1. SESSION 7
Joint Design
This session discusses joint design for jointed plain concrete pavements. Historically this is an item that is often ignored or overlooked, with designers often blindly adopting available design standards. However, joint design is a critical part of pavement design and is intricately linked with slab thickness and the support conditions, and therefore must be considered as a unique element for each design project. Indeed, many (if not most) pavements that fail prematurely do so because of poor joint design, such as excessive joint spacing or inadequate load transfer, and not because of inadequate slab thickness.

2. Objectives Identify types of joints Determine suitable joint spacings
Determine load transfer requirements
Develop joint reservoir designs
Define tie bar requirements for longitudinal joints
The objectives for this session are listed here. Note that the focus of this session is on the design of transverse joints, although some discussion will be included on longitudinal joint design.

3. Types of Joints Contraction joint Construction joint Expansion joint
To begin this session, let’s first define the primary types of joints used in concrete pavements. The three types are listed here and are roughly named to describe the function that they perform. Lets look at each of these by testing your knowledge...

4. (transverse or longitudinal)
Joint Type?
Butt Joint
Slab
Thickness
Dowel or Tie Bar
What type of joint is this? Yes, a construction joint (or butt joint) where two adjacent slabs butt up against each other. Used at end of days paving or as a means of facilitating construction (e.g., paved shoulder that is paved after mainline pavement). Thus, can also be placed either transversely or longitudinally.
Construction Joint
(transverse or longitudinal)

5. Contraction Joint (transverse or longitudinal)
Joint Type?
Initial
Sawcut
Slab
Thickness
Dowel Bar
or Tie Bar
What type of joint is this? That’s right, a contraction joint. Contraction joints, also called weakened plane joints, are created by forming a notch (either through sawing or forming) in the surface of the pavement, thus creating a weak area below which the pavement will crack. This joint can be placed either transversely or longitudinally and is the most common type of joint.
Contraction Joint (transverse or longitudinal)

6. Joint Type? Expansion Joint Expansion Joint with Filler Material
Slab
Thickness
Dowel Bar
What joint type is this? Yes, it is an expansion joint. Expansion joints contain an open area to allow expansion of slab. Once the predominant type of transverse joint in concrete pavements (up through the 1940s), today it is most commonly used where?? That’s right, near bridges or other fixed structures. Expansion joints can also be either transverse or longitudinal.
Expansion Joint

7. Elements of Joint Design
Transverse joints
Joint spacing
Load transfer design
Sealant reservoir design
Longitudinal joints
Tie bar design
These are the primary elements of concrete pavement joint design that we will be covering in this session. Again, we will be focusing on transverse joint design.

8. JPCP Joint Spacing Short enough to prevent mid-slab cracking
Intricately linked with:
Slab thickness
Base support
Climatic conditions
Generally between 3.6 and 6.1 m (12 and 20 ft)
So lets discuss transverse joint design, beginning with joint spacing. The purpose of transverse joint spacing is to control the development of random transverse cracking. Joints should be short enough to prevent that cracking, yet long enough to reduce doweling and sawing/forming costs. While joint spacing historically has just been selected, studies have shown that the spacing is linked to slab thickness, base support, and climatic conditions. Generally spacings between 3.6 and 6.1 m (12 to 20 ft) are used, although thicker slabs can have longer joint spacings (e.g., airport pavements).
What spacing is used here? How has it worked?

9. Example Joint Spacing Guidelines
Here are some example joint spacing guidelines from the NCHRP 1-32 design catalog. Note that this is for a wet/dry freeze climate and that variables include subgrade support and slab thickness. For example, for a k = 300 psi/in (81 Mpa/m) and a 12 in (305 mm) slab, this says the maximum joint spacing should be no more than 17 ft (5.2 m). Of course, these guidelines should be tempered with local experience.
Ask:
--What effect does climate have on joint spacing recommendations?
--Why can thicker slabs have longer joint spacing?
--Why do stiffer support conditions require shorter joint spacing?

10. Uniformity of Joint Spacing
Uniform joint spacing
Joints spaced at fixed intervals
Variable joint spacing
3 or 4 joint spacings in a repeating pattern, e.g., m ( ft)
Intended to reduce rhythmic response of vehicles
Joints may be placed either at fixed intervals (e.g. 6.1 m [20 ft]) or at variable spacings ( m [ ft]). The variable joint spacings came out of research conducted in the late 1950s as a means of reducing the rhythmic response of vehicles to faulted joints. It is not clear whether it is appropriate for today’s vehicles and suspension systems, or whether it is appropriate for pavements with doweled joints (where faulting is presumed to be controlled). Because of the problems in constructing variable spaced joints, and the greater costs for dowel bar insertion on these designs, the current trend is now back to uniform joint spacings.

11. Joint Orientation Perpendicular Joints perpendicular to centerline
Skewed
Joints placed at an angle to pavement centerline (counterclockwise skew)
May be beneficial for nondoweled joints
Limit skew to minimize corner breaks (maximum 1:10)
Another variation in joint design is the layout of the joints. Joints can be placed perpendicular to the centerline of the pavement, or at an angle to the pavement in a counterclockwise skew. The idea is that only one wheel passes over the joint at a time, thus reducing the load on the joint. Some evidence exists that suggests that skewed joints may be beneficial in reducing faulting of nondoweled pavements, but again its effectiveness is questioned for doweled pavements. It is important to limit the skew to minimize the risk of corner breaks (no more than 1:6 (0.6 m per 3.7 m lane [2 ft per 12 ft lane]), although some agencies suggest no more than 1:10 (0.4 m per 3.7 m lane [1.2 ft per 12 ft lane]).

12. Example Variable Spacing and Skewed Joints
Traffic 1 10
This drawing illustrates the concept of skewed joints and variable joint spacing (describe). Minimizing the skew angle minimizes the chances for corner breaks. 1:10 maximum skew recommended.
3.6 m
4.6 m
4.0 m
4.3 m
(12 ft)
(15 ft)
(13 ft)
(14 ft)

13. Skewed Joints
Photo of a project on I-80 near Reno, Nevada showing cracking of acute corners on a pavement with skewed joints (2:12 skew). Curling/warping of longer slabs contributes to this problem.

14. Load Transfer
Ability of joint to convey wheel load from one side to the next
Reduces deflections
Reduces pumping, faulting
Methods
Dowels
Aggregate interlock
The next joint design item is load transfer, defined as the ability of a joint to convey wheel loads from one side to the next. The purpose is to reduce the magnitude of deflections, which in turn can reduce pumping and faulting.
Load transfer is accomplished through dowel bars placed across the transverse joints at the mid depth of the slab, or by aggregate interlock, the interlocking of aggregate particles of abutting joint faces. Note that underlying base courses (especially stabilized bases) also contribute to load transfer but are not considered a formal load transfer method per se.

15. Load Transfer Illustration
Wheel
Load
Direction of Traffic
Approach Slab
Leave Slab
Unloaded
LT =
Loaded
This is a schematic of the concept of load transfer, which is defined as the deflection (or stress) on the unloaded side divided by the deflection (or stress) on the loaded side.
In the top figure, no load transfer exists, meaning that the approach slab moves freely and gains no support from the leave slab. In the bottom figure, the leave slab assists the approach slab in supporting the wheel load as it moves across the joint.
Discuss how load transfer is measured in the field and the difference between deflection and stress load transfer.
100% Load Transfer
Approach Slab
Wheel
Load
Direction of Traffic
Leave Slab

16. Load Transfer Recommendations
Dowels recommended for most highway pavements (slab thickness > 200 mm [8 in])
Minimum 32 mm diameter (38 mm preferred)
Corrosion inhibitor required
Recommendations for load transfer design are given here.
Dowel bars are recommended for all medium and high traffic facilities (pavements that are thicker than 200 mm [8 in] ).
A minimum 32 mm (1.25 in) diameter dowel bar is recommended, with recent movement toward larger bars (38 mm [1.5 in]). A common rule of thumb has been the dowel diameter should be 1/8 of the slab thickness (e.g., 305 mm [12 in] slab should have 38 mm [1.5 in] dowel).
It is also recommended that the dowels be coated with a corrosion inhibitor to prevent corrosion and subsequent lock up of the dowels (which can cause severe joint damage). Epoxy is the most commonly used corrosion inhibitor, although there are some concerns expressed by a few agencies regarding the long-term performance of epoxy coated dowels.

17. 12 dowels @ 0.3 m (1 ft) center to center
Dowel Layout
Conventional Spacing
Traffic
Traffic
Outer Traffic Lane
Inner Traffic Lane
m (1 ft) center to center
This is a schematic showing the conventional dowel layout. In this configuration, dowels are spaced at uniform 0.3 m (1 ft) intervals across the transverse joints. The dowels are either placed on baskets or implanted into the plastic concrete.
Where a widened slab exists, some agencies may place a dowel bar in the outside widened area, depending on the width of the widening. Other agencies do not (e.g., Minnesota widens their PCC slabs 0.46 m [1.5 ft] and does not place a dowel bar in the widened area).

18. Alternative Dowel Layout
Cluster Spacing
Traffic
Traffic
Outer Traffic Lane
Inner Traffic Lane
5 0.3 m
(1 ft) center to center
4 0.3 m
As an alternative, this figure shows the use of cluster spacing of dowel bars. In this case, it is attempted to cluster the dowels in the wheelpaths where the traffic is expected to travel. Studies have shown that this layout produces about the same magnitude of stresses and deflection as the conventional spacings in response to traffic loading. However, preliminary studies from instrumented dowels in Ohio indicate that, for a 254-mm (10-in) slab with 6.4-m (21-ft) joints, the environmental stresses on dowels are much greater than the load stresses (the slab faces rotate in response to curling and warping, and the dowel bars resist that rotation). This suggests that fewer dowels in a clustered arrangement may not be sufficient to resist environmental forces in a new pavement design. Thus, this dowel layout should be applied with caution. (The use of clustered dowel spacing is still appropriate for the installation of retrofitted load transfer because the environmental forces/stresses that develop soon after construction are not a concern on an aged pavement.)
Other issues to consider in conventional vs. cluster spacing debate:
--Does traffic always travel in the wheelpath? There can be considerable traffic “wander.”
--Just based on the reduced number of dowels, cluster spacing could cost about 75% (18/24) of conventional spacing. On a long project, this could represent a significant cost savings but can we guarantee performance??

19. Joint Sealing and Reservoir Design
Purposes of joint sealing
Reduce moisture infiltration
Keep out incompressibles
Cost-effectiveness of sealing?
The final part of transverse joint design is joint sealing. The purpose of joint sealing is to reduce moisture infiltration (thereby reducing pumping and faulting) and prevent incompressibles from getting into the joints (thereby preventing spalling, blowups, and other pressure damage).
Lately the cost effectiveness of joint sealing has been called into question, that is, do the benefits outweigh the costs of sealing and resealing over the life of the pavement. Some agencies believe it is not cost effective and therefore do not seal joints; WI in particular does not seal transverse or longitudinal joints, but often employs permeable bases (and in some cases has a course subgrade that provides some bottom drainage) and short joint spacings, which together are believed to contribute to the performance of nonsealed joints.
Also the relative effectiveness of sealing in preventing moisture infiltration is not known. A Minnesota study showed that, within about 2 or 3 weeks of sealing, the same amount of water was infiltrating a pavement system as before sealing. More field studies are needed.
What are the feelings of this group on the effectiveness of joint sealing?

20. Consideration Factors
New or rehabilitation design
Climate
Joint design
Base and subgrade type and drainability
Local experience
Others?
What are some of the factors to be considered in determining whether to seal joints?
--Rehab design with wide joints are not candidates for unsealed joints
--Wetter climates may require sealing
--Narrow joints with dowels and short joint spacing are more appropriate for unsealed joints
--Drainability of base and subbase may provide bottom drainage for unsealed joints
--Local experience indicates relative effectiveness/ineffectiveness of sealed/unsealed joints
Are there others?

21. Joint Channel Design Unsealed joints
Crack control sawcut (3 mm [1/8 in]
If joints are to be left unsealed, it is recommended that the joint channel be narrow (3 mm [0.12 in]), as shown in the photo (I-90 near LaCrosse, WI). This is the typical width of the initial crack control sawcut, so no widening cut is performed. Short joint spacings are needed to minimize joint movement which could otherwise allow larger incompressible materials into the joint.

22. Joint Channel Design (continued)
Sealed joints
Crack control sawcut (3 mm [1/8 in])
Joint reservoir sawcut (typ. 10 to 15 mm [0.4 to 0.6 in] wide)
If joints are to be sealed, the initial crack control sawcut is still made, but this is then followed up with a widening cut to establish the joint reservoir. Typically a widening cut of 10 to 15 mm (0.4 to 0.6 in) is made. Narrow widening cuts are preferred to provide opportunity for future resealing (which may require some partial joint widening). Some saw blades are available such that the crack control sawcut and the widening cut can be done in a single operation.

23. Joint Reservoir Sawcut
Widening Cut
Depth of Widening Cut (25 to 38 mm) (1 to 1.5 in)
Here is a schematic illustration of the widening cut used to produce the joint reservoir. The width of the cut will depend upon the sealant to be used and the anticipated joint movements (to be discussed next). The widening cut can be done as a separate operation after the initial sawcutting, or may be accomplished during the initial sawing using a specially equipped blade. The depth of the widening cut is about 25 to 38 mm (1 to 1.5 in) deep.
Crack Control Sawcut

24. Joint Reservoir Design
Selection of sealant material
Estimation of joint movements
Determination of required joint width
The design of the joint reservoir system is based on these three factors, as will be discussed in the following slides.

25. Sealant Materials Rubberized asphalt (ASTM D3405) Silicone
Preformed compression seals
Placed in state of compression
Must be compressed 20 to 50% of normal width over service life
These are the common sealant materials used today. Performance studies suggest longer lives for silicone and preformed seals, although some silicone sealant installations have not performed well due to improper installation (joint must be clean) or an incompatibility between the concrete coarse aggregate and the silicone seal. Preformed seals have shown good performance in field studies up to 15 years. Note that preformed seals and silicone sealants are typically not used in longitudinal joints because of their higher costs and less movement.
What sealant type is used by your agency?

26. Joint Reservoir Width 3 to 6 mm (1/8 to 1/4 in) Recess Depth Backer
Rod
Joint Sealant
Here an example of a joint reservoir for hot-poured or silicone sealants. Note the initial sawcut at the bottom and the widened reservoir at the top (the widening cut is about 25 to 38 mm [1 to 1.5 in] deep). The backer rod (commonly a closed cell polyethylene or a polyurethane foam) is placed at the bottom of the joint reservoir to prevent three sided adhesion and to establish the proper shape factor (W:D). The shape factor is important to the performance of the sealant material. Note that a joint reservoir is not required for longitudinal joints where less movement occurs.
Shape Factor = W / D

27. Example Compression Seal Installation
Compressed Width
Reservoir Depth
Preformed Compression Seal
Here is an example of a joint reservoir for a preformed compression seal. The seal is designed to compressed between 20% and 50% of its normal width. Thus, the seal should have an uncompressed width of about twice the width of the reservoir. For example, a 13 mm (0.5 in) seal will be compressed to a width between 6.5 and 10 mm (0.25 and 0.4 in). The joint width and appropriate seal size are designed so that the compression seal will be able to handle the expected openings.
If the seal is compressed too little, it will drop down in the joint; if it is compressed too much, “compression set” will occur and the seal will not function. Compression set occurs when the seal takes on the compressed width and is no longer capable of returning to its original uncompressed width.

28. Estimating Joint Movements
L = C L ( T + )
L = Joint opening, in
C = Adj. factor (0.8 gran. base, 0.65 stab.)
L = Joint spacing, in
 = Thermal coef. of expansion (3.8 to
6.6 x 10-6), in/in/oF
T = Temperature range, oF
 = Drying shrinkage coefficient (2 to
8 x 10-4), in/in
Here is the traditional equation used to estimate joint movements in a new concrete pavement. The equation shows that joint openings increase with increasing slab length, temperature differences (computed as the temperature at the time of PCC placement minus the average minimum temperature), shrinkage, and thermal coefficient of expansion. For joints longer than about 15 m (50 ft), significant openings can occur.
It should be noted that the openings computed using this equation will represent average movements, and some extreme openings above and below this value are expected. Field studies have shown this provides reasonable estimates of long-term openings, although short-term openings may be significantly different, primarily because not all of the joints crack immediately after sawing (nonuniform cracking and opening of joints due to environmental conditions, base types, etc.). Also, skip sawing of transverse joints may also cause excessive joint movements.

29. Required Joint Reservoir
Hot-poured/silicone sealants
Required joint width
W = L / S
W = Required joint width
L = Joint opening
S = Allowable sealant strain
Required sealant depth
Apply proper shape factors
Once the joint sealant has been selected and the expected joint opening has been computed, the required joint width is computed from this formula. The allowable sealant strain depends on the sealant type:
--Rubberized asphalt 0.15 to 0.50 (W:D of 1:1)
--Silicone sealant 0.3 to 0.5 (W:D of 2:1)
The shape factors are used to determine the required depth of sealant, for example, if the required joint width is 12.5 mm (0.5 in), and the shape factor is 1:1, then the depth is mm (0.5 in).

30. Required Joint Reservoir (continued)
Compression seals
Select uncompressed seal width
USW > L / (Cmax - Cmin)
Cmax = 0.5 (typ); Cmin = 0.2 (typ)
Determine width of sawcut
W = (1 - Pc) * USW
Pc = % of compression at installation
For compression seals, the uncompressed seal width (USW) should be selected based upon the anticipated joint openings and the maximum and minimum recommended compression of the seal (Cmax and Cmin), which are generally 0.5 and 0.2, respectively. Be sure to round up to the next available actual seal size (e.g., if calculate 16.5 mm [0.65 in], use a 17.1 mm [11/16 in] seal).
The sawcut width is then determined based on the anticipated state of compression of the seal at the time of compression (Pc), which is based largely on the expected temperature range and the installation temperature. Pc may be estimated as:
Pc = Cmin + [(Install temp - Min. temp) / (Max temp - Min. temp)] * (Cmax - Cmin)
This design procedure is sensitive to the installation temperature. To reduce the sensitivity, select an USW one or two sizes larger than the calculated value, and reduce joint openings by using shorter joints.
Because of the development of nonuniform joint openings, several different sizes of compression seals should be on hand. See FHWA Technical Paper 89-04, Preformed Compression Seals for PCC Pavement Joints for more information on the design of preformed seals.

31. Longitudinal Joint Design
Contraction (sawed) joints
Between lanes or between lane - shoulder
Adequate sawing depth/timing
Effective tie bar system
Construction (butt) joint
Commonly between lane and shoulder
Briefly a few words on longitudinal joint design…Essentially there are two types of longitudinal joints:
Contraction joints--most common type between traffic lanes. Usually sawed, but some were formed using parting strips (plastic inserts) that were not always effective in establishing joint. An effective tie bar system is needed. Tie bars may be mechanically implanted or placed by a ferris wheel or placed on chairs ahead of the paver.
Construction joints--most common between a concrete pavement and a separately paved concrete shoulder, or between two concrete lanes that were paved separately. Again an effective tie bar system is needed. Tiebars may be bent bars that are installed during the paving of the first lane and then straightened after paving (note that many agencies are now prohibiting bending because it is detrimental to the steel and to any corrosion-inhibiting coatings), may be drilled into the first-paved lane, or may use two-component bars that are threaded together after paving of the first lane. Some of the construction joints may also be formed as keyway joints, but these have not always shown good performance (male keyway shears off, particularly on thinner slabs). For slabs less than 254 mm (10 in), keyways are not recommended. Tiebars are still needed with keyways to hold the male and female portions together.

32. Longitudinal Contraction Joint
Joint Formed
by Sawing D
D/2
Deformed Tie Bar
(Minimum No. 5 Bar)
Mainline Pavement
Mainline Pavement or PCC Shoulder
Illustration of a longitudinal contraction joint. Technically, tiebar spacing is a function of the pavement thickness and the distance to the nearest free edge. However, for most Interstate-type pavements, the FHWA recommends a minimum No. 5 bar (16 mm [0.62 in] diameter) spaced at 762-mm (30-in) intervals in order to keep slabs held tightly together. These bars should be 762-mm (30-in) long.
Some agencies restrict the number of lanes that can be tied together to reduce the potential for restraint cracking. Some advocate that no more than 3 lanes be tied together.

33. Longitudinal Construction Joint
Deformed Tie Bar
(Minimum No. 5 Bar)
Butt Joint
Possible Key Way
PCC Shoulder
Mainline Pavement
Illustration of a longitudinal construction joint. Again minimum No. 5 bar (16 mm [0.62 in] diameter) is recommended at 762-mm (30-in) spacings. Note that a keyway is sometimes included, but as discussed before it is not recommended for slabs less than 254 mm (10 in).

34. Summary Joint types Joint spacing guidelines
Load transfer recommendations
Joint sealant system
Longitudinal joint requirements
To recap, these are the key topics that have been covered in this session. What do you remember about each of these? How does this relate to your practices? How can you use this in the future?
Note urban jointing (e.g., around manholes and other utilities) and intersection joint layout design are not addressed here. Refer to the available ACPA literature on these topics.