自密实混凝土抗冻性能研究

通过宏观和微观两方面研究C60自密实混凝土抗冻性。宏观研究表明C60自密实混凝土不能经受300次快速冻融循环,需掺入引气剂以保证其抗冻耐久性,自密实混凝土抗冻性能与高强混凝土类似,冻融破坏的主要因素是在冻融过程中温度变化而产生的疲劳应力所造成。微观研究表明C60自密实混凝土的相对耐久性指数与混凝土含气量、气泡间隔系数及微观孔径分布存在一定关系。     

“重点工程混凝土安全性的研究”成果简介之二“混凝土耐久性能的研究”

“混凝土抗冻性的研究”（96－535－02－01）混凝土抗冻性研究取得以下主要成果：　　1．通过控制水灰比、含气量、气泡参数等指标,开发了F300的高抗冻混凝土和F600的超抗冻混凝土,掺粉煤灰的高抗冻混凝土已成功地应用于三峡大坝工程,并取得良好的技术经济效果。　　2.进行了C100、C80、C60和C60引气高强混凝土抗冻性的研究,C100和C80高强混凝土具有极高的抗冻性,而C60混凝土不能满足高抗冻混凝土的要求,C60引气和C80混凝土的抗冻标号可达F1000以上,而C100混凝土冻融循环2160次才达到破坏（断裂）,研究了高强混凝土冻融过程中…     

聚合物改性多孔混凝土冻融特性研究

多孔混凝土路面的冻融破坏主要表现为内部冻胀及表面脱落,为研究聚合物改性多孔混凝土的冻融特性,采用快冻法对不同配比的基准混凝土、含砂混凝土及聚合物混凝土进行低温冻融循环试验,对比分析试验前后多孔混凝土试件的外观及质量损失。结果表明：基准混凝土的抗冻性主要由粗集料的冻融特性决定;而掺入机制砂或聚合物的多孔混凝土,其抗冻性由集料及胶结料的冻融特性共同决定;可通过选用抗冻性好的优质集料,增加混凝土中胶结料的含量,改善季冻地区聚合物改性多孔混凝土的抗冻性能。     

干燥环境条件下大掺量矿物掺合料高强混凝土的抗冻性

用快冻法研究了单掺铝酸盐膨胀剂和钢纤维及其复合对大掺昔矿物掺合料混凝土抗冻性的影响.结果表明:大掺量矿物掺合料高强混凝土(HSC)具有较好的抗冻性,但是其抗冻性对养护环境相对湿度(RH)非常敏感,干燥养护条件下的冻融寿命仅有标准养护的38%～40%.单掺2%钢纤维在保证其抗冻性不变的基础上,可改善其抗冻性对养护环境RH的敏感性.单掺10%膨胀剂能够降低大掺量矿物掺合料HSC的抗冻性对养护环境湿度的敏感性,但是其冻融寿命降低了34%.此外,在改善大掺量矿物掺合料高强混凝土的抗冻性及其湿度敏感性方面,复合掺加膨胀剂和纤维的效果并不比单掺膨胀剂或纤维的效果更好.     

高强钢纤维混凝土抗冻性能试验研究

采用快冻法,研究了钢纤维掺量对高强混凝土抗冻性能影响规律。结果表明:普通高强混凝土抗冻性能达不到F400的要求;随冻融循环次数的增加,高强钢纤维混凝土相对动弹性模量没有下降趋势,即相对动弹性模量不适于评价其冻融损伤;适量钢纤维的掺入,可显著抑制混凝土质量损失率,保障其冻后抗压强度和劈拉强度。结合本次试验,钢纤维掺量为1.5%时,对混凝土抗冻性能改善效果最佳。     

快速检测混凝土抗冻性能试验方法研究

采用全面浸水快速冻融试验方法评价单面受冻的水工混凝土抗冻性能存在局限性,单面冻融试验往往更接近现场实际。传统浸水冻融试验周期长,不能及时评价混凝土的抗冻性,采用盐冻可以模拟更为苛刻的冰冻劣化条件,与水冻相比试验周期缩短。采用水和盐溶液作为冻融介质,进行碾压混凝土快速冻融和单面冻融试验,寻找水冻-盐冻、快速冻-单面冻的对应关系,以此为基础探索快速检测碾压混凝土抗冻性能的试验方法。     

钢纤维混凝土抗冻性能试验研究

为了研究钢纤维混凝土的抗冻性能,采用快冻法进行了0％、0.5％、1.0％、1.5％、2.0％五种不同钢纤维掺量的混凝土在水中和3.5％氯化钠溶液中冻融试验.通过分析冻融循环次数和钢纤维体积率对钢纤维混凝土冻融后质量损失、劈裂强度损失和相对动弹性模量变化的影响,分析了冻融环境下钢纤维对混凝土的增强机理.并且用压汞法和SEM从微观上研究了钢纤维混凝土的孔径分布特征,讨论了微观结构对其抗冻性能的影响.研究表明,在冻融循环作用下掺入适量的钢纤维能够减小混凝土内部的孔隙率、增加密实度,有效阻止混凝土内部微裂缝的产生与发展,提高混凝土的抗冻性能.钢纤维掺量对混凝土抗冻性影响显著,掺量为1.5％时,钢纤维对混凝土抗冻性能改善效果最好.     

路面混凝土冻融破坏评价指标参数敏感性分析

评价混凝土冻融破坏的指标参数较多,为研究这些评价参数对抗冻性的敏感程度,测试了不同含气量和粉煤灰混凝土的抗冻性能,着重从含气量、气泡间距系数、相对动弹性模量、相对耐久性指数、抗折强度损失率、剥落量6个方面分析混凝土冻融破坏。结果表明,含气量、气泡间距系数均是评价混凝土抗冻性的敏感指标,水冻环境和盐冻环境下分别增加抗折强度损失率和剥落量作为评价混凝土抗冻性的敏感指标。     

混杂纤维/束高强混凝土的抗冻性

为提高混凝土的抗冻性及促进工业废料的应用,利用快冻法对混凝土试件进行冻融试验,测试冻融前后试件的相对动弹性模量（E_r）和抗压强度,研究单掺玄武岩纤维束（BMF）、双掺短切玄武岩纤维（CBF）和BMF以及在双掺的较优配比基础上继续掺加硫酸钙晶须（CSW）对高强混凝土抗冻性能的影响.结果表明：素高强混凝土（PC）在经历150次冻融循环后的E_r值为52.4%,视为冻融破坏;单掺、双掺和三掺均提高了高强混凝土的抗冻性,三掺的效果最好;当BMF体积分数为0.30%、CBF掺量为0.15%、CSW掺量为水泥质量的3%时,高强混凝土试件（CSW3CBF0.15BMF0.3）的抗冻性最佳,经历300次冻融循环后的E_r值为86.6%,仍未冻融破坏;试件CSW3CBF0.15BMF0.3在经历150次冻融循环后的抗压强度损失率仅为2.5%,远低于PC（30.9%）;CSW、CBF和BMF发挥混掺效应,多尺度阻止了高强混凝土的冻胀开裂.     

MgSO4腐蚀环境作用下混凝土的抗冻性

采用快冻法研究了高强的混凝土(HSC)、大掺量矿物掺合料混凝土(HVMAC)和掺入引气剂、高效减水剂、混杂纤维和膨胀剂的高耐久性混凝土(HDC)在水、5％(质量分数)MgSO4溶液中的抗冻性能.结果表明:对于高水胶比、低掺量矿物掺合料HSC,MgSO4溶液因冰点降低效应、化学腐蚀反应产物填充混凝土表层的毛细孔对试件表层...     

碳化与冻融交替作用下的混凝土抗压强度

通过室内单一碳化、单一冻融,以及碳化与冻融交替作用下的混凝土耐久性循环试验,对比分析了混凝土相对抗压强度、相对动弹性模量和碳化深度等指标的变化规律.结果表明:在碳化与冻融交替作用下,混凝土相对抗压强度要比单一冻融作用时大,但增加程度有限;混凝土相对动弹性模量要比单一冻融作用时小,碳化深度则比单一碳化作用时大.碳化与冻融交替作用下的混凝土抗冻耐久性较之单一冻融作用下有所下降,抗碳化能力较之单一碳化作用下有所减弱.最后建立了碳化与冻融交替作用下以碳化时间和冻融循环次数为变量的混凝土抗压强度拟合模型.     

盐冻环境下钢筋混凝土黏结性能的梁式试验

采用240 mm×150 mm×1 200 mm梁式黏结试件,通过0,50,75,100次快速冻融循环试验研究了盐冻循环作用对钢筋混凝土黏结强度,黏结刚度,初始滑移值,极限滑移值,破坏形态等指标的影响规律,并采用最小二乘法拟合得到盐冻作用后的黏结滑移本构方程.结果表明:随着冻融次数的增加,钢筋混凝土初始滑移和极限黏结强度均逐渐降低,且前者降幅更为显著;冻融循环次数越多,相同黏结应力水平下滑移量越大,黏结刚度越低,滑移量增长也越快;箍筋能有效地抑制和延缓盐冻融作用环境下纵筋与混凝土黏结性能的劣化程度.     

The characteristics of air void and frost resistance of RCC with fly ash and expansive agent

In order to assure the outer concrete of Longtan dam in China possesses excellent of frost resistance, the losses of strength, mass and air void characteristics of roller compacted concrete (RCC) containing fly ash, superplasticizer and a novel MgO-bearing expansive agent (HNM) were studied using the freezing–thawing method ASTM C666. The results show that there is a linear correlation between strength and mass losses in RCC subjected to cycles of freezing and thawing.There is a relationship between the air void spacing factor and the frost resistance of RCC. However, for RCC containing fly ash and superplasticizer a spacing factor of 0.25 mm is not necessary. Using a water:binder ratio of 0.48 in RCC containing 50% fly ash and 8% HNM a durability factor of over D₃₀₀ can be achieved provided the spacing factor is less than 0.4 mm.     

The effects of air entrainment and pozzolans on frost resistance of 50–60 MPa grade concrete

An experimental investigation was conducted using an air-entraining agent and pozzolans such as silica fume and fly ash, to meet the design strengths 50 and 60 MPa, as well as frost resistance to 300 cycles of freezing and thawing. Among a series of concretes of grade 50 or 60 MPa, only a small part could resist 300 cycles of freezing and thawing. It was demonstrated that frost resistance might be independent on strength of concrete. By means of mercury intrusion porosimeter, the pore structure characteristics of six concretes were identified. Air entrainment, no matter whether the pozzolans were used, caused an increase in cumulative pore volume, and also an increase in the mean pore size. It is revealed that, as to concrete at a 0.32 water/binder ratio, air entrainment should be a main approach to enhance frost resistance, although the pozzolans could be used to increase long-term strength of concrete.     

钢纤维活性粉末混凝土抗冻性能试验研究

通过试验以钢纤维0%,0.5%,1%,2%,3%五种不同掺量掺入活性粉末混凝土中,研究了钢纤维对混凝土抗冻性能的影响规律,试验冻融循环次数和钢纤维体积率对钢纤维混凝土冻融后相对动弹性模量变化、质量损失和劈裂强度损失的影响,分析了冻融环境下钢纤维对混凝土的增强机理。结果表明,活性粉末混凝土在冻融循环作用下,掺入钢纤维可以改善活性粉末混凝土的抗冻性能。钢纤维以2%的掺量与粉煤灰、硅灰复合掺入混凝土中,可以配制高抗冻性能的活性粉末混凝土。     

Minimum curing time for preventing frost damage of early-age concrete

When fresh concrete is exposed to extremely low temperatures, the free water in the concrete is cooled below its freezing point and transforms into ice, leading to a decrease in the compressive strength of concrete. When freezing takes place after an adequate curing time, the decrease in compressive strength does not occur. In other words, the concrete can resist the frost damage. Of the many influencing factors, the age of concrete at the beginning of freezing and curing temperatures is significantly important with regard to the loss of compressive strength. In this study, tests were performed to examine how these factors affect the compressive strength of concrete frozen at early-ages as well as to investigate the source of frost damage in fresh concrete. The results from the tests showed that the loss of compressive strength decreases when the onset of freezing was delayed and the curing temperature was high. Moreover, the results showed that the curing temperature after the freezing period does not affect the resistance against frost damage but it affects the strength development. Finally, we propose a new method to predict the minimum curing time based on the development theory of frost resistance with decrease of saturation degree of capillary pores and using the hydration degree curves at an early age.     

混凝土冻融循环的交流阻抗研究

研究了３种混凝土在冻融循环过程中交流阻抗谱的变化,据此以对混凝土在冻融循环过程中微结构的变化和混凝土经冻融循环而遭受破坏的机制有所了解,交流阻抗方法还可用于经少量冻融循环次数后对混凝土长期抗冻性能进行推测。     

西藏水工混凝土抗冻性能的研究(续)

地处高寒地区的西藏，尽夜温差较大，水利水电工程中混凝土的冻融破坏非常严重。为研究解决这一工程问题，采用正交试验设计方法，就当地混凝土材料和引气剂设计了几种水利水电工程常用的不同强度等级混凝土。通过快速冻融试验，测试了冻融对混凝土的抗压强度、抗折强度及动弹性模量的影响，并通过显微测孔分析了混凝土的微现结构。最后，就不同因素对混凝土强度和抗冻性能的影响进行了评价，并提出了几种抗冻混凝土的推荐配合比。     

Minimum curing time prediction of early-age concrete to prevent frost damage

The purpose of this study is to propose a method that predicts the minimum curing time required for early-age concrete to prevent frost damage. Tests were performed to examine the primary factors that affect the compressive strength of concrete frozen at early ages and to investigate the source of frost damage in early-age concrete. The test results showed that the rate of the decrease in the compressive strength decreases as the start of the frost damage is delayed and when the water–cement ratio is lower. In addition, the results showed that concrete made with Type III cement was less susceptible to frost damage compared with concrete made with ordinary Portland cement. Furthermore, it was found that frost damage occurred through the formation of ice lenses.When early-age concrete is being damaged as a result of freezing, a phase transition of free water into ice appears in the capillary pores of the concrete and leads to a decrease in the compressive strength. Accordingly, the frost resistance of fresh concrete can be determined based on the saturation degree of the capillary pores. A method for predicting the minimum curing time is suggested using the concept of the critical saturation degree of capillary pores.