Expert: Dan Griffin - 1/15/2009

Question
Need to install new side walk, It's very cold and live near train (rail carts) lots of traffic, so there's allot of vibration. What should the weather (temp.) be? How strong can I make it? What should I use?

Answer
Victoria,  there are so many rules about concrete.
It would be best to wait until you had a forcast that does not go below freeezing for at least the first 3 days after the concrete is poured.  The air temperature should be 40 degrees F and rising before starting the pour.  Concrete can be poured in cold weather, but the costs accelerate and may require to heat the subgrade, provide heat for the concrete, and concrete blankets, etc.

I would use 3500 psi concrete with air entrainment.  Finishers may want calcium to hurry the initial set.  Concrete should not exceed a .45 water/cement ratio - about a 4" slump.  Excess water makes the concrete easier to work, but much weaker.  Concrete will be stronger if properly cured.  The best cure is to keep the concrete under water for the first 7 days, but you can't do that in freezing weather - use a chemical curing compound like Conspec 1.  

The most important element of good concrete is the preparation of the ground underneath the concrete.  NO sod, avoid loam if possible, make sure the ground is undisturbed and/or compacted to very hard density.  Make sure the bottom of the slab is as smooth and flat as the top - no wheel ruts, lumps, clods, etc.  A bed of sand, gravel, red select, or other proper fill material helps grade the subgrade.  If the subgrade is properly prepared I would not use any mesh/fiber/ or steel.  If you know the concrete will be driven on by trucks,etc I would increase the depth of concrete to 5" and still would not use rebar.

Exterior concrete should always have air entrainment.  Make sure you have at least 1/4" of fall per foot in at least one directon to get water off the cocnrete.  Once the concrete is placed, struck off, and bull floated do NOT do anything else to the concrete until the bleed water goes off the slab (about 2 hours, more if cold).  

Make sure you have a contraction joint (saw cut or jointer) that breaks the concrete into sections that are close to square.  If the walk is 5' wide, then a joint every 5 feet.  Use an expansion joint at each end and at least every 60' to allow for expansion once it gets hot.

I hope this answers most of your questions.  Don't hesitate to re-ask.