What on earth is ‘DEF’?

A hidden threat for concrete precast elements

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 of concrete associated with the delayed formation of this mineral. DEF typically results from elevated temperatures during curing (above about 70°C), as high temperature inhibits 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 which normally forms within the concrete as it cures.
  • Elevated curing temperatures (above 70°C) can inhibit the curing process and prevent full ettringite formation. (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 taken place at the curing stage, the reaction may recommence later in the life of the structure, leading to an internal expansion of the concrete and external cracking. This reaction is triggered and driven by the ingress and movement of moisture.
  • Cracking of the concrete leads to a cycle of degeneration, where reinforcing steel is exposed to corrosive or reactive elements. In short, the service life of the structure is significantly undermined.

True, it’s not as simple as it sounds.

When concrete starts to degenerate, it’s often difficult or controversial to dictate what the primary cause may be. 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 up the moisture within the pores of the concrete 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, instead of being rejected, can be treated by spray application and sent to the site with confidence.

Interestingly, this same treatment will also prevent ASR later in life, and for the same reasons: the ASR process is dependent 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 for the life of the concrete and helps ensure the intended service life of the structure is preserved.

Note, too, that if DEF cracking does begin to show on an existing structure, hydrogels may still be able to help, depending on the degree of advancement of the condition. 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!

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