Experimental Study on Impact Resistance of Polyvinyl Alcohol and Polypropylene Hybrid Fiber Reinforced Concrete
Xie Yongliang, Zhao Liang, Zhang Yong
(Department of Airport Engineering and Support, Air Force Logistics College, Xuzhou, Jiangsu 221000, China)
Abstract
In this paper, a drop hammer tester was used to conduct impact tests on concrete mixed with polyvinyl alcohol (PVA) fibers and polypropylene (PP) fibers of different dosages and lengths. Displacement-time and equivalent force-time curves of materials with different mix proportions were obtained. Image analysis shows that the addition of PVA and PP fibers significantly improves the impact resistance of concrete. Meanwhile, the effects of fiber dosage, length, type and hybrid interaction on the impact resistance of test blocks were investigated.
Keywords
Polyvinyl alcohol fiber; Polypropylene fiber; Impact resistance
Introduction
Fiber reinforced concrete is a composite material composed of fibers and concrete. Concrete has disadvantages such as low tensile strength, poor ductility and high brittleness. Adding fibers can improve its mechanical properties including tensile strength, impact strength, flexural performance and ductility, thus attracting extensive attention.
PVA fiber can not only enhance the crack resistance of cement-based brittle materials, but also significantly reduce brittleness and improve ductility and toughness, meeting material requirements for future engineering. Su Jun et al. conducted flexural toughness tests on PVA fiber reinforced concrete and found that PVA fibers obviously improved flexural toughness. Hu Xingyu studied the properties of PVA-basalt hybrid fiber reinforced concrete and found that compressive and flexural strengths increased with fiber volume fraction and length.
Polypropylene fiber is widely used due to its light weight, low cost, good toughness and crack resistance. Li Xueying et al. added PP fibers into concrete and found that the splitting tensile strength increased by 45%, flexural strength by 19% and tension-compression ratio by 46% at 28 days compared with plain concrete. Liu Weidong conducted abrasion and impact resistance tests on PP fiber concrete and proved that PP fibers significantly toughen and crack-resist concrete, improving impact resistance and wear resistance.
In this study, PVA and PP fibers were added into concrete with controlled dosages and lengths. Their effects on impact resistance were studied through impact tests, providing references for strength improvement and engineering applications.
1 Experimental Design
1.1 Raw Materials
(1) Cement: Zhonglian P32.5 composite Portland cement produced by Xuzhou Zhonglian Co., Ltd.
(2) Sand: Xuzhou construction river sand with fineness modulus of 2.6.
(3) Coarse aggregate: Crushed stone with continuous gradation of 5–10 mm, cleaned and air-dried before mixing.
(4) Water: Tap water of Xuzhou.
(5) Water reducer: Naphthalene-based high-concentration powder water reducer from Hengshui Youyi Chemical, Hebei, with water reduction rate of 18–25%.
(6) PVA fiber and PP fiber: Bundled monofilament fibers with diameter of 15–30 μm and lengths of 6 mm, 9 mm and 12 mm.
1.2 Experimental Scheme
Two dosages and three lengths of PVA and PP fibers were selected as variables. Several groups of tests were designed to study the effects of fiber dosage and length on impact resistance. Material proportions were calculated by absolute volume method according to concrete specifications. Group 0 was the control group without fibers.
1.3 Specimen Preparation and Curing
According to designed mix ratios and dosages, raw materials were weighed and fibers were added to avoid agglomeration and ensure uniform distribution. After mixing with water, the mixture was cast into molds in three layers with manual tamping and vibration for 30–60 seconds. Plastic film was used to retain moisture. Specimens were demolded after 24 hours and cured in an SHBY-40A standard curing box at (20±2)℃ and humidity of about 95% for 28 days. After curing, specimens were dried for impact tests.
2 Experimental Analysis
2.1 Impact Test
A 1000 J JLW-800 drop hammer impact testing machine was used. Specimen size was 71 mm×71 mm×71 mm. Drop height was 30 cm. Test data were converted by a SKY-Ⅱ data processor, and A-D curves were obtained using supporting software for analyzing force-time and displacement-time relationships under impact failure.
2.2 Failure Patterns of Specimens
Under a drop height of 30 cm, failure patterns of fiber-reinforced specimens were significantly different from plain concrete (Group 0). Plain concrete suffered severe damage with large spalling around the impact center. In contrast, specimens with PVA and PP fibers showed much less damage due to fiber ductility, which effectively mitigated fracture.
Visual differences in impact performance were observed with varying fiber dosage and length, but quantitative conclusions required further data analysis.
2.3 Data Analysis
Impact tests were carried out on 25 groups of specimens with different mix proportions at 30 cm drop height, including single PVA fiber concrete, single PP fiber concrete and hybrid PVA-PP fiber concrete.
With increasing PVA fiber length, specimen displacement increased from about 1.7 mm at 6 mm to 2.05 mm at 12 mm, an increase of about 21%. Shorter PVA fibers performed better, with an optimal length of 6 mm.
For PP fibers, displacement first decreased then increased. It dropped from 1.8 mm at 6 mm to 1.4 mm at 9 mm (a 22% reduction) and rose to 1.55 mm at 12 mm (an 11% increase). The optimal PP fiber length was 9 mm.
At a fiber dosage of 1 kg/m³, maximum impact force of PVA fiber concrete slightly decreased while PP fiber concrete showed little change compared with plain concrete. At 2 kg/m³, maximum impact force of both increased from 160,000 N to 170,000 N, an increase of 6.3%. Excessive fiber dosage did not further improve impact resistance; the optimal dosage for both single fibers was 2 kg/m³.
Under the same dosage and length, PP fiber concrete exhibited smaller maximum displacement and higher maximum equivalent force than PVA fiber concrete. Therefore, PP fibers improve impact resistance more significantly than PVA fibers.
Hybrid fiber tests showed that simply increasing total or individual fiber dosage did not always enhance performance. The combination of 1 kg/m³ PVA fiber + 1 kg/m³ PP fiber achieved the most obvious improvement in impact resistance, with the smallest displacement and largest equivalent force.
3 Conclusions
Based on drop hammer impact tests on PVA and PP fiber reinforced concrete with varying dosages and lengths, the following conclusions are drawn:
(1) For PVA fibers, impact displacement increases with fiber length; the optimal length for impact resistance is 6 mm. For PP fibers, displacement first decreases then increases; the optimal length is 9 mm within 6–12 mm.
(2) The optimal dosage for both single PVA and PP fiber concrete is 2 kg/m³. Impact resistance does not increase indefinitely with fiber dosage and has an upper limit.
(3) Polypropylene fibers improve concrete impact resistance more significantly than polyvinyl alcohol fibers based on displacement-time and equivalent force-time curves.
(4) Increasing hybrid fiber dosage unilaterally or excessively does not guarantee better performance. The hybrid mix of 1 kg/m³ PVA fiber + 1 kg/m³ PP fiber provides the most significant improvement in impact resistance.
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