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Alkali Aggregate Reaction Phenomenon

In: Miscellaneous

Submitted By Thanu57
Words 1684
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1.1 General background

“Studying the effect of deterioration on the performance of structures is important because much of the nation’s essential infrastructure is aging. The alkali-aggregate reaction phenomenon is the cause of considerable damage to concrete structures built during the late 1920s to early 1940s” (Osama, Mohamed, 2001). “Numbers of concrete structures in California, USA, was observed to develop severe cracking within a few years of their construction, although quite acceptable standards of construction and quality control of materials were employed. It was a major scientific achievement with far-reaching consequences when Stanton in 1940 was able to demonstrate the existence of alkali-aggregate reaction as an intrinsic deleterious process between the constituents of a concrete. The damage was the result of cracking throughout the concrete, manifested at the surface as extensive map cracking or pattern cracking or surface popouts and spalling. Such problems where confined mostly to certain regions of the country” (Osama, Mohamed et al., 2001). ASR was since been discovered in a number of concrete structures around the world, and has become truly a concrete durability problem with an international dimension.

Alkali Aggregate reaction (AAR) is a chemical reaction occurring between alkalis derived from Portland cement and certain aggregates which are susceptible to this reaction.

1.2 Types of reactions

AAR has been divided into two types of reaction depending on the types of minerals involved.
Alkali-Carbonate Reaction (ACR)
The Alkali-Carbonate Reaction is a chemical reaction between hydroxyl ions associated with the alkalis sodium and potassium in the cement and certain dolomitic textures in the aggregate resulting in expansion and eventually cracking of the hardened concrete.

Alkali-Silica Reaction (ASR)
It is of more concern because aggregates containing reactive silica materials are more common. In ASR, aggregates containing certain forms of silica will react with alkali hydroxide in concrete to form a gel that swells as it adsorbs water from the surrounding cement paste or the environment. These gels can swell and induce enough expansive pressure to damage concrete.
“Most of the structures severely cracked by AAR are exposed to the weather or underground in contact with damp soil. This is because for significant expansion to occur presence of moisture is essential. Apart from the moisture, High content of alkali in the concrete and reactive minerals are also essential.” (ACI Committee 221, 1998)
From the above it can be seen that AAR expansion to occur in concrete, it is necessary to have * sufficient moisture supply, * high content of alkali in concrete, and * pessimum amount of reactive minerals in aggregate.

“It is also found that, when there is sufficient moisture and alkali, maximum expansion of concrete due to AAR occurs when the content of reactive minerals in aggregate is within a sensitive region, some refer to this as "pessimum" content. Content of reactive minerals below or greater than the pessimum value, AAR expansion reduces” (Neville, 1990).

“Certain factors influence the behavior of concrete affected by AAR regardless of the type of rock or minerals. In all cases increases in expansion result from increases in available alkali, water is a requirement, and higher temperatures of exposure or accelerated curing increase expansion. However there are significant differences in behavior between different types of material. Concrete containing opal, crystobalite, tridymite and man made glasses, often expands relatively rapidly whereas the rate of expansion of concrete made with chalcedony, greywacke and related rocks is often very slow” (Gillott, 1995).

1.3 Tests methods
Numerous tests have been devised to ascertain whether a particular aggregate or concrete mix will develop alkali-silica reactivity. These tests centre on checks concerning either the aggregate alone or the concrete mix or a simplified variant of it.

Common Test Methods to assess ASR
• Petrographic Examination (ASTM C 295)
• Chemical Test (ASTM C289 – 03)
• Accelerated Mortar Bar Test (ASTM C 1260 - 05a)
• Concrete Prism Test (CPT)
• Accelerated Concrete Prism Test (ACPT)
Petrographic examination is the process of identifying the types of minerals in aggregate or concrete section by observation using microscope or other aids. This method can identify types of minerals in the aggregate and give suggestions as for whether the aggregate is potentially reactive or not. Because the uncertainties involved in the test, the method is generally used as a screening test as a part of an investigation.
Many proposed test methods using chemical analyses to identify potential reactivity of aggregate. ASTM C389-87 chemical test (ASTM 1987a), for example, evaluates aggregate reactivity by measuring the amount of dissolved silica and the reduction of alkalinity in the reaction alkali solution. (ACI Committee 221, 1998)
Accelerated Mortar Bar Test
The aggregate to be evaluated is crushed to sand size and mortar bars (25mm x 25mm x 285 mm) are made with it. After the mortar has set, the length of the bars is measured and the bars are stored, immersed in sodium hydroxide solution, at 80°C. The change in length of the bars is monitored periodically for 14 days. Expansions greater than 0.15% at 14 days are considered to be indicative of potentially deleterious expansion of the aggregate in concrete having high alkali content. This test is appropriate only for alkali-silica reactive aggregates. Although this mortar test does not exactly replicate how concrete will behave, the test is a good indicator of potential reactivity and has the great advantage of being quick.
Concrete Prism Test
The concrete prism test is applicable to both alkali-silica and alkali-carbonate reactive aggregates. The coarse aggregate is made into concrete prisms (75mm x 75mm x 275 mm). After de-molding, the length of the prisms is measured. They are then placed in sealed containers over water to maintain a humidity of over 95%. The containers are stored in an enclosure maintained at 38°C and the length of the prisms is monitored periodically for one year. Expansion greater that 0.04% at one year considered being indicative of potentially reactive aggregates.
“It should be noted that the methods that have been proposed so far have their limitations. Some succeeds in identifying reactivity for certain aggregates whereas fails for others. Therefore it is difficult to ascertain an aggregate is absolutely non-reactive using the currently available testing methods”
“The most evident manifestations of deleterious AAR in a concrete structure are map cracking, displacement of structural member due to internal expansion of the concrete, and popouts. However, these features should not be used as the only indicators in the diagnosis of AAR in the concrete structure. Cracking in concrete is essentially the result of the presence of excessive tensile stress within the concrete” (ACI Committee 221, 1998)
“The external appearance of the crack pattern in a concrete member is closely related to the stress distribution within the concrete. The distribution of strain is, among other things controlled by the location and type of reinforcement, and the structural load imposed upon the concrete. Expansion of a concrete element will tend to occur in the direction of least restraint. Cracks caused by the expansion due to AAR tend to align parallel to the direction of maximum restraint” (ACI Committee 221, 1998)
Controlling the alkali-aggregate reaction, researches tend to pay attention towards the prediction and precautionary measures. So it is necessary to develop a method that can be used for predicting the effect of alkali-aggregate reaction in an easily available way. In order to fulfill this method, we are proposing general purpose finite element package SAP2000 and computer language to predict the expansion of alkali-aggregate reaction.
1.4 Remedies

Minimizing the chances of AAR in new structures
“There are several preventative actions which will reduce the risk of occurrence of all three varieties of AAR. Most of the actions require detailed knowledge of the performance of either the cements or aggregates proposed to be used. The most recommended action is the inclusion of fly ash, so that if a potential for reaction does exist, the fly ash will mitigate its effects” ( Barry Butler,1998).
This reaction can be controlled by: * Avoiding susceptible aggregates. Local experience may show that certain types of rock contain reactive silica. Typically rock types that may be susceptible are: siliceous limestone, chert, shale, volcanic glass, synthetic glass, sandstone, opaline rocks and quartzite. River rock is also typically susceptible. * Pozzolanic admixture. By reacting with the calcium hydroxide in the cement paste, a pozzolan can lower the pH of the pore solution. Additionally, the silica contained in a pozzolan may react with the alkali in the cement. This reaction is not harmful because it essentially skips the expansive water attraction step. * Low-alkali cement. Less alkali available for reaction will limit gel formation. * Low water-cement ratio. The lower the water-cement ratio, the less permeable the concrete. Low permeability will help limit the supply of water to the alkali-silica gel.

Remedial measures in existing structures
“Remedial measures varied depending on the degree of distress and the function of the concrete. Damage may be so severe that complete replacement of the concrete is the only solution. If the expansion rate is declining and the structural effects appear small, repair may be attempted by the partial replacement of concrete, installation of improved reinforcement including steel plate adhesion, injection of resin, or cementitious material to decrease permeability, and the use of waterproof surface coatings and cutting of transverse slot by means of a diamond wire. Opening joints or cutting slots by sawing to relieve internal stresses has been the most common remedial action to keep mass-concrete structures in service. In this process, a continuous loop of wire embedded with cutting diamonds makes the desired cut.” (Gillott, 1995)

1.5 Aim
This project is aimed at developing a SAP based finite element procedure to investigate the potential of Alkali Aggregate reaction in concrete and use it to investigate the potential of AAR in a 3-D concrete structure.

1.6 Scope
Scope of the project is to predict the development of stress and deformation due to Alkali Aggregate reaction in 3-D structures by using general purpose finite element package “SAP 2000”.…...

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