素混凝土抗冻性能试验研究

为了进一步研究约束混凝土和无约束混凝土的抗冻性能差异,以约束混凝土模型为参照,制作相同配合比的素混凝土立方体块并在相同条件下进行0、20、40、60、100、120次的冻融腐蚀试验。主要分析冻融腐蚀下素混凝土试块抗压强度,质量损失以及吸水率等性能随冻融次数的变化规律,并建立素混凝土抗压强度和质量损失冻融损伤预测模型。试验结果表明:随冻融循环次数增加,素混凝土的抗压强度呈抛物线下降,干燥质量和饱水质量呈线性下降,而吸水率则呈双折线下降。从材料层次方面,粉煤灰对混凝土抗压强度损失,质量损失率和吸水率影响较大,而水灰比对以上指标影响较小。掺15%粉煤灰的抗压强度损失比不掺粉煤灰的混凝土大约20%~30%;质量损失率比不掺粉煤灰的大5%~6%,粉煤灰不利于抗冻性能。相同粉煤灰含量,低水灰比抗压强度损失,质量损失率和吸水率变化比高水灰比小,使用粉煤灰混凝土时易降低水灰比。     

早期受冻混凝土的抗氯离子渗透性能与力学性能相关性研究

对早期受冻混凝土的抗氯离子渗透性能和力学性能进行测试,分析了指标的变化规律及单因素条件变化下混凝土的渗透指标与力学指标的关系,研究发现:粉煤灰和水泥用量对混凝土的力学性能影响较小;随着含气量的增加,混凝土的力学性能先提高后降低;水灰比、水泥用量和粉煤灰掺量变化时,混凝土的动弹模量与电通量呈负相关关系,粉煤灰掺量变化时,两者相关性较差;含气量变化时,混凝土的动弹性模量与电通量有较好的非线性相关关系。因此,水灰比和水泥用量变化时,可以用电通量指标评价早期受冻混凝土的抗冻性能。     

混凝土干湿循环时间与抗腐蚀效果关系的探讨

通过混凝土抗硫酸盐侵蚀试验法研究了C50混凝土试件质量、相对动弹性模量损失率及试件抗压强度耐蚀系数与干湿循环时间的关系。研究结果表明干湿循环次数小于30次混凝土试件相对动弹性模量、抗压强度耐蚀系数损失率及质量变化率均较小,在30~120次之间,其变化率最大,120次干湿循环后相对动弹性模量、试件抗压强度耐蚀系数损失率及质量变化率又变得较小。     

冻融损伤混凝土力学性能衰减规律

为研究冻融作用对混凝土力学性能的影响,采用快冻法将混凝土盐冻或水冻至不同损伤程度后,测其动弹性模量、抗折强度、抗压强度和劈裂抗拉强度.以动弹性模量为损伤变量,分析了冻融损伤与抗折、抗压强度之间的关系,采用回归分析的方法建立了抗折强度衰减方程.结果表明,混凝土抗压、抗折、劈裂抗拉强度以及动弹性模量均随冻融循环作用次数的增加而逐步降低;抗折、劈裂抗拉强度以及动弹性模量的衰减速率随冻融循环作用次数的增加而不断增大,但抗压强度的衰减速率则是先增大后减小;在冻融循环次数相同的情况下,含气量越高、水灰比越低,混凝土的强度损失越小,其力学性能损失的大小顺序依次为:抗折强度和劈裂抗拉强度、动弹性模量、抗压强度;冻融损伤与抗折强度的相关性较好,但与抗压强度的相关性较差,当冻融损伤小于40%时,混凝土的抗压强度一般不低于其初始强度的70%.     

混凝土动弹性模量与超声声速及抗压强度的关系研究

混凝土抗压强度是衡量混凝土性能的综合指标,但目前判定混凝土冻融破坏采用的指标主要是混凝土动弹性模量,而工程中混凝土动弹性模量却无法测试,因此建立混凝土抗压强度与动弹性模量的关系对于实际工程耐久性评估具有重要的意义。通过不同水胶比、粉煤灰掺量、含气量混凝土在3.5%NaCl溶液中的快速冻融试验,测试了混凝土的动弹性模量、超声声时、抗压强度,根据测试结果建立了混凝土动弹性模量与超声声速及抗压强度之间的关系,为混凝土结构冻融损伤的评定提供了理论依据。     

冻融循环后再生混凝土的力学性能及损伤模型研究

通过改变冻融循环次数,研究了普通混凝土和粗骨料取代率100%的再生混凝土力学性能及抗冻性能的影响规律,以抗压强度损失率、质量损失率和动弹性模量损失率作为损伤变量建立冻融损伤模型,并针对内蒙地区对再生混凝土抗冻性进行寿命预测。研究结果表明：再生混凝土的抗压强度和劈裂抗拉强度与冻融循环次数呈反比关系,冻融循环每增加50次,抗压强度平均下降33.1%,劈裂抗拉强度平均下降33.0%;在冻融前期,普通混凝土与再生混凝土力学性能相差不大,但随着冻融次数的增加,二者性能均出现劣化且再生混凝土劣化程度大于普通混凝土,根据动弹性模量损失率建立的冻融损伤模型拟合精度更高,冻融损伤模型的建立可以直观清晰地反映再生混凝土宏观的力学性能变化。     

盐碱-干湿条件下混凝土强度及耐久性问题研究

分析了纳米CaCO_3掺入量、盐碱浓度等因素对混凝土在盐碱-干湿循环后的强度及耐久性的影响,通过150次的干湿循环,研究了混凝土抗压强度耐蚀系数、质量损失率、相对动弹性模量等耐久性指标随干湿循环的变化规律。试验结果表明,掺入1%的纳米CaCO_3可以使28 d混凝土试件获得较高的抗压强度,盐碱-干湿循环条件下2%掺量的纳米CaCO_3混凝土试件强度下降最低。纳米CaCO_3对混凝土干湿循环中后期的耐久性有明显的提高作用,其最佳掺量为2%,掺量超过2%后则会造成耐久性指标的下降。盐碱浓度对纳米混凝土耐久性的影响随干湿循环次数的增加而增大,且相对动弹模量较质量损失率指标更易受盐碱浓度的影响。     

早期受冻掺合料混凝土服役性能试验研究

为评定早期冻融损伤对混凝土结构服役期承载性能及耐久性能的影响规律,通过不同掺合料种类及掺量、不同起冻时刻的混凝土早期受冻试验,对比了不同受冻时刻混凝土服役期的表面损伤形态,研究了起冻时刻、掺合料掺量对早期受冻混凝土服役期抗压强度、劈拉强度、动弹性模量、渗透性的影响规律,探讨了早期受冻对混凝土服役性能的损伤机理。结果表明:起冻时刻对混凝土抗压强度、劈拉强度、动弹模的影响程度均表现为2h0.5h≈8h1d3d,劈拉强度损失最明显,动弹模损失次之,抗压强度损失最不显著;掺入20%粉煤灰或15%矿粉或20%煤矸石的混凝土早期受冻后的强度和动弹模损失最小,略小于普通混凝土;随着掺合料掺量的增加,同时刻受冻混凝土试件的抗渗水性和抗渗气性变差;未冻混凝土试件的抗渗性最好,3d受冻试件次之,1d受冻试件再次,而0.5、2、8h受冻试件最差甚至超出AutoClam设备的测试范围。     

无机水泥类脱空修补材料冻融损伤试验研究

采用典型无机类压浆材料,并控制其水灰比和含气量分别为0.38、0.44、0.50和1.6%、3.0%、5.0%、7.0%。以试验数据统计分析为基础,总结冻融环境下动弹性模量、抗压、抗折、质量增长率和体积膨胀率的变化趋势。试验结果表明压浆材料的抗压强度、抗折强度、动弹性模量的劣化速率随冻融循环次数的增加呈现出先增大后减小的趋势,而质量变化率和体积膨胀率则不断增加;在冻融循环作用次数相同时,采用高含气量可有效降低抗折强度、动弹性模量的力学损伤,而有效降低抗压强度的损伤则可通过降低水灰比的方法来实现;试件体积膨胀率和质量变化率最大值均在5%以内,这表明压浆材料在冻融循环条件下,具有良好的稳定和密实性。     

不同劣化作用对混凝土力学性能的影响

为探讨混凝土力学性能在不同劣化作用下的变化规律,对3种类型混凝土分别在冻融循环、干湿循环、碳化和氯盐侵蚀条件下的抗压强度和动弹性模量进行了试验研究,结果表明:不同劣化作用对混凝土力学性能的影响机理不同,冻融循环对混凝土力学性能损伤较大,冻融循环300次时,3种混凝土抗压强度损失率依次为30.1%、51.4%、23.3%,动弹性模量损失率依次为2.8%、10.1%、21.6%,氯盐侵蚀和短期内的干湿循环对混凝土的力学性能损伤不明显,而碳化对混凝土力学性能有促进作用。标准养护条件下混凝土抗压强度与动弹性模量具有较好的相关性,但在不同劣化因素作用下,这种相关性随混凝土类型、劣化因素类型而发生改变。     

The compressive strength of cement blocks permeated with an organic phase change material

The authors investigate the feasibility of constructing walls and partitions with enhanced heat storage capability from cement blocks permeated with an organic phase change material (pressed stearic acid). The blocks are lightweight, easy to produce and are cured in the same way as cement or concrete.The ratio of components (cement, stearic acid (SA) and water) was optimized with respect to compressive strength. With normal portland cement, the best compressive strength 2.2 MPa was obtained with a ratio C:SA:W of 1:0.8:0.65 (i.e. 32% stearic acid) after 28 days of curing. This was increased to 4.4 MPa when high early strength cement was used, about as strong as Sipporex, lightweight concrete or lightweight clay bricks.As expected, the compressive strength decreased linearly with temperature between 25°C and 100°C, at a rate of 15 kPa/°C. The drying shrinkage of the modules was found to be 50% greater than for cement mortars. The addition of fibreglass gave only a 15% increase in the compressive strength. Samples containing 35% fatty acid showed surface deterioration after 200 thermal cycles (between ?5°C and 6 °C) while none was observed with those containing 32%. The dynamic modulus of elasticity did not change after 200 cycles.     

高温作用后钼尾矿混凝土单轴受压性能试验研究

为研究钼尾矿混凝土高温后的单轴受压力学性能,进行了不同目标温度（20,200,300,400,600,800 ℃）条件下钼尾矿混凝土的轴心抗压强度、峰值应变、弹性模量、泊松比、破坏形态及质量变化的试验研究。结果表明：钼尾矿混凝土试件的质量损失率随温度的升高而增加,在800 ℃时质量损失率平均为6.52%;轴心抗压强度随温度的升高而逐渐降低,800 ℃时与常温相比平均降低70.04%,且随钼尾矿掺量的增加而降低;而峰值应变随温度的升高先减后增;弹性模量和泊松比都随温度的升高而降低,在800 ℃时弹性模量和泊松比平均比常温降低88.22%和35.66%。对于弹性模量,大体上随着钼尾矿掺量的增大而减小;而对于泊松比,钼尾矿掺量100%的混凝土略大于掺量50%的混凝土。根据试验结果,建立了钼尾矿混凝土高温后的单轴受压应力-应变本构方程。     

Silica fume admixture effect on the dynamic properties of concrete

The construction of concrete structures using silica fume (SF) additions is a common practice because it is well known that this pozzolanic material improves the compressive strength of concrete, among many other mechanical and physical properties. The response of a structure under dynamic actions is highly determined by the dynamic mechanical properties of its building material. In this work, it has been studied the influence of SF additions in quantities ranging from 0% to 15% of cement mass on the dynamic and static mechanical properties of concrete, such as: resonant frequencies, dynamic and static modulus of elasticity, damping ratio, compressive strength and flexural strength. The results prove that SF additions or replacements reduce both the dynamic modulus of elasticity and damping ratio of concrete. The dynamic elastic properties of concrete, with and without SF, present higher values than their static counterparts. These differences are smaller in concretes containing SF.     

Effect of moisture and temperature on the mechanical properties of concrete

Concrete mechanical properties are determined under laboratory conditions of ideal air temperatures between 20 and 22 °C and relative humidity between 40% and 60%. This paper describes the development of concrete mechanical properties when cured under different environmental conditions. Tests to measure modulus of elasticity, compressive strength, and split tensile strength were conducted at varying temperatures and humidity conditions to examine their effects on normal concrete. An environmental chamber was constructed in the laboratory using available materials. The chamber works in conjunction with a freezer to provide chilled air and a heat gun to provide hot air. The heating and cooling functions were controlled via a microcontroller. The moisture content in the concrete specimens was controlled by massing the specimens. The results indicate that concrete strength and modulus of elasticity are inversely related to temperature as well as moisture content in the concrete. Concrete modulus of elasticity was directly related to concrete compressive strength in both temperature and moisture testing. Mathematical formulas were developed for modulus of elasticity, compressive strength, tensile strength, and Poisson’s ratio.     

Strength reduction factors for structural rubbercrete

Many researches have been carried out to study the fresh and hardened properties of concrete containing crumb rubber as replacement to fine aggregate by volume, yet there is no specific guideline has been developed on the mix design of the rubbercrete. The experimental program, which has been developed and reported in this paper, is designed and executed to provide such mix design guidelines. A total of 45 concrete mixes with three different water to cement ratio (0.41, 0.57 and 0.68) were cast and tested for fresh and mechanical properties of rubbercrete such as slump, air content, unit weight, compressive strength, flexural strength, splitting tensile strength and modulus of elasticity. Influence of mix design parameters such as percentage of crumb rubber replacement, cement content, water content, fine aggregate content, and coarse aggregate content were investigated. Three levels of slump value (for conventional concrete mixes) has been selected; low, medium and high slump. In each slump level, water content was kept constant. Equations for the reduction factors (RFs) for compressive strength, flexural strength, splitting tensile strength and modulus of elasticity have been developed. These RFs can be used to design rubbercrete mixes based on the conventional mix (0% crumb rubber content)     

钢纤维混凝土高温后SHPB试验研究

采用改进的分离式Hopkinson压杆装置,结合应变直测技术,分别对常温以及经历400℃和800℃高温的普通混凝土和钢纤维混凝土（SFRC）进行了单轴冲击压缩试验,减少了传统的Hpokinson压杆试验中的入射波的高频震荡,使得应变率的波动性明显减小。经历400℃和800℃后,普通混凝土的峰值应力和弹性模量均有较大程度的降低,同时,试件的能量吸收能力大幅度下降;钢纤维混凝土的峰值应力也有较大程度的降低,但是弹性模量降低较少。     

再生粗骨料取代率对混凝土基本性能的影响

系统研究了相同水灰比情况下再生粗骨料取代率对混凝土基本性能的影响。试验中再生粗骨料取代率分别为0，30％，50％，70％和100％，保持混凝土的水灰比不变。主要研究了再生粗骨料取代率对混凝土立方体坍落度、抗压强度、棱柱体抗压强度、峰值应变和泊松比、弹性模量、劈裂抗拉强度以及抗折强度的影响。试验结果表明，再生粗骨料取代率对上述各性能指标均有一定影响，但程度不同。同时发现，除抗折强度外，普通混凝土各基本力学性能指标问的关系均不适用各种再生骨料取代率混凝土。     

高贝利特水泥混凝土性能

与硅酸盐水泥混凝土比较,研究了高贝利特水泥混凝土不同龄期的抗压强度,抗拉强度和抗拉弹性模量;高贝利特水泥混凝土的抗冻性、抗渗性和抗硫酸镁侵蚀性能。结果表明,高贝利特水泥混凝土7d龄期的抗压强度低,90d龄期的抗压强度是硅酸盐水泥混凝土的117．6％;28d龄期高贝利特水泥混凝土的抗拉强度和抗拉弹性模量分别是硅酸盐水泥混凝土的116．6％和94．8％;高贝利特水泥混凝土的抗冻性与硅酸盐水泥混凝土基本相同;抗渗和抗硫酸镬侵蚀性能优。高贝利特水泥混凝土早期强度低,后期强度增长率大,抗拉强度高,弹性模量低。高贝利特水泥混凝土的耐久性和后期力学性能优于硅酸盐水泥混凝土。     

冻融循环作用后钢筋混凝土梁受弯性能试验研究

为研究冻融循环作用后的钢筋混凝土梁受弯性能,对经历0、50、100、125次冻融循环的混凝土立方体试块进行抗压强度试验;设计并制作了48根钢筋混凝土梁试件,分别经历0、50、100、125次冻融循环后进行静力受弯性能试验;分析混凝土质量损失率、相对动弹性模量、相对抗压强度等参数与冻融循环次数的关系。研究冻融循环次数、混凝土强度等级对钢筋混凝土梁受弯性能的影响。结果表明：混凝土试块质量、相对动弹性模量和立方体抗压强度均随冻融循环次数的增加而下降,高强度等级混凝土可延缓冻融循环造成的破坏;部分设计适筋混凝土梁经历冻融循环作用后发生超筋破坏。推导了受弯梁经历冻融循环作用后发生超筋破坏的界限配筋率计算式,并通过试验验证了其正确性;冻融循环作用后钢筋混凝土梁的开裂弯矩、受弯承载力仍可采用现行混凝土结构设计规范相关理论计算。