Contraction Joints In Structural Topping 1

[Bubik]

Hi all.

My questions are regarding contraction joints in structural topping. I am completely new to this topic. Contraction was to lay bonded structural topping (the thickness between 25 up to 50 mm on an in-situ slab) and forgot to apply contraction joints. The topping cracked and they have to lay a new one. The slab has openings as shown on the drawings.

1. They intend to apply contraction joints every 3 m. ,every joint running along the shorter side of the room . The contraction joints will not be aligned with the openings edges. Can it be like that? Are the joints not supposed to match the edges of the openings?
2. They intend to put a mesh ,just in case. I always thought that in bonded system a mesh is not used. If it is ok to use mesh what maximum thickness would be allowed so there is not too much steel in the concrete.

3. Aren’t joints between the topping and the walls supposed to be also applied?

 

[hokie66]

If it is a structural topping, it should not have contraction joints. The topping is in compression, so you don't want to interrupt it. Cracking is normal.

 

[Bubik]

All they apply is 'topping' (cement+sand+water=screed?) bonded to the slab, maybe I got the terminology wrong.

I thought that we always apply joints to screeds thou?

 

[SlideRuleEra]

...bonded structural topping (the thickness between 25 up to 50 mm on an in-situ slab)...

A bonded concrete overlay on existing concrete should be a constant thickness. Patch and prepare the existing slab so that the overlay will be a constant thickness.

Match the location of any existing joints. Extra joints are ok.

Don't use mesh.

Proper, timely concrete curing is essential.

See pages 34 though 37 in "Guide To Concrete Overlays, Third Edition" (free .pdf) by the U.S. National Concrete Pavement Technology Center.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dik]

Good pub...

Dik

 

[Bubik]

Thank you guys

SlideRuleEra Are you saying that extra contraction/control joints are ok then.

Are you saying that extra contraction/control joints are ok then.Also,when you lay topping you lay it in stages so for example you divide it into areas,lets say you put a stopper every 3 meters. So apart from those stoppers every 3meters, is it ok to put extra contraction joints every 6 meters? it is what I read in one of the guides.

Once again sorry for being awkward but I am completely new to it.

 

[dik]

I've often put them in parallel to the span of the 'composite member, and at mid span between the composite members... also with HC slabs. Used a Goldblatt 'groover' and not sawcut

Dik

 

[JAE]

PeterOron - is the slab an elevated slab spanning between structural beams?
Or is this is a slab that is on the ground?

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[SlideRuleEra]

...extra contraction/control joints are ok?

Per JAE's question, I will assume the question is about a slab on grade. Extra "joints" are ok. I suggest that a name (contraction, control, expansion, etc.) not be used for the joints in the overlay. The bonded overlay must perform exactly like the underlying slab. Overlay joints are there to make that happen... that is all.

You have read other guides... suggest reading the pages noted in the guide linked to above.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Bubik]

JAE
it is on first floor spanning between beams

 

[hokie66]

With those two long openings in your suspended slab, it is a bit difficult to know how the slab is supposed to work. How thick is the slab? How it is reinforced?

Do you need the topping for strength or just for leveling? If for strength, then shrinkage control joints make the situation worse. You would be adding mass without adding strength.

 

[Settingsun]

This thread is missing the reason for the work. What doesn't the building have now that it will have after the topping is placed? (Drainage, flat enough to lay tiles, increased strength etc).

I'm going to guess the structural system is beams parallel to the openings and a one-way slab spanning in the short direction. The 3m joint spacing makes a bit of sense for that. This information is also needed to answer the question properly.

 

[Bubik]

Sorry for the lack of information

The screed is purely for levelling purposes and beams run along the shortest side of the building (across) (obviously there are beams around the openings as well). The building is an electrical substation. Although the screed is for levelling purposes it will have to withstand quite heavy equipment (transformer) before finally placed onto the openings.