Bonding properties of UHPC-high strength rebar based on beam test
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摘要: 为了研究高强钢筋与超高性能混凝(Ultra-high-performance concrete, UHPC) 的黏结性能，通过梁式搭接试验，设计制作了9根搭接梁，分析了钢筋搭接长度、钢纤维掺量、机械锚固措施对搭接梁中高强钢筋与UHPC黏结性能的影响。结果表明：采用UHPC连接的搭接梁，搭接段受拉钢筋与混凝土具有更优异的黏结性能；随着钢筋搭接长度的增加，搭接梁的峰值荷载提高，但平均黏结强度逐渐减小；搭接梁的峰值荷载和黏结强度随着钢纤维掺量的增大而增大；采用机械锚固措施处理后的搭接梁，具有更高的峰值荷载和黏结强度，其中采用弯钩处理的搭接梁峰值荷载和黏结强度提升最为明显，分别提高了212.4%，199.4%，并且搭接钢筋发生屈服。根据搭接梁达到峰值荷载时轴力和弯矩的平衡条件，计算出搭接梁中钢筋的最大拉应力，进一步建立钢筋与UHPC平均黏结强度的计算方法，并与中心拉拔试验，对拉搭接试验结果进行对比。Abstract: To study the bonding properties of high-strength steel bars and ultra-high-performance concrete (UHPC), nine groups of lap beams were designed and fabricated. The influences of specific variables on the bonding properties were analyzed, including lap length, steel fiber volume content, and mechanical anchoring measures. The experimental results show that the tensile steel bars in the lap section of the lap beams connected with UHPC have better performance on the bonding properties with concrete. Increasing the lap length promotes peak load but the average bonding strength of the lap beams on the contrary. With the increase of steel fiber volume content, the peak load and bond strength of the lap beams increase. Mechanical anchoring adopted lap beams show a higher peak load and bonding strength, where bent hooks treated lap beam shows the highest enhancement, with peak load and bond strength improved by 212.4% and 199.4%, respectively, and it is worth mentioning that the hooked lap steel bars yielded. Based on the equilibrium condition of axial force and bending moment at the peak point, the maximum tensile stress of the steel bar in the lap beam was calculated. A calculation method of the average bonding strength between the steel bar and UHPC was then established. The proposed method calculation results were compared with the centre pull-out test and brace lap test results subsequently.
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图 1 梁大样图及俯视图(单位：mm)
Figure 1. Sample and top view of beam (Unit: mm)
1-C40 Precast concrete sections; 2-A8@100; 3-Rough surface concave and convex depth 6 mm; 4-Post-cast section
图 2 梁截面配筋图及键槽示意图(单位：mm)
Figure 2. Beam cross-section reinforcement diagram and keyway schematics (Unit: mm)
图 8 UHPC-高强钢筋梁式搭接试件破坏示意图
Figure 8. Schematic diagram of UHPC-high strength rebar beam lap test specimen failure
图 9 UHPC-高强钢筋梁式搭接试件荷载-挠度曲线
Figure 9. Load-deflection curve of UHPC-high strength rebar beam lap test specimen
图 10 UHPC-高强钢筋梁式搭接试件跨中混凝土应变分析
Figure 10. Strain analysis of plane section of UHPC-high strength rebar beam lap test specimen
图 11 UHPC-高强钢筋梁式搭接试件搭接段钢筋应变分析
Figure 11. Strain analysis of steel bar in lap section of UHPC-high strength rebar beam lap test specimen
图 12 钢筋应力计算示意图
Figure 12. Schematic diagram of reinforcement stress calculation
$ h $-Lap beam section height; $ {h_{\text{0}}} $-Effective height of lap beam section; $ {d'} $-Distance from the point of action of the combined forces of the compression reinforcement to the outer edge of the beam; $ {A_{\text{s}}} $-Cross-sectional area of tensile reinforcement; $ {A_{\text{s}}^{\prime}} $-Cross-sectional area of compression reinforcement; $ {x_{\text{c}}} $-Height of compression zone of beam section; $ m $-The depth of the extreme UHPC tensile fiber below the neutral axis; $ {\varepsilon _{\text{t}}} $-Calculated tensile strain of UHPC; $ {\varepsilon _{\text{u}}} $-Compressive strain of UHPC at the extreme compression fiber; $ {\varepsilon _{\text{s}}} $-Actual strain in tensile reinforcement; $ {\varepsilon _{\text{s}}^{\prime}} $-Actual strain in compression reinforcement; $ {f_{\text{y}}} $-Yield stress of tensile reinforcement; $ {f_{\text{y}}^{\prime}} $-Yield stress of compression reinforcement; $ {f_{\text{t}}} $-Measured uniaxial tensile strength of dumbbell specimens; $ {\sigma _{\text{c}}} $-Compressive stress of UHPC at the extreme compression fiber; $ {T_{\text{s}}} $-Combined force of tensile reinforcement; $ {T_{{\text{UHPC}}}} $-Combined force of UHPC in the tension zone; $ {C_{\text{s}}} $-Combined force of compression reinforcement; $ {C_{{\text{UHPC}}}} $-Combined pressure of UHPC in the pressure zone
图 15 机械锚固措施对黏结强度的影响
Figure 15. Influence of mechanical anchoring measures on bond strength
表 1 梁式搭接试验试件参数设计及钢筋应力计算结果
Table 1. Parameter design of beam lap test specimen and results of reinforcement stress calculation
Number Type L Lap form Vf/% Peak load /kN fs/MPa Yield or not B1
B2C80
UHPC3d
3dStraight rebar lap
Straight rebar lap2%
2%23.5
95.2-
187Not
NotB3 UHPC 8d Straight rebar lap 2% 207.8 449 Not B4 UHPC 12d Straight rebar lap 2% 231.1 494 Not B5 UHPC 3d Straight rebar lap 3% 145.3 302 Not B6 UHPC 3d Hook treatment 2% 297.4 560 Yield B7 UHPC 3d Anchor plate 2% 124.1 257 Not B8 UHPC 3d One side weld 2% 124.3 257 Not B9 UHPC 3d Two side weld 2% 188.0 403 Not Notes: Type-Type of post-cast concrete in lap section; L-Lap length; d-Steel bar diameter; Lap form-Different mechanical anchorage measures; Vf-Fibre volume fraction; fs-Calculated tensile strength of rebar. 
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表 2 UHPC材料性能
Table 2. Material properties of UHPC
Vf/% fcu/MPa fc/MPa ft/MPa 2 123.3 113.2 6.22 3 135.6 122.7 7.01 Notes: fcu-Cubic compressive strength; fc-Prismatic compressive strength; ft-Tensile strength.
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表 3 钢筋力学性能
Table 3. Mechanical properties of reinforcement
Strength grade Diameter/
mmYield strength/
MPaUltimate strength/
MPaHPB300 8 357 529 HRB500 20 560 715
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表 4 UHPC-高强钢筋梁式试验对比
Table 4. Test comparison of UHPC-high strength rebar beam lap test
Number Type L Vf/% Lap form Center pull-out test Brace lap test Beam lap test τu2/τu1 τu3/τu2 τu1/MPa Failure mode τu2/MPa Failure mode τu3/MPa Failure mode B1
B2C80
UHPC3d
3d0%
2%Straight rebar lap
Straight rebar lap19.2
35.6SPF
SPF12.1
23.1SPF - BOF 0.63 - SPF 15.6 BOF 0.65 0.68 B3 UHPC 8d 2% Straight rebar lap 20.8 SPF 16.2 SPF 14.0 BOF 0.78 0.86 B4 UHPC 12d 2% Straight rebar lap 14.4 RF 12.3 RF 10.3 BOF 0.85 0.84 B5 UHPC 3d 3% Straight rebar lap 49.7 SPF 27.6 SPF 25.2 BOF 0.56 0.91 B6 UHPC 3d 2% Hook treatment - - 50.7 RF 46.7 BEF - 0.92 B7 UHPC 3d 2% Anchor plate - - 25.2 SPF 21.4 BOF - 0.85 B8 UHPC 3d 2% One side weld - - 27.1 SPF 21.4 BOF - 0.79 B9 UHPC 3d 2% Two side weld - - 28.4 SPF 33.6 BOF - 1.18 Notes: All the above specimens are made of HRB500 grade rebar, diameter is 20 mm, the concrete protective layer is 1.5d; τu1-Bond strength obtained by center poll-out test, τu2-Bond strength obtained by brace lap test, τu3-Bond strength obtained by beam lap test; SPF and RF represent the splitting pull-out failure and steel bar rupture failure respectively; BOF and BEF represent the bonding failure of steel bars and the bending failure of lap beams respectively.
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表 5 UHPC-高强钢筋搭接长度计算
Table 5. Lap length calculation of UHPC-high strength rebar
Lap length Vf/% Mechanical anchoring measures 3% Straight
rebar lapHook treatment Anchor plate One side weld lsy 9.8d 11.6d 6.7d 9.4d 8.6d lsu 13.7d 16.0d 10.9d 13.6d 12.6d Notes: lsy-Minimum lap length of steel bar yield; lsu-Minimum lap length of steel bar rupture.
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