w/c ratio for slab-on-grade mix

[PG35]

Our office standard is a w/c ratio of 0.45.

A ready mix supplier is telling my client that this will increase the chances of slab curling because the mix is more likely to dry unevenly (especially on a poly base). He also said drying shrinkage is more likely to be a problem. Finally, he said that the lack of bleed water will make the surface cure and not allow air or water to escape. This will cause blistering.

He is suggesting a mix with a w/c ratio of 0.55.

Can anyone comment on this?

 

[jike]

This is my understanding also. You will have a greater chance of shrinkage cracks as well. It only takes a certain amount of cement to hydrate and then the rest contributes to shrinkage. I can't quite remember now but I thought our office standard was a w/c of 0.52. I assume this is an indoor slab and not exposed to freeze-thaw. Are you using a water-reducing admixture?

 

[civilperson]

Put six inches of moist sand over the poly base and put cure compound over the top surface after finishing and use w/c ratio of less than 0.5.

 

[vincentpa]

If it is exterior and the slab will have deicing salts, 0.45. If not, 0.5 or .52. See ACI 318 chapter 4.

 

[PG35]

Jike,

This is an interior Slab-on-grade. Are you saying that the extra cement contributes to shrinkage? I thought that the extra water in a higher w/c ration contributes to shrinkage.

 

[PG35]

I specified that a mid-range water reducer is to be used.

 

[strguy11]

W/C ratio is a component of curing, but you also need to look at the gradation of the aggregate. ACI has some good data on this.

 

[vincentpa]

smaller aggregate and more cement will create more shrinkage cracking. You should read ACI 360.1 and 360.2. Also good reading is PCA "Slabs on Ground." There are many factors that go into good concrete slabs that will reduce cracking and curling. It is too much to explain in one thread. Get the three items I've posted here and start reading. Even then your slabs will curl and crack. It's the nature of the beast. Concrete is like a lion or a bear. You can't truly tame it. It's still a wild animal.

 

[Taro]

I think the ready mix supplier may be giving you bad advice. As jike noted, a w/c ratio of 0.55 is in excess of what is required to hydrate the cement and will cause increased shrinkage. There are more effective ways to deal with proper placing, finishing and curing the concrete than to increase the w/c ratio. But the cynic would point out that these other measures are more expensive than simply adding more water. I would call the ready mix supplier's technical people and ask them to explain their concerns to you directly. Something may be getting lost in the translation from the client.