Garage Floor - Concrete placement & joints.

[Dougal Thompson]

Hi,

What a great site, I've puzzled with many of the questions answered over 25 years of DIY building!

At present I'm building a garage/workshop. the floor area is 6.50m wide x 10.50m long. Blockwork is up to floor level to provide shuttering on three sides.

Access for the concrete truck is limited to the front entrance which would make it difficult to place the rear bay if divided in three bays of 3.5m x 6.50m.

My questions are:

1) Would it be OK to do the full area in a single pour (well 2 deliveries of 5.5m3 one after the other for a 150mm slab) and cut two crack joints with a diamond saw at 3.50m intervals? Or do I really need two proper movement joints in this size of slab (if so is there a way of introducing such joints during a pour?) I will have expansion joints around the perimeter.

I would intend to hire a roller striker with a 7.00m pole & a power float. The finish will be a dry shake topping (Don Construction Monoshake)

2) If crack joints OK, how long after the pour should they be cut?

3) What is the calculation for the necessary width of expansion joint at the perimeter?

4) I require a channel in the concrete for later fitting of a steel channel for track hung sliding doors which I intend to achieve using a temporary batten former 50mm wide x 40mm deep. Any advice on placement & removal?

I think that's got all my problems off my chest for today, hope you can help.

Thanks, Dougal.

[TonyMcC]

I would guess that a couple of crack control joints would be fine, as you have the movement/expansion joints at the slab edges, but I'm not an engineer, so I can't guarantee that is right - I can only speak from past experience.

The CCJs can be cut as soon as the slab is hard enough, which usually means 3 days minimum, depending on the type of concrete being used. If you cut them while the concrete is still 'green', the cut edges have a tendency to spall and crumble.

I'm not aware of a calculation to detrmine expansion joint width - they are generally 12-25mm wide, depending on which type of compressible material is used. On a slab such as yours, I'd suggest that 12mm exp. joints to the perimeter would be ample.

As for the batten used to form a channel, I think you might struggle to hold such a former in place. Obviously, I can't see just where it would go, but any above-surface supports or formwork will interfere with the use of the roller striker. Would it not be easier to cut this channel after the slab has cured?

[Dougal Thompson]

Thanks for the re-assurance. I guess I have nothing to loose trying one way or another to keep the channel batten in place, I can always give up and cut the channel along with the CCJ's!