Benoit Fournier

Flexural Performance of Steel-Reinforced Recycled Concrete Beams

A new method of mixture proportioning is used to investigate the flexural performance of reinforced concrete beams made with coarse recycled concrete aggregate (RCA). In this method, RCA is treated as a two-phase material comprising residual mortar and natural aggregate; therefore, when proportioning the mixture, the relative amount and properties of each phase are considered separately. Several reinforced concrete beams are built and tested using concrete mixtures designed by the new method and their deflection; cracking, yielding, and ultimate moments; crack spacing; cracking patterns; and failure modes are studied. The results show that at both the serviceability and ultimate limit states, the flexural performance of beams made of RCA-concrete proportioned by the new method is comparable to that of beams made of conventional natural aggregate concrete; and the general flexural theory and current code provisions for flexural design are applicable, without alteration, to the reinforced recycled concrete beams.

Proposed Method for Determining the Residual Mortar Content of Recycled Concrete Aggregates

Recycling concrete from demolition of existing structures and using it as recycled concrete aggregates (RCAs) in structural-grade concrete have significant economic and environmental benefits. Currently, only a small portion of the concrete waste is reused in building construction, while most of it is used as either pavement base course or sent to landfills for disposal. The lack of confidence in the material properties of the concrete produced with RCAs is generally the main reason for its under-utilization in structural concrete. It has been demonstrated in the literature that the amount of residual mortar attached to the original (or “virgin”) aggregate particles is one of the factors affecting the material properties of RCAs. Therefore, before using RCAs in new concrete, it is crucial that the residual mortar content (RMC) is determined accurately; however, currently there is no standard procedure to determine this quantity. In this paper, an experimental method is proposed to determine the RMC of RCAs. The method comprises a combination of mechanical and chemical stresses that disintegrate the residual mortar and destroy the bond between the mortar and the natural aggregates. The mechanical stresses are created through subjecting RCA to freeze-and-thaw action, while the chemical degradation is achieved through exposure of the RCA to a sodium sulphate solution. The results of the proposed test procedure are validated by means of comprehensive image analysis. With the proposed approach, the attached residual mortar can be adequately removed, and the residual mortar content can be determined.

Performance Limits for Evaluating Supplementary Cementing Materials Using Accelerated Mortar Bar Test

The accelerated mortar bar test (AMBT) was originally developed for the purpose of identifying alkali-silica reactive aggregates, but has been widely used to evaluate the preventive action of supplementary cementing materials (SCM). Indeed, a modified version of the AMBT for testing the effectiveness of pozzolans and slag for controlling expansion due to alkali-silica reaction (ASR) was recently developed and published as an ASTM standard test method (ASTM C 1567). In this paper, results from accelerated mortar bar tests on reactive aggregate-SCM combinations are compared with the performance of the same combination of materials in concrete structures, field-exposed concrete blocks, and laboratory expansion tests on concrete prisms (ASTM C 1293). It is concluded that the use of a 14-day expansion limit of 0.10% in the AMBT produces an outcome that agrees well with the performance of concrete in the laboratory or under field conditions. Combinations of reactive aggregates and SCM that pass this limit when tested in mortar have a very low risk of resulting in damage when used in concrete. Furthermore, the minimum level of SCM required to control expansion with a given reactive aggregate can be determined using the 14-day expansion limit and the result is in good agreement with the amount of SCM required to prevent cracking in concrete. Extending the duration of the test (for example to 28 days) is overly conservative and results in estimates of much higher levels of SCM (by 1.5 times on average) to control expansion than that actually required in concrete.

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.

Effectiveness of Lithium-Based Products in Concrete Made with Canadian Natural Aggregates Susceptible to Alkali-Silica Reactivity

To evaluate the effectiveness of lithium-based products to counteract alkali-silica reaction (ASR), a total of 87 concrete mixtures were made incorporating 12 reactive aggregates of various types and degrees of ASR, and using various dosages of LiNO 3 and Li glass in combination or not with supplementary cementing materials (SCM). The concrete prisms were tested at 38 °C (100 °F), according to CSA A23.2-14A or ASTM C 1293, and also at 60 °C (140 °F) to evaluate the possibility of accelerating the testing procedure. Using LiNO 3 at the [Li]/[Na + KJ ratio of 0.74 recommended by the manufacturer, satisfied the 2-year 0.04% expansion limit criterion (CSA A23.2-28A) at 38 °C (100 °F) for six aggregates; three aggregates required a ratio between 0.74 and 1.11, while a ratio 1.11 was not effective with the other three aggregates. The lithium glass was not effective. The ternary silica fume/slag cement tested was effective and the fly ashes and slag as well provided they were in sufficient quantities and they had a proper composition. Most LiNO 3 -SCM combinations did not show significant synergetic effect. The required LiNO 3 dosage is not related to the aggregate reactivity. The 6-month expansion at 60 °C (140 °F) correlated well with the 2-year expansion at 38 °C (100 °F) for the control and SCM mixtures, but not for the LiNO 3 mixtures.

Influence of Specimen Geometry, Orientation of Casting Plane, and Mode of Concrete Consolidation on Expansion Due to ASR

Concrete specimens of different sizes and shapes were made with various reactive aggregates and stored under conditions favorable to the development of alkali-silica reactivity (ASR), with their expansion measured with time along the three directions. They have been cast vertically (cylinders and prisms) or horizontally (prisms and larger blocks), using a vibrating table, a vibrating needle, or rodding. The expansion due to ASR was always greater in the direction perpendicular to the casting plane. The higher the number of flat and elongated particles in the reactive aggregate, the higher the coefficient of anisotropy, defined as the ratio between the expansions perpendicular and parallel to the casting plane. This coefficient was constant through the course of the expansion. It was generally higher for the cylinders than for the prisms, and still less for larger blocks. Consolidation by rodding induced anisotropy coefficients distinctly smaller than consolidation using a vibrating table, while a vibrating needle induced intermediate values; however, all methods gave constant volumetric expansion at least up to an important expansion level. For prisms cast horizontally and measured axially in accordance with the concrete test CSA A23.2-14A or ASTM C 1293, consolidation using rodding induced long-term (axial) expansions greater by 71% compared with consolidation using a vibrating table. In order to reduce the experimental variability of the test, only one method of consolidation should be allowed. When evaluating field concrete affected by ASR, it appears important to consider the orientation with respect to the casting plane of the core samples subjected to mechanical or residual expansion tests.

PROPOSED GUIDELINES FOR THE PREVENTION OF ALKALI-SILICA REACTION IN NEW CONCRETE STRUCTURES

The approach now under consideration by the Canadian Standards Association’s Technical Group on Alkali-Aggregate Reactions (AARs) to prevent the risk of deterioration associated with the use of potentially reactive aggregates in concrete is described. This approach involves a risk-evaluation process based on three factors judged critical for the development of damage in concrete structures due to alkali-silica reaction. The factors are (a) the degree of reactivity of the particular reactive aggregate, (b) the size of the concrete element and the environmental conditions it will face, and (c) the expected service life of the structure. The proposed preventive action against AARs will depend on the actual risk level thus determined. The action will involve one or a combination of the following approaches: (a) limit the alkali content in the concrete to a selected level, or (b) use an effective supplementary cementing material (SCM) (or an effective combination of SCMs) in sufficient amounts.

Use of the Accelerated Mortar Bar Test to Evaluate the Effectiveness of LiNO 3 Against Alkali-Silica Reaction—Part 1: Pore Solution Chemistry and Influence of Various Parameters

For the time being, the only reliable test method to evaluate the effectiveness of lithium nitrate against alkali-silica reaction (ASR) in concrete is the concrete prism test (CPT) CSA A23.2-14A or ASTM C1293, extended to two years. In its actual form, the more commonly used accelerated mortar bar test (AMBT) CSA A23.2-25A or ASTM C1260 is not able to predict this effectiveness and needs to be modified to improve it’s reliability. Part I of this study, which involves a large variety of reactive aggregates from Canada and the United States, aims to evaluate the partition of various ions (OH−, Na+, K+, Li+, and silica species) between the mortar pore solution and the soak solution in the AMBT, and the effect of a number of experimental parameters on the expansion of mortar bars with/without lithium nitrate, e.g., the presence of lithium in the original mortar bars, the Li concentration in the soak solution, the concentration and the composition (NaOH versus KOH) of the soak solution, the cement alkali content, the water-to-cement ratio, and the initial 24-h soaking in pure water. The second part of this study (Part II) compares the above AMBT results with the CPT results for the same aggregates and aims to propose modifications to the AMBT when testing lithium nitrate against ASR to better correlate with the most realistic CPT results.

Applicability of the Accelerated Mortar Bar Test for Alkali-Silica Reactivity of Recycled Concrete Aggregates

Using recycled concrete aggregate (RCA) as a replacement for natural aggregate in new concrete is a promising way to increase the overall sustainability of new concrete. This has been hindered, however, by a general perception that RCA is a sub-standard material because of the lack of technical guidance, specifically related to long-term durability, on incorporating RCA into new concrete. The goal of this research was to determine whether current testing methods (namely, ASTM C1260) for assessing natural aggregate susceptibility to alkali-silica reactivity could be used to assess the potential reactivity of concrete incorporating RCA. Seven different RCA sources were investigated. It was determined that ASTM C1260 was effective in detecting reactivity, but expansion varied based on RCA processing. Depending on the aggregate type and the extent of processing, up to a 100 % increase in expansion was observed. Replicate testing was performed at four university laboratories to evaluate the repeatability and consistency of results. The authors recommend modifications to the mixing and aggregate preparation procedures when testing the reactivity of RCA using ASTM C1260.

Evaluation Protocol for Concrete Aggregates Containing Iron Sulfide Minerals

Several cases of concrete deterioration involving sulfide-bearing aggregates have been reported over the years. However, no specific guidelines are currently available to enable making a precise decision on the deleterious potential of aggregates containing iron sulfide minerals. The aim of this paper is to provide an innovative assessment protocol to evaluate the potential deleterious effects of iron-sulfide-bearing aggregates prior to their use in concrete. The findings of this paper are based on tests developed within the past few years as part of a major research project. The protocol is divided into three major phases: 1) total sulfur content measurement; 2) oxygen consumption evaluation; and 3) an accelerated mortar bar expansion test. Tentative limits are proposed for each phase of the protocol, which still need to be validated through the testing of a wider range of aggregates.