Question 2 of 7: Jointing of unreinforced concrete pavements, and pumping

Question 2: Jointing of unreinforced concrete pavements, and pumping (12 + 8 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) — The jointing system of a jointed plain concrete pavement

A jointed plain concrete pavement, JPCP, carries no distributed reinforcement. It survives instead by being deliberately cracked in a pattern the designer chooses, at joints, rather than in a pattern that shrinkage and temperature would choose for it. Every element the question lists exists to serve one of three purposes: to relieve volume change, to carry load across the break the volume change creates, or to hold the break tight enough that it keeps carrying that load for thirty years. Understanding which purpose each element serves is the whole of the subject, and it is also the reason two superficially similar bars — dowels and tie bars — behave in opposite ways.

Transverse contraction joints are sawn across the pavement at right angles to the centreline, typically at 4 to 5 m spacing on a Canadian highway slab, and always within a few hours of placing while the concrete is still gaining strength. The saw cut penetrates one-quarter to one-third of the slab depth, creating a plane of weakness at which the drying shrinkage and the thermal contraction of the first winter are relieved by a single clean crack running down from the notch. The spacing is not arbitrary. Too long a slab develops a mid-panel transverse crack, at which point the designer has lost control of the crack pattern and gained an unsealed, undowelled break; too short a slab wastes joints, and joints are the most expensive and the most vulnerable metre of any concrete pavement. A useful working rule is that the panel length should not exceed about 24 times the slab thickness, and that panels should be kept close to square, because a long narrow panel cracks transversely at its midpoint regardless of what the sawing plan says.

Dowel bars are plain, smooth, round steel bars, commonly 32 mm in diameter and 450 mm long at 300 mm centres, placed at mid-depth across every transverse contraction joint. They exist to transfer shear from the leaving slab to the approaching slab, so that a wheel crossing the joint deflects both panels together rather than punching the leading edge downwards. Crucially, a dowel is smooth and is usually coated or greased over at least one half of its length so that it deliberately does not bond to the concrete. It must be free to slide axially as the joint opens and closes with the seasons; a dowel that bonds to both slabs restrains that movement, and the pavement then cracks somewhere the designer did not choose. Dowels must also be placed parallel to the centreline and to the surface, because a misaligned dowel locks the joint just as effectively as a bonded one. Load transfer is the single best predictor of how long a jointed pavement lasts: a joint with good dowels halves the corner deflection and therefore roughly halves the flexural stress that drives fatigue cracking.

Longitudinal joints run parallel to the centreline, normally on the lane lines, and control the longitudinal cracking that a wide slab would otherwise develop from differential moisture and temperature through its depth, aided by warping. They are formed either by sawing a wide paved mat or as a construction joint between adjacent paving passes. Tie bars are what distinguish a longitudinal joint from a transverse one. They are deformed reinforcing bars, typically 15M at 750 to 900 mm centres, fully bonded into both slabs. Their purpose is the opposite of a dowel’s: they are meant to prevent movement, holding the two lanes together so that the joint faces stay in intimate contact and transfer load by aggregate interlock rather than opening into a lane-separation crack. A tie bar is not a load-transfer device in the dowel sense and must never be substituted for one. Conversely, tie bars must not be used across a transverse joint, where they would restrain the very contraction the joint was cut to permit.

Expansion, or isolation, joints are full-depth gaps, commonly 20 to 25 mm wide, filled with a compressible board and sealed at the surface. They allow the slab to grow without generating compressive stress against a fixed object. Modern practice uses them sparingly — a plain pavement on a properly cut contraction-joint pattern never closes its joints enough to need general expansion relief, and a redundant expansion joint simply becomes a place where the adjacent joints progressively open, lose aggregate interlock and admit water. They are retained where the pavement abuts a structure: at bridge approach slabs, at drainage structures, at the junction with an existing pavement, and at asymmetric intersections where slabs of different orientation meet. Where an expansion joint is used, dowels are still required across it, and one end must be fitted with an expansion cap so the bar has somewhere to travel.

Construction joints are the joints made where paving stops, whether planned at the end of a day’s run or forced by a breakdown. A planned transverse construction joint is made to coincide with a contraction joint location, formed full depth against a header and doweled exactly as a contraction joint would be. An emergency joint placed mid-panel is a weakness that will be visible in the ride for the life of the pavement, which is why paving crews carry headers and why the specification insists that if the mixer stops the crew places the joint at the nearest planned location. Longitudinal construction joints between paving lanes are keyed or butted and tied.

All of these joints are then sealed, which is the maintenance item that determines whether the design assumptions survive. A sealed joint keeps surface water out of the base and keeps incompressibles out of the joint reservoir. An unsealed joint admits both, and the consequences are the subject of part (b).

Part (b) — Pumping of joints

Pumping is the ejection of water and fine-grained material through a joint, a crack or the pavement edge under the action of a passing wheel load. It is the mechanism by which a jointed concrete pavement loses its foundation, and it is progressive: every cycle removes a little more support, which increases the deflection, which increases the volume expelled on the next cycle.

Three conditions must be present simultaneously. There must be free water beneath the slab, which arrives through unsealed joints and cracks, through the shoulder, or from a high water table; there must be a fine-grained, erodible material under the slab, typically a silt or clay subgrade or an untreated granular base with excessive fines; and there must be frequent heavy axle loads producing appreciable slab-end deflection. Remove any one of the three and pumping does not occur, which is precisely how it is designed against.

The mechanism is one of rapid pressure cycling. As a heavy axle approaches a joint, the leaving slab deflects downward and displaces the water trapped in the void beneath it. Because the loading is fast, the water cannot drain away slowly; it is forced sideways and upward at high velocity, scouring fines into suspension. The wheel then crosses the joint, the leaving slab rebounds and the approaching slab deflects, and the flow reverses. Over thousands of cycles the slurry is ejected at the joint and appears on the surface as the characteristic fan of grey or buff staining beside the joint or along the pavement edge after rain.

The consequences follow directly from the loss of support. A void forms first under the leaving slab, which then loses vertical support at its end and settles: the joint faults, with the approach slab standing proud of the departure slab, producing the rhythmic bump that travellers associate with old concrete pavement. The unsupported corner is now a cantilever, so corner breaks and diagonal corner cracking follow. If the joint has dowels, the concentrated bearing at the top of the dowel socket crushes and the dowel loosens, so load transfer falls and deflections rise further. In the limit the panel is broken up and the pavement fails structurally even though the concrete itself may still be sound.

Prevention is straightforward once the three conditions are understood. Water is excluded by sealing all joints and cracks and by maintaining the seal, and is removed by a permeable drainage layer with edge drains and positive outlets, all of which is standard practice in the Canadian provinces where freeze-thaw compounds the problem. Erodible fines are eliminated by placing the slab on a stabilised base — cement-treated, asphalt-treated or lean concrete — or on a well-graded open granular base with the fines limited by specification, and by providing a separation geotextile over a fine-grained subgrade. Deflections are limited by adequate slab thickness, by properly aligned dowels at every transverse joint, and by tied concrete shoulders or a widened outer lane, which move the wheel path away from the free edge where deflections are greatest. Where pumping has already started, the remedies are slab stabilisation by pressure grouting to fill the voids, retrofitted dowel bars to restore load transfer, and diamond grinding to remove the faulting; none of these succeeds unless the drainage that caused the problem is corrected first.