Experimental and theoretical study on flexural behavior of precast UHPC-RAC composite beams
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摘要: 超高性能混凝土(Ultra-high performance concrete，UHPC)和再生混凝土(Recycled aggregate concrete，RAC)，碳足迹低，属于“低碳混凝土”。将再生混凝土梁受拉侧或侧壁的部分RAC用UHPC替换，形成“绿色低碳”UHPC-RAC组合截面，以提高力学性能。采用工厂预制工艺，制作了预制UHPC-RAC组合梁。通过四分点抗弯性能试验，分析了受拉UHPC厚度、UHPC-RAC结合面粗糙度和侧壁UHPC高度，对预制UHPC-RAC组合梁破坏机制、承载力、变形和初始刚度的影响规律，提出了承载力计算公式。研究表明：与RAC梁相比，预制UHPC-RAC组合梁随受拉UHPC厚度的增加，形成的UHPC-RAC穿筋结合面，限制了开裂后UHPC剥离脱落；增加界面粗糙度，阻滞了水平裂缝的延展，初始刚度可提高16.6%；随受拉UHPC钢纤维拔出，荷载-位移曲线下降明显，待再生混凝土压溃后，仍有较高的残余强度。预制UHPC-RAC组合梁的开裂荷载和极限荷载，分别增加63.1%和22.9%，截面抗弯刚度、初始刚度均得到明显改善。组合截面内钢筋、UHPC和RAC协同受力，应变沿截面高度线性变化，符合平截面假定；将截面应力等效分布后，推导了预制UHPC-RAC组合梁的受弯承载力计算公式，计算结果与试验值吻合较好。Abstract: Ultra-high performance concrete (UHPC) and recycled aggregate concrete (RAC), with a low carbon footprint, belong to the "Low Carbon Concrete". A portion of RAC on the tensile side or side wall of the RAC beam was replaced with UHPC to form a "green and low carbon" UHPC-RAC composite section to improve mechanical pro-perties. The precast UHPC-RAC composite beam was fabricated by precast technology. The influences of tensile UHPC thickness, roughness of UHPC-RAC joint surface and UHPC height of side wall on failure mechanism, bearing capacity, deformation and initial stiffness of precast UHPC-RAC composite beams were analyzed by four-point flexural tests, then the calculation formula of bearing capacity were proposed. The results show that, comparing precast UHPC-RAC composite beams with RAC beams, the UHPC-RAC bonding surface traversed by the stirrups with the increase of UHPC thickness on the tensile side limits the peeling off of UHPC after cracking. The increasing roughness of the interface further retards the extension of horizontal cracks and improves the initial stiffness about 16.6%. With the failure of steel fiber pulling out in the UHPC on the tensile side, the load-displacement curves decrease obviously. The precast composite beams still have a high residual strength when the compression concrete is crushed. Compared with RAC beams, the cracking load and ultimate load of precast UHPC-RAC composite beams are increased by 63.1% and 22.9%, respectively, and the section flexural stiffness and initial stiffness are signifi-cantly improved. In the composite section, reinforcement, UHPC and RAC work collaboratively, and the strain changes linearly along the section height, conforming to the assumption of plain section. After equivalent section stress distribution, the calculation formula of the bending capacity of the precast UHPC-RAC composite beams is deduced, and the calculated results are in good agreement with the experimental values.
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图 4 预制组合梁侧面和底面裂缝形态
Figure 4. Crack morphologies on the side and bottom of the precast composite beams
图 5 预制组合梁的表面应变云图
Figure 5. Surface strain cloud map of the precast composite beams
The left fulcrum of the prefabricated composite beam is the starting point and the coordinate is 0 mm; and the right fulcrum is the end point and the coordinate is 2000 mm; The coordinates 500 mm to 1500 mm are pure curved areas
图 6 预制组合梁纯弯区的典型破坏形态
Figure 6. Typical failure patterns in the pure bending zone of the precast composite beams
图 8 不同因素影响下预制组合梁的初始刚度
Figure 8. Initial stiffness of the precast composite beam effecting by different factors
图 9 不同因素影响下预制组合梁跨中纯弯区的竖向变形
Figure 9. Vertical deformation of the pure bending zone in the span of precast composite beam effecting by different factors
图 10 预制组合梁表面RAC/UHPC的应变
Figure 10. Strain of RAC/UHPC on the precast composite beam surface
图 12 预制组合梁的受拉钢筋应变
Figure 12. Tensile reinforcement strain of the precast composite beams
图 14 不同受拉UHPC厚度预制组合梁的应力-应变分布：(a) 截面；(b) 截面应变；(c) 截面应力
Figure 14. Stress-strain distribution of prefabricated composite beams with different tensile UHPC thicknesses: (a) Section; (b) Strain distribution of section; (c) Stress distribution of section
h—Cross section height; b—Cross section width; ht—Height of tensile zone; h0—Section effective height; hu—Thickness of UHPC on tensile side; xn—Actual height of compression zone; as—Distance from resultant force point of tensile reinforcement to edge of tensile zone; Ast—Section area of tensile reinforcement; εcu—Ultimate compressive strain of RAC; εut—Tensile strain of UHPC; εs—Tensile strain of tensile reinforcement; αc1 and βc1—Characteristic parameters of equivalent rectangular stress pattern about RAC; fcc—Axial compressive strength of RAC; fy—Yield strength of the reinforcement; fut—Tensile strength of UHPC; Ccc—Pressure of RAC; Tst—Tensile force of the reinforcement; Tu1—Tensile force of the UHPC on the tensile side; Mu—Ultimate bending moment of the combined section
图 15 U型预制组合梁的应力-应变分布：(a) 截面；(b) 截面应变；(c) RAC截面应力；(d) UHPC截面应力
Figure 15. Stress-strain distribution of U-shaped precast composite beams: (a) Section; (b) Strain distribution of section; (c) Stress distribution of section of RAC; (d) Stress distribution of section of UHPC
bf—Width of side wall UHPC; εut,0—Peak tensile strain of UHPC; εu0—Peak compressive strain of UHPC; εc0—Peak compressive strain of RAC; εut,p—Ultimate tensile strain of UHPC; σut,p—Ultimate Stress of UHPC; λ—Ratio of εut,p to εcu; fuc—Axial compressive strength of UHPC; Cuc—Pressure of UHPC; Tu2—Tensile force of the side wall UHPC as plastic state; Tu3—Tensile force of the side wall UHPC as elastic state; Mcu—Ultimate bending moment obtaining from RAC and reinforcements; Muu—Ultimate bending moment obtaining from tensile UHPC and compressive UHPC; λxn—Height of the elastic phase of concrete; εcu—Ultimate compressive strain of RAC; αu1—Ratio of the stress value of the UHPC rectangular stress diagram in the compression zone to the design value of the UHPC axial compressive strength; βu1—Influence coefficient of UHPC strength.
图 16 浅U型预制组合梁的应力-应变分布：(a) 截面；(b) 截面应变；(c) RAC截面应力；(d) UHPC截面应力
Figure 16. Stress-strain distribution of shallow U-shaped prefabricated composite beams: (a) Section; (b) Strain distribution of section; (c) Stress distribution of section about RAC; (d) Stress distribution of section about UHPC
hu1—Height of the side wall UHPC as plastic state
表 1 再生混凝土(RAC)的配合比
Table 1. Mixture ratio of recycled aggregate concrete (RAC)
kg/m3 Recycled coarse aggregate
replacement ratioWater cement ratio Cement Coarse aggregate Sand Water Natural Regeneration 50% 0.42 488.1 570.99 570.99 614.92 227.84 
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表 2 RAC、超高性能混凝土(UHPC)和受拉钢筋力学性能
Table 2. Mechanical properties of RAC, ultra-high performance concrete (UHPC) and tensile reinforcements
MPa Material categories Compressive strength Tensile strength Material categories Yield strength Ultimate strength RAC 37.9 — Tensile
reinforcement473.4 603.4 UHPC 113 9.70
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表 3 预制组合梁的设计参数
Table 3. Design parameters of the precast composite beams
Specimen number UHPC Longitudinal bar Stirrup Roughness/mm Tensile thickness/mm Side wall height/mm RAC-B1 — — 3C14 C8@100 — UHPC13-RAC/S/T-B2 13 — 3C14 — UHPC21/S-RAC/T-B3 21 — 3C14 — UHPC35/S/T-RAC-B4 35 — 3C14 — UHPC35/S/T-I2-RAC-B5 35 — 3C14 2.0 UHPC35/S/T-I4-RAC-B6 35 — 3C14 4.0 UHPC35/S/T-RAC-B7 35 160 3C14 — UHPC35/S/T-RAC-B8 35 300 3C14 — Notes: S—Stirrups; T—Tensile reinforcements; / —S or T locates in RAC or UHPC; I—UHPC-RAC interface.
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表 4 预制组合梁的受弯性能参数
Table 4. Flexural performance parameters of the precast composite beams
Specimen number Fcr/kN δcr/mm Fy/kN δy/mm Fu/kN δu/mm K/(kN·mm-1) μ $ F_{\text{u}}^{\text{c}} $/kN Fu/$ F_{\text{u}}^{\text{c}} $ RAC-B1 46.6 3.2 230.19 9.78 270.4 24.73 22.44 2.53 273.91 0.99 UHPC13-RAC/S/T-B2 73.2 4.2 262.22 10.34 316.2 27.33 23.51 2.64 296.62 1.07 UHPC21/S-RAC/T-B3 76.8 3.6 264.44 10.67 300.2 27.68 25.05 2.59 311.07 0.97 UHPC35/S/T-RAC-B4 72.2 4.8 287.36 11.18 335.8 24.97 25.28 2.23 330.89 1.01 UHPC35/S/T-I2-RAC-B5 75.6 4.4 238.44 9.39 318.4 20.13 25.49 2.14 330.89 0.96 UHPC35/S/T-I4-RAC-B6 78.8 3.0 280.79 9.45 326.8 20.82 26.17 2.21 330.89 0.99 UHPC35/S/T-RAC-B7 75.2 4.8 290.62 10.95 361.2 28.24 26.51 2.57 353.84 1.02 UHPC35/S/T-RAC-B8 80.2 4.0 288.62 10.73 366.8 29.81 26.82 2.78 394.58 0.93 Notes: Fcr—Cracking load; Fy—Yielding load; Fu—Ultimate load; δcr—Cracking displacement; δy—Yielding displacement; δu—Ultimate displacement; K—Initial stiffness; μ—Ductility factor; $ F_{\text{u}}^{\text{c}} $—Calculated value.
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