Alkali Silica Gel Research Articles

Microscopic analysis of alkali–aggregate reaction products in a 50-year-old concrete

Fifty-year-old concrete from a large dam was examined in the scope of an investigation program concerning the properties of granite as aggregate material for concrete. Site inspection, which was developed in order to detect possible signs of deterioration of the concrete, revealed the existence of efflorescence and exudations. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses were attempted to identify the composition of these materials and their morphology. From the analyses, it was concluded that some of the exudations were composed by alkali–silica gel. In these samples, an interesting behavior was observed in different moments after a 3-month interval. It was noticed that the initially noncrystalline alkali–silica gel transformed into sodium-rich needles and tablets after a few months kept in a desiccator in the laboratory. Therefore, it was concluded that the materials identified corresponded to different stages of evolution of an alkali–aggregate reaction product.

Alkali reactivity of mortars containing chert and incorporating moderate-calcium fly ash

This paper reports the results of an experimental program, which aimed to investigate the alkali reactivity of chert and the effect of a moderate-calcium fly ash on the alkali–silica reaction. To determine the expansions, mortar bars were cast and tested in accordance with ASTM C1260. Mortar aggregate was replaced by chert, in controlled amounts, to find out the pessimum limit, if any. To evaluate the degree of cracking, sonic pulse velocity measurements and petrographic analysis were carried out on the cracked bars and on the thin sections taken from these bars, respectively. In the next series of tests, limestone and chert were blended together as mortar aggregate and cement was replaced by different dosages of fly ash to examine the changes in the mortar bar expansion as well as in the chemistry of reaction products. Microstructural observations were done on polished sections using a scanning electron microscope, equipped with energy dispersive X-ray analysis. The results showed that the chert used in this investigation had a pessimum proportion in the range of 5–15%. Sufficient fly-ash additions suppressed the expansion caused by chert. The study also revealed out that as the CaO/Na 2O eq of alkali–silica gel increased, the expansivity of the gel decreased.

Electrochemical chloride extraction: efficiency and side effects

Some specimens of reinforced concrete cast with an alkali-resistant aggregate, previously maintained in a solution of NaCl, were subjected to an electrochemical chloride extraction (ECE). The chloride profiles before and after treatment were determined. Likewise, alkali ions profiles before and after treatment were determined. After treatment, some specimens were stored in a controlled atmosphere (60 °C and 100% RH) in order to accelerate the alkali–silica reaction, if any. Results of chloride content after treatment show that about 40% of the initial chloride is removed within 7 weeks. About one-half of the chloride close to steel was removed, but at the same time, significant amounts of alkali ions were observed around the steel. Microstructural observations by scanning electron microscopy (SEM) showed that after treatment, new cementitious phases containing higher concentrations of sodium, aluminum and potassium were formed. Moreover, alkali–silica gel was observed in the specimens stored at 60 °C and 100% RH. It may be possible that the ECE accumulates locally high amounts of alkali ions that stimulate the alkali–silica reaction even though the concrete contained nominally inert siliceous aggregates. The specimen expansions were not recorded, but no cracks were observed.

The effects of potassium and rubidium hydroxide on the alkali–silica reaction

Expansion of mortar specimens prepared with an aggregate of mylonite from the Santa Rosa mylonite zone in southern California was studied to investigate the effect of different alkali ions on the alkali–silica reaction in concrete. The expansion tests indicate that mortar has a greater expansion when subjected to a sodium hydroxide bath than in a sodium–potassium–rubidium hydroxide bath. Electron probe microanalysis (EPMA) of mortar bars at early ages show that rubidium ions, used as tracer, were present throughout the sample by the third day of exposure. The analysis also shows a high concentration of rubidium in silica gel from mortar bars exposed to bath solutions containing rubidium. The results suggest that expansion of mortar bars using ASTM C 1260 does not depend on the diffusion of alkali ions. The results indicate that the expansion of alkali–silica gel depends on the type of alkali ions present. Alkali–silica gel containing rubidium shows a lower concentration of calcium, suggesting competition for the same sites.

Alkali-Silica Reaction—Possible Cause of Map Cracking in Gunite

In this case study, a number of gunite swimming pools exhibited extensive map or alligator cracking that caused rough and uneven surfaces. The nature of the cracking pointed to alkali-silica reaction (ASR) as the cause of the cracking. Four test methods, the uranyl acetate method, petrographic examination, chemical analysis by electron microprobe, and X-ray diffraction, were used to investigate the presence of ASR within the gunite. Results showed (1) some evidence of ASR gel within the gunite using uranyl acetate; (2) the potential existence of alkali silica gel throughout the cement and within the fractures of the sand grains based on petrographic examination; (3) elevated levels of total alkali, indicating a potential for ASR to occur based on chemical analysis; and (4) tiny inclusions of thermonatrite (commonly found in ASR gel) based on X-ray diffraction. Results from each of the four test methods suggest, albeit inconclusively, the existence of ASR in the gunite. Additionally, the results of all of the four test methods did not contradict each other. Accordingly, it can be stated with some certainty that the distress in the gunite was due to ASR.

Alkali–aggregate reaction in concrete containing high-alkali cement and granite aggregate

The paper discusses results of the research into the influence of high-alkali Portland cement on granite aggregate. The deformation of the concrete structure occurred after 18 months. The research was carried out by means of a scanning electron microscope equipped with a high-energy dispersive X-ray analyzer that allowed observation of unpolished sections of concrete bars exhibiting the cracking pattern typical of the alkali–silica reaction. Both the microscopic observation and the X-ray elemental analysis confirm the presence of alkali–silica gel and secondary ettringite in the cracks.

Resistance of alkali-activated slag concrete to alkali–aggregate reaction

This paper presents an investigation into durability of alkali-activated slag (AAS) concrete exposed to alkali–aggregate reaction (AAR). In the concrete prism test method (ASTM C 1293), reactive aggregates introduced in the AAS matrix reacted with alkali–silica gel formed around aggregates. The expansion of AAS concrete prisms was 0.04% after 50 days and 0.1% after 22 months. The expansion in OPC concrete was about 0.03% after 22 months of experiment. It was found that AAS concrete had lower resistance to alkali–aggregate attack than that of OPC concrete of similar grade.

Delayed Ettringite Formation at Brewer Stadium, Boone, North Carolina

Abstract Brewer Stadium, Appalachian State University, Boone, NC, was constructed in 1962 using steam-cured precast lightweight concrete channel sections for the bleacher seats. Spalling of the bleachers was observed within a few years. By 1995, approximately 375 of 860 precast elements exhibited sufficiently severe cracking and delamination that reconstruction of the stadium was the most economical alternative. The destructive process causing the cracking and delamination of the 375 units is DEF. The (fibrous, crystalline) ettringite occurs in the concrete intergrown in the paste, in veins and cracks throughout the mortar, filling voids in the mortar, and as rims in the paste around coarse- and fine-aggregate particles. Also observed, in the same spatial relationships as the crystalline ettringite, and commonly intergrown with it, was a clear, gel-like material with low-order, mottled birefringence. This appears to be the same proto-ettringite material as described by Mielenz et al. (1995). Visibly undistressed members exhibit incipient forms of the same DEF features that are well developed in the distressed members. Sulfate analyses show no difference between distressed and undistressed members. The physical and chemical circumstances of the failure conform to the model presented by Heinz and Ludwig, whereby cements capable of promoting DEF meet the sulfate limitations of ASTM Specification for Portland Cement (C 150), and DEF occurs due to excessive temperature reached during heat curing of concretes made with cements of a particular sulfate to alumina ratio. These results support the view that revisions to current standards for portland cement are unnecessary, and that what is necessary is adherence to maximum temperature guidelines during production of heat-cured concrete products. Petrographic evidence supports the view that expansion due to DEF is caused by swelling of a proto-ettringite gel—by water absorption, similar to alkali-silica gel—that forms in the paste and that later, following expansion and crack formation, crystallizes in the cracks and peripheral separations, forming the familiar veins and rims of fibrous ettringite. This mechanism speaks to the origin of the expansive force in the general case of sulfate attack, and to why the formation of the delicate, fibrous, ettringite crystals in the cracks is not the mechanism that exerted sufficient force to cause the cracks in which the ettringite occurs.

Concrete Capillary Porosity as It Relates to Durability of Pavements Built with Alkali-Silica-Reactive Aggregates

Several concrete pavements have been constructed in southwestern Minnesota using aggregates that recently have tested as being potentially deleteriously alkali-silica reactive. Standard ASTM test procedures were used. Most of these pavements fail to meet their design life. However, the rate of deterioration varies. Six pavement sites varying in age and performance were selected for comparison. Alkali-silica gel is present in all six pavements, even in the 36-year-old pavement still in good condition. Similar aggregate types were used for all six pavements. The coarse aggregate was a low-grade metamorphic quartzite, and the fine aggregate was a glacial sand obtained from various local sources. The relative capillary porosity of the concrete paste was examined using fluorescent dye epoxy impregnated with thin-section samples from each pavement site. Preliminary results suggest that the capillary porosity is linked to the durability of the concrete pavement. The best-performing concretes had a homogeneous capillary porosity that was moderate to low, and the poorer-performing concretes had paste with a capillary porosity that varied from high to low.

Quantity of Alkali Silica Gel and Its Effect on Concrete Properties

A quantitative gel fluorescence technique was used to investigate relationships between the extent of alkali silica reactivity (ASR) and properties of concrete cores of varying ages taken from over 20 bridges or pavements in North Carolina. The cores had varying degrees of ASR and were composed of a wide variety of aggregates and cementitious materials. The technique, combining image processing and analysis with a Strategic Highway Research Program qualitative gel fluorescence method, is described and results are discussed. The effects of lamp intensity, thresholding, video averaging, and color based discrimination to differentiate fluorescing areas are reviewed. Preliminary statistical analysis of the technique is presented. While no useful correlation was found between the mechanical properties of cores and the apparent gel content, a strong correlation was found between the percent area of fluorescing material and the log10 expansion of cores after immersion in an alkaline solution for 90 days.

On the mechanical behaviour of concrete subjected to alkali-aggregate reaction

A simple continuum theory is presented for the description of mechanical effects of alkali-aggregate reaction in concrete. The formation of alkali-silica gel leads to a progressive expansion of the material. The rate of expansion depends, in general, on the chemical composition of concrete mix, in particular on the available alkali content of the cement matrix. The progress in the reaction is assumed to be coupled with the degradation of mechanical properties, the latter described within the framework of elastoplasticity. The formulation is illustrated by some numerical examples evaluating the influence of the reaction on the mechanical characteristics of the material.

Composition of alkali silica gel and ettringite in concrete railroad ties: SEM-EDX and X-ray diffraction analyses

Alkali silica gel and ettringite observed in concrete railroad ties were analysed by SEM-EDX and X-ray diffraction. The compositions, represented in the Si CaNa + K system, of clear gel were in the range of 53 to 63 atomic% Si and 20 to 30 atomic% Ca. The K + Na contents were constant and about 15% (excluding oxygen, water and trace elements). Analyses of crystallized gel showed a constant ratio between Si, Ca and Na + K of 70: 15: 15. Analyses of the Si and Ca content, of crystallized gel in cracks inside the aggregate particles, and clear gel at the mouth of the cracks and in air voids revealed a compositional difference with the crystallized gel as higher Si and lower in Ca. The content of Si decreases and Ca increases in gel away from the aggregate particles. The gel compositions are in the range published in the literature. The composition of ettringite is close to stoichiometrical values for ettringite, however, slightly lower in sulfur. The X-ray diffraction analyses identified the ettringite as crystalline.

A simple treatment to distinguish alkali—silica gel from delayed ettringite formations in concrete

Alkali-silica gels or formations of late-developing Sample preparation and visual ettringite (delayed ettringite formation (DEF)) that fill examination sockets around aggregate particles may sometimes be observed by an optical microscope on unpolished or polished sawn concrete surfaces only as a ‘gel’ or as dark circles around the aggregate particles. They may thus be indistinguishable from each other. It has been found that the preparation of an unpolished or polished sawn concrete surface by carbon deposition (for SEM work) allows alkali-silica gel reaction and DEF to be distinguished by the nuked eye. However, X-ray elemental analysis recommended to confirm the visual observation.

Development of alkali silica reactions in concrete structures with time

This technical note considers the effects of the geometrical dimensions of a concrete structure on the development of alkali silica reactions (ASR) within it. It is known from many examinations of concrete structural elements, suffering from ASR, that damage begins only after a certain time of exposure, the time of initiation. When started, damage increases to a maximum level during the development period. Finally, a rest period is reached, where nothing more happens, except perhaps for some self-healing of cracks. The author outlines a hypothesis, involving alkali silica gel, to explain how this process develops. To examine this hypothesis, information was collected from the literature about observations and measurements on various specimens, ranging from mortar bars to gravitation dams. These specimens had different types of aggregate, and were exposed to different climates. After an Arrhenius correction to 20 degrees Celsius, the results were plotted on a log-log chart. The plot shows a large scatter, because no corrections were made for other relevant factors. Nevertheless, the results do not contradict the hypothesis. A procedure is proposed for predicting when a specific structure, containing ASR aggregate, will require repair.

Undispersed Granulated Silica Fume in Concrete - Chemical System and Durability Problems

AbstractGranulated condensed silica fume is not easily dispersed. In contact with the pore fluids of the cement paste the granulates rapidly turn into alkali-silica gel nodules. With time this gel absorbs Ca and at least the rim of the AS-nodules transforms into a C-S-H product. The gel by itself does not cause expansion as it forms before the cement paste hardens. However, if exposed to alkali salts the AS-nodules may become further enriched in alkalies, which in turn may trigger an expansive reaction and cracking.

Simultaneous presence of alkali — silica gel and ettringite in concrete

Simultaneous presence of alkali—silica gel and ettringite in concrete has been reported by several authors. However, the proposed interpretations of this phenomenon, and its relationship to concrete durability, differ. Based on field experience and experimental research data, it is shown that the two phenomena are independent and can be experimentally distinguished.

Detection of ASR in opened fractures of damaged concrete

Indisputable evidence of alkali-silica reaction (ASR) in field concrete depends on positive identification of alkali-silica gel, the reaction product of ASR. Experimental results are given on the unambiguous detection of gel by microscopic techniques using fractured concrete surfaces.

MICROSCOPIC OBSERVATION OF THE PRODUCTS OF THE ALKALI-SILICA REACTION IN THE DYED THIN SECTIONS OF CONCRETE CORES

Dyeing the products of the alkali-aggregate reaction made it possible to distinguish them from the components of the concrete under a microscopy. Cyanoacrylate mixed with a coloring substance was used to dye the alkali-silica gel red.Cyanoacrylate reacts readily with water contained in gel but not with the components of the concrete. Examination of dyed thin sections revealed a few concrete cores with expansion of alkali-silica reactive aggregates. This method appears to be useful for observing the alkali-aggregate reaction.

A simple staining technique for the identification of alkali-silica gel in concrete and aggregate

A simple technique is described for staining alkali-silica gel reaction product formed as a result of the interaction between alkali solution and siliceous concrete aggregate particles. The method involves allowing the developing gel to take up copper ions from a solution of cuprammonium sulphate. After appropriate washing the gel remains stained blue. Alternatively, the gel may be dissolved in hydrochloric acid and the copper content, which is proportional to the volume of gel, estimated colorimetrically by neutralising the acid and reforming the blue cupprammonium complex under standard conditions.

Porosity/Diffusivity And Alkali-Silica Reaction

AbstractPhysical and chemical processes are responsible for the alkali-silica reaction in concrete. Physical processes include (i) migration of hydroxyl and alkali ions (ii) swelling by water imbibition of the alkali-silica gel produced, (iii) development of cracks. Chemical processes involve (i) neutralisation of acidic silanol groups and attack of siloxane bridges, both by OH- ions (ii) formation of an alkali silicate. These reactions result in a breakdown in the silica structure and diffusion of some silicate and impregnate the cement paste filling the capillary pores. The increase in permeability due to the opening and progression of cracks facilitates secondary chemical deteriorations like the formation of ettrinqite, carbonation and corrosion of steel reinforcement. Recent studies have emphasized the role of Ca(OH)2 preventing the dissolution or miqration of freed silica out of the structure. Only the removal of Ca(OH)2 is able to stop the expansion of concrete.The paper will present some new interpretations of alkali-silica reaction related to the diffusivity of various ions from the pore solution.