Physical–mechanical properties, and mesostructure of plain and fibre reinforced self-compacting concrete

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Abstract

This study analyses the mesostructural homogeneity of plain and steel fibre reinforced self-compacting concrete (SFR-SCC) to be used in slender elements of considerable height, avoiding the use of conventional reinforcement. Tests in the fresh state include the measurement of the rheological properties and self-compactability through engineering tests. Slender columns were filled with plain and SFRC-SCCs (up to 50 kg/m3 of fibres). In the hardened state, studies at the mesostructural level include the quantification through image analysis of the distribution of the coarse aggregate and fibres along the height of the columns, and measures of modulus of elasticity, compressive strength and ultrasonic pulse velocity. The physical–mechanical properties did not vary significantly along the height of the columns, though a decrease was observed in the superior third of the elements, the compressive strength was the most affected parameter. The aggregate distribution was slightly more homogeneous in the case of fibre concretes. The variation of the fibre density along the columns was relatively high, with no identifiable tendency.

Introduction

The elimination of vibration for the compaction of fresh concrete makes the use of self-compacting concrete (SCC) beneficial in terms of cost reduction and improvement of the work environment. Furthermore, due to its intrinsic low porosity, SCC usually has high performance properties also in terms of mechanical behavior and durability. These properties could even be elevated improved if steel fibers are incorporated, thus obtaining steel fiber reinforced self-compacting concrete (SFR-SCC).

As inherent characteristics, SCC has the capacity to fill the formwork and encapsulate the reinforcement, with no blocking or segregation of its components, consolidating by its own weight, with no need of vibration [1]. In this manner, the flowability, passing ability and the resistance against segregation are the essential properties of SCC. However, as it can be foreseen, the first two characteristics are in opposition to the last one [2]; it is clear that the SCC matrix should be sufficiently viscous to avoid segregation of the coarse aggregate but have sufficient mobility to assure an appropriate filling of the element.

Rheological studies on fresh SCC have shown that to achieve self-compacting characteristics, an adequate combination of shear stress and plastic viscosity is necessary. If the viscosity is too low, an increase of shear stress is recommended to avoid segregation. On the other hand, if viscosity is too high, a low shear stress would be necessary [3]. Thus, an adequate balance of the fundamental rheological parameters, such as shear stress and plastic viscosity, govern the behaviour SCC. The development of SFR-SCC has been reported by few authors [4], [5], [6]. It has been observed that the incorporation of steel fibres modifies the rheological parameters of concrete [7].

Considering concrete as a composite material, it is usual to study the properties of the material from different levels of observation, i.e. micro-, meso-, and macro-levels. At the mesostructural level, concrete is assumed as a composite material formed by a mortar matrix, the aggregate fraction larger than 5 mm, the interfacial transition zone, and fibres if dealing with FRC. With regard to SFRC, recent works report the importance of a homogeneous distribution of the fibres [8] and its effects under uniaxial and flexural tension [9], [10].

The importance of the homogeneity of the material is evident for any application, since it will affect the material properties. Particularly, an important influence of the mesostructure on the mechanical and durability-associated transport properties of concrete has been identified [11], [12]. However, this acquires a major significance in the case of SCC, where the correct filling and compactness in the hardened state will specifically depend on the rheological properties of the material in the fresh state.

The objective of this study is to analyse the mesostructural homogeneity of plain and SFR-SCC for use in slender elements of considerable height, such as foundation and retaining walls, columns, building panels, etc. As it can be foreseen when thinking in SCC as a suspension of coarse aggregate in a mortar matrix, this type of elements could induce segregation. The use of steel fibres was considered to contemplate the possibility of avoiding the conventional reinforcement in this type of applications, either partially or totally.

Results of physical properties in the fresh state include fundamental rheological parameters obtained with a concrete BML viscometer, and self-compactability measures from engineering tests such as slump-flow, V-funnel, and J-ring. In the hardened state, studies at the mesostructural level include the quantification through image analysis of the distribution of the coarse aggregate and fibres along the height of the application-oriented slender elements. The studies are supported by measures of modulus of elasticity, compressive strength and ultrasonic pulse velocity, as a traditional non-destructive method to evaluate concrete homogeneity.

Section snippets

Experimental program

To study the tendency of segregation in plain and steel fibre reinforced self-compacting concrete, slender in height elements were chosen, i.e. round columns of 150 mm diameter and 2500 mm height. The prototype columns were filled with a plain SCC and two SFR-SCCs obtained from the same batch; incorporating 25 and 50 kg/m3 of fibres. Two columns were filled with each of the three mixes.

Results in the fresh state include the yield stress and plastic viscosity, slump-flow, V-funnel time, and J-ring

Materials and fabrication process

The concrete making materials involve four fractions of crushed limestone aggregates; 0–2 mm and 0–5 mm sands, and 5–12 mm and 12–18 mm gravels. A blended cement conforming CEM II 32.5 R, a limestone filler and a policarboxilate type superplasticizer. Hooked-ended steel fibres were 50 mm in length and 1 mm diameter. The final mix proportions of the component materials are indicated in Table 1.

Concrete with the specified composition but without the superplasticizer and filler was delivered by a

Fresh state

The SCC rheological properties, i.e. yield stress and plastic viscosity, were measured by means of a BML Viscometer 3 on concrete samples of approx. 20 l in volume. The viscometer measures the torque and rotation velocity at increasing and decreasing velocities, and calculates the yield stress (τ0) and plastic viscosity (μ) in accordance to the Bingham model. The software includes an additional step in which it verifies the tendency of concrete to segregate, at an intermediate rate.

Fresh state

Fig. 5 shows the aspect of the SCC after the slump-flow and J-ring tests. Table 2 presents the fresh properties of the concretes used to fill the columns. As it was explained above, it should be kept in mind that the different concretes were obtained from the same base batch. Thus, a lapse took place between the fresh state evaluation, filling of columns, and casting of cylinders for each of the concretes. During this lapse, the rheological properties of the material noticeably varied, which is

Conclusions

This work has analysed the variation of physical–mechanical properties of SCC and SFR-SCC along the height of slender columns, and related such variations to the mesostructure of the material. The study was part of a preliminary test program aimed to evaluate the material homogeneity that can be achieved when plain and steel fibre reinforced SCC is used in slender elements of considerable height, such as columns, foundation and retaining walls, building panels, etc.

The utilised SCC and SFR-SCCs

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