浅谈大体积混凝土施工的裂缝控制措施

随着建筑施工技术的飞速发展，特别是泵送混凝土的大量使用，使混凝土浇筑的体积由几百立方米逐渐增大到几万立方米。由于其体积大，表面积小，产生较大的温度应力和收缩应力，导致大体积混凝土产生温度裂缝，影响结构安全和正常使用。因此对大体积混凝土施工的裂缝控制措施提出了更高的要求。为有效克服大体积混凝土裂缝，确保工程质量，本文从裂缝产生的原因入手，从选料、配合比设计、施工方法、温度控制、养护等方面采取综合性控制措施。阐述一些看法。     

建筑工程大体积混凝土施工技术及泵送设备应用分析

在现代建筑工程中,大体积混凝土的应用日益广泛,特别是在高层建筑、大型水利水电工程及各类重要基础设施的建设中。大体积混凝土的施工技术及其泵送设备的应用,是确保工程质量、加快建设进度的关键环节。深入分析大体积混凝土施工技术的特点与挑战,并探讨泵送设备在其中的应用,力图为工程技术人员提供理论与实践指导,以促进建筑工程领域的技术进步和发展。     

房屋建筑中大体积混凝土施工技术

本文介绍了大体积混凝土的概念及其施工技术,对其构造原则进行阐述,探讨房屋建筑中大体积混凝土施工技术的应用,以此保证房屋建筑工程的整体质量。     

浅谈现浇混凝土裂缝产生原因及预防措施

论述现浇混凝土施工过程中混凝土出现的质量问题,分析混凝土裂缝产生的原因,从而提出防治措施,为大体积混凝土的施工过程提出质量控制要求。     

基于Cscan技术的LNG储罐基坑混凝土密实性检测

为了解决大体积混凝土质量检测的技术难题,提出了一种无损检测技术——声波散射混凝土扫描成像技术(Cscan)。从基本原理、现场采集、数据处理及结果的工程解释等方面对Cscan技术进行了详细研究,并开展了其对某液化天然气(LNG)储罐基坑混凝土密实性的检测试验,验证其准确性。结果表明,Cscan技术对大体积混凝土的检测精度较高,在只具有单一可测面的现场条件下具有良好的应用效果。     

大体积混凝土在工程质量中的控制

根据影响大体积混凝土在工程质量中的因素 ,指出为了确保其质量 ,必须从混疑土原材料的选用起就开始实行控制 ,进行混凝土的配合比设计 ,并制定了一些质量控制措施。     

不同冲击条件下早龄期混凝土的力学特性

利用大杆径分离式霍金森压杆(SHPB)试验装置,对1、3、7、14、28 d这5组龄期下的混凝土进行冲击压缩试验,研究早龄期混凝土(C20)在不同冲击条件下的力学特性。结果表明:龄期7 d以前,各龄期50%临界入射能的冲击有助于提高混凝土的强度和弹性模量;龄期7 d以后,无论是75%还是50%临界入射能的冲击,都会使混凝土的强度和弹性模量降低;在单次冲击条件下,混凝土的变形能力和单位体积吸收能与龄期呈对数关系增长;在多个龄期冲击条件下,混凝土的变形能力和单位体积吸收能都受到不同程度的影响,其中单位体积吸收能下降了50%。     

山地风电场基础混凝土质量检测方法

山地风电场基础混凝土结构具有体积大、配筋量大、钢筋分布密集等特点,加上现场施工条件的局限,常会出现混凝土浇筑不密实、基础环脱空等质量缺陷.针对山地风电场的基础混凝土施工缺陷尚无系统性的、成熟的检测和评估方法的问题,在充分考虑山地风机基础混凝土结构特点的基础上,通过对多个风电场的成功检测实例,系统性地提出了山地风电场基础...     

超声波层析成像技术在三峡工程混凝土质量检测中的应用

介绍了采用超声波层析成像(CT)技术检测体积很小或形状不规则的混凝土缺陷及超声波CT成像原理。使用该方法检测了三峡大坝某段混凝土缺陷的位置，确定了缺陷的范围，并采用单孔波速测试和钻孔录像进行了验证。试验证明，该方法能有效检测混凝土缺陷。     

浅谈防止大体积混凝土温度裂缝的技术措施

大体积混凝土由温度应力产生裂缝是我们长期关注并期待更好地解决的问题,本文就总结前人的经验、结合实际,提出几种常用的技术措施。     

A new sensing skin for qualitative damage detection in concrete elements: Rapid difference imaging with electrical resistance tomography

In this paper we investigate whether a thin layer of electrically conductive materials that is painted to the surface of concrete elements can be used as sensing skin to detect and locate cracking and damage in the concrete substrate. Cracking of the concrete results in the rupture of the sensing skin, thus locally increasing its electrical resistivity. We monitor the local change in the electrical resistivity of the sensing skin using electrical resistance tomography. In this work, we utilize difference imaging scheme. Experiments on polymeric substrates as well as on concrete substrates are performed. The results indicate that the developed sensing skin can be successfully used to detect and locate cracking and damage on concrete and potentially other nonconductive substrates.     

紫铜厚大结构件钨极氩弧焊热裂纹形成机理

对紫铜厚大结构件钨极氩弧焊(GTAW)热裂纹形成机理进行了研究.对热裂纹的动态形成过程进行观察和分析,在普罗霍罗夫理论基础上优化了热裂纹形成判据,并建立了基于刚性拘束热裂纹试验的紫铜厚板GTAW有限元模型.结果表明,内部变形率Δε是促使开裂的内因,得到HS201焊缝金属在脆性温度区间内(BTR)的横向拉伸应力及Δε的变化规律,并与其高温延性进行对比,得到不预热工艺下热裂纹产生的原因.为了防止热裂纹的出现,模拟并分析了不同预热温度下HS201焊缝金属的Δε的变化规律,预热温度由500℃降为420℃即可避免热裂纹出现.     

Stress corrosion cracking and hydrogen embrittlement susceptibility of an eutectoid steel employed in prestressed concrete

The stress corrosion and hydrogen embrittlement behavior of AISI 1080 steel employed in concrete prestressing tendons was studied with different experimental techniques. A simulated concrete pore solution, with and without contaminants such as chloride, sulfate and thiocyanate ions was used. For comparison purposes the standard 20% ammonium thiocyanate solution was also employed. Polarization curves, slow strain rate tests and fracture mechanics tests were used to evaluate the influence of parameters such as potential, temperature (between 0 and 100 °C), and tempering temperature of the steel. The results have shown that the fracture mechanism of the stress corrosion cracking process is associated with hydrogen action.     

Advances in optimisation of mechanical properties in cast TiAl alloys

The room temperature tensile properties of a number of cast TiAl-based alloys, which have been grain-refined either by additions of boron or by a new quenching and ageing treatment, have been assessed. It has been found that in the alloys which have been grain-refined by boron addition, the size of the titanium boride particles, which is strongly influenced by the size of the cast samples, is an important factor in limiting the tensile ductility. The tensile properties of samples which have been quenched and aged can be significantly improved and it has been shown that this grain refinement technique, which is based on the massive transformation, can be applied to a wide range of alloys. Quenching to above the ductile-to-brittle temperature, but below the temperature at which the massive transformation occurs, reduces the tendency for cracking during this type of heat treatment. The potential for producing cast components using these two approaches to grain-refinement is discussed.     

混凝土锈胀开裂寿命预测模型及其应用研究

采用圆孔扩张理论对混凝土钢筋锈胀过程进行分析,推导了不同钢筋锈胀量对应的塑性区边界混凝土应力及塑性区半径计算公式,在此基础上建立了混凝土锈胀开裂寿命准则及寿命预测模型。锈胀开裂寿命模型分析表明,预测寿命值与钢筋相对保护层厚度m和混凝土等级有直接关系。随着相对厚度m增加,锈胀开裂预测寿命值增加,并且前期增加较快,后期增加较缓慢。混凝土强度等级对预测寿命值影响显著,随混凝土强度增大,混凝土锈胀开裂寿命增大。工程应用表明,该模型预测寿命可为工程的中度维护提供参考。     

Experimental and probabilistic analysis of time to corrosion-induced cover cracking for marine reinforced concrete structures

Corrosion-induced concrete cover cracking is an important indication of durability limit state for marine reinforced concrete structures. In this paper, two analytical models predicting the time from corrosion initiation to cover cracking and their main differences were introduced. Based on an accelerated corrosion test, two models’ applicability and variability were compared and discussed with experimental data. Considering the random nature of influencing factors, a probabilistic model was developed by using Monte Carlo simulation technique. The results showed that the cracking time could be modelled by the Weibull distribution. Finally, the probability analytical technology was applied to a marine reinforced concrete pier with four different durability design levels. It is found that both the mean and 90% confidence interval of the cracking time increase when the durability design specifications change from low level to extreme level, which indicates that the accurate prediction of cracking time with a deterministic model will become more difficult accordingly.     

A mathematical model for prediction of time from corrosion initiation to corrosion cracking in RC structures

Prediction of time from corrosion initiation to corrosion cracking is a crucial factor in evaluating concrete structures' durability and integrity deteriorated by reinforcement corrosion. This paper presents a mathematical model to predict the time from corrosion initiation to corrosion cracking. A dynamic corrosion rate model based on Bulter–Volmer kinetics and a rust expansion model based on Faraday's law are developed. An analytical model is developed to demonstrate the mechanical consequences of rust buildup around the rebar. The time to corrosion cracking is found to be a function of surrounding concrete material properties, chemical composition of rust, ratio of concrete cover thickness to rebar diameter, and corrosion current density. Comparisons with available experimental results are conducted to show the accuracy and reliability of the present model. Further parametric analysis is carried out to show the influences of various parameters included in the model on the time to corrosion cracking.     

Distribution of millscale on corroded steel bars and penetration of steel corrosion products in concrete

This study investigated a reinforced concrete specimen that had deteriorated in an artificial environment for 2 years. The steel/concrete interface and corrosion-induced cracks were observed by SEM to investigate the millscale on the rebar surface and the distribution of rust. The millscale was not further oxidised before the surface cracking of the concrete cover. The penetration of corrosion products into concrete and the formation of a corrosion layer proceeded simultaneously. The rust did not fill the corrosion-induced cracks in concrete. Instead of the three-stage model, a two-stage model is proposed to describe the concrete cracking process induced by steel corrosion.     

Modellversuche zur Spannungskorrosion hochfester Spannstähle in nicht karbonatisiertem Beton

Model experiments on stress corrosion cracking of high strength steels in prestressed non-carbonated concrete The author deals with the mechanisms of stress corrosion cracking of stressing steels in alcaline media (pH 12.6) containing ions which promote stress corrosion cracking. The tests have revealed that this ion effect increases as the stability of their adsorptive bonding to iron atoms is increased. Sulfides can give rise to stress corrosion rupture in the anodic range when the concrete electrolyte becomes saturated by CaS. In the presence of chlorides stress corrosion cracking must be taken into account when chloride concentrations exceed 350 mg/l. Consequently, adequate protection is feasible by limiting to values not exceeding 300 mg/l the chloride content of the water used for concrete preparation. In the case of nitrates there is no danger of stress corrosion cracking. However, at nitrate concentrations exceeding 0.1 M/l the passivation layer is destroyed and then uniform corrosion sets in, so that subsequent rupture because of reduced load bearing cross sections may occur.     

Damage analysis and cracking model of reinforced concrete structures with rebar corrosion

The damage of concrete cover in reinforced concrete structures induced by reinforcing steel corrosion is investigated in this study. The damage process of the concrete cover can be divided into two distinct stages: the non-cracking stage and the partial cracking stage. An analytical model based on damage mechanics and elastic mechanics is developed to predict the concrete cracking due to steel corrosion. Based on this model, the expansive pressure and the radial loss of steel bar are discussed. Parametric studies are carried out to examine the effects of the correlative factors on the expansive pressure and the steel loss.