Microstructural analysis of the adhesion mechanism between old concrete substrate and UHPFC

The performance of any repaired concrete structure, and thus its service life, depends on the quality of the interfacial transition zone of the composite system formed by the repair material and the existing concrete substrate. In this work, the properties of the interfacial transition zone between normal concrete (NC) substrate as an old concrete and ultra-high performance fiber-reinforced concrete (UHPFC) as a repair material was investigated. Pull-off and splitting cylinder tensile tests were performed to quantify the bond strength in direct and indirect tensions, respectively. The microstructure of the interfacial transition zone was also studied using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS). Different types of NC substrate surface preparation methods were used. An optical three-dimensional surface metrology device was used to estimate the substrate roughness parameters. Based on the results, high interfacial bond strength was achieved on the 3rd, 7th, and 28th days. The pull-off test results revealed that all failures occurred in the substrate, regardless of the substrate surface roughness. The majority of failures in the split tensile test also occurred in the substrate. SEM/EDS proved that the use of UHPFC as a repair material chemically, physically, and mechanically improved the repaired interfacial transition zone to become stronger and denser, as well as more uniform, and durable. Moreover, the use of UHPFC increased the service life of repaired structures and minimized the number and extent of interventions to the lowest possible level.     

Comparison of methods for evaluating bond strength between concrete substrate and repair materials

This investigation was aimed at studying the effect of test methods on bond strength between concrete substrate and repair material. Four test methods with cementitious or modified-cementitious repair materials, and two surface roughnesses were studied. The methods used were pull-off, slant shear, splitting prism and a new direct shear named Bi-Surface shear test. While the coefficient of variation (COV) for each type of test was acceptable, the bond strengths from some tests were up to eight times larger than those obtained from others. It is imperative that the bond tests be selected such that they represent the state of stress the structure is subjected to in the field. The new test method was easy to carry out and had reasonable results and can be developed by further investigations.     

UHPC与普通混凝土试件界面黏结抗冻性能试验研究

超高性能混凝土(UHPC)因为其较高的强度和优良的耐久性被认为是极具潜力的结构修补用材料之一。同时,UHPC与普通混凝土(NC)之间的界面黏结性能,是影响UHPC在混凝土加固修复工程中应用可靠性的关键因素。针对严寒环境,本试验对超高性能混凝土与普通混凝土(以下简称UHPC-NC)黏结试件开展－60 ℃的冻融循环试验,分析冻融循环后试件的宏观形态变化、质量变化率。通过黏结强度试验,获得界面的黏结强度以及相应的界面破坏模式。试验主要分析－60 ℃冻融循环对UHPC-NC试件界面黏结性能的影响,以及界面的不同处理方式(钢丝刷刷毛、高压水射流冲毛及劈裂)对抗－60 ℃冻融循环作用的影响,同时,对冻融作用下UHPC-NC试件的界面损伤机理进行初步探索。试验结果表明:－60 ℃冻融循环对UHPC-NC试件黏结强度有较大影响,劈裂组试件的界面黏结强度在经历10次、15次、20次冻融循环后分别下降为界面黏结基准强度的72.94%、55.62%及44.33%,界面黏结强度呈现先急速下降再缓慢下降的趋势;界面粗糙度越高,界面的剩余黏结强度越大,经历20次冻融循环后,劈裂组试件的剩余黏结强度为高压水射流冲毛试件的2.03倍。     

预制与后浇混凝土粘结的劈拉性能试验研究

通过对预制与后浇混凝土粘结的劈拉试验研究,分析了后浇混凝土的浇筑方式、粘结面粗糙度及温度对混凝土粘结劈拉性能的影响规律。结果表明：同种工况条件下,水平向浇筑混凝土粘结劈拉强度明显大于竖直向浇筑粘结劈拉强度,粘结劈拉强度随粘结面的粗糙度的增加而显著增大,随温度的升高而急剧下降,到300℃时,比常温下降了50％。为阐明混凝土粘结劈拉机理提供详实的数据支撑,并为装配式混凝土结构发展提供参考。     

Bond characterization between concrete substrate and repairing SFRC using pull-off testing

In the last years, an emerging repair and strengthening technique for concrete slabs has been used, consisting of applying a thin layer of steel fibre reinforced concrete (SFRC) onto the slabs. The performance of the strengthened structural system depends on the bonding behaviour between old and new concretes. Adhesives based on epoxy resins currently make this liaison. The prices of these adhesives are quite different depending, mainly, on the percentage of pure resin that they include. In the present paper, three commercial adhesive compounds of distinct prices and properties were selected to bond concrete substrate and repairing SFRC. The bond behaviour was assessed from pull-off tests and the influence of the strength class of concrete substrate and repairing SFRC was analysed. Finally, the performance of the adhesives was evaluated considering both the bond strength and their prices.     

Adhesion of Different Concrete Repair Systems Exposed to Different Environments

This article presents the results of experiments conducted to assess the effects of aging under dry laboratory conditions, underwater storage, cyclic freeze-thaw, and temperature changes on the adhesion between the repair and the substrate concrete. The repair systems considered for these studies included ordinary sand/cement (S/C) mortar, with and without bonding agents, two polymer modified cementitious mortars, and two resinous mortars. The specimens were concrete slabs of 600 × 300 × 100 mm dimensions with saw cut face 600 × 300 mm on which a repair layer of 20 mm had been applied. In order to eliminate the effects of surface texture and surface strength of the concrete on the adhesion of repair systems, repair applied surfaces were all saw cut surfaces of concrete with 28-day design compressive strength of 65 MPa. Tensile bond strengths of these specimens were measured using a direct tensile test (pull-off) method. The results indicated that the tensile bond strengths of different repair systems under dry laboratory conditions, and stored underwater, ranged from 1.51 to 5.27 Mpa. Exposure to 300 cycles of freeze-thaw and to 200 cycles of temperature changes resulted in 6 to 100 percent reduction in their tensile bond strengths.     

Adhesion of Different Concrete Repair Systems Exposed to Different Environments

This article presents the results of experiments conducted to assess the effects of aging under dry laboratory conditions, underwater storage, cyclic freeze-thaw, and temperature changes on the adhesion between the repair and the substrate concrete. The repair systems considered for these studies included ordinary sand/cement (S/C) mortar, with and without bonding agents, two polymer modified cementitious mortars, and two resinous mortars. The specimens were concrete slabs of 600 × 300 × 100 mm dimensions with saw cut face 600 × 300 mm on which a repair layer of 20 mm had been applied. In order to eliminate the effects of surface texture and surface strength of the concrete on the adhesion of repair systems, repair applied surfaces were all saw cut surfaces of concrete with 28-day design compressive strength of 65 MPa. Tensile bond strengths of these specimens were measured using a direct tensile test (pull-off) method. The results indicated that the tensile bond strengths of different repair systems under dry laboratory conditions, and stored underwater, ranged from 1.51 to 5.27 Mpa. Exposure to 300 cycles of freeze-thaw and to 200 cycles of temperature changes resulted in 6 to 100 percent reduction in their tensile bond strengths.     

The significance of nanoparticles on bond strength of polymer concrete to steel

Polymer concrete (PC) is a commonly used material in construction due to its improved durability and good bond strength to steel substrate. PC has been suggested as a repair and seal material to restore the bond between the cement annulus and the steel casing in wells that penetrate formations under consideration for CO₂ sequestration. Nanoparticles including Multi-Walled Carbon Nano Tubes (MWCNTs), Aluminum Nanoparticles (ANPs) and Silica Nano particles (SNPs) were added to an epoxy-based PC to examine how the nanoparticles affect the bond strength of PC to a steel substrate. Slant shear tests were used to determine the bond strength of PC incorporating nanomaterials to steel; results reveal that PC incorporating nanomaterials has an improved bond strength to steel substrate compared with neat PC. In particular, ANPs improve the bond strength by 51% over neat PC. Local shear stresses, extracted from Finite Element (FE) analysis of the slant shear test, were found to be as much as twice the apparent/average shear/bond strength. These results suggest that the impact of nanomaterials is higher than that shown by the apparent strength. Fourier Transform Infrared (FTIR) measurements of epoxy with and without nanomaterials showed ANPs to influence curing of epoxy, which might explain the improved bond strength of PC incorporating ANPs.     

Overblasting effects on surface properties of low-carbon steel

The influence of overblasting on surface properties of low-carbon steel was investigated. In detail, surface roughness, contact angle, Vickers-hardness, degree of surface contamination, and pull-off strength were evaluated. It was shown that overblasting had notable effects on the surface properties. Most importantly, the surface roughness was significantly reduced, and the microhardness of the substrate increased. Also, overblasting increased surface wettability. The influence on the pull-off strength of applied coating systems was only marginal.     

Effect of Surface Roughness on the Adhesive Strength of the Heat-Resistant Adhesive RTV88

Heat-resistant adhesive RTV88 is a hyper-elastic material and so far there have been little research on using RTV88 in adhesive joints. In this study, the effect of surface roughness on the adhesive strength of RTV88 was examined. Aluminum adherends were first sandblasted in order to generate rough surfaces, and then tensile–shear tests on Al/RTV88 single lap joints were performed. The shear strength was shown to be influenced by surface roughness. Peel failure was dominant when the surface roughness was at a low level. However, cohesive failure was the major type of failure when the surface roughness was at a high level. Effective area, peel failure area, and cohesive failure area were introduced to explain the effects of surface roughness on the adhesive strength. An empirical relation for the failure force was proposed, based on these parameters. Tensile tests of the RTV88 bonding was performed in order to obtain the necessary data. Finally, the empirical relation for the failure force was verified by tensile–shear test results.     

A study on the lap shear strength of a co-cured single lap joint

In this paper, the lap shear strength of a co-cured single lap joint subjected to a tensile load was investigated by experimental analysis. Co-cured joint specimens with several different bonding parameters such as bond length, surface roughness, and stacking sequence of the composite laminate were fabricated and tested. The dependence of the lap shear strength of the co-cured joint on the bonding parameters was investigated from the experimental results. The failure mechanism of the co-cured single lap joint was partially cohesive failure. The lap shear strength of the co-cured single lap joint was significantly affected by the bond length and the stacking sequence of the composite laminate. However, the effect of surface roughness on the lap shear strength of the co-cured single lap joint was not so significant.     

等离子喷涂硅灰石涂层结构和性能的研究

采用等离子喷涂技术，在Ti-6Al-4V基体上制备了硅灰石涂层，利用SEM和XRD分析技术对涂层的形貌，结构和相组成进行了研究，按ASTMC－633标准对涂层的结合强度也进行了测试，将涂层试样浸泡于模拟体液中以评估其生物活性，利用SEM及配备的能谱仪（EDS），XRD和IR对浸泡后涂层表面产物的形貌，结构和相组成等进行了分析，结果表明，等离子喷涂硅灰石涂层具有粗糙的表面和层状结构，涂层内部存在一些气孔和微裂纹，涂层的主晶相是三斜晶系硅灰口，也存在玻璃相，硅灰石涂层和Ti-6Al-4V基体热膨胀系数相近，因此涂层和T-6Al-4V基体具有较高的结合强度，其值可达约39MPa，模拟体液浸泡试验显示，硅灰石涂层表面能形成含有碳酸根的羟基磷灰石层，这表明硅灰石涂层会有良好的生物活性，可作为生物活性涂层的候选材料。     

Pull-off strength of thermoplastic fiber-reinforced composite coatings

The paper aims at presenting the results of pull-off strength tests of fiber-reinforced polymer composite coatings laminated on steel substrates. It contains the measured data on the thickness of manufactured coatings and the substrate surface’s roughness, according to the various methods of surface’s preparation with the means of abrasive blasting. The microstructure analysis of material’s cross-sections and damage analysis of samples after failure were also performed. The highest pull-off strength’s values for composite coatings were obtained for joints with the substrate modified by abrasive blasting with corundum F60 or simply degreased. To establish the compatibility of substrates with coatings the wettability of the chosen materials was tested and work of adhesion was calculated on its base. Concerning the wettability, it was found that the most preferable joints were characterized by the similar thermodynamic work of adhesion and consisted of the coating’s matrix (SBS) and the steel substrate degreased with acetone or modified with corrundum abrasive blasting and then degreased.     

G／Epoxy复合材料胶接强度研究

使用G／Epoxy作为底材研究了垫板、结构胶黏剂厚度和底材表面处理对拉伸剪切强度的影响。使用光学显微镜观察了断口形貌。结果表明加垫板能减小试验过程中由于加载偏心引起剥离应力,测试结果较大;结构胶黏剂的厚度和底材表面处理对拉伸剪切强度影响十分明显,随着厚度的增大而减小,经打磨表面裸露出纤维的试样拉伸剪切强度很低。结构胶黏剂厚度较小时以内聚破坏为主,随着厚度的增加破坏模式转变为粘接破坏。     

Abrasion erosion of concrete by water-borne sand

Hydraulic concrete structures frequently experience long-term abrasive erosion by water-borne sand, resulting in surface damage and eventually limiting their service life. In this study, the investigation of abrasion erosion tests on concrete with various water to cementitious material ratios (w/cm) was performed. The effects of the constituent materials on concrete structure abrasion erosion resistance were studied. The test results show that: (1) reduction in the w/cm ratio increases the tested concrete abrasion resistance; (2) the splitting tensile strength is a viable indicator for concrete abrasion resistance; (3) high permeability concrete exhibits weak abrasion resistance; and (4) concrete and low strength concrete made with coarser aggregate exhibit greater abrasion resistance.     

Investigation of the relationship between statistical characteristics of substrate surface roughness and the strength of adhesive joints

The relationship between statistical characteristics of butadiene styrene rubber (BSR) surface roughness and shear strength of adhesive joints has been investigated. The assumption of stationary normal distribution of coordinates of surface points was made to determine the statistical characteristics of surface roughness. The profile length above the selected level l ₁ (u) was introduced as a new surface roughness parameter to characterize adhesive penetration depth. The validity of simulated l ₁ (u) value was verified experimentally. A good correlation between experimental and calculated results was found. A relationship between adhesive penetration depth and the bonding pressure during adhesive joint preparation was also obtained. The dependences among lap shear joint strength, bonding pressure and roughness characteristic l ₁ (u) were determined.     

Effects of Cyclic Loading,Freeze-Thaw and Temperature Changes on Shear Bond Strengths of Different Concrete Repair Systems

An experimental study was performed to evaluate the residual shear bond strengths between different cementitious and resinous repair materials and substrate concrete after being subjected to cyclic loading, freeze-thaw, and temperature changes. In this paper, techniques and results of test methods that induce shear along the repair/concrete interface are discussed. In addition to the effect of surface preparation on the strength of the old concrete surface, which proved the saw cut surface as the most suitable substrate concrete for shear bond strength assessment, by means of cylindrical shear and friction-transfer methods, the effects of cyclic loading, freeze-thaw, and temperature changes on the shear bond strengths of six different repair systems are illustrated. Analysis of the results indicated that: in order to avoid fatigue failure, the maximum safe stress level to be applied should be between 20 to 40% of the original shear bond strength of the repair system, and the critical stress level differs for different repair materials; 300 freeze-thaw cycles can reduce the shear bond strength of a resin mortar by up to about 80%; and 200 cycles, of temperature changes can reduce the original shear bond strength of a cementitious mortar by up to about 90%.     

高温下钢筋与混凝土粘结锚固性能试验研究

为研究温度对钢筋与混凝土粘结性能的影响,通过制作30个标准立方体试块、8个温度场试件及15个中心拉拔试件,分别完成了室温(20℃)、100℃、200℃、400℃、600℃下标准立方体试块抗压试验与抗拉劈裂试验、拉拔试件温度场试验及中心拉拔试验.分析了高温作用对混凝土抗压强度与抗拉强度的影响,根据温度场试验研究结果,提出一种简易的高温下中心拉拔试验方法,在此基础上研究了高温下钢筋与混凝土粘结性能退化规律.基于Harajli模型综合考虑温度对粘结强度、峰值滑移及试件破坏模式的影响,提出了高温下钢筋与混凝土粘结-滑移本构模型.试验结果表明:高温下混凝土强度、钢筋与混凝土的粘结强度随温度升高整体呈下降趋势,但在100℃时发生陡降现象,高温下钢筋与混凝土的粘结强度变化趋势与混凝土抗拉强度相近.最后提出了高温下钢筋与混凝土的粘结-滑移本构模型,并验证了模型的适用性.     

Interfacial Characteristics of Sisal Fiber and Polymeric Matrices

Sisal-fiber-reinforced composites, as a class of eco-composites, have attracted much attention from materials scientists and engineers in recent years. In this article, the effects of fiber surface treatment on fiber tensile strength and fiber-matrix interface characteristics were determined by using tensile and single fiber pullout tests, respectively. The short beam shear test was also employed to evaluate the interlaminar shear strength of the composite laminates. Vinyl ester, epoxy, and high-density polyethylene (HDPE) were chosen as matrix materials. To enhance the interfacial strength, two kinds of fiber surface-treatment methods, namely, chemical bonding and oxidisation, were used. The results obtained showed that different fiber surface-treatment methods produced different effects on the tensile strength of the sisal fiber and fiber-matrix interfacial bonding characteristics. Hence, valuable information on the interface design of sisal fiber–polymer matrix composites can be obtained from this study.     

Variables that Determine the Fiber‐Matrix Bond Strength in Ethylene‐Type Ionomer Composites

The effects of some variables, namely, ion concentration, matrix tensile strength, matrix yield strength and the matrix tensile modulus on the fiber‐matrix bonding strength were determined for six ionomers (coded PEA‐1 to PEA‐6) bonded to surface‐modified poly(p‐phenylene terephthalamide) (PPTA) fibers. The results obtained show that the mean bonding shear strength of the ionomers correlates well with both their ultimate tensile strengths and their tensile yield stresses. However, correlation of the bonding shear strengths with the matrix yield stresses reveals that the bonding shear strength was about 1.1 times that of the matrix tensile stress. Failure criteria for all the materials predict maximum shear stress to be either 0.5 or 0.577 of the tensile yield stress, hence a value greater than unity cannot be interpreted nor theoretically justified. It was found that the bonding shear strength of the ethylene‐type ionomer PEA‐6 compared to carboxymethyl surface‐modified PPTA is about 20% lower than the bonding shear strength of this resin against sized PPTA fibers. The reduction of entanglements and/or ionic crosslinking across the bound polymer/bulk polymer interface leads to a weak interface with a subsequent decrease in the measured shear strength.