Patrice Rivard

Characterization of the ASR rim: Application to the Potsdam sandstone

Abstract Chemical properties of the reaction rim associated with alkali–silica reaction (ASR) were investigated using microprobe and scanning electron microscope (SEM). The studied aggregate is the Potsdam sandstone, a Cambrian siliceous sandstone well known for its reactivity. This particular rock is composed of well-crystallized quartz grains surrounded by a poorly crystallized siliceous cement that is considered to be the reactive constituent. Research was conducted on laboratory concrete specimens having reached various expansion levels and on some samples taken from an ASR-affected dam. Results indicate that the dark rim surrounding reactive particles is mainly composed of silica. This suggests that the reaction rim is formed by the precipitation of dissolved silica. Some alkalis and calcium were detected inside the thin intergranular joints in concentration ranging from 1% to 10%. These ions come from the cement paste and play a major role in dissolving original reactive silica.

Alkali mass balance during the accelerated concrete prism test for alkali–aggregate reactivity

The alkali mass balance was calculated in concrete specimens submitted to the storage conditions of the Canadian standard CSA A23.2-14A concrete prism test for expansion because of alkali-aggregate reaction (AAR). The alkali concentration of both the concrete pore solution expressed under high pressure and the water below specimens in storage pails (bottom water) was measured. Measurements were conducted over a 1-year period that corresponds to the length of the above test. Two reactive aggregates were testes [Potsdam sandstone (PO) and Spratt limestone (SP)]. Each aggregate was incorporated in two concrete mixtures (mass concrete and structural concrete), for a total of four batches. Significant alkali leaching occurred at 38 degrees centigrade while performing tests in high moisture storage conditions even though prisms were covered with plastic sleeves. After 52 weeks, the alkali loss ranged from 12% to 25% of the original Na(2)O(e) content of the concrete, depending on the mixture proportioning and the aggregate type. After estimation of the proportion of alkalis fixed in cement hydrates, it appears that about 23% to 39% of the original alkalis released by the cement are quickly sorbed on aggregate surfaces or have rapidly migrated inside aggregate particles, which may have been incorporated with time in the AAR product. After 52 weeks at 38 degrees centigrade, the pore solution alkalinity expressed from mass concrete made with PO was 250 mmol/l, whereas the alkalinity was 270 mmol/l in mass concrete incorporating SP. Since prisms of both mixtures were still expanding at 1 year, these alkalinity values are above the thresholds required for sustaining AAR in these concrete mixtures.

Valorization of unauthorized sea disposal dredged sediments as a road foundation material.

The main objective of this study is to show the ability of fine dredged material (mainly silty material) to be used in road construction project. This paper is divided into three parts. In the first part, the physical, the mineralogical and the mechanical characteristics of the used fine dredged sediments, as well as their chemical composition and environmental impacts are presented. In the second part, the methodology developed to design the road made from dredged fine sediment is developed. The third part of the paper focuses on the presentation of the road construction and the interpretation of analyses made on cores drilled samples from the road and measurements of the deflection of the road. The environmental assessment, based on leaching tests, is also performed at different issues.

The Damage Rating Index Method for ASR Affected Concrete—A Critical Review of Petrographic Features of Deterioration and Evaluation Criteria

The Damage Rating Index method has recently been used with success in several cases of damage evaluation in structures affected by alkali-silica reaction in Canada and in Brazil. Although this petrographic method is starting to be widely used and is in the process of becoming integrated as a Canadian standard, it has not been modified yet from the original design. An evaluation of the method is presented in this paper. According to data obtained from many petrographic examinations, the number of cracks in coarse aggregates (filled or not with silica gel) seemed to show to best correlation with the expansion measured on laboratory concrete specimens made with Spratt limestone. The reaction rim is not a real “damage” feature and should not be considered as one but as a “degree of reaction” feature. In an attempt to improve the DRI method for assessing damage related to ASR, a new parameter should be introduced, which takes into account cracks running from aggregate particles to cement paste. The geological nature of the rock used as concrete aggregate may influence the reaction mechanism as well as the petrographic features related to ASR. Comparing concrete specimens subjected to ASR, which incorporate different aggregate types may, in some instances, be influenced by the type of reaction produced by the various reactive rocks and minerals in each aggregate.

QUANTITATIVE PETROGRAPHIC TECHNIQUE FOR CONCRETE DAMAGE DUE TO ASR: EXPERIMENTAL AND APPLICATION

An automatic petrographic test procedure for quantifying damage in concrete affected by alkali-silica reaction (ASR) is presented in this paper. A computer image analysis program was designed to quantify the degree of microcracking and the amount of silica gel resulting from ASR. The procedure involves the petrographic examination at a magnification of ×20 of polished concrete sections impregnated with fluorescent epoxy resin for cracking and uranyl acetate coated sections for the determination of their silica gel content. Also, the data were compared to the results obtained from a semi-quantitative petrographic method, i.e., the Damage Rating Index commonly used in Canada for evaluating the condition of concrete affected by ASR. The petrographic examination was first carried out on laboratory sections cut and polished from concrete prisms incorporating two different aggregate types, the Spratt limestone and the Potsdam sandstone. Both methods were also applied to specimens prepared from a core collected from a large dam affected by ASR. Good correlation was obtained between analytical parameters derived from the image analysis method and the expansion levels of the laboratory test prisms; however, no explicit relation was found to date between the amount of gel as measured in this study and the expansion level. This study shows that the quantitative petrographic method using the image analysis and the Damage Rating Index Method can be used to estimate the condition and current expansion of concrete specimens cored from concrete structures affected by ASR.

Nonlinear Acoustic Technique of Time Shift for Evaluation of Alkali-Silica Reaction Damage in Concrete Structures

Recent studies have indicated that nonlinear acoustic techniques appear to be promising for the detection and characterization of early microcracks produced by damage mechanisms like alkali-silica reaction (ASR). Most of the nonlinear acoustic techniques currently used are not suitable for field investigation due to their complicated implementation. A research program has been developed on the theory and application of a new nonlinear acoustic technique, named Time Shift (TS), for field experiences. The technique is based on the wave interaction between high-frequency/ low-amplitude probe waves and low-frequency/high-amplitude pump waves generated by an impact. Previously, this method has been validated in the laboratory on small specimens. To validate TS for further field applications, tests were conducted in a laboratory test setup involving three large reinforced concrete slabs in which the damage associated with ASR was monitored over time. The required pump waves were produced in the lab by a hydraulic actuator, which is a simulation of field traffic loads. Indeed, traffic load is a potentially suitable source for generating the pump waves in concrete structures, which may dispel the difficulty of perturbation of concrete elements in the field. Experimental results show that this approach for applying the TS method appears to be reliable for detecting ASR damage over time in reinforced concrete structures.

Geometric Effect of Asperities on Shear Mechanism of Rock Joints

Three-dimensional tracking of changes of asperities is one of the most important ways to illustrate shear mechanism of rock joints during testing. In this paper, the changes of the role of asperities during different stages of shearing are described by using a new methodology for the characterization of the asperities. The basis of the proposed method is the examination of the three-dimensional roughness of joint surfaces scanned before and after shear testing. By defining a concept named ‘tiny window’, the geometric model of the joint surfaces is reconstructed. Tiny windows are expressed as a function of the x and y coordinates, the height (z coordinate), and the angle of a small area of the surface. Constant normal load (CNL) direct shear tests were conducted on replica joints and, by using the proposed method, the distribution and size of contact and damaged areas were identified. Image analysis of the surfaces was used to verify the results of the proposed method. The results indicated that the proposed method is suitable for determining the size and distribution of the contact and damaged areas at any shearing stage. The geometric properties of the tiny windows in the pre-peak, peak, post-peak softening, and residual shearing stages were investigated based on their angle and height. It was found that tiny windows that face the shear direction, especially the steepest ones, have a primary role in shearing. However, due to degradation of asperities at higher normal stresses and shear displacements, some of the tiny windows that do not initially face the shear direction also come in contact. It was also observed that tiny windows with different heights participate in the shearing process, not just the highest ones. Total contact area of the joint surfaces was considered as summation of just-in-contact areas and damaged areas. The results of the proposed method indicated that considering differences between just-in-contact areas and damaged areas provide useful insights into understanding the shear mechanism of rock joints.

Effect of drying–rewetting on the alkali concentration of the concrete pore solution

Abstract Experience has shown that the pore solution alkalinity of resaturated concrete was lower than expected. Laboratory-concrete specimens were made with reactive aggregates, stored over water at 38 °C, and then broken into two pieces. The pore solution of half-specimens was extracted by high-pressure squeezing. The other half-specimens were dried at ambient air and rewetted in humid air to their initial weight. The pore solution was then extracted and compared with the composition results of the first extraction. The results obtained from this investigation confirmed that a certain part of the alkali ions in pore solution that had become fixed by drying are not subsequently extracted after rewetting. The alkali concentration [Na+K] was reduced from 34% to 61% by the drying and rewetting treatments.

Comparison of Joint Shearing Resistance Obtained with the Barton and Choubey Criterion and with Direct Shear Tests

Some researchers have criticized the use of the Barton and Choubey criterion. Kulatilake et al. (1995) reported that the JRC does not have the capability of capturing nonstationarity profiles. Hong et al. (2008) reported that the roughness could be underestimated for relatively rough joint profiles (JRC[14). Despite these limitations, we think that this criterion is suitable for analysis and design in rock engineering. The three input parameters of this criterion require simple, and less time-consuming tests, compared with direct shear tests. Barton and Choubey (1977) reported that:

Evaluation of damages due to alkali-silica reaction with nonlinear acoustics techniques

Studies recently indicated that certain tools from nonlinear acoustics seem to be promising for the characterization of cracks or micro-cracks developed in concrete structures. Only a few of them are currently applied in the field because their implementation causes various problems. This paper proposes a new method based on nonlinear acoustics to characterize the concrete damage associated with the alkali-silica reaction (ASR), a common deterioration process affecting numerous concrete structures. This method consists in quantifying the influence of an external mechanical disturbance on the propagation of an ultrasonic compression wave in the material. When a damaged material is subjected to continuous ultrasonic pulses while a punctual mechanical perturbation is generated, the pulse velocity is suddenly reduced due to the opening of the micro-cracks. Then it slowly and gradually returns to its initial value while the micro-cracks are closing. This variation is associated to the nonlinear attenuation obs...