Residual mechanical resistance of concrete blocks and laying mortars after high temperatures

This paper reports an experimental campaign to evaluate the residual mechanical resistance after high temperatures of two structural masonry components: block and mortar. Residual compressive strength and deformation modulus of four different hollow concrete blocks and two different mortar mixes after heating at high temperatures are investigated. The test method used was the one recommended by RILEM TC 200 for mortars and an adaptation of the same method proposed by Medeiros et al. suitable for the geometry of hollow blocks. Despite the sharp drop in the deformation modulus after heating blocks and mortar, no different behaviours are observed in the deformability of the materials caused by the variables studied. The same cannot be said in relation to the variation of the residual compressive strength of the blocks, which is affected by the variables: initial nominal compressive strength and width of the concrete block. Regarding laying mortars, the results confirmed the small influence of compressive strength on the evolution of residual mechanical strength. The data and analyses reported here on the residual mechanical properties of hollow concrete blocks produced from a concrete mixture of very dry consistency, vibro-pressed and with normal weight aggregates are relevant, since the data found in the literature generally refer to the wet cast concrete material and in cylindrical bodies.     

保护层厚度对混凝土中钢筋锈蚀的影响

混凝土中的钢筋锈蚀破坏是混凝土结构失效破坏的最主要原因.通过电化学测定,研究了保护层厚度对钢筋锈蚀的影响,研究表明:增大混凝土保护层厚度能有效延缓混凝土中钢筋的锈蚀,延长钢筋开始锈蚀的时间,保护层厚度越大,越能延长钢筋开始锈蚀的时间;进入钢筋开始锈蚀阶段后,保护层比较厚的混凝土中钢筋锈蚀速率增长的较慢;在稳定锈蚀阶段,保护层厚度越大.稳定腐蚀电流密度就越小,保护层厚度和稳定腐蚀电流密度存在一定的函数关系,且数据相关性良好.     

Thermal and mechanical properties of light‐weight concrete exposed to high temperature

The extensive studies devoted so far to normal‐strength light‐weight aggregate concrete (LWAC or LWC) have exhaustively clarified its behaviour in ordinary conditions. However, the introduction of high‐performance light‐weight aggregate concrete (HPLWAC or HPLWC), containing such pozzolanic components as microsilica and fly ash, raises some concerns, for instance about the behaviour at high temperature and after cooling. To investigate the temperature‐induced mechanical damage, both in compression and tension, of silica‐fume HPLWCs, a systematic research program was carried out at the Politecnico di Milano on materials residual behaviour (testing after cooling down to room temperature). Three concrete mixes (Normal‐Strength Concrete = NSC, f_c²⁰ = 30 MPa; Light‐Weight Concrete = LWC, f_c²⁰ ≈ 40 MPa; and High‐Performance Light‐Weight Concrete = HPLWC, f_c²⁰ ≈ 60 MPa), five temperature levels (20, 105, 250, 500 and 750 °C, no loads applied during heating), one thermal state (after cooling), three nominally‐equal tests for each case (for repeatability) were planned, bringing the total number of specimens to 120 (45 tested in compression, 45 in direct tension and 30 in indirect tension by splitting). At the same time, the thermal diffusivity of the materials was evaluated up to 750 °C (4 specimens). The results show that HPLWC is somewhat more temperature‐sensitive than both NSC and LWC, but this extra sensitivity is counterbalanced by HPLWC's lower diffusivity. Its better insulation properties are advantageous for the concrete in axially‐loaded members and for the tension bars in the beams, as demonstrated by the thermo‐mechanical analyses of three typical R/C sections (rectangular, T and slab sections) carried out in the second part of the paper. Copyright © 2012 John Wiley & Sons, Ltd.     

Retained properties of concrete exposed to high temperatures: Size effect

Concrete as a construction material is likely exposed to high temperatures during fire. The retained properties of concrete after such exposures are still of great importance in terms of the serviceability of structures. This paper presents the effects of high temperatures on the physical, mechanical, and microstructural properties of concrete. Specimens with different sizes were exposed to high temperatures ranging from 200 to 1200^°C. The compressive strength, splitting tensile strength, ultrasonic pulse velocity, and rebound numbers of the specimens were determined. The microstructures of the specimens were examined by scanning electron microscope (SEM) analyses. The test results indicated that the retained compressive strength of concrete considerably decreased with increase in temperature. The effect of specimen size on the retained compressive strength was not pronounced. The retained splitting tensile strength of concrete remarkably reduced as the temperature was increased. The specimen size played an important role on the retained splitting tensile strength of concrete up to 400^°C. The test results revealed that ultrasonic pulse velocity (UPV) test can be successfully used in order to check the uniformity of fire‐damaged structures. The rebound numbers decreased with increase in exposure temperature. SEM studies on specimens exposed to 800^°C revealed significant changes in the microstructure of the concrete. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of elevated temperatures on the residual mechanical properties of high-performance mortar

The effect of high temperature on the mechanical properties of high-strength mortar was investigated. Specimens were heated up to elevated temperatures (300, 600, 900 °C ) without loading. After being exposed to these oven temperatures, the residual modulus of elasticity, flexural strength and compressive strength of the specimens were determined. The effect of the rate of heating, duration of exposure to maximum temperature and the role of graphite powder, which is known as a high-temperature refractory material on the behavior of the mortar specimens, were observed. Temperatures up to 600 °C resulted in considerable losses in mechanical properties, and at 900 °C, specimens lost almost all of their strength. Higher rate of heating and exposure to the maximum temperature for a shorter period of time resulted in higher residual properties. The useful effect of graphite addition on the residual compressive strength and modulus of elasticity of the mortar specimens as percentages of initial values for each of the heating cycles are clearly observed.     

Flexural properties after exposure to elevated temperatures of a ground granulated blast furnace slag concrete incorporating steel fibers and polypropylene fibers

Experiments were carried out to investigate the flexural properties of fiber‐reinforced ground granulated blast furnace slag (GGBFS) concrete after exposure to high temperatures. On the basis of experimental observation, the effect of GGBFS content, the steel fiber dosage, the polypropylene (PP) fiber dosage, and the strength grade on the residual strength of concrete after exposure to elevated temperatures were systematically examined. Test data indicate that exposure to high temperatures causes deterioration in the flexural strength of concrete; inclusion of GGBFS, PP fibers, and steel fibers, all effectively improve the residual flexural strength of concrete after fire. The optimum amounts of GGBFS, PP fibers, and steel fibers are identified respectively for better fire resistance of concrete. The strength losses of concretes characterized by different strength grades are very close to one another. Equations are proposed to predict the residual flexural strength of concrete incorporating GGBFS, PP fibers, and steel fibers after being heated to temperatures up to 800°C. Copyright © 2013 John Wiley & Sons, Ltd.     

矿渣颗粒对砂浆性能的影响

研究了矿渣颗粒以不同比例替代河砂时对砂浆流动度和强度的影响规律。结果表明:(1)采用补水法制备含矿渣颗粒的砂浆,可以克服因矿渣颗粒吸水造成的流动度降低;(2)随着矿渣颗粒掺入量的增加,砂浆3d,7d的抗折、抗压强度下降,而28d抗折强度随矿渣颗粒替代河砂比例的增加变化不大,28d砂浆抗压强度随着矿渣颗粒替代量的增加是先减小后增加;(3)矿渣颗粒在建筑砂浆中具有陶粒的某些性能,通过吸水返水作用可以提高建筑砂浆的强度增进率。     

高强钢绞线网-聚合物砂浆加固小偏心受压混凝土柱的极限承载力分析

高强钢绞线网-聚合物砂浆加固方法具有高强、防火、耐腐蚀、施工简便等特点．在9根钢筋混凝土小偏心受压柱加固试验的基础上,借鉴约束混凝土的基本原理,提出了钢绞线约束混凝土峰值应力、应变的计算方法;采用合理的材料本构关系,基于平截面假定,提出了高强钢绞线网-聚合物砂浆加固小偏心受压柱极限承载力的简化计算公式;其计算结果与试验值吻合良好,可用于加固工程实践．     

Several properties of mineral admixtured lightweight mortars at elevated temperatures

The improvement of thermal and mechanical properties of mortars including expanded perlite aggregate (EPA) containing either clinoptilolite, a type of natural zeolite (NZ), waste glass powder (GP) or blast furnace slag (BFS) cured at elevated temperature was analyzed using thermal conductivity, compressive strength, flexure strength and dry unit weight. EPA mortar specimens were prepared by replacing a varying part of the portland cement with the above minerals. All mortar samples were prepared and cured at 23±1°C lime saturated water for 28 days. The maximum thermal conductivity of 1.3511W/mK was determined with the control samples containing plain cement. GP has shown 1 and 4% decrease for both 10, 20% GP and 25% EPA, respectively. Both BFS and NZ have a decreasing effect on thermal conductivity. The experiments were carried out, in which the samples were subjected to temperature of 300, 500 and 800°C for 2 h, then cooled in air. The results indicated that all the mortars exposed to temperature of 500 and 800°C shown a significant decrease in thermal conductivity, compressive strength and flexure strength. However, compared with the mortars including 25% EPA, adding the other admixtures at all level replacement decreased thermal conductivity, compressive strength, flexure strength and dry unit weight as a function of replacement percent. Copyright © 2010 John Wiley & Sons, Ltd.     

温度和电流密度对高速镀铬钢板性能的影响

对二次冷轧钢板依次进行电镀铬、氧化铬,制得无锡镀铬钢板。总结了电镀铬过程中温度和电流密度的相关性。研究了温度和电流密度对铬镀层厚度、铁溶出量、表面形貌等性能的影响。结果表明,电流密度与温度之间具有相关性,生产过程中必须使二者相匹配。当电流密度为(45±5)A/dm2、温度为(40±5)°C时,所得镀铬板的铁溶出量为50～60mg/m2,耐蚀性和结合力良好。     

Explosive spalling and residual mechanical properties of fiber-toughened high-performance concrete subjected to high temperatures

In this paper, an experimental investigation was conducted to explore the relationship between explosive spalling occurrence and residual mechanical properties of fiber-toughened high-performance concrete exposed to high temperatures. The residual mechanical properties measured include compressive strength, tensile splitting strength, and fracture energy. A series of concretes were prepared using OPC (ordinary Portland cement) and crushed limestone. Steel fiber, polypropylene fiber, and hybrid fiber (polypropylene fiber and steel fiber) were added to enhance fracture energy of the concretes. After exposure to high temperatures ranged from 200 to 800 °C, the residual mechanical properties of fiber-toughened high-performance concrete were investigated. For fiber concrete, although residual strength was decreased by exposure to high temperatures over 400 °C, residual fracture energy was significantly higher than that before heating. Incorporating hybrid fiber seems to be a promising way to enhance resistance of concrete to explosive spalling.     

Effect of temperature and cooling regime on mechanical properties of high‐strength low‐alloy steel

This paper presents results from experimental studies on the effect of temperature on mechanical properties of high‐strength low‐alloy ASTM A572 steel commonly used in structural members in bridges. A set of high‐temperature tensile strength tests and post‐temperature exposed residual strength tests is carried out on ASTM A572 steel coupons in 20–1000 °C temperature range. The residual strength tests on high‐temperature exposed steel coupons are carried out after subjecting the coupons to two methods of cooling, namely, air cooling and water quenching. Results from these tests indicate that temperature‐dependent strength and stiffness degradation in A572 steel follow the same trend as that of carbon steel but with some variations. A572 steel recovers almost 100% of its room temperature yield strength when heated to temperature up to 600 °C, regardless of the method of cooling, while the extent of strength degradation in coupons subjected to heating beyond 600 °C is dependent on heated temperature and method of cooling. Data generated in these tests are utilized to generate high‐temperature stress–strain and residual stress–strain response of A572 steel. These results are also utilized to propose temperature‐dependent strength, elastic modules, and residual strength reduction factors of A572 steel, which can be used in evaluating residual response of fire‐exposed steel structures. Copyright © 2015 John Wiley & Sons, Ltd.     

Residual compressive strength of fire‐damaged mortar after post‐fire‐air‐curing

The effects of cooling regimes and post‐fire‐air‐curing on compressive strength of mortar were investigated. Mortars were made with CEN reference sand, CEM I 42.5 R cement and natural spring water. The sand–cement and water–cement materials' ratios were chosen as 3.0 and 0.50 for all mixtures, respectively. At 28 days, the specimens were heated to maximum temperatures of 400, 600, 800 and 1000°C. Specimens were then allowed to cool in the air, furnace and water. After cooling, the specimens were air‐recured. Compressive strength test was carried out before air‐recuring and after 7 days of air‐recuring. The highest reduction in compressive strength was observed at 1000°C regardless of cooling regime. Gradual cooling regime in air and furnace without post curing showed almost no difference in terms of compressive strength reduction for four elevated temperatures. Shock cooling in water caused significant reduction in compressive strength compared with both gradual cooling regimes without post curing. After air and furnace cooling regimes, 7 days air‐recured specimens showed further reduction in compressive strength for four elevated temperatures. Specimens cooled in water and subjected to 7 days air‐recuring showed significant strength gain approximately 39, 100 and 130% for 400, 600 and 800°C elevated temperature, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

煤矸石机制砂细度模数对胶砂性能影响研究

为了将煤矸石作为机制砂合理应用,使用煤矸石机制砂,研究了煤矸石机制砂的基本性质,设计了两种水胶比,配制了8个细度模数,研究了不同水胶比下煤矸石机制砂的细度模数对胶砂性能的影响.研究表明:水灰比较低时,煤矸石机制砂的各项性能都相较于较高的水灰比都更好,尤其是在抗压强度方面.水灰比相同时,细度模数为2.7和2.8时胶砂的各...     

Compressive strength of lumber at high temperatures

A model was developed to predict the residual strength in compression parallel to grain for dimension lumber subjected to axial loads at elevated temperatures while braced to prevent buckling. Prediction of the time‐dependent temperature profile within the cross‐section of a lumber member was achieved by adapting a two‐dimensional heat transfer model. An extensive literature review examined seven models proposed by various authors to predict residual strength under various temperature regimes. With knowledge of the temperature‐history and material properties, it was possible to predict the time to failure by crushing of axially loaded members exposed to elevated temperatures. Initially, predicted times to failure did not show as good an agreement with those measured in an experimental programme as had been hoped. However, when a new residual strength‐temperature model was developed and used to simulate the axial load capacity of nominal 38 mm × 89 mm studs exposed to fire, the results obtained showed good agreement with experimental results. Copyright © 2004 John Wiley & Sons, Ltd.     

氧化钙高温改性磷石膏对水泥基胶凝材料性能的影响研究*

磷石膏是磷化工产业过程中产生的固体废弃物，其主要成分为CaSO4 ·2H2O，具有产量大、组分复杂等特征，阻碍了其在胶凝材料中的应用。本研究利用氧化钙的碱性来去除磷石膏中的酸性杂质，探究氧化钙改性磷石膏的机理，对磷石膏水泥基胶凝材料的物理性能进行考察，结合SEM分析胶凝材料水化产物情况。结果表明：氧化钙能够消除磷石膏中的酸性杂质，减弱其对二水石膏晶体定向生长的抑制作用；掺入氧化钙后胶凝材料的安定性合格，初凝时间和终凝时间均下降；氧化钙的掺入有利于水泥基胶凝材料强度的发展，最佳的氧化钙掺量为2%；SEM结果显示，氧化钙消除了杂质的抑制作用，水化作用增强，层状的二水石膏晶体出现，有利于胶凝材料强度的发展。     

Influence of PFA on cracking of concrete and cement paste after exposure to high temperatures

Cracking is a visible type of damage to concrete that has significant adverse effects on the mechanical and durability properties of concrete. An experimental study on the identification and quantification of cracking in postheated concrete was conducted to provide a better understanding of the mechanisms of damages to concrete after exposure to high temperatures. In addition to the quantification of the residual compressive and tensile strengths of concrete after high temperature exposure, both macroscale and microscopic cracks were observed and measured. The crack patterns in different concretes, including concrete made with different water to binder (w/b) ratios and PFA dosages, were classified. Also examined was the cracking in the corresponding hardened cement pastes (hcp's) prepared without adding aggregates. The relation of cracking with deterioration of the durability properties of concrete, with respect to the chloride diffusion test results, was discussed. Crack density, a quantitative term, which had been introduced to study the microcrack properties in concrete, was adopted for measuring the severity of cracking. Severe cracking of concrete was observed after exposure to 450 °C and higher temperatures. The presence of PFA reduced the extent of these thermal cracks.     

The influence of aggregate type on the physical and mechanical properties of high‐performance concrete subjected to high temperature

This paper presents the results of an experimental study on the behaviour of high‐performance concretes after high temperature exposure. The high temperature exposure is related to the potential risk of fire, and mechanical properties analysis is needed afterwards to assess the residual strength of the material. The results presented in the paper show the properties evolution of four concretes made with four different aggregate types: basalt, granite, dolomite and riverbed gravel. The mix compositions allow comparisons, because the cement paste and mortar compositions and their volumes remain the same for all the four concretes. Moreover, the aggregate particle size distribution was chosen to be quasi identical so that this factor does not affect the concrete behaviour. The decrease of tensile strength value with the increase of temperature is more pronounced than compressive strength reduction thus, the exponential and power function equations were proposed to describe f_tT–f_cT relationship. The change of modulus of elasticity in relative values is similar, although the initial values of modulus are different and correspond to the aggregate type. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of power ultrasound on the corrosion of aluminium and high speed steel

The influence of power ultrasound on the corrosion of aluminium and high speed steel in chloride medium has been investigated. Open circuit potential, polarization curves and electrochemical impedance spectroscopy were employed to study the corrosion process before, during and after ultrasonic irradiation as well as scanning electron microscopy of the corroded surfaces. Cavitation led to formation of pits, erosion and increased mass transport. The effect of ultrasound power, chloride concentration, horn tip–metal distance and of tip diameter were assessed. The principal effect of ultrasound on aluminium is destruction of the oxide film and on high speed steel removal of corrosion products and increased solution mass transport; the influence of ultrasound is greatest at low chloride concentrations. For both metals it was shown that for chloride ion concentrations up to 0.1 M the influence of ultrasound power is most evident, followed by the distance between probe tip and metal, then [Cl^–] and lastly the size of the probe tip. The potentialities of using ultrasound as a rapid diagnostic test of corrosion resistance are indicated.     

Influence of heat treatment on the corrosion of high speed steel

The corrosion behaviour of M2 high speed steel, as-received as well as heat-treated under different conditions, has been studied in 0.1 M KCl aqueous solution by corrosion potential measurements, Tafel curves and electrochemical impedance. Heat treatment leads to an increase of the corrosion resistance of high speed steel; the higher the tempering temperature after quenching, the higher the corrosion resistance of the steel. X-ray diffraction, energy dispersive X-ray analysis and scanning electron microscopy were also used to further understand the corrosion behaviour of the different samples. Based on these analyses, the microstructure of the different samples was correlated with their corrosion properties.