Causes, Identification and Prevention of Alkali-Silica ReactionMay 31, 2017
Alkali Silica Reaction (ASR) is a cause of cracking and serious expansion in concrete. It is a rare occurrence but understanding how to identify the problem can help avoid it in the first place and save having to seal or replace concrete that has already been poured. Sourcing your concrete from an experienced supplier with years of concrete mixing behind them is the best place to start when preventing ASR. Let’s take a more in depth look at the issue.
ASR is the most common form of Alkali Aggregate Reaction (AAR) and is caused by a reaction between hydroxyl ions in the cement solution and reactive silicas in the aggregate, such as chert, quartzite, opal and strained quartz crystal. These silicas are only present in high volume in substandard aggregates.
When the hydroxyl ions and silicas react they form a calcium alkali silicate gel within the microscopic cracks in the concrete structure. This gel increases in volume by absorbing water and expands the internal cracks. In severe cases, the resulting pressure can lead to failure of the concrete.
Professionally mixed concrete uses high quality ingredients to avoid ASR and professionals understand the conditions that contribute to its occurrence. Conditions that cause ASR are:
- A high alkali content in the cement
- Presence of a reactive aggregate, such as chert
- Excess water within the curing concrete
Generally, identification of ASR by the naked eye is possible when the concrete is between 5-10 years old. The cracks that form on the concrete surface are referred to as ‘map pattern cracking’ due to its resemblance to a road network marked on a map. The map pattern cracking is most obvious in concrete that is not reinforced with steel rebar.
A more scientific route to identification involves examining the concrete using Scanning Electron Microscopy (SEM). After SEM, the concrete is analysed by X-ray microanalysis in a laboratory and this confirms the presence of ASR gel within the interior of the concrete structure. A certain diagnosis of ASR can only be made under lab conditions with the correct equipment.
Using a trusted concrete mixing company with years of experience will mitigate the risks of ASR. Their proven track record for quality pours of concrete demonstrate their chosen ingredients, specifically the quality aggregates, make for concrete that will last for many decades.
It is always important to choose an established concrete supplier who uses professional mixing equipment. A concrete mixing plant that is computer controlled will always produce superior concrete to alternative hand-mixed batches, and this always results in a pour that is unlikely to suffer from structural problems. The best prevention for ASR, therefore, is to pick a concrete supplier with accreditation from industry standards agencies, such as the British Kitemark.
EKA Concrete supply high quality concrete mixes throughout Brighton, Crawley, Horsham and Reigate. With over 20 years of experience delivering concrete to domestic and commercial sites, we assure you of top tier service and durable concrete mixes for any application. From foundation mixes to screed, our concrete is certified by British Standards Kitemark for your peace of mind. With our fleet of vehicles and concrete pumps, we can access hard-to-reach sites with speed and accuracy, and offer onsite mixing for exactly the correct amount of concrete: no over-ordering charges guaranteed. For larger batches, use our handy concrete calculator to the left of this page. For more information, contact our team today, they’re happy to help.This entry was posted in concrete tips. Bookmark the permalink. ← Top tips for laying a shed base Types of concrete finishes and how to achieve them →