Effect of utilizing unground and ground normal and black rice husk ash on the mechanical and durability properties of high-strength concrete

The aim of the present study is to investigate the effects of utilizing different processings of normal rice husk ash (RHA) and black rice husk ash (BRHA) on the mechanical and durability properties of high-strength concrete (HSC). Mechanical and durability properties of HSC were evaluated on concrete mixes containing unground BRHA and RHA and ground BRHA and RHA, their average particles sizes being 165, 85, 67 and 24 µm, respectively. The replacement of ordinary Portland cement with the ashes was adopted at 20%. The results showed that incorporating any form of RHA and BRHA in HSC reduced the slump value. The surface areas of RHA and BRHA, not their carbon content, determined the dosage of superplasticizer needed to achieve a targeted slump value. Concrete with unground and ground RHA incorporated exhibited 30% higher compressive strength while unground BRHA produced 30% lower compressive strength than that of the control concrete. Incorporating unground and ground RHA showed a synergy between filler and pozzolanic effect and had insignificant difference in mechanical and durability properties of the concretes. Meanwhile, incorporating ground BRHA showed a dominant filler effect in the concrete. Overall, the improvement of splitting tensile strength and modulus of elasticity of both RHA and GBRHA concrete showed a similar trend to that of the compressive strength of RHA concrete. The durability of concretes with unground and ground RHA and ground BRHA incorporated showed better performance than that of the control concrete. The material with 20% ground BRHA as partial cement replacement in HSC of Grade 50 could be used without any reduction in the mechanical and durability properties. Use of unground BRHA is not recommended because it did not improve these properties.     

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。      

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.     

Influence of mix proportions and curing conditions on tensile splitting strength of high strength concretes

The effect of the composition of high strength concretes with low water to binder ratio and silica fume on the development of splitting tensile strength was studied. A statistical approach was employed to develop formulation which could adequately describe the relations between splitting tensile strength and the concrete composition, when cured in two different regimes: water curing at 20°C and sealed curing at 30°C. Autogenous shrinkage was induced in the second type of curing but was largely eliminated in the first one. The relations were presented as nomograms which could be used as a basis for mix design. The correlation between tensile splitting strength and compressive strength could not be described in terms of a simple linear relation with a characteristic constant. For the range of variables studied, the ratio between tensile and compressive strength varied over a large range of 0.08 to 0.12. As a result, the relations developed here for tensile strength are quite different in nature than those for compressive strength in a previous study. Analysis of the data suggest that tensile strength is sensitive to effects which induce autogenous shrinkage to a much greater extent than compressive strength. It is proposed that this may be the main reason for the different trends observed for the relations between the composition of the low water/binder ratio concretes and their compressive and tensile strength.     

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.     

High strength concrete containing natural pozzolan and silica fume

Various combinations of a local natural pozzolan and silica fume were used to produce workable high to very high strength mortars and concretes with a compressive strength in the range of 69–110 MPa. The mixtures were tested for workability, density, compressive strength, splitting tensile strength, and modulus of elasticity. The results of this study suggest that certain natural pozzolan–silica fume combinations can improve the compressive and splitting tensile strengths, workability, and elastic modulus of concretes, more than natural pozzolan and silica fume alone. Furthermore, the use of silica fume at 15% of the weight of cement was able to produce relatively the highest strength increase in the presence of about 15% pozzolan than without pozzolan. This study recommends the use of natural pozzolan in combination with silica fume in the production of high strength concrete, and for providing technical and economical advantages in specific local uses in the concrete industry.     

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.     

Comparison between high strength concrete and normal strength concrete subjected to high temperature

Two normal strength concretes and three high strength concretes, with 28-day compressive strengths of 28, 47, 76, 79 and 94 MPa respectively, were used to compare the effect of high temperatures on high strength concrete and normal strength concrete. After being heated to a series of maximum temperatures at 400, 600, 800, 1000 and 1200°C, and maintained for 1 hour, their compressive strengths and tensile splitting strengths were determined. The pore size distribution of hardened cement paste in high strength concrete and normal strength concrete was also investigated. Results show that high strength concrete lost its mechanical strength in a manner similar to or slightly better than that of NSC. The range between 400 and 800°C was critical to the strength loss of concrete with a large percentage of loss of strength. Microstructural study carried out revealed that high temperatures have a coarsening effect on the microstructure of both of high strength concrete and normal strength concrete.     

Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete

Rice husk ash (RHA) has been used as a highly reactive pozzolanic material to improve the microstructure of the interfacial transition zone (ITZ) between the cement paste and the aggregate in high-performance concrete. Mechanical experiments of RHA blended Portland cement concretes revealed that in addition to the pozzolanic reactivity of RHA (chemical aspect), the particle grading (physical aspect) of cement and RHA mixtures also exerted significant influences on the blending efficiency. The relative strength increase (relative to the concrete made with plain cement, expressed in %) is higher for coarser cement. The gap-grading phenomenon is expected to be the underlying mechanism. This issue is also approached by computer simulation. A stereological spacing parameter (i.e., mean free spacing between mixture particles) is associated with the global strength of the blended model cement concretes. This paper presents results of a combined mechanical and computer simulation study on the effects of particle size ranges involved in RHA-blended Portland cement on compressive strength of gap-graded concrete in the high strength/high performance range. The simulation results demonstrate that the favourable results for coarser cement (i.e., the gap-graded binder) reflect improved particle packing structure accompanied by a decrease in porosity and particularly in particle spacing.     

Properties of lightweight aggregates produced with cold-bonding pelletization of fly ash and ground granulated blast furnace slag

Pelletization is a worldwide process used in producing artificial aggregates although its usage is not common in Turkey. In this study, lightweight aggregates (LWAs) were manufactured through cold-bonding pelletization of ground granulated blast furnace slag (G) and two types of fly ash with different finenesses (Fly ash A and B). Ordinary Portland cement (PC) was used as a binder at varying amounts from 5 to 20 % by weight. A total of 20 cold-bonded lightweight aggregates were produced at room temperature with different combinations of PC, FA and/or G. The hardened aggregates were tested for specific gravity, water absorption, and crushing strength. Thereafter, lightweight concretes (LWCs) were produced with water to cement ratio of 0.50 and a cement content of 400?kg/m³ by using such lightweight aggregates. The hardened concretes were tested for compressive strength at 28 and 56?days to explore the effect of aggregate types on the compressive strength development. Test results revealed that the amount of cement content had a significant effect on the strength of LWAs which in turn governed the variation in compressive strength of the LWCs. The highest 28 and 56-day compressive strengths of 43 and 51?MPa, respectively were achieved for the concretes including LWAs produced from the blend of 40 % slag, 40 % FA-A and 20 % PC.     

Mechanical properties of young concrete: Part I: Experimental results related to test methods and temperature effects

Results of several test series on mechanical properties of young concrete are presented. Six different concrete mixes were tested systematically, and a number of other concrete mixes less extensively, all with w/b≈0.40. The program included compressive strength, tensile strength, splitting tensile strength and E-modulus determination both in compression and tension. Because temperature influences the rate of property development, and also the “final value” of a given property, the specimens were subjected to realistic temperature regimes the first few days as well as isothermal conditions. The test methods are described, and results obtained by the different test methods are compared. It is recommended that the temperature sensitivity constants should be determined from compressive strength tests on specimens exposed to realistic temperature histories. These parameters depend strongly on the cement type and the silica fume content. It was found that high performance concretes were quite robust to the negative effects of elevated temperatures. This was particularly true for concretes with pozzolana. In part two of the paper model parameters for an equation to be used in calculation programs are determined, and a test program for crack risk estimation is proposed.     

Self compacting concrete from uncontrolled burning of rice husk and blended fine aggregate

This paper presents an experimental study on the development of normal strength Self compacting concrete (SCC) from uncontrolled burning of rice husk ash (RHA) as a partial replacement to cement and blended fine aggregate whilst maintaining satisfactory properties of SCC. Experiments on the fresh and hardened state properties have been carried out on RHA based SCC from uncontrolled burning. The dosages of RHA are limited to 0%, 20%, 30% and 40% by mass of the total cementitious material in the concrete. The experiments on fresh state properties investigate the filling ability, the passing ability and the segregation resistance of concrete. The experiments on hardened state properties investigate the compressive and the splitting tensile strengths. The water absorption level of the concrete with changing RHA levels has also been monitored. The experimental studies indicate that RHA based SCC developed from uncontrolled burning has a significant potential for use when normal strength is desired.     

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%.     

Hardened properties of self-consolidating high performance concrete including rice husk ash

This paper mainly presents the key hardened properties of self-consolidating high performance concrete (SCHPC). Various SCHPCs were produced with different water/binder (W/B) ratios, rice husk ash (RHA) contents, and air contents. The required filling ability and air content were achieved in all freshly mixed SCHPCs. The hardened SCHPCs were tested for compressive strength, ultrasonic pulse velocity, water absorption, total porosity, and true electrical resistivity. The effects of W/B ratio, RHA content, and air content on these hardened properties were observed. Test results revealed that the compressive strength, ultrasonic pulse velocity, and electrical resistivity increased whereas the water absorption and total porosity decreased with lower W/B ratio and higher RHA content. In addition, the air content decreased the compressive strength, ultrasonic pulse velocity, water absorption, and total porosity but increased the electrical resistivity. Based on the overall effects of rice husk ash, the optimum RHA content for SCHPC has been defined.     

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.     

Comparative study on strength,sorptivity, and chloride ingress characteristics of air-cured and water-cured concretes modified with metakaolin

This paper reports an investigation in which the performance of plain and metakaolin (MK)-modified concretes were studied under two different curing regimes. The purpose of this study is to evaluate the effectiveness of MK in enhancing the strength and permeation properties of concrete. MK was used to replace 0–20% of Portland cement by weight in concrete with two water-binder (w/b) ratios of 0.35 and 0.55. The change in compressive strength, sorptivity, and chloride ingress with age at all cement replacement levels under both air and water curing are compared with those of the control concrete. The results indicated that the inclusion of MK greatly reduced sorptivity and chloride permeability of concrete in varying magnitudes, depending mainly on replacement level of MK, w/b ratio, curing condition, and chloride exposure period. It was found that under the inadequate or poor curing, MK-modified concretes suffered a more severe loss of compressive strength and permeability-related durability than the plain concretes.     

钢纤维及陶粒掺量对轻质混凝土基本力学性能的影响

制备了3种强度等级共13组配合比的钢纤维增强轻质混凝土（Steel Fibre Reinforced Lightweight Aggregate Concrete,SFRLAC）,测量了立方体抗压强度、劈裂抗拉强度和轴心抗压强度,得到了SFRLAC轴心受压应力-应变曲线。试验结果表明,钢纤维能小幅度提高轻质混凝土（Lightweight Concrete,LC）的抗压强度,随陶粒比率（Haydite Ratio,V_h）的增大,抗压强度降低,且强度等级越低,降幅越大。钢纤维能显著提高LC的劈裂抗拉强度,钢纤维对低强度等级LC劈裂抗拉强度的贡献优于对高强度等级LC的贡献。低强度等级SFRLAC （LC30和LC40）的劈裂抗拉强度受V_h的影响较大,而高强度等级SFRLAC （LC50）与之相反。当V_h达到80%时,V_h不再是影响SFRLAC劈裂抗拉强度的主要因素,而钢纤维的增强效应显著。试块的破坏形态表明钢纤维能改善LC的塑性。V_h对抗拉强度的降低效应远大于对抗压强度的降低效应。建立了SFRLAC轴心抗压强度与立方体抗压强度的关系式。SFRLAC应力-应变曲线综合体现了钢纤维的增强效应和陶粒的削减作用,陶粒降低LC的峰值应力和韧性,钢纤维主要提高LC的韧性。     

Transport and mechanical properties of self consolidating concrete with high volume fly ash

This paper presents the transport and mechanical properties of self consolidating concrete that contain high percentages of low-lime and high-lime fly ash (FA). Self consolidating concretes (SCC) containing five different contents of high-lime FA and low-lime FA as a replacement of cement (30, 40, 50, 60 and 70 by weight of total cementitious material) are examined. For comparison, a control SCC mixture without any FA was also produced. The fresh properties of the SCCs were observed through, slump flow time and diameter, V-funnel flow time, L-box height ratio, and segregation ratio. The hardened properties included the compressive strength, split tensile strength, drying shrinkage and transport properties (absorption, sorptivity and rapid chloride permeability tests) up to 365 days. Test results confirm that it is possible to produce SCC with a 70% of cement replacement by both types of FA. The use of high volumes of FA in SCC not only improved the workability and transport properties but also made it possible to produce concretes between 33 and 40 MPa compressive strength at 28 days, which exceeds the nominal compressive strength for normal concrete (30 MPa).     

Decoupling the physical and chemical effects of supplementary cementitious materials on strength and permeability: A multi-level approach

Supplementary cementitious materials (SCMs) represent an alternative for the industry to achieve sustainability by reducing cement contents without significant compromises of the mechanical properties and enhancing durability. SCMs play a dual role during hydration: a physical effect promoting nucleation and cement hydration and a chemical effect through pozzolanic activity. Rice husk ash (RHA) and natural pozzolans (NP) were evaluated using compressive strength and durability tests in a multi-level experimental program. RHA increased the strength more than NP, which is well explained by its prominent chemical effect (78%) assessed by isothermal calorimetry and its high amorphous silica content. Both RHA and NP produced significant reductions in the permeability of the concrete, which is mostly explained by the chemical effect. Decoupling the physical and chemical effects of a SCM allows for optimisation of its manufacturing process.     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.