You are correct that the lower the water/cement ratio the stronger the mix. However, in my opinion I would tend to agree that strength of the mix is not really a huge issue with a sidewalk/patio. One thing to note that a drier mix will be more difficult to finish and you may sacrifice that.

If it were me I would make sure that the base and steel were done properly, over the mix design. The base is the stone or dirt under the concrete. The fill you mention could be concrete or stone, but it should be placed in small lifts and compacted very well. The other area that is often overlooked or done improperly is the steel reinforcement. Either welded wire fabric (this is probably the screen fabric or screening you mentioned) or individual tied bars should work, though my preference would be individual tied bars if costs were similar. The main thing is that any steel reinforcement needs to be in the middle of the concrete to do any good. That means the reinforcement should be supported by chairs to keep it up off the ground. With the welded wire fabric the sqaures are small so it makes it difficult for the contractor to walk around the concrete to pour/work/finish. Individual bars typically have larger squares so if they are careful they can step between if necessary (still should be on chairs though). From what I have seen this area is often overlooked and done improperly. The contractor walks through concrete pushing the steel to the bottom, where it does absolutely no good. Most contractors will also promise to manually lift the steel once they have finished walking on it, right before they begin to finish it, but more often than not this is overlooked.

Then finally make sure they use proper jointing. This is not as big a problem, but they need control joints to control cracking and expansion joints to allow for proper movement. I won't go into detail here, but a good contractor usually gets this one right.

As far as selecting a contractor, just make sure you call their references. If they have references from several years ago, even better, so you can see how their is performing over time.

Good luck,

Riles

I like 6 sack mix 3/4" aggregate 5" slump. For non-structural app's, 3/8" aggregate is OK. I use a pumper to place the crete. The 3/8" agg will flow through a small diameter hose that I can handle by myself. A pumper costs $200 cash for the job. It will save $ on labor and standby time for the delivery truck.

Mist the curing concrete multiple times a day for 2 weeks to prevent cracking. In the direct sun, this is VERY important.

Everything Riles says is correct. He is an engineer.

Riles thank you so much for all your help and guidance! Just had a question as to what a "small lift" is?

Aidan, thanks for the detailing of the type of mixture. Is "slump" the thickness of the slab?

How would it be best to go about to insure that the steel is in the middle of the concrete when the installers are in the middle of what is a very laborious project? I don't want to totally tick them off.

I did read somewhere that the fill should be roadbase to give it some air so the concrete dries from both the top and underside. That didn't make much sense though since the road base is suppose to be so compacted.

I would put a minimum of 4-6" stone under all concrete. Depending on how deep your fill areas are, I would probably limit that to 8" thick lifts, then compact and repeat as necessary to complete fill.

The slump Aidan refers to is just a measurement of the consistency of the wet concrete as it arrives. It goes back to your question on water/cement ratios.

As far as the steel goes, I would ask them prior whether or not they plan to set the steel on chairs or not. That is probably your best insurance. You can also watch them to make sure they do it.

I would prefer the fill to be either dirt of gravel. A large mass of concrete will tend settle/expand/contract differently and will tend to crack the concrete at the edges of this mass. This can all be dealt with, by isolation, or expansion joints, but they should have a plan on how to deal with that.

Riles

6 sack mix- six 94 lb pound bags of portland cement per cubic yard of concrete. This should achieve minimum 4,000 psi

3/4" or 3/8" aggregate- the size of gravel rocks added to the concrete. Larger aggregate results in stronger cured concrete. Smaller aggregate is easier to work with.

5" slump- how much an unsupported column of the wet concrete will "slump" The column starts out 12" tall, the cylinder is removed, the slump is how much it falls. 12" slump would be pure liquid, 0" slump is solid. 5" is just right, not too wet or dry.

Those are the basic specs you call out when ordering a batch of concrete. You can add all other sorts of plasticizers, lightweight aggregate, curing additives, colors, etc. to get special concrete, but the parameters I gave you are the basics.

Sorry I haven't responded back, 12 hour work days and company over can get in the way :)

I thought I'd get the bids by now to compare what exactly I was getting. I realize the job is pretty labor intensive so it may be hard to bid out. I'm glad I got many out here to check it over because I've only gotten 2 actual bids that were miles apart in price.

How long should I give, before I realize I'm not going to even get a bid? One person came back to recheck some measurements. He was saying it was $500 per day for the pump and it would be cheaper to send the cement down a makeshift chute to get down to the backyad. Most wanted to do the pour in one day, he wanted to do it in two to insure the concrete didn't get set too soon. He seems very careful, but some of his practices seem to go against the mainstream of what I'm hearing. What would you think? I haven't gotten a list of his references yet (nor a bid).

I can't answer the first question, but a follow up call to some of the folks might shed some light on whether or not they plan to submit a bid. Give them a deadline.

$500 for a pump sounds about right. When I got one for my deck I think I was able to find someone for $300/min., so you might be able to check around and save a little. The pump truck is usually a totally different contractor that the prime subcontracts with. You may be able to contact them direct.

One pour vs. two pours wouldn't matter to me as long as it is placed properly and finished properly.

Riles

The pump guy is your best friend the day of the pour. He manages the project, coordinates the delivery of the concrete, negotiating the price down to a contractor rate. Concrete companies do not like to deal directly with the public. A poorly planned pour can be a financial disaster.

I think you have the knowledge to do this project as your own general contractor. Sub out the labor to a pump guy and crew of finishers.