摘要
基于压入法原理,提出了1种混凝土超早期弹性模量的测试方法. 采用数值分析方法设计试件尺寸,选用30°球锥形压头作为混凝土超早期弹性模量的测试压头,优化压入深度为5 mm. 通过改变混凝土的水胶比和粉煤灰掺量,并与超声波法相对比,对该方法进行验证. 以初凝时刻的弹性模量0值为起点,传统应力-应变方法最早测得的弹性模量为终点,用这2点之间的连线预测混凝土超早期弹性模量.该预测法与压入法试验结果相比,误差均在30%以内.
开裂是影响混凝土耐久性的重要因
水泥基材料超早期弹性模量分为动弹性模量和静弹性模
一些学者采用纳米压痕法研究水泥基材料的力学行
本文讨论了采用压入法测试混凝土超早期弹性模量的可能性,以期为水泥基材料超早期力学性能的研究提供一种简单、高效的测试方法.
ISO 14577‑1:2015《Metallic materials—Instrumented indentation test for hardness and materials parameters—Part 1: Test method》按压入深度h和载荷P,将压入法分为:纳米范畴,h≤200 nm;显微范畴,P<2 N、h>200 nm;宏观范畴,2 N≤P≤30 kN. 宏观压入试验借助传感器,记录压头在加载-卸载过程中的P‑h曲线. 对该曲线的分析普遍采用Oliver‑Pharr方
(1) |
式中:hf为残余压入深度;B和m为拟合参数.
对
(2) |
式中:S为接触刚度;hmax为最大压入深度.
压头接触深度ho&p的计算式为:
(3) |
式中:ε为压头形状参数,对于圆锥形压头,ε=0.73;对于球锥形压头,ε=0.75.Pmax为最大压入载荷.
根据接触力学与弹性力学求得接触刚度S与折合弹性模量Er的关系,其计算式为:
(4) |
其中 | (5) |
式中:β为压头的形状参数,对于轴对称压头,β=1;A为刚性压头的接触面积;E和ν分别为测试材料的弹性模量和泊松比;Ei和νi分别为压头的弹性模量和泊松比.
需要说明的是,本研究属于压入法的宏观测试,可认为压头的制作误差与磨损的影响相对较小,故接触面积不必修正,近似为压头与试件材料接触的投影面积.
相比水泥石和砂浆,混凝土中因粗骨料的存在,其具有明显的不均匀性. 为减小尺寸效应的影响,通过数值模拟方法设计混凝土试件尺寸,以满足基本假设中条件(4)的要
本文选用有限元软件Abaqus进行仿真分析,研究试件尺寸对压入行为的影响,以确定合理的压入试件尺寸. 控制压入深度不变,改变试件半径和厚度,分析压入荷载的差异. 考虑到所使用的钢制圆锥压头和圆柱体混凝土试件均为旋转轴对称体,建立轴对称模型. 压头选用30º球锥压头,钢制压头的弹性模量为200 GPa,泊松比为0.30. 假定混凝土为线弹性体,其超早期弹性模量为300 MPa、泊松比为0.35. 接触类型选择面-面接触,接触属性的切向行为设定为无摩擦,法向行为设定为硬接触. 边界条件为:试件对称轴被赋予轴对称边界;试件底面约束轴向位移,即相当于将试件置于刚性底座上. 由于压入试验本质上是非线性接触问题,所以选取适合用于接触分析的C4X4R(四节点双线性缩减积分带沙漏控制的四边形)单元,单元形状选为四边形. 控制hmax为5 mm,并将试件半径R保持为hmax的20倍(100 mm).试件底部为固定端,不受外力,得出不同厚度H试件的最大压入荷载Pmax.
试件厚度与最大压入深度的比值H/hmax对压入行为的影响如
H/hmax | 60.0 | 50.0 | 20.0 | 10.0 | 7.5 | 5.0 |
---|---|---|---|---|---|---|
Fmax/kN | 0.998 7 | 1.011 0 | 1.073 0 | 1.135 0 | 1.193 0 | 1.344 0 |
Relative error/% | 0 | 1.2 | 7.4 | 13.6 | 19.5 | 34.6 |
Note: Relative error=[(indentation value-maximum size specimen indentation value)/maximum size specimen indentation value]×100%.
控制hmax为5 mm、H为150 mm,建模并设计试件半径. 试件半径与最大压入深度的比值R/hmax对压入行为的影响如
R/hmax | 100 | 60 | 30 | 20 | 10 | 5 |
---|---|---|---|---|---|---|
Fmax/kN | 1.061 | 1.061 | 1.057 | 1.047 | 0.976 | 0.787 |
Relative error/% | 0 | 0 | -0.3 | -1.3 | -8.0 | -25.8 |
为便于试验且兼顾经济性,将模具设计成半径为300 mm、高度为150 mm的圆柱形模具,模具材料选为PVC材质. 每个试件以圆心为原点,环向间距200 mm均匀打点,模具尺寸及压入位置如

图1 模具尺寸及压入位置
Fig.1 Mold size and press‑in position(size:mm)
原材料选用秦皇岛浅野P·O 42.5R普通硅酸盐水泥、秦皇岛热电厂Ⅱ级粉煤灰和自来水、细度模数为2.7的中砂、粒径为5~20 mm的玄武岩碎石. 水泥和粉煤灰的化学组成和比表面积如
Material | Chemical composition(by mass)/% | Specific surface area/( | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
CaO | SiO2 | Al2O3 | SO3 | Fe2O3 | MgO | K2O | TiO2 | P2O5 | ||
Cement | 62.30 | 19.70 | 5.84 | 4.14 | 3.13 | 3.03 | 0.87 | 0.49 | 0.12 | 350 |
Fly ash | 4.18 | 48.50 | 41.00 | 0.38 | 2.91 | 0.14 | 0.65 | 1.45 | 0.20 | 390 |
Specimen No. | Mix proportion/(kg· | mW/mB | |||
---|---|---|---|---|---|
Stone | Sand | Cement | Fly ash | ||
C‑0.38 | 1 200 | 676 | 400 | 0 | 0.38 |
C‑0.42 | 1 200 | 676 | 400 | 0 | 0.42 |
C‑0.44 | 1 200 | 676 | 400 | 0 | 0.44 |
C‑0.46 | 1 200 | 676 | 400 | 0 | 0.46 |
C‑0.48 | 1 200 | 676 | 400 | 0 | 0.48 |
C‑F0.44 | 1 200 | 676 | 320 | 80 | 0.44 |
考虑混凝土粗细骨料粒径的大小及特

图2 不同形状压头示意图
Fig.2 Sketch maps of indenters with different shapes(size:mm)
为保证压入时间相同,采用多台压力机并用不同压头同时测试混凝土超早期试
将压入时间设为1 mi

图3 采用90°圆锥压头测得的C‑0.46混凝土试件的荷载-位移曲线
Fig.3 Load‑displacement curves of C‑0.46 concrete specimen with cone indenter of 90º
由
不同压头和不同压入深度试件的弹性模量曲线如

图4 不同压头和不同压入深度的弹性模量曲线
Fig.4 Elastic modulus at different indenters and indentation depths
针对不同粉煤灰掺量、水胶比和初凝时间的混凝土试件,采用优选后的加载速率5 mm/min和30°球锥压头,进行压入法试
分别选用水胶比0.42、0.44和0.46,制备混凝土试件进行压入法测试,并与对应的传统应力-应变方法进行对比,结果见

图6 不同配合比混凝土试件的弹性模量曲线
Fig.6 Elastic modulus curves of concrete specimens with different mix proportions
参考C‑0.46混凝土试件的初凝时间测试试验,将混凝土中5 mm以上的粗骨料筛除,参照现有超声波测试方法,采用康科瑞NM‑4A非金属超声波检测分析仪,测试剩余砂浆的纵横波波速. 其中横波(Vs)测试主频为100 kHz,试件长200 mm;纵波(Vp)测试主频为50 kHz,试件长25 mm. 根据纵横波测试结果求得C‑0.46混凝土试件是动弹性模量

图7 C‑0.46混凝土试件的弹性模量曲线
Fig.7 Elastic modulus curves of C‑0.46 concrete specimen
由
混凝土在初凝之前,由于水泥基材料没有固化,还具有流动性和可塑性,其弹性模量值接近于

图8 各混凝土试件超早期弹性模量曲线对比
Fig.8 Comparison of ultra‑early elastic modulus curves of various concrete specimens
(1)通过Abaqus 仿真分析,研究了混凝土超早期压入行为的尺度效应,得出可忽略尺寸影响的最小混凝土压入试件的尺寸(半径100 mm,厚度150 mm). 以此为依据,设计了混凝土超早期弹性模量压入法测试模具尺寸和压入位置.
(2)设计制造了5种压头,试验表明,30º 球锥压头比90º 球锥压头更加适合于压入法测试混凝土超早期弹性模量,压入深度选为5 mm为宜.
(3)对于不同配合比的混凝土,采用压入法与传统应力-应变方法测得的超早期弹性模量的相对误差均在20%以内,证明采用压入法测试混凝土超早期弹性模量具有可行性. 通过超声波测试混凝土的动弹性模量与压入法静弹性模量曲线趋势一致.
(4)混凝土的超早期弹性模量与时间呈线性函数关系. 基于该结论,提出1种相对误差在30%以内的混凝土超早期弹性模量的简单预测方法——测试初凝时间并设定其弹性模量为0 MPa,以传统应力-应变方法最早测得的弹性模量为终点,通过此2点数值所在直线对在混凝土超早期任意时间的弹性模量进行预测.
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