JOINT DESIGN
Chapter 9 Joint Design, Layout, Construction, Sawing and Sealing
Overview of Contraction Joints
Contraction joints (Figure 9.1 – Types B, C and D) control the location of pavement cracking caused by drying shrinkage and/or thermal con- traction. Contraction joints also are used to re- duce the stress caused by slab curling and warping. Load transfer usually is accomplished in contraction joints by aggregate interlock. How- ever, dowel bars may be used for load transfer at contraction joints under certain conditions; typi- cally at the last 3 joints in a runway or taxiway (consult appropriate design criteria for specific guidance on the use of dowels). Thickness de- sign procedures for FAA and military concrete pavements are based on load transfer through aggregate interlock. Dowels may be required when the natural subgrade k value is less than 100 psi/in. and no base is used.
Overview of Construction Joints
Construction (Figure 9.1 – Type E) separate abutting construction placed at different times, such as at the end of a day’s placement or be- tween paving lanes. Load transfer at construc- tion joints is achieved through the use of dowels.
Isolation Joint Considerations
When joints are designed according to the rec- ommendations of this guide, isolation joints are not required transversely or longitudinally in air- field pavements except at special locations. In- troduction of “expansion” joints on a regular spacing may allow slabs to migrate if contraction joints in interior areas of a concrete pavement open excessively. This unintended consequence degrades the effectiveness of aggregate inter- lock at the contraction joints and reduces the overall performance of the pavement. Thus, the traditional and misleading term “expansion” joint has been modified to “isolation” joint to be more accurate and descriptive and the FAA no longer
The purpose of an isolation joint is to separate intersecting pavements or to isolate structures within or along the pavement. Isolation joints provide freedom for lateral panel movement without any mechanical interconnection that might damage the pavement, structure or fixture.
To be effective, the pre-molded compressible filler should meet the requirements of ASTM D1751, D1752, or D994, and must cover the en- tire thickness of the concrete slab. If an isolation joint is placed within the pavement area and will carry active traffic loads (such as where the pavement abuts a structure like a building) or where horizontal and vertical differences in movement of the pavements are anticipated, a Type A – Thickened Edge is necessary to re- duce edge stress in the pavement. If the isola- tion joint is used along a pavement penetration, building or other non-load area, then a simple butt joint (Type A – Undoweled) typically is used.
All intersections of runway, taxiway, or apron pavements require a Type A – Thickened Edge to separate the facilities, which expand and con- tract along different axes. The concrete panels on both sides of the joint are thickened by 25 percent. The thickened edges are tapered back to the nominal thickness over a minimum dis- tance of 10 ft for FAA and over a minimum 5 ft for military designs, but it is preferable to taper the thickness over the length or width of a full panel.
Longitudinal Joint Considerations
Longitudinal joints are those joints parallel to the lanes of construction and usually the direction of traffic. They are either contraction joints (Type B, C or D) that are sawed between the construction joints or construction joints (Type E) that are formed as the edges of construction lanes. If the new pavement is placed adjacent to and abut- ting an existing concrete pavement, the longitu- dinal joint at or near the interface also might
The pavement thickness and the overall width of the pavement feature (runway, taxiway or apron) are the primary factors determining the spacing of longitudinal joints. The spacing of longitudinal (and transverse) joints also depends upon shrinkage properties of the concrete, soil condi- tions, subbase materials and climatic conditions.
A longitudinal joint spacing that divides the pave- ment section evenly is most advantageous, reli- able and recommended. For example, 37.5 ft wide construction lanes can be used with inter- mediate longitudinal contraction joints at 12.5 or 18.75 ft, depending upon pavement thickness.
Chapter 9 – Joint Design, Layout, Construction, Sawing and Sealing
Contraction:
Type D – Undoweled or Dummy T Type C – Doweled
T/2 ± d/2 T Smooth Dowel: Size Depends Upon Slab Thickness
Type B – Tied or Hinged
T Deformed Tie Bar: 5/8 in. dia., 30 in. long (16 mm dia., 760 mm long)
T/2 ± d/2
Use only on pavement ≤ 9 in. (225 mm)
Note: Use an initial sawcut depth of T/4 on unstabilized (granular) subbases and T/3 on stabilized subbases.
Note: All contraction joints use joint reservoir.
Construction:
Type E – Doweled Butt T/2 ± d/2 T Smooth Dowel: Size Depends Upon Slab Thickness
Note: All construction joints use joint reservoir.
Isolation:
Type A – Undoweled
1/2 – 1 in.
(12 – 25 mm) max.
Fixture or Structure
T Type A – Thickened Edge
T
Note B
3/4 in. (19 mm)
Note A Non-Extruded Pre-Molded
Compressible Insert
Note A: 1.25 T to nearest 1 in. (25 mm) but at least T + 2 in. (50 mm)
Note B: To nearest joint; 10 ft (3 m) minimum
Note: All isolation joints use joint reservoir.
Non-Extruded Pre-Molded Compressible Insert
Construction versus Contraction Joints:
• Contraction joints are cut in the pavement to induce a crack. Load transfer across this crack is achieved through aggregate interlock.
• Construction joints have smooth finished edges, thus dowels must be used for load transfer (aircraft greater than 30,000 lb).
Figure 9.1. Cross sections of different joint types.
Longitudinal joints should be designed to mini- mize width changes for a slipform paver; chang- ing the width of a paver can take up to 3 days, and incurs unnecessary costs and schedule de- lays. Unlike years ago when equipment choices were limited, modern slipform paving equipment permits construction widths up to 50 ft.
All longitudinal construction joints should be Type E doweled joints unless they serve as an isolation joint. For runways and aprons, which are typically wide pavement areas, undoweled joints (Type D) are acceptable for intermediate longitudinal contraction joints unless the joint is one of the last three joints before a free edge or isolation joint; for this exception, a doweled joint (Type C) is recommended. For all narrow (75 ft or less) taxiway pavements on unstabilized (granular) bases, and thinner than 9 in., tied joints (Type B) are acceptable for intermediate longitudinal contraction joints. For taxiway pave- ment greater than 9 in., doweled joints (Type C) are required in intermediate longitudinal contrac- tion joints adjacent to a free edge.
Keyed construction joints should not be used in airfield pavements.Experience on airfield pave- ments with keyed longitudinal construction joints shows that keyways provide limited strength and often break, becoming a maintenance problem.
Keyways perform particularly poorly if they are either too high or too low in the slab. The female side of the key often cracks to the pavement sur- face, creating a small sliver of loose concrete.
Over time, failed keyways break into small frag- ments, which results in a high potential for for- eign object damage (FOD).
Transverse Joint Considerations
Transverse contraction joints (Type C or D) cre- ate a weakened plane at planned locations per- pendicular to the direction of paving in order to control where cracks form. Sawing the pave- ment creates transverse contraction joints. The FAA’s current recommendation is for a saw cut depth of one-fourth the pavement thickness. Ex- perience shows that saw cuts to one-fourth the pavement thickness are effective under moder- ate prevailing paving conditions. Increasing the depth of cut to one-third the pavement thickness where hard aggregates or a stabilized base are used, as recommended by this guide, provides increased control against the development of uncontrolled (random) cracking.
Joint Spacing
Slab sizes should be validated through the FAA design procedure, which accounts for subgrade and base support. Designers must consider the thickness, slab size, support beneath the slab, aircraft loading and daily temperature fluctua- tions. It should be noted that the climate and concrete aggregate common to some geo- graphic regions will impact the cracking potential of the slabs and should be considered during the design process. For example, concrete made from granite and limestone coarse aggregate is much less sensitive to temperature change than concrete made from siliceous gravel, chert, or slag aggregate. A less temperature-sensitive concrete does not expand or contract much with temperature change, allowing for a longer spac- ing between pavement contraction joints without any greater chance of random cracking.
Chapter 9 – Joint Design, Layout, Construction, Sawing and Sealing
The primary function of all joints is to control cracking. Plain concrete pavement joints are spaced to reduce thermal and shrinkage re- straint stresses such that no uncontrolled crack- ing occurs between joints as a result of these restraint stresses. The restraint stress that influ- ences joint spacing is dependent on:
1. Concrete temperature and moisture gra- dients (top and bottom of slab).
2. Drop in concrete temperature (relative to the temperature at concrete final set).
3. Concrete shrinkage.
4. Slab/base interface friction.
5. Modulus of base/subgrade reaction.
6. Pavement thickness.
Joint spacing requirements are also affected by concrete properties. Concrete properties that af- fect restraint stress magnitudes are:
1. Modulus of elasticity (generally ranging from 3.5 to 5.5 million psi; assumed to be 4.0 million psi for most designs).
2. Coefficient of thermal expansion (gener ally ranging from 5.0 to 6.5 x 10-6 in./in./deg. F).
3. Shrinkage coefficient (generally ranging from 250 to 350 x 10-6 in./in.).
4. Density (generally 142 to 150 lb/ft3for air-entrained concrete).
Stresses in pavements increase with a greater modulus of base/subgrade reaction (k-value).
For high strength stabilized bases, the allowable joint spacing needs to be designed in the range of 4 to 6 times (typically 5 times) the pavement radius of relative stiffness (l), which can be eas- ily calculated using the Radius of Relative Stiff- ness Calculatorat www.apps.acpa.org.
Based on the above considerations, maximum joint spacing is plotted in Figure 9.2 as a func- tion of slab thickness and modulus of base/sub- grade reaction, assuming a concrete modulus of elasticity of 4,000,000 psi and a joint spacing equal to five times the radius of relative stiffness.
The maximum joint spacing decreases with in- creasing stiffness of the base/subgrade.
Note: This figure is for illustrative purposes only and should not be used for design.
Figure 9.2. Maximum joint spacing for pavements on stabilized bases.
Pavements with light cans require special atten- tion. Blockouts used to install light cans can re- strain slab movement and increase restraint stresses associated with moisture and thermal changes. Design engineers typically add em- bedded steel to slabs containing light cans.
While the embedded steel will not prevent re- straint cracking around light can blockouts, the steel will hold any cracks that may develop tight and reduce the potential for crack spalling. Be- cause most slab movements occur near longitu- dinal and transverse joints, jointing patterns need to be such that light cans are located more than 4 ft from planned joints, if possible; cracks tend to emanate from light cans if the light cans are positioned closer than 4 ft to joints.
Joint locations should be marked on the base, edge of the slab, or on the forms. When paving an apron, runway or wide taxiway, it can be diffi- cult to transfer the joint locations across pilot lanes. Surveying can be used to transfer joint locations across paving lanes. Small deviations in transferring joint locations across the pave- ment can result in joints at a skew. Joint loca- tions need to be carefully marked and joints need to be constructed at the proper locations.
Aspect Ratio Limit
Performance has shown that it is desirable to have panels with approximately equal trans- verse and longitudinal joint spacings. When slabs are long and narrow, they tend to crack under traffic into smaller pieces of nearly equal dimensions. Panels are not likely to develop an intermediate crack if the length-to-width ratio does not exceed 1.25. This ratio may be difficult to maintain within intersections and can be dis- regarded in favor of common-sense jointing pat- terns.
Construction (Butt) Joint Considerations Transverse construction joints are necessary at the end of paving each day or where paving op- erations are suspended for 30 minutes or more.
If the construction joint occurs at or near the lo- cation of a transverse contraction joint, a dow- eled butt joint (Type E) is recommended. A construction joint occurring in the middle of the normal joint interval should not be used unless the pavement is cut back to normal joint spac- ing.
Dowel Bars
Dowel bars are used to transfer wheel loads across a joint to the adjacent panel, reducing de- flection (and stress) at the joint and preventing differential displacement of the abutting panels.
Dowel bars are smooth bars that must be placed near the neutral axis (mid-depth) of a slab and in careful alignment to allow adjacent slabs to move when expanding or contracting from ther- mal and moisture changes.
The need to use dowel bars depends upon the joint type and its location in the airfield pavement facility. The following joints require dowel bars:
1. All Type E construction joints
2. Transverse contraction joints near the free edges of a facility, such as a runway or taxiway.
Tiebars
Tiebars are deformed steel bars. Tiebars should not be used to “tie” together panels of pavement features built on stabilized bases because doing so increases restraint to pavement movement from thermal and moisture changes. The use of tiebars is allowed for FAA pavements that are less than 9 in. thick.
Chapter 9 – Joint Design, Layout, Construction, Sawing and Sealing