old concrete and hydration

[ticas]

It is recommended to add water to fresh concrete for 7 days to keep it moist and effect good curing. What if it's not done. Can one add water to the concrete 2 months later to continue the chemical reactions that could have stopped because of insufficient water during the 7 days curing period? Or would adding water late just damage the concrete? What damage can possibly occur (or not)?

 

[Ron]

The hydration process in concrete progresses over a long period of time. Initial hydration during the first few weeks is critical for strength gain. If insufficient water is provided for curing, there will be a delay in the hydration of all the cement in the concrete mix; however, it will eventually occur.

In a concrete mix, about half of the water that is in a typical mix is needed for hydration. The rest of the water is for the convenience of placement. With that in mind, strength gain will occur even if no wet curing is done. The ultimate strength of the concrete will be lower if this occurs; however, it is likely that the design strength will still be met.

The curing process, with wet curing, does a lot more to help the durability of the concrete than to affect the strength, although with wet curing, strength is gained faster and more consistly.

There will be unhydrated cement particles that remain in the concrete after initial curing. If you re-wet the concrete, some of these will hydrate, but they will have little effect on the strength. With hydration though, the durability will improve.

 

[ticas]

But when you re-wet the concrete months later, it will just be the surface (let's say bare) that gets wet. How would that affect the concrete particles many inches inside the center of a beam for instance Remember you just wet the surface. And I don't think it would be permeable enough to get many inches inside.

You also stated "If insufficient water is provided for curing, there will be a delay in the hydration of all the cement in the concrete mix; however, it will eventually occur.". How will it occur when it is not re-wet with water later? maybe you are referring to the moisture in the humidity in the air supplying the hydration or some kind of spontaneous chemical reactions?

 

[Ron]

ticas....the amount of water in the mix when it is placed is sufficient for hydration to occur for strength gain. Curing with moisture helps to reduce the amount of water that evaporates from the surface of the concrete, thus increasing durability and reducing initial drying shrinkage. If too much water evaporates, then hydration is affected and the water that was initially there for hydration leading to strength gain and durability is lost...that is why curing is important; whether it is wet curing or curing by prevention of evaporative moisture loss (curing compound). Curing compounds are less effective than wet curing.

When you re-wet concrete, the water will go into the concrete. It is not impervious and will allow water to migrate through its pore structure. The more water you put on the surface, the more that will be absorbed.

 

[ticas]

We always read how strength gain increases through the years and that is why the concrete foundation constantly wet or exposed to water is much stronger (at least not enough to cause heaving or foundation damage from water freezing and expanding or bar corrosions). Is a myth since you said water added later won't affect the strength?

But what if too much water evaporates in the initial cure and not even enough for strength gain. Then would adding water months later cause the chemical reactions to proceed or still not?

 

[Ron]

No, it isn't a myth. Continuously wetted concrete will continue to hydrate and gain strength; however, the rate of strength gain decreases with time...approaching a flat line at about a year...though some increase will continue.

If wetting is not continuous, then hydration will slow or even stop, depending on the amount of moisture available. If it stops, then rewetting will restart it; however, it will not increase at the same rate as when continously wet.

 

[ticas]

But you stated earlier that "There will be unhydrated cement particles that remain in the concrete after initial curing. If you re-wet the concrete, some of these will hydrate, but they will have little effect on the strength. With hydration though, the durability will improve."

And above you stated that "Continuously wetted concrete will continue to hydrate and gain strength"

You earlier said there would be little effect on the strength but later said it will gain strength. Please clarify because they seem to contract each other. Thanks.

 

[Compositepro]

I've read that reaction with carbon dioxide in the air will continue to increase the strength of lime and concrete over years and centuries.

 

[hokie66]

Compositepro,
Don't know where you read that, but it is off base. CO2 is an enemy of concrete, not a friend. It tends to lower the pH of concrete over time, and thus the protection of the reinforcement is reduced.

 

[Compositepro]

Hokie66, now I had to look into it further. I read about it when doing research on lime chemistry. It seems that slow carbonation can be beneficial for lime mortar and concrete. Rapid carbonation will degrade concrete due to differential expansion and, as you said, the loss of corrosion protection of the iron reinforcement. With slow carbonation cracks and porosity will seal.

This article was interesting.

http://www.fpmccann.co.uk/sites/all...n_Factsheet/Carbonation_Factsheet_ Part 1.pdf

It points out that eventually all lime and concrete will carbonate (on a geologic time scale).