Constitutive model of recycled brick aggregate geopolymer concrete under compression before and after elevated temperature
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摘要: 再生砖骨料地聚物混凝土(RBGC)是一种极具潜力的可持续建筑材料，但高温前后RBGC性能的相关研究较少。本文首先探究了胶凝材料用量和粗骨料类型对地聚物混凝土轴压本构模型的影响，发现随着胶凝材料的增加，RBGC抗压强度、劈拉强度、弹性模量和峰值压应变的变化幅度均小于普通骨料地聚物混凝土(NAGC)，RBGC的应力-应变曲线上升段线性段更长，下降段应力下降速度更快。其次研究了高温后RBGC和NAGC的力学性能，800℃时，RBGC强度和刚度损失分别比NAGC小22.1%和18.3%，发现RBGC表现出更好的耐高温性能，两种混凝土力学性能指标的计算应采用不同的模型。这是由于砖骨料温度膨胀系数接近地聚物砂浆，且高温下RBGC内部温度梯度小。最后，通过修正下降段形状参数，确定了高温前后RBGC和NAGC的应力-应变关系模型，模型与试验结果吻合较好。Abstract: Recycled brick aggregate geopolymer concrete (RBGC) is a sustainable building material with great potential, but there are few studies on the performance of RBGC before and after high temperatures. Firstly, the influence of the amount of cementitious material and the type of coarse aggregate on the axial compression constitutive model of geopolymer concrete was explored. It is found that with the increase of cementitious material, the compressive strength, split tensile strength, elastic modulus and peak compressive strength of RBGC decrease. The change amplitude of strain is smaller than that of ordinary aggregate geopolymer concrete (NAGC). The linear section of the rising section of the stress-strain curve of RBGC is longer, and the stress decreases faster in the falling section. Secondly, the mechanical properties of RBGC and NAGC after high temperature were studied. At 800°C, the strength and stiffness loss of RBGC are 22.1% and 18.3% smaller than that of NAGC respectively. And it is found that RBGC shows better high temperature resistance, and different models should be used to calculate the mechanical performance indicators of the two concretes. This is because the temperature expansion coefficient of brick aggregate is close to that of geopolymer mortar, and the internal temperature gradient of RBGC is small at high temperatures. Finally, by correcting the shape parameters of the descending section, the stress-strain relationship model of RBGC and NAGC before and after high temperature is determined, and the model is in good agreement with the test results.
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图 4 高温前GC应力-应变曲线
Figure 4. Compressive stress-strain curves of GC before elevated temperatures
图 5 NAGC和RBGC弹性模量和抗压强度的关系
Figure 5. Relationship between elastic modulus and compressive strength of NAGC and RBGC
图 6 NAGC和RBGC高温后典型破坏形态
Figure 6. Typical failure patterns of NAGC and RBGC after high temperature
图 7 再生砖骨料(RB)和地聚物浆体的热膨胀应变
Figure 7. Thermal expansion of recycled brick aggregate (RB) and geopolymer paste
图 8 高温后地聚物混凝土应力-应变曲线
Figure 8. Stress-axial strain curves of GC exposed to elevated temperatures
图 9 NAGC和RBGC温度对相对抗压强度、弹性模量、峰值应变和劈拉强度的影响
Figure 9. Effect of temperature on the relative compressive strength, elastic modulus, peak strain and splitting strength of NAGC and RBGC
图 10 提出的应力-应变关系与试验结果和文献模型的比较
Figure 10. Comparison of proposed stress-strain relationships with experimental results
表 1 粗骨料和细骨料的物理性能
Table 1. Physical properties of coarse and fine aggregates
Type Particle
size/mmSpecific
gravity24 h water
absorption/%Fineness
modulusCrush
index/%Cylinder compressive
strength/MPaNatural river sand ≤5 2.67 1.2 2.6 — — Limestone 5~20 2.64 0.83 — 7.9 — Recycled brick aggregate 5~20 1.74 10.92 — 21.3 4.7 
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表 2 配合比设计和力学性能
Table 2. Mix proportion design and mechanical properties
No. Mix proportion/(kg·m−3) Dry density/
(kg·m−3)Elastic
modulus/GPafc
/MPaBinder SS SH Sand Coarse
aggregateAdded
waterSP NAGC-1 300 188 75 568 1224 35.5 2.5 2231 21.58 26.10 NAGC-2 400 188 75 504 1224 35.5 4.5 2244 25.17 39.13 NAGC-3 500 188 75 450 1224 35.5 5.5 2287 27.60 50.29 RBGC-1 300 188 75 568 1105 73.5 3.5 1893 11.03 21.23 RBGC-2 400 188 75 504 1105 73.5 4.5 1915 13.38 29.97 RBGC-3 500 188 75 450 1105 73.5 5.5 1931 15.39 34.50 Notes：GC stands for geopolymer concrete, RB and NA stand for limestone crushed stone and recycled brick aggregate; the numbers 1 to 3 represent the target strength from lowest to highest; fc is the compressive strength of NAGC and RBGC.
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表 3 NAGC和RBGC抗压强度、弹性模量、峰值应变和劈拉强度拟合公式
Table 3. Fitting formulas of compressive strength, elastic modulus, peak strain and splitting tensile strength of NAGC and RBGC
Properties NAGC RBGC Compressive strength $ \dfrac{{{f_{{\text{cT}}}}}}{{{f_{\text{c}}}}} = \dfrac{1}{{1 + 13.75{{(T/1000)}^{3.70}}}} $ $ \dfrac{{{f_{{\text{cT}}}}}}{{{f_{\text{c}}}}} = - 0.84(T/1000) + 1.02 $ Elastic modulus $ \dfrac{{{E_{{\text{cT}}}}}}{{{E_{\text{c}}}}} = \dfrac{1}{{1 + 50.40{{(T/1000)}^{4.03}}}} $ $ \dfrac{{{E_{{\text{cT}}}}}}{{{E_{\text{c}}}}} = - 1.03(T/1000) + 1.02 $ Peak strains $ \dfrac{{{\varepsilon _{{\text{PT}}}}}}{{{\varepsilon _{\text{P}}}}} = 1 + 4.20{(T/1000)^{2.78}} $ $ \dfrac{{{\varepsilon _{{\text{PT}}}}}}{{{\varepsilon _{\text{P}}}}} = 1 + 1.16{(T/1000)^{1.76}} $ Splitting tensile strength $ \dfrac{{{f_{{\text{stT}}}}}}{{{f_{{\text{st}}}}}} = - 1.13(T/1000) + 1.00 $ $ \dfrac{{{f_{{\text{stT}}}}}}{{{f_{{\text{st}}}}}} = \left\{ \begin{gathered} - 0.69(T/1000) + 1.02{\text{,25}}^\circ C \leqslant {\text{T}} \leqslant 400^\circ C \\ - 1.52(T/1000) + 1.36{\text{,400}}^\circ C < {\text{T}} \leqslant 800^\circ C \\ \end{gathered} \right. $ Notes：fc , Ec , εp and fst are compressive strength, elastic modulus, peak strain and splitting tensile strength of the concrete at room temperature respectively; fcT , EcT , εpT and fstT are compressive strength, Elastic modulus, peak strain and splitting tensile strength of the concrete after high temperature respectively.
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表 4 NAGC和RBGC回归参数α,β和n
Table 4. Regression parameters α,β and n of NAGC and RBGC
Type Parameter Temperature(℃) 25 200 400 600 800 NAGC n 1.50 1.48 0.99 0.95 1.06 α 2.24 2.09 2.33 3.24 2.94 RBGC n 5.47 4.47 3.33 6.39 5.14 β 44.09 9.27 - - - α - - 7.54 5.73 4.62
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