The laboratory investigation of concrete affected by TSA in the UK

Procedures that have been successfully employed by Geomaterials Research Services Ltd. in the determination of the distribution of the thaumasite form of sulfate attack (TSA) in concrete samples taken from bridge and other motorway structures throughout the UK are described. Electron microprobe analysis has been used to provide confirmation of the presence of thaumasite and to investigate the distribution of sulfate compounds including gypsum and ettringite in cement paste at the surfaces of concrete affected by sulfate attack. Electron microprobe analysis has the advantage that it is capable of detecting very small quantities of thaumasite. Electron microprobe analysis used in conjunction with petrographic analysis is regarded as the most effective tool for the diagnosis of TSA and other forms of sulfate attack.

The analysis of numerous cores taken from bridge foundations throughout the UK shows that the development of TSA is often accompanied by the formation of calcium carbonate in cement paste. High levels of chloride in the cement paste of TSA damaged concrete suggest the importance of run-off moisture as a contributory factor in the development of TSA.     

海水浸泡对FRP筋-珊瑚混凝土粘结性能的影响

开展了30℃海水浸泡条件下玻璃纤维增强树脂基复合材料（GFRP）筋、碳纤维增强树脂基复合材料（CFRP）筋与珊瑚混凝土粘结性能的试验研究,分析了纤维增强树脂基复合材料（FRP）筋-珊瑚混凝土粘结滑移曲线特征、破坏形态及粘结强度变化。试验结果表明,海水浸泡后FRP筋力学性能和粘结性能均表现为不同程度的降低。随浸泡时间增加,GFRP筋表层树脂与纤维间的孔隙率明显增大,并逐渐出现脱粘现象,纤维本身遭受到侵蚀,而CFRP筋仅表面基体有少许损伤,其耐久性明显优于GFRP筋;FRP筋-珊瑚混凝土粘结强度呈现出先增加后减小的趋势,且后期下降速率逐渐变小,部分GFRP筋-珊瑚混凝土试件的破坏模式逐渐由筋被拔出转变为筋材断裂;增加保护层厚度能有效地减缓海水对GFRP筋的侵蚀,有利于保持GFRP筋-珊瑚混凝土间的粘结性能。     

Is concrete a poromechanics materials?—A multiscale investigation of poroelastic properties

Materials and Structures - There is an ongoing debate, in Concrete Science and Engineering, whether cementitious materials can be viewed as poromechanics materials in the sense of the porous media...     

Experimental tests on the shear behaviour of dowels connecting concrete slabs to stone masonry walls

An experimental investigation has been conducted on the behaviour of dowels used to connect concrete slabs to stone masonry walls in order to transfer horizontal shear forces. A technique for embedding the dowels in the stone block without injection of grouting material or resin has been developed. Special experimental equipment has been designed in order to allow the execution ofin situ load tests on representative ancient buildings. Monotonic loading tests have been carried out on eight specimens with the purpose of measuring both stone block displacement and dowel deformation     

Utilization of oil-contaminated sands in asphalt concrete for secondary roads

The oil well sabotage during the Iraqi occupation and following the Iraqi withdrawal contaminated the Kuwaiti desert with crude oil covering millions of square meters. Thus, the desert has a huge amount of oil-contamined sand that must be cleaned up or treated in order to restore it to its natural beauty. The oil-contaminated sand was utilized to mix asphalt concrete in the laboratory to determine the feasibility of using sand contaminated with oil as feed stock in theproduction of asphalt construction materials. The products developed in experimental testing did meet international standards. The mixes were asphalt concrete of a quality that can be used for secondary roads, road beds, road subbases, impermeable layers for landfill and containment facilities, or as stabilizers for steep embankments. Gases emitted during the production of asphalt concrete utilizing oil-polluted sands are considered environmentally acceptable. Further studies are required to determine the feasibility of producing asphalt concrete from oil-contaminated sand on a pilot-plant scale as a means of utilizing the millions of square meters of oil-polluted sand.     

A laboratory investigation of plasma carburizing in hydrogen-methane mixtures

A current intensive glow discharge, without auxiliary heating, has been used to study the carburization kinetics of 99.9% pure iron in the temperature range 950-11 OO°C. The treatments were all carried out at a total pressure of 1500 Pa in an atmosphere of methane and hydrogen, in the range 1-10% CH₄. Spark emission spectrometry was used to generate the resultant carbon profiles. It has been found that the surface carbon concentration is a function of temperature, time and methane partial pressure. A t low partial pressures a limit has been found to the availability of active carburizing species from the plasma, and this is reflected in a low carburization rate. Some limitations on the method of processing are also discussed.     

An experimental investigation on the FPZ properties in concrete using digital image correlation technique

This paper presents an experimental investigation on the properties of the fracture process zone (FPZ) in concrete using the digital image correlation (DIC) technique. Based on the experimental results, it is found that the FPZ length increases during crack propagation but decreases after the FPZ is fully developed. The FPZ length at the peak load and the maximum FPZ length increase with an increase in specimen height, but decrease by increasing the notch depth to specimen height ratio. It is also found that the crack extension length at the peak load is about 0.25 times the ligament length.     

铜线性能及键合参数对键合质量的影响

为了提高铜线性能及其键合质量,采用拉力-剪切力测试仪、扫描电镜等研究了不同力学性能铜线及相应的键合参数对其键合质量的影响,分析了不同伸长率和拉断力、铜线表面缺陷、超声功率和键合压力对铜线键合质量的作用机制.结果表明:伸长率过小和拉断力过大会造成焊点颈部产生微裂纹,从而导致焊点的拉力和球剪切力偏低;表面存在缺陷的铜线其颈部经过反复塑性大变形会造成铜线表面晶粒和污染物脱落而出现短路和球颈部断裂;键合过程中键合压力过大能够引起的焊盘变形,同时较大的接触应力引起铝层溢出;过大的超声功率使键合区域变形严重产生明显的裂纹和引起键合附近区域严重的应力集中,致使器件使用过程中产生微裂纹而降低器件的使用寿命.     

Numerical investigation of statistical variation of concrete damage properties between scales

Concrete is typically treated as a homogeneous material at the continuum scale. However, the randomness in micro-structures has profound influence on its mechanical behavior. In this work, the relationship of the statistical variation of macro-scale concrete properties and micro-scale statistical variations is investigated. Micro-structures from CT scans are used to quantify the stochastic properties of a high strength concrete at the micro-scale. Crack propagation is then simulated in representative micro-structures subjected to tensile and shear tractions, and damage evolution functions in the homogenized continuum are extracted using a Helmholtz free energy correlation. A generalized density evolution equation is employed to represent the statistical variations in the concrete micro-structures as well as in the associated damage evolution functions of the continuum. This study quantifies how the statistical variations in void size and distribution in the concrete microstructure affect the statistical variation of material parameters representing tensile and shear damage evolutions at the continuum scale. The simulation results show (1) the random variation decreases from micro-scale to macro-scale, and (2) the coefficient of variation in shear damage is larger than that in the tensile damage.     

Effect of fibre characteristics on physical,mechanical and microstructural properties of geopolymer concrete: A comparative experimental investigation

The main aim of this work is to comparatively reveal the effect of fibre type, length and content on compressive strength and microstructure of structural geopolymer concrete (GPC) produced under constant mixture and curing parameters in order to address the significant gap in present literature. Firstly, GPCs with different NaOH concentrations (i.e., 6, 9, 12 and 15 M) and activator solution/binder (a/b) ratios (i.e., 0.45 and 0.55) were produced in ambient curing condition, and optimum production parameters were determined based on the preliminary evaluations. Then, glass and polypropylene fibres in 6-mm length (GS6 and PP6) and polyamide and polypropylene fibres in 12-mm length (PY12 and PP12) were included in GPCs at ratio of 0.4%, 0.8% and 1.2% (by volume). Compressive strength, apparent porosity, bulk density, ultrasonic pulse velocity (UPV), X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis of GPC samples were carried out comparatively. The inclusion of GS6 fibre enhanced the compressive strength thanks to fibre surface being covered by geopolymer gel and the strong adhesion between GS fibre and geopolymer matrix. SEM images of fibre reinforced GPC (FRGPC) also confirmed the experimental findings, which were attributed to improvement in compressive strength. Regardless of the fibre type, the maximum compressive value strength was obtained from GPC specimens with 0.4% fibre and then decreased. Higher fibre inclusions led to poor compaction, workability issues and inhomogeneous fibre dispersions. A very good relation (R² = 0.98) was acquired between UPV and compressive strength values of GPC/FRGPC samples.     

A classification of studies on properties of foam concrete

Though foam concrete was initially envisaged as a void filling and insulation material, there have been renewed interest in its structural characteristics in view of its lighter weight, savings in material and potential for large scale utilization of wastes like fly ash. The focus of this paper is to classify literature on foam concrete in terms of constituent materials (foaming agent, cement and other fillers used), mix proportioning, production methods, fresh and hardened properties of foam concrete. Based on the review, the following research needs have been identified: (i) developing affordable foaming agent and foam generator, (ii) investigation on compatibility between foaming agent and chemical admixtures, use of lightweight coarse aggregate and reinforcement including fibers, (iii) durability studies, and (iv) factors influencing foam concrete production viz., mixing, transporting and pumping.     

钢筋砼粘结锚固性能的试验研究

通过钢筋与混凝土拉式粘结试验,测试了高强度带肋钢筋与不同强度等级混凝土的粘结强度,分析了带肋钢筋与混凝土的粘结受力机理;采用双线性软化本构对开裂区的软化行为进行描述,建立了综合考虑开裂区及未开裂区混凝土影响的粘结强度理论计算模型;研究了开裂区不同径向位移分布对计算结果的影响,并将计算结果与试验结果进行对比,验证了计算模型的有效性。结果表明：模型采用基于等效弹性假设的开裂区径向位移分布时,计算值与试验值最为吻合,但却过高的估计了低强度混凝土试件的粘结强度;为确保有足够的安全储备,建议采用弹性假设作为开裂区混凝土径向位移分布。     

A comprehensive investigation into the effect of aging and coarse aggregate size and volume on mechanical properties of self-compacting concrete

The popularity of self-compacting concrete (SCC), as an innovative construction materials in concrete industry, has increased all over the world in recent decades. SCC offers a safer construction process and durable concrete structure due to its typical fresh concrete behavior which is achieved by SCC’s significantly different mixture composition. This modification of mix composition may have significant effect on the hardened mechanical properties of SCC as compared to normal vibrated concrete (NVC). Therefore, it is necessary to know whether the use of all rules and relations that have been formulated for NVC in current design codes based on years of experience are also valid for SCC. Furthermore, this study represents an extensive evaluation and comparison between mechanical properties of SCC using current international codes and prediction equations proposed by other researchers. Thus, in this experimental study, major mechanical properties of SCC are investigated for twelve SCC mixes with wide spectrum of different variables i.e. maximum coarse aggregate size, coarse aggregate volume and aging. In the present study, an extensive body of data reported by many researchers for SCC and NVC has been used to validate the obtained results.     

Laboratory investigation of rheological properties and scaling resistance of air entrained self-consolidating concrete

An experimental investigation was undertaken to analyze the influence of various admixtures on the rheological properties and scaling resistance of self-consolidating concrete. Such concrete is intended for use as a repair material for filling highly restricted areas, such as forms with closely spaced reinforcing steel bars. Several self-consolidating concrete mixtures having slump flow of 550+50 mm were prepared with water-to-cement ratios varying between 0.35 and 0.41. The mixtures were cast with 0 and 3 percent silica fume, with and without air-entraining admixture. All concretes incorporated superplasticizer and viscosity-modifying admixture to enhance deformability and stability. Rheological parameters (yield value and plastic viscosity) were measured using a concrete viscometer. The air content, unit weight, and consistency were also determined. The consistency was assessed using the slump flow and L-Flow methods. Tests performed on hardened concrete included compressive strength at 28 days (ASTM C 39), scaling resistance (ASTM C 672), durability to freezing and thawing (ASTM C 666) and measurement of the air-void parameters (ASTM C 457). Relationship between the simple slump flow and yield value and plastic viscosity measurements determined using a concrete viscometer are also discussed. In general, the laboratory test results indicate that it is possible to produce a frost durable, self-consolidating concrete with low yield value and high plastic viscosity (for such fluid concrete) which can be use as a repair material to fill highly restricted areas.     

An investigation on bonding mechanism and mechanical properties of Al/Ti compound materials prepared by insert moulding

Al/Ti metallic composites prepared by insert moulding are attracting more attention now because of their low production costs, low energy consumption, simple production procedure and high interface bonding strength. However, the insert moulding of pure Al and pure Ti has not been reported so far though it can be considered as a fundamental in studying Ti-alloy/Al-alloy interface bonding. Therefore, the insert moulding of pure Al and pure Ti is intended in this paper and the corresponding microstructure, elements distribution and mechanical properties of the interface are also analyzed. As a result, a good metallurgical bonding can be achieved at the interface of Al and Ti, which is mainly comprised of intermetallic compounds TiAl₂ and TiAl₃ formed in the transition zone around Ti insert and Al matrix, respectively, depending on different heat treatment parameters and cooling conditions. It is shown that the hardness of the interface layer varies with the types of interface sublayers. For the compact sublayer, the hardness is higher than those of both base metals (Al and Ti) with the maximum value reaching HV520. However, the hardness of the granular interface sublayer depends on the proportion of the intermetallic compounds and aluminum matrix. The average shear strength of the interface layer could reach about 60 MPa, which is higher than that of the aluminum matrix (43 MPa) tested in this experiment. The result also shows that shear crack initiates at bottom face of the specimen (adjacent to locator) in the aluminum matrix nearby the interface.     

Occurrences of thaumasite in laboratory and field concrete

The mineral thaumasite (CaSiO₃ · CaCO₃ · CaSO₄ · 15H₂O) has been observed by the authors in a number of concrete samples including (i) an 80 year-old aqueduct in Manitoba, (ii) a 33 year-old pavement in Ontario, (iii) test samples exposed to marine (tidal) conditions, and (iv) laboratory samples exposed to wet–dry cycles in sulphate solution. The source of carbonate differed for these cases being variously derived from de-dolomitization of dolostone aggregate, carbonate ions in the seawater, or atmospheric CO₂ through the process of carbonation. In some cases the thaumasite mineral was found in close association with the mineral ettringite (3CaO · Al₂O₃ · 3CaSO₄ · 31H₂O).

The paper discusses the results of detailed analyses using optical and electron microscopy. Although the mineral thaumasite can be readily identified by these techniques, it is possible that the thaumasite form of sulphate attack is frequently misdiagnosed as conventional sulphate attack due to the similarities in crystal structure and appearance of the minerals thaumasite and ettringite.     

Investigation of thermo-mechanical properties of adhesive used for bonding CF fabrics to concrete members using post-curing techniques

Carbon fibre reinforced polymers (CFRPs) have been increasingly employed for structural strengthening using bonding adhesives. The work in this paper reports the use of post-curing temperature on adhesive properties in CFRP/concrete systems which use a bonding adhesive under a sustained load to achieve thermal and mechanical performance. An increase in the adhesives glass transition temperature were achieved for post-curing, and as expected it was found that the improvement in the glass transition was greater for samples post-cured at high temperatures, compared to those which had only been exposed to moderately elevated temperatures. However, the single-lap shear test results at elevated temperatures under sustained loads showed a greater improvement in the bond-loss temperature compared with those which were post-cured at only moderately elevated temperatures, with higher temperatures negatively affecting the concrete properties, particularly at the adhesive/concrete interface.     

A numerical investigation on the size effect of fiber-reinforced concrete specimens in crack propagation

The paper addresses a numerical investigation on the size effect in fiber-reinforced concrete specimens that is based on an alternative approach for cohesive crack propagation. A discrete version of the Hellinger?CReissner variational principle manages mode I crack growth in the case of piece-wise linear cohesive softening equations. A three-point bending test is investigated according to the mechanical properties of fiber-reinforced mixtures that have been characterized in the experimental literature. The achieved results point out that each segment of the considered cohesive laws plays an important role in the control of the size effect, depending on the dimension of the specimen.