单面干燥条件下混凝土的收缩变形分布特征

通过固定多组位移传感器,研究了单面干燥条件下不同水灰比混凝土试件距离表面不同深度处的收缩变形,同时埋置湿度传感器,研究了不同深度处混凝土中相对湿度变化.结果表明:混凝土试件内层(距表面55 mm和100 mm)的收缩和相对湿度降低明显小于混凝土近表层(距表面10 mm)的;水灰比越小,沿混凝土干燥表面垂直方向的收缩与湿度梯度差越大:单面干燥条件下,不同深度混凝土层的收缩与其相对湿度降低值之间均存在较好的线性关系,在达到相同湿度条件时的表层混凝土收缩小于内层混凝土.     

混凝土超早期收缩试验与模拟

通过测定混凝土早期变形随龄期的发展规律,定义了基于变形的凝结时间和基于内部湿度发展的临界时间,研究了3个强度等级混凝土的凝结时间和临界时间随水胶比的变化规律,并对湿度饱和期收缩进行了模拟。结果表明：混凝土水胶比越大,凝结时间越长,临界时间也越长,湿度饱和期收缩越小;凝结时间与临界时间之差随水胶比的减小而减小;基于水泥水化程度和刚度修正的混凝土早期收缩模型,能够反映混凝土早期收缩发展特征,模拟结果与试验结果吻合良好,模型可以较好地预测混凝土湿度饱和期收缩的发展。     

再生骨料混凝土在剪力墙上应用的可行性试验研究

再生骨料混凝土应用在钢筋混凝土剪力墙上,不但要满足力学方面的性能要求,还要满足混凝土材料方面的耐久性能要求.本次试验从材料稳定性方面考虑,研究了再生骨料混凝土在约束条件下剪力墙上产生的干燥收缩裂缝,并利用混凝土表面应变片和内部应变片分别对约束条件下再生骨料混凝土剪力墙以及对比用普通混凝土剪力墙进行了干燥收缩应变测试.试验结果表明:混凝土的内部收缩应变明显小于外表面的收缩应变,并且随着再生粗细骨料置换率的增大,钢筋混凝土剪力墙上产生的微裂缝比普通混凝土剪力墙多.     

不同骨料用量与尺寸对混凝土干缩性能影响

试验以不同骨料用量与尺寸为变量,研究了C30混凝土配合比条件下对混凝土干缩性能的影响,试验结果表明,随着混凝土干燥收缩龄期的增加,不同组配C30混凝土的干燥收缩长度变化率都有所增加;再生骨料组配混凝土的干燥收缩长度变化率要比普通混凝土大;在相同混凝土配比条件下随着骨料用量(体积率)的增加,不同组配混凝土的干燥收缩长度变化率都有所下降趋势,普通天然骨料体积变化率对混凝土干燥收缩长度变化率影响更明显,骨料用量越多,干燥收缩率越小;但再生骨料随着体积率的增大对混凝土干燥收缩长度变化率的影响相对有限;在相同配比条件下适当增加粗骨料的用量在一定程度上可以缓解混凝土的干燥收缩.     

硫酸钠掺量对混凝土早期收缩开裂的影响

研究了分别掺加占胶结料质量分数1％,2％的硫酸钠对混凝土早期收缩开裂的影响。用非接触式混凝土收缩测量仪及板式混凝土开裂架进行实验。用电子显微镜对硫酸钠混凝土试件早期亚微观结构进行较为直观的研究分析。结果表明：硫酸钠对混凝土早期收缩开裂在不同孔径中的作用不尽相同。硫酸钠对混凝土早期收缩的影响与水胶比及硫酸钠的掺量有一定的关系。硫酸钠掺量相同情况下,水胶比较小混凝土早期收缩值较大;在相同水胶比情况下,硫酸钠掺量较大混凝土早期收缩值较小。硫酸钠对混凝土早期开裂的影响与养护方式相关,空气中养护与密封条件下养护这2种养护制度相比,在空气中养护混凝土表面出现裂纹的时间较早,裂纹较宽且较多。在这2种养护制度下,水分蒸发速度的不同决定了硫酸钠结晶初始深度的不同,是硫酸钠对混凝土早期开裂的主要影响机理。     

轻质骨料对泡沫混凝土性能的影响及其内养护机制

为降低泡沫混凝土干燥收缩,采用轻质骨料(页岩陶粒及粉煤灰陶粒)制备泡沫混凝土,研究了轻质骨料对泡沫混凝土抗压强度、干燥收缩、内部湿度及孔结构等的影响,并分析了轻质骨料的内养护机制。结果表明,轻质骨料能够调节泡沫混凝土的内部湿度,约束基体的变形,降低泡沫混凝土的干燥收缩。而轻质骨料也会在泡沫混凝土中引入缺陷,导致其强度降低。轻质骨料中大于100 nm的墨水瓶孔是内养护效果的控制因素,大于100 nm的墨水瓶孔数量越多,轻质骨料释水能力越强,内养护效果越好,制备的泡沫混凝土干燥收缩越小。     

超细矿物掺合料对高强混凝土干燥收缩影响

开展了微珠、超细矿粉及硅灰三种超细矿物掺合料对高强混凝土干燥收缩变化规律的研究,采用干燥收缩模型对试验结果进行拟合,并预测其收缩终值.结果表明:微珠和超细矿粉均能减小高强混凝土干燥收缩,且随掺量增加干燥收缩减小越明显;而硅灰的加入会增大高强混凝土的干燥收缩,其掺量越高,混凝土干燥收缩程度越大.采用指数方程得到的预测曲线随着龄期的发展与实测收缩值接近,拟合优度均达到99％以上.     

戈壁地区不同养护方式下混凝土箱梁养护效果试验研究

为研究戈壁地区混凝土箱梁在不同养护方式下的各项性能及在这种特殊环境下的实用性,对实验原材料在现场进行验证试验研究其养护效果.结果表明:与塑料薄膜和土工布养护相比,橡塑板养护早期强度增长更快;不同养护方式下混凝土水化热均在20 h后达到峰值,并呈现正弦规律的周期性变化,给出了箱梁的水化热温度分布规律;箱梁应变的产生是由混凝土自收缩和温度共同作用的结果,通过曲线规律分析出了收缩应变和温度应变的作用阶段;通过微观结构测试,分析出了不同养护方式下混凝土孔结构的分布特征,具有较小孔间距系数的橡塑板养护力学性能优于其他两种养护方式.     

养护环境及置换率对再生混凝土干燥收缩裂缝的影响

试验以不同环境、不同再生粗骨料置换率为变量对C30再生混凝土抵抗干燥收缩裂缝性能进行了基础性试验研究,并与普通混凝土试件进行对比.试验结果表明,在相对湿度(RH60％)相同条件下,再生混凝土与普通混凝土随着干燥环境温度的提高,自由干燥收缩长度变化率逐渐加大;在相同干燥温度条件下,随着再生粗骨料置换率的增加,混凝土自由干燥收缩率也会加大;在环境相对湿度(RH60％)相同条件下,混凝土的干燥温度越高,早期干燥收缩裂缝产生的龄期就越早;在混凝土干燥温度相同条件下,再生粗骨料置换率越大,早期干燥收缩裂缝产生的龄期就越早,再生粗骨料的置换率对再生混凝土极限抗拉强度的影响有限.     

大温差地区不同养护方式下混凝土桥墩温度场及变形性能分析

我国西北大部分地区多为干燥、大风、大温差的特殊气候环境,这种特殊的气候环境给混凝土早期开裂带来了不利的影响,混凝土桥墩的开裂受到多方面因素的影响,水化热温差过大是引起混凝土桥墩早期开裂的主要原因,因此,研究混凝土桥墩早期温度场对防止桥墩早期裂缝有重要的指导意义,不同的养护方式下混凝土桥墩内外温差分布不同.本次试验采取三种不同的养护方式,结果表明:自然养护会导致大温差地区混凝土桥墩产生较大的早期温度应力应变,可能会引起温度裂缝,土工布包裹养护对混凝土有保温作用,可以改善混凝土内部温差,新材料包裹养护效果最好,早期良好的保温保湿效果使桥墩内部温度分布较为均匀,并能大幅度减小温度应力应变,起到良好的养护作用.     

Autogenous shrinkage of high-strength concrete containing silica fume under drying at early ages

The present study investigated experimentally autogenous shrinkage of high-strength concrete containing silica fume under drying at early ages. The influence of drying on hydration of cementitious materials in the high-strength concrete with water-binder ratios of 0.25, 0.35 and 0.45 was evaluated based on bound water content (BWC), which was exposed to drying at the ages of 0.5, 1.0 and 3.0 days, respectively. By establishing the relationship between the BWC and autogenous shrinkage strain under sealed conditions, autogenous shrinkage strain under drying conditions and drying shrinkage strain were separated from total shrinkage strain, and, then, the contribution of autogenous shrinkage in total shrinkage was discussed. The results showed that the percentage of autogenous shrinkage was macroscopically 50-20% based on the present method, while that was 70-30% based on the conventional superposition principle (SP). The latter resulted in overestimating autogenous shrinkage strain under drying conditions.     

外加剂对混凝土收缩与开裂性能影响的试验研究

针对传统混凝土中加入矿物外加剂会导致混凝土开裂的问题,提出在传统方法上加入粉煤灰和硅灰。为验证方法的可行性,采用不同实验配方比,并结合单掺和复掺方式,对上述方法进行验证。实验表明:复掺30%~40%的粉煤灰和矿粉可有效降低混凝土的开裂面积;硅灰与粉煤灰和矿粉在二元或三元复合下可降低砂浆自收缩和干燥收缩,且混凝土强度不会降低。由此得出混凝土收缩与开裂和砂浆收缩存在显著相关性,砂浆的自收缩和干燥收缩是影响开裂的主要因素。     

混凝土内部湿度场与自约束应力场的研究

为防治混凝土自约束作用导致的开裂现象,对混凝土内部湿度场和自约束应力场开展了研究。通过位移传感器、温湿度传感器对不同强度、养护条件下的混凝土收缩、内部温度和湿度进行测试。之后,通过理论推导,建立混凝土内部湿度、应变与内部湿度关系的理论计算模型。研究表明,混凝土内部湿度随着水泥水化作用和干燥作用的增强而降低。混凝土湿度扩散系数是表征其内部湿度的函数,随着与干燥面的间距增加,混凝土内部湿度扩散系数增大。混凝土的收缩变形与内部湿度之间存在显著的相关性,计算模型与试验结果拟合良好。混凝土内部湿度场的存在导致了应变梯度的存在,进而使混凝土内部产生自约束应力。相同环境条件下,高强混凝土内部自约束应力高于普通强度混凝土。     

Integrative Study on the Effect of Internal Curing on Autogenous and Drying Shrinkage of High-Strength Concrete

High-strength concrete generally has a low water-to-cement ratio, which in turn increases the possibility of early-age cracking due to its high autogenous shrinkage. In this article, the effect of internal curing using presoaked lightweight aggregate (PSLWA) in high-strength concrete on shrinkage and interior humidity is investigated by continuously measuring the deformation and interior humidity of the test specimen under plastic film sealing and surface drying conditions since specimen cast. Four mixture proportions with an induced curing water-to-cement ratio (W _IC/C) of 0, 0.04, 0.08, and 0.12 were used in experiments. The experimental results show that the decrease in interior humidity was gradually reduced with an increase in W _IC/C. Accordingly, both autogenous and drying shrinkage of concrete gradually decreased with an increase in W _IC/C. Internal curing cannot completely eliminate autogenous shrinkage because part of the autogenous shrinkage is developed within the humidity-saturated stage in early-age concrete.     

圆环法研究再生微粉混凝土收缩性能

采用圆环试验方法研究了再生微粉掺合料对混凝土收缩开裂趋势的影响,并测定了再生微粉混凝土早期干燥收缩性能。试验结果表明,圆环试验能给混凝土提供了完全的、均匀的约束,能合理评价混凝土抵抗自收缩和干燥收缩综合作用开裂的能力。再生微粉掺合料能明显降低混凝土收缩开裂趋势及干燥收缩性能;且微粉细度越大,再生微粉混凝土抗收缩抗裂性能越好。试验结合SEM试验结果,分析了再生微粉掺合料减小水泥基材料收缩开裂趋势的原因。     

Total deformation of loaded drying concrete

Usually the total time-dependent deformation of a loaded drying concrete specimen is subdivided into two components, these two being creep and shrinkage. But it turns out that the sum of both, pure creep and pure shrinkage, is always less than the deformation under load and simultaneous drying. Until now the question remains whether there does exist a special mechanism of drying creep or load induced shrinkage. Creep under sealed conditions can be analytically expressed by means of rate theory. It is shown here that tensile stresses in the drying outer shell usually overcome tensile strength of concrete. Thus crack formation takes place and the internal stress is redistributed. Theoretical predictions are compared with experimental results. It may be concluded that creep and shrinkage of a loaded drying specimen cannot be separated. The total deformation is a consequence of the superposition of internal and external state of stress.     

半干硬混凝土外方内圆偏心约束收缩试验

采用外方内圆偏心约束试验方法对半干硬高掺量橡胶集料混凝土及未掺橡胶的半干硬普通混凝土进行约束收缩试验,并用静态电阻应变仪采集钢环内表面0°区、90°区、180°区在120 h内的环向应变值.试验结果表明:6组外方内圆偏心试件开裂截面均处在试件最窄处(0°区);半干硬橡胶混凝土早期收缩量及收缩率小于半干硬普通混凝土;半干硬橡胶混凝土试件开裂时间晚于半干硬普通混凝土,其前者裂缝宽度小于后者;所测应变值也首次验证了:0°区>90°区>180°区这一规律.计算0°区/90°区和0°区/180°区应变相对值,发现这两个值随时间变化趋于稳定.这表明,用静态线弹性有限元模拟外方内圆偏心约束收缩试验有其合理性.     

A thermal active restrained shrinkage ring test to study the early age concrete behaviour of massive structures

In massive concrete structures, cracking may occur during hardening, especially if autogenous and thermal strains are restrained. The concrete permeability due to this cracking may rise significantly and thus increase leakage (in tank, nuclear containment…) and reduce the durability.The restrained shrinkage ring test is used to study the early age concrete behaviour (delayed strains evolution and cracking). This test shows, at 20 °C and without drying, for a concrete mix which is representative of a French nuclear power plant containment vessel (w/c ratio equal to 0.57), that the amplitude of autogenous shrinkage (about 40 μm/m for the studied concrete mix) is not high enough to cause cracking. Indeed, in this configuration, thermal shrinkage is not significant, whereas this is a major concern for massive structures.Therefore, an active test has been developed to study cracking due to restrained thermal shrinkage. This test is an evolution of the classical restrained shrinkage ring test. It allows to take into account both autogenous and thermal shrinkages. Its principle is to create the thermal strain effects by increasing the temperature of the brass ring (by a fluid circulation) in order to expand it. With this test, the early age cracking due to restrained shrinkage, the influence of reinforcement and construction joints have been experimentally studied. It shows that, as expected, reinforcement leads to an increase of the number of cracks but a decrease of crack widths. Moreover, cracking occurs preferentially at the construction joint.     

Drying shrinkage of portland cement pastes I. Microcracking during drying

The occurrence of microcracking of portland cement pastes during drying has been studied by comparing the effects of specimen thickness on shrinkage and cracking using light microscopy. Increases in specimen thickness tended to impede drying and wetting, but there were only slight changes (less than experimental errors) in total and reversible shrinkage once equilibrium was attained. Although microcracking occurred at the beginning of drying whenever the thin specimen (thickness <2mm) was suddenly exposed to low relative humidity (～50%), the cracks eventually closed up. It was concluded that no matter whether or not this microcracking happened, the shrinkage of the specimen after reaching equilibrium was unrestrained.