R. Troli –Construction joints in concrete pavements
CONSTRUCTION JOINTS IN CONCRETE SLABS
Ing. Roberto Troli
CONSTRUCTION JOINTS IN CONCRETE SLABS Construction joints are placed in a concrete slab to define the extent of the individual placements, 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.
Not allowed movements
Allowed movements
(a)
(b)
Figure 1 – Relative movements which must be (a) allowed and (b) not allowed by a construction joint for concrete slabs
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R. Troli –Construction joints in concrete pavements
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 slabs, the most common being the chipping of the concrete edge (Figure 2) caused by heavy-wheeled traffic.
wheel chipping
Vertical displacement
Figure 2 - Typical damage due to the lack of construction joints
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R. Troli –Construction joints in concrete pavements
Figure 2a and 2b – Some examples of dilapidated state of the construction joints
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. 2.
3.
4. 5.
Placing of a side formwork before casting the slab (Figure 3a). The formwork must be vertical, straight and as high as the concrete slab. 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. 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. Placing of the second side of the slab (Figure 3c); 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 (Figure 3d). DIFFICULTIES IN THE PLACING OF A DOWELLED JOINT
Correct creation of a dowelled joint involves the execution of a series of delicate and important operations. Often a lack of time or care and insufficient remuneration causes workers to make mistakes that can produce serious defects in the slabs.
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R. Troli –Construction joints in concrete pavements
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. (Figure 3a). These operations require care, attention to detail and time.
transverse
connection bar
dowel
dowel 1 1
formwork
wedges
grease or sheath (b)
(a)
2
sealed cut
1
(c)
(d)
Figure 3 – Steps required in the formation of a “dowelled joint”
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.
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R. Troli –Construction joints in concrete pavements
Figure 4 – Incorrect fabrication of a construction joint
Figure 5 –Lack of alignment of dowels in a construction joint cast with a partial formwork
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R. Troli –Construction joints in concrete pavements
cracks
dowel Figure 6 – Cracks caused by a lack of alignment of dowels
concrete corbel
crack
concrete reflow under a partial formwork
Figure 7 – Damage caused by partial formwork
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R. Troli –Construction joints in concrete pavements
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 4). 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 position 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 11-b).
crack
h/3 2h/3
rebar mat (a)
(b)
Figure 8 – (a) Incorrect and (b) Correct position of the re-bar mat
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R. Troli –Construction joints in concrete pavements
IDEAL WORK CONSTRUCTION JOINT The Ideal Work Joint is a pre-assembled, dowelled joint produced by Ideal Work. The Ideal Work Joint solves all the problems associated with the placing of a dowelled joint. The Ideal Work Joint, thanks to its patented design, allows the two slabs to move in parallel and at the correct angle to the dowels (Figure 9b).
Dowel
Ovoidal PVC muff
Figure 9b – Upside view of a bi-directional joint Allowed movements
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R. Troli –Construction joints in concrete pavements
Figures 10a ÷ 10c shows images of an IDEAL WORK JOINT.
Figure 10a Image of an IDEAL WORK JOINT.
Figure 10b Ideal Work joint
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R. Troli –Construction joints in concrete pavements
Figure 10c Sketch showing correct placement of an IDEAL WORK JOINT.
The placing of The Ideal Work Joint is very simple. A thick (≠ 3mm) L-shaped double steel plate, available in standard depths of 10, 15 and 20 cm and custom measures on request, does away with the placing of side formwork giving considerable savings in time. It is sufficient to lay the joint on a properly leveled sub-grade, ensuring that the upper edge of the joint is aligned with the desired height of the casting, and insert wooden picket troughs in the steel stirrups on both sides of the joint to resist the concrete movement (figure 11a). A steel ring welded on both sides of the joint allows the rapid insertion of oval PVC muffs that lodge the steel dowels. The dowels are made from LG 800 - Ø 20 or 18 mm smooth bars, depending on the thickness of the slab. After casting the first side of the slab, the pickets are removed, steel dowels inserted in the PVC muffs and the second side of the slab cast. The following day it is possible to remove the plastic screws that join the two plates and then seal the joint with silicone or resins (Figure 11c).
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R. Troli –Construction joints in concrete pavements
PVC muff
stirrups Wood pickets
(a)
1st concrete side
rebar mat
dowel
(b)
Silicone sealing
2nd concrete side
(c) Figure 11 – Procedures for making construction joints using THE IDEAL WORK JOINT system. Some images of Ideal Work Joint’s past applications are shown in the following pages.
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R. Troli –Construction joints in concrete pavements
Placing and leveling an Ideal Work Joint
Placement of the first portion of a concrete slab
Concrete placing beneath the joint
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R. Troli –Construction joints in concrete pavements
Hand screeding and finishing operations
Insertion of the steel dowels through the joint
IDEAL WORK JOINT showing no damage after several years in use
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R. Troli –Construction joints in concrete pavements ®
IDEAL WORK SYSTEM®: “THE SLIDING JOINT ” In very long joints or in the case of crossing of two right-angled dowelled joints, the formation of cracks may occur due to restrained movements along the joint (Figures 11 and 12).
cracks
Restrained displacement
Figure 12 – Cracks induced by restrained displacement along the joint
cracks
dowel
construction joints
Figure 13 – Cracks induced by the crossing of two right-angled construction joints
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R. Troli –Construction joints in concrete pavements
This problem cannot be resolved using normal “in place” dowelled joints. Double movement Ideal Work Joint has been designed in order to resolve these problems. This special joint differs from the other joints because of the ovoidal shape of the PVC muff cross section (Figure 13). The presence of two plastic plates inside the muff allow for a right location of the bar (orthogonal to the joint plane) even in the absence of displacements. This kind of joint is specifically recommended in the case of very long joints or in the case of two orthogonal joints crossing each other. The double movement joint, however, is recommended even for the making of normal construction joints, seen that parallel joint movements can occur due to thermal changes on all types of pavements.
Plastic plates which maintain the bar in a right position in the absence of transverse displacements
Ovoidal muff steel bar Figure 14a – Cross section of an ovoidal muff
When the floor “moves”, the plastic plates are broken so transverse movements of the dowel became possible
ovoidal muff dowel Figure 14b – Functional sketch of a bi-directional joint
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R. Troli –Construction joints in concrete pavements
ADVANTAGES OF USING “THE IDEAL WORK JOINT” The use of The Ideal Work Joint gives many advantages over the ‘on-site’ placing of a dowelled joint. 1) It is no longer necessary to place side formwork as The Ideal Work Joint acts as formwork during placing. 2) Perfect alignment of dowels is ensured by the presence of PVC muffs inserted in steel rings welded to the joint. 3) The “C”-shaped stirrups, welded on both sides of the joint, provide a support for the re- bar mat which can be correctly located within the slab. 4) The two thick steel sheets which constitute The Ideal Work Joint act as strengthening for the concrete upper edges of the slab avoiding the chipping induced by hard-wheeled traffic. 5) Introducing steel re-bars into the C-shaped stirrups make it possible to create a reinforced concrete curb that strengthens the edges of the slabs. 6) No more problems of concrete flowing through or under the formwork. The result is a perfectly vertical cut between the two slabs. 7) There is no need to wait for sufficient hardening of the concrete to seal the upper side of the joint, as it is not necessary to make a saw cut along it. The sealing can be done immediately after the second casting or the next day. In conclusion, therefore, by using The Ideal Work Joint it is possible to save considerable time and money in casting operations, ensure quality of joints and slab integrity and reduce future maintenance to an absolute minimum all for a low initial expenditure.
REFERENCES [1] M. Collepardi, L. Coppola, “Mix Design del Calcestruzzo”, Appendice 6, Edizioni Enco, Spresiano, Settembre (1991). [2] A. M. Neville, “Properties of Concrete”, Fourth Edition - Longman Group Lim., Harlow, England (1995). [3] “Guide for Concrete Floors and Slabs Construction”, ACI 302.1R-89, ACI Manual of Concrete Practice (1994). [4] “Design of Slabs on Grade”, ACI 360R-92, ACI Manual of Concrete Practice (1994). [5] O. Belluzzi, “Scienza delle Costruzioni”, Volume 3°, Edizioni Zanichelli, Bologna (1941).
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