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| Problem connected to traditional joints |
COSTRUCTION JOINTS IN CONCRETE SLABS
Construction joints are placed in a concrete slab to define the extent of the individual place-ments, generally in conformity with a predetermined joint layout.
They must be designed in order to allow displacements between both sides of the slab but, at the same time, they have to transfer flexural stresses produced in the slab by external loads.
Construction joints must allow horizontal displacement right-angled to the joint surface that is normally caused by thermal and shrinkage movement. At the same time they must not allow vertical or rotational displacements. Figure 1 summarizes which displacement must be allowed or not allowed by a construction joint.
DILAPIDATED STATES OF CONSTRUCTION JOINTS IN CONCRETE SLABS.
Incomplete or wrong execution of construction joints may lead to the formation of defects and damages in a concrete slab, the most common being the chipping of the concrete edge (Figure 2) caused by heavy-wheeled traffic.
HOW TO MAKE A CONSTRUCTION JOINT
The most common way to make a construction joint is the so-called “dowelled joint”. This is created by introducing horizontal metal bars (dowels), between two concrete slabs which are laid down at different times and aligned to be parallel in both the horizontal and the vertical planes. Dowels force concrete on both sides of the joint to deflect equally when subjected to a load, allowing the joint to open as the slabs shrink.
A dowelled joint is normally made using the following procedures:
1.Placing of a side formwork before casting the slab. The formwork must be vertical, straight and as high as the concrete slab.
2.Smooth dowels of appropriate size must be introduced through suitable holes drilled into the formwork with a diameter and spacing dependant on the slab thickness. It is important to ensure that the dowels remain properly aligned during the placing and finishing operation.
3.The day after placing the first side of the slab the formwork is removed and grease is applied to the dowel external surface in order to facilitate relative displacements between bars and concrete. As an alternative to grease, it is possible to use a plastic sheath of suitable diameter.
4.Placing of the second side of the slab;
5.No later than two days after placing the concrete, a saw cut is made on the upper side of the joint and filled with silicone sealant or other equivalent methods.
DIFFICULTIES IN THE PLACING OF A DOWELLED JOINT
The first mistake is committed setting the side formwork. A partial formwork, located on the upper side of the slab, is preferred to a complete formwork, as it seems easier this way to place the dowels across the joint. Placing the dowels with a complete formwork requires the drilled holes in the wooden form. Calibrated holes hinder the removal of formwork the day after casting, whereas oversized holes do not maintain the alignment of dowels if the clearance between hole and bar is not filled with wood shims. These operations require care, attention to detail and time.
Dowels, therefore, are often placed over a curb made with fresh dry concrete and partial formwork consisting of a wooden plank laid over. (Figure 4).
The incorrect fabrication of a dowelled joint can cause serious damage in to the slabs. Firstly, it is almost impossible to maintain the bars in the proper position during the casting operations (Figure 5). The lack of parallelism between the bars and the subsequent incorrect angle between them and the joint layer may induce the formation of cracks as described in Figure 6.
An irregular surface at the joint interface may cause further damage with the formation of concrete corbels as shown in Figure 7. This type of damage can occur either with or without the bars.
RE-BAR MAT AND THE CONSTRUCTION JOINTS
Another frequently made mistake in the creation of dowelled joints is the placing of the re-bar mat across the joint (Figure 8a). In this way, steel re-bars inhibit every horizontal displacement that must be allowed and may cause the opening of cracks (Figure 8a). It is known that the right posi-tion of the mat is on the upper side of the slab at 1/3 the slab depth below the surface (Figure 8b).
The “C”-shaped stirrups, welded on both sides of the joint, provide support for the re-bar mat that can then be correctly located in the slab (Figure 8-b).
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Costruction Joints 
