Experimental Investigation on Relations Between Impact Resistance and Tensile Properties of Cement-Based Materials Reinforced by Polyvinyl Alcohol Fibers

: Cement-based material is brittle and is easily damaged by an impact load with a few blows. The purpose of this paper is to study the relations between the impact resistance and tensile properties of cement-based materials reinforced by polyvinyl alcohol ﬁber (PVA-FRCM). A drop-weight test and uniaxial tension test were performed. The relations were studied based on the experimental results, including the relation between the blow number and the tensile stress at the ﬁrst visible cracking ( σ c ) and the relation between the blow number and the tensile strain at the ultimate failure ( ε f ). Results showed that the blow number for the ﬁrst visible crack for disc impact specimens increases obviously with the increase of σ c of slab specimens. The crater diameter and blow number for ultimate failure of the disc specimens increase with the increase of ε f of slab specimens. For the PVA-FRCM specimens with larger σ c and ε f , much more blows are needed to cause both the ﬁrst visible crack and ultimate failure. Polyvinyl alcohol ﬁbers can reinforce impact resistance and tensile properties of cement-based materials.


Introduction
Cement-based material, such as concrete, is brittle and is easily damaged by impact load with a few blows [1,2]. Thus, various fibers, e.g. steel fiber, polypropylene fiber, and carbon fiber, are added into the mixture to improve its impact resistance. The effects of the cellulose, polypropylene, and steel fibers on the impact resistance of the cement-based material were studied by Rahmani [3], among which the effect of steel fiber was the most significant. The mean failure blows were increased from 48 to 228 after steel fibers were added. Nataraja et al. [4] conducted the drop-weight test on steel fiber-reinforced concrete. It was known from the test that the failure blows increased with the increase of the steel fiber volume content. The impact resistance of the concrete with steel fiber volume content of 1.5% was about 20-25 times to the one of its plain counterpart. Wang et al. [5,6] used the steel fibers to improve the impact resistance of light weight aggregate concrete. The mean failure blows were increased by 13 times when the steel fiber volume content was 2%. Mastali et al. [7,8] reinforced the self-compacting concrete with the regenerated carbon fiber and the recycled glass fiber. The impact resistance increased 6.48 times for the cylinder specimens with regenerated carbon fiber content of 1.25% and 6.14 times for the ones with regenerated glass fiber volume content of 1.25%.

Materials
Components of the PVA-FRCM in this study contain the PVA fiber, cement, fly ash, quartz sand, hydroxyl-propyl methyl cellulose (HPMC) and superplasticizer. In the PVA-FRCM mixture, the type of PVA fiber is KII-REC15. The properties of the PVA fiber are shown in Table 1. Portland cement with a model of P.O 42.5R was produced in Hohhot. The chemical composition of cement is presented in Table 2. Class I fly ash from the power station in Erdos was adopted and its chemical composition is listed in Table 3. Quartz sand originated from Tongliao was adopted. The sand size was from 0.075 mm to 0.106 mm. HPMC was produced in Jinan. 3301E superplasticizer was produced in Dalian.  The mixture proportion of PVA-FRCM is shown in Table 4. The water-binder ratio of the mixture was fixed at 0.26 throughout the test. In the mixture proportion, PVA fiber volume contents (V f ) were 0, 0.5%, 1.0%, 1.5%, and 2.0%. The specimens were named in F0, F0.5, F1, F1.5, and F2 to represent the above PVA fiber volume contents respectively. HPMC and superplasticizer were added into the mixture at a cement weight of 0.05% and 2% respectively.

Specimen Preparation
When the specimens prepared, the cement, fly ash and quartz sand in a dry state were mixed firstly in a 100-liter mixer with rotating blade for two minutes. Then the 80% of the water and the superplasticizer were added into the mixture. The remaining water and the HPMC were added after 2 min of mixing. After mixed for another one minute, the PVA fibers were added manually and gradually for ensuring the most random dispersion in the cementitious matrix. Then all ingredients were continuously mixed for five minutes before the mixture was poured into the mold.
There were two types of specimens in this test. One type of specimen was for the uniaxial tension. This type of specimen had a shape of dumbbell. For the test section of the specimen, it was just a rectangular slab. Its length was 90 mm, width was 30 mm and thickness was 15 mm. The other type of specimen was for the drop-weight test and it had a shape of disc. Its diameter was 152 mm and its thickness was 63.5 mm. For the first type, three specimens were prepared for each V f . Six specimens were prepared for each V f of the second type. The average cubic compressive strength of PVA-FRCM was about 50 MPa at the age of 28 days.
All the specimens were kept in the laboratory for 24 hours and were cured in the natural water with a room temperature of 24 ± 3 • C for 28 days after demolding. After curing, all the specimens were stored indoor for another 28 days with a room temperature of 24 ± 3 • C and a relative humidity of 85 ± 10%. Taking into account the use of fly ash, to minimize the effects of fly ash on the early-age properties of PVA-FRCM such as modulus of elasticity or early-age shrinkage, both drop-weight and uniaxial tension tests were conducted at age of 56 days of the specimens in this study.

Drop-Weight Test
The drop-weight test apparatus was designed according to the recommendation of American Concrete Institute (ACI)Committee 544 [19], as shown in Figure 1. The 64mm diameter impact sphere was placed on the top of the centre of the disc specimen. The PVA-FRCM disc was collided repeatedly by the impact hammer dropping through the impact sphere. The impact hammer weights 4.54 kg. The distance was 457 mm from the bottom of impact hammer to the top of impact sphere.
The impact test operation was continuous until the disc specimen touched three of the four steel lugs. The blow numbers for the first visible crack (N c ) and for the ultimate failure (N f ) were recorded during the test. The failure pattern and impact crater diameter of PVA-FRCM disc specimen were identified.

Uniaxial Tension Test
The uniaxial tension test was accomplished on the universal testing machine. Loading apparatus are shown in Figure 2. Two iron plates were fixed to both ends of the slab specimen by epoxy. In order to ensure the uniaxial tension, two ball joints were connected to the iron plates at the ends of the specimen. The loading rate was 0.1 mm/min and kept constant from the beginning. The tensile load was measured by a load cell. The tensile deformation was measured by linear variable differential transformers (LVDTs). The data acquisition system was used to acquire the load and deformation values simultaneously. The crack patterns of specimen were captured into pictures.  Figure 3 shows the failure patterns of PVA-FRCM disc specimens with different Vf. Subjected to impact hammer dropping repeatedly, disc specimens were broken into three or four pieces. The orientations of cracks have a common origin at the center of crater and exhibited about 90 degrees, 120 degrees and 180 degrees angle between each other. The disc specimens containing more PVA fibers, e.g. F1.5 and F2, failed with more debris and their fibers were pulled out, while the PVA fibers were snapped for the specimens with less PVA fibers. The impact craters were observed in the center of disc specimens when the disc specimen failed, as shown in Figure 4. The specimen F0 had the smallest impact crater diameter. The specimen F2 had the biggest impact crater diameter of 59 mm that was nearly the same size of the diameter of the impact sphere. The diameters of impact craters for all specimens are listed in Table 5. The relationship between V f and average diameter of impact crater is shown in Figure 5. The confidence level is 95%, and uncertainty of the test methods ranges from 2.0 to 4.8. It is found from the histogram that as the V f increases the diameter of the impact crater becomes larger and larger. This phenomenon indicates that the energy consumed by the specimen during dropping repeatedly would become more, when more PVA fibers are added into the disc specimen.

Blow Numbers
The blow numbers for the first visible crack (N c ) and ultimate failure (N f ) of PVA-FRCM disc specimens are listed in Table 5. Because the specimen F0 had no PV fiber added, its average values of N c and N f were only one and two respectively. It is evident that the disc specimen without PVA fiber is very brittle and its impact resistance is almost negligible. When the specimens are properly reinforced with PVA fibers, the average values of N c and N f increase dramatically. For disc specimens with V f of 2%, the average values of N c and N f reached 1662 and 3018, which appeared good toughness performance.
The effect of V f on the blow numbers is shown in Figure 6. When the confidence level is 95%, uncertainty of N c is no more than 320, one of N f do not exceed 270 and one of N f -N c is within 370. It can be found that N c and N f of disc specimens increased evidently with the increase of V f . Especially for the specimens with V f of more than 1%, the increases of N c and N f became remarkable. The conclusion can be drawn that the addition of PVA fibers can improve the impact resistance of cementitious materials. Besides, the difference ∆ between N c and N f increased simultaneously with the increase of V f . It increased from 1 to 21, 123, 779, and to 1456 finally, while V f increased from 0 to 0.5%, 1%, 1.5%, and 2%. This indicates that V f has a strong influence on the blow number difference between N c and N f of the PVA-FRCM disc specimens.

Impact Energy
The impact energies for the first visible crack and ultimate failure of the disc specimens under the impact load were calculated by the Equation (1) and (2) respectively.
where, W 1 and W 2 are the impact energies for the first visible crack and ultimate failure respectively, m is the mass of the impact hammer (4.54 kg), g is the gravitational acceleration (9.8 m/s 2 ), h is the falling distance of the impact hammer (457 mm), and the impact energy is in the unit of joules (J). The impact energy W 1 and W 2 were calculated and summarized in Table 5. The influence of V f on the average impact energy are shown in Figure 7. It can be observed that the impact energy for all PVA-FRCM disc specimens increased with V f . The impact energy had a small increase when V f is less than 1%, while it had a remarkable increase when V f is more than 1%. According to the overall tendency, the impact energy at the ultimate failure increases faster with the increase of V f than the one at the first visible crack. This indicates that V f has larger effect on the impact energy at the failure then the one at the first cracking.

Uniaxial Tension Test Results and Analysis
This study examined the uniaxial tension test results on the slab specimens, e.g. crack pattern, stress-strain curve, stress and strain at the first visible crack moment (σ c and ε c ), and stress and strain at the failure moment (σ f and ε f ). For the specimen F0, a brittle failure happened under uniaxial tension because no PVA fibers were added. Its stress and strain were so small that they could hardly be captured.

Crack Pattern under Uniaxial Tension
The crack patterns of the specimens except specimen F0 under uniaxial tension are shown in Figure 8. It can be seen clearly that crack numbers were different for the specimens with different V f . For the specimen F0.5, only one crack appeared during uniaxial tension test. The first visible crack was exactly the failure crack. When more PVA fibers were added into the specimen, the crack numbers became more. Especially when V f reached 2%, lots of visible cracks appeared on the surface of the specimen uniaxial tension. The crack numbers of the specimen F1 and F1.5 were in between. It is indicated that V f has an influence on the crack pattern of the specimens under uniaxial tension.

Stress-Strain Curves under Uniaxial Tension
The stress-strain curves of all specimens expect for the specimen F0 under uniaxial tension are shown in Figure 9. All stress-strain curves contained many small curve segments. A gradual ascending followed by a sudden descending existed in every small curve segment. Only two curve segments appeared in the stress-strain curves of the specimen F0.5 and F1, while five for the specimen F1.5 and twenty for the specimen F2. The specimens with more PVA fibers had more curve segments. Comparing the number of curve segments in Figure 9 with the number of cracks in Figure 8, it can be found that they were approximately equal. The main reason is probably that the PVA fibers were actually pulled out gradually under uniaxial tension, rather than all PVA fibers being pulled out of the cement matrix simultaneously. When some of the PVA fibers were pulled by the tension transferred by the cement mix, the stress-strain curves exhibited the ascend gradually. Just after these PVA fibers were pulled out, the stress-strain curves exhibited the descend suddenly.

Influence of V f on Strain and Stress
All specimens with PVA fibers showed a good toughness performance. Even V f of the specimen was only 0.5%, its ε f could reach 0.77%. When the specimen F2 failed, its ε f reached up to 6.39%. As shown in Figure 10, ε f increased significantly with the increase of V f . Unlike ε f , ε c changed little when V f increased. The relation between ε c and V f exhibited an approximate horizontal line. As shown in Figure 11, both σ c and σ f increased clearly with the increase of V f . But the growth of σ f was faster than the one of σ c . After V f increased from 0.5% to 2%, σ f increased by 1.68, 2.01, and 2.87 times, while σ c increased by 1.48, 1.7, and 2.06 times. Because the small curve segment exactly represents the pullout or snap process of some PVA fibers, the more curve segments exhibits and the greater pulling capacity needs. So σ f of the specimen F2 was the biggest. σ f of the specimen F0.5 was the smallest. Figure 11. Influence of V f on tensile stress.

Criteria on the First Crack and Failure
As mentioned above, σ c increased significantly but ε c changed barely with the increase of V f . So σ c is decisive on the first crack of specimens. After first cracking, strain and stress continue to increase until the specimen fails. The test results in Table 6 show an increasing strain growth (ε ∆ ) and almost constant ε ∆ /ε f of all specimens with the increase of PVA fiber contents. By comparison, the stress growths (σ ∆ ) were less and the values of σ ∆ /σ f of all specimens ranged from 0 to 0.28. The difference between σ f and σ c was small. Thus, the stress is unfit to describe the failure process of specimens, and ε f is more appropriate.   Figure 12 shows the elements of PVA-FRCM disc specimen subjected to the dropping hammer impact. The disc specimen can be considered as many concentric circle elements (M). Each circle element (M) carries the circumferential tensile load from the impact hammer dropping. The circle element (M) can be modelled as many slab elements (N) in the radial direction and each slab element carries uniaxial tensile load. Thus, the disc specimen under the impact load can be simplified into many slabs under uniaxial tensile load in the tangential direction. Whenever the first crack appears on the slabs, it is exactly the first crack on the disc. When the strain of slab increases under the uniaxial tensile load, the crater diameter of disc specimen will also increase under the impact hammer dropping. When the slabs are pulled into pieces, the disc specimen fails.  Table 7 lists the major test results of the tensile properties and impact resistance. The tensile properties, σ c and ε f , and the impact resistance, blow numbers and crater diameter, are tabulated. At the first crack moment, σ c of the slab specimen increased from 1.78 MPa to 2.64 MPa, 3.04 MPa and 3.67 MPa when more PVA fibers were added. Meanwhile, the blow number N c of the disc specimen increased from 11 to 88, 805, and up to 1662. The relation between N c of the disc specimen and σ c of the slab specimen is shown in Figure 13. Although the dataset is relatively small, the general change trend can be observed that the N c increased obviously with an increase of σ c . The N c of disc specimen depends largely on the σ c of slab specimen. This phenomenon indicates that much more blows are needed to cause the appearing of first visible crack when the σ c of PVA-FRCM is larger.

Relation between Blow Number N f and Strain ε f
Besides, it can be found that ε f of the slab specimen increased from 0.77% to 1.16%, 1.93%, and 6.39% after more PVA fibers were added. The diameter of the crater in the center of disc specimen increased at the failure moment with the increase of PVA fiber content. It can be seen from Table 7, the crater diameter increased from 29.53 mm to 35.95 mm, 55.8 mm, and 59.28 mm when V f increased. Corresponding to the crater diameter, the blow number N f of the disc specimen increased from 32 to 211, 1548, and up to 3018. The N f of disc specimen and ε f of slab specimen exhibited a parabolic relationship, as shown in Figure 14. The N f of disc specimen increased obviously with an increase of ε f . However, this is a convex parabola which is different from the concave relation between N c and σ c . It might be predictable that the increase of ε f can apparently increase N f only within a certain range, and this effect would not be apparent exceeding that range. When the diameter of the impact crater approaches the diameter of the impact sphere, the range would be coming. According to the presented relationships between impact resistance and tensile properties of PVA-FRCM in this study, impact resistance properties of PVA-FRCM could be conventionally evaluated or predesigned by its tensile properties without conducting extra dynamic test.

Conclusions
Relationships between the impact resistance and tensile properties PVA-FRCM were discussed by analyzing the experimental data of the drop-weight test and the uniaxial tension test performed in this study. The conclusions are as follows: (1) When the disc specimen with more PVA fibers fails under the impact hammer dropping, the fibers are pulled out, the diameter of impact crater is larger, more debris appears, and more energy is consumed. The blow number increases with the PVA fiber volume content for the disc specimens. And this increase of the blow number for the ultimate failure is faster than the one for the first visible crack.
(2) In the uniaxial tension test, the slab specimen with more PVA fibers has better toughness performance and more cracks. The number of small curve segments in the tension stress-strain curves is roughly equal to the number of cracks. σ c is decisive on the first crack of specimens. ε f is more appropriate to describe the failure process of specimens.
(3) Based on the analysis on the relationship between the impact resistance and tensile properties, the blow number for the first visible crack N c depends largely on σ c of slab specimen and increases obviously with an increase of σ c . The crater diameter and blow number at ultimate failure moment N f increase with an increase of ε f of slab specimen. When both σ c and ε f of PVA-FRCM are larger, much more blows are needed to cause both the first visible crack and ultimate failure.
(4) According to the presented relationships between impact resistance and tensile properties of PVA-FRCM in this study, impact resistance properties of PVA-FRCM could be conventionally evaluated or predesigned through its tensile properties without conducting extra dynamic test.