Effect of delayed curing followed by immersion?

[Tomfh]

Theoretician question-

Supposing we pour two thin concrete slabs, say 25mm
thick, allow one to rapidly dry out after placement, and immerse the other in water immediately after it sets.

The one that is left to dry will turn out weaker if tested in say a years time.

But what if after say 3 month we immerse the dried slab into a water bath and leave it for an extended period? Presumably the water will enter the slab whose pores never closed up, and restart the hydration? Similar to when a cracked slab undergoes “autogenous healing”?

How much would the strength catch up to the slab that was cured from the start? Would it get close? Or would it already be dead in some way, and unable to ever recover?

I don’t understand concrete chemistry and the processes of hydration well enough to understand these questions, but would like to find out.

 

[LittleWheels]

This has been discussed here before e.g. thread507-344206

It is a complicated subject and it is possible to dig up supporting evidence for both 'strength gain restarts' and 'strength gain is permanently reduced'. I lean towards 'strength gain is permanently affected, though sufficient rehydration can lessen this'. In most cases, it will be a surface or near-surface effect (<10mm?), depending on how long the poorly cured concrete is left dry. Concrete deeper in the section should be in comparatively good shape, although drying shrinkage cracks may be an issue.

[URL unfurl="true"]https://www.understanding-cement.com/concrete-curing.html[/url] puts things pretty clearly. Drying shrinkage and resultant cracking, along with increased permeability has an impact near the surface. Partially hydrated concrete will have calcium silicate hydrate coating the unhydrated cement particles, which inhibits how well moisture can reach the cement later. It is a lot easier to subsequently saturate partially-hydrated concrete in the laboratory, compared to in-the-field.

 

[Tomfh]

Thanks for the links.

It sure seems complex!

That microscope photo is the most interesting thing I’ve seen so far.

I wonder if you placed that concrete in water and left it there would the “desiccated microstructure” region rehydrate? Maybe high pressure to force the water in?

All impractical in the real world of course, I’m just trying to understand what’s going on.

Why does this slab has a permanently “desiccated microstructure”, but a cracked slab will undergo autogenous healing and bridge larger cracks than these empty pore? Does cracking provide access to cement grains that were otherwise locked off in a poorly cured state?

 

[LittleWheels]

"Why does this slab has a permanently “desiccated microstructure”, but a cracked slab will undergo autogenous healing and bridge larger cracks than these empty pore? Does cracking provide access to cement grains that were otherwise locked off in a poorly cured state?"

That is my guess too. Note: guess, not knowledge.