of drying shrinkage and temperature shrinkage, there always appear cracks or even plate fracture and broken body in CSM base [6]. en, the cracks extend to the top layer of pavement, which would lead to the cracking and damage of asphalt pavement, the main cause of early damage to the road [7,8].
erefore, improving the crack resistance and mechanical properties of CSM base is a subject worthy of study.
ere are many prevention measures for the cracks of CSM base, for example, optimizing the gradation of aggregate [9], controlling cement dosage [10], improving the quality of construction [11], strengthening early-age curing condition [7], adding modifiers [12], and paving geotextile cloth [13,14]. Although these measures have a positive effect on improving the crack resistance of CSM base, reflective cracking of pavement is still serious. Adding a new fiber is a simple and efficient way to improve the cracking resistance and mechanical performance of CSM.
At present, fibers, which are widely used as base materials and surface materials in road engineering, include polyester [15], basalt [1], polypropylene [16], polyacrylonitrile [17], lignin [18], and metallic [19]. Lots of researches have shown that fibers can significantly improve mechanical performance and cracking resistance of material stabilized with inorganic binders. Li et al. [20] studied the crack resistance of CSM reinforced with polypropylene fiber. e results indicated that the addition of polypropylene fiber could enhance dry shrinkage crack resistance and thermal shrinkage crack resistance. Liu et al. [21] studied the impact of fiber diameter on the performances of CSM. e results indicated that, with increasing fiber diameter, the average dry shrinkage coefficient of polyester-reinforced CSM first increased and then decreased, while the temperature shrinkage coefficients increased. Simultaneously, the dispersal uniformity of fiber as a technical problem needed to be solved, which had attracted great attention all along [22,23].
Polyvinyl alcohol (PVA) fiber is a synthetic, semicrystalline, biodegradable, and water-soluble polymer [24,25]. As a new ecological and environmental road material, PVA is characterized by its advantages of high strength, high elastic modulus, high cement affinity, strong alkali and weather resistance, good durability, green environmental protection, and lower price, so it is applied widely in road engineering field [26]. PVA fiber-reinforced cementitious composite is first developed and produced, which is a new cement-based composite material with high toughness, narrow cracks, and self-healing ability [27]. At the same time, the study on pavement base material of multiscale fiber-reinforced cement-based materials is also reported [28,29].
It should be noted that most of the investigations, as mentioned earlier, involve the influence of PVA fiber and others on mechanical performances and pavement performance of CSM and cement concrete. However, the reinforcement effect of PVA fiber on CSM material is rarely reported, especially the improvement effects on crack resistance and mechanical properties. e main objective of this study is to reveal the improvement effect of PVA on the mechanical performances and crack resistance of CSM material. To achieve this goal, mixing proportion design of PVA fiber-reinforced CSM (PFRC) is studied to obtain the maximum dry density and the optimum water content from a heavy compaction experiment. en three series of CSM mixtures (cement content: 3.2%, 3.6%, and 4.0%) are prepared by incorporating four different contents (0, 0.6, 0.9, and 1.2 kg/m 3 ) of PVA fiber. Meanwhile, the effect of fiber length on the mechanical performances and crack resistance of CSM was studied firstly; the four different lengths of fibers (12,18,24, and 30 mm) were selected as test materials.

Aggregate.
Aggregate is crushed limestone (0-31.5 mm) from Wazikou stone material factory in Xiaoxian County of Anhui Province. All aggregates are divided into five sizes according to particle size. e basic physical performances of aggregate meet the requirements of the relevant criteria, as shown in Table 2. e gradation of aggregate is displayed in Table 3.

PVA Fiber.
PVA fiber used in this study is supplied by Anhui Wanwei Group Co., Ltd., and it is a kind of synthetic fiber made from polyvinyl alcohol by special technology. PVA fiber is shown in Figure 1. e physical properties of PVA fiber are shown in Table 4.
PVA fiber dispersion equipment is shown in Figure 2. e clustering of PVA fiber includes three types: monofilament, bundle, and reticulation, as shown in Figure 3. e correlation researches indicate that the uniform 3-dimensional net structures of the interlaced distribution fibers can help to improve the performance of the mixtures [34]. So the ideal cluster condition of fiber is monofilamentous.

Mixture Proportions Design.
In order to determine the maximum dry density and the optimum water content of PFRC, heavy compaction experiment was performed according to Chinese Standard [35]. In order to reveal the influence of PVA fiber on the mechanical performance of CSM, three series (total 39 groups) of PFRC were designed in this experiment.

Sample Preparation.
To ensure uniform distribution of PVA fiber in the mix, aggregate and fiber are mixed for 5 minutes firstly; then cement and water are added and continued to be mixed for 30 minutes. e rotational speed of the stirring shaft is 48 (r/min). e mixing quality of CSM mixture is shown in Figure 4. e prepared mixture is put into a test mold to vibrate, and then the surface of the specimen is flatted. Mold is removed after 48 h maintenance, and then specimens are cured in the standard curing environment with temperature of 20 ± 2°C and relative humidity of 95%.

Heavy Compaction Test.
Chinese TJR-III multifunction electric compaction apparatus was employed in heavy compaction test [35]. e mass of driving hammer is 4.5 kg, and drop height is 45 cm, with 98 × 3 times hammering for each proportion. e volume of compacting cylinder is 2177 cm 3 . Permissible maximum nominal particle size is 37.5 mm.
To study the influence of different contents of cement and fiber on the compaction curves of CSM, six different test mixture proportions were designed in this study. PVA is nonmetallic fiber, and when the fraction is too large, it easily agglomerates. erefore, three series of CSM mixtures (cement content: 3.2%, 3.6%, and 4.0%) were prepared by incorporating two different contents (0 and 1.2 kg/m 3 ) of PVA fiber, and the length of fiber is 30 mm. Five samples for each test mixture proportion were fabricated with different moisture content. Afterwards, the mixtures were sealed with double PVC plastic bags for four hours. us, the curve of moisture content versus dry density was made and optimum moisture content and maximum dry density were determined by parallel experiments. Testing methods [30] Negative pressure screen method Vicat apparatus method Method of testing cements for determination of strength (ISO method)    Advances in Civil Engineering

Unconfined Compressive Strength (UCS) Test.
In the specimens fabricating stage, nearly 6279 g of raw materials was mixed and blended according to the compaction parameters determined by compaction tests. en 150 mm × 150 mm cylindrical specimens were fabricated by pressing forming method. Before UCS test, the specimens were stored in a curing chamber that maintained the temperature of 20 ± 2°C and relative humidity of 95% for 7, 28, and 90 days, respectively. On the last day, the specimens were immersed in water with a temperature of 20 ± 2°C. UCS test was carried out after standard curing [35,36], and the loading rate is 1 mm/min. e laboratory temperature is maintained at 20 ± 5°C, and the relative humidity should be 50-70%. e conditions are the same in other mechanical experiments in this paper. Each mixing proportion was conducted on 13 cylindrical specimens, and then the average within the error range was recorded.

Compressive Resilience Modulus (CRM) Test.
In this test, the preparation method and curing conditions of specimens are the same as those of UCS test [35]. e curing age is 90 days. Two ends of cylindrical specimens are completely smoothed with cement paste. Fifteen specimens of each mixing proportion were placed for more than 8 hours after leveling and immersed in water for 24 h. e specific pressure remained between 0.5 and 0.7 MPa. In order to make the loading roof contact close with the surface of the specimens, two preloadings were carried out by applying half of the maximum load. en the maximum load was divided into 5-6 stages. e first-stage load was applied and maintained for 1 minute. e load was unloaded while   the reading was recorded by micrometer or displacement sensor so that the specimen could fully rebound, and the displacement reading was rerecorded until 30 s. Loading and unloading of the specimens were carried out step by step. e CRM test of PFRC is shown in Figure 5.

Splitting Strength
Test. e tensile strength of a composite can be indirectly measured by splitting strength test. is test has become increasingly prevalent as it can be more easily run. Furthermore, the failure pattern observed in this test reflects the tensile properties of materials [37][38][39][40]. e splitting strength test was done at loading rate of 1 mm/min [35]. e splitting strength test of PFRC is as shown in Figure 6. e splitting strengths of 13 cylindrical specimens (the size of test specimen: 150 mm × 150 mm) were tested after standard curing. e layering width is 18.75 mm. When the specimens were destroyed, the maximum pressure was recorded to calculate splitting strength.

Flexural Tensile Strength (FTS) Test.
e sketch map of FTS test and test apparatus are shown in Figure 7. e three dividing points loading test was done by a flexural testing machine at a loading rate of 50 mm/min [35]. Fifteen beam specimens of 100 mm × 100 mm × 400 mm with a span of 300 mm (at the ages of 7, 28, and 90 days) for each mixing proportion were applied to study FTS.

Drying Shrinkage Test (DST).
e sketch map of DST and test apparatus are shown in Figure 8. ree small beam specimens with the size of 100 mm × 100 mm × 400 mm for each mixing proportion are applied to study dry shrinkage property, which were cured in the standard curing room for 7 days. en vernier calipers were used to measure the initial length of the small beam specimens three times to offer the arithmetic mean as the datum length. e shrinkage measuring apparatus was put into a shrinkage chamber that maintained the temperature of 20 ± 1°C and relative humidity of 60 ± 5% together with the specimens.

Temperature Shrinkage Test (TST).
e sketch map of TST and test apparatus are shown in Figure 9. e temperature shrinkage coefficient (TSC) of PFRC is studied in the dry state. ree small beam specimens with the size of 100 mm × 100 mm × 400 mm for each mixing proportion were applied to study temperature shrinkage property, which were cured in the standard curing room for 7, 28, and 90 days, respectively. en the small beam specimens were dried to a constant weight. e TSTs were done by temp shock at a cooling rate of 0.5°C/min for three hours [34]. e test starts from high temperature and cools down step by step. Four temperature levels were tested, which are 60-50, 50-40, 40-30, and 30-20°C, respectively. e dial indicator reading was read five minutes before the end of heat preservation.

Heavy Compaction Test Analysis.
e results of the heavy compaction test are shown in Table 5. e results show that when the cement content is the same, the optimum moisture content and the maximum dry density of CSM with fiber content of 1.2 kg/m 3 are very close to those of CSM without fiber. e differences in maximum dry density and optimum moisture content are not more than 0.08 g/cm 3 and 0.5%, respectively. So the content of PVA fiber in a certain range may be considered as unaffected for the maximum dry density and the optimum moisture content.

UCS Analysis.
e studies show that the reinforcement effect of fiber on UCS of CSM has a great relationship with fiber content, fiber length, cement content, and curing age [1,41], and therefore the influence factors of UCS of PFRC are discussed from the above four aspects. e trend charts of 7-, 28-, and 90-day UCS of CSM with different fiber content and length are shown in Figure 10. In the legend, take 3.2-7 for example; 3.2-7 denotes that cement content is 3.2%, and the curing age is 7 days. e standard deviation of the specimens is 6.3-14.4%, which meets the requirements of standard specification (<15%). e addition of PVA fiber in CSM greatly improves the UCS. In general, the addition of fiber results in an increase in the load-carrying capacity of composite samples. is performance boost stems from the existence of the transverse confinement effect between fiber and cement-based material [42]. e trend chart of UCS average growth rate of CSM with different fiber content is shown in Figure 11. Compared with fiber content of 0%, the UCS average growth rates of CSM with fiber content of 0.6, 0.9, and1.2 kg/m 3 are 19.92%, 19.46%, and 18.63%, respectively, and the UCS average growth rates of CSM with fiber lengths of 12, 18, 24, and 30 mm are 18.91%, 19.32%, 20.58%, and 19.18%, respectively. As far as UCS is concerned, the optimal PVA fiber content is closely related to fiber length.
In the legend of Figures 12 and 13, take 0.6-7 for example; 0.6-7 denotes that fiber content is 0.6 g/cm 3 , and the curing age is 7 d. Figure 12 shows that, with the increase of fiber length, the trend of UCS of CSM increases first and then decreases. Compared with CSM without PVA, the UCS of PFRC with fiber lengths of 12, 18, 24, and 30 mm increases by 15.1%, 7.2%, 16.7%, and 11.8%, respectively, on average. As far as UCS is concerned, the optimum length of PVA fiber is 24 mm. Figure 13 shows that the UCS of PFRC nonlinearly increases with increasing cement content. Compared with cement content of 3.2%, the UCS of PFRC with cement content of 3.6% and 4.0% increases by 18.4% and 35.4%, respectively, on average. It reveals that cement content has a certain influence on PVA fiber enhancement of UCS of CSM base.
From Figure 10, it can be found that the UCS of all series of PFRC increases as the age increases. Compared with 7, 28, and 90 d, UCS of PFRC with cement content of 3.2%, 3.6%, and 4.0% increases by 29.8% and 52.5%, 24.9% and 45.2%, and 22.5% and 37.8%, respectively. For 7 d, the UCSs of PFRC with cement content of 3.2%, 3.6%, and 4.0% are about 2.5-3.2 MPa, 3.1-3.8 MPa, and 4.0-4.8 MPa, respectively; all of them can meet the technical specification requirements for heavy traffic on the expressway as subbase material (>2.5 MPa) and base material (>3.0 MPa) [43]. Dial indicator e small beam specimen   Advances in Civil Engineering

CRM Analysis.
e standard deviation of the specimens is 5.7-13.5%, which meets the requirements of standard specification (<15%). Figure 14 shows that the CRM of PFRC with different fiber length increases first and then decreases with the increase of fiber content, but the CRM of PFRC with fiber length of 24 mm irregularly changes. Compared with CSM without fiber, the CRM of PFRC with fiber content of 0.6, 0.9, and 1.2 kg/m 3 averagely 12-0.6 12-0.9 12-1. Advances in Civil Engineering 7 increases by 4.64%, 3.96%, and 2.08%, respectively. e optimum fiber content corresponding to the CRM of PFRC varies with the fiber length. So the optimum content range of PVA fibers is 0.6-0.9 kg/m 3 . In the legend, take 3.2-7 for example; it denotes that the cement content is 3.2%, and the curing age is 7 d. Figure 14 shows that when fiber content is constant, the CRM of PFRC with cement content of 3.2% and 3.6% increases first and then decreases with the increase of fiber length, but the CRM of PFRC with cement content of 4.0% irregularly changes. e optimum fiber length corresponding to the CRM of PFRC varies with fiber content. So the optimum length range of PVA fiber is 18-24 mm.
When the fiber lengths are 12 and 30 mm, the CRM of PFRC increases along with the increasing of cement content, but when the fiber lengths are 18 and 24 mm, the CRM of PFRC increases first and then decreases with the increase of cement content. So the optimum cement content of PFRC with fiber lengths of 18 mm to 24 mm is 3.6%.

Splitting Strength Analysis.
e relationships between splitting strength of PFRC and fiber content are shown in Figure 15. In the legend, taking 7-3.2 as an example, it denotes that the curing age is 7 days, and cement content is 3.2%. e standard deviation of the specimens is 4.9-13.8%, which meets the requirements of standard specification (<15%). e results show that the splitting strength of PFRC with different fiber length increases first and then decreases with the increase of fiber content. Compared with CSM without PVA, the splitting strength of CSM with fiber content of 0.6, 0.9, and 1.2 kg/m 3 averagely increases by 4.73%, 12.50%, and 6.98%, respectively. So the optimum fiber content of PFRC is 0.9 kg/m 3 . Figure 15 shows that the splitting strength of PFRC increases along with the increase of fiber length. e optimum fiber length of PFRC is 18-24 mm. e splitting strength of PFRC increases nonlinearly with the increase of cement content. Compared with cement content of 3.2%, the splitting strength of PFRC with cement content of 3.6% and 4.0% after curing age of 28 days averagely increases by 6.82% and 28.88%, respectively. It implies that cement content is a key effect factor on splitting strength of PFRC. As the cement content increases, the hydrous binding materials increase gradually and the density of the whole composite increases gradually. Besides, the proportion of the interface zone also increases gradually, and then the strength and density of the interface zone increase gradually [44]. For the above reasons, as cement content increases, there is an increasing tendency in the splitting strength of PFRC.

FTS Analysis.
e relationships between FTS of PFRC with fiber content of 0.9 kg/m 3 and fiber length are shown in Figure 16. e legend of Figure 16 has the same meaning as that of Figure 15. e standard deviation of the specimens is 8.2-14.6%, which meets the requirements of standard specification (<15%). e results show that the FTS of PFRC increases first and then decreases with the increase of fiber length. So the optimum fiber length of PFRC is 18 mm to 24 mm. Compared with CSM without PVA, the FTS of PFRC with fiber length of 24 mm increases by 8.67%. e relationships between FTS of PFRC with fiber content of 0.9 kg/m 3 and cement content are shown in Figure 16. e results show that the FTS of PFRC increases along with the increasing of cement content. Compared with cement content of 3.2%, the FTS of PFRC with cement content of 3.6% and 4.0% after curing age of 28 days averagely increases by 1.18% and 3.83%, respectively.

Drying Shrinkage Property Analysis.
In the legend of Figure 17, taking 3.2-12 as an example, it denotes that the curing age is 7 days, and fiber length is 12 mm. e relationships between dry shrinkage coefficient (DSC) of PFRC and curing age are shown in Figure 17(a). e results show that the DSC of PFRC increases with the increment of age, but the speed of the DSC increase is reduced in the later period. e relationships between DSC of PFRC and fiber length are shown in Figure 17(b). e results show that, with the increase of PVA fiber length, the DSC decreases firstly and then increases, and there is optimum fiber length.

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Compared with CSM without PVA, the DSC of PFRC with 12, 18, 24, and 30 mm averagely decreases by 8.38%, 15.79%, 22.49%, and 17.96%, respectively. So when the fiber length is 24 mm, the DSC of PFRC is the smallest. e relationships between the DSC of PFRC and cement content are shown in Figure 17(c). e results show that, with the increase of cement content, the DSC of PFRC increases gradually. Compared with CSM without PVA, the DSC of PFRC with cement content of 3.2%, 3.6%, and 4.0% decreases by 19.8%, 15.9%, and 4.6%, respectively.

Temperature Shrinkage Property Analysis.
e trend charts of TSC of PFRC with different temperature levels are shown in Figure 18. e legend of Figure 18 has the same meaning as that of Figure 17. e results show that, with the increase of temperature levels, the TSC of PFRC increases gradually. Compared with temperature levels of °C, the TSCs of PVA fiber-reinforced CSM in temperature levels of , , and (60-50]°C averagely increase by 12.1%, 26.9%, and 41.4% respectively. Figure 18 shows that, with the increase of curing age, the TSCs of PFRC with different fiber length increase\ gradually. Compared with curing age of 7 days, the TSCs of PFRC after curing age of 28 and 90 days averagely increase by 30.7% and 44.7%, respectively. e incorporation of PVA can effectively improve the anti-temperature-shrinkage property of CSM. Compared with CSM without PVA, the TSCs of PFRC with fiber lengths of 12, 18, 24, and 30 mm averagely reduce by 9.4%, 12.0%, 36.9%, and 11.8%, respectively. So the optimum fiber length is 24 mm.

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Compared with CSM without PVA, the TSCs of CSM with cement content of 3.2%, 3.6%, and 4.0% averagely reduce by 15.2%, 18.2%, and 19.4%, respectively. Considering factors such as TSC, economy, and crack resistance effect comprehensively, the optimum cement content is 3.6%.

Analysis of Crack Resistance Mechanism of PVA.
A large number of experimental studies show that fiber can significantly enhance the crack resistance of concrete. e superposition method of stress intensity factor, k (k-superposition), based on the principle of linear elastic fracture mechanics, is widely applied in the fracture mechanism model of fiber-reinforced concrete [45,46]. Based on the theory of fracture mechanics and the displacement mode of crack surface, the crack propagation can be divided into three types: open type, shear type, and tear type ( Figure 19). e drying shrinkage stress and temperature shrinkage stress of CSM base are easy to lead to open cracks, and the cracks are basically transverse cracks. Generally, their distribution also has a certain regularity [47].
Without considering creep and stress relaxation, shrinkage stress of PFRC is mainly composed of dry shrinkage stress and temperature shrinkage stress. When the shrinkage stress exceeds the ultimate tensile strength of the material, cracks will occur in the base: where σ s is shrinkage stress, MPa;E w is bending modulus; α d is dry shrinkage coefficient;α w is temperature shrinkage coefficient.
According to formula (1), the crack resistance index K L is calculated as follows: where σ K is antishrinkage stress, MPa; σ S is shrinkage stress, MPa; R c is flexural tensile strength, MPa; E c is compressive resilience modulus, MPa; a d is dry shrinkage coefficient; a w is temperature shrinkage coefficient. In this paper, the crack resistance index K L is proposed to characterize the crack resistance of PFRC. e larger K L , the better the crack resistance. e stress mode of CSM base is shown in Figure 20. e crack resistance index K L of CSM with cement content of 3.6%, fiber length of 24 mm, fiber content of 0.9 kg/m 3 , and fiber content of 0 kg/m 3 is shown in Table 6. As shown in Table 6, compared with CSM without PVA, the crack resistance of PFRC with fiber content of 0.9 kg/m 3 improves by 44.4%.

Conclusions
On the basis of analyses of experimental results on crack resistance and mechanical properties of PFRC, the main findings and conclusions of this paper are summarized as follows: (1) Fiber content, fiber length, cement content, and curing age are the main factors affecting the mechanical properties and crack resistance of CSM. e mechanical properties and crack resistance of CSM are obviously improved by adding PVA fiber. In terms of the improvement of the crack resistance of CSM, there are optimal values for cement content, fiber content, and fiber length, which separately are 3.6%, 0.9 kg/m 3 , and 24 mm under the test conditions. (2) e incorporation of PVA fiber can effectively improve the anti-drying-shrinkage property and the temperature shrinkage property of CSM. When PVA fiber content is 0.9 kg/m 3 , the optimum fiber length of PFRC is 24 mm. With the increase of temperature levels, the TSC of PFRC nonlinearly increases. (3) e crack resistance and toughening effect of PVA fiber are very remarkable. Taking crack resistance index as an evaluation index, compared with CSM without PVA fiber, the crack resistance of PFRC with fiber content of 0.9 kg/m 3 is improved by 44.4%.
Adding fiber is one of the important means to improve the crack resistance of CSM base. However, the technical process and production cost of PFRC should be considered to further improve meeting the needs of practical application. In addition, the homogeneity of fiber distribution is one of the important factors affecting the road performance of CSM base. Using digital image processing technology to evaluate the homogeneity of fiber distribution is a forward-looking and intelligent technology with real-time, continuous, and quantitative characteristics, which is an important research direction.
Data Availability e data supporting the findings of this study are included in the article.

Conflicts of Interest
e authors declare no conflicts of interest.  Figure 20: e stress mode of CSM base [44].
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