question about footings

[snalks]

The actual design footings for a three floor building calls for 2ft deep footing with reinforcement at 1 ft -10" from top.

The bearing rock is about 6ft from required elevation. the contractor wants to fill the footing pit with 6ft of concrete(4000psi) with the reinforcement provided at 1ft-10in from top. Is this an acceptable practice?

 

[jxzohio]

Your contractor is right. I will do exactly the same. 4000 psi plain concrete behaves just like bedrock after cure. I have no problem with it.

 

[GeoPaveTraffic]

Agreed.

 

[Taro]

With 6 feet of concrete, you may need to consider this "mass concrete". Additional reinforcing or other measures may be needed to deal with the heat of hydration and other effects.

 

[dmoler]

I make them pour it in 2 phases. First phase is to pour the concrete to bring to bottom of footing grade. Then after concrete set for 7 days, the footing may be formed and poured. A cold joint is needed between the 'mass' concrete and the actual footing.

 

[SlideRuleEra]

Concur with dmoler. This is exactly how seal concrete / footings in cofferdams are constructed for bridge and other heavily loaded foundations.

http://www.SlideRuleEra.net

 

[BigH]

Your concrete doesn't need to be "maximum" strength. It can be of a low strength variety - a cement stabilized base course if you so wish. You certainly don't need reinforcement - it is, in effect, just a stronger variation of engineered fill - and why don't you just use a DOT standard base course material rather than concrete? Properly placed and compacted, it would be more than capable of supporting a 3 storey building.

 

[GeotechPE]

I see this situation often. I agree with dmoler and BigH - first pour to reestablish the proposed bearing elevation with 1,500 psi concrete 'flowable fill' and a second pour for the actual foundation as previosly deigned and reinforced. Place concrete 5 to 7 days apart. Contractors usually fine working with aggregate in excavations greater than 4 to 5 feet deep difficult and time consuming. Flowable fill is quick. 1,500 psi concrete has adequate strength and is more economical than 4,000 psi concrete.