Showing papers on "Compressive strength published in 2007"

Solid solution alloys of AlCoCrFeNiTix with excellent room-temperature mechanical properties

Abstract: Alloys with composition of AlCoCrFeNiTix (x: molar ratio; x=0,0.5,1,1.5) were designed by using the strategy of equiatomic ratio and high entropy of mixing. The alloy system is composed mainly of body centered cubic solid solution and possesses excellent room-temperature compressive mechanical properties. Particularly for AlCoCrFeNiTi0.5 alloy, the yield stress, fracture strength, and plastic strain are as high as 2.26GPa, 3.14GPa, and 23.3%, respectively, which are superior to most of the high-strength alloys such as bulk metallic glasses.

Mechanical behaviour of concrete made with fine recycled concrete aggregates

Abstract: This paper concerns the use of fine recycled concrete aggregates to partially or globally replace natural fine aggregates (sand) in the production of structural concrete. To evaluate the viability of this process, an experimental campaign was implemented in order to monitor the mechanical behaviour of such concrete. The results of the following tests are reported: compressive strength, split tensile strength, modulus of elasticity and abrasion resistance. From these results, it is reasonable to assume that the use of fine recycled concrete aggregates does not jeopardize the mechanical properties of concrete, for replacement ratios up to 30%.

Workability and strength of coarse high calcium fly ash geopolymer

Abstract: In this paper, the basic properties viz., workability and strength of geopolymer mortar made from coarse lignite high calcium fly ash were investigated. The geopolymer was activated with sodium hydroxide (NaOH), sodium silicate and heat. The results revealed that the workable flow of geopolymer mortar was in the range of 110 ± 5%–135 ± 5% and was dependent on the ratio by mass of sodium silicate to NaOH and the concentration of NaOH. The obtained compressive strength was in the range of 10–65 MPa. The optimum sodium silicate to NaOH ratio to produce high strength geopolymer was 0.67–1.0. The concentration variation of NaOH between 10 M and 20 M was found to have a small effect on the strength. The geopolymer samples with high strength were obtained with the following practices: the delay time after moulding and before subjecting the sample to heat was 1 h and the optimum curing temperature in the oven was 75 °C with the curing duration of not less than two days.

Evaluation of bagasse ash as supplementary cementitious material

Abstract: The utilization of waste materials in concrete manufacture provides a satisfactory solution to some of the environmental concerns and problems associated with waste management. Agro wastes such as rice husk ash, wheat straw ash, hazel nutshell and sugarcane bagasse ash are used as pozzolanic materials for the development of blended cements. Few studies have been reported on the use of bagasse ash (BA) as partial cement replacement material in respect of cement mortars. In this study, the effects of BA content as partial replacement of cement on physical and mechanical properties of hardened concrete are reported. The properties of concrete investigated include compressive strength, splitting tensile strength, water absorption, permeability characteristics, chloride diffusion and resistance to chloride ion penetration. The test results indicate that BA is an effective mineral admixture, with 20% as optimal replacement ratio of cement.

Durability of alkali-activated fly ash cementitious materials

Abstract: The study described in the present paper addresses the durability of alkali-activated fly ash (AAFA) cement under different conditions: specifically, cement performance is measured in a number of aggressive environments (deionized water, ASTM seawater, sodium sulphate and acidic solutions) and with respect to alkali–silica reaction-induced expansion. The chief parameters studied are: weight loss, compressive strength, variations in volume, presence of the products of degradation and microstructural changes. The results show that AAFA pastes perform satisfactorily in aggressive environments and that degradation in these materials is distinctly different from such processes in OPC paste. These mortars are also compliant with the 16-day expansion limit stipulated in ASTM standard C1260-94 on potential alkali–silica reactivity.

Removal of cement mortar remains from recycled aggregate using pre-soaking approaches

Abstract: With a rising tide of adoption of recycled aggregate (RA) for construction, investigation on ways to improve the quality of RA has been overwhelming. The adoption of RA brings benefits including savings in the limited landfill spaces and the use of natural resources. However, the poorer quality of RA often limits its utilization to low grade applications such as sub-grade activities, filling materials and low grade concrete. The major reason that affects the quality of RA is the large amount of cement mortar remains on the surface of the aggregate, resulting in higher porosity, water absorption rates and thus a weaker interfacial zone between new cement mortar and aggregates, which weakens the strength and mechanical performance of concrete made from RA. This paper attempts to study three pre-soaking treatment approaches; namely ReMortarHCl, ReMortarH2SO4 and ReMortarH3PO4 in reducing the mortar attached to RA. The results show that the behaviour of RA has improved with reduction in water absorption, without simultaneous exceeding the limits of chloride and sulphate compositions after the treatment. This work has also compared the compressive strength, flexural strength and modulus of elasticity of concrete made from the approaches, which shows marked improvements in quality when compared with those using traditional approaches.

An experimental study on optimum usage of GGBS for the compressive strength of concrete

Abstract: This paper presents a laboratory investigation on optimum level of ground granulated blast-furnace slag (GGBS) on the compressive strength of concrete. GGBS was added according to the partial replacement method in all mixtures. A total of 32 mixtures were prepared in four groups according to their binder content. Eight mixes were prepared as control mixtures with 175, 210, 245 and 280 kg/m3 cement content in order to calculate the Bolomey and Feret coefficients (KB, KF). For each group 175, 210, 245 and 280 kg/m3 dosages were determined as initial dosages, which were obtained by removing 30 percent of the cement content of control concretes with 250, 300, 350, and 400 kg/m3 dosages. Test concretes were obtained by adding GGBS to concretes in an amount equivalent to approximately 0%, 15%, 30%, 50%, 70%, 90% and 110% of cement contents of control concretes with 250, 300, 350 and 400 kg/m3 dosages. All specimens were moist cured for 7, 14, 28, 63, 119, 180 and 365 days before compressive strength testing. The test results proved that the compressive strength of concrete mixtures containing GGBS increases as the amount of GGBS increase. After an optimum point, at around 55% of the total binder content, the addition of GGBS does not improve the compressive strength. This can be explained by the presence of unreacted GGBS, acting as a filler material in the paste.

Effects of the molecular architecture of comb-shaped superplasticizers on their performance in cementitious systems

Abstract: The objective of this study is to link the molecular structure of polycarboxylate-ether-type superplasticizers with the performance of cementitious systems in order to develop new products with enhanced properties, e.g. improved water reduction with a wide range of cements or a reduced retardation of cement hydration. Different experimental superplasticizers have been synthesized varying length and density of the polyether chains as well as the molecular weight of the polymer. The influence of these polymers on the properties of cement pastes and mortars was determined using various characterization methods like mortar flow, rheological and calorimetric measurements, adsorption measurements and mortar compressive strength. Characteristic connections between molecular structure of the polycarboxylate-type water reducers, adsorption behaviour, workability and retarding effect have been determined allowing the synthesis of new superplasticizers with improved performance.

Mechanical Properties of Steel Fiber-Reinforced Concrete

Abstract: This paper presents the results from an experimental program and an analytical assessment of the influence of addition of fibers on mechanical properties of concrete. Models derived based on the regression analysis of 60 test data for various mechanical properties of steel fiber-reinforced concrete have been presented. The various strength properties studied are cube and cylinder compres- sive strength, split tensile strength, modulus of rupture and postcracking performance, modulus of elasticity, Poisson's ratio, and strain corresponding to peak compressive stress. The variables considered are grade of concrete, namely, normal strength 35 MPa, moderately high strength 65 MPa, and high-strength concrete 85 MPa, and the volume fraction of the fiber Vf=0.0, 0.5, 1.0, and 1.5%. The strength of steel fiber-reinforced concrete predicted using the proposed models have been compared with the test data from the present study and with various other test data reported in the literature. The proposed model predicted the test data quite accurately. The study indicates that the fiber matrix interaction contributes significantly to enhancement of mechanical properties caused by the introduction of fibers, which is at variance with both existing models and formulations based on the law of mixtures. 85 MPa with various fiber dosages Vf=0, 0.5, 1.0, and 1.5%. An empirical relationship for various mechanical properties of SFRC has been proposed. The proposed model attempts to bring out the significance of fiber matrix interaction in all the strength properties. This study reports the experimental results of the strength properties of SFRC, namely, cube and cylinder compressive strength, split tensile strength, modulus of rupture, modulus of elasticity, Poisson's ratio, and strain corresponding to peak com- pressive stress. Empirical relationships were developed for vari- ous strength properties based on the regression analysis of the 60 test data. It is expected that these proposed models would be helpful in assessing the strength properties of fiber-reinforced concrete based on the matrix strength and fiber-RI.

Key Parameters for Strength Control of Artificially Cemented Soils

Abstract: Often, the use of traditional techniques in geotechnical engineering faces obstacles of economical and environmental nature. The addition of cement becomes an attractive technique when the project requires improvement of the local soil. The treatment of soils with cement finds application, for instance, in the construction of pavement base layers, in slope protection of earth dams, and as a support layer for shallow foundations. However, there are no dosage methodologies based on rational criteria as exist in the case of the concrete technology, where the water/cement ratio plays a fundamental role in the assessment of the target strength. This study therefore aims to quantify the influence of the amount of cement, the porosity and the moisture content on the strength of a sandy soil artificially cemented, as well as to evaluate the use of a water/cement ratio and a voids/cement ratio to assess its unconfined compression strength. A number of unconfined compression tests, triaxial compression tests, and measurements of matric suction were carried out. The results show that the unconfined compression strength increased linearly with the increase in the cement content and exponentially with the reduction in porosity of the compacted mixture. The change in moisture content also has a marked effect on the unconfined compression strength of mixtures compacted at the same dry density. It was shown that, for the soil-cement mixture in an unsaturated state (which is usual for compacted fills), the water/cement ratio is not a good parameter for the assessment of unconfined compression strength. In contrast, the voids/cement ratio, defined as the ratio between the porosity of the compacted mixture and the volumetric cement content, is demonstrated to be the most appropriate parameter to assess the unconfined compression strength of the soil-cement mixture studied.

Engineered Cementitious Composites with High-Volume Fly Ash

Abstract: Engineered cementitious composites (ECCs) are a breed of high-performance fiber-reinforced cementitious composites (HPFRCC) with significant strain-hardening behavior under tension. As ECCs impart ductility and durability to the structure, the high cement usage in the mixture causes environmental and economical impacts. In this paper, the mechanical performance of ECCs incorporating high volume fly ash and bottom ash is reported. Emphasis is placed on the influence of fly ash content on the key micromechanics properties relevant to composite ductility. It is revealed that a high volume faction of fly ash tends to reduce the polyvinyl alcohol (PVA)fiber/matrix interface bond and matrix toughness in favor of attaining high tensile strain capacity. The limit of cement substitution with ash is constrained by compressive strength development.

Mechanical properties and durability of mortar and concrete containing natural pozzolana and limestone blended cements

Abstract: The benefits of limestone filler (LF) and natural pozzolana (NP) as partial replacement of Portland cement are well established. Economic and environmental advantages by reducing CO2 emission are well known. However, both supplementary materials have certain shortfalls. LF addition to Portland cement causes an increase of hydration at early ages inducing a high early strength, but it can reduce the later strength due to the dilution effect. On the other hand, NP contributes to hydration after 28 days improving the strength at medium and later ages. Hence, ternary blended cement (OPC–LF–NP) with better performance could be produced. In this paper, mortar prisms in which Portland cement was replaced by up to 20%LF and 30%NP were tested in flexure and compressive strength at 2, 7, 28 and 90 days. Some samples were tested under sulfate and acid solutions and for chloride ions permeability. Results show that the use of ternary blended cement improves the early age and the long-term compressive and flexural strengths. Durability was also enhanced as better sulfate, acid and chloride ions penetration resistances were proved. (A) “Reprinted with permission from Elsevier”.

A review of the hardened mechanical properties of self-compacting concrete

Abstract: Data from more than 70 recent studies on the hardened mechanical properties of self-compacting concrete (SCC) have been analysed and correlated to produce comparisons with the properties of equivalent strength normally vibrated concrete (NVC) The significant scatter obtained in much of the data is a consequence of the wide range of materials and mixes used for SCC, but clear relationships have been obtained between cylinder and cube compressive strength, tensile and compressive strengths, and elastic modulus and compressive strength It is also clear that limestone powder, a common addition to SCC mixes, makes a substantial contribution to strength gain Bond strength of SCC to reinforcing and prestressing steel is similar to or higher than that of normally vibrated concrete Variation of in situ properties in structural elements cast with SCC is similar to that with NVC, and the performance of the structural elements is largely as predicted by the measured material properties The analysis has shown that sufficient data have been obtained to give confidence in the general behaviour of SCC, and future studies need only be focused on specific or confirmatory data for particular applications

Compressive Behavior of Ultra-High-Performance Fiber-Reinforced Concrete

Abstract: An experimental program aimed at determining ultra-high performance fiber-reinforced concrete (UHPFRC) uniaxial compressive behaviors was conducted. Overall stress-strain behavior, strain capacity, modulus of elasticity, and strength of both steam-treated and untreated UHPFRC were determined by analyzing results of compression tested cylinders. Enhanced stiffness and exceptional compressive strength were exhibited by UHPFRC according to study results. The authors present modulus of elasticity as compression strength function and strength gain with time predictor equations. The authors present UHPFRC stress-strain response linearity and establish an ascending branch compressive stress-strain behavior equation.

Recycled aggregate concrete as structural material

Abstract: The use of recycled aggregates in concrete opens a whole new range of possibilities in the reuse of materials in the building industry. The utilisation of recycled aggregates is a good solution to the problem of an excess of waste material, provided that the desired final product quality is reached. The studies on the use of recycled aggregates have been going on for 50 years. In fact, none of the results showed that recycled aggregates are unsuitable for structural use. However, some hypothetical problems related to durability aspects resulted in recycled aggregates being employed practically only as base filler for road construction. This paper focuses on the possibility of the use of recycled aggregate concrete as a structural material. For that purpose an experimental study of the shear behaviour and strength of beams made with recycled aggregate concrete was studied. Twelve beam specimens with the same compression strength, four concrete mixtures using different percentages of recycled coarse aggregates (0%, 25%, 50% and 100%) and three different transverse reinforcement arrangements were cast and tested up to failure. Analytical predictions of the experimental results were carried out using a numerical model based on the modified compression field theory and simplified models such as those proposed by Cladera & Mari, the Canadian standard CSA and the Eurocode-2. The results obtained indicate that a substitution of less than 25% of coarse aggregate, scarcely affects the shear capacity of RC beams, provided that all measures related to dosage and durability aspects have been adopted.