Influence of the rheological properties of paste on the early-age tensile creep of high-volume fly ash concrete
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摘要: 混凝土中浆体的流变特性是影响其拉伸徐变的重要因素之一。通过纳米压痕技术分析大掺量粉煤灰水泥基浆体(掺量为60%)微观力学性能、微观徐变等流变特性；同时实验研究相同水泥基浆体的大掺量粉煤灰混凝土(HVFAC)拉伸徐变时变规律，提出考虑浆体流变特性的HVFAC拉伸徐变ZC模型预测表达式。结果表明：水泥基浆体在相同测试龄期时，粉煤灰对其微观徐变的发展有促进作用；而等质量粉煤灰替代时，微观徐变随着测试龄期的后延更快趋于收敛；粉煤灰、加载龄期对徐变发展的影响与其不含骨料水泥基浆体的微观徐变影响规律具有一致性。Et,28d/(EV+EH)、Et,28d/χφ、φ与相对抗压强度fc(t0)/fc,28d、C、τ与测试龄期相关性分析表明这些参数与函数y=axb有较好的吻合度。水泥基浆体流变特性参数与ZC拉伸徐变模型参数相结合建立考虑水泥浆体流变特性的HVFAC拉伸徐变发展的预测表达式能反映模型单元体结构。Abstract: It is one of important factors affecting on concrete tensile creep that the rheological properties of concrete paste. The microscopic mechanical properties and rheological characteristics of cementitious paste containing high volume fly ash (60%) were analyzed by nanoindentation. And the same time, the tensile creep law dependent on ages of high-volume fly ash concrete containing same cementitious paste was experimental studied. The predictive function expressions of the HVFAC tensile creep considering the rheological properties of paste were proposed. The nanoindentation results show that the fly ash promotes the paste’s micro creep development of paste at the same test ages, and the micro creep of paste tends to converge quickly with the delay of test age which the cement is replaced with the same quality fly ash. The influence of fly ash and loading age on the development of HVFAC creep is consistent with the influence law of cementitious paste’s micro creep without aggregate. Base on the correlation analysis on the parameters of Et,28d/(EV+EH), Et,28d/χφ, φ and relative compressive strength fc(t0)/fc,28d, the parameters of C, τ and test ages, it shows that these parameters are in good agreement with the function y = axb. The tensile creep prediction expression of ZC model considering the rheological properties of cement paste can reflect the structure of the model unit cell.
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Key words:
- nanoindentation /
- tensile creep /
- rheological properties of paste /
- fly ash /
- ZC model
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图 1 基于纳米压痕技术表征浆体微观徐变行为
Figure 1. Characterization of paste microscopic creep behavior based on nanoindentation technique
hmax—Maximum indentation depth; hf—Residual indentation depth after complete unloading; hc—Indentation contact depth; S: The contact stiffness
图 2 ZC模型单元体徐变机理及混凝土中浆体微观徐变
Figure 2. The creep mechanism of ZC model the paste micro creep of concrete
The EM(t) and EV(t) is the elastic coefficient of elastic element Maxwell and Kelvin, respectively; The ηM(t) and ηV(t) is the viscous coefficients of viscous element Maxwell and Kelvin respectively; and the EH(t) is the elasticity coefficient in Hooke's element.
图 3 持荷阶段的HVFAC拉伸徐变实验场景
Figure 3. Experimental scene of tensile creep specimens of HVFAC during load holding stage
图 4 P0、P60试样微观力学性能的平均值变化
Figure 4. Mean changes in micromechanical properties of P0 and P60 specimens
图 5 不同粉煤灰掺量下的微观徐变经时变化：(a) P0、(b) P60
Figure 5. Variation of microscopic creep through time for different fly ash dosages: (a) P0, (b) P60
图 6 FA0、FA60拉伸徐变实验值与ZC模型预测值
Figure 6. Tensile creep experimental values and ZC model predictions of FA0 and FA60
图 7 FA0、FA60模型参数与相对抗压强度的相关性
Figure 7. Fitted relationships between the parameters in the model and relative compressive strength for FA0 and FA60
表 1 水泥和粉煤灰的化学成分
Table 1. Chemical compositions of fly ash and cement
Material Mass fraction/wt % CaO SiO2 Al2O3 MgO Fe2O3 SO3 K2O Na2O Others Cement 62.04 22.07 4.23 4.01 3.04 2.71 0.762 0.392 0.746 Fly ash 4.43 51.63 33.98 1.16 4.40 0.260 0.905 0.888 2.347 
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表 2 净浆配合比
Table 2. Proportions of paste
No. W/(C+FA) FA/% C/% P0 0.4 0 100 P60 60 40 Notes: W—Water; C—Cement; FA—Fly ash; P0—Paste group without fly ash; P60—Paste group with 60% of fly ash replacing cement by equal mass.
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表 3 HVFAC配合比(kg/m3)
Table 3. Proportions of HVFAC (kg/m3)
No. W/(C+FA) Cement Fly ash Water Fine aggregate Coarse aggregate Superplasticizer FA0 0.4 450 0 180 654 1113 0.720 FA60 180 270 180 654 1113 0.855 Notes: FA0—Group of concrete without fly ash; FA60—Group of concrete with 60% of fly ash replacing cement by equal mass.
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表 4 HVFAC基本力学性能
Table 4. Basic Mechanical Properties of HVFAC
No. Age of testing/d Compressive strength/MPa Splitting tensile strength/MPa Tensile elastic modulus/GPa FA0 3 46.24 4.00 37.36 7 56.02 4.80 37.80 28 67.75 5.79 40.43 FA60 3 11.56 1.51 24.30 7 16.83 1.71 26.28 28 24.48 2.74 28.28
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表 5 P0、P60的微观徐变参数
Table 5. Microscopic creep parameters for P0, P60
No. Age of testing /d C /GPa τ /s P0 3 97.1 6.0 7 106.5 8.1 28 110.4 9.3 P60 3 89.0 2.6 7 99.5 3.1 28 108.9 4.7 Notes: C—Contact creep modulus, τ—Characteristic time of the paste.
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表 6 FA0、FA60的ZC模型参数
Table 6. ZC model parameters for FA0, FA60
No. Loading age/d φ 1/χφ/
10−6·MPa−11/(EV+EH) /
10−6·MPa−1FA0 3 0.191 24.60 11.50 7 0.159 16.39 8.89 28 0.148 6.04 3.98 FA60 3 0.218 56.47 19.65 7 0.204 29.39 14.93 28 0.165 17.82 5.01 Notes: φ—Coefficient affecting the growth rate of the coefficient of viscosity; The parameter of 1/χφ and 1/(EV+EH) is the final specific creep of the spring assemblies Maxwell and Kelvin, respectively.
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