Movement Joints in Ground Bearing Slabs and RC Retaining Walls
Movement Joints in Ground Bearing Slabs and RC Retaining Walls
(OP)
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
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






RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
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.
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
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.
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
302 Guide for Concrete Floor and Slab Construction—ACI 302.1R-04
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
I know there is a big difference between residential driveways and garage slabs, and commercial buildings and warehouses, but CJs really work!
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
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.
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
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.
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
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.
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
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...
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
RE: Movement Joints in Ground Bearing Slabs and RC Retaining Walls
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