Viktor Mechtcherine

Simulation of Fresh Concrete Flow Using Discrete Element Method (DEM)

The behaviour of fresh concrete during its mixing, transport, placement, and compaction can ultimately have significant effects on its mechanical performance, durability, surface appearance, and on its other properties after hardening. In concrete construction many problems result from the improper filling of formwork, insufficient de-airing, concrete segregation, etc. The importance of these issues has increased year after year since formwork is becoming continually more complex. Steel reinforcement has become denser, and the range of workability has been considerably broadened by the use of self-compacting concrete (SCC) and other novel concrete materials. Consequently, on the one hand, modern material design must match particular demands resulting from the geometrical and technological conditions to which the material is subjected. On the other hand, the concrete working techniques and, in some cases, the geometry of structures can be optimised in considering the usage of particular concretes with their special rheological properties. So, in order to build concrete structures efficiently and with high quality, the consistency of the fresh concrete should comply with the requirements posed by the structure’s geometry and by the methods of transport, placing, and compaction. Computer simulation of fresh concrete behaviour and the working processes could provide a powerful tool in optimising concrete construction and developing new concrete technologies [1].

Mechanical Behavior of SHCC under Impact Loading

This paper presents the results of short-time dynamic measurements on a strain-hardening cement-based composite (SHCC) at strain rates 140 to 180s− 1. The dynamic experiments were performed by means of a Hopkinson bar. Uniaxial, quasi-static tensile tests served as reference. The mechanical performances under dynamic and quasi-static loadings are compared and also related to the observed crack patterns and the condition of fracture surfaces.

Durability of Structural Elements and Structures

Strain-hardening cement-based composites (SHCC) have a great potential for the application in structures exposed to severe mechanical or environmental loading. This chapter summarises the current knowledge on the durability of the composite components as well as steel reinforcement when used in combination with SHCC.

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.

Mineral-Based Coating of Plasma-Treated Carbon Fibre Rovings for Carbon Concrete Composites with Enhanced Mechanical Performance

Surfaces of carbon fibre roving were modified by means of a low temperature plasma treatment to improve their bonding with mineral fines; the latter serving as an inorganic fibre coating for the improved mechanical performance of carbon reinforcement in concrete matrices. Variation of the plasma conditions, such as gas composition and treatment time, was accomplished to establish polar groups on the carbon fibres prior to contact with the suspension of mineral particles in water. Subsequently, the rovings were implemented in a fine concrete matrix and their pull-out performance was assessed. Every plasma treatment resulted in increased pull-out forces in comparison to the reference samples without plasma treatment, indicating a better bonding between the mineral coating material and the carbon fibres. Significant differences were found, depending on gas composition and treatment time. Microscopic investigations showed that the samples with the highest pull-out force exhibited carbon fibre surfaces with the largest areas of hydration products grown on them. Additionally, the coating material ingresses into the multifilament roving in these specimens, leading to better force transfer between individual carbon filaments and between the entire roving and surrounding matrix, thus explaining the superior mechanical performance of the specimens containing appropriately plasma-treated carbon roving.

Recovery against Environmental Action

Autogenic self-healing has been defined in chapter 1 as a self-healing process where the recovery process uses materials components that could also be present when not specifically designed for self-healing (own generic materials).

Effect of Superabsorbent Polymers on the Workability of Concrete and Mortar

The use of Superabsorbent Polymers (SAP) to promote internal curing of concrete is one of the techniques used to mitigate its autogenous shrinkage. SAP absorbs water from the fresh mixture and releases it in a later stage when the relative humidity of the concrete pore system decreases due to the cement hydration. Several studies indicate that besides leading to a substantial reduction in autogenous shrinkage the addition of dry SAP to concrete also affect other properties of concrete such as workability, mechanical behavior and durability. This chapter gives an overview of the effect of SAP on the workability of High Performance Concrete (HPC), Ultra High Performance Concrete (UHPC) and OPC mortars. Considering that at present only few results are available in the literature regarding the influence of SAP on the workability of concrete, serious research efforts will be necessary to fully understand the rheological behaviour of concrete containing SAP.

Effects of Superabsorbent Polymers on Shrinkage of Concrete: Plastic, Autogenous, Drying

This chapter deals prospectively with one of the main applications of SAP in concrete construction, the mitigation of the autogenous shrinkage of concrete. In particular, the effects of using SAP as an additive for internal curing in cementitious materials with a low water-to-cement ratio and a low-permeability microstructure are presented and discussed. Since the addition of SAP, often in conjunction with extra water, influences not only autogenous shrinkage but also other types of volumetric changes, this is addressed as well in this chapter. Furthermore, the development of stresses due to restrained autogenous shrinkage is also considered for concretes with and without internal curing.

Practical Applications of Superabsorbent Polymers in Concrete and Other Building Materials

Superabsorbent polymers (SAP) possess a number of features that make them attractive for use in many different applications. The aim of this chapter is to present an overview of existing and foreseen opportunities for the use of SAP in many different functions to improve the performance and durability of the built environment. Two case studies are also presented in this chapter: one on a thin-wall architectural structure in Germany, and another on shotcreting of wall panels in Denmark.

Hardening Process of Binder Paste and Microstructure Development

Superabsorbent Polymers (SAP) added into high performance concrete prevents or reduces self-desiccation (or autogenous shrinkage) of concrete. The mechanism behind is free water release from saturated SAP leading to an increase of relative humility and promoting further hydration. Compared to plain cement paste, the addition of SAP changes the hydration process and the development of microstructure in concrete. In this chapter, the degree of hydration of cement in cement composites containing SAP is reviewed. The influence of SAP on the development of microstructure characteristics, i.e. porosity, pore size distribution, morphology and connectivity of bulk cement paste, interfacial transition zone between cement paste and SAP and the voids introduced by SAP is analyzed. It is concluded that the addition of SAP in the mixture increases the hydration degree of cement particle, this leads to a reduction of capillary porosity in the matrix. The additional of SAP meanwhile increase the void in the ITZ between SAP and matrix. Micro-computer tomography image shows that the voids introduced by SAP in the mixture are homogenously distributed.