Effect of macro fibers on flexural toughness and crack surface topography of concrete
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摘要: 为了研究结构型聚丙烯纤维、结构型钢纤维及混杂纤维(包括结构型聚丙烯纤维和结构型钢纤维)对混凝土弯曲韧性及裂缝表面形态的影响，参照RILEM TC 162-TDF[
10 ]标准进行混凝土开口梁的三点弯曲试验。利用激光扫描仪对每个试件裂缝表面形态进行信息采集。基于裂缝表面信息，计算了四种裂缝表面粗糙程度参数(即裂缝粗糙度(RN)、分形维数(D)、z坐标正态分布的标准差(σz)和裂缝曲折度(τ))，并比较分析了裂缝表面粗糙程度参数与弯曲韧性参数之间的相关性。研究表明：随着纤维掺量的增多，混凝土弯曲韧性和裂缝表面粗糙程度均随之增大。与结构型聚丙烯纤维和结构型钢纤维相比，混杂纤维在提高混凝土弯曲韧性和增大混凝土裂缝表面粗糙度方面均展现出正混杂效应。与分形维数D、z坐标正态分布标准差σz和裂缝曲折度τ相比，裂缝粗糙度RN与纤维增强混凝土梁弯曲韧性参数的相关性最为显著，且存在指数函数关系。基于该函数关系，可借助纤维增强混凝土梁的弯曲试验快速估测裂缝表面的粗糙程度。Abstract: In order to study the effect of different macro fibers (including macro polypropylene fiber, macro steel fiber and hybrid fiber of polypropylene fiber and steel fiber) on the flexural toughness and crack surface topography of concrete, three-point bending test was conducted on notched beams in accordance with RILEM TC 162-TDF[10 ]. The laser scanning equipment was utilized to measure the information of crack surface topography. The four roughness parameters (roughness number (RN), fractal dimension (D), standard deviation of height distribution (σz) and crack tortuosity (τ)) were calculated according to the information of crack surface topography and the correlation between four crack roughness parameters and flexural toughness parameters were then analyzed and compared. The results show that with the increasing of fiber content, the flexural toughness and crack surface roughness increase. Compared to polypropylene fiber and steel fiber, the hybrid fiber illustrates the positive synergistic effect on the flexural toughness and crack surface roughness of concrete. Compared to ractal dimension D, standard deviation of height distribution σz and crack tortuosity τ, the correlation between RN and flexural toughness of fiber reinforced concrete beam is the most satisfactory. Meanwhile, the relationship between roughness number RN and flexural toughness parameters follows the exponential function, which can be applied to quickly estimate crack surface roughness by the bending test of fiber reinforced concrete.-
Key words:
- macro fiber /
- flexural toughness /
- laser scanning /
- topography of crack surface /
- roughness
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图 1 结构型钢纤维(SF)和结构型聚丙烯纤维(PP)的形状
Figure 1. Geometries of macro steel fiber(SF) and macro polypropylene fiber(PP)
图 2 混凝土开口梁弯曲试验
Figure 2. Flexural test of concrete notched beam
LVDT—Linear variable differential transformer
图 7 混凝土试件裂缝表面z坐标分布直方图
Figure 7. Distribution histogram of z coordinates from concrete cracked surface
图 9 普通混凝土(NC)、钢纤维增强混凝土(SF/NC)、聚丙烯纤维增强混凝土(PP/NC)和混杂纤维增强混凝土(SF-PP/NC)试件的荷载-挠度曲线
Figure 9. Load-deflection curves of normal concrete(NC), steel fiber reinforced concrete(SF/NC), polypropylene fiber reinforced concrete(PP/NC) and hybrid fiber reinforced concrete(SF-PP/NC) specimens
图 10 NC、SF/RC、PP/NC和SF-PP/NC试件裂缝表面的重构图
Figure 10. Reconstruction views of cracked surface of NC, SF/RC, PP/NC and SF-PP/NC specimens
图 11 NC、SF/NC、PP/NC和SF-PP/NC试件的粗糙程度参数
Figure 11. Roughness parameters of NC, SF/NC, PP/NC and SF-PP/NC specimens
图 12 混凝土裂缝表面粗糙程度参数与弯曲韧性参数的统计关系
Figure 12. Statistical relationship between roughness parameters and flexural toughness parameters of concrete
图 13 NC、SF/NC、PP/NC和SF-PP/NC试件的裂缝粗糙度RN对比
Figure 13. Comparison of crack surface roughness number RN of NC, SF/NC, PP/NC and SF-PP/NC specimens
图 14 SF/NC、PP/NC和SF-PP/NC试件裂缝粗糙度与弯曲韧性的关系
Figure 14. Relationship between cracked surface roughness and parameters of flexural toughness of SF/NC, PP/NC and SF-PP/NC specimens
表 1 混凝土基准配比
Table 1. Basic mix proportion of concrete
kg·m–3 Cement Fly ash Fine aggregate Coarse aggregate Water Superplasticizer 390 155 822 848 272.5 5.5 
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表 2 纤维性能参数
Table 2. Properties of fibers
Type Shape of fiber Length/mm Diameter/mm Aspect ratio Tensile
strength/MPaElastic
modulus/GPaNumber/
(104 pieces·kg−1)Macro PP Straight 45 0.75 60 490 3.9 5.76 Macro SF Hooked-end 35 0.55 65 1 150 200.0 1.45
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表 3 28天龄期混凝土试件抗压强度
Table 3. Compressive strength (28 d) of concrete specimens
Composite SF PP Compressive strength/MPa NC — — 37.3 SF20/NC 20 kg/m3 (0.25vol%) — 41.7 SF40/NC 40 kg/m3 (0.5vol%) — 35.4 SF60/NC 60 kg/m3 (0.75vol%) — 34.1 PP2.3/NC — 2.3 kg/m3 (0.25vol%) 36.5 PP4.6/NC — 4.6 kg/m3 (0.5vol%) 35.4 PP6.9/NC — 6.9 kg/m3 (0.75vol%) 37.5 SF20-PP2.3/NC 20 kg/m3 (0.25vol%) 2.3 kg/m3 (0.25vol%) 37.2 SF40-PP2.3/NC 40 kg/m3 (0.5vol%) 2.3 kg/m3 (0.25vol%) 38.5
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表 4 NC、SF/NC、PP/NC和SF-PP/NC试件的比例极限荷载、极限荷载、能量吸收能力、等效抗弯强度和裂缝表面纤维根数
Table 4. Load at limit of proportionality, ultimate load, energy absorption capacity, equivalent flexural tensile strength and parameters of flexural toughness of NC, SF/NC, PP/NC and SF-PP/NC specimens
Fiber content/vol% Specimen FL/kN Fu/kN DfBZ,2/(N·m) feq,2/MPa DfBZ,3/(N·m) feq,3/MPa Nf 0 NC 10.0 10.0 0 0 0 0 0 0.25 SF20/NC 9.1 9.1 2.0 2.23 11.3 2.52 60.0 PP2.3/NC 9.7 9.7 1.1 1.20 7.4 1.65 50.7 0.50 SF40/NC 11.4 11.9 4.3 4.82 25.3 5.63 125.0 PP4.6/NC 9.8 9.8 1.4 1.53 9.7 2.16 108.0 SF20-PP2.3/NC 11.2 13.0 5.1 5.67 29.5 6.56 134.7 0.75 SF60/NC 12.4 18.4 9.1 10.12 45.1 10.03 199.3 PP6.9/NC 10.6 10.6 2.3 2.51 13.9 3.10 169.0 SF40-PP2.3/NC 12.3 15.7 6.2 6.89 36.2 8.04 194.3 Notes: FL—Maximum load in the interval of 0.05 mm; Fu—Ultimate load; DfBZ,2, DfBZ,3 —Energy absorption of the influence of fiber at the deflections of (δL+0.65 mm) and (δL+2.65 mm), respectively; feq,2, feq,3—Equivalent flexural tensile strengths by the deflections of (δL+0.65 mm) and (δL+2.65 mm), respectively; Nf—Number of fibers on the cracked surface.
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表 5 NC、SF/NC、PP/NC和SF-PP/NC试件的裂缝表面粗糙程度参数
Table 5. Roughness parameters of NC, SF/NC, PP/NC and SF-PP/NC specimens
Parameter NC SF20/NC SF40/NC SF60/NC PP2.3/NC PP4.6/NC PP6.9/NC SF20-PP2.3/NC SF40-PP2.3/NC RN 1.20
(Cv=2.5%)1.47
(Cv=4.8%)1.89
(Cv=2.0%)1.98
(Cv=3.3%)1.23
(Cv=5.9%)1.58
(Cv=3.2%)1.74
(Cv=2.3%)1.96
(Cv=0.9%)2.04
(Cv=1.6%)D 2.068
(Cv=3.5%)2.082
(Cv=1.9%)2.124
(Cv=5.0%)2.149
(Cv=4.0%)2.076
(Cv=2.0%)2.117
(Cv=1.7%)2.125
(Cv=2.3%)2.143
(Cv=2.9%)2.157
(Cv=5.1%)σz/mm 2.173
(Cv=3.4%)2.411
(Cv=3.0%)2.805
(Cv=1.9%)2.994
(Cv=2.8%)2.250
(Cv=2.2%)2.542
(Cv=1.0%)2.587
(Cv=5.5%)2.896
(Cv=3.5%)3.384
(Cv=2.5%)τ 0.824
(Cv=1.1%)0.758
(Cv=1.8%)0.742
(Cv=2.2%)0.740
(Cv=3.3%)0.792
(Cv=6.7%)0.777
(Cv=6.1%)0.754
(Cv=2.6%)0.722
(Cv=1.8%)0.715
(Cv=2.1%)Note: Cv—Coefficient of variation; RN—Roughness number; D—Fractal dimension; σz—Standard deviation of height distribution; τ—Crack tortuosity.
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