Interfacial Weakness Criterion by Indentation in 3D Printed Concrete.

Three-dimensional (3D) printable concrete requires cementitious material that must have suitable but self-contradictory properties to be printable such as fluidity to facilitate pumping along with stiffness and strength to ensure buildability, both having a great influence on the cohesion of the interfacial zone. A pool of characterization tests was developed over the last decades for layered 3D printed structures to quantify and qualify the interfacial region. Although destructive tests are typically selected to capture actual interfacial bonding strength, nondestructive testings were also used. Indentation tests were preferred in this study to locally determine the mechanical properties of the center part of two consecutive layers, the edge of the layer and the interfacial zone. As results, it was found that the previously deposited layer is harder than the upper one. The hardness of the edges of the printed filament can decrease ∼50% over few hundred microns compared to the core of the material. Moreover, this decrease in hardness is also observed at the interface. From the hardness-distance profile measured perpendicularly to the plan of the interface, we propose an interfacial weakness criterion, which has been successfully applied in various conditions of 3D printed concrete elaboration.

Quantification of the Hardened Cement Paste Content in Fine Recycled Concrete Aggregates by Means of Salicylic Acid Dissolution.

Adherent hardened cement paste attached to recycled concrete aggregates (RCA) generally presents a higher porosity than natural aggregates, which induces a lower porosity in the properties of RCA. The characterization of the adherent hardened cement paste content (HCPC) in the fine RCA would promote better applications of RCA in concrete, but the determination of HCPC in fine RCA is not well established. A simple method based on salicylic acid dissolution was specifically developed to quantify the HCPC in RCA, especially for RCA containing limestone aggregates. The results demonstrated that the soluble fraction in salicylic acid (SFSA) was equal to the HCPC for white cement and slightly lower for grey Portland cement, which was also confirmed by a theoretical approach using modelling the hydration of cement paste with the chemical equations and the stoichiometric ratios. The physical and mechanical properties of RCA (e.g., water absorption) were strongly correlated to the SFSA. For industrial RCA, SFSA did not give the exact value of HCPC, but it was sufficient to correlate HCPC with the other properties of RCA. The water absorption could be estimated with good accuracy for very fine RCA (laboratory-manufactured RCA or industrial RCA) by extrapolating the relationship between water absorption and HCPC, which is very important for concrete formulation.

Evaluation of PIRs Post-Fire Pull-Out Strength in Concrete Exposed to ISO 834-1 Fire.

Post-installed rebars (PIRs) using mortar can offer bond strength at ambient temperature equal or higher to that of cast-in place rebars. However, high temperatures have the effect of weakening the bond, typically governed by the chemical and physical properties of the mortar which is often sensitive to temperature increase. Therefore, the behavior of PIRs in a fire situation becomes vulnerable. Moreover, after exposure of PIRs to high temperature, the heat transfer continues during the post-fire phase, which might endanger the construction after a fire event. In order to evaluate the evolution of the pull-out capacity during fire, Pinoteau et al. have developed the bond resistance integration method (Pinoteau's RIM) to predict the bond resistance value of a rebar subjected to various temperatures in accordance with the fire exposure curves. Therefore, accurate temperature profiles during the post-fire phase are needed to ensure a correct calculation of the post-fire behavior of the PIR connection. This paper presents 3D finite element thermal simulations of PIRs in concrete exposed to ISO 834-1 fire conditions then cooled with ambient air. Numerical thermal profiles are then compared to the experimental results (i.e., post-fire pull-out tests). The proposed model provides guidelines for conducting numerical simulations to determine the thermal entry data necessary for predicting thermal profiles in PIRs during heating and cooling phases. Then, the post-fire pull-out capacity of PIRs in concrete is calculated using Pinoteau's RIM, and compared to experimental post-fire pull-out results.

Valorization of Fine Recycled Aggregates Contaminated with Gypsum Residues: Characterization and Evaluation of the Risk for Secondary Ettringite Formation.

Fine recycled aggregates (FRA) (0/4 mm) are up to now not valorized on a high enough level because of characteristics like an elevated water absorption, higher fines content, and the presence of contaminations. Leftover gypsum residues from the construction site can cause internal sulfate attack when FRA are incorporated into new structures. Concern about this deteriorating reaction plays an important role in the rejection of FRA. In this study, samples of FRA from different recycling centers were characterized and incorporated into mortars. They were then subjected to swelling tests in order to evaluate the development of sulfate attack. Reference materials with different amounts of sulfates were used as a comparison. Results showed a variable sulfate content in industrial FRA, depending heavily on the source of the materials. In all but one case, the total amounts surpassed the acceptable sulfate contents specified in the European standard EN 206, meaning the FRA would be rejected for reuse in concrete. Nevertheless, swelling tests demonstrated that these contamination levels did not pose a risk for sulfate attack. These results indicated that the incorporation of FRA leads to acceptable mechanical performances and that the sulfate limit could be reviewed to be less strict.

Optimization of the formulation of an original hydrogel-based bone cement using a mixture design.

Highly swelling polymers, i.e. superabsorbent hydrogels, are hydrophilic, three dimensional networks that can easily absorb a significant amount water, fluid or drug. They are widely used in various applications such as foods, cosmetics, and medical devices. Bone cements are used in orthopaedics as a filling biomaterial or as a grout enhancing the embedding of a prosthesis into bone and fixation is achieved by mechanical interlock with metal or bone surfaces. Recently, hydrophilic bone cements have attracted the attention for bone tissue-engineering applications. Here a bone cement containing an acrylic hydrogel (HEMA) as a liquid phase and a blend of corn starch, cellulose acetate and bioceramic filler as a solid phase is investigated by means of a mixture design which is a special topic within statistical Design of Experiments (DoE). Output variables of interest, complex shear modulus, compressive modulus and swelling rate related to rheological, mechanical and swelling properties respectively, are measured for each cement formulation. Applying the mixture design strategy enables to assess the impact of the three powder components on each variable of interest and to determine the optimal formulation in order to achieve the required properties of this HEMA-based bone cement, especially the rheology adapted to the desired clinical application, but also appropriate mechanical and swelling properties.