What is alkali silica reaction and how does it occur? How does it affect concrete durability? How can it be prevented? How is damage assessment conducted for alkali silica reaction? In this article, you will find everything you need to know about Alkali Silica Reaction.
What is Concrete Durability?
The ability of building materials and structures to fulfill their function for the duration of their service life without degradation is called durability. One of concrete’s properties is compressive strength, and increased compressive strength is positively correlated with durability. However, even a quality concrete element that provides the required strength in terms of load can degrade in a short period of time to the point of being useless if certain factors are ignored in the design stage. What’s more, it can lead to maintenance and repair costs. One of the parameters that can shorten the service life of concrete is Alkali Silica Reaction (ASR). In recent years, numerous studies have been conducted regarding Alkali Silica Reaction demonstrating the negative impact it has on concrete durability. So, what is Alkali-Silica Reaction?
What is Alkali-Silica Reaction?
The reaction between the reactive silica in the aggregates and the alkali components in cement and the alkalis in concrete pore solution is called alkali-silica reaction (ASR). Alkali silica gel is formed as a result of this chemical reaction. One characteristic of this gel is its ability to absorb a limitless amount of water. When alkali silica gels absorb large quantities of water, it causes significant expansion inside hardened concrete.
Figure 1. Formation of Alkali-Silica Reaction
How Does Alkali-Silica Reaction Affect Concrete?
Alkali silica reaction is a reaction that happens very slowly. The reaction takes months and even years. Cracks in the form of maps are observed in the concrete, which is exposed to the alkali silica reaction, after about 1 year. Moisture and temperature also affect how long alkali silica reaction takes to occur.
- These cracks have a negative effect on the strength and durability of concrete. They also compromise the aesthetic appearance of concrete.
- The cracks that form leave the concrete defenseless against freezing-thawing, sulfate attack, physical wear, etc., and in the long-term render the concrete unsuitable for use.
What Factors Impact Alkali-Silica Reaction?
Alkali silica reaction occurs when 4 factors occur simultaneously.
- The alkali content of the concrete
- Reactive aggregate
- Ambient temperature
Figure 2. Factors That Impact Alkali-Silica Reaction
1. Alkali Content of the Concrete
- The amount of alkali in cement is calculated with the formula (Na2O+0,658 K2O).
- When cements that contain 0.6% or more alkali are used with reactive silica aggregate, the result is formation of alkali-silica gel.
- Mineral admixtures, chemical admixtures, mixing water and thawing agents can increase the amount of alkali in the concrete.
2. Reactive Aggregate
- Alkali reactivity depends on the mineral content of aggregate with silica.
- Fine-grained silica have a large surface area in contact with alkali, and the result is greater reactivity.
- In general, aggregates that present a large surface area, with micro-cracks and micro-pores in its structure are more prone to reaction.
- Moisture in the concrete facilitates the reaction of alkali ions and allows the expansion of alkali silica reaction gel.
4. Ambient Temperature
- As with most chemical reactions, higher temperatures accelerate alkali-silica reaction.
How is Damage Assessment Conducted for Alkali Silica Reaction?
In the assessment of alkali silica reaction damage, the cracks that have formed in the concrete, surface residue, discoloration and disintegration of pieces on the surface of the concrete are taken into consideration. The gel that forms in the cracks of concrete undergoing reaction is generally transparent with a resin-type texture. The gels eventually become carbonated when they are exposed to air and turn white and dry.
The standard methods used to Assess Potential alkali silica reaction are:
- ASTM C1260*
- ASTM C227
- ASTM C289
- ASTM C1263
- Petrographic and Chemical Analyses are used.
*ASTM C1260 method provides results in 16 days and is a rapid analysis technique.
What Precautions Should be Taken to Prevent Alkali Silica Reaction?
- The first measure that must be taken to prevent the risk of alkali silica reaction is to be sure that the aggregates do not contain reactive silica. The potential alkali reactivity of aggregates is determined using standard testing procedures.
- If the aggregate contains reactive silica, the alkali ratio of the cement that will be used must be examined. Check to see whether the alkali value of the cement is less than 0.6%.
- Concrete permeability should be minimized by reducing the water/cement ratio, using mineral additives and implementing an appropriate curing process.
- If fly ash, blast furnace slag or mineral admixtures will be inputs, care must be taken to ensure that these additives do not increase the total alkali content.
- Protective coatings can be used with concretes that will be in contact with moisture. In this way, alkali entry is prevented.
Why You Should Pay Attention to Alkali Silica Reaction?
Alkali Silica Reaction negatively affects the durability of concrete and for this reason it presents a hazard if care is not taken. Controlling the amount of alkali in concrete, selecting appropriate aggregate and paying attention to the mixing ratios will prevent alkali silica reaction and ensure that the finished product is concrete with excellent durability.