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Joints

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There are numerous types of joints utilized in concrete pavements and all of them serve a specific purpose. All joints are designed in some way or another to help the pavement achieve its design life. Jointed plain concrete pavement (JPCP) is most representative of how concrete pavements utilize joints, but all concrete pavement types, including jointed reinforced (JRCP) and continuously reinforced concrete pavement (CRCP), use joints for a number of reasons. This page looks at the purpose and history of joints as well as their proper design and construction.

The primary focus of this page is roadway pavements. While much of the information is applicable to airport pavements, there are some significant differences that are featured in the airfield jointing page.

Why Joint Concrete Pavements?

The first reason for jointing concrete pavements is because concrete shrinks. This happens under a few different mechanisms. Drying shrinkage is the result of water being used for hydration while it is also lost to the atmosphere through evaporation. Thermal shrinkage is the product of the heat of hydration of the concrete. As cement is mixed with water, the mixture gets hot. As the concrete sets and cools, heat is given off and the pavement shrinks. Chemical shrinkage occurs because the products of the cement hydration (concrete) occupy less volume than the reactants (cement and water in addition to aggregates).

Shrinkage itself is not an issue and ultimately wouldn't be an issue if pavements were not subject to restraint. If the concrete is free to shrink, as it would be if it were constructed on a frictionless base, then shrinkage does not cause any issues. However, friction from the subbase or subgrade provides resistance to the shrinkage forces, which in turn builds up internal stresses. As shrinkage progresses over time, the internal stresses build up, eventually reaching the tensile strength of the concrete and the pavement will begin to crack. Without joints, the concrete will naturally begin to crack at about a 40-80 ft interval to relieve the built-up internal stress. As shrinkage continues to progress, cracking will continue at an interval of 15-20 ft. This interval will occur not only transversely, but longitudinally as well because the restraining forces applies to all directions. This cracking can occur completely due to shrinkage, without any load being applied.

To control the cracking, concrete pavements can be jointed at an interval less than what the concrete would crack naturally. This cuts the pavement into slabs or panels that are small enough that the internal stresses are minimized and random or mid-slab cracking is eliminated. The joints can be sawed to make slabs about 15-18 ft long and typically around 12 ft wide. The timing of the saw cut is important as sawing too early results in ravelling and sawing too late results in natural cracking beginning.

While controlling cracking due to shrinkage is the primary reason we joint concrete pavements, there are a number of other reasons. One such reason is that jointing allows division of the pavement into construction lanes or increments. This can be through longitudinal joints that occur at the edges of a slipform paver as it passes or at a header at a transverse joint at the end of a day's paving. This helps the contractor construct the pavement. Since many times longitudinal joints happen to occur at the same location as the edges of lanes, they often help delineate lanes for motorists. However, delineating lanes is not one of the reasons to joint pavements. It can be beneficial to place longitudinal joints in the center of lanes if it eliminates a pass of the paver or moves the loading away from the edge of the slabs.

Another reason to joint concrete pavements is to accommodate slab movement. Isolation joints provide room for a concrete pavement to expand and contract without pushing up against adjacent structures. The expansion and contraction of concrete occurs in cycles as weather fluctuates with humidity and temperature.

Joints can also provide load transfer through placed dowels because joints can be sawed over the location of the dowels. The dowels can be placed with baskets on grade or with a dowel bar inserter (DBI). Such load transfer is not provided over a crack because cracks occur at unplanned locations. If a pavement is not jointed, then planning where the dowels need to be placed becomes difficult. To restore load transfer over a crack, dowel bars can be retrofitted into the pavement if aggregate interlock will not provide enough load transfer. This process is known as dowel bar retrofit (DBR)

A Brief History of Jointing

Dowelled Jointed Reinforced Concrete Pavement (JRCP)

The earliest concrete pavements were typically built in 6-8 ft squares and were about 6 in. thick. There was no such thing as a structural design and the main purpose was to try and solve rutting problems and 6 in. was arbitrarily chosen. The 6-8 ft spacing of the slabs was not a design consideration but rather a limitation of mixer capacity. This was the birth of construction joints or flat edge joints created by forms between these slabs. As the speed of vehicles increased, the roughness of these joints began to be more apparent. Additionally, to increase productivity and minimize costs larger slabs with cracks, acting as contraction joints, promoting load transfer through aggregate interlock became more popular. Utilizing contraction joints and promoting load transfer through aggregate interlock began to fix some of the roughness issues.

Issues with the load transfer provided through aggregate interlock occurred as cracks began to open. To combat this, steel was added to help restrain the slabs and hold the cracks together. This was the birth of JRCP around 1913. The amount of steel in JRCP is typically between 0.06% and 0.25% by cross sectional area and is calculated based on subgrade drag theory. The steel is not used to promote load transfer, rather it is a means to hold the cracks tightly together to promote aggregate interlock. Joints are typically constructed at intervals of 40-100 ft and intermittent cracks, serving as contraction joints, occur every 15-20 ft.

Dowelled Jointed Plain Concrte Pavement (JPCP)

The intermittent cracks still occurring in the JRCP were still random and difficult to maintain. This lead to the beginning of JPCP shortly after 1913. For JPCP, the contraction joints were sawed at intervals less than the 15-20 ft the pavement would naturally crack at. This lead to joints that were straight lines, like construction joints, and the crack would propagate down from the bottom of the saw cut. This would still allow for aggregate interlock across the joint. Early JPCP did not utilize any steel for reinforcement or load transfer and construction joints were still placed at a 40-100 ft interval due to paving limitations.

To obtain adequate load transfer at the construction joints of JRCP and JPCP, dowel bars were added around 1917. This also helped eliminate other joint distresses that were being seen. Later it became common practice to dowel the intermediate contraction joints as well.

Continuously Reinforced Concrete Pavement (CRCP)

Due to the poor performance of the dowelled JRCP, more steel was added to create CRCP around 1923. The amount of steel increased from 0.06-0.25% by cross sectional area up to 0.6-0.85% steel by cross sectional area. The increase in the amount of steel actually increased the restraining forces applied on the concrete which resulted in shorter crack spacings of 2-6 ft. However the additional steel did hold these cracks tightly to continue to promote load transfer through aggregate interlock.

Design Challenge JPCP JRCP CRCP
Transverse Joint Spacing 13-18 ft 22-100+ ft N/A
Transverse Crack Spacing N/A 15-20 ft 2-6 ft
Shrinkage Accounted for by Jointing Cracking Cracking
Reinforcing Steel N/A 0.06-0.25% 0.6-0.85%
Expansion Joints Used No Sometimes Maybe
Tie bars Used in Longitudinal Joints Yes Yes Yes
Longitudinal Joint Spacing 12-14 ft 12-14 ft 12-14 ft
AASHTO 62-93 Design Yes Yes Yes
AASHTOWare Pavement ME Design Yes No Yes
ACPA StreetPave Yes No Yes

Joint Spacing

For JPCP, the maximum joint spacing needs to be determined to ensure that all joints are cut at an interval such that stresses will be relieved and no random cracking will occur. The following mechanistic equation was developed based on the radius of relative stiffness (l) along with such parameters as the modulus of elasticity of the concrete (E), slab thickness (h), modulus of the subgrade (k), and Poisson's ratio (μ) in order to determine the maximum joint spacing:

Radius of Relative Stiffness English.png

In order to be conservative, the ratio of joint spacing to the radius of relative stiffness should be limited to 4-5 although 7 can work in the field.

A simpler way of calculating joint spacing is an empirical method based on the thickness of the slab and the subbase/subgrade support layer immediately beneath the concrete. To find the maximum joint spacing, the thickness is multiplied by a support constant which ranges from 24 for subgrades and unstabilized subbases to 12-15 for thin bonded overlays on asphalt. A free web-based application, based on this equation, called the Maximum Joint Spacing Calculator is available.

There are a few other joint spacing guidelines that can help improve performance. The ratio of transverse to longitudinal spacing should be less than 1.5, although square slabs will typically result in the best performance. Additionally, the maximum joint spacing of transverse joints should be limited to 15 ft. unless local history shows longer panels can work.

Sawcuts

Saw cut timing and depth are crucial to creating a joint. Proper timing and depth will ensure a joint activates without causing distress or deterioration. Additionally, timing and depth will help the long term performance of the joints and the pavement as a whole.

Sawcut Timing

Sawing Window

To know when to saw the joints, one must consider the crack control or sawing window as in the figure. The concrete begins gaining strength soon after it is placed as the cement begins to hydrate and the aggregates begin to interlock. The strength gain will look essentially as it does in the figure. The internal stress will be more cyclical because it depends on many factors including temperature, wind speed, solar radiation, etc. The internal stress does not begin to build up until some time later after the hydration process of the cement is far enough along that the concrete material can sustain some tensile forces. The delay in the hydration reaction is what provides time for transportation, placement, consolidation, and finishing of the concrete.

Eventually the concrete develops enough strength that joints can be sawed into the surface without causing significant deterioration to the surface of the pavement known as raveling. At this point on the strength curve, the minimum strength to avert excessive saw cut raveling is achieved and the sawing window begins. The sawing window is the window of opportunity that a contractor has to saw joints before the internal stress curve catches up to the concrete strength curve and cracking occurs. If the internal stress reaches the concrete strength, the pavement will begin to crack at about a 40-80 ft interval in the transverse direction. This initial cracking alleviates some of the built up stress from the shrinkage of the concrete. As the shrinkage continues, the stresses will continue to build and the cracking will progress to a 15-20 ft interval unless joints are sawed.

There are many factors that can affect the sawing window. Weather is a significant one. A sudden rise or drop in temperature can shorten the window by affecting the set temperature of the concrete or the shrinkage rate. High winds and low humidity can also lead to higher evaporation rates leading to faster drying shrinkage. In addition to weather, the support layers (subgrade/subbase) can affect the sawing window by changing the friction and restraint the pavement is subject to. They can also absorb water out of the concrete speeding up drying shrinkage. Other factors affecting the sawing window include the concrete mixture, whether or not lanes are being paved next to existing lanes, and saw blade selection.

One other important factor affecting the sawing window is curing which can influence how the concrete material properties develop. Proper curing can effectively lengthen the sawing window by increasing the strength development and delaying the development of internal stresses. This is done by controlling the internal temperature and moisture content of the concrete by controlling the evaporation rate. Evaporation rate can be calculated using ACPA's Evaporation Rate Calculator application to determine if changes to the curing application need to be applied to maintain proper moisture content and temperature.

There is currently no standard test to determine when is the appropriate time to make the saw cuts. Typically this is left to the expertise of sawing crews. Someone with experience can scratch the surface of the concrete and determine if it has enough strength to resist raveling. A tool known as HIPERPAVE can be used to evaluate the strength and stress development of the pavement to help predict the sawing window.

Sawcut Depth

Sawcut depth recommendations for unstabilized and stabilized bases

Sawcut depth is also essential to creating a properly performing joint. If a joint is sawed at the correct time but not to the correct depth, the weakened plane intended to introduce a crack may not propagate through the depth of the pavement and the concrete may crack elsewhere to alleviate the internal stress build-up. Typical recommendations are between 1/4 and 1/3 of the slab thickness depending on the type of support. A granular subbase typically requires a sawcut depth of 1/4 the slab thickness and a stabilized subbase requires a depth of 1/3 the slab thickness. The frictional resistance between the slab and the stabilized subbase is greater so a deeper cut is necessary to present a weakened plane that reliably controls crack formation using conventional sawing equipment.

Early entry saws can have a shallower sawcut depth on the range of 1/6 to 1/5 of the slab thickness. The minimum depth needs to be at least 1.25 in. If dust is seen with an early entry saw, the depth should be reverted to 1/4 the thickness.

Types of Joints

There are three basic types of joints that encompass most of all joints constructed and they are contraction, construction, and isolation joints. Each of these joint types can be used in the transverse or longitudinal direction. There are also a number of specialty joints typically used in transitions from asphalt to concrete or in CRCP.


Contraction Joints

Contraction Joints

Longitudinal and transverse contraction joints are the most used types of joints due to the use of JPCP. Contraction joints are used to control the formation of cracks. Typically these are placed every 15-20 ft in the transverse direction and every 12-14 ft in the longitudinal direction. These joints utilize aggregate interlock as one form of load transfer, but they can also be dowelled to provide mechanical load transfer. Transverse contraction joints are typically dowelled and longitudinal contraction joints are typically tied together with a tie bar. Aggregate interlock alone is an acceptable means of load transfer when the pavement will service less than 80-120 trucks/ day, less than 4-5 millions ESALs, and the slab thickness is 7 in. or less. Longitudinal contraction joints are used to control cracking when more than 12-15 ft of pavement is being placed in one pass of a paver.

Contraction joints are placed by placing a saw cut at the desired location of the joint. As the concrete shrinks, the saw cut propagates through the pavement to "activate" the joint. This retains aggregate interlock between the slabs to help promote load transfer between the slabs. More information on load transfer can be found in the joint mechanics page.

Construction Joints

Construction Joints

Transverse construction joints are used for the end of a paving run or at an interruption. The locations are often called headers in practice. Unlike contraction joints, these joints have a flat interface so some form of embedded steel is required to either provide load transfer or to prevent the joint from opening. Dowels may be placed in fresh concrete or placed into holes drilled into a previously placed header. Tie bars can also be placed in fresh concrete or epoxied into holes drilled into a previously placed header.

Transverse construction joints can either be formed or sawed. If a slipform paving machine is allowed to run out of concrete, the resulting header would need to be sawed to create a construction joint. Dowels could be drilled in or tie bars could be epoxied in if it is not where a planned joint was intended.

Longitudinal construction joints are used for joining lanes paved in separate passes. Tie bars are typically used with longitudinal construction joints to make sure the joint remains tight and the lanes do not begin to separate.

Keyways are an option that have been used with both transverse and longitudinal construction joints. Keyways were originally designed to promote load transfer. One issue with using keyways is that they can be difficult to construct properly. Another issue is that even if they are properly constructed, the male portion of the joint can be sheared off and create further problems.

Isolation Joints

Transverse isolation joints are mainly used for isolating structures within the pavement. They are typically used at bridges, intersections, roundabouts, and for small in-pavement objects like utilities. Transverse isolation joints may use a thickened edge to compensate for there being no load transfer. Dowels can also be used for load transfer so long as an expansion cap is used to allow the slabs to expand and contract independently of each other. A sleeper slab can also be used to aid in supporting the free edges. Longitudinal isolation joints are used to isolate structures adjacent to the pavement. Isolation joints are typically 1/2 - 1 in. wide.


Specialty Joints

Specialty Transition Joint

There are a few additional types of joints that are occasionally used. These specialty joints have very specific purposes and thus are less common than contraction, construction, and isolation joints. The first type of specialty joints are used for transitions between concrete and asphalt pavements. Dowels or thickened edges can be used to help mitigate increased stress levels due to a lack of load transfer between the pavements. Another possibility is for the asphalt pavement to extend over a thinned portion of concrete to help the transition.

Another type of specialty joint is specific to CRCP. This type of specialty joint is known as terminal joints and they are used to help transition between sections. Two terminal joint designs include the wide flange steel beam terminal joint and the lug anchor terminal joint.

Joint Sealing

Joint sealing is the process of applying a sealant to minimize the infiltration of surface water and incompressibles into the joint system. Joint sealing is required by most agencies for most applications and always required for airfields. There are some special applications that may not require joint sealing. More information can be found on the joint sealing page.

Joint Mechanics

Load transfer across joints is essential to long term performance of concrete pavements. The primary factors that contribute to load transfer are aggregate interlock, the use of dowels and tie bars, and subbase support. Tie bars do not directly impact load transfer, but they can impact aggregate interlock by maintaining a tight joint.

For additional information on load transfer and dowel technologies, visit the joint mechanics page.


Joint Construction

Joint construction involves the placement of load transferring and reinforcing steel in addition to the sawing of the joint. Additionally, sealing joints is an optional feature that can be constructed.

Steel Placement

The first step in constructing joints is determining how the steel is to be placed in the pavement. Steel can either be pre-placed before paving or inserted during the paving. For pre-placed steel, baskets are utilized for dowels and tie bars (though tie bars can also use chairs). CRCP steel is tied onto chairs for pre-placed applications. For insertion, dowel bar inserters (DBI) are used along with tube feeders for CRCP pavements. The DBI vibrates the dowels into the concrete and the vibrates the concrete to consolidate the portion directly above the dowel. In the case of construction joints, dowels can be drilled and epoxied into the joint before paving continues. Dowel bar retrofit (DBR) can also be used to restore load transfer to pavements or cracks to reduce deflections and stresses. Tie bars should not be placed within 6 inches of the tip of the nearest dowel bar in a transverse joint, regardless of tie bar spacing. This can impact the effectiveness of the dowels to transfer loads and can also interfere with joint opening and closing.


Sawing

Joint cutting with conventional walk behind saw

After the steel is placed and the concrete is paved, the next step in construction is to form or saw the joints. Joints are typically sawed rather than formed or cut into the surface to avoid roughness issues. For both early entry and conventional saws the concrete must gain the proper strength to avoid ravelling the surface. The most common saw blades are diamond or abrasive (carborundum). The diamond blades require water cooling and the diameter of abrasive blades will decrease over time as the abrasive material wears away. Blades should be matched to the concrete mainly depending on the aggregate hardness and the power output of the saw.

The most common practice is to first saw the transverse joints and then cut the longitudinal joints. This is because the longitudinal stresses do not develop as quickly because the width of concrete being paved is not as significant as the length. This helps alleviate the critical internal stresses before cracking occurs. Occasionally skip sawing can be used if the pavement is gaining strength to rapidly. Skip sawing would saw approximately every third transverse joint to take advantage of the concrete's natural 40-80 foot cracking pattern to help relieve some of the initial stress before going back and sawing the other joints. However, this can create dominant joints which may open more than others and cause other joint issues.

The sawing speed should be controlled to ensure that the proper depth of cut is attained. Harder aggregates may require a slower speed. A saw operator that attempts to speed up the cutting may end up pushing the saw too fast causing the blade to ride up out of its full cut. If the proper depth is not obtained there is a higher risk of cracking. The depth should be checked as a cut too shallow may not relieve stresses which could result in random cracking. A cut too deep means unnecessary effort and expense went into cutting the joint which could cause equipment wear and reduced aggregate interlock. Additionally, it is important to check the depth as the blade's diameter can wear down.

Starting and stopping the saw cut requires special attention. Early entry saws may require the saw cut to stop about a half inch short of the pavement edge to prevent a blow out. This is due to early entry saws being up-cut saws. Another special consideration for saw cutting is wind conditions. During windy conditions, the wind should be oriented with the direction of sawing. This will ensure that the saw is moving away from the pavement that is drying more quickly.

Sealed Joint Construction

Sealing joints is required by most agencies for most applications and is always required for airfields. Sealing joints does require some additional steps for construction. For background on sealing joints in addition to details on construction aspects see the joint sealing page.

Transverse Construction Joints

Formed transverse construction joint

Transverse construction joints or headers are constructed at the end of a day's paving or when significant paving delays are encountered (typically due to equipment malfunction). These joints can either be formed or sawed, but there is no way to account for them in the layout planning because their location is not planned ahead of time. When placing next to an existing pavement, best practice is to match the header with an existing transverse joint. If the header is formed, the dowels or tie bars can protrude through the form or false-dowels attached to the form face can be used and dowels can be inserted upon form removal. The concrete around the form must be well-consolidated to ensure proper performance of the joint. When using a formed header, at least six hours should elapse before paving resumes.

If the header is not formed, then the paver will continue through the header and the excess concrete must be sawed back and removed. Dowel and tie bar holes are then drilled and then installed. For ease of removal, it can be helpful to place a separator between the subbase or subgrade and the concrete past the header. This will ensure that the concrete beyond the construction joint can be removed in a timely fashion and paving can continue at the proper time.

When paving is to resume, the paving equipment is re-positioned over the joint to start the next placement. Some hand placement and hand vibration will be necessary on the start-up side of the header. The previously-placed header should serve as a guide for surface finishing to ensure a smooth transition.

Related Pages

Related Resources and Materials

Webinars (On-Demand Education and Training):

Publications:

Research and Technology Updates:

Web Apps:

From the National Concrete Pavement Technology Center: