Don’t you love acronyms? No, us neither.
However, they do have some uses, and ‘D.E.F.’ is easier to say (and spell) than Delayed Ettringite Formation.
The formation of the mineral ettringite is a normal product of early cement hydration. Delayed ettringite formation (DEF) causes expansion and cracking in concrete. DEF typically results from elevated curing temperatures (above about 70°C), as high temperatures inhibit the normal early formation of ettringite.
DEF can cause cracking in heat-cured precast concrete components, several years after construction.
Importantly, external moisture is required for the phenomenon to occur. The availability of this moisture will dictate the rate and the extent of the resulting internal expansion.
Let’s run through that again!
- Ettringite is a mineral that normally forms in concrete as it cures.
- Elevated curing temperatures (above 70°C) can inhibit the curing process and prevent the formation of full ettringite. (Steam-cured precast elements are at particular risk… more so if they are to be in close proximity to water – e.g. bridges or wharves.)
- When this formation hasn’t fully occurred during curing, the reaction may recommence later in the life of the structure, leading to internal expansion of the concrete and external cracking. This reaction is triggered and driven by moisture ingress and movement.
- Cracking in concrete leads to a cycle of deterioration, exposing reinforcing steel to corrosive or reactive agents. In short, the structure’s service life is significantly undermined.
True, it’s not as simple as it sounds.
When concrete starts to deteriorate, it’s often difficult or controversial to determine the primary cause. Chlorides may be present. ASR and DEF have many similarities in their activity, complicating diagnosis. It’s even harder if the degenerative cycle has become severe, when the causal diagnosis may become moot in any case. So it’s important to keep a balanced outlook on risk factors.
Having said all that —
If excessive curing temperatures are recorded, the DEF risk factor is more or less built in. It’s highly likely that curing has been impacted and inhibited to some extent.
So, does that structural element need to be rejected? Single bridge beams, for example, are incredibly costly – not something to lightly toss in the trash.
There is a solution.
Penetrating hydrogels.
Concrete hydrogel treatments bind moisture within the concrete’s pores and prevent further moisture from entering. By binding the moisture into a hydrogel, you effectively prevent the whole DEF process from commencing – provided treatment is applied early enough. Precast elements, rather than being rejected, can be treated with a spray application and sent to the site with confidence.
Interestingly, this same treatment will also prevent ASR later in life, for the same reasons: the ASR process depends on the availability of moisture for its reactivity. Hydrogels also protect against chlorides and other contaminants – these need moisture to carry them to the reinforcing steel. Very simple principle – many benefits.
And it’s a cost-effective process – particularly compared to the expense of re-making the failures.
 |
Hydrogel treatment remains effective throughout the life of the concrete and helps ensure that the structure’s intended service life is preserved.
Note, too, that if DEF cracking begins to appear on an existing structure, hydrogels may still be able to help, depending on the condition’s stage. Hydrogel treatment can seal existing non-structural cracks up to 0.5mm, and will still immobilise moisture, helping arrest the reaction within the concrete.
Intrigued? Get in touch today to learn more!
If you manage civil infrastructure, try our on-demand webinar “Resilient Reconstruction“.
