What is Alkali silica reactivity
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 Alkali silica Reactivity 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 Alkali silica reactivity or 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 Alkali silica Reactivity 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 Reactivity is diagnosed primarily by four main features
1. Presence of alkali silica reactive aggregates
2. Crack pattern
3. Presence of alkali silica gel in cracks and/or voids
4. 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 Alkali silica Reactivity 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.
Alkali silica Reactivity 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.
Alkali silica Reactivity 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.
