Boshoff Wp

Transfer of Fluids, Gases and Ions in and Through Cracked and Uncracked Composites

The durability of strain-hardening cement-based materials (SHCC) is strongly influenced by the transport of different substances through the material. Since numerous fine cracks are formed in SHCC, the relationship between the crack pattern and different transport properties has been the subject of experimental investigations, including tests of water and gas permeability, chloride ingress, and capillary absorption. It appears to be insufficient to consider only the average or maximum crack width when transport through SHCC is to be modelled. Therefore, the determination of certain crack pattern parameters has been proposed that take into account both crack widths and distances. These parameters may be linked to certain transport properties of the respective material.

Numerical simulation of ductile fiber-reinforced cement-based composite

Strain Hardening Cement-based Composite (SHCC) is a type of High Performance Concrete (HPC) that was developed to overcome the brittleness of conventional concrete. Even though there is no significant compressive strength increase compared to conventional concrete, it exhibits superior behavior in tension. The primary objective of the presented research is to develop a constitutive model that can be used to simulate structural components with SHCC under different types of loading conditions. In particular, the constitutive model must be efficient and robust for large-scale simulations. The proposed model, based on previous research Vorel and Boshoff (2012), for plane stress is outlined and the further focus of this paper is on the mesh objectivity of the model. It is shown to be mesh size independent.

Computational modelling of real structures made of Strain-hardening Cement-based Composites

Fibre reinforced cement-based composites is a large group of composites with a variety of properties. The purpose of adding fibres is to overcome the brittleness of the concrete by improving the post-cracking behaviour and enhancing ductility. This paper deals with the group Strain-hardening Cement-based Composites (SHCC) which exhibits excellent mechanical behaviour showing tensile strain-hardening and multiple fine cracks. The primary objective of the presented research is to verify a developed constitutive model utilised within the nonlinear and sequentially linear framework. Specifically, this paper presents the studies involving the tree-point bending test and the shear behaviour of reinforced SHCC elements tested on beam specimens monotonically loaded by an anti-symmetrical moment.

Cracking Behavior of Strain-Hardening Cement-Based Composites Subjected to Sustained Tensile Loading

Strain-hardening cement-based composites (SHCCs) are a special type of fiber-reinforced concrete (FRC) that exhibit superior ductility and crack control when compared with conventional concrete, even when compared with conventional FRC. SHCC exhibits fine, multiple cracking under a tensile load, which makes it a promising material for concrete applications that require superior durability. This paper reports on tensile creep tests on notched SHCC specimens, and it is shown that the crack widths and tensile deformation increase significantly over time when subjected to a sustained tensile load. This increased deformation is a result of the widening of cracks and the initiation of additional cracks over time. Two phases of the deformation that increase over time can also be distinguished, which are separated by a relative sudden increase of the logarithmic deformation rate after approximately 18 hours. This is a concern for durability applications of SHCC and should be addressed.

Introduction: Crack Distribution and Durability of SHCC

Inherent crack control to fine widths in strain-hardening cement-based composites (SHCC) suggests that structural elements produced from SHCC or steel-reinforced SHCC (R/SHCC) may be rendered durable by limiting the ingress rates of potentially deleterious substances. Recently, it has been reported that, while the average crack width in SHCC is maintained up to large tensile strains in excess of 3%, the maximum crack width may equal or exceed those are considered to be limiting in terms of durability. Also, the typical range in SHCC average crack width, from 50 to 100 μm, has been shown to be a threshold in water permeability, at which width permeability is restricted to several orders lower than that expected for crack widths ranging from 0.2 to 0.3 mm — a typical reinforced concrete crack width limit in durability standards. However, it has recently been shown that capillary absorption in dry, pre-cracked SHCC is a quick process, with water penetrating into fine cracks within minutes of exposure. In addition to describing these findings, this chapter sets the scene for later chapters on improved ingress rate characterisation and the actual deterioration of cracked SHCC or R/SHCC. Guidelines for the pre-cracking of SHCC towards durability testing are derived, based on the results of recent comparative testing. These include the specimen shape, size, test set-up, crack measurement to sufficient resolution, and crack width distribution presentation. Finally, the field performance of repairs, structures and structural elements produced from SHCC and R/SHCC in the past decade is reported.

Tensile Creep of Cracked Steel Fibre Reinforced Concrete: Mechanisms on the Single Fibre and at the Macro Level

This paper reports on tests done to investigate the mechanisms causing the increased tensile creep of cracked fibre reinforced concrete (FRC) members when a sustained load is applied. It is important to understand the mechanisms as this will pave the way for improvements that can be made to reduce this creep and will also assist in creating prediction models as it will be based on the fundamental mechanisms involved. This paper presents results of uni-axial tensile creep tests of cracked steel fibre reinforced concrete (SFRC) and also tests at the single fibre level. Single fibres were embedded in the matrix and pull-out at different rates and sustained loading was also applied to the single, embedded fibres. It was found that the single fibre pull-out creep test results can be directly linked to the uni-axial tensile tests. It was also shown that the pull-out creep is proportional to the load up to at least 50 % of the ultimate load after which the pull-out creep increases non-linearly. This can be ascribed to micro-cracking around the hooked-end of the fibre. Lastly, it is postulated that the interfacial transition zone between the fibre hooked-end and the concrete matrix plays a significant role in the pull-out creep behaviour.

Macro-Synthetic Fibre Reinforced Concrete: Creep and Creep Mechanisms

The creep of cracked macro-synthetic fibre reinforced concrete has been investigated in uniaxial tension at five stress levels (30, 40, 50, 60 and 70 %) of the residual uniaxial tensile strength under controlled temperature and relative humidity of 23 ± 1 °C and 65 ± 5 % respectively. Specimens were prepared and cracked before they were subjected to the various stress levels. One mix design has been used throughout the investigation and the fibre was used at a volume of 1 %. Test results of the investigation has revealed that while the creep response is a function of the applied stress level, significant creep has been recorded after 8 months even at lower stress levels. The mechanism responsible for the time-dependent crack opening of macro-synthetic fibre reinforced concrete was investigated through time-dependent fibre pull-out and fibre creep tests. The result of this investigation has shown that macro-synthetic fibre creep and the time-dependent fibre pull-out are mechanisms responsible for the creep of cracked macro-synthetic fibre reinforced concrete.

The Effect of Crack Patterns on the Corrosion of Steel Reinforced SHCC

Ingress Potential Indices (IPI) based on crack width distributions have recently been proposed for water permeation and chloride diffusion into cracked SHCC. Reasonable correlation between chloride content and the IPI Cl index has been reported, based on chloride profiles determined for multiply cracked R/SHCC beams. In these beams, significant concentration levels of chloride at the surface of embedded reinforcing steel bars were recorded, but low corrosion rates and steel bar damage in the form of mass loss and pitting area and depth. Thus, IPIs appear to be reasonable indices of actual chloride ingress, but not of actual corrosion rates and damage. This has been postulated to be due to an altered electrochemical process of dominantly micro-cell corrosion in finely cracked SHCC. In this contribution, the effect of crack patterns on the corrosion of steel reinforcement in SHCC is discussed. This paper also presents a postulation that three different zones exist where different types of corrosion take place. These zones are directly linked to the crack pattern. The final aim of this project is to create an index which can relate the crack patterns of steel reinforced SHCC to actual chloride-induced corrosion.

Creep and Creep Fracture of Engineered Cement-based Composites / Kriechen und Kriechversagen von technologisch entwickelten Zement gebundenen zusammengesetzten Werkstoffen (ECC)

This paper studies rate dependence of Engineered Cement-based Composites (ECC). This new class of fibre reinforced cement-based composites, with the distinctive pseudo strainhardening behaviour in tension, is currently the focus of intense research internationally, for its potential in durable construction and repair material. Creep fracture of concrete is a wellknown phenomenon. As early as the 1960s it was shown that delayed fracture may occur for concrete structures at sustained loads equal to and higher than roughly 80 % of the peak load resistance in compression and flexure. More recently, evidence has been produced that an even lower creep fracture limit may exist for concrete, potentially as low as 60 % of the peak short term resistance in flexure. Such characterisation and consideration of loading rate dependence, creep and creep fracture are essential for design guidelines for ECC and form the focus of this paper. A comprehensive experimental program has been launched to characterise the time dependent behaviour of a particular ECC, including creep and creep fracture, as well as shrinkage. In this paper, results are presented of direct tensile tests, as well as three point bending tests towards establishing the creep and creep fracture behaviour of ECC.