TECHNICAL REPORT 34 – CONCRETE INDUSTRIAL GROUND FLOORS – 4Th EDITION
JOINTS IN CONCRETE PAVEMENTS
Recognized worldwide as a relevant publication, this fourth edition brings us updated information on the design and construction of concrete pavements.
What matters to MGSI and its technical department, the 4th edition of this guide brings new methodologies for the calculation of reinforced slabs with metallic fibres and new advices on the use of joints in concrete pavements.
With regard to joints, once again it is confirmed that the use of joints with a continuous load transmission system (point 6.5.2) is not recommended, the so-called omega joints, due to the concrete breaking problems that have occurred in pavements using this system.
Another important consideration is the fact that it is advised against sawing joints on fibre-reinforced slabs (section 7.1) unless there are adequate means of load transfer. Sawn joints on fibre-reinforced pavements, without proper load transfer device, are at risk of opening sufficiently that the slab loses its bearing capacity and operates as a free edge. In this situation the slab has significant deformations which leads to cracking and damages in the edges of the joint.
LOAD TRANSFER AT JOINTS
Since was found that the concrete rupture occurred at lower loads than those calculated according to the previous version of TR34, the fourth version has new information that must be taken into account when calculating the punching resistance.
It is clear from section 6.5.4 that the improvement of steel fibres in punching resistance cannot be taken into account. The calculation of punching resistance is therefore only performed considering unreinforced concrete which leads manufacturers of joints with load transmission systems to reformulate the technical data sheets of their products.s.
In addition to the structural strength of the pavement, the designer must minimize the opening of cracks in the pavement. The only way to guarantee a crack-free floor is to consider a post-tensioned floor, which results in higher construction costs. That being said, every precaution must be taken to ensure that the construction of the floor is as correct as possible to limit cracking.
Since joints are the most vulnerable points on any pavement, attention must also be paid to the detail of joint edge protection due to impacts caused by the wheels of load vehicles (point 11.9). The use of construction/expansion joints that incorporate arris protection devices in your system are particularly important.
Since the concrete will always shrink, it is not possible to completely eliminate the use of joints. However, in order to avoid problems associated with saw joints, areas separated by joints every 35m can be accomplished. This construction method is generally associated with the use of steel fibers although a fiber-mesh combination can be used. In this case, considering an open range in order of 0.5mm/m, care must be taken to ensure a sufficient load transmission for openings values of 18mm. The TR34 4th Edition specifies some points that must be followed in order to minimize cracking (point 11.1):
- Control sub-base flatness;
- Use of uncoupling membrane over the sub-base;
- Optimized concrete mix;
- Attention in layout and type of joints;
- Use of isolation joints with singular points of restriction (column bases, machines foundations, etc.).
LOAD TRANSFER AT JOINTS
Joints between adjacent slab panels must be properly executed in order to ensure effective load transfer thus limiting the differential deformations between panels. In case of load transfer is not ensured there will be pathologies associated to the wheels impact on the arris of the slabs.
In case ground-slabs, the number of effective transmission dowels is calculated as 0.9L for each side of the applied load, L is the radius of relative stiffness calculated according to section 7.5 of the Tr34 4th edition.
Is not possible a load transmission more than 50%. The plate immediately below the load assumes the main load and this value decreases linearly for the remaining plates. If there is a second load that is at a distance less than 0.9L, the overlapping of stress must be considered.
The subgrade modulus K has extreme influence on the L value. With larger k the smaller L will be, so fewer dowel plates will withstand the load.
This publication is the result of a detailed review of all design and construction aspects of slabs on ground by a multidisciplinary team of engineers, contractors, materials specialists and users.