42907 Chap.9 Alkali silica reaction and its prevention
Alkali silica reaction and its prevention
Week 7 covers a reaction that needs three conditions simultaneously, a reactive form of silica in the aggregate, enough alkali in the pore solution and enough moisture, and that cannot be stopped once it has started.
The chapter follows gel formation through its three steps, sets out the field signs led by map cracking, and compares the two Australian test methods with their numeric limits, including the hornfels that is slowly reactive on one and non reactive on the other. Mitigation runs through alkali limits and the recommended supplementary binder levels.
What this chapter covers
- 01
Two reaction families, silica and carbonate, and why only the first matters in Australia
- 02
The three necessary conditions, and that they must occur simultaneously
- 03
Reactive forms of silica against well crystallised quartz
- 04
The pessimum content: why more reactive aggregate does not always mean more expansion
- 05
Alkali migration under wetting and drying, and local reaction at low overall alkali content
- 06
Gel formation in three steps, and a gel that imbibes water and swells
- 07
Six field signs, led by map cracking, and what restraint does to the pattern
- 08
Why petrography can never clear an aggregate
- 09
The accelerated mortar bar test: one molar sodium hydroxide at 80 degrees, and its expansion limits
- 10
The concrete prism test: a year at 38 degrees, and its own limit on its own basis
- 11
Greywacke, hornfels and basalt, and the aggregate that changes class between the two tests
- 12
Field exposure as the benchmark, and why laboratory limits keep being revised
- 13
Five mitigation levers, and the two a designer can actually pull
- 14
The cement alkali limit, the concrete alkali recommendation and the sodium oxide equivalent
- 15
Recommended supplementary binder levels, their alkali ceilings, and how much of that alkali reaches the pore solution
Writing the reaction paragraph when the project says it does not govern
- +1Yes, and say why it does not govern rather than omitting it. A durability plan silent on one of the four deterioration processes reads as an oversight, and one sentence recording the non reactive status closes it.
- +1Name the condition that is absent. The reaction needs three conditions at once; moisture is abundant on a marine structure and alkali is present in any cement, so the absent condition is the reactive aggregate. Saying which one is missing is what makes the claim checkable.
- +1Do not claim more than the source supports. Write that no reactive aggregate is specified for the project; do not write that the aggregate has been tested and found non reactive unless a test result exists.
- +1Keep the alkali limits in the specification anyway. The cement alkali limit and the concrete alkali recommendation cost nothing, they are standard practice, and they guard against alkali arriving from somewhere other than the cement.
Key terms
- Alkali silica reaction
- A reaction between the hydroxide ions accompanying sodium and potassium in the pore solution and reactive forms of silica in the aggregate, producing a gel that swells and cracks the concrete.
- Reactive silica
- Amorphous or heavily defective forms of silica, including opal, chalcedony, cryptocrystalline quartz and cristobalite. Well crystallised quartz is not reactive, so the mineralogy rather than the chemistry decides.
- Pessimum content
- The proportion of reactive aggregate that produces the largest expansion. Well above or well below it the damage is small or absent, so expansion is not a monotonic function of how much reactive material is present.
- Map cracking
- The random pattern of closed polygons that is the signature of unrestrained expansion at a concrete surface. Where restraint is present the cracks reorient along it and the signature pattern disappears.
- Accelerated mortar bar test
- A rapid screening test holding mortar bars in one molar sodium hydroxide at 80 degrees for up to 21 days. Its conditions are severe enough that aggregates with good field performance can fail it.
- Concrete prism test
- A slower test holding concrete prisms sealed at high humidity and 38 degrees for a year on a realistic mix. It is perceived as the more reliable of the two, and it carries its own expansion limit on its own basis.
- Sodium oxide equivalent
- The combined alkali content of a cement, calculated as the sodium oxide plus 0.658 times the potassium oxide. Australian cement is limited to 0.6 per cent on this basis.
- Low alkali cement
- Portland cement whose sodium oxide equivalent does not exceed 0.6 per cent. Using it is not always successful, because significant alkali also arrives from aggregates, admixtures, mixing water and external salt.
Alkali silica reaction and its prevention FAQ
Can the reaction be stopped once it has started?
No. The guidelines state that there is nothing is known that will halt alkali aggregate reaction in practice once it has begun, and that remedial work is aimed at appearance and at keeping the member structurally competent. That is the sharpest contrast in the subject: corrosion can be arrested by cathodic means or by removing the chloride, whereas this has to be designed out before the concrete is batched.
Which test result do I act on when the two disagree?
The prism, with the reasoning recorded. The accelerated test is a screen whose conditions are deliberately brutal, so a reactive result there means further testing is required rather than rejection; the prism runs a year on a realistic mix and is described as the more reliable. The subject documents exactly this disagreement for a hornfels, which is slowly reactive on the mortar bar and non reactive on the prism.
What a report must not do is quote only the result it prefers.
Why do supplementary binders work against this reaction?
In two independent ways. They supply reactive silica that consumes alkali harmlessly in a fine, well distributed form, and they dilute the cement that was carrying the alkali. The guidelines are explicit that a material supplying enough reactive silica remains effective even if it does not significantly dilute the alkali content.
That is also why the alkali content of the material itself has to be checked: a high alkali fly ash gives back with one hand what it takes with the other.
Is a low alkali cement enough on its own?
Usually not. The guidelines record that using low alkali cement to control the reaction is not always successful, because significant alkali arrives from aggregates, from admixtures, from bore or recycled mixing water, and from sea spray and de icing salts, and because highly reactive rocks can expand even with a low alkali cement.
Internationally the proposed concrete alkali limit ranges from 2 to 5 kilograms per cubic metre and no universally acceptable limit exists.
Why does the required amount of reactive aggregate not simply scale with the damage?
Because of the pessimum effect. There is a proportion of reactive particles that produces maximum expansion, and well above or well below it the damage is small or does not occur at all. Some aggregates show a sharp pessimum, some a broad one, and some none at all, with the maximum at a fully reactive aggregate.
The practical consequence is that diluting a reactive aggregate can make things worse before it makes them better, so mitigation is not a matter of reducing the dose.
Assessment move
Learn the three conditions as a set that has to be complete, because every mitigation lever in the chapter is an attempt to remove one of them and the argument only works if you can name which. Practise reading test results against the right limits, and always name the standard beside any expansion figure, since the mortar bar and the prism carry different numbers on different bases.
Keep the hornfels case in mind as the worked illustration of why the faster test is a screen. For mitigation, memorise the recommended replacement levels and their alkali ceilings as a pair rather than singly, and remember that the effectiveness criterion is a measured expansion rather than a dosage. Finally, be ready to write a short, honest paragraph for a project where the reaction does not govern.
Working through Alkali silica reaction and its prevention in 42907? Sia is AskSia’s AI Engineering tutor — ask any 42907 Alkali silica reaction and its prevention question and get a clear, step-by-step explanation grounded in how 42907 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
