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Re: Plain Concrete Questions

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Sleiman Serhal wrote:
> Regards,
> I'd like to get some opinions on the following:
> 1. To what maximum slump would u recommend vibrating the concrete ?
> Mindess & Young write that vibration is needed only for concretes with a
> slump less than 50mm. Isn't this too low a value ? Do you think it's a
> typo and should be 5" instead ?

I think you mean at what maximum slump is it no longer necessary to
vibrate the concrete. For most slabs a good working slump for a
reasonable mix design will be in the three (3) inch to five (5) inche
range and ideally should be internally vibrated or externally vibrated
with a screed to assure consolidation. Check our ACI 318-95 and ACI 360
and ACI 302 for more accurate information on slump requirements and
floor slab construction considerations.
> 2. How cold should it be to justify the use of air entrainment. I know
> the definition of "cold weather concrete" from ACI 306 , but it applies
> to the weather conditions during casting and until concrete reaches
> 500psi.
> Can anyone quantify the terms "mild," "moderate" & "severe" exposure as
> specified in PCA's "Design & Control of Concrete Mixtures?"

I think Any concrete which is to be exposed should be air entrained.  In
general, air is not very expensive, decreases the permeabilty and
increases the durability.  PCA pulled their criteria from ACI 201 Guide
to Durable Concrete you might get a better feel from this than the foot
note on page 60 of the PCA Guide. I like to think of exposure in these

	Severe - Bridge Decks in most American Climates, Sidewalks and
pavements which will be de-iced.
	Moderate - All exterior concrete except above.
	Mild - Most concrete inside.

> 3. I am to cast a slab in a few weeks in an area where snow occurs for
> about 4 months in a year with mild temperatures, maybe an average of 8°C
> (46°F) and a max. low of -5°C (25°F) in the coldest month. This slab
> will be exposed to one winter season only since next year, the
> construction of several floors above that one is planned. Do u think I
> should use air entraining admixtures in the slab ? Also, do u think I
> should protect the steel dowels in waiting so as not to corrode, or do u
> think the time span is too short to cause significant corrosion.

For one exposure cycle air may not be needed, but may be a good idea. 
When you say dowels do you mean the floor dowels or the reinforcing bars
which extend through the floor and are to connect to the column.

On floors, dowel "slide" for load transfer.  They are usually coated
with a bond breaking compound on one side and cast in a sleeve on the
other.  I be less worried about corrosion and more worried about the
bond breaking compound not working good after four months.  Have the
contractor clean the dowel and reapply a bond breaking compund if they
are suspect.  A little corrosion (rust) which is not flaking or
powerding but simply roughing the surface can actually improve the bond
strength between the reinforcing steel and the concrete.  Four months
will cause some corrosion, but I wouldn't expect huge amounts, unless
there is some significant cathodic current going on or exposure to Cl.  

> Thanks in advance,
> Moni Serhal, MSCE

Arvel Williams, P.E.