CIVL1810 Chap.9 Road Pavements: Flexible, Rigid and Their Joints
Road Pavements: Flexible, Rigid and Their Joints
A pavement is a hard wearing top layer over supporting material, laid so traffic can cross the ground repeatedly. It is stacked in layers whose quality falls with depth: what is near the tyre carries best, what is below it fans the load sideways. Everything else in Week 5 follows from that principle. The subgrade is judged on strength, gradation and drainage capacity and can be removed, replaced or stabilised.
Above it, the base and surface courses grade upward in quality. The family split is flexible against rigid, and it is a statement about who does the load distribution rather than about which is stronger.
What this chapter covers
- 01
Pavements defined, and where they are used beyond roads
- 02
The layered structure principle: spreading a concentrated wheel stress until each lower layer is below its material strength
- 03
Aggregate, asphalt, asphalt pavement and concrete pavement defined
- 04
The subgrade judged on strength, gradation and drainage capacity, and what good soil does when wet
- 05
Poor soil options: remove to reach better material, replace, or stabilise to increase stiffness
- 06
The four subgrade preparation steps and why grading affects compaction
- 07
Compaction plant: static and vibrating smooth wheeled rollers and the sheepsfoot roller
- 08
Subbase, base and surface courses and the material each one uses
- 09
Flexible pavements: the structure flexes and the base and subbase do the distribution
- 10
Rigid pavements: concrete panels distribute the load and little work is done by the layers below
- 11
Stated disadvantages of rigid pavements including capital cost, curing before traffic, and polished aggregate
- 12
Why concrete pavements crack, and contraction joints as control rather than prevention
- 13
Jointed reinforced concrete: thinner slab and fewer joints
- 14
Continuously reinforced concrete: no contraction joints, hairline cracks held by steel
- 15
Steel fibre reinforced concrete: fibres control shrinkage cracking but increase curling
- 16
Initial and final cracking spacings in unreinforced, unjointed concrete
- 17
Asphalt as a thermoplastic, elastic, adhesive binder, and asphaltic concrete laid by paving machine
Diagnosing cracking from its spacing
- +1Identify the first stage. Cracks running across the slab at intervals of 10 to 50 m, appearing not long after drying begins, are the first of the two waves, caused by drying shrinkage in a slab restrained by friction against the road base, which induces tensile stress exceeding the concrete's tensile capacity.
- +1Identify the second stage. Transverse cracks about 5 m apart with longitudinal cracks at 3 to 5 m centres is the final cracking stage, reached once the concrete has completely dried out, with temperature and moisture gradients adding warping stresses.
- +1State the remedy correctly. The cracking cannot be stopped, only located. Contraction joints at typical spacings of 3.7 to 6.1 m would have forced the cracks to occur where they were wanted, or continuous reinforcement would have held hairline cracks together at 1.5 to 3 m spacing instead.
Key terms
- Pavement
- The hard wearing top layer together with the material supporting it, laid so vehicles or people can cross the ground repeatedly. Used for roads, car parks, airports and industrial areas.
- Subgrade
- The existing material beneath a pavement, assessed on strength, gradation and drainage capacity. A good soil retains most of its load bearing capacity when wet, which is the condition that governs.
- Base and subbase courses
- The moderately stiff layers that help carry traffic and minimise deflection. The subbase typically uses relatively fine aggregate and the base moderately coarse aggregate, either unbound or bound with asphalt.
- Flexible pavement
- A pavement whose total structure bends to accommodate traffic loads. Its surface layer cannot adequately redistribute concentrated stress, so the base and subbase do much of the distribution and the surface mainly protects them from moisture and traffic.
- Rigid pavement
- A pavement of concrete panels whose high modulus of elasticity does not allow appreciable flexing. The panels distribute the load, little work is done by the layers below, and the panels must not move vertically relative to one another.
- Contraction joint
- A deliberate control point that forces shrinkage cracking to occur where it is wanted rather than randomly, at a typical spacing of 3.7 to 6.1 metres.
- Continuously reinforced concrete pavement
- A rigid pavement with sufficient steel that contraction joints need not be provided, because the steel holds together hairline cracks forming at about 1.5 to 3 metres.
- Steel fibre reinforced concrete pavement
- Concrete containing a large volume of small steel fibres mixed in during batching, which control shrinkage cracking and reduce the need for contraction joints but increase curling of the slabs.
Road Pavements: Flexible, Rigid and Their Joints FAQ
Does rigid mean stronger than flexible?
No, and treating it as a strength ranking is the commonest error on this topic. The distinction is about where the load distribution happens. In a flexible pavement the base and subbase do much of the distribution, because the surface layer offers limited rigidity.
In a rigid pavement the concrete panels do most of it, with little done by the base and subbase, which is why a rigid pavement tolerates a weaker base and a flexible pavement does not.
Why does the wet condition govern the subgrade assessment?
Because a good soil is defined as one that retains most of its load bearing capacity when wet, and the design case is the worst credible condition rather than the best. A soil that performs well dry and collapses saturated will fail in service. That is also why drainage capacity sits alongside strength and gradation as one of the three assessment criteria.
What are the trade offs of a continuously reinforced pavement?
In its favour: it has the lowest maintenance requirement if constructed properly, it reduces stresses on the base and subbase so lower quality road base material can be used, and it performs well where there would otherwise be differential settlement. Against it: slab repairs after damage are difficult, because there are no joints to cut back to, and the materials are very difficult to recycle when the pavement is removed.
Why is polished aggregate listed as a disadvantage of rigid pavements?
Because the coarse stone mixed in to give grip is worn smooth by traffic over time, at which point the surface turns treacherous in the wet. It sits alongside the very high capital cost and the requirement to let the slab mature fully before any vehicle is let onto it as the three stated disadvantages.
Exam move
Anchor everything to the layered structure principle, because it answers most of the topic. Write it out in full once, then use it to explain why the courses grade upward in quality, why compaction of the subgrade is key, and why the flexible and rigid families differ.
Build a two column comparison of flexible against rigid covering surface material, why the name, who distributes the load, what the surface layer is for, and whether joints are needed. Then learn the four concrete systems as a progression from many joints to none, with the trade at each step. Keep the crack spacings on your numbers card, since a cracking question is usually diagnosed by its spacing.
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