Set-on-demand concrete

Abstract

The ability to control the fresh-state properties of concrete in real-time provides the user greater control over processing and placing of concrete. Based on the principles of magnetorheology, a smart cement-based material system has been developed wherein magnetic particles are added and an external magnetic field is applied to control the fresh-state properties of concrete. Rheological tests were conducted on paste mixtures containing magnetic particles and when a magnetic field was applied, it was determined that the shear resistance of the paste could be altered significantly. The response of the paste was found to be dependent on the magnetic field strength and the concentration of the magnetic particles. Furthermore the magnetic particles used did not have any effect on cement hydration products or on compressive strength results. Such a material can be useful in applications in which controlling the fresh-state behavior of concrete is critical.

Introduction

Today's concrete is no longer a simple combination of cement, aggregates and water. With increased use of various types of supplementary cementing materials and chemical admixtures, material incompatibility problems have been observed in concrete construction [1], [2], [3], [4]. As a result, some of the greatest problems in concrete manufacturing occur when concrete does not stiffen, set or harden on time [3]. However, what if it were possible to create a concrete in which the contractor is able to control in real-time its stiffening/setting behavior? Cement-based magnetorheological (MR) fluids can potentially be used in civil engineering applications to act as a “set-on-demand” material, allowing the user greater control over the processing (e.g. mixing, pumping, and placing) of concrete. MR fluids undergo large, reversible and fast changes in their rheological properties when subjected to an external magnetic field. Jacob Rabinow first discovered MR fluids in 1948 [5]. MR fluids typically consist of micrometer sized magnetic particles that are suspended in a carrier fluid. As very few elements possess ferromagnetic properties, the particles used are limited to iron and iron based materials. In the presence of a magnetic field, the magnetic dipoles of these particles align along the magnetic field lines and the response time of MR fluids is generally within a few tens of milliseconds [6]. The interaction between the dipoles causes the particles to form columnar structures, parallel to the applied field (see Fig. 1). These chain-like structures restrict the motion of the fluid, thereby increasing the solid-like characteristics of the suspension. The mechanical energy needed to yield these chain-like structures increases as the applied field increases resulting in a field dependent yield stress.

A detailed review of properties and applications of MR fluids has been published elsewhere [7], [8], [9], [10], [11]. Although MR fluids are not common in civil engineering applications, they have been used in a variety of areas, such as automotive clutches [5], cancer treatment [12], drilling fluids [13], body armor [14], gun recoil system [15], precision polishing [16], prosthetic knee dampers [17], seat dampers, fluid brakes, vibration damper [18], and seismic vibration control [19]. In such applications, a magnetic field can be generated by passing current through an electric coil or through the use of permanent magnets. The magnetic field generated through an electric coil can be altered in real time by varying the magnitude of applied current. This is beneficial in applications where different levels of magnetic field would be required at different periods of time. However, at high levels of applied current (or in other words, high levels of magnetic field), the electric coils have to be cooled to counteract the heat that is produced. On the contrary permanent magnets generate a constant magnetic field without the use of electricity and also without the generation of any heat. In addition, permanent magnets can be manufactured in any desired shape and size [20], [21], [22], [23]. Similar to the set-ups used in the above-mentioned applications [12], [13], [14], [15], [16], [17], [18], [19], a set-up consisting of electric coils or permanent magnetics could be used for generation of magnetic fields in-situ for cement-based materials. It is envisioned that cement-based magnetorheological fluids can potentially be used in applications to act as a “set-on-demand” material, allowing the user greater control over the processing (e.g. casting and pumping) of concrete. Such a material can be useful in civil engineering applications such as:

• drilled shaft construction of bridges

• formwork pressure reduction of self-consolidating concrete;

• casting of narrow channels or long casting distances;

• improving segregation resistance of aggregates in concrete;

• to avoid sloughing or slumping of shotcrete from its receiving surface.

Traditionally in MR fluids, Newtonian carrier liquids, such as mineral oil, synthetic oil, water or ethylene glycol, are used to suspend the magnetic particles. The viscosities of these carrier liquids generally range from 0.2 to 0.3 Pa s at 25 °C [11]. As such, when a magnetic field is applied the transformation in the material from a liquid-like to a solid-like material is amplified. However, the lack of yield stress and the low viscosity of these Newtonian carrier liquids, as well as the differences in density between the magnetic particles and the carrier fluids, typically make particle sedimentation a major concern in MR fluids. While reducing the size of the magnetic particles has been shown to reduce sedimentation, using smaller particles usually leads to a reduction in the yield stress of the field-induced fluid [24], [25]. Other approaches to increasing sedimentation resistance in MR fluids are to use additives, such as stabilizing additives, thixotropic additives [26] and nano-scale particles [27], [28], [29], or by choosing a carrier fluid with a yield stress [30]. Cement paste, mortar and concrete have been characterized as being yield stress fluids [31], [32], [33], and they are typically modeled as a Bingham fluid [34]. This inherent yield stress of cement-based suspensions can be beneficial in minimizing sedimentation of the magnetic particles.

The research presented in this paper presents an innovative approach to control the fresh state performance of cementing operations. Through better control of fresh state properties the quality and durability of the cementing operations can be improved [35]. The aim of the current research was (a) to investigate the viscoelastic behavior of cement-based MR fluids over time and (b) to evaluate the influence of magnetic particles and magnetic field on cement hydration. In a traditional MR fluid, the dosage of magnetic particles generally varies from 40% to 50% of the fluid volume [36]. A unique aspect of this work is the use of small dosages of magnetic particles (less than 5% of paste volume) to investigate the changes in rheological properties with application of magnetic field. The dosages were kept to a minimum to maximize cementitious content and to minimize increase in cost.