Pumping is the ejection of water and soil particles through the joints or cracks of a pavement when passing loads deflect the slab. The PAVER manual for concrete airfields defines it as the ejection of material by water through joints or cracks. The deflection of the slab under passing loads causes it. The water carries particles of gravel, sand, clay or silt. The result is a progressive loss of pavement support.
How pumping happens
In a rigid pavement, free water can collect between the concrete and an impermeable layer under it. The load of a passing wheel puts this water under pressure and moves it. The moving water erodes the subsurface material and forms voids under the concrete. Pumping increases as the volume and speed of the traffic increase.
What it looks like
Surface staining and base or subgrade material on the pavement close to joints or cracks are evidence of pumping. Pumping can occur at cracks as well as at joints. Pumping near joints indicates poor joint load transfer, a poor joint seal, or ground water.
The PAVER manual says that the joint seal must be identified as defective before pumping can be said to exist. Pumping debris at a joint also counts as medium-severity joint seal damage. The manual defines no degrees of severity for pumping. A pumping joint between two slabs counts as two slabs, and each extra pumping joint around a slab adds one slab.
How it is recorded in the LTPP system
The FHWA LTPP manual records water bleeding and pumping as one distress. It is Distress Type JCP 16 in jointed concrete pavement and ACP 15 in asphalt pavement. The inspector records the number of occurrences and the length of the affected pavement. The minimum length is 1 m. The manual uses no severity levels. The reason is that the amount of bleeding and pumping changes with the moisture conditions.
What it causes
Pumping causes a progressive loss of support that can lead to cracking. Three related distresses follow from it:
- Corner breaks. Load repetition and curling stresses usually cause cracks at the slab corner. Loss of support, which pumping can cause, adds to them.
- Faulting. This is a difference in elevation at a joint or crack. It can result from the loss of fines.
- Damage to the base. Water that enters through a damaged joint seal can cause pumping or deterioration of the base.
Prevention and repair
The base of a rigid pavement provides uniform support for the slab and prevents pumping of fine-grained soils. AC 150/5320-6G requires a stabilized base when the design aircraft have gross loads of 100,000 lb (45,359 kg) or more. It recommends a drainage layer for pavements that serve aircraft above 60,000 lb. This applies in areas with excessive subsurface moisture and soils of low permeability.
AC 150/5320-6G says that a rigid pavement with a stable subgrade and no pumping needs no corrective measures. If panels pump or rock under aircraft traffic, chemical or cement grout can go into the voids in the subgrade. Consult an experienced pavement or geotechnical engineer first. A dowel bar retrofit or reconstruction may be required for extensive areas. In a full depth repair, water infiltration and pumping can damage the base and subbase, and they then need repair (AC 150/5380-6C, paragraph 6.3.1.2).
Pumping in an airfield pavement survey
The PAVER manual for concrete airfields lists pumping as distress 69. The inspector looks for the stains and the deposits at the joints and cracks of each slab. Pumping has no severity level, so the record only shows that it exists.
Frequently Asked Questions
- What does pumping look like?
- Pumping shows as surface staining and as base or subgrade material on the pavement close to joints or cracks. The ejected water carries particles of gravel, sand, clay or silt. The stain is the evidence, because the process itself happens under the slab.
- Does pumping have severity levels?
- No. The PAVER manual for concrete airfields defines no degrees of severity, and it is sufficient to indicate that pumping exists. The FHWA LTPP manual also uses no severity levels, because the amount of pumping changes with the moisture conditions.
- Can pumping occur if the joint seal is intact?
- Not by the PAVER definition. The joint seal must be identified as defective before pumping can be said to exist. Pumping debris at a joint is itself a sign of medium-severity joint seal damage.
- How is pumping repaired?
- If panels pump or rock under aircraft traffic, FAA AC 150/5320-6G allows injection of chemical or cement grout into the voids in the subgrade. Consult an experienced pavement or geotechnical engineer first. A dowel bar retrofit or reconstruction may be required if the areas are extensive.
References
- faa-paver-concrete
- Concrete Surfaced Airfields, PAVER Distress Identification Manual FAA (attachment to AC 150/5380-7B)
- faa-ac-150-5380-6c
- AC 150/5380-6C, Guidelines and Procedures for Maintenance of Airport Pavements FAA
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
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