Casting Concrete

[normm]

I will appreciate if you can give your views on the following 2 questions :

1. I am planning to construct a shallow reinforced concrete base only about 2m from a busy railroad track. The base will be cast at night and rail traffic will resume after about 2 hours carrying about 12 trains per hour. The rail traffic could transmit some ground vibrations to green concrete. For various reasons precasting the base has been ruled out. Can you suggest what precautions, if any, can be taken to protect green concrete.

2. What should be the minimum time gap between 2 adjacent concrete pours, say between the base and the wall which starts from the base. Does it depend only on the base reaching sufficient strength to be able to support the load from the self weight of the wall, or are there other factors to be taken into account?

 

[JedClampett]

Why don't you use a set accelerating admixture? Or a high early strength cement?
I'd talk to your concrete supplier. This is a common issue for bridge repair work.

 

[paddingtongreen]

I remember reading about a McDonalds that needed a new entry ramp. They closed at midnight, excavated, formed and placed a 10,000# concrete ramp with accelerators, and re-opened to traffic at six the next morning.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[SkiisAndBikes]

Overnight concrete placement should not be a problem with the correct mix design. A colleague of mine worked on an international airport runway rehab project removing and replacing full panels. Demolition started at 11 pm and the runways had to be reopened by 6 am each work day.

To achieve your requirements in item 1, the mix design will gain strength rapidly. You likely could plan to cast the walls the following day, dependent on you mix design and/or lab tests on the proposed mix design combined with the anticipated loading.

 

[vandede427]

I just saw an article in some eng magazine. I believe the headline was "3000psi In An Hour."

Jed's right; use an accelerating admixture.

 

[normm]

Thanks folks for your quick comments.

You are right that using an acclerating admixture is the appropriate thing. Most of the reports that I have heard are for simple flat rectangular pours like airport runway slab etc where it will work without any problem.

But I have a situation with a rectangular section base with odd shape plinths on top of the base about 1 m high which will require formwork. Normally the base will be stage 1 pour and plinths will be stage 2-pour which can be next day as SkiisAndBikes suggests. But it still makes it a 2-pour operation. I have been asked if the whole lot can be done in one pour by doing some formwork on top of the plinths on top of the base. If I use an acclerator in a one-pour operation, the base may have already started to set when I am about to pour the plinths. May be I should avoid this situation.

The safe thing seems to be a two-stage operation,using acclerator. This is unfortunately still slow because of the 2 stages involved.

 

[HeavyCivil]

Ditto - use type III Portland. Not sure I understand the pour but obviously do it monolithic if you can.