International Concrete Abstracts Portal

Foamed geopolymers possess excellent fire resistance similar to ordinary geopolymers and can also provide good thermal insulation when using lightweight aggregates, making them ideal for fire-resistant coatings. However, foamed geopolymers are susceptible to significant and uneven shrinkage, which can result in cracks and breakage when heated due to the dehydration of the weakly bound water in the C-A-S-H or N-ASH gels. To address this issue, this study incorporated the expanded perlite particles (EPP) and replaced a portion of fly ash with crushed stone powder (CSP) in a ground granulated blast furnace slag/fly ash-based geopolymer that was foamed using fine metallic silicon powder. The EPP and CSP acted as inactive precursors to form a stable particle skeleton with a continuous gradation in the foamed geopolymers. Experimental results indicate that the suitable inclusion of CSP and EPP effectively mitigated geometrical deterioration and cracking under elevated temperatures. The foamed geopolymers with these additives showed no warping or cracking, even as they decreased in dimension and bulk density when heated on a one-side surface. It is considered that continuous gradation of CSP and EPP particles formed a stable particle skeleton within the foamed geopolymer to effectively diminish the shrinkage caused by the thermal decomposition of gels.

Serviceability checks in Reinforced Concrete (RC) elements involves the verification of crack width mainly aimed to limit the exposure of the steel reinforcement to corrosion and chemical attack and, thus, improve durability. Classical approaches for assessing the crack width in RC elements provide the calculation of two terms: 1) the average crack spacing, and 2) the average difference between the strain in the steel reinforcement and in the concrete in tension referred to the average crack spacing. A similar approach can be assumed valid also for RC elements strengthened with externally bonded Fiber Reinforced Polymer (FRP) materials, taking into account the additional tension stiffening effect provided by the external reinforcement.

This paper presents the comparisons of some existing code formulations for predicting crack spacing and crack width in RC elements with the experimental results of a database collected by the Authors and concerning tests on RC beams and ties externally bonded with different types and configurations of FRP materials. The paper is mainly aimed to check the reliability of the existing equations provided by codes in order to address the future assessment of reliable design provisions for cracking verifications in RC elements strengthened with FRP materials. The comparisons have evidenced, indeed, some useful issues for the design provisions: 1) larger scatter in the predictions of crack width than in crack spacing and, in particular, for ties, 2) limited effect of shrinkage on crack width, 3) necessity of taking into account the external reinforcement in crack spacing formulations, 4) good reliability of mechanical models for calculating cracks width.

A special concrete was used to erect the MAXXI building in Rome designed by Zaha Hadid and her team with long, inclined, curvilinear walls. Due to the very congested reinforcements, the original concrete issued by Zaha Hadid and her team was self-compacting concrete (SCC). However, irregular cracks -caused by the restrained drying shrinkage- appeared on the surface of this concrete a few days after removing the formworks. On the other hand, due to aesthetic reasons, neither saw cuts in the hardened concrete to produce regular contraction joints -carried out to avoid the irregular cracks caused by a restrained drying shrinkage- were accepted by the Architects. Therefore, a special 3-SC mixture was developed and used; it is characterized to be: - a self-compacting concrete based on the use of an acrylic superplasticizer, a viscosity modifier to avoid the bleeding risk, and a special particle size distribution of the aggregates; - a self-compressive concrete due to the use of a CaO-based expansive agent; - a self-curing concrete based on the use of a shrinkage-reducing admixture (SRA). This concrete called 3-SC, because it is 3 times “self”, was very successful in producing a crack-free concrete surface even in the very long, curvilinear, and inclined walls: after 18 years of building the long, inclined, curvilinear walls of the MAXXI museum have been carefully examined and during the last inspection their surface resulted to be still sound and crack-free. However, just before the building’s inauguration in 2009, in very few areas some micro-cracks were observed on the concrete surface and considered to be dangerous for the future of the building. Therefore, the concrete surface was treated with a transparent varnish in order to avoid the ingress of the aggressive humid air to protect the steel reinforcements from the corrosion promoted by the carbonation process.

Dredged river sediments can be considered as a promising alternative for conventional aggregates in concrete. However, the effect of sediments’ properties and particularly their organic matter (OM) content on those of the concrete have to be assessed. Indeed, the organic weight fraction of organic matter in sandy sediments dredged in the Seine watershed is highly variable as it varies from 0 wt% to 10 wt% of the dry matter. This research aims at assessing the effects of humic substances (HS) on the early age behavior of a cement paste. HS are organic compounds resulting from the chemical, physical and microbiological transformation of animals and plants residues and which are also the most representative component of sediments organic matter. A soluble HS, potassium humate, is added as partial substitution of cement; 0.2 wt%, 0.5 wt% and 1 wt%. The results indicate that HS causes a delay of both chemical and autogenous shrinkages by retarding the hydration process. In addition, the chemical shrinkage amplitude is not significantly affected by the presence of HS in the mixture, while the autogenous shrinkage is decreased especially for the high w/b (water/binder; binder=cement+HS) ratios, due to bleeding. Furthermore, for high rates (2% and 3%), this bleeding could generate a delay of setting between the top and the bottom of the sample causing cracks due to a restrained shrinkage in the upper part of the sample.

Compared to normal concrete, packing density optimised Ultra High Performance Concretes have a high shrinkage up to 1 mm/m due to their high cement content. Especially in the first 24 hours approximately 80 % of the final shrinkage is reached which reduces the early strength due to microcracks. Instead of additives within the scope of this research work, parts of normal portland cement (NPC) were substituted by Calcium Sulfoaluminate (CSA) Cement and Calcium Aluminate (CA) Cement with the aim to reduce shrinkage of UHPC-mixture as well as a fast setting. CSA-cements with low CO2 footprint are characterised by their fast strength development and expansion behaviour due to early ettringite formation. X-ray diffraction was used to study the phase development. The influence on the shrinkage value was measured by shrinkage tests. In addition, the development of the microstructure was investigated by scanning electron microscopy. Finally, the influence on the strength development was correlated by ultrasonic measurement. These techniques allow a prediction of the setting process in the early stages. Finally, an environmentally friendly NPC-CSA blend could be developed which, in addition to high early strength, also achieves low shrinkage. Furthermore, the influence of the ettringite formation on the microstructure could be determined.