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Alkali-silica reactivity (ASR), a pervasive durability problem that occurs in portland cement concrete, is responsible for the premature deterioration of various types of concrete structures in the United States and around the world. While lithium compounds have been recognized for more than 50 years as being effective in preventing concrete expansion due to ASR, there has been increased interest in recent years in using them to both treat existing structures and as a preventive measure in new concrete construction.
The best way to avoid ASR in new concrete is to take precautions in the mix design. These include testing aggregates for reactivity; use of low-alkali cements, suitable pozzolans like ASTM C-618 Class F fly ash, and lithium-based admixtures.
How lithium inhibits ASR?
Lithium nitrate reaction with reactive silica and moisture is similar to other alkalies such as sodium and potassium. Unlike ASR, however, the gel formed by Lithium Nitrate with alkalis does not absorb excess moisture, preventing harmful expansion.
From the industry's point-of-view, use of an ASR inhibitor not only better serves clients, but also can protect from potential legal action that could arise if ASR is not mitigated when state-of-the-art remedies are available.
The cracks radiate from the interior of the aggregate out into the surrounding paste.  The cracks are empty (not gel-filled) when formed. Small or large amounts of gel may subsequently exude into the cracks.  Formation of the alkali silica gel does not cause expansion of the aggregate by itself. Observation of gel in concrete is therefore no indication that the aggregate or concrete will crack. Only moisture can cause the expansion and resultant cracking

Alkali silica reaction is diagnosed primarily by four main features

  • Presence of alkali silica reactive aggregates

  • Crack pattern

  • Presence of alkali silica gel in cracks and/or voids

  • Ca(OH)2 depleted paste

From this brief overview of the literature on the mechanisms, the following
summarizes present knowledge on ASR:
  • ASR is a reaction between the OH in the pore solution with amorphous or poorly
    crystallized silica in the aggregates.
  • The reaction product imbibes water and expands.
  • The presence of water or RH higher than 80’% is necessary for the gel formed to
    expand and induce concrete cracking.
  • Some siliceous mineral admixtures deplete the alkalis horn the pore solution, lowering
    the pH, therefore decreasing the likelihood of ASR.
  • The aggregate type and size distribution play a significant role in the expansion
    measured in concretes
  • Other factors influencing the cracking due to ASR include air entrainment and possibly
    WlC.
ASR-induced cracking can be confused with other forms of cracking. As a result, inspectors sometimes misdiagnose the problem and then apply rehabilitation techniques that may actually make the ASR problem worse. Inspectors and engineers therefore need better, more accurate tools for identifying ASR in existing concrete structures and for deciding on the best treatment for existing and current structures. Several such tools were developed under the Strategic Highway Research Program (SHRP):

Handbook for the Identification of Alkali-Silica Reactivity in Highway Structures, an easy-to-use field guide for distinguishing ASR-related cracking in various types of concrete roads and structures.

• A fast and simple test for detecting the presence of ASR in concrete.

• A fast and reliable means of determining an aggregate’s potential reactivity to alkalis.

Eliminating or Minimizing Alkali-Silica Reactivity, a report that identifies available options for alleviating ASR damage in concrete pavement and structures.

ASR expansion can be reduced to acceptable levels by use of Type F fly ash and by use of lithium nitrate additive in accordance with the manufacturer’s recommendations.
The only indisputable evidence that ASR has developed in concrete is the presence of ASR gel reaction products. In the early stages of reactivity, or under conditions where only small quantities are produced, ASR gel is virtually undetectable by the unaided eye, and revealed only with difficulty by a skillful observer using a microscope. Thus, ASR may go unrecognized in field structures for some period of time, possibly years, before associated severe distress develops to force its recognition and structure rehabilitation. Use of uranyl (uranium) acetate fluorescence method has been developed. This method can be used to monitor possible ASR prior to development of serious distress and to confirm ASR existence.

ASR is uniquely characterized by production of a gel-like reaction product. It is composed of essentially of silica, the alkalis (sodium and potassium), and calcium in the presence of water. Uptake of water by the gel is the primary factor determining volume changes associated with ASR. The gel may be present in large or minute amounts in aggregates, aggregate sockets, air voids, fractures, and on the surfaces of externally formed concretes. By application of uranyl acetate solution to a surface containing the gel, the uranyl ion substitutes for alkali in the gel, thereby imparting a characteristic yellowish-green glow when viewed in the dark using short wavelength (254 nanometer) ultraviolet light. ASR gel fluoresces much more brightly than the cement paste due to the greater concentration of alkali and, therefore, the uranyl ion in the gel.

The presence of ASR gel will be revealed in UV light by a yellowish-green fluorescent glow. Deposits will be localized in cracks, air voids, certain aggregate particles and, in severe cases, as broad films in aggregate particles and fractured surfaces. Such films on sawed and cored surfaces may reflect as "smear" from sawing or cutting. Fractured surfaces eliminate this effect and most clearly reveal undisturbed ASR gel deposits.

 

Damage due to alkali-silica reaction (ASR) in concrete is a phenomenon that was first recognized in the U.S. since 1940 and has since been observed in many countries. Despite numerous studies published, the mechanism is not yet clearly understood. Nevertheless, the three major factors in concrete have been identified, i.e., the alkalies contained in the pore solution, reactive amorphous or poorly crystallized silica present in certain aggregates, and water. It was found that air content is the most important variable (other than the three majors factors cited above) that increase expansion of concretes affected by ASR.
2.1. ASR Mechanisms
Most researchers agree that the main reaction of ASR is the reaction between certain forms of silica present in the aggregates and the hydroxide ions ( OH) in the pore water of a concrete. Very early in the hydration of cement calcium ions are incorporated in the hydration products but potassium and sodium stay in solution and
eventually they are partially incorporated into calcium silicate hydrate (C-S-H) and monosulfate (AFJ). Hydroxide ions from the hydration of portland cement result in a pore solution having a pH of at least 12.5. Soluble alkalies raise the pH to about 13 or higher. Also, the amount of alkalis present in the pore water is related to the amount of soluble alkalis present in the cement. If the silica is well crystallized the vulnerable sites are only at the exterior surface of the aggregate (Figure 1a), but in the case of poorly crystallized silica, there are many vulnerable sites in the aggregate structure, leading to disintegration of the silicate network. To keep a neutral charge balance, the cations Na+ and K+ diffuse toward the hydroxide ions to react with them and the resulting product is a gel-like material.  The migration of cations of Na+ and K+ is slow, therefore the migration of Ca2+ takes place. If the gel is high in calcium then the gel is not expansive when exposed to water and, therefore, may not induce cracking in concrete. This theory rests on the assumption that calcium could be available. Diamond found that there is very little calcium in the pore solution. This is expected since the high pH causes the volubility of Ca(OH)2 to be depressed. Nevertheless, calcium could be dissolved from the solid phase of cement paste to produce a gel. Most researchers do not mention the distinction between “safe” and “swelling” gel but there are acknowledgments that there are more than one composition of gel produced by ASR.

The formation of the gel per se is not deleterious. The deterioration of the concrete structure is due to the water absorption by the gel and its expansion. The RH must be higher than 80% for the gel to swell although it can be formed at lower relative humidity. According to Hobbs the progression of the swelling of ASR gel follows the general patterns. As the tensile strength of the system is exceeded, cracks will form and propagate. As there is not a preferential direction for cracks to propagate and also the sites of crack initiation are randomly distributed in the specimen, map cracking will be characteristic of ASR deterioratio. The sites of
the cracks are determined by the location of the reacting silica on the aggregates and the availability of OH in the vicinity.
Aggregates exhibiting this type of reactivity contain various forms of reactive silica. For convenience, CSA Standard A23.1-Appendix B, divides alkali-silica reaction into two categories according to the type of reactive silica involved.

1. Alkali-silica reaction that occurs with poorly crystalline or metastable silica minerals and volcanic or artificial glasses: Aggregates containing such materials (see Table 5-7 and CSA A23.1-Appendix B) may cause deterioration of concrete when the reactive component is present in amounts as small as 1%. Cracking of concrete containing these aggregates and a high alkali content is usually seen within 10 years of construction.
2. Alkali-silica reaction that occurs with various varieties of quartz such as chalcedony, cryptocrystalline, and macrogranular quartz: Aggregates containing such forms of quartz may cause deterioration of the concrete when the reactive component is present in amounts as small as 5% by mass of the aggregate and there is a high alkali content. Cracking of the concrete may be seen within 10 years of construction. Canadian experience has been that this category also includes several slowly expanding aggregates in which micro-crystalline quartz is thought to be the reactive component. Rocks such as greywacke, argillite, quartz-wacke, quartzite, hornfels, granite and granite gneiss are some. See Table 5-7 and CSA A23.1-Appendix B for a more complete list. Such rock types may not show cracking and deterioration for up to 20 years. In other cases however, particularly when exposed to deicing salts, cracking may occur in 5 years or less.

Visual Symptoms of Expansive ASR. Typical indicators of ASR might be any of the following: a network of cracks ( Fig.5-20); closed or spalled joints; relative displacements of different parts of a structure; or fragments breaking out of the surface of the concrete (popouts) ( Fig. 5-21). Because ASR deterioration is slow, the risk of catastrophic failure is low. However, ASR can cause serviceability problems and can exacerbate other deterioration mechanisms such as those that occur in frost, deicer, or sulphate exposures.

Mechanism of ASR. The alkali-silica reaction forms a gel that swells as it draws water from the surrounding cement paste. Reaction products from ASR have a great affinity for moisture. In absorbing water, these gels can induce pressure, expansion, and cracking of the aggregate and surrounding paste. The reaction can be visualized as a two-step process:

1. Alkali hydroxide + reactive silica gel -> reaction product (alkali-silica gel)
2. Gel reaction product + moisture -> expansion

The amount of gel formed in the concrete depends on the amount and type of silica and alkali hydroxide concentration. The presence of gel does not always coincide with distress, and thus, gel presence does not necessarily indicate destructive ASR.

Factors Affecting ASR. For alkali-silica reaction to occur, the following three conditions must be present:

1. reactive forms of silica in the aggregate,
2. high-alkali (pH) pore solution, and
3. sufficient moisture.

If one of these conditions is absent, ASR cannot occur.

Examples of reactive aggregates are chalcedony, porous flint and some types of sandstone. The reaction leads to the formation of a gel that has a tendency to absorb increasing amounts of water, which causes swelling. The swelling forces the concrete to expand, resulting in gel surfacing and loss of concrete integrity.
  ASR