Patrick Fontana

Evolution of Phases and Micro Structure in Hydrothermally Cured Ultra-High Performance Concrete (UHPC)

Thermal curing of Ultra-High Performance Concrete (UHPC) has a strong influence on its mechanical properties. By applying a water vapor saturation pressure additional to the increased temperature the curing conditions are strongly enhanced and lead to a significant improvement of the degree of hydration of the cement paste. Increasing temperature accelerates the formation of crystalline calcium silicate hydrates by dehydrating the cement paste and ends in the formation of gyrolite, truscottite and xonotlite at 200 °C and 15 bars. Thereby the micro structure undergoes an obvious change. Cement paste consists of close networked crystal fibers with dimensions up to 1 μm. By filling cracks and small pores with crystalline C-S-H phases, flaws in the matrix are healed and generate a more homogeneous micro structure. Additionally, autoclaving encourages dissolution processes at quartz grains, which produces a better cohesion between fillers and the fine crystalline cement paste. As a consequence, autoclaving enhances compressive and flexural strength significantly but, compared to simple heat treatment at 1 bar, with very low scatter of the test results.

Plastic Shrinkage Cracking Risk of Concrete – Evaluation of Test Methods

Concrete pavements subjected to fast evaporation during the first hours after concrete placing are prone to plastic shrinkage cracking. In this paper the mechanisms by which evaporation of water leads to capillary pressure, and finally to shrinkage and cracking, are briefly discussed. In the main part, the paper presents results of a study on the evaluation of three different measurement setups. With two of these setups usually the efficiency of fibres in reducing plastic shrinkage cracking of concrete is assessed in Germany and Austria. The third one is an advanced setup that was developed in collaboration of BAM and Empa. It allows performing measurements of parameters that are important for the evaluation of the cracking risk.

Detection of early-age cracking due to restrained autogenous shrinkage

The influence of the mix composition on the early-age autogenous shrinkage of high-strength cement paste with and without silica fume and the resulting crack formation were investigated. The mechanisms of autogenous shrinkage were studied systematically by linking the measured deformations to hydration kinetics and to pore structure data. The time t0 which assigns the transition from the plastic state to a solid structure was determined by analysis of the measured strain rate. It was demonstrated that the autogenous shrinkage correlates reasonably well with the self-desiccation and the thereby expected capillary stresses. The crack formation was detected using acoustic emission (AE) analysis. AE measurements were performed simultaneously on hardening silica fume cement pastes with and without external restraint of deformation. The results showed that the intense autogenous shrinkage during the acceleration period of cement hydration was associated with the begin of micro cracking shortly after t0, irrespective of the existence of external restraint. The cracking was more pronounced in the case of restraint.

Numerical modelling of UHPC and TRC sandwich elements for building envelopes

In this paper a modelling approach is presented to reproduce the mechanical behaviour of sandwich panels via finite element analysis. Two types of panels were investigated in this scope of work. The first sandwich element was a textile reinforced concrete (TRC) panel with cellular lightweight concrete insulation and the second configuration was an ultra-high performances concrete (UHPC) panel with aerated autoclaved concrete insulation. The goal was to obtain a reliable numerical strategy that represents a reasonable compromise in terms of sufficient accuracy of the element characteristics and the computational costs. The results show the possibility of describing the composite action in a full sandwich panel. The achieved modelling approach will later be used for the optimization of TRC and UHPC panels in terms of minimizing the thickness, identifying the number and location of connectors, as well as evaluating varying anchorage systems.

Influence of Hydrothermal Curing on Micro Structure and Mechanical Properties of Ultra-High Performance Concrete

Thermal curing of Ultra-High Performance Concrete (UHPC) has a significant influence on its mechanical properties. When additionally a water vapour saturation pressure is applied (autoclaving), the curing conditions are strongly enhanced, leading to an improved hydration of cement and secondary cementitious materials. Autoclaved UHPC exhibits a cement paste matrix of close networked C-S-H crystal fibres. Flaws are filled with crystalline reaction products generating a more homogenous micro structure. As a consequence, the mechanical properties of UHPC are improved significantly. The partial reaction of quartz filler grains may contribute by improving the cohesion with the cement paste.

Experimental investigations on the initial shear strength of masonry with earth mortars

In this paper, a comparative study on the initial shear strength of masonry with earth mortars is presented. Triplet tests were carried out to characterise the shear bond strength of five different types of earth mortar, three purely mineral and two with vegetable additives (wood and straw chaff), using calcium silicate blocks. In spite of their lower bulk densities, mortars with chaffs reached a value of compressive strength comparable to the values shown by the purely mineral mortars. The characteristic initial shear strengths of all the tested earth mortars were between two and five times higher than the minimum values for initial shear strengths required by standards. To assess the influence of blocks pre-wetting, a comparison between calcium silicate blocks and earth blocks was performed to evaluate the results obtained from the standard test procedure compared to the more common practice of using earth mortars in combination with earthen blocks.

In-plane shear behaviour of earthen materials panels strengthened with polyester fabric strips

An experimental investigation was carried out to study the in-plane shear behaviour of earthen material panels strengthened with polyester fabric strips. Strengthened panels were developed to exploit the strength potential of earthen materials and to solve its lack of tensile strength, significantly improving not only strength but also ductility. Three earthen materials were considered: cob, earth block masonry (EBM) and rammed earth (RE). As first approach the strengthening configuration, based on different adhesive materials, was tested only for cob panels. As part of the study the results of a big testing campaign of unstrengthened Panels were considered. Seven strengthened panels were tested in diagonal compression/shear.A unique reinforcement orientation was used. The results of these tests are presented in this paper, and include the load-displacement behaviours, crack patterns, failure modes. The results showed that the reinforcement was the most effective in EBM panels, with increase in strength and ductility observed. In RE and cob panels the reinforcement did not likely contribute significantly to the shear resistance, due to a lack of embedment length of the strips. Instead, in EBM it was likely that the vertical reinforcement acted in tension to restrain shear induced dilation and to restrain sliding.

Zwangsinduzierte Rissbildung in Hochleistungsbeton durch autogenes Schwinden

In diesem Beitrag werden Untersuchungsergebnisse vorgestellt, die den Einfluss von Mikrosilica auf die Selbstaustrocknung und das damit verbundene autogene Schwinden der Bindemittelmatrix von Hochleistungsbetonen beschreiben. Die dadurch hervorgerufenen Schadigungsprozesse wurden mit Hilfe von zerstorungsfreien Prufmethoden verfolgt. Dabei zeigte sich, dass eine durch auseren Zwang bedingte Mikrorissbildung durch Schallemissionsanalyse detektiert werden kann.