The effect of realistic curing temperature on the strength and E-modulus of concrete

The strength and E-modulus of concrete are decisive parameters when it comes to ultimate limit state design, serviceability limit state design, and early age crack assessment. The properties of concrete are generally determined in the laboratory under 20 °C isothermal conditions and then used as the basis for calculations under realistic temperature conditions. It is well-known, however, that the curing temperature affects both the rate of property development in concrete and the “final value” of a given property. The current study investigated the effect of a realistic temperature history on the compressive cube strength, the tensile strength, and the tensile E-modulus for two concretes, a reference concrete and a fly ash concrete. Concrete specimens were subjected to either (1) 20 °C isothermal curing conditions, or (2) realistic temperature curing conditions for 14 days and then 20 °C isothermal conditions, until they were tested after 28 and 91 days. Parallel tests performed in a Temperature-Stress Testing Machine were also used to evaluate the results. The reference concrete showed a general reduction in strength and E-modulus when subjected to a realistic curing temperature, whereas the fly ash concrete showed an 11% increase in the 28-day E-modulus when cured under realistic temperature conditions. Furthermore, in both isothermal and realistic curing temperature conditions, the fly ash concrete showed a pronounced property development beyond 28 days, which could not be described by the material model currently used.     

纤维对超高性能混凝土残余强度及高温爆裂性能的影响

采用普通原材料制备56 d龄期抗压强度为140~160 MPa的空白组超高性能混凝土、钢纤维超高性能混凝土及混杂纤维超高性能混凝土,测定其遭受高温作用后的残余抗压强度和劈裂抗拉强度,并对100%含湿量的混凝土试块进行高温爆裂试验。此外,测定大小2种加热速率对超高性能混凝土高温爆裂行为的影响。结果表明:所配制混凝土的残余抗压强度均随着目标温度的升高呈现先增大再降低的趋势,800℃高温后的残余抗压强度约为常温强度的30%。钢纤维与混杂纤维混凝土的残余劈裂抗拉强度亦呈现先升高再降低的趋势,800℃高温后的残余劈裂抗拉强度分别为常温强度的15.1%和35.4%。空白组混凝土的残余劈裂抗拉强度随着目标温度的升高而单调下降,800℃高温后的强度值约为常温强度的20.3%。7.5℃/min加热速率下,100%含湿量的3种混凝土试块均发生了严重高温爆裂,单掺钢纤维可以改善超高性能混凝土的高温爆裂,但不能避免爆裂的发生,而混杂纤维对超高性能混凝土高温爆裂的改善效果并未显著优于钢纤维。2.5℃/min加热速率下,混杂纤维可避免部分超高性能混凝土试块发生爆裂。      

Low-volume wet-process sprayed concrete: hardened properties

This paper, which reports on part of a 3-year research project into wet-process sprayed mortars and concretes for repair, investigates the hardened performance of wet-process sprayed fine concretes. It follows on from an earlier paper by the authors on the performance of hardened wet-process sprayed mortars and some comparisons with these are made here (Austin SA, Robins PJ, Goodier CI (2000). Magz Concr Res 52:195–208). Work has also been completed by the authors on the pumping and rheology of the fine concrete mixes presented here (Austin SA, Goodier, CI, Robins PJ (2005). Mater Struc, RILEM 38:229–237). Nine laboratory-designed fine concretes were pumped and sprayed through a wet-process piston pump and one through a dry-process pump. The properties measured included compressive and flexural strength, tensile bond strength, hardened density, elastic modulus, sorptivity and drying and restrained shrinkage. In situ test specimens were extracted from 500 × 500 × 100 mm deep sprayed panels. Hardened property tests were also conducted on corresponding cast specimens and, where possible, on specimens that had been sprayed directly into a cube or beam mould. The compressive strengths of the cast cubes, although very similar, were usually slightly greater than the in␣situ cubes, the opposite of what was found for wet-sprayed mortars (Austin SA, Robins PJ, Goodier CI (2000). Magz Concr Res 52:195–208). Inconsistent results for compressive and flexural strengths obtained from spraying directly into a steel mould suggest that this method is not as reliable when using a piston pump as it is when using a low-output worm pump (Austin SA, Robins PJ, Goodier CI (2000). Magz Concr Res 52:195–208). The bond strength of all the mixes exceeded 2.1 MPa at 7 days. The values for modulus of elasticity, when compared with the compressive strength, were similar to published data for this relationship. The sorptivity values showed only a slight relationship with the compressive strength. The mixes exhibited a wide range of drying shrinkage, but the data from the restrained specimens suggest an actual repair is influenced as much by ambient conditions as it is by the mix proportions.     

Mechanical properties of young concrete: Part II: Determination of model parameters and test program proposals

The paper concerns testing and modelling of the mechanical properties required as input to calculation programs made for crack risk estimation of hardening concrete structures. The results from several test series on mechanical properties of young concrete as described in Part I of this paper, are further evaluated. Model parameters for the modified CEB 1990 Model Code-equations, are determined for six concrete mixes, all having a w/b-ratio on 0.40, for compressive strength, tensile strength and E-modulus. To make the models applicable for young concrete, at ₀-parameter is introduced to fix the time at which significant mechanical properties are present. A test program to determine the model parameters is proposed, based on the experience that compressive strength tests have the smallest statistical scatter and that they are simplest to carry out.     

Effects of fly ash fineness on the mechanical properties of concrete

The present study reviews the effects of fly ash fineness on the compressive and splitting tensile strength of the concretes. A fly ash of lignite origin with Blaine fineness of 2351?cm²/g was ground in a ball mill. As a consequence of the grinding process, fly ashes with fineness of 3849?cm²/g and 5239?cm²/g were obtained. Fly ashes with three different fineness were used instead of cement of 0%, 5%, 10%, and 15% and ten different types of concrete mixture were produced. In the concrete mixtures, the dosage of binder and water/cement ratio were fixed at 350?kg/m³ and 0.50, respectively. Slump values for the concretes were adjusted to be 100 ± 20?mm. Cubic samples were cast with edges of 100?mm. The specimens were cured in water at 20°C. At the end of curing process, compressive and splitting tensile strengths of the concrete samples were determined at 7, 28, 56, 90, 120 and 180?days. It was observed that compressive and splitting tensile strength of the concretes was affected by fineness of fly ash in short-and long-terms. It was found that compressive and tensile strength of the concretes increased as fly ash fineness increased. It was concluded that Blaine fineness value should be above 3849?cm²/g fineness of fly ash to have positive impact on mechanical properties of concrete. The effects of fly ash fineness on the compressive and splitting tensile strength of the concretes were remarkably seen in the fly ash with FAC code with fineness of 5235?cm²/g.     

The use of oil shale ash in Portland cement concrete

An experimental investigation was undertaken to study the potential use of Jordanian oil shale ash (OSA) as a raw material or an additive to Portland cement mortar and concrete. Different series of mortar and concrete mixtures were prepared at different water to binder ratios, and different OSA replacements of cement and/or sand. The compressive strength of mortar and concrete specimens, cured in water at 23 °C, was determined over different curing periods which ranged from 3 to 90 days. The results of these tests were subjected to a statistical analysis. Equations were developed by regression analysis techniques to relate the effect of batch constituents on the strength developments of OSA mortars and concretes. The models were checked for accuracy by comparing their predictions with actual test results.The obtained results indicated that OSA replacement of cement, sand or both by about 10% (by wt) would yield the optimum compressive strength, and that its replacement of cement by up to 30% would not reduce its compressive strength, significantly. It was found that OSA on its own possesses a limited cementitious value and that its contribution to mortar or concrete comes through its involvement in the pozzolanic reactions. The statistical model developed showed an excellent predictability of the compressive strength for mortar and concrete mixes.     

Engineering property and structural design relationships for new and developing concretes

The paper reports the findings of a study carried out to investigate the effect of new and developing concrete technology solutions,e.g. (i) use of particle packing techniques and fillers to minimise voids, (ii) use of cement additions attained from industrial by-products and (iii) use of high range water-reducing admixtures which enable lower cement contents, on the engineering and structural performance of concrete and implications for structural design. The test programme considered 54 concrete mixes in three series to assess the impact of these on the tensile strength, flexural strength and modulus of elasticity of concrete, and in parallel, 37 mixes to measure these effects on the shear resistance of reinforced concrete beams. The results indicate that the influence of the concretes on compressive strength were generally inproportion to the effects on other engineering properties and were in line with current design assumptions on the behaviour of concrete. Furthermore, EC2 equations for predicting the shear strength of reinforced concrete beams, based on compressive strength, were also found to be appropriate for the range of concrete mixes considered. Overall, the work has demonstrated that new and developing concrete technology solutions can be utilised effectively within the framework of present design procedures and compressive strength is an appropriate parameter for assessing the structural performance of these concretes.     

Bond performance of reinforcing bars in inorganic polymer concrete (IPC)

The basic mechanical and chemical properties of fly-ash-based inorganic polymer concretes (IPC) have been studied widely, but, key engineering and structural properties of the material for instance modulus of elasticity, compressive, tensile, flexural strengths and bonding strength of the material to reinforcement have received little attention. Structural applications of reinforced IPC depend on the bond performance of the material to the reinforcement. Due to their difference with ordinary Portland cement (OPC) based concrete in terms of chemical reaction and matrix formation it is not known whether IPC exhibit different bonding performance with the reinforcement. Simply relying on compressive strength of the material and extrapolating models and equations meant for OPC based concrete may lead to unsafe design of structural members. To that end, 27 beam-end specimens, 58 cubic direct pullout type specimens and number of laboratory test specimens were tested to evaluate bonding performance of IPC with reinforcement. The results of beam-end specimens and direct pullout type specimens correlate favourably, although the results of direct pullout tests are in general more conservative than those of beam-end specimens. Overall, it can be concluded that bond performance of IPC mixes are comparable to OPC based concrete and therefore IPC and steel can be used as a composite material to resist tension in addition to compression.     

Valorization of co-combustion fly ash in concrete production

The purpose of this paper is to compare the effects of two different Supplementary Cementing Materials (SCMs) on mechanical and durability-related properties of structural concrete. Three mixes were produced, where coal and co-combustion fly ashes were used as partial substitute of cement (20% in volume) and compared with a control/reference concrete. Performances investigated included fresh concrete properties, compressive and tensile strength, elastic modulus, permeability, capillarity and drying/wetting resistance. Results indicate that both the SCMs can be classified as low-carbon fly ashes, and their use in concrete improves the workability of the mixes. A slight reduction of mechanical strength was observed for the concretes including both the SCMs. In addition, concrete transport properties were also slightly reduced when co-combustion fly ash was used. Wetting-drying cycles affected significantly the durability of all the mixes: compressive strength after these cycles was significantly lowered, and the cracks occurred due to the thermal stress applied, appeared to be filled by needle-shape crystals of ettringite.     

Long-term strength development of concrete in arid conditions

A long-term investigation into the development of the compressive strength of various concretes, subjected to Kuwait hot and arid environmental conditions is reported. The main parameters investigated included, w/c ratio, cement type and content, and admixture type and its dosage. Other parameters investigated included the effects of using different water curing periods, curing compounds, and casting season. Forty-seven different mixes were placed on the roof of the laboratory building and were exposed to the environment. Compression tests on 100 mm cubes were carried out over a period in excess of five years.The results generally showed that the compressive strength of the concrete increased with age. The gain in strength at 1800 days above that at 28 days varied considerably depending on the concrete constituents and curing procedure. Concretes made with white Portland cement achieved higher compressive strengths than those made with ordinary or sulphate resisting Portland cements. Also, the type and dosage of admixture influenced the compressive strength of concrete. An increase in the water-curing period was more effective in improving the 28-day compressive strength than the 1800-day strength. The use of curing compounds or silica fume appeared to influence the early age strength more than the long-term strength. Compression test results from selected mixes at the age of 10 years indicated that there was little or no increase in strength during the previous five years.     

The role of 0–2 mm fine recycled concrete aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate concrete

The aim of this study is to investigate the role of 0–2 mm fine aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate (RCA) concrete with normal and high strengths. Normal coarse and fine aggregates were substituted with the same grading of RCAs in two normal and high strength concrete mixtures. In addition, to keep the same slump value for all mixes, additional water or superplasticizer were used in the RCA concretes. The compressive and splitting tensile strengths were measured at 3, 7 and 28 days. Test results show that coarse and fine RCAs, which were achieved from a parent concrete with 30 MPa compressive strength, have about 11.5 and 3.5 times higher water absorption than normal coarse and fine aggregates, respectively. The density of RCAs was about 20% less than normal aggregates, and, hence, the density of RCA concrete was about 8–13.5% less than normal aggregate concrete. The use of RCA instead of normal aggregates reduced the compressive and splitting tensile strengths in both normal and high strength concrete. The reduction in the splitting tensile strength was more pronounced than for the compressive strength. However, both strengths could be improved by incorporating silica fume and/or normal fine aggregates of 0–2 mm size in the RCA concrete mixture. The positive effect of the contribution of normal sand of 0–2 mm in RCA concrete is more pronounced in the compressive strength of a normal strength concrete and in the splitting tensile strength of high strength concrete. In addition, some equation predictions of the splitting tensile strength from compressive strength are recommended for both normal and RCA concretes.     

Modified water/cement ratio law for compressive strength of fly ash concretes

Experimental data are presented which suggest that the development of compressive strength of fly ash concretes can be explained by superposition of two independent mechanical pore-filling mechanisms in the cement—fly ash paste. It is also suggested that the traditional water/cement ratio law for ordinary Portland cement concretes can be applied to fly ash concretes, provided that a slight modification is introduced. This will be of assistance in the design of fly ash concrete mixes for compressive strength.     

Effects of cold-bonded fly ash aggregate properties on the shrinkage cracking of lightweight concretes

This paper presents an experimental study on the restrained shrinkage cracking of the lightweight concretes made with cold-bonded fly ash lightweight aggregates. Two types of fly ash having different physical and chemical properties were utilized in the production of lightweight aggregates with different strengths. Afterwards, lower strength aggregates were also surface treated by water glass and cement–silica fume slurry to improve physical and mechanical properties of the particles. Therefore, a total of eight concrete mixtures were designed and cast at 0.35 and 0.55 water–cement ratios using four types of lightweight coarse aggregates differing in their surface texture, density, water absorption, and strength. Ring type specimens were used for restrained shrinkage cracking test. Free shrinkage, creep, weight loss, compressive and splitting tensile strengths, and modulus of elasticity of the concretes were also investigated. Results indicated that improvement in the lightweight aggregate properties extended the cracking time of the concretes resulting in finer cracks associated with the lower free shrinkage. Moreover, there was a marked increase in the compressive and splitting tensile strengths, and the modulus of elasticity.     

A study on durability properties of high-performance concretes incorporating high replacement levels of slag

This paper presents an experimental study of combined effects of curing method and high replacement levels of blast furnace slag on the mechanical and durability properties of high performance concrete. Two different curing methods were simulated as follows: wet cured (in water) and air cured (at 20°C and 65% RH). The concretes with slag were produced by partial substitution of cement with slag at varying amounts of 50–80%. The water to cementitious material ratio was maintained at 0.40 for all mixes. Properties that include compressive and splitting tensile strengths, water absorption by total immersion and by capillary rise, chloride penetration, and resistance of concrete against damage due to corrosion of the embedded reinforcement were measured at different ages up to 90 days. It was found that the incorporation of slag at 50% and above-replacement levels caused a reduction in strength, especially for the early age of air cured specimens. However, the strength increases with the presence of slag up to 60% replacement for the 90 day wet cured specimens. Test results also indicated that curing condition and replacement level had significant effects on the durability characteristics; in particular the most prominent effects were observed on slag blended cement concrete, which performed extremely well when the amount of slag used in the mixture increased up to 80%.     

Residue strength,water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures

In this paper, the effects of high temperature exposure of recycled aggregate concretes in terms of residual strengths, capillary water absorption capacity and pore size distribution are discussed. Two mineral admixtures, fly ash (FA) and ground granulated blast furnace (GGBS) were used in the experiment to partially replace ordinary Portland cement for concrete production. The water to cementitious materials ratio was maintained at 0.50 for all the concrete mixes. The replacement levels of natural aggregates by recycled aggregates were at 0%, 50% and 100%. The concretes were exposed separately to 300 °C, 500 °C and 800 °C, and the compressive and splitting tensile strength, capillary water coefficient, porosity and pore size distribution were determined before and after the exposure to the high temperatures. The results show that the concretes made with recycled aggregates suffered less deteriorations in mechanical and durability properties than the concrete made with natural aggregates after the high temperature exposures.     

Performance of metakaolin concrete at elevated temperatures

An experimental investigation was conducted to evaluate the performance of metakaolin (MK) concrete at elevated temperatures up to 800 °C. Eight normal and high strength concrete (HSC) mixes incorporating 0%, 5%, 10% and 20% MK were prepared. The residual compressive strength, chloride-ion penetration, porosity and average pore sizes were measured and compared with silica fume (SF), fly ash (FA) and pure ordinary Portland cement (OPC) concretes. It was found that after an increase in compressive strength at 200 °C, the MK concrete suffered a more severe loss of compressive strength and permeability-related durability than the corresponding SF, FA and OPC concretes at higher temperatures. Explosive spalling was observed in both normal and high strength MK concretes and the frequency increased with higher MK contents.     

Strength development of concrete with rice-husk ash

This paper presents a study on the development of compressive strength up to 91 days of concretes with rice-husk ash (RHA), in which residual RHA from a rice paddy milling industry in Uruguay and RHA produced by controlled incineration from the USA were used for comparison. Two different replacement percentages of cement by RHA, 10% and 20%, and three different water/cementicious material ratios (0.50, 0.40 and 0.32), were used. The results are compared with those of the concrete without RHA, with splitting tensile strength and air permeability. It is concluded that residual RHA provides a positive effect on the compressive strength at early ages, but the long term behavior of the concretes with RHA produced by controlled incineration was more significant. Results of splitting tensile and air permeability reveal the significance of the filler and pozzolanic effect for the concretes with residual RHA and RHA produced by controlled incineration.     

Measurement and prediction of the strength of rubberized concrete

Illegally discarded piles of automobile tires are sources of potential hazards. Current disposal methods are wasteful and costly as they require either consumption of landfill space or continuous costly maintenance. A solution to the problem of scrap-tire disposal is the potential use of tire chips and crumb rubber as mineral aggregate substitutes in Portland cement concrete mixes. In this study, some of the engineering properties of rubberized concrete were examined and a neural network was developed to predict its compressive and tensile strengths. Rubberized concrete was found to possess good esthetics, acceptable workability, and a smaller unit weight than normal concrete. However, rubberized concrete did not perform as well as normal concrete under repeated freeze-thaw cycles. It exhibited lower compressive and tensile strength than that of normal concrete. Unlike normal concrete, rubberized concrete had the ability to absorb a large amount of plastic energy under compressive and tensile loads. It did not demonstrate the typical brittle failure, but rather a ductile, plastic failure mode. Test results were analyzed so that a model can be developed to predict the strength of rubberized concrete. Two neural network models were developed to predict the reduction in the compressive and tensile strength as a result of replacing mineral aggregate with rubber aggregate. A maximum difference of 9·2% between test results and model prediction was detected during the testing of the neural networks.     

同时确定钢纤维高强混凝土的断裂韧度及拉伸强度

考虑钢纤维高强混凝土试件细观非均质性对宏观断裂的影响机制,将钢纤维掺量、长度、直径及钢纤维抗拉强度等细观层面的钢纤维特征参数,引入钢纤维高强混凝土宏观断裂模型的虚拟裂缝扩展量的具体计算公式,从而发展了考虑钢纤维特性的可同时确定钢纤维高强混凝土的断裂韧度与拉伸强度的模型及方法。采用变化参数为钢纤维掺量和混凝土水灰比的三点弯曲试件,基于所提模型,同时确定了钢纤维高强混凝土的断裂韧度与拉伸强度,确定值与试验拉伸强度值以及尺寸效应模型计算的断裂韧度吻合良好。基于测试数据离散性为钢纤维高强混凝土固有属性的事实,采用确定的断裂韧度及拉伸强度,建立起钢纤维高强混凝土塑性——准脆性——线弹性不同结构断裂模式的±20%全曲线,其可涵盖实验室条件下的所有试验数据。该文所提模型及方法适用于钢纤维高强混凝土及高强混凝土,可为钢纤维高强混凝土等复合材料真实断裂韧度与拉伸强度的确定,及个性化结构断裂破坏的预测等关键科技问题提供依据。     

Testing of concrete by rebound method: Leeb versus Schmidt hammers

Hardness is considered as an important property of concrete; it can be used to estimate compressive strength of concrete in situ. The classic Schmidt rebound hammer is the most popular nondestructive method to measure concrete surface hardness, while the Leeb rebound hammer has been extensively studied in geological and metallographic fields over decades, and its use for testing concrete is almost not known. The national and international standards for the measurement of hardness are reviewed. Concrete made different w/c ratios (0.33, 0.4 and 0.5) were tested by both methods. The simple linear correlation between rebound numbers (both Schmidt and Leeb) and concrete compressive strength are proposed. Schmidt rebound number was differently correlated with compressive strength for concretes with different w/c ratios, while the Leeb rebound numbers were more consistent and could be applied in predicting concrete compressive strength within 10% error for all w/c ratios. It was also concluded that Schmidt test can be considered as a semi-destructive method, because of significant strength reduction (in average by 10.5 MPa) that was observed after application of Schmidt hammer impact on specimens, while the Leeb rebound test procedure did not result in any damage of concrete. This difference can be explained by the dramatic difference in impact energy of the two hammers (2207 and 11 N·mm - for Schmidt rebound hammers of N-type and Leeb hammers of D-type, respectively). Moreover, the classic Schmidt rebound hammer is not recommended to be used on the concrete specimens, which are aimed for compressive tests at early age (less than 3 days) or when expected compressive strength is less than 7 MPa. These constraints do not apply to lower impact Leeb rebound devices, which can be considered as perfectly invasive (non-destructive). At the same time, as expected, Leeb rebound test is sensitive to the surface conditions, such as carbonation and surface moisture.