Thierry Chaussadent

Comparaison et validité des méthodes de mesure de la carbonatation

ABSTRACT The modeling of concrete carbonation needs physical parameters well determined, especially those of the real carbonation front and the quantity of calcium carbonate formed after carbonation. This paper proposes a comparison between different carbonation measurement methods. Obtained results are slightly different from used method. Furthermore, the interest and the limit of each technique are showed.

Pore Structure and Moisture Properties of Cement-Based Systemsfrom Water Vapour Sorption Isotherms

This paper presents results obtained for hardened cement pastes and concretes, ordinary and very-high-performance materials, concerning pore structure and moisture properties. Because of the inadequacy of classical pore investigation methods (mercury intrusion, nitrogenadsorption) to provide the exact pore network characteristics of cement-based systems, notably in the case of very-high-performance materials, a more pertinent type of experiment was carried out: water vapour desorption and adsorption measurements. From these experiments, isotherms were determined at hygrometrical equilibrium and at constant temperature (T = 23 °C) for every desorption/adsorption path. The analysis of these curves gives textural parameters in the mesopore range, not accessible by most of other methods. Thus, from the adsorption curve, the specific surface area accessible to water molecules was calculated by the B.E.T. method. The statistical thickness of the water film adsorbed on the solid surface was also determined. And finally, application of the B.J.H. method on the desorption branch provides relevant porosity values and pore size distribution of the hardened materials, and especially intrinsic characteristics of C-S-H hydrates. This makes it possible to point out the similarities and the differences between the materials, regarding microstructure and moisture properties, and to quantify the influence of mix parameters such as water-cement ratio or silica fume content.

Influence du rapport E/C sur l'hydratation, la microstructure et les déformations endogènes de pâtes de ciment durcies

ABSTRACT This work deals with the influence of the W/C ratio on the cement hydration and on the chemical characteristics and the microstructure of hydrated compounds. Results obtained with various techniques show that the W/C ratio has a significant influence on the structural characteristics of the hydrated cement pastes. Whatever the time of hydration, it appears that the materials have a different behaviour when W/C is higher than 0.40. This limit could correspond to a “percolation threshold” of water within the capillary pore network. When W/C increases, the amount of calcium hydroxide, referred to a unit mass of initial cement, increases whereas the CaO/SiO2 of C-S-H has a tendency to decrease. Moreover, for the autogenous deformations recorded during hydration, a swelling phase is exhibited before shrinkage phase with high W/C ratios. In the case of low W/C ratios, only shrinkage is observed.

La microfissuration superficielle a-t-elle une influence sur la profondeur de carbonatation des bétons?

ABSTRACT This work deals with the influence of the superficial microcracking on concrete carbonation. Three types of concrete with strengths ranging between 25 and 60 MPa were subjected to different thermo-hygrometric treatments to create a superficial microcracking, then, to accelerated carbonation tests for 3 months. After their period of preconditioning treatments, the quantification of the superficial microcracking and the measure of the water absorption of the concrete were carried out to observe correlations with carbonation depth. Results show that the influence of the superficial microcracking is strongly dependant of the concrete strength and begins to be significant when concrete strength reaches about 50 MPa.

Comparaison de différentes méthodes de mesure du degré d'hydratation de pâtes de ciment Portland durcies

ABSTRACT This study, achieved as a part of the research project “Transfers in concrete and durability of structures”, deals with comparative measurements between two methods which evaluate the degree of hydration of different hardened Portland cement pastes: classical measurement of the chemically bound water, and image analysis on polished sections by scanning electron microscope in backscattered mode (SEM—BSE). The water/cement ratio ranging from 0.25 to 0.6 and time from 28 days to 2 years, are the two majors parameters of this study. The results indicate a good agreement between the two methods. Assumptions, advantages and limitation of each method are discussed, especially, when applied to other types of cements and to concrete.

Corrosion Processes of Carbonated Chloride-Contaminated Reinforced Concrete and Electrochemical Chloride Extraction (ECE) Efficiency

Civil engineering structures and historical buildings can suffer from corrosion of the embedded reinforcing steel once the concrete cover is totally carbonated and/or when chloride ions have reached the steel/concrete interface. In practice, these two types of contamination can be encountered separately or combined requiring implementation of proper repair methods. In this research, carbonated and chloride-contaminated reinforced concrete specimens were studied by three main analysis means: electrochemical characterizations, Raman microspectrometry and scanning electron microscopy (SEM). The specimens were contaminated in a two-step process involving addition of chlorides followed by carbonation of the complete cover. Electrochemical chloride extraction (ECE) was then performed as a repair treatment (1 A/m2 of steel surface during 8 weeks). The efficiency of the treatment and its impact on the steel/concrete interface and cement matrix were studied during the treatment, after steel depolarization and on the long term (several months) in order to evaluate the durability of the treatment. Electrochemical characterizations showed an increase of corrosion rates comparing the specimens after fabrication and after carbonation (from a negligible level of 0.1 µA/cm2 to values in excess of 10 µA/cm2). SEM observations confirmed this significant increase with the identification of a corrosion layer on most of the steel/concrete interface after carbonation. The ECE efficiency was evidenced by a decrease of chloride content below the practical threshold value of 0.4% by weight of cement after a two weeks treatment. Simultaneously a realkalisation ring was observed around the reinforcement bar having a diameter of about 1 cm after four weeks. Results obtained after depolarization showed that a two weeks ECE treatment allowed the stabilization of the corrosion state of the rebar.

Accelerated Biodeterioration Test for the Study of Cementitious Materials in Sewer Networks: Experimental and Modeling

Important deteriorations have been observed in concrete sewers, due to hydrogen sulfide (H2S) production. Hydrogen sulfide environment involves the selection of sulfur-oxidizing bacteria (bacteria able to oxidize the reduced sulfur compounds) in contact with the cementitious materials. These biological reactions lead to a local production of sulfuric acid and, as a consequence, to the dissolution of cement matrix and its mineralogical transformations (gypsum and ettringite formation). This phenomenon disturbs the sewer system and leads to expansive works of rehabilitation. As a consequence, a project was initiated in order to propose more efficient solutions. The main objectives of this project are to set up an accelerated test and to develop an associated model. To date, experimental studies and some improvements of the model previously setting up were performed. The first study describes the impact of several parameters, including type of cementitious materials, on hydrogen sulfide adsorption. These abiotic tests involve monitoring hydrogen sulfide concentration as a function of time. This experiment was realized in a hermetic chamber with five types of mortars (cast with calcium aluminate cement (CAC), blended Portland cement (CEM III, CEM IV and CEM V) and super sulfated cement (SSC)) and under different relative humidity. The second study is deterioration state of mortars characterization, thanks to some analyses (SEM – EDX). After three months of exposition, different types of sulphur species are observed on mortar surfaces, which vary with the nature of mortar. All these experiments allow providing improvements to model previously setting up. Abiotic tests measurements are used to determine mathematical law, which modelises hydrogen sulphide adsorption on each type of cementitious material.

Extension of Petersen matrix to the modelling of chemical equilibrium involved in concrete carbonation

Concrete is a porous material in which several chemical reactions may develop. One way to handle such a complex set of reactions is to use a modified form of Petersen matrix notation which is described in the paper. This formulation of Petersen matrix is able to handle the case where some of the reactions involved in the set of reactions have no analytical expression, but are described by chemical equilibrium. In order to handle this new situation, the reaction rate is tuned during the simulations in such a way that the chemical equilibrium remains verified. This tuning is performed thanks to a numerical proportional integral (PI) controller technique. This extension of the Petersen matrix is applied to the case of concrete carbonation modelling. The PI coefficients are given for each equilibrium controlled kinetics. The simulation results obtained thanks to a finite-difference solver are compared to experimental results taken from the literature.

Durability of Polymer Modified Repair Mortars on Concrete Structures

Polymer modified mortars (PMM) used as repair products present higher intrinsic properties than classic mortar due to polymer effect in the cementitious matrix. But evolution of their adhesives properties is not well known. This article deals with adhesive behavior of two PMMs made in laboratory with styrene acrylate (SA) and ethylene vinyl acetate (EVA) polymers. It is highlighted that adhesion of these mortars depends on polymer amount in the mortar, on the environmental conditions, and on the roughness of the concrete support. This adhesion is improved in warm environment and with high roughness of support surface. The influence of polymer amount is not the same for the two types of polymers but in the two cases, bond strength remains low when low amount of polymer are added.