Natural carbonation of self-compacting concretes

This experimental work involves a study of the carbonation depth in self-compacting concretes at different ages and also analyses their porous microstructure, since these aspects are directly related to each other. Eight different concretes were used, four self-compacting (SCC) and four normally-vibrated (NVC). The carbonation rate was found to be lower in SCC than NVC, due to the fact that limestone fines produce less porosity and a finer microstructure. The difference between both types of concrete tends to disappear as their fines content becomes similar. It was also observed that, under the test conditions (RH 60.8%), for pore sizes under 0.065 μm CO₂ diffusion in the interior of the concrete is not significant. An expression is proposed to estimate carbonation rate in concrete from the volume of pores over 0.065 μm and the threshold diameter.     

自密实混凝土无腹筋梁抗剪性能

为研究自密实混凝土无腹筋梁的抗剪性能和裂缝开展形态,进行了集中荷载作用下12根无腹筋钢筋混凝土简支梁(8根自密实混凝土和4根普通混凝土)的剪切破坏试验,变量为混凝土强度和剪跨比。探讨了《混凝土结构设计规范》(GB 50010―2010)、Zsutty拟合公式、美国规范(ACI318-11)抗剪承载力计算公式对自密实混凝土无腹筋梁抗剪承载力计算的适用性和准确性。收集了在集中荷载作用下的130根自密实混凝土和798根普通混凝土矩形截面无腹筋梁剪切破坏试验数据,将自密实混凝土和普通混凝土无腹筋梁抗剪承载力进行了对比。结果表明:自密实混凝土梁和普通混凝土梁的裂缝发展、破坏形态大致相同,自密实混凝土梁斜裂缝断面更为光滑;Zsutty拟合式计算结果与本文试验结果最接近;GB 50010—2010计算结果与本文试验结果也比较吻合,但偏于不安全;美国规范ACI 318-11计算公式偏差较大;自密实混凝土梁受剪承载力略低于普通混凝土梁。     

Permeability properties of self-compacting rubberized concretes

The paper presented herein was carried out to investigate the permeability characteristics of self-compacting rubberized concretes with and without fly ash. At a water–cementitious material (w/cm) ratio of 0.35, the self-compacting concretes (SCCs) were produced by replacing the fine aggregate with four designated crump rubber contents of 0%, 5%, 15%, and 25% by fine aggregate volume. Moreover, the SCCs with fly ash were produced by partial substitution of cement with fly ash at varying amounts of 20% to 60%. Totally, 16 concrete mixtures were cast and tested for permeability related properties such as chloride ion permeability, water sorptivity, and water absorption. The tests were conducted at 28 and 90 days after casting. Tests results revealed that using the crumb rubber aggravated all of the measured properties of self-compacting rubberized concretes (SCRCs) without fly ash. However, with the combined use of the crump rubber and fly ash, the concretes had better resistance to the chloride ion permeability, water sorptivity, and water absorption.     

Homogeneity of self-compacting concretes used in tunnel strengthening – A case study

This work presents the results of an experimental study carried out to evaluate the homogeneity of the self-compacting concrete (SCC) used for the strengthening of the Montblanc tunnels, in Spain. These tunnels are part of the new high-speed railway line Madrid–Zaragoza–Barcelona–French border, partly under construction. The experimental program involved non- and semi-destructive techniques; georadar scanning combined with the extraction and laboratory testing of core samples. The results indicate that the great filling ability capacity of SCC allows obtaining an adequate material homogeneity in this type of applications, characterised by very high reinforcement ratios and difficult casting conditions.     

Strength and drying shrinkage properties of self-compacting concretes incorporating multi-system blended mineral admixtures

Drying shrinkage can be a major reason for the deterioration of concrete structures. The contraction of the material is normally hindered by either internal or external restraints so that tensile stresses are induced. These stresses may exceed the tensile strength and cause concrete to crack. The present study investigated compressive strength and particularly drying shrinkage properties of self-compacting concretes containing binary, ternary, and quaternary blends of Portland cement, fly ash (FA), ground granulated blast furnace slag (GGBFS), silica fume (SF), and metakaolin (MK). For this purpose, a total of 65 self-compacting concrete (SCC) mixtures were prepared at two different water to binder ratios. It was observed that drying shrinkage lessened with the use of FA, GGBFS, and MK while incorporation of SF increased the drying shrinkage.     

Flowing ability of the mortars formulated from self-compacting concretes incorporating rice husk ash

This paper presents the flowing ability of the mortars formulated from various self-compacting concretes (SCCs) incorporating rice husk ash (RHA). The mortars were produced with different RHA contents and water–binder (W/B) ratios, as used in their parent SCCs. The flowing ability of the mortars was determined with respect to the flow spread at various dosages of a polycarboxylate based high-range water reducer (HRWR). The effects of RHA, W/B ratio, and HRWR on the flowing ability of mortars were observed. The mortar flowing ability decreased with the higher RHA content and W/B ratio, but increased with the greater HRWR dosage. However, the excessive HRWR dosages were not conducive to the mortar flowing ability due to the segregation problem in the form of bleeding. The overall mortar flowing ability results were useful to examine the suitability of RHA, and to fix the HRWR dosages for different SCCs.     

Placing conditions,mesostructural characteristics and post-cracking response of fibre reinforced self-compacting concretes

The benefits of adding fibres to concrete, evidenced in the post-cracking behaviour, are strongly influenced not only by the type and content of fibres but also by their orientation. The objective of this study is to evaluate the influence of the casting/placing procedure on the post-peak behaviour of fibre reinforced self-compacting concrete, and its relationship with the mesostructural characteristics of the material (type, distribution and orientation of fibres). Three concretes were prepared using two types of steel fibres of different lengths (50 mm and 30 mm) and a structural type polymer fibre. Beams of 150 × 150 × 600 mm were cast in three different ways: filling the moulds from the centre in accordance with the EN 14651 Standard, pouring concrete from one end of the mould after a flowing along a 5 m length and 150 mm diameter pipe, and finally, filling the moulds vertically. Flexural tests according to the European Standard indicate that the three types of fibres achieve a preferential orientation along horizontal planes, like in conventional vibrated fibre reinforced concrete. The mechanical response of beams cast with longer steel fibres was strongly affected by the casting procedure while the flexural performance of the other two fibre concretes, was less affected. Such results are well in accordance with the density of fibres measured by fibre counting in different cut planes.     

自密实混凝土解决小截面柱的加固问题

通过选择合适的原材料、试验的试配和施工工艺的优化来解决加固柱加固截面尺寸小、钢筋密,一至二层柱同时浇筑的施工难题。由于结构形式的特殊性,普通混凝土达不到其设计的及无法正常振捣的施工要求,因此笔者设计自密实混凝土解决小截面柱的加固问题。     

Post-cracking behaviour of steel and macro-synthetic fibre-reinforced concretes

The present paper describes the results of an experimental investigation on the performances of concrete specimens reinforced with either steel or macro-synthetic fibres under three-point bending. Steel fibres are often used to improve the flexural toughness of concrete and are used in various structural applications while synthetic fibres are more often used to reduce crack opening due to shrinkage. Macro-synthetic fibres have been proposed more recently with the aim of creating an alternative to steel fibres in structural applications but their use is still limited.In the tests performed, specimens cast with the same concrete mix, but containing different dosages of either steel or macro-synthetic fibres, were used and compared. In general steel fibres were more efficient in increasing the toughness of concrete than macro-synthetic fibres, even though their results were significantly more scattered.Test results were used to calculate the parameters of stress-crack opening relations via inverse analysis using a cracked hinge model. This numerical model provided results, in terms of force-Crack Mouth Opening Displacement (CMOD) curves, in very good agreement with the experimental data.     

自密实混凝土与钢筋局部黏结性能试验研究

自密实混凝土由于材料组分和免振捣的特点,其与钢筋黏结性能备受关注。采用电液伺服加载方法分别进行了10个单调荷载作用和10个重复荷载作用下的钢筋混凝土试件拉拔试验,其中16个为自密实混凝土试件,4个为普通混凝土对比试件,研究自密实混凝土强度、荷载作用形式对钢筋与自密实混凝土局部黏结性能的影响,包括局部黏结应力的分布、残余黏结应力等。试验结果表明：沿锚固长度黏结应力分布在单调荷载作用下呈单波峰状,而在重复荷载作用下呈波浪状;自密实混凝土强度越高,黏结应力分布越不均匀,各级荷载卸载时的残余黏结应力也较大;随着循环荷载和次数的增加,黏结应力分布更均匀,残余黏结应力逐渐向自由端移动;自密实混凝土与钢筋的黏结性能略优于普通混凝土与钢筋的黏结性能。采用最小二乘法得到钢筋与自密实混凝土局部黏结应力分布的位置函数并与试验结果对比,表明其具有较好精度。     

碳纤维布加固锈蚀钢筋与混凝土黏结力试验研究

采用碳纤维布加固是钢筋混凝土结构加固的常用方法,而加固后钢筋与混凝土的黏结力是衡量其加固效果的关键点。钢筋采用电化学加速锈蚀方法,研究了碳纤维布加固锈蚀钢筋与混凝土的黏结力,对比分析了钢筋具有相同理论锈蚀率下的3组试验方法:未加固锈蚀试件、锈蚀后加固和锈蚀前加固。试验结果表明:碳纤维布加固试件能够显著提高其黏结力,尤其是钢筋锈蚀前加固试件;碳纤维布加固使得试件破坏状态由劈裂破坏转变为钢筋拔出破坏;随着钢筋锈蚀率的增大,未加固试件的黏结力显著下降;钢筋锈蚀前加固试件,其黏结滑移曲线下降段较为平缓。     

Mechanical and elastic behaviour of concretes made of recycled-concrete coarse aggregates

In this paper an investigation of mechanical behaviour and elastic properties of recycled-aggregate concretes is presented. These concretes were prepared by alternatively using two different (coarse and finer coarse) recycled-aggregate fractions both made of recycled concrete coming from a recycling plant in which rubble from demolition is collected and suitably treated. Several concrete mixtures were prepared by using only virgin aggregates (as reference), 30% finer coarse recycled aggregate replacing fine gravel and 30% coarse recycled aggregate replacing gravel. Five different water to cement ratios were adopted as: 0.40, 0.45, 0.50, 0.55 and 0.60. Concrete workability was in the slump range of 190–200 mm. Compression tests were carried out after 28 days of wet curing. In addition, concrete elastic modulus and drying shrinkage were evaluated. Results obtained showed that structural concrete up to C32/40 strength class can be manufactured by replacing 30% virgin aggregate with recycled-concrete aggregate. Moreover, a correlation between elastic modulus and compressive strength of recycled-aggregate concrete was found and compared to those reported in the literature. Finally, on the basis of drying shrinkage results, particularly if finer coarse recycled-concrete aggregate is added to the mixture, lower strains could be detected especially for earlier curing time.     

Bond behaviour of FRP-to-clay brick masonry joints

The out-of-plane bending and in-plane shear response of unreinforced modern clay brick masonry walls retrofitted with fibre-reinforced polymer (FRP) strips is often governed by debonding failure mechanisms. Hence, it is necessary to quantify the fundamental interface bond–slip model, which describes the debonding behaviour of the FRP-to-masonry interface. This paper presents the results of a series of 29 pull tests investigating the use of externally bonded (EB) and near surface mounted (NSM) retrofitting techniques. Test variables included: surface preparation; geometric properties; location of FRP (relative to perpend joints and cores); bonding agent of bed joints (mortar and quick drying paste); bonding method for glass fibre sheets (plate bonding and dry lay-up); and FRP material. A discussion of the test results and preliminary practical recommendations are also provided. A model used to predict the intermediate crack debonding resistance was verified against the test data. The model is generic in that it is applicable to both the EB and NSM retrofitting techniques. This generic model was shown to give very good ultimate strength predictions for the series of 29 pull tests conducted as part of this research.     

Bond strength of hot-dip galvanized reinforcement in normal strength concrete structures

Hot-dip galvanizing is one of the methods used to solve the problem of corrosion of reinforcement in concrete structures. The few research reported in the literature on the effect of galvanizing on bond strength of reinforcing bars anchored in pullout specimens, indicates contradictory results. The primary objective of this paper is to evaluate experimentally the effect of hot dip galvanizing on the bond capacity of tension lap splices. The test results may have an implication on the development length and splice length design provisions of galvanized bars in building design codes. To achieve the objective, six concrete beam specimens were tested in positive bending. For each of three bar sizes 20, 25, and 32 mm, two companion beams identical except for whether the bars were black or galvanized, were tested. Each beam was designed with bars spliced at midspan in a constant moment region. The nominal concrete strength was 28 MPa. The mode of failure was splitting of the concrete cover in the splice region. The test results indicated that the use of galvanized bars has a negligible negative effect on bond strength of reinforcement in normal strength concrete.     

补偿收缩纤维自密实混凝土在通道桥加固改建工程中的应用

在单独使用复合防冻剂存在减水不足的情况下,通过添加高效减水剂,提高混凝土减水率,在此基础上又加入聚丙烯纤维和膨胀剂的双重措施配制出符合冬施要求的泵送自密实混凝土,解决了其硬化后易收缩、开裂的问题;同时利用豆石代替碎石,实现自密实混凝土不离析、不堵泵。实践表明采用该技术路线配制的补偿收缩纤维自密实混凝土的工作性、力学性能和耐久性均能满足北京健翔桥加固改建工程的要求。     

Mechanical behaviour and fibre dispersion of hybrid steel fibre reinforced self-compacting concrete

In this study, mixture design, workability, fibre dispersion/orientation, mechanical properties and fracture behaviour of hybrid steel fibre reinforced self-compacting concretes (HSFRSCCs) were investigated. Three different types of steel fibres with and/or without hooked-ends were added to the mixtures in two different volume fractions (0.75 and 1.5% of the total volume of concrete). The results of slump flow, U-box, V-funnel and J-ring tests have shown that increasing the fibre content of the concretes slightly reduced the workability of HSFRSCC, and the main influencing factor on flowability is the geometry of fibres. The addition of fibres, although did not change the final flowability, decreased the rate of flowability. The results from the experimental tests showed that the flexural strengths increased slightly with increasing strength of long fibres, whereas the splitting tensile strength remained unchanged. The concretes with high strength, long steel fibres show behaviour of enhanced toughness and ductility compared to that with normal strength steel fibres. The orientation and distribution of fibres in concrete have been investigated by image analysis and it was observed that fibres dispersed homogeneously in all concrete series without any clumping. With increasing the amount of fibres, the fibres were more vertically orientated relative to the bending loading direction, resulting in enhancement in the mechanical properties of concrete.     

碳纤维布约束下钢纤维混凝土与腐蚀钢筋黏结性能的试验研究

为研究碳纤维布加固对腐蚀钢筋与钢纤维混凝土黏结性能的效用,采用外加电流法对96个预埋钢筋-钢纤维混凝土试件进行加速腐蚀,腐蚀完成后横向包裹两层单向碳纤维布;通过拉拔试验研究纤维布约束对腐蚀钢筋与钢纤维混凝土黏结性能的影响规律,并与普通混凝土试验结果进行对比分析,探讨钢筋种类、保护层厚度、腐蚀率、纤维布约束及钢纤维等对黏结性能的影响规律。研究表明:①碳纤维布约束使螺纹钢筋试件破坏模式由钢纤维混凝土劈裂破坏转变为钢筋拔出破坏,且碳纤维布约束使钢纤维混凝土试件的极限黏结强度提高,黏结-滑移曲线的下降段变缓;②钢筋种类、保护层厚度显著影响钢纤维混凝土试件的峰值黏结强度,螺纹钢筋试件的峰值黏结强度约为光圆钢筋试件的2.0～2.7倍,保护层厚度由40mm增大到60mm时,试件峰值黏结强度提高;③当腐蚀率小于5%时,腐蚀钢筋与钢纤维混凝土峰值黏结强度随着腐蚀率增大有所提高;当腐蚀率大于5%时,腐蚀钢筋与钢纤维混凝土峰值黏结强度降低;④在相同约束条件下,钢纤维混凝土峰值黏结强度比普通混凝土提高22.5%～61.5%,在峰值拉拔荷载下对应的滑移量较大,且光圆钢筋钢纤维混凝土试件黏结-滑移曲线的下降段比普通混凝土更为平缓。     

钢筋混凝土夹芯复合梁受弯性能的试验研究

为了研究集中荷载作用下的钢筋混凝土夹芯复合简支梁的抗弯破坏形态、变形性能和极限承载力,对8根试件进行了正截面受弯试验研究,研究了纵筋配率、翼缘高度等因素对受弯极限承载力的影响,得出复合梁的延性和整体性较好的结论.     

Bond behaviour between recycled aggregate concrete and steel rebars

Thirty six pullout tests were carried out in order to investigate the bond behaviour between recycled aggregate concrete (RAC) and steel rebars. Three recycled coarse aggregate (RCA) replacement percentages (i.e., 0%, 50% and 100%) and two types of steel rebar (i.e., plain and deformed) were considered in this paper. Based on the test results, the influences of both RCA replacement percentages and the rebar surface on the bond strength between the RAC and steel rebars were investigated. It was found that under the equivalent mix proportion (i.e., the mix proportions are the same, except for different RCA replacement percentages), the bond strength between the RAC and the plain rebar decreases with an increase of the RCA replacement percentage, whereas the bond strength between the RAC and the deformed rebar has no obvious relation with the RCA replacement percentage. The empirical bond stress versus slip relationship between RAC and steel rebars was established through regression analysis using the experimental data.     

Numerical modelling of bond behaviour in RC structures affected by reinforcement corrosion

Steel corrosion in reinforced concrete structures leads to severe degradation processes which usually affect both the ultimate and serviceability limit state performance of the whole construction. Numerical modelling of such a behaviour requires effective non linear models able to capture all the main effects of corrosion (i.e. cracking, reduction in bond strength, reduction in steel cross-section, bond degradation, etc.). This paper discusses these topics by focusing on the effects of corrosion on bond behaviour. In particular, a coupled mechanical-environmental damage model is used to simulate the deterioration of concrete (i.e. cover cracking and reduction of mechanical properties), while the effects of corrosion on bond behaviour have been dealt with using two different approaches, one based on a “frictional type” law and the other on a “damage type” law. A comparison between experimental pull-out test data and numerical results verifies that the proposed procedures can effectively simulate the effects of corrosion on bond behaviour, mainly for the “damage type” approach. Finally the numerical simulation of some experimental tests of corroded beam has been carried out.