Relation between abrasion resistance and flexural strength of high volume fly ash concrete

In this paper, abrasion of high volume fly ash (HVFA) concretes made with 50% and 70% of cement replacement with fly ash was assessed in terms of its relation to flexural tensile strength. Comparisons were made between normal Portland cement (NPC) concrete and fly ash concrete. Comparisons were also made between fly ash concretes. Investigation results have shown that the abrasion resistance increased as flexural tensile strength increased. Analysis of the results showed that, for concrete with tensile strength of greater than 4–5 MPa, the abrasion resistance of HVFA concrete with 70% replacement with cement was found to be higher than that of counterpart control NPC concrete and concrete made with 50% fly ash. The comparison between the relation of abrasion to compressive strength and abrasion to flexural tensile strength made in terms of R² of the linear regression showed that a stronger relation existed between abrasion and flexural tensile strength than that of abrasion to compressive strength of the concrete studied.

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Résumé L'étude a pour but d'estimer la relation entre la résistance à l'usure avec la résistance à la traction en flexion des bétons contenant de 50% et 70% de cendres volantes. On a comparé le béton pur au béton contenant des cendres volantes. Des comparaisons ont été faites également entre les différents bétons contenant des cendres volantes. Les resultats de la recherche ont montré que la résistance à l'usure augmente à mesure que la résistance à la traction en flexion de 4∼5 MPa ont une résistance à l'usure plus grande s'ils contienneint 70% de cendres volantes que s'ils étaient purs ou contenaient 50% de cendres volantes. La comparaison entre les relation de la résistance à l'usure en compression et de la résistance à l'usure en traction a été établie en termes de R² de la régression linéaire. On a prouvé qu'une relation plus forte a été obtenue entre la résistance à l'usure et la résistance à la traction en flexion par rapport à la résistance à l'usure en compression du béton étudié.

     

Effect of fire exposure on cracking,spalling and residual strength of fly ash geopolymer concrete

Fly ash based geopolymer is an emerging alternative binder to cement for making concrete. The cracking, spalling and residual strength behaviours of geopolymer concrete were studied in order to understand its fire endurance, which is essential for its use as a building material. Fly ash based geopolymer and ordinary portland cement (OPC) concrete cylinder specimens were exposed to fires at different temperatures up to 1000 °C, with a heating rate of that given in the International Standards Organization (ISO) 834 standard. Compressive strength of the concretes varied in the range of 39–58 MPa. After the fire exposures, the geopolymer concrete specimens were found to suffer less damage in terms of cracking than the OPC concrete specimens. The OPC concrete cylinders suffered severe spalling for 800 and 1000 °C exposures, while there was no spalling in the geopolymer concrete specimens. The geopolymer concrete specimens generally retained higher strength than the OPC concrete specimens. The Scanning Electron Microscope (SEM) images of geopolymer concrete showed continued densification of the microstructure with the increase of fire temperature. The strength loss in the geopolymer concrete specimens was mainly because of the difference between the thermal expansions of geopolymer matrix and the aggregates.     

Residual strength of reinforced concrete columns exposed to fire

Abstract

Several methods were employed to evaluate the residual strength of reinforced concrete columns exposed to different durations of fire. These methods included the analytical method, ultrasonic tests, hammer tests and load tests. Fifty columns were involved in the tests. Calculated temperatures and residual strengths of the test columns were compared with those measured. Comparisons were also made between results from load tests and those from nondestructive tests. The results showed that using analytical procedures is acceptable while the nondestructive test methods are accurate only for shorter durations of fire.     

External reinforcement of high strength concrete columns

With the technology development on the compressive strength of concrete over the years, the use of high strength concrete has proved most popular in terms of economy, superior strength, stiffness and durability due to many advantages it could offer. However, strength and ductility are inversely proportional [J. Mater. Civil Eng. 11 (1999) 21]. High strength concrete is a brittle material causing failure to be quite sudden and ‘explosive' under loads. It is also known that structural concrete columns axially compressed rarely occur in practice. The stress concentrations caused by the eccentric loading further reduce the strength and ductility of high strength concrete. Therefore, studies for high strength concrete columns under eccentric loading are essential for the practical use.

This paper experimentally investigates a number of high strength concrete columns that are externally reinforced with galvanised steel straps and fibre-reinforced polymers subjected to concentric and eccentric loading. The experimental results show that external reinforcement can enhance the properties of high strength concrete columns.     

Influence of activated fly ash on corrosion-resistance and strength of concrete

Various activation techniques, such as physical, thermal and chemical were adopted. By adopting these methods of activation, hydration of fly ash blended cement was accelerated and thereby improved the corrosion-resistance and strength of concrete. Concrete specimens prepared with 10%, 20%, 30% and 40% of activated fly ash replacement levels were evaluated for their compressive strengths at 7, 14, 28 and 90 days and the results were compared with ordinary Portland cement concrete (without fly ash). Corrosion-resistance of fly ash cement concrete was studied by using anodic polarization technique. Electrical resistivity and ultrasonic pulse velocity measurements were also carried out to understand the quality of concrete. The final evaluation was done by qualitative and quantitative estimation of corrosion for different systems. All the studies confirmed that upto a critical level of 20–30% replacement; activated fly ash cement improved both the corrosion-resistance and strength of concrete. Chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.     

Compressive strength and chloride resistance of self-compacting concrete containing high level fly ash and silica fume

The influence of high-calcium fly ash and silica fume as a binary and ternary blended cement on compressive strength and chloride resistance of self-compacting concrete (SCC) were investigated in this study. High-calcium fly ash (40–70%) and silica fume (0–10%) were used to replace part of cement at 50, 60 and 70 wt.%. Compressive strength, density, volume of permeable pore space (voids) and water absorption of SCC were investigated. The total charge passed in coulombs was assessed in order to determine chloride resistance of SCC. The results show that binary blended cement with high level fly ash generally reduced the compressive strength of SCC at all test ages (3, 7, 28 and 90 days). However, ternary blended cement with fly ash and silica fume gained higher compressive strength after 7 days when compared to binary blended fly ash cement at the same replacement level. The compressive strength more than 60 MPa (high strength concrete) can be obtained when using high-calcium fly ash and silica fume as ternary blended cement. Fly ash decreased the charge passed of SCC and tends to decrease with increasing fly ash content, although the volume of permeable pore space (voids) and water absorption of SCC were increased. In addition when compared to binary blended cement at the same replacement level, the charge passed of SCC that containing ternary blended cement was lower than binary blended cement with fly ash only. This indicated that fly ash and silica fume can improve chloride resistance of SCC at high volume content of Portland cement replacement.     

Workability and strength behaviour of concrete with cold-bonded fly ash aggregate

This paper discusses the development of empirical models for workability and compressive strength of cold-bonded fly ash aggregate concrete in terms of mixture proportioning variables such as cement content, water content and volume fraction of cold-bonded aggregate through statistically designed experiments based on Response Surface Methodology. Factor level of cement is taken from 250 to 450 kg/m³ to introduce weak as well as strong matrix phase in the concrete. Apart from water content, workability of concrete is highly influenced by main and interaction effect of volume fraction of cold-bonded aggregate in the composition. Response surface indicate that increase in cement content causes to change the predominant failure mode from mortar failure to aggregate fracture and concrete strength decreases with increase in volume fraction of aggregate at higher cement contents. The models developed have been found useful in arriving typical relationship to establish a mixture proportioning methodology for cold-bonded fly ash aggregate concrete.     

Transport properties of high volume fly ash roller compacted concrete

This paper describes research on the transport properties of high-volume fly ash roller compacted concrete (RCC). The mixes were developed through incorporating 50–260 kg/m³ cement and high volumes of fly ash ranging from 40% to 85% by mass of the total cementitious material. The concretes were investigated for permeability, absorption, sorption and chloride diffusion. The study showed that RCCs of moderate cement and moderate fly ash contents had lower values of permeability, absorption, sorption and chloride diffusivity.     

Workability and strength of coarse high calcium fly ash geopolymer

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.     

Determination of gas permeability of high performance concrete containing fly ash

In this paper, gas permeability of high performance concrete (HPC) with water-binder ratio (w/b) varying from 0.25 to 0.35 and level of fly ash (FA) replacement ranging from 0% to 60%, were determined. Pore structure of high performance concrete was also investigated with mercury intrusion porosimetry (MIP). It showed that w/b ratio was found to have a varying influence on gas permeability. In this research, it achieved higher gas permeability at w/b of 0.30 than that at w/b of 0.25 or 0.35. The incorporation of fly ash increased the gas permeability of HPC. There’s a better interdependence between gas permeability and porosity than that between gas permeability and threshold diameter and mean diameter, respectively.     

The permeability of fly ash concrete

Oxygen permeability tests were carried out on plain ordinary Portland cement (OPC) and fly ash concretes at three nominal strength grades. Prior to testing the concretes were subjected to a wide range of curing and exposure conditions. The results emphasize the importance of adequate curing to achieve concrete of low permeability, especially when the ambient relative humidity is low. In addition, the results demonstrate the considerable benefit that can be achieved by the use of fly ash in concrete. Even under conditions of poor curing, fly ash concrete is significantly less permeable than equal-grade OPC concrete, the differences being more marked for higher-grade concretes. Attempts were made to correlate strength parameters with permeability but it is concluded that neither the strength at the end of curing nor the 28-day strength provides a reliable indicator of concrete permeability. A reliable correlation was established between the water to total cementitious material ratio [w/(c+f)] and the permeability of concretes subjected to a given curing and exposure regime.     

Efficiency of fly ash in concrete

Earlier efforts towards an understanding of the efficiency of fly ash in concrete has led to the introduction of rational methods. Based on the results available on some of the more recent pulverised fuel ashes, the authors evaluated the efficiency of fly ash in concrete over a wide range of percentage replacements (15–75%). It was clearly shown that the overall efficiency of fly ash cannot be adequately predicted using a single efficiency factor at all percentages of replacements. The overall efficiency factor (k) has been evaluated at all percentages of replacements considering the general efficiency factor (k_e) and the percentage efficiency factor (k_p). This study resulted in a quantitative assessment of the behaviour of fly ash in concrete, especially for the 28 day compressive strength at different percentages of replacement.     

Prediction of compressive strength of concrete with fly ash as sand replacement material

Fly ash (FA) acts as a partial replacement material for both Portland cement and fine aggregate. The published information on FA as sand (fine aggregate) replacement material (SRM) is limited and rational guidelines to estimate the compressive strength of concrete are not available. This aspect was investigated and a formula to predict the compressive strength of concrete at 28 day is suggested in this paper. This formula, containing cementing efficiency factor, k, of FA, is useful also when the quantity of FA used is more than that of sand replaced. Application of the formula to the test data in published literature, indicate that it can estimate the compressive strength of concrete containing different levels of sand replacement by fly ash.     

Heat deformations of fly ash concrete

When dealing with concrete resistance to high temperatures it is important for design purposes to know the elastic parameters, such as the temperature–strain curves and the modulus of elasticity.Concretes containing a high volume of fly ash differ from conventional mixes in the cementitious phase. This results in a different behaviour under heating compared to plain Portland cement concretes. To find the elastic response of fly ash concrete four series of concrete mixtures were manufactured: one with cement only, another with 30% by mass partial replacement of cement by fly ash, and two with 30% and 40% by mass replacement of cement by ground fly ash. Tests were carried out on cylinders (150 × 300 mm). A high-calcium fly ash was used.The conditions were selected so that the applied level of stress corresponded to 25% or to 40% of the ultimate compressive strength of concrete, and a transient type of temperature regime was followed. Based on the experiments the critical temperature, the residual deformation and the modulus of elasticity were determined.The results indicate that concretes containing a high volume of fly ash are more sensitive to high temperatures, since they developed greater deformations. The fineness of the fly ash used also seems to influence the degree of deformation in an adverse way.     

FRP confined concrete columns: Behaviour under extreme conditions

The behaviour of concrete columns wrapped with fibre reinforced polymer (FRP) materials when exposed to several extreme conditions is evaluated. Cold regions environments, FRP repair of corroding reinforced concrete columns, and fire resistance are all considered. For the cold regions exposure, FRP wrapped cylinders (152 × 305 mm) are exposed to temperatures as low as −40 °C or to up to 300 cycles of freeze-thaw (−18 °C to +15 °C). The combination of freeze-thaw exposure with sustained loading is also examined. For FRP wrapping of corroding reinforced concrete columns, the results of tests on cylinders and larger-scale circular columns (300 × 1200 mm) are presented. The specimens are corroded and then wrapped with FRP sheets. The rate of corrosion is monitored both before and after wrapping. The final extreme condition that is considered is fire exposure. Tests on full-scale reinforced concrete columns (400 × 3800 mm) exposed to a standard fire are described and discussed. Overall, the results demonstrate that FRP confined concrete columns tested in concentric axial compression have adequate performance under several extreme conditions such as low temperature, freeze-thaw action, corrosion of internal reinforcement, and fire exposure.     

Effect of transversal reinforcement in normal and high strength concrete columns

This paper deals with an experimental investigation and numerical simulation of reinforced concrete columns. Behavior of normal and high strength columns is studied with special attention paid to the confinement effects of transversal reinforcement in columns with square cross section. Character of a failure, strengths, ductility and post-peak behavior of columns are observed in experiments and also in numerical solution. Three-dimensional computational model based on the microplane model for concrete was constructed and compared with experimental data. Results of numerical model showed good agreement in many aspects and proved capabilities of the used material model.     

Behaviour of concrete-filled steel tubular columns incorporating fly ash

The subject of this work is to investigate the effect of fly ash on the strength of concrete filled steel tubular columns from 28 to 365 days. A contrast study was carried out on concrete filled steel tubular columns incorporating 10–40 wt% fly ash, and for control Portland cement concrete filled steel tubular columns. The effect of pre-coating the inner surface of steel tubes with a thin layer of fly ash was also studied. Assessments of the concrete mixes were based on the compressive strength and the bond strength. The results show that a lower replacement with fly ash can improve both bond strength and compressive strength, while a higher replacement with fly ash requires a relatively longer time to achieve similar beneficial effects. Pre-coating the inner surface of steel tubes with a thin layer of fly ash can notably improve the bond strength. The microstructure of the interface between concrete and steel tube was also studied by using scanning electron microscopy analyzer.     

Behaviour of FRP wrapped normal strength concrete columns under eccentric loading

Eccentric loads are common for columns in buildings and other types of structures. Columns that are in the border of buildings, especially corner columns and columns near opening are usually subject to a combination of axial load and bending moment, thus creating an equivalent eccentric load. Fibre Reinforced Polymers have been used in strengthening/retrofitting columns and other types of elements. The results by and large are satisfactory. Most of the research studies undertaken in strengthening columns are based on concentrically loaded columns. In effect, the behaviour of FRP wrapped columns under the influence of eccentric loading is less known compared to concentrically loaded columns. This paper presents results of testing six normal strength concrete columns under eccentric loading. The columns are wrapped with different number of layers of FRP. Results show that wrapping a column with an adequate number of FRP layers will result in higher strength, ductility and energy absorption than a column reinforced with steel bars.