Movement joint

[mammut]

Hi,

Question about movement joints:
What should be the minimal distance between movement joints for the reinforced concrete slab on the ground (thickness-150mm)

Thanks

 

[civilperson]

150%-200% of the thermal expansion/contraction distance.

 

[hemis]

Please, what is the thermal expansion/contraction distance of a 150mm-thick reinforced concrete slab?

 

[civilperson]

The thickness is irrelevant,(the joint goes through it), the length and width of the slab are the important dimensions and expansion is roughly 5/8" per 100 degrees F of temperature change per 100 feet of length.

 

[civilperson]

My answer was for width of expansion joint. The spacing can be whatever is convenient.

 

[civilperson]

Control joints and construction joints are normally 20-25 times the thickness of the slab, (10 feet to 12.5 feet).

 

[hokie66]

mammut,

I do not agree with the advice given by civilperson to place control or construction joints 10 to 12.5 ft apart. More normal practice is to use in the order of 20 ft or 6 metre spacings. Joints are the main problem area for slabs on ground, so more joints is not necessarily better.

Expansion joints are not usually required, as natural shrinkage of the concrete provides plenty of space for the concrete to expand into when hot.

 

[Zambo]

I agree that joints are the main problem for gound bearing slabs. The number of construction joints will depend on the casting method, which may also depend on the flatness requirement.

What is your slab for? How do you intend to cast it?

I have previously worked on a 150m x 50m factory floor slab that we cast in 3nr 50 x 50m bays, no expansion joints just 2 construction joints. We achieved a good finish.

 

[mammut]

==>Zambo

I have similar situation: factory floor - 120x40m.
It will be casted in 4 bays -40*30m

 

[Zambo]

mammut,

by what method will you cast the slab? Your bay width of 30m means that you cannot span a levelling beam between the side forms. Tis means that you will need to use a lazer level and then spot check the concrete level as you proceed.

 

[kslee1000]

Zambo:

I assume you have reinforcing accross the construction joints. What was your casting sequence - in series, or skip the middle (cast earliest or the last)? Did you have saw cut control joints after curing?

 

[Zambo]

kslee1000

the slabs were cast in series. The reinforcing did pass through the construction joints. We didn't saw cut after casting.

The floor slab is a factory floor with dry shake floor hardener. As mentioned earlier joints can be a problem with floors trafficed by heavy forklifts etc. By forming just a construction joint skilled concrete finishers can achieve a very good smooth joint which is durable.

We did get 3 or 4 cracks progated at manhole corners and one at a vertical pipe penetration. These cracks may not have been stopped by saw cut control joints. But in any event treating the cracks with epoxy was preferable to dealing with the longterm maintenance issues of joints.

 

[kslee1000]

zambo:

Thanks for the very informative answer, I have always doubted the effectiveness of control joints, especially on reinforced slabs/walls unless the slabs/walls are subjected to drastic and fast heating/cooling cycles, or for architectural reasons. Otherwise I think the practices of using smaller bars in closer spacing and ensure proper curing/finishing can prevent most of those undesirable movements to occur, if not 100%.

We always provide a lot of additional bars (parallel & diagonal) around the openings/penetrations, it seems to work.

 

[hokie66]

kslee,

I agree that the approach used by Zambo has merit, but though it may indeed by best practice, it is not standard practice. If you go this way, I suggest you advise your client of your reasons for eliminating joints, and also give the client advice on expected cracking and necessary remediation, if any.

 

[kslee1000]

hokie66:

I have no question on the winsdom of "standard practices", just want to collect some others successful stories. I am still convinced that with properly designed/placed reinforcing steel or mesh and good curing method, the potential for wide structural/shrinkage cracks is next to null. Compared to saving the construction cost (for avoiding joints in reinforced concrete), and the headaches (for maintaining/fixing the broken joints), I think a few fine cracks should be acceptable to most reasonale persons.

Have you ever seen cracks occurred just next to the expansion/control joint, and/or midway in between joints? And/or the crumbling joints with broken sealant on industrial floors (as zombo and I was talking about)? I have no doubt that you do (not necessary your design), then how do you explain the phenomenon to people don't know much about concrete?

I admit I am still following the rule of generally "feel-good" practice, just have some doubts.

 

[Zambo]

kslee1000,

on a forum like this the posts are quite brief and of course any points made should be followed up by some hard research. I was describing a particular project, I will use the same approach again in similar circumstances.

You mention having the rebar close together to control cracking. This has made me feel the need to raise a point. I was talking about a heavily loaded slab which is heavily reinforced accordingly. With a lightly loaded slab cast on ground there may be insufficient rebar for the reduced joint approach.

I have seen proposals to use steel fibres for lightly loaded ground bearing slabs, this is intended to control cracking sufficently to enable large bay sizes and therefore reduced construction/control joints.

It is well worth getting in contact with a company which specialises in factory floor slabs while at the design stage. They can advise on similar projects carried out and the tolerances which large bay sizes can be cast to. Of course they won't be responsible for design, so at the end of the day you still have to decide which approach is best for your client.

 

[kslee1000]

zambo: Agree, thanks for further clarification/suggestion.

mammut: apologize for diverting the conversation from your orignal question.