Tehmina Ayub

Effects of Different Mineral Admixtures on the Properties of Fresh Concrete

This paper presents a review of the properties of fresh concrete including workability, heat of hydration, setting time, bleeding, and reactivity by using mineral admixtures fly ash (FA), silica fume (SF), ground granulated blast furnace slag (GGBS), metakaolin (MK), and rice husk ash (RHA). Comparison of normal and high-strength concrete in which cement has been partially supplemented by mineral admixture has been considered. It has been concluded that mineral admixtures may be categorized into two groups: chemically active mineral admixtures and microfiller mineral admixtures. Chemically active mineral admixtures decrease workability and setting time of concrete but increase the heat of hydration and reactivity. On the other hand, microfiller mineral admixtures increase workability and setting time of concrete but decrease the heat of hydration and reactivity. In general, small particle size and higher specific surface area of mineral admixture are favourable to produce highly dense and impermeable concrete; however, they cause low workability and demand more water which may be offset by adding effective superplasticizer.

Mechanical characteristics of hardened concrete with different mineral admixtures: a review.

The available literature identifies that the addition of mineral admixture as partial replacement of cement improves the microstructure of the concrete (i.e., porosity and pore size distribution) as well as increasing the mechanical characteristics such as drying shrinkage and creep, compressive strength, tensile strength, flexural strength, and modulus of elasticity; however, no single document is available in which review and comparison of the influence of the addition of these mineral admixtures on the mechanical characteristics of the hardened pozzolanic concretes are presented. In this paper, based on the reported results in the literature, mechanical characteristics of hardened concrete partially containing mineral admixtures including fly ash (FA), silica fume (SF), ground granulated blast furnace slag (GGBS), metakaolin (MK), and rice husk ash (RHA) are discussed and it is concluded that the content and particle size of mineral admixture are the parameters which significantly influence the mechanical properties of concrete. All mineral admixtures enhance the mechanical properties of concrete except FA and GGBS which do not show a significant effect on the strength of concrete at 28 days; however, gain in strength at later ages is considerable. Moreover, the comparison of the mechanical characteristics of different pozzolanic concretes suggests that RHA and SF are competitive.

Effect of Chopped Basalt Fibers on the Mechanical Properties and Microstructure of High Performance Fiber Reinforced Concrete

This paper presents the mechanical properties and the microstructure of the high performance fiber reinforced concrete (HPFRC) containing up to 3% volume fraction of chopped Basalt fibers. Three types of the concrete were prepared, out of which, the first type was prepared by utilizing 100% cement content. The other two types of the concrete were prepared by replacing 10% cement content with silica fume and the locally produced metakaolin. Using each concrete type, four mixes were prepared in which Basalt fibers were added in the range of 0–3%; that is, total twelve mixes of the HPFRC concrete were prepared. From each of the twelve concrete mixes, total twelve specimens were cast to determine the mechanical properties of the HPFRC including compressive strength (cube and cylinder), splitting tensile strength, and the flexural strength. In this way, a total of 108 specimens were cast and tested in this study. Test results showed that the addition of the Basalt fibers significantly increased the tensile splitting strength and the flexural strength of the HPFRC, while there was slight improvement in the compressive strength with the addition of Basalt fibers. The microstructure of HPFRC was examined to determine the interfacial transition zone (ITZ) between the aggregates and the paste by using field emission scanning electron microscope (FESEM), which showed the improvement of the ITZ due to the addition of the Basalt fibers.

Strength development of high-strength ductile concrete incorporating Metakaolin and PVA fibers.

The mechanical properties of high-strength ductile concrete (HSDC) have been investigated using Metakaolin (MK) as the cement replacing material and PVA fibers. Total twenty-seven (27) mixes of concrete have been examined with varying content of MK and PVA fibers. It has been found that the coarser type PVA fibers provide strengths competitive to control or higher than control. Concrete with coarser type PVA fibers has also refined microstructure, but the microstructure has been undergone with the increase in aspect ratio of fibers. The microstructure of concrete with MK has also more refined and packing of material is much better with MK. PVA fibers not only give higher stiffness but also showed the deflection hardening response. Toughness Index of HSDC reflects the improvement in flexural toughness over the plain concrete and the maximum toughness indices have been observed with 10% MK and 2% volume fraction of PVA fibers.

Effectiveness of U-Shaped CFRP Wraps as End Anchorages in Predominant Flexure and Shear Region

U-shaped external anchorage have been used to enhance the shear resistance and performance of RC beams strengthened by externally bonded CFRP plates but the reported results are not adequate yet to draw rational conclusions. One factor that could affect the ultimate strength and mode of failure is the depth of the end anchorage used. Varying end anchorage depths were considered in this study to investigate the effectiveness of U-shaped end anchorages along with determining ultimate load carrying capacity and failure modes of strengthened beams in predominant flexure and shear loading regions. Full depth U-shaped anchorages at plate cut-off points were found to be effective in strengthened RC beams in the predominant shear loading region while depth of the end anchorage was not found to be factor that affects the load carrying capacity of the strengthened RC beams.

Finite element modelling of FRC beams containing PVA and Basalt fibres: A comparative study

The endeavour of current study is to compare the flexural behaviour and three dimensional (3D) finite element analysis (FEA) and the results of FEM are compared with the experimental results of 07 HPFRC beams. Out of seven (07), 01 beam of plain concrete without fibres was cast as a control beam. Three (03) beams containing 1, 2 and 3% volume of PVA fibres were prepared by using HPFRC mixes while, the remaining other three (03) beams were prepared using HPFRC mixes containing 1, 2 and 3% volume of Basalt fibres. In order to ensure flexural failure, three-point bending load was applied at the mid span of all beams. The maximum flexural load and corresponding deflection and strains at the mid span attained prior to the failure were obtained as flexural test results. The FEM results were obtained by simulating all beams in ATENA 3D program and verified through flexural test results. Both of the results of FEM and Experiment showed good agreement with each other.

Behavioral Study of RC Beams Designed for Shear Using CFP and ACI Code Models

Compressive Force Path concept is a proposed shear design method to explain shear behavior in reinforced concrete beams. This concept identifies 04 behaviors based on the shear span to beam depth (a/d) ratio and provides detailed shear design and transverse reinforcement detailing procedure for each behavior. Therefore, author of this paper intended to use this concept as a practical tool for the designing of RC beams particularly for Type II (2 ≤ a/d < 5) and Type III (1 < a/d < 2) behaviors to validate the concept. Total 08 beams of 100×200×1800 mm size beams were cast; out of which, 04 beams were designed according to ACI Code approach while, rest were designed and detailed using CFP concept strategy. The beam sizes in this study are identical and all parameters are constant except shear span ‘a’. The two-point loading test results of RC beams showed that the shear resistance of concrete is better estimated by the CFP concept with a good prediction of cracks pattern, load carrying capacity and actual behavior of the beams in shear as compared to the beams designed according to ACI Code approach. However, most of the beams, particularly a/d ratio less than 4.44 were observed to be deficient in serviceability and failed in shear in spite of attaining theoretical predicted loads.

Flexural Modelling and Finite Element Analysis of FRC Beams Reinforced with PVA and Basalt Fibres and Their Validation

A flexural capacity model for fibre-reinforced concrete (FRC) beams reinforced with PVA and basalt fibres is suggested for the rectangular beam sections. The proposed models are based on the concept of equivalent stress block parameters for both compressive and tensile stresses, similar to Eurocode and ACI code. The parameters are defined by allowing the conversion of the stress-strain models into equivalent rectangular stress blocks, similar to Eurocode 2. The flexural model is suggested to determine the loading capacity of 21 FRC beams containing up to 3% volume fraction of PVA and basalt fibres without reinforcing bars. In order to investigate the accuracy of the proposed flexure models, finite element analysis (FEA) of the same beams was carried out using the compressive and tensile stress-strain curves. Furthermore, 21 FRC beams subjected to three-point bending were tested. The results of the flexural models showed good agreement with the load-carrying capacity of the tested FRC beams, and the results of FEA of all beams showed a good correlation with the experimental results in terms of the maximum load, load versus midspan deflection patterns, and the maximum tensile strains.

Microstructure characteristics of concrete incorporating metakaolin and PVA fibers and influence on the compressive strength

In this paper, microstructure of concrete is investigated using metakaolin (MK) as cement replacing material and Polyvinyl Alcohol (PVA) fibers. Total ten (10) mixes of concrete are examined by varying PVA fiber aspect ratio. It was found that MK refines the pore structure, improves interfacial transition zone (ITZ) due to its pozzolanic effects, reduces portlandite (Ca(OH)2) content and bridges the gap between matrix and aggregates due to finer particle size. Due to improvement in ITZ, the compressive strength was improved. There was no indication of Ca(OH)2 around the PVA fibers in the presence of MK and the interface between the fiber and matrix was observed very narrow.