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Annual North Queensland Concrete Engineering and Te chnology Seminar
Postponed fromPostponed fromPostponed fromPostponed from Friday, 11 th February 2011
North Queensland Regional Committee In partnershi p with
Correct use of joint systems in concrete slabs and pavements
Presented byBruce Ireland, B EngProduct Development ManagerDanley Systems
There are a number of JOINT SYSTEMS used in concret e slabs and pavements. Among these are:
�Mechanical systems for protecting the edges of join ts
�Filling systems to prevent ingress of detritus or “s tuff” getting into opened joints
�Mechanical systems for load transfer across the joi nt�Mechanical systems for load transfer across the joi nt
This presentation will only address selected aspects of load transfer across the joint
For those of you who have experienced my Design for Load Transfer at Joints in Concrete Slabs and Pavementspresentation; or some of my other presentations …..
….. bear with me – there’s some new stuff here
When you want to move stuff around, you can use ….
…. a Morris Minor ute …
.... or a Toyota Landcruiser cab chassis with tray ...
…. or perhaps a Nissan 4 tonne …. or perhaps a Nissan 4 tonne dropside tray
Each one eminently suitable for particular jobs, and with unique attributes
Each of these vehicles has:
� A body with tray to carry the load � Wheels for movement� Steerable front axle to allow movement in any direc tion
in the horizontal plane
Similarly, load transfer systems need: Similarly, load transfer systems need:
� A dowel to carry the load� A mechanism [sleeve] to allow for movement� Sleeves to accommodate movement in any direction in
the horizontal plane
[i.e. body, wheels and steering]
System Carryload
[body?]
Allowmovement?[wheels?]
In 2 directions?[steerable?]
Round dowels[greased, shrink-wrapped or with tubular sleeves]
Yes Yes NO
Dowel systems …
with tubular sleeves]
Square dowels [with rectangular sleeves] Yes Yes Yes
Plate dowels [rectangular, square or rotated with sleeves or tapered]
Yes Yes Yes
Consequences of movement restraint:
Joint lock up – leading to cracked slabs
Designs should allow for independent‘floating’ panels that accommodate ‘floating’ panels that accommodate natural concrete curing shrinkage both perpendicular and parallel to the joints
Ah, but you say “This joker is only trying to promote his products
I’ll beat him at his own gameI’ll beat him at his own game
I’ll still use round dowels because they are cheap, and use them with a rectangular sleeve”
Round dowel through a square aperture in rectangular dowel sleeve
A 20 mm round dowel blocks off 78% of the opening; or, in other words leaves 22% open area in the corners of the dowel sleeve that can allow the concrete slurry to enter the sleeve
Round dowel through a rectangular aperture in a rectangular dowel sleeve
If the dowel sleeve has inside dimensions of 20 mm high x 30 mm wide to allow for +/- 5 mm sideways movement, open area is 48%, so more risk of concrete slurry entering sleeve
Consequences of concrete slurry entering sleeve:
Dowel movement restrained
Joint lock-up
Cracked slabs
Even for a square dowel through a rectangular aperture in a rectangular dowel sleeve
For dowel sleeve with inside dimensions of 20 x 30 mm wide to allow for +/- 5 mm sideways movement, open area is 33%, allowing entry of concrete slurry
Square dowel through a square aperture in a rectangular dowel sleeve
For 20 mm square dowel in nominal 20 mm square opening, open area is virtually zero, so there is virtually zero risk if concrete slurry entering the sleeve.
But square dowels are more expensive than round dowels ….Yes, the mass per metre [and hence cost per dowel] of say 20 mm square dowels may be 127% of the mass of 20 mm roun d dowels
But testing has shown that 20 mm square dowel have approximately 120% greater capacity than 20 mm round dowels in th e same thickness and strength of concrete
So square dowels can be spaced at greater centres t han round So square dowels can be spaced at greater centres t han round dowels to achieve the same load transfer capacity p er metre of joint
Specified dowel Equivalent dowel
R16 or R20 at 300 crs S16 or S20 at 350 crs
R16 or R20 at 400 crs S16 or S20 at 475 crs
R16 or R20 at 450 crs S16 or S20 at 525 crs
R16 or R20 at 500 crs S16 or S20 at 600 crs
This means that fewer square dowels and sleeves wil l be needed
Because site labour has a major impact on the overa ll cost of the supply and installation of the dowel systems, the m inimal cost impact of the square dowels themselves is almost always mo re than exceeded by the savings provided by the reduction in labour costs to install fewer dowels
And don’t forget the benefits of actually having a system that really works
This is what the owner of the facility is expecting of your design or
installation
Dowel spacing …Spacing dowels further apart in thin concrete elements prevents overlap of shear cones, fully utilising the concrete capacity available to each dowel.
When dowels are closely spaced in thin concrete elements, shear in thin concrete elements, shear cones overlap, reducing the available concrete capacity per dowel.
Also, spacing dowels close together in thin concrete elements can lead to “zipper” type failures along the line of the dowels.
Dowel length …I frequently see specifications for 20 mm dowels 600 or 800 mm long
I suspect that the logic for this is the longer the dowel, the greater the load resistance
In practical terms, the highest loading occurs in the first 50 mm loading occurs in the first 50 mm from the joint face [A to B on thediagram at right], so dowels 300 to 400 mm long are usually more than appropriate
What is the outcome of specifying long dowels? Wasted resources. Higher costs. Specials that take longer to source.
Dowel positioning during installation …Dowels should be located in the half-thickness with a placement tolerance of +/- 5% of thickness
e.g. for 250 mm thick slabdowels need to be 125 mm down from the top edge of the formwith a tolerance of +/- 12 mm
Consequences of incorrect positioning:Consequences of incorrect positioning:These dowels are 75 mm down from the top surface of a 300 mm thick slab
Concrete will perform as a 150 mm thick slab
A FEW WORDS OF CAUTION ….
For those of you who use the 2009 edition of the Cement Concrete and Aggregates Association CCAA T48 Guide to Industrial Floors and Pavements – design, construction and specification
Be very cautious when using Appendix C Design of dowelled joints
I believe this section of the guide is flawed, for these reasons:
Firstly ….Appendix C recommends that loads are resisted by do wels within an area of 90% of the Radius of Relative Stiffness fro m the centre of the load
The formula for determining Radius of Relative Stif fness contains a factor for modulus of sub-grade reaction [k] in the divisor
So, the smaller the sub-grade modulus the greater t he extent of relative stiffness
But edges of concrete slabs curl, so there is no su pport from the sub-grade around the perimeter of each slab, and the mo dulus of sub-grade reaction is zero
With zero in the divisor, the Radius of Relative St iffness is infinitely large – so cannot be appropriate
Secondly ….While Appendix C does require bearing capacity of t he concrete at the dowel be checked, it does not require a check to be carried out on the tension shear [bursting] capacity of the concrete a djacent to the dowel, which is far more critical than bearing
Failure in concrete in bearing is rarely the mode o f failure – it is almost always caused by shear load failure of the concrete
And T 48 gives no guidance on design of shear tensi on [bursting] capacity of concrete at dowels …..
160
180
200
220
240
260
280
300
320
340
360
380
400
Des
ign
capa
city,
φφ φφ
V [k
N]
Exa
mpl
e in
CC
AA
T48
App
endi
x C
of
roun
d do
wel
s at
300
mm
cen
tres
in 4
0 M
Pa
conc
rete
to A
S 3
600
and
AS
410
0
369 kN bending capacity in steel of dowel for 3 mm joint width in example
94 kN bearing capacity of dowel on concrete
0
20
40
60
80
100
120
140
160
100 150 200 250 300
Slab Thickness [mm]
Des
ign
capa
city,
Exa
mpl
e in
CC
AA
T48
App
endi
x C
of
roun
d do
wel
s at
300
mm
cen
tres
in 4
0 M
Pa
conc
rete
to A
S 3
600
and
AS
410
0
concrete67.3 kN shear capacity of steel in dowel
Bursting capacity of concrete adjacent to dowel41 kN in 230 thick slab
Perhaps the best dowel load capacity information is contained in the load charts in the Danley® “High Performance Dowel
Systems” brochure
