Question 2 of 6: Joints in Portland Cement Concrete Pavements

Question 2: Joints in Portland Cement Concrete Pavements (4 + 3 + 5 + 3 + 4 + 1 = 20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — Four purposes of joints (4 marks)

Concrete is strong in compression and weak in tension, and a pavement slab is a large, thin, fully restrained plate that is asked to change length and shape continuously with temperature and moisture. Jointing is the designer's decision to place the inevitable cracks where they can be controlled rather than letting the slab choose. The four purposes are:

1. To control the location and geometry of cracking. Restrained drying shrinkage and thermal contraction generate tensile stress that will exceed the modulus of rupture within days. A sawn contraction joint creates a weakened plane of about one quarter to one third of the slab depth, so the crack forms directly beneath the saw cut, vertically, and stays tight enough to transfer load by aggregate interlock.
2. To accommodate slab movement. Joints allow the slab to expand and contract longitudinally with the daily and seasonal temperature cycle, and to curl and warp under the temperature and moisture gradients through the slab depth, without generating destructive stress in the concrete or in adjacent structures.
3. To provide load transfer between slabs. A properly detailed joint — dowelled or keyed, or relying on aggregate interlock at short spacings — carries a share of the wheel load across to the neighbouring slab, halving the critical edge and corner stresses and the corner deflections that drive pumping and faulting.
4. To isolate the pavement from adjoining fixed elements and to divide the work for construction. Isolation joints keep the pavement free of bridge abutments, manholes, catch basins, curbs and buildings, which would otherwise restrain movement and crack the slab. Construction joints define the ends of each day's paving run and the boundaries between paving lanes.

Part (b) — Three reasons for concrete pavement cracking (3 marks)

1. Restrained volume change with inadequate jointing. If the joint spacing is too long, or the saw cut is too shallow or made after the concrete has gained too much strength (outside the sawing window), the tensile stress from drying shrinkage and thermal contraction exceeds the modulus of rupture at an unplanned location and a random transverse crack forms. Late sawing is the single most common construction cause.
2. Load-associated fatigue and loss of support. Repeated heavy axle loads on a slab that is too thin for the traffic, or on a slab whose foundation has been eroded, accumulate fatigue damage until a bottom-up transverse or longitudinal crack develops. Loss of support usually arises from pumping: free water beneath the slab is ejected under a passing wheel, carrying fines with it, leaving a void at the joint or edge and creating a cantilever that cracks.
3. Materials-related durability distress. D-cracking from freeze-thaw susceptible coarse aggregate, alkali-aggregate reactivity, plastic shrinkage cracking caused by rapid surface drying before final set, and inadequate curing all produce cracking that has nothing to do with the traffic loading. In the Canadian climate D-cracking and the use of a non-air-entrained mix in a de-icing salt environment are the classic examples.

Part (c) — Five considerations governing joint spacing (5 marks)

1. Slab thickness. The long-standing rule of thumb is that the transverse contraction joint spacing should not exceed roughly 21 to 24 times the slab thickness (for example 4.5 m for a 200 mm slab), because the shrinkage stress that a slab can carry scales with its thickness.
2. Coarse aggregate type and thermal properties. The coefficient of thermal expansion of the concrete is dominated by the coarse aggregate. Siliceous gravels expand roughly twice as much as limestone, so slabs made with high-expansion aggregate need shorter joint spacing to keep joints tight enough for aggregate interlock.
3. Restraint offered by the base or subbase. A stiff, high-friction stabilised base restrains the slab and increases the frictional resistance to contraction, raising the shrinkage stress for a given length; a granular base with an interlayer offers less restraint and permits somewhat longer panels.
4. Climate and the resulting temperature and moisture gradients. Large diurnal temperature ranges and severe winters increase both the axial movement and the curling stresses. Curling stress rises with the panel length relative to the radius of relative stiffness (Bradbury's coefficient climbs steeply until the length reaches roughly six to eight radii), so cold climates and high solar exposure argue for shorter panels.
5. Slab geometry and the load transfer provided. Panels should be as nearly square as possible: a length-to-width ratio above about 1.25 (1.5 for unreinforced slabs) invites diagonal cracking. Lane width, the position of longitudinal joints, whether the joints are dowelled, and whether the slab is plain, reinforced or continuously reinforced all change the permissible spacing; and joints must be aligned with those in adjacent lanes and located to intercept structures, inlets and manholes.

Part (d) — Dowel bars (3 marks)

What they are. Dowel bars are short, smooth, round, plain steel bars — typically 32 mm in diameter, about 450 mm long and spaced at 300 mm centres — installed horizontally at mid-depth of the slab and parallel to both the pavement centreline and the pavement surface. They are epoxy-coated or stainless-clad in Canadian practice because of de-icing salt. One half of each bar is coated with a bond breaker or fitted with a sleeve so that it is deliberately not bonded to the concrete on that side.

Where they are used. Across transverse joints — contraction joints in the wheel paths of jointed plain concrete pavement carrying significant truck traffic, transverse construction joints, and expansion joints (where the sleeved end also carries an expansion cap). They are placed by dowel basket or by an automatic dowel bar inserter on the paver, and they are not used in longitudinal joints, where tie bars serve a different function.

Their purpose. To transfer shear across the joint from the loaded slab to the unloaded slab while allowing the joint to open and close freely. Because they are smooth and debonded on one side they carry load without restraining longitudinal movement. The result is a large reduction in corner deflection and in the critical edge stress, which in turn suppresses faulting, pumping and corner breaks and can double the life of a joint compared with reliance on aggregate interlock alone. This should be contrasted with the tie bar — deformed, bonded on both sides, used across longitudinal joints — whose purpose is the opposite: to hold two lanes tightly together and prevent the joint from opening or the lanes from separating, not to permit movement.

Part (e) — The four types of joint (4 marks)

1. Contraction (control) joints. Sawn or formed transverse weakened planes at regular spacing (typically 4 to 5 m) whose purpose is to induce the shrinkage and thermal crack at a chosen location. They are the most numerous joint by far.
2. Construction joints. Joints at the end of a day's paving or at a planned interruption. Transverse construction joints are butt joints, always dowelled; longitudinal construction joints between paving lanes are keyed or tied.
3. Expansion (isolation) joints. Full-depth joints containing a compressible filler board 12 to 25 mm thick, placed where the pavement abuts a bridge, structure, manhole or curb, or at some intersections. They allow unrestrained expansion and prevent the pavement from pushing against a fixed object; dowels through an expansion joint carry expansion caps.
4. Longitudinal (warping or hinge) joints. Joints parallel to the direction of paving that divide the pavement into lane-width panels, control longitudinal cracking caused by warping, and are held together with deformed tie bars so that aggregate interlock is preserved and lane separation is prevented.

Part (f) — Purpose of the joint sealant (1 mark)

The sealant occupies the sawn reservoir at the top of the joint and has two functions: it keeps surface water out of the joint and hence out of the base and subgrade, which prevents saturation, pumping, loss of support and freeze-thaw damage and protects the dowels from corrosion by chloride-laden water; and it keeps incompressible solids out — sand, grit and stone chips — which would otherwise fall into the open joint in winter and prevent it from closing in summer, generating very high compressive stress and causing spalling or blow-ups. The sealant must remain elastic and bonded through the full range of joint movement, which is why the reservoir is proportioned to a shape factor (typically a width-to-depth ratio of about 2:1 over a backer rod) rather than simply filled.