Movement Joints in Ground Bearing Slabs and RC Retaining Walls

[sybie99]

Hi Guys and girls

When designing concrete buildings we usually limit the distance between movement joints to approx 50m, that is just a rule of thumb that most companies I have worked for in the UK go by.

When looking at ground bearing slabs opinions are varied. Also with regards to reinforced concrete retaining walls. WHat guidelines do you use? I know TR34 (UK) gives excellent guidelines for industrial slabs, but what about regular ground bearing slabs, so too retaining walls. Does minimum reinforcement not restrict cracking?

I would love to now your opinions, or better still ways of dealing with MJ's based on theory.

Cheers

Seb

 

[ash060]

If you are talking about slabs-on-grade than you will most likely need joints much closer than 50m.

One of my old bosses used about 3 ft per inch of slab thickness for normally loaded slabs. I guess that be about 1 meter per 25mm in metric. It seems like a good rule.

 

[steellion]

Lightly loaded slabs-on-grade: Joints at 15 feet on center max, with an aspect ratio not exceeding 1.5:1. I typically specify a saw-cut joint with a contractor option of a construction joint if I'm not worried about where the construction joint will be.

 

[patswfc]

Sybie,

Get a copy of 'Concrete ground floors - their design, construction and finish' by Deacon.

Table 5 shows that ground bearing slabs can be poured up to 81m in length between free movement joints depending on the thickness of the slab and the mesh used.
This would then need to be divided into smaller panels using restrained movement joints (saw cut joints). These should be positioned to create sub panels with aspect rations up to the 1.5:1 limit mentioned by stellion.

Figure 16 suggests that the saw cut joints should be not be spaced at greater than 6m if the slab is not reinforced (standard practice in UK) but can be increased to 10m centres if the slab is reinforced.

Similar to TR34 this document is geared towards industrial applications, warehouses, etc., were the slab will be exposed and the joint types and spacings recommended are to control the serviceability (appearance) of the slab.


If you have lightly loaded slabs with finishes that are going to hide the appearance of the slab (e.g. a floating screed, carpet) then I think that you can relax the limits recommended in these documents within reason.

 

[a2mfk]

I recently built up my library of concrete slab-on-grade (I hate slab-on-GROUND) articles for my forensic work and a pool deck replacement project. I'll post a few, maybe they will be a good start. I am also about to order this which I recommend:

302 Guide for Concrete Floor and Slab Construction—ACI 302.1R-04

 

[a2mfk]

ACI 302 excerpt

 

[a2mfk]

From doing forensic investigations of residential properties, the number one thing I can say about concrete SOG is control joints, control joints, control joints... There is one specific neighborhood we go to a lot where the driveways have CJs at 5-10' o.c. and they rarely crack unless there is very unusual changes in geometry. Same thing with garage slabs.

I know there is a big difference between residential driveways and garage slabs, and commercial buildings and warehouses, but CJs really work!

 

[hawkaz]

I'm not farmiliar with the term movement joint- Do you mean a shrinkage and temperature control joint (normally sawcut), or a full blown expansion/cold joint?

Regarding your cracking/reinforcement question, most slab on grade cracking (other than minor shrinkage cracking)occurs when the slab curls at the edges and at joints. Slab dowels can help with this, but I prefer a diamond dowel/dowel basket system at these joints. Slab reinforcing doesn't really help control this type of cracking.

 

[omw7]

We recently cast a slab in Singapour which was 180m by 80m without movement joints. This was based on slabs being poured in a checker pattern and stitched together at a later date so as to avoid the problems linked to cement shrinkage. The reinforcement was calculated to take account of further long term shrinkage.
The temperature was not a problem as the ground temperature of the Singapour is virtually constant at about 26 degrees equal to the average ambient temperature. We were between 6m and 2m below ground level. Assumptions were made as to the maximum temperature variation.
The french codes allow a diagonal dimension of 40m unless account is made for shrinkage and temperature. For me, these are the two variables for the in-plane shrinkage of slabs.
The other variable is differential settlement and unequal loading patterns. An FE model is often very useful to calculate the necessary reinforcement but will only give good results based on good and conservative assumptions.

 

[rscassar]

Depends on applications.

I call up external slabs subject to light vehicular traffic with 6000mm (20') between saw joints, panel ratio 1.5:1 with light WWF reinforcement located 1.5" cover from top.

I have specified on residential/commercial projects with floor coverings with no saw joints and a movement joint at 30,000mm spacing (100') with heavier WWF top reinforcement and slab thickenings every 5000mm (16'). Of course the floor is covered and nobody can see the random cracking until the floor covering is removed.

 

[sybie99]

Thanks people

It seems there really are so many factors that each situation is different and needs to be looked at in isolation. It would be useful if there was a publication that gave a standard which can be used giving joint requirements for different elements, for the time being it seems that for any given scenario it is likely that different engineers will have different ways of dealing with cracking and use of MJ's.

The debate goes on...

 

[hokie66]

There are such publications, but they vary markedly from country to country. Slabs on grade (or slabs on ground if you prefer) are not really structural elements, so agreement across the profession has not been achieved. US engineers like lots of joints, while many others like myself think joints are the main problem, and don't object to normal shrinkage cracking, as long as the width of the cracks are controlled by reinforcement.

 

[enko]

I personally would like to make a finite element model including the termic design as a load case and produce the combination as the respective code allows.

I belive the 50 m rule (or even 60 m )gets justified easily after the model results.

The main input data to be clarifed is the dT (temperature difference) for the specifik location