The resistance of high alumina cement against corrosion of steel in concrete

The present study concerns the resistance of high alumina cement (HAC) concrete to chloride-induced corrosion, in terms of corrosion behaviour, binding of chloride ions and chloride transport. The corrosion resistance was evaluated using concrete specimens containing steel rebar exposed to a salt environment. The binding capacity of chloride ions was measured at 28, 58 and 91 days by the water extraction method. The rate of chloride transport was expressed into apparent diffusion coefficient and surface chloride content. The strength test was also performed at up to 365 days. As a result, it was found that the compressive strength for HAC concrete was always higher than for OPC, although a reduction of the strength for HAC concrete was observed at 28 days. The corrosion rate in HAC mortar and concrete indicated the lower values than OPC ones in spite of the lower chloride binding capacity of HAC paste. For chloride transport, there is only marginal difference in the diffusivity of chloride ions between in HAC and OPC concrete.     

Effect of electric arc furnace dust on the properties of OPC and blended cement concretes

This paper presents results of a study conducted to evaluate the mechanical properties and durability characteristics of ordinary Portland cement (OPC) and blended cement (silica fume and fly ash) concrete specimens prepared with electric arc furnace dust (EAFD). Concrete specimens were prepared with and without EAFD. In the silica fume cement concrete, silica fume constituted 8% of the total cementitious material while fly ash cement concrete contained 30% fly ash. EAFD was added as 2% replacement of cement in the OPC concrete and 2% replacement of the total cementitious content in the blended cement concretes. Mechanical properties, such as compressive strength, drying shrinkage, initial and final setting time, and slump retention were determined. The durability characteristics were evaluated by measuring water absorption, chloride permeability, and reinforcement corrosion. The initial and final setting time and slump retention increased due to the incorporation of EAFD in both OPC and blended cement concretes. The drying shrinkage of EAFD cement concrete specimens was more than that of concrete specimens without EAFD. The incorporation of EAFD was beneficial to OPC concrete in terms of strength gain while such a gain was not noted in the blended cement concretes. However, the strength differential between the blended cement concretes with EAFD and the corresponding concretes without EAFD was not that significant. The water absorption and chloride permeability, however, decreased due to the incorporation of EAFD in both the OPC and blended cement concretes. The corrosion resistance of OPC and blended cement concrete specimens increased due to the addition of EAFD.     

Current distribution in reinforced concrete cathodic protection system with conductive mortar overlay anode

The current distribution was studied in a designed three-layer reinforced concrete cathodic protection (CP) system, with carbon fibre reinforced cement (CFRC) as the conductive mortar overlay anode. The influence of steel bars’ initial corrosion state, concrete resistivity and magnitude of impressed current density on the current distribution was discussed, respectively. Testing results show that the initial corrosion rate of steel has a great effect on the protection current distribution. In addition, there exists a threshold for corrosion rate, beyond which the uniformity of protection current distribution worsens markedly. Higher concrete resistivity not only deteriorates the current distribution, but also has an adverse effect on the rebars far away from the anode. However, the magnitude of impressed current density only affects the uniformity of current distribution to some extent when the rebars are corroding. This study could be useful to the cathodic protection of reinforced concrete structures in field condition.     

Corrosion of exposed rebars,associated mechanical degradation and correlation with accelerated corrosion tests

In a large number of steel reinforced concrete buildings in Greece, spalling of the cement has left the steel reinforcement (rebars) exposed to the atmosphere. This has led to corrosion of the exposed rebars, especially in coastal areas, with questions regarding their remaining load-carrying capacity. This work addresses the problem of corrosion and strength degradation of such exposed rebars. A large number of samples (sections of exposed rebars) were collected from buildings up to 96 years old, and weight loss measurements, tensile testing and fractographic analysis were carried out. Accelerated corrosion testing (salt spray) was performed on new similar grade rebars in order to establish a correlation with the naturally corroded exposed rebars.It was found that exposed rebars suffer from uniform corrosion followed by degradation of mechanical properties. In certain cases the properties were below the minimum threshold required by the international standards. A rough estimate of the correlation factor between natural corrosion of exposed rebars and salt spray corrosion has been derived. This correlation factor could be used, under certain limitations, to estimate the condition, in terms of mechanical property degradation and structural integrity, of coastal buildings with exposed rebars in Greece.     

NaCl除冰盐作用下混凝土的抗冻能力分析

通过存质量浓度为3.5%的NaCl溶液中的冻融循环试验,测试了普通混凝土、引气混凝土、粉煤灰混凝土和硅灰混凝土的相对动弹性模量和质量损失率,分析了不同混凝土的抗盐冻的能力.结果表明:适当的引气能够明显改善混凝土的抗盐冻性能,掺加20%粉煤灰时,混凝土仍具有较高的抗盐冻能力,掺量达到40%时混凝土的抗盐冻性能明显下降,掺加10%硅灰明显提高混凝土的抗盐冻能力.     

Strength and water permeability of concrete containing palm oil fuel ash and rice husk–bark ash

In this paper, palm oil fuel ash and rice husk–bark ash, which are by-products from electricity generating power plants and disposed as wastes in landfills, were used as a partial cement replacement. They were ground and incorporated into concrete at the levels of 20%, 40% and 55% by weight of binder. Compressive strength and water permeability of concretes containing ground palm oil fuel ash (GPOA) and ground rice husk–bark ash (GRBA) were investigated. From the tests, the replacement of Portland cement by both materials resulted in the higher water demand in concrete mixtures as compared to ordinary Portland cement (OPC) concrete with compatible workability. The compressive strengths of concretes containing 20% of GPOA and GRBA were as high as that of OPC concrete and were reduced as the increase in the replacement ratios. Although the compressive strengths of concrete with the replacement of GPOA or GRBA up to 40% were lower than OPC concrete, their water permeabilities were still lower than that of OPC concrete. These results indicate that both of GPOA and GRBA can be applied as new pozzolanic materials to concrete with an acceptable strength as well as permeability.     

Corrosion behavior of epoxy/zinc duplex coated rebar embedded in concrete in ocean environment

The corrosion behavior of epoxy/zinc duplex coated rebar embedded in concrete is evaluated comparing with the black steel, galvanized and epoxy coated rebars for a long term in ocean environment. The effect of mechanical damages of epoxy coatings on the corrosion protection is examined. The epoxy coated and epoxy/zinc duplex coated rebars show the higher anti-corrosion performance than other types of rebars. However, once the epoxy coating is mechanically damaged, the more serious corrosion may occur in the damaged area of epoxy coated rebar in concrete. The epoxy/zinc duplex coating remains a good corrosion protection to steel in concrete even when suffering from some mechanical damages.     

钢筋在碱激发矿渣砂浆中的早期腐蚀行为

研究了模拟海水干湿循环过程中海水拌和碱激发矿渣(SAAS)砂浆、自来水拌和碱激发矿渣(TAAS)砂浆及矿渣水泥复合(CS)砂浆的强度变化.同时,通过腐蚀电位、线性极化电阻、电化学阻抗谱法研究了低碳钢筋(LC)和低合金耐蚀钢筋(LA)在上述3种砂浆中的早期腐蚀行为.结果表明:模拟海水干湿循环过程中,SAAS砂浆表现出较CS砂浆更高的强度保证;SAAS砂浆中钢筋早期腐蚀行为与TAAS砂浆无明显区别;钢筋在SAAS砂浆及CS砂浆中的极化电阻及电荷转移电阻在6个月的干湿循环过程中整体呈上升趋势,表现出较低的腐蚀程度.     

The use of inorganic conversion coatings to enhance the corrosion resistance of reinforcement and the bond strength at the rebar/concrete

The performance of concrete specimens reinforced with uncoated rebars or rebars coated with inorganic conversion coatings was investigated. The corrosion resistance of rebars and the bond strength at the rebar/concrete interface for uncoated rebars as well as rebars coated with three different inorganic conversion coatings were evaluated according to the corresponding ASTM G109 standard. The results showed that different inorganic conversion coatings give significant enhancements of corrosion resistance and increased interface bond strength compared to uncoated ordinary steel rebars. However the extent to which each conversion coating improves the corrosion resistance of the rebars and the interface bond strength inevitably depends on the chemical composition of the applied inorganic conversion coating.     

人工海水对于凝石的侵蚀性探讨

研究将矿渣硅酸盐水泥混凝土与"凝石"混凝土进行对比实验,分别对0.34、0.45、0.55三种水灰比的混凝土进行海水腐蚀实验,系统地研究了试验混凝土的抗压强度、抗折强度、吸水率、失重率的变化规律及原因,综合展现了"凝石"混凝土抗海水侵蚀的能力。     

Durability of sulfur concrete in various aggressive environments

Durability of sulfur concrete with different fillers, as well as Portland cement concrete, was tested in the solutions of HCl, H₂SO₄, and NaCl. Regarding mass changes, in the solutions of HCl and H₂SO₄ sulfur concrete with talc and fly ash exhibited higher durability, while in NaCl samples with alumina and microsilica were better. The type of filler did not affect durability regarding compressive strength. Strength loss was higher in the solution of HCl comparing to H₂SO₄, while negligible in NaCl which is in accordance with apparent porosity increase. Portland cement concrete after two months lost 20% of mass.     

海工混凝土的氯离子扩散性与寿命评估

通过现场海洋曝露试验和实验室海水浸泡试验,采取分层取样和化学分析方法,应用氯离子三维扩散理论,研究了普通混凝土和高性能混凝土在海洋大气区、潮汐区、水下区和实验室海水浸泡下的C1-扩散系数变化规律.结果表明,混凝土的C1-一扩散系数随着曝露时间的增加而降低,高性能混凝土的抗C1-扩散性优于普通混凝土.在Khatri计算模型的基础上,提出了考虑劣化效应系数的海工混凝土使用寿命计算模型.该模型计算结果与Clear经验模型基本吻合,解决了Khatri计算模型结果与实际寿命不相符的问题.     

混凝土中钢筋加速锈蚀试验适用性研究

钢筋锈蚀导致其屈服强度降低、力学行为改变,影响钢筋与混凝土之间的粘结性能,钢筋锈蚀量影响钢筋混凝土的失效模式。为研究锈蚀钢筋混凝土结构的相关性能,需要在较短时间内得到所需的锈蚀构件。通过对4种不同工况下混凝土中钢筋电化学加速锈蚀方法进行对比试验,得到了锈蚀后钢筋表面形态特征,分析了模拟自然环境条件下钢筋锈蚀的适用性。试验表明：全浸泡外加电流加速锈蚀方法使钢筋纵向、径向表面形成均匀锈蚀,而自然环境锈蚀钢筋表面锈蚀相对不均匀,坑蚀更明显,两者差异显著;利用全浸泡外加电流加速锈蚀方法进行锈蚀钢筋与混凝土粘结-滑移本构关系和锈蚀钢筋混凝土构件承载能力等研究不合适;半浸泡和贴面外加电流加速锈蚀方法能较好模拟自然环境锈蚀;加速锈蚀试验方法的理论锈蚀质量高于试验锈蚀质量。图12表1参7     

镁基海水海砂混凝土技术研究

镁基海水海砂混凝土以海砂为唯一骨料,以原状海水为拌合水,具有较高强度。硫氧镁水泥MOS含量为16%时,28d抗压强度达59MPa以上、抗折强度达10MPa以上;含量增至20%时,28d抗压强度达72MPa以上、抗折强度达16MPa以上。试验证明,使用较少的镁基胶凝材料即可使海水海砂混凝土达到较高强度,满足多种建材产品的强度要求。同样,氯氧镁水泥MOC含量为25%时,7d抗压强度达68MPa以上,28d抗压强度达94MPa以上。海水拌合海砂混凝土以镁基胶凝材料通过改性剂及掺合料控制海砂中游离氯离子的含量,克服硫酸盐硫酸根离子与混凝土孔隙中的钙离子发生反应,提高混凝土材料的强度和耐久性。以原状海水代替淡水直接拌合,就地取材,以盐治盐,施工成本低廉,经济优势明显。     

Estimation of the permeability of silica fume cement concrete

Deterioration and durability of concrete structures mainly depend on permeability of concrete. Silica fume (SF) as a mineral admixture for high performance concrete produces more discontinues and impermeable pore structure in concrete. The higher permeability reductions with silica fume are due to pore size refinement and matrix densification, reduction in content of Ca(OH)₂ and cement paste-aggregate interfacial refinement. During the hydration process the transition interfacial zone is gradually densified due to pozzolanic reaction between silica fume and calcium hydroxide. Based on a microstructure model, a procedure for predicting the permeability of high strength silica fume cement concrete is developed by considering water-to-binder ratio, silica fume replacement ratio and degree of hydration as major influencing factors. Results of the permeability calculated using the procedure is verified with the available literature. Subsequently, effects of silica fume on the permeability of concrete are evaluated. Finally, optimum silica fume replacement ratios that reduce the permeability of concrete reasonably are proposed for durable concrete.     

模拟混凝土环境下GFRP筋抗压性能加速老化试验研究

参照ACI 440.3R-04提供的试验方法,将90根玻璃纤维增强塑料（GFRP）筋分别放入40℃、60℃和80℃的模拟混凝土溶液中进行加速老化试验,侵蚀时间分别为3.65 d、18.0 d、36.5 d、92.0 d和183.0 d,分析了温度、侵蚀时间、SiO2含量等参数对GFRP筋受压力学性能的影响。研究表明：侵蚀183.0 d后,40℃、60℃、80℃模拟混凝土环境下的GFRP筋抗压强度较侵蚀前分别下降了29.59%、39.12%和47.62%,其抗压弹性模量分别下降了10.12%、12.47%和19.06%。采用扫描电子显微镜（SEM）对侵蚀前后GFRP筋的微观形貌进行了观测,发现侵蚀后GFRP筋的劣化区域内纤维与周围树脂之间出现了明显的脱粘现象,而且随着温度的提高这种脱黏现象更加明显。采用X射线荧光光谱分析仪（XRF）分析了侵蚀前后GFRP筋的SiO2含量变化,结果表明随着侵蚀时间的增加,模拟混凝土环境下GFRP筋中SiO2含量呈递减趋势;侵蚀前GFRP筋中SiO2含量为62.11%,在40℃、60℃和80℃模拟混凝土环境下侵蚀183.0 d后,GFRP筋中SiO2含量较侵蚀前分别下降到52.05%、50.66%和47.65%。基于XRF分析提出了模拟混凝土环境下GFRP筋抗压强度的预测模型。     

Effect of unburnt carbon on the corrosion performance of fly ash cement mortar

The increase in activated carbon content accelerated the corrosion of rebar (steel) in ordinary Portland cement (OPC) mortar containing fly ash and different percentages of carbon. The alkalinity of the cement was greatly affected with increased carbon contents, and when the quantity of carbon was increased, cement lost its characteristic colour. For the exposure period of one year, more than 60% of the surface area was rusted when the carbon content was increased beyond 8%. Corrosion rates comparable to OPC were obtained for up to 6% carbon level. Therefore it was concluded that the upper limit of replacement for various admixed carbon systems, under aggressive alternate wetting and drying conditions with 3% NaCl, was 6–8%.     

CNT CF水泥基材料辅助阳极对钢筋混凝土阴极保护影响

应用腐蚀电位、腐蚀电流密度和交流阻抗谱,研究了碳纳米管(CNT)-碳纤维(CF)水泥基材料铺覆层作为辅助阳极对混凝土中钢筋的阴极保护效果.结果表明:所施加的电流能引起钢筋电位足够的负移,从而有效地保护钢筋.与碳纤维水泥基材料相比,采用CNT-CF水泥基材料作为辅助阳极来实施阴极保护的钢筋试样腐蚀电流密度更小,保护效果更佳.随着龄期的增长,在实施阴极保护钢筋试样的Nyquist图中,中频区容抗弧半径增大,电荷转移电阻增大.采用CNT-CF水泥基材料铺覆层作为辅助阳极对混凝土中钢筋的阴极保护具有显著的增强效果.     

高性能混凝土经飞机除冰液长期作用的化学腐蚀

飞机除冰液对机场高性能混凝土(HPC)长期作用的腐蚀性未知,采用浸泡腐蚀法经过4 a测定了普通水泥混凝土(OPC)和HPC分别在水、25%乙二醇(飞机除冰液)和25%商用飞机除冰液(LBR-A除冰液)中的抗腐蚀性。结果表明,除冰液与OPC表层的CH发生化学腐蚀反应,产生结晶腐蚀产物,使得混凝土试件质量不减反增,LBR-A除冰液对混凝土表层的腐蚀性比乙二醇更强一些。但除冰液的长期作用并不能对水泥混凝土的内部结构造成实质性影响,仅限于表层的逐层剥蚀。因此,在商用飞机除冰液的长期腐蚀作用下,用硅酸盐水泥掺加40%粉煤灰(FA)配制的HPC抗腐蚀性最好,用抗硫酸盐硅酸盐水泥掺加40%FA配制的HPC抗腐蚀性次之,OPC抗腐蚀性较差。     

Obtaining progressive chloride profiles in cementitious materials

The ingress of chloride into concrete is controlled by its absorption and diffusion characteristics. The authors have carried out an investigation to monitor the rate of ingress of chlorides during a 48-week cyclic wetting and drying regime using a variety of cement blends, viz. pulverised fuel ash, ground granulated blast furnace slag, metakaolin and microsilica. Chloride profiles were obtained by analysing concrete dust samples extracted from different depths from the surface that was exposed to the chloride exposure regime. The resistivity of the concrete was determined by measuring conductance between pairs of electrodes, in order to assess if this could be used to determine the presence of chlorides. The water absorption (sorptivity) of the concrete was also measured in order to determine if any correlation existed with the chloride ingress. The chloride profiles depended on both the duration of exposure and the type of cementitious material. A linear relationship was established between the depths to a constant chloride concentration of 0.2% by weight of cementitious material and the square root of time, yielding a rate of chloride ingress coefficient. This coefficient was found to depend on the type of cementitious material. The lowest value was obtained for slag concrete and the highest value was found for the OPC concrete. This corresponded well with the results from the resistivity profiles which indicated slag cement was best at resisting chloride penetration and the OPC concrete the worst. The sorptivity decreased significantly after the testing regime, due to pore refinement caused by continued hydration and the binding of chloride ions, leading to a reduction in continuous porosity. No correlation was found between the sorptivity and the rate of chloride ingress, which indicates that the dominant mechanism responsible for the transport of the chloride ions was not absorption.