Casting concrete replacement front door step in situ

Hi Folks

My Victorian terraced house opens directly onto the street, at a 15deg turn in the road, with 3 steps from (incl) road level up to the front door level (incl), set in a brick porch.

During recent building work, the fella managed to crack the large curved central step, that is somewhat triangular in plan, tightly fitted with a small cut out for a rollback on the front elevation which forms a curved pillar on one side, and separated from my neighbour on the other side by a half brick wall. Anyway, it's a complex shape, so I have thought the best way to replace would be to cast in situ. First, grind out the old step, then create a metal framework bolted together and either side into the brickwork, with 1" or so angle irons, put some kind of thin boarding in sufficient to fill to the spaces between the metal bars and also support concrete tipped in to the top, with some wooden formwork at the front.

I have full access below into my cellar, and have had some metal grilles made up to insert as risers to give good ventilation.

1) Are there any types of steel/finish I should avoid to prevent chemical reaction/damage with the concrete?
2) What sort of mix/product name of concrete should I use? I think standard Portland 4:1 may be a bad idea, considering the heavy use and potential for chipping.
3) Should I use a different concrete material for the topmost layer to give a finer finish?
4) How do I ensure a really flat top/sharp 90deg angle to the front?
5) How to make sure it cures properly -need something reasonably fast as potential for local children to sit on it.

Many thanks...

Can you make a wooden frame around the step like in the pic below.


Also buying an edging trowel should put the curve on the step for you.

If you are going to use it a lot I would purchase stainless steel otherwise just get a cheap one, B&Q should stock them.

If you make a frame and mix cement/concrete/chippings after pouring it in make sure you tamper it down to get rid of the bubbles and to pack the step down in all areas.

Water generally rises to the surface, after several hours float the step off.
In this weather you would be crazy to be concreting outside, the water would no doubt freeze in the mix which would not set properly.

If you do have to make the step then cover it on a night with a blanket or carpet or something similar, this should stop any frost damaging the step.

If the step looks a bit rough to the sides when you take away the frame you could use a bit of builders sand and cement mixed to fill any areas and give a neater finish.

Here is a link to fixing a step which I found on google.
http://home.howstuffworks.com/home-improvement/repair/how-to-do-brick-and-concrete-repairs3.htm

(I am just going by my own step which I did a couple of months ago but I am sure someone will post the correct method your looking for)

Hi Always Learning

Thanks for some good/useful tips there. I should say the 'curve' I mention is part of the shape in plan view, though the original did have a curve as you suggest in cross-section. The step is surrounded by brick walls (incl door!) on three sides, and bridges a large cavity below into my cellar, hence my questions about strength (of concrete and metal bars) and reinforcement! Please can folks specifically answer the numbered questions? Ta!