Question 6 of 7: Field Compaction Control and Asphalt Pavement Distress

Question 6: Field Compaction Control and Asphalt Pavement Distress (10 + 10 = 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.

Given.

Find. Whether the field compaction meets the specification, and whether the correct remedy is to add water or to let the material dry back, with the reasoning.

Approach. Convert the measured wet density to a dry density, express it as a percentage of the modified Proctor maximum, compare with the 95 percent requirement, and then place the field water content relative to optimum on the compaction curve to decide the remedy.

1. Part (a) — convert the field measurement to a dry density. A nuclear gauge or sand-cone test returns total density, which includes the pore water: $$\rho_{d} = \frac{\rho_{t}}{1 + w} = \frac{2080}{1 + 0.13} = \frac{2080}{1.13}$$ $$\boxed{\rho_{d} = 1840.7\ \text{kg/m}^{3}}$$
2. Express the result as relative compaction. $$RC = 100\,\frac{\rho_{d}}{\rho_{d,\max}} = 100 \times \frac{1840.7}{1960}$$ $$\boxed{RC = 93.9\ \text{percent} < 95\ \text{percent required}}$$ so the compaction is not satisfactory and the lift must be reworked before the next one is placed.
3. Quantify the shortfall. The specification demands $$\rho_{d,\text{req}} = 0.95 \times 1960 = 1862\ \text{kg/m}^{3}$$ so the lift is 21.3 kg/m³ light, about 1.1 percent. It is a modest shortfall — the kind that a moisture correction plus additional passes normally recovers — rather than evidence of an unsuitable material.
4. Locate the field point on the compaction curve. The field water content is $$w = 13\ \text{percent} \quad\text{against}\quad OMC = 11\ \text{percent}$$ so the material is 2 percentage points wet of optimum. On the wet side of the modified Proctor curve the dry density falls as water is added, because the additional pore water occupies volume the compactive effort can no longer expel: the soil approaches the zero-air-voids line and further rolling merely pumps the water about.
5. Decide the remedy. Adding water would move the material further down the wet branch and reduce the achievable dry density still more. The correct action is therefore $$\boxed{\text{do NOT add water — aerate and allow the road to dry back toward } w \approx 11\ \text{percent, then re-roll}}$$ Quantitatively, roughly $$\rho_{d}\,(w - OMC) = 1840.7 \times 0.02 = 36.8\ \text{kg of water per cubic metre}$$ must be removed, achieved by scarifying, harrowing and letting the lift air-dry. Once at optimum, the wet density that corresponds to the specified dry density is 1862 × 1.11 = 2067 kg/m³, which becomes the field control value for the retest.
6. Confirm the diagnosis independently. Taking a specific gravity of solids G_s = 2.70, the zero-air-voids density at 13 percent water is $$\rho_{zav} = \frac{\rho_{w}G_{s}}{1 + w\,G_{s}} = \frac{1000 \times 2.70}{1 + 0.13 \times 2.70} = 1998\ \text{kg/m}^{3}$$ so the field point contains only 100 × (1 − 1840.7/1998) = 7.9 percent air by volume. There is little air left to expel, which is precisely why more rolling at this water content will not raise the density — the material is close to saturation and the deficiency is a moisture problem, not a compactive effort problem. At 11 percent water the zero-air-voids density rises to 2082 kg/m³, leaving ample room to reach the required 1862 kg/m³.

Final Results.

Part (b) — Five distress types used to evaluate asphalt pavements

Pavement condition surveys in Canada follow the TAC Pavement Asset Design and Management Guide and the LTPP Distress Identification Manual, in which each distress is recorded by type, severity and extent. Five that between them cover the structural, material and environmental failure modes are described below. The reason each appears matters more than the label, because the cause dictates the treatment: a structural distress needs added thickness, a mixture distress needs a mix redesign, and an environmental distress needs a different binder grade.

1. Fatigue (alligator) cracking. A network of interconnected cracks that begins in the wheel paths and progressively subdivides into pieces resembling alligator hide. It is caused by repeated tensile strain at the underside of the bound layer under traffic loading: each axle bends the pavement slightly, and after enough repetitions a micro-crack initiates at the bottom of the asphalt and propagates upward. Its appearance is therefore a direct statement that the structure is too thin for the traffic it has received, or that the support beneath it has weakened — often through loss of subgrade strength in spring thaw. Because it is load-associated and bottom-up, surface treatments do not arrest it; only a structural overlay or reconstruction will.

2. Rutting (permanent deformation). A longitudinal depression in the wheel path, sometimes with shoulders of displaced material along its edges. Two mechanisms produce it, and the shape distinguishes them. If the rut is narrow with humps beside it, the asphalt mixture itself is shoving: the binder is too soft for the service temperature, the aggregate skeleton has too little coarse angular material to interlock, or the mixture is over-asphalted so that the binder rather than the stone carries load. If the rut is wide and dish-shaped with no humps, the deformation is in the unbound layers or the subgrade, and the cause is inadequate thickness or poor drainage. Rutting matters beyond ride quality because a rut holds water and creates a hydroplaning hazard.

3. Transverse (thermal) cracking. Cracks running roughly perpendicular to the centreline at fairly regular spacing, and the dominant distress on Canadian highways. They are not caused by traffic at all. When the pavement cools, the asphalt layer tries to contract but is restrained by friction against the layer beneath, so tensile stress builds; when that stress exceeds the tensile strength of the binder at that temperature, the layer cracks right across. The controlling variables are the low-temperature grade of the binder and the rate of cooling, which is why the Superpave performance grade carries an explicit low-temperature designation such as PG 58-34. Repeated thermal cycling widens the cracks, admits water to the base, and in a freezing climate leads to the secondary distresses of spalling and frost heave.

4. Potholes and raveling. Raveling is the progressive loss of aggregate from the surface, leaving it rough and open-textured; a pothole is a bowl-shaped hole where the surface and often part of the base have been lost entirely. The initiating cause of raveling is loss of adhesion between binder and aggregate, from oxidative hardening of the binder with age, from stripping where water displaces the binder film off the stone surface, or from inadequate compaction at construction leaving the mix too open. Potholes then develop where water enters through raveled or cracked areas, saturates the base, and traffic loading pumps it out, removing fines and undermining the surface until it collapses. Freeze-thaw accelerates the process by expanding the trapped water.

5. Bleeding (flushing). A film of free binder on the pavement surface, shiny and often confined to the wheel paths, which becomes tacky in hot weather. It occurs when the mixture contains more binder than its void structure can hold: traffic densifies the pavement in service, the air-void content falls toward zero, and the excess binder has nowhere to go but up. Over-asphalted mix design, an over-applied tack or prime coat, and low in-place air voids are the usual causes. The consequence is a serious safety one — a bled surface has markedly reduced skid resistance in the wet — and the treatment is to restore texture by applying hot sand, milling, or a corrective surface course.

Two further distresses are worth naming for completeness because condition surveys record them routinely: longitudinal cracking, which along a paving joint indicates poor joint construction and along a wheel path is usually the first stage of fatigue cracking; and block cracking, a rectangular pattern covering the whole surface including areas outside the wheel paths, which signals age-hardening of the binder rather than any load effect. In a Canadian condition survey each of these is scored by severity and extent and combined into a pavement condition index, and the resulting index drives the choice between routine maintenance, resurfacing and rehabilitation.