wet curing concrete slab 5

[eng003]

so it is said wet curing maintains the moisture in your concrete slab for the hydration process and will help your concrete gain a higher and predictable strength. I have three questions regarding this:
1. Doesn't increasing your water to cement ratio decrease your strength? or does the water not get into the mix after placed?
2. What is the most common practice for residential slabs wet-cure or not?
3. I placed some concrete not intending to wet cure and then accidentally splashed some water in the middle which eventually spalled up, so why would more water be a good thing?

 

[Ron]

Let's take these one at a time....

[highlight #FCE94F][1. Doesn't increasing your water to cement ratio decrease your strength? or does the water not get into the mix after placed?][/highlight]
Once the concrete has set up, has been finished and there is no bleed water present on the surface, additional water will not get into the mix (thus, the w/c ratio is not changed), except at the very top surface. That's why wet curing usually involves wetting burlap or sand on top of the concrete, not flooding the concrete surface with water. After about 8 to 12 hours, it is ok to flood the surface.

[highlight #FCE94F][2. What is the most common practice for residential slabs wet-cure or not?][/highlight]

Most residential slabs receive little or no supplemental curing and almost never get wet cured.

[highlight #FCE94F][3. I placed some concrete not intending to wet cure and then accidentally splashed some water in the middle which eventually spalled up, so why would more water be a good thing?][/highlight]

You likely splashed water on before the concrete set up, thus locally increasing the water-cement ratio right at the surface, causing a condition known as scaling.

The wet curing process is intended to maintain a moist condition to allow full hydration of the cement. I have seen concrete that was several years old and when a petrographic examination was done, found lots of unhydrated cement. This means that there was insufficient moisture in the concrete to allow full hydration to occur. With full hydration comes greater strength and durability.

 

[eng003]

got it! thank you Ron

 

[jwilki]

Hi eng003,
Here is my go at answering some of these questions. Keep in mind I'm located in Australia where water is a precious resource.
1) Increasing the water to cement ratio does decrease strength however in typical water curing techniques (Ponding, Sprinkling or wet covering) the concrete slab is already formed, vibrated and has a surface finish. The water has no chance to mix into the concrete.
2) As mentioned I'm in Aus and it is uncommon to have a water cured residential slab or footing. Contractor do not want to maintain any wet hessian, sprinklers or create a pond and we face highly reactive soil that need special attention once you start wetting up the ground. Typical curing methods involve hand sprayed curing compounds.
3) Not sure on this one, but part of the curing technique is to maintain a wet surface (avoid spraying warm concrete with cold water as this may cause a thermal shock and lead to cracking) and avoid drying-wetting of the slab (this can cause volume changes which can contribute to surface crazing).

Hope that helps and correct me if I;m wrong.

 

[msquared48]

I wet cure all my slabs. I just drink an lot of beer and let nature take its course.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

The only thing that Ron failed to note, and it may only be my opinion, is that wet curing is probably the best form of curing... it is also costly and not often done.

mike... filtering it through your kidneys, probably causes some damage to the concrete <G>.

Dik

 

[Ron]

Yes, dik. You are correct on both the curing and the filtering!

Mike...good for the kidneys...bad for the concrete!

 

[msquared48]

Actually it works great for exposed aggregate!

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[rjenyoy12]

Yes, one method of curing for concrete slab is through wetting. There is no problem of spalling as long as the concrete has attained its initial setting.

 

[dik]

Mike:
Yes, one method of exposing the aggregate is through wetting... <G>.

Dik

 

[civeng80]

The most common and economical way of curing concrete slabs in Australia is to spray curing oil over the fresh concrete.

 

[dik]

Common here... but, not oil, a specially prepared curing compound manufactured by several of the construction chemical manufacturers here...

Dik

 

[hokie66]

civeng80,
Be careful which 'curing oil' is used. If it is the PVA type, you may as well use Mike's method.

 

[dik]

Catch the publication from Cement Concrete Aggregates Australia...



Dik

 

[civeng80]

Please ignore the new thread, it should have been a comment on this thread.

Cheers ! and Happy new year to all.

 

[dik]

The use of PVA is shown in the article attached above... one of the least effective curing compounds.

Dik

 

[hokie66]

civeng80,
Happy New Year to you as well.

I redflagged the other thread you started, but to answer your question, I consider PVA used as a curing compound to be useless if not worse. It is cheap, so contractors like it, and to some, it gives a false sense of confidence.

The link which dik posted tells the story, especially the chart on Page 4.

The two most effective curing compounds are chlorinated rubber and wax emulsion based. Both have shortcomings in that they interfere with bond of floor finishes. Chlorinated rubber is also quite noxious to applicators, so needs protective gear.

 

[Qshake]

wet curing is often mis applied and often overlooked for some cheap/quicker alternates. Wet curing should be done immediately with little finishing (only that which you must have) and then begin wet curing and continue for 7 to 14 days. I can only venture that contractors are neither fond of the labor in laying wet burlap or keeping it moist for 7-14 days. Shame, it's such a simple thing.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.