What is a Transverse Joint?

Purpose

A transverse joint is a joint across a concrete pavement lane that divides it into panels and controls where the concrete cracks. Changes in temperature and moisture cause volume change and warping of a panel, which causes stress. Joints divide the pavement into panels of set size to reduce these stresses and to limit random cracking (AC 150/5320-6G, 3.16.8). The same joint appears in road, apron, taxiway and runway pavements.

FHWA gives joints four possible functions: control of cracking, load transfer, isolation of structures that move differently, and delineation of lanes or shoulders. FHWA also notes that many distresses of jointed concrete pavement start at the joints. They include faulting, pumping, spalling, corner breaks and blowups.

Transverse and longitudinal joints

A transverse joint runs across the lane, at a right angle to the direction of paving and traffic. A longitudinal joint runs along the lane, between two lanes or two paving passes. FHWA names the joint types by function, and then adds the orientation. A contraction joint or a construction joint can be either transverse or longitudinal. Tie bars hold a longitudinal joint closed (AC 150/5320-6G, 3.16.10.1). Dowels carry load across a transverse joint.

Joint types

AC 150/5320-6G sorts all pavement joints into three categories by function: isolation, contraction and construction joints. It gives each a letter type (Table 3-5). FHWA adds a fourth category, the expansion joint.

Contraction joints

A contraction joint is the most common joint in jointed plain concrete pavement. It is a weakened plane, and the concrete cracks under it when it shrinks or cools. AC 150/5320-6G says that contraction joints also reduce the stress from curling and warping. The AC has three contraction types for the transverse direction:

• Type C is a doweled contraction joint. The AC uses it in the last three joints from a free edge. It also uses it in two or three joints on either side of an isolation joint.
• Type D is the dummy contraction joint, with no dowels. The AC uses it for all other contraction joints.
• Type B is a hinged joint. A transverse Type B joint is only for panels thinner than 9 in (230 mm) that use a “tension ring”. The AC describes the tension ring in 3.16.9.2.

Construction joints

A construction joint is the joint that results when new concrete is placed against hardened concrete. FHWA says that transverse construction joints, also called header joints, are typically made at the end of each day. They are also made at a weather or equipment stop. There is no aggregate interlock at a header joint, so it needs mechanical load transfer. If the header lines up with the contraction joints of the next lane, FHWA uses the same dowel size and spacing as in those joints.

Item P-501 requires a transverse construction joint at the end of each day. It also requires one at any stop of more than 30 minutes, or when the concrete may reach initial set before fresh concrete arrives. The joint goes at a planned contraction or expansion joint. If placing stops between planned joints, the contractor removes the excess concrete back to the previous planned joint. FHWA gives the reason. A header that lines up with the contraction joints of the next lane prevents short slabs and sympathy cracking in an adjacent tied lane.

Types E and F are the construction joints in AC 150/5320-6G. Type E has dowels, and the AC requires it for pavements that serve aircraft of 30,000 lb (13,610 kg) or more. Type F is a butt joint. The AC allows it for aircraft under 30,000 lb on a stabilized base, or for a concrete overlay of a flexible pavement.

Isolation and expansion joints

An isolation joint separates a pavement from a structure, or from a pavement that moves in another direction. A taxiway that meets a runway is an example. AC 150/5320-6G also puts an isolation joint where an extension is planned. Type A has a thickened edge. Type A-1 has reinforcement and is for panels thicker than 9 in (230 mm). FHWA says that an isolation joint has a full-depth compressible filler and no dowels or tie bars. Item P-501 adds that an isolation joint slips and does not work by compressing the filler. It names a thickened edge as the preferred load transfer.

FHWA defines an expansion joint as a special joint that lets new pavement expand without high compressive force. Unlike an isolation joint, it usually has dowels and moves only in the direction of expansion. FHWA advises against overuse of expansion joints. Too many of them let the contraction joints around them open over time. Then the sealant fails, water and debris enter, and load transfer drops. AC 150/5320-6G does not list an expansion joint type.

How a contraction joint works

Crews saw a groove in the hardened concrete. The groove makes a weak vertical plane. When the slab shrinks or cools, a crack starts at the bottom of the groove and runs down through the slab. FHWA describes the crack face as irregular. Load passes from one panel to the next by shear on this face, which is called aggregate interlock. FHWA lists the factors. They are the gradation, hardness and angularity of the aggregate, the strength of the concrete when the joint opens, and the width of the joint. Interlock falls fast when the joint is wider than 0.03 in (about 0.8 mm). Repeated heavy traffic can also abrade and smooth the crack face, which reduces interlock over time.

Sawing depth and timing

FHWA gives the usual depth as one-quarter to one-third of the slab thickness. It cites AASHTO 1993 for one-quarter for transverse joints and one-third for longitudinal joints. Some guidance accepts a shallower cut of at least 25 mm (1 in) if the saw cuts early.

Timing matters as much as depth. FHWA describes a sawing window. If the saw cuts too early, the concrete ravels. If it cuts too late, random cracks form from stress that has built up. FHWA says that conventional sawing is generally between about 4 and 12 hours after paving. The time varies with the mix and the weather. An early-entry saw is lighter and can start 1 to 4 hours after placement.

Item P-501 sets the airfield rules. Sawing starts as soon as the concrete is hard enough to cut without chipping, spalling or tearing, and before uncontrolled shrinkage cracking. It then continues without a break until all joints are cut. The first cut is at least 1/8 in (3 mm) wide, to the depth on the plans. Crews widen the top of the joint later for the sealant. If the contractor uses two cuts, the contractor applies curing compound in the first cut at once. The sealant reservoir is sawed after the curing period.

Dowels

Dowels carry load across a joint and prevent relative vertical displacement of the panels (3.16.10.2). FHWA adds that a dowel lets the joint open and close with temperature and moisture. The AC requires dowels in the last three transverse contraction joints from a free edge. It also requires them in all construction joints, unless the edge is thickened or reinforced. FHWA says that aggregate interlock without dowels suits only low-volume local roads. It defines low volume as fewer than 100 trucks per day.

Table 3-6 gives dowel size by panel thickness. A panel of 7.5 to 12 in (191 to 305 mm) uses dowels of 1 in (25 mm). They are 18 in (460 mm) long, at 12 in (305 mm) spacing. Item P-501 requires epoxy-coated dowels with a bond-breaker. It limits the alignment error to 1/4 in per foot (6 mm per 0.3 m). The AC says to hold the dowels in a wire basket anchored to the base, or to place them with an automated dowel bar inserter. FHWA says that poor consolidation around the dowels leaves voids and reduces load transfer.

Spacing

The maximum joint spacing depends on panel thickness and base type (Table 3-7).

Without a stabilized base:

With a stabilized base:

The AC says that panel thickness controls the spacing, and that panels should be nearly square when there is no embedded steel. The transverse spacing should not be more than 1.25 times the longitudinal spacing. On a federally funded project, a spacing above the table value needs an engineering analysis. The AC also says that smaller panels generally perform better over time. The AC adds that a shorter spacing may be needed to keep a clearance between joints and in-pavement objects such as light bases. FHWA reports that US highway practice centers on 15 ft (4.6 m) panels, and that many states cap the panel length at 15 ft.

Sealant

The AC says to seal all joints, to keep water and foreign material out. FHWA gives the same purpose, to limit the entry of water, deicing chemicals and incompressible material. FHWA also reports that highway agencies disagree about whether sealing pays, and that the answer depends on the project. On an airfield, the AC does not leave this open. Item P-501 widens the top of the sawed joint to make a reservoir for the sealant.

Transverse joints in asphalt pavement

An asphalt pavement also has transverse joints. A transverse joint in asphalt is the joint where new asphalt meets the end of an earlier pass. It does not control cracking. Item P-401 asks for a continuous bond between the courses and the required density at every joint. Crews form a transverse joint with a bulkhead or by tapering the course. They cut the tapered edge back on a straight line to expose a vertical face. They coat the contact faces with tack coat before they place fresh asphalt against the joint. P-401 also says that transverse joints in one course must be offset by at least 10 ft (3 m) from those in the course below. Transverse joints in adjacent lanes must also be offset by at least 10 ft (3 m).

Distresses at transverse joints

A survey to ASTM D5340 gives a Pavement Condition Index from 100 (no defects) down to 0. The FAA PAVER concrete manual defines several distresses at joints:

• Joint seal damage is any condition that lets soil or rock collect in the joint or water enter it. Examples are stripped, hardened or missing sealant. AC 150/5380-6C lists causes: wrong joint width, wrong sealant, poor application, a dirty joint and aging.
• Joint spalling is the breakdown of the slab edge within 2 ft (0.6 m) of the joint. Incompressible material in the joint causes it, and so does weak concrete at the joint from overworking. Misaligned dowels also cause it.
• Corner breaks are cracks that cross the whole slab thickness and meet the joints within half the slab length of the corner. Load repetition, loss of support and curling cause them.
• Faulting is a difference in elevation across a joint. On a runway or taxiway, more than 1/2 in (13 mm) is high severity. On an apron, more than 1 in (25 mm) is high severity. The manual does not rate a step that the builder made at construction.
• Pumping is ejection of water and fine material through the joint. The joint seal must be recorded as defective before pumping is recorded.

A blowup is buckling or shattering at a joint where incompressible material blocks the slab from expanding. Crews repair it at once because it can damage aircraft.

Joints on an airfield

Airfield lighting also depends on joints. For a new runway, AC 150/5340-30J puts the base of a centerline light at least 2 ft (0.6 m) from any pavement joint. ICAO Annex 14 recommends that grooves in a runway surface run perpendicular to the centre line, or parallel to transverse joints that are not perpendicular. EASA guidance on the inspection of paved areas also lists the condition of joint sealing and cracking and spall of concrete (GM1 ADR.OPS.B.015).