The effect of recycled concrete aggregate properties on the bond strength between RCA concrete and steel reinforcement

The purpose of this study was to investigate the influence that replacing natural coarse aggregate with recycled concrete aggregate (RCA) has on concrete bond strength with reinforcing steel. Two sources of RCA were used along with one natural aggregate source. Numerous aggregate properties were measured for all aggregate sources. Two types of concrete mixture proportions were developed replacing 100% of the natural aggregate with RCA. The first type maintained the same water–cement ratios while the second type was designed to achieve the same compressive strengths. Beam-end specimens were tested to determine the relative bond strength of RCA and natural aggregate concrete. On average, natural aggregate concrete specimens had bond strengths that were 9 to 19% higher than the equivalent RCA specimens. Bond strength and the aggregate crushing value seemed to correlate well for all concrete types.     

Alteration of alkali reactive aggregates autoclaved in different alkali solutions and application to alkali-aggregate reaction in concrete (II) expansion and microstructure of concrete microbar

The effect of the type of alkalis on the expansion behavior of concrete microbars containing typical aggregate with alkali-silica reactivity and alkali-carbonate reactivity was studied. The results verified that: (1) at the same molar concentration, sodium has the strongest contribution to expansion due to both ASR and ACR, followed by potassium and lithium; (2) sufficient LiOH can completely suppress expansion due to ASR whereas it can induce expansion due to ACR. It is possible to use the duplex effect of LiOH on ASR and ACR to clarify the ACR contribution when ASR and ACR may coexist. It has been shown that a small amount of dolomite in the fine-grained siliceous Spratt limestone, which has always been used as a reference aggregate for high alkali-silica reactivity, might dedolomitize in alkaline environment and contribute to the expansion. That is to say, Spratt limestone may exhibit both alkali-silica and alkali-carbonate reactivity, although alkali-silica reactivity is predominant. Microstructural study suggested that the mechanism in which lithium controls ASR expansion is mainly due to the favorable formation of lithium-containing less-expansive product around aggregate particles and the protection of the reactive aggregate from further attack by alkalis by the lithium-containing product layer.     

The role of alkali content of Portland cement on the expansion of concrete prisms containing reactive aggregates and supplementary cementing materials

This paper presents results covering the effects of alkali content of Portland cement (PC) on expansion of concrete containing reactive aggregates and supplementary cementing materials (SCM). The results showed that the alkali content of PC has a significant effect on expansion of concrete prisms with no SCM. When SCM is used, the expansion was found to be related to both the chemical composition of the SCM and, to a lesser extent, the alkali content of the PC. The concrete expansions were explained, at least partly, on the basis of the alkalinity of a pore solution extracted from hardened cement paste samples containing the same cementing blends. An empirical relation was developed correlating the chemical composition (Ca, Si and total Na₂O_e) of the cementing blend (PC + SCM) and the alkalinity of the pore solution. Results from accelerated mortar bar test (ASTM C 1260) and a modified version thereof are also presented.     

Selection of an effective ASR-prevention strategy for use with a highly reactive aggregate for the reconstruction of concrete structures at Mactaquac generating station

Mactaquac Generating Station was constructed in the mid 1960's and is located in the province of New Brunswick in Eastern Canada. The effect of ASR expansion on the concrete structures of the station were first noticed approximately 10 years after construction and, ASR was conclusively diagnosed in 1986. Since 1985, various remedial measures have been undertaken to mitigate the effects of concrete expansion. Eventually reconstruction of the concrete structures will be necessary and current projections are that replacement should be complete by 2030. Due to the lack of any suitable locally-available non-reactive aggregate, consideration is being given to using the same source of reactive aggregate for reconstruction.This paper describes a research study to determine the optimum strategy for preventing deleterious ASR expansion with this aggregate. The options being evaluated include the use of pozzolans and slag, limiting the alkali content of the concrete, and the use of chemical admixtures. Methods of evaluation include accelerated laboratory tests and field exposure of large blocks.     

Effects of aggregate size and alkali content on ASR expansion

Attempts to model ASR expansion are usually limited by the difficulty of taking into account the heterogeneous nature and size range of reactive aggregates. This work is a part of an overall project aimed at developing models to predict the potential expansion of concrete containing alkali-reactive aggregates. The paper gives measurements in order to provide experimental data concerning the effect of particle size of an alkali-reactive siliceous limestone on mortar expansion. Results show that no expansion was measured on the mortars using small particles (under 80 µm) while the coarse particles (0.63-1.25 mm) gave the largest expansions (0.33%). When two sizes of aggregate were used, ASR-expansions decreased with the proportion of small particles. Models are proposed to study correlations between the measured expansions and parameters such as the size of aggregates and the alkali and reactive silica contents. The pessimum effect of reactive aggregate size is assessed and the consequences on accelerated laboratory tests are discussed.     

Characterization of the ASR rim: Application to the Potsdam sandstone

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.     

Effects of externally supplied lithium on the suppression of ASR expansion in mortars

Lithium salts are being externally supplied for mitigating the progress of deterioration of ASR-affected concrete structures. However, it is not clear whether the sodium or potassium in the ASR gel in concrete is replaced by the lithium supplied from the outside. In this article, we examine changes in the composition of the ASR gel, previously formed in mortar specimens, after they are immersed in LiOH solution, using backscattered electron (BSE) imaging and energy-dispersive X-ray (EDX) analysis, associated with length change measurement of the mortar prisms. The intrusion of lithium ions into mortar specimens containing a reactive aggregate could arrest their further expansion within a relatively short time after immersion in 0.50 N LiOH solution. The alkali ions incorporated in most ASR gels, located not far away from interfaces between the cement paste and reactive aggregate particles, appear to be replaced by the lithium ions supplied from the solution. However, the ASR gel within the reacted aggregate particles did not appear to have been affected by the lithium ions.     

Summary of research on the effect of LiNO3 on alkali-silica reaction in new concrete

This paper summarizes findings from a research study conducted at the University of New Brunswick in collaboration with the University of Texas at Austin, and CANMET-MTL, on the effect of LiNO₃ on ASR in new concrete. The studies included expansion testing, silica dissolution measurements and microstructural examinations of cement systems containing glass and two different reactive aggregates (NB and NS). Only a small proportion of the data are presented here for the purpose of highlighting the principal findings of this investigation.Based on these findings, it is proposed that the inhibiting effect of LiNO₃ against ASR in new concrete is attributed to the formation of two reaction products in the presence of lithium, these being a crystalline lithium silicate compound (Li₂SiO₃) crystal and a Li-bearing, low Ca silica gel. These two phases could serve as a diffusion barrier and protective layer to prevent the reactive silica from further attack by alkalis.It was found that the reason the two reactive aggregates selected responded differently to LiNO₃ was due to the difference in their textural features. The NB aggregate contained reactive volcanic glass particles, the surface of which was immediately and equally available to sodium, potassium and lithium, and thus a Li-Si barrier was able to form quickly. The reactive phase in the NS aggregate was microcrystalline and strained quartz, which was embedded in a dense matrix of a non-reactive predominantly alumino-silicate phase and was not easily accessible to lithium.     

Modeling the hydration of concrete incorporating fly ash or slag

Granulated slag from metal industries and fly ash from the combustion of coal are industrial by-products that have been widely used as mineral admixtures in normal and high strength concrete. Due to the reaction between calcium hydroxide and fly ash or slag, the hydration of concrete containing fly ash or slag is much more complex compared with that of Portland cement. In this paper, the production of calcium hydroxide in cement hydration and its consumption in the reaction of mineral admixtures is considered in order to develop a numerical model that simulates the hydration of concrete containing fly ash or slag. The heat evolution rates of fly ash- or slag-blended concrete is determined by the contribution of both cement hydration and the reaction of the mineral admixtures. The proposed model is verified through experimental data on concrete with different water-to-cement ratios and mineral admixture substitution ratios.     

ASR expansion behavior of recycled glass fine aggregates in concrete

The alkali-silica-reaction (ASR) expanding behavior of different types of glass, all derived from cullet with different chemical composition, has been investigated. The glass reactivity was determined in different alkaline solutions based on sodium and/or calcium hydroxide to simulate concrete environment. The expansion of mortar containing different amounts of the investigated glass as fine aggregate has been carried out in different conditions: data collected underline a different response of glass towards the alkaline environment. Soda-lime glass shows negligible expansion, lead-silicate glass always generates expanding trends while boro-silicate glass has different behaviors depending on its colour. An attempt to link the behavior to the solubility and chemical reactivity of the glass is proposed.     

Diagnosing delayed ettringite formation in concrete structures

There has been a number of cases involving deteriorated concrete structures in North America where there has been considerable controversy surrounding the respective contributions of alkali–silica reaction (ASR) and delayed ettringite formation (DEF) to the observed damage. The problem arises because the macroscopic symptoms of distress are not unequivocal and microscopical examinations of field samples often reveal evidence of both processes making it difficult to separate the individual contributions. This paper presents the results of an investigation of a number of concrete columns carrying a raised expressway in North America; prior studies had implicated both DEF and ASR as possible causes of deterioration. Although the columns were not deliberately heat-cured, it is estimated that the peak internal temperature would have exceeded 70 °C and perhaps even 80 °C, in some cases. The forensic investigation included scanning electron microscopy with energy-dispersive X-ray analysis and expansion testing of cores extracted from the structure. Small-diameter cores stored in limewater expanded significantly (0.3 to 1.3%) and on the basis of supplementary tests on laboratory-produced concrete specimens it was concluded that expansion under such conditions is caused by DEF as the conditions of the test will not sustain ASR. In at least one column, DEF was diagnosed as the sole contributory cause of damage with no evidence of any contribution from ASR or any other deterioration process. In other cases, both ASR and DEF were observed to have contributed to the apparent damage. Of the columns examined, only concrete containing fly ash appeared to be undamaged. The results of this study confirm that, under certain conditions, the process of DEF (acting in isolation of other processes) can result in significant deterioration of cast-in-place reinforced concrete structures.     

Use of gasification residues in compacted concrete paving blocks

Research was done to determine the possible uses of fine-grained solid gasification residues in concrete products. Due to the residues' chemical composition, compacted concrete paving blocks were studied as one viable non-structural application of adding the residue. Six different residues were tested in the laboratory at up to 25% replacement by dry weight of either cement or aggregate. After these preliminary laboratory tests, one residue was used in full-scale field tests at the Lakan Betoni factory where 10 and 15% of straw-derived residue was used as a cement replacement. Tensile strength, compressive strength, resistance to freezing-thawing cycles, absorption and leaching were evaluated. The addition of the residue improved the workability of the concrete, provided a beneficial dark coloring and did not adversely affect most properties. The freeze-thaw resistance should be improved by reducing the factory compaction effort to ensure sufficient void space.     

Test methods for evaluating preventive measures for controlling expansion due to alkali-silica reaction in concrete

This paper provides a critical evaluation of the various methods available for testing the efficacy of measures for preventing expansion due to alkali-silica reaction (ASR) in concrete containing deleteriously reactive aggregate. The ideal test method should be rapid, reliable and capable of determining the influence of aggregate reactivity, alkali availability and exposure conditions. None of the currently available or commonly used methods meet all of these criteria. The shortcomings of the different test methods are discussed and suggestions are made for modifying the concrete prism test and accelerated mortar bar test to make these tests more acceptable.     

Porosity of recycled concrete with substitution of recycled concrete aggregate: An experimental study

In this paper, we present the experimental analysis of samples of recycled concrete (RC) with replacement of natural aggregate (NA) by recycled aggregate originating from concrete (RCA). The results of the tests of mechanical properties of RC were used for comparison with tests of mercury intrusion porosimetry (MIP), in which the distribution of the theoretical pore radius, critical pore ratio, the surface area of the concrete, threshold ratio and average pore radius were studied at ages of 7, 28 and 90 days. The results showed some variation in the properties of the RC with respect to ordinary concrete. Porosity increases considerably when NA is replaced by RCA. Additionally, a reduction in the mechanical properties of the RC is seen compared with ordinary concrete when porosity increases.     

The role of activating solution concentration on alkali–silica reaction in alkali-activated fly ash concrete

To enable commercial use of alkali-activated fly ash concrete, its durability must be better understood. Alkali–silica reaction is a primary concern since highly alkaline solutions are generally used for activation. This study investigated the effect of NaOH activating solution concentration on pore solution alkalinity and subsequent alkali–silica reaction in alkali-activated fly ash concrete. It was found that pore solution alkalinity increased with increasing activating solution NaOH concentration, and this effect was amplified at concentrations above an optimum, defined as the concentration that resulted in the highest mortar compressive strength. Expansion of concrete prisms containing highly reactive fine aggregate and activating solution concentrations above the optimum concentration was approximately three times that of concrete with optimum activating solution concentrations, but only about 5% of the expansion observed in the ordinary portland cement control. The low expansion may be attributed to the low calcium levels in the alkali-activated fly ash concrete.     

Microstructural analysis of recycled concrete using X-ray microtomography

This paper reports an investigation of the recycled concrete (RC) microstructure using synchrotron microtomography (μCT) at the Advanced Light Source combined with Scanning Electron Microscopy analysis. The study evaluated the influence of 50% of recycled concrete aggregate (RCA) and its water absorption compensation on the RC microstructure. The following variables were studied: a) the compressive strength of the original concrete used to obtain the RCA (40 and 80 MPa) and b) the initial moisture condition of the RCA (Saturated Surface Dry and Oven Dry). The microtomographic images showed the mixtures cast with RCA in the dry condition developed an evident macropore network surrounding the RCA particle that was not observed in the mixtures using RCA in the SSD condition. SEM images confirmed the initial findings from μCT and showed that the thickness of the interfacial transition zone in RC is in the same order of magnitude as the reference concrete.     

Measurements of strain and humidity within massive concrete cylinders related to the formation of ASR surface cracks

This study aims at proving the validity of a notion that the formation of non-expansive near-surface layer is responsible for surface cracking in ASR-affected concretes by a laboratory experiment. Relationship between the progress rate of the front of non-expansive layer toward inner portions and the formation of the first surface cracks was scrutinized by measuring relative humidity (R.H.) values and strains within a massive concrete cylinder (?450 mm × 900 mm) with reactive aggregates under a dry environment. It was presumed from the measurements that a non-expansive layer of about 40 mm had been formed at the first cracking. Thereafter, the environmental humidity was raised to > 95% R.H. Pursuit of the growth of surface cracks and subsequent measurements of strains and R.H. values within the concrete cylinder under the moist environment suggested that the re-saturation continuously gave rise to the generation of tensile stresses in near-surface regions leading to active extension of surface cracks.     

The current state of the accelerated concrete prism test

Expansions due to alkali-silica reaction (ASR) in the accelerated concrete prism test (ACPT-60 °C) show a significant reduction at 13 weeks compared to 52 week testing in the standard concrete prism test (CPT-38 °C). Previous work indicated that increased leaching, higher mass loss and a reduction in the pH were observed when temperature was increased from 38 to 60 °C. After further investigation the authors have revealed that non-reactive fine aggregate from certain sources combined with the same reactive coarse aggregate exhibited further reduction in expansion in the ACPT. Expansion data for a wide range of reactive coarse aggregates in 38 and 60 °C testing regimes is shown. Data investigating the Spratt reactive coarse aggregate combined with seven different non-reactive sands will be shown to demonstrate the dramatic effect of the non-reactive sand. Selected pore solution analyses will be given to further elucidate this issue.     

Chemo-mechanical modeling for prediction of alkali silica reaction (ASR) expansion

The effect of the size of the aggregate on ASR expansion has already been well illustrated. This paper presents a microscopic model to analyze the development of ASR expansion of mortars containing reactive aggregate of different sizes. The attack of the reactive silica by alkali was determined through the mass balance equation, which controls the diffusion mechanism in the aggregate and the fixation of the alkali in the ASR gels. The mechanical part of the model is based on the damage theory in order to assess the decrease of stiffness of the mortar due to cracking caused by ASR and to calculate the expansion of a Representative Elementary Volume (REV) of concrete. Parameters of the model were estimated by curve fitting the expansions of four experimental mortars. The paper shows that the decrease of expansion with the size of the aggregate and the increase of the expansion with the alkali content are reproduced by the model, which is able to predict the expansions of six other mortars containing two sizes of reactive aggregate and cast with two alkali contents.     

Use of perlite powder to suppress the alkali-silica reaction

Reported below are the results from a study aimed at mitigating the deleterious alkali-silica reaction by using perlite powder as an admixture. The expansion of mortar bars containing various amounts of silica fume (SF), expanded perlite, and natural perlite was studied. Two kinds of reactive aggregates were used in the study: highly reactive river aggregate containing opal and marginally reactive monzo-diorite aggregate. Expanded perlite and silica fume were tested with both aggregate, separately; on the other hand, natural perlite was tested only with monzo-diorite aggregate. The bars were cast in accordance with ASTM C1260, accelerated mortar bar method, and were stored in NaOH solution for 30 days. Length changes were measured and reported. The results showed that both expanded and natural perlite powder (NPP) have potential to suppress the deleterious alkali-silica expansion.