SHCC3: Strain hardening cementitious composites

Strain-Harding Cement Composites (SHCC) are classes of fibre-reinforced cement-based composites in which crack formation in the cement-based matrix is controlled by fibres bridging carcks to the extent that multiple, closely fine cracks form at increasing tensile deformation and force. RILEM TC-FDS is focussing on the durability afforded to the structural system in service conditions. This must be appropriately modelled and characterised to enable recommendation of rational design guidelines for durability design with SHCC. In addition the TC is dealing with a standard test procedure for such characterisation to verify compliance of a material to specified durability requirements. Topics : - Test methods for mechanical characterization - Structural design and performance - Durability characterization and design - Theoretical considerations and computational methods - Practical application

Elastic modulus of the Alkali-Silica reaction rim in a simplified calcium-alkali-silicate system determined by nano-indentation

This work aims at providing a better understanding of the mechanical properties of the reaction rim in the alkali-silica reaction. The elastic modulus of the calcium alkali silicate constituting the reaction rim, which is formed at the interface between alkali silicate and Ca(OH)(2) in a chemically-idealized system of the alkali-silica reaction, was studied using nano-indentation. In addition, the corresponding calcium to silica mole ratio of the calcium alkali silicate was investigated. The results show that the elastic modulus of the calcium alkali silicate formed at the interface increased with the increase of the calcium to silica mole ratio and vice versa. Furthermore, the more calcium that was available for interaction with alkali silicate to form calcium alkali silicate, the higher the calcium to silica mole ratio and, consequently, the higher the elastic modulus of the formed calcium alkali silicate. This work provides illustrative evidence from a mechanical point of view on how the occurrence of cracks due to the alkali-silica reaction (ASR) is linked to the formation of the reaction rim. It has to be highlighted, however, that the simplified calcium-alkali-silicate system in this study is far from the real condition in concrete

A real-time height measurement and feedback system for 3D concrete printing

Recent years have seen a rapid growth of additive manufacturing methods for concrete construction. Generally, these methods are based on a linear sequence of design → print path definition → actual printer actions in a print environment. However, printing experiments show that a large number of parameters influence the printing process. Not all of these can be predicted accurate on forehand. Therefore, a method is introduced that allows real-time adjustment of the print process. As a proof-of-concept, a measurement system for the nozzle height has been developed and tested. Because this variable relates to machine properties, environmental conditions as well as material behaviour, it is a crucial parameter to control. In two case study prints, the effectiveness of the device was shown. In one study, the printer could follow a range of irregular curves in the print bed, whereas only a simple flat rectangular print path had been programmed. In the other, it was shown the print path could be adjusted to vertical deformation of the previous layers of printed filament in a tubular object of several dozen layers. Thus, premature failure through irregular loading of the object during printing was avoided. Further expansion of the use of real-time measurement devices may be anticipated in the future. Besides more advanced geometrical measuring, chemical and physical conditions such as concrete temperature (both before and after deposition), surface wetness, and environment humidity, can be recorded. Combined with the machine action log, this should result in a detailed set of as-built data of the printed object, allowing e.g. for a geometrical clash control with the design as well as other quality controls

Impact of surface roughness on the debonding mechanism in concrete repairs

Surface roughness of the existing concrete substrate was considered to have the greatest impact on the bond strength in repair systems. However, the influence of this parameter has been subject for debates in recent years. The effect of concrete surface roughness is not quite clear, nor there exist a clear relation between the surface roughness and the adhesion in multilayer systems. In order to understand and explain this relation, simple numerical experimentation is used. Repair systems with different roughness parameters are simulated in order to get load displacement diagrams and crack debonding propagation. The influence of roughness on the composite response in simulated direct tension, shear and three point bending test using a lattice model, is studied. Results indicate that roughness has different influence on tensile and shear bond strength. In addition, although it seems to have negligible influence on load bearing capacity of the composite system in bending, it enables more monolithic response and slower debonding propagation