The Lugeon Test: Methodology, Interpretation, and Applications in Geotechnical Engineering

 Random pavement cracking is primarily caused by tensile stresses that exceed the concrete's tensile strength, often resulting from a combination of design deficiencies, construction practices, and environmental factors.

Source: Unsplash

Specific causes of random pavement cracking include:

• Tensile Stresses from Volume Change and Bending

  • Cracks form when the tensile stresses that build up within the concrete exceed its tensile strength. These stresses develop from the restraint of the concrete's volume change at early ages, and the restraint of bending caused by temperature and moisture gradients through the concrete. Early volume changes are particularly associated with the concrete's drying shrinkage and temperature contraction.
  • Large forces due to drying shrinkage and environmental changes in concrete pavements are significant enough to cause cracks to form even without traffic loading. Without properly designed joints, concrete pavements would typically be "riddled with cracks within one or two years after placement".

• Improper Saw Timing

  • There is an optimal "sawing window" for contraction joints. If the sawing of joints is delayed beyond the period when significant concrete shrinkage occurs, it can induce random cracks within the pavement, often at intervals greater than 8-10 meters. Conversely, sawing too early can cause raveling along the cut, though not random cracking.

• Insufficient Saw Cut Depth

  • If the saw-cut depth at a joint location is less than required, it may not adequately weaken the concrete, leading to cracking of the concrete elsewhere in the slab.

• Long Joint Spacing

  • Pavements designed with long transverse joint spacing may develop cracks at locations other than the saw cuts due to tensile stresses resulting from temperature curling. Optimal joint spacing is dependent on factors like slab thickness, subbase stiffness, and concrete strength.

• Misaligned or Locked Dowel Bars

  • Misalignment of dowel bars at a transverse joint can induce a crack away from the saw cut if the dowels physically lock two slabs together and restrict their contraction. This issue can arise even if the saw timing and depth are adequate.
  • Locked transverse joints, which can occur due to dowel bar corrosion, improper alignment, or inadequate lubrication, prevent the slabs from moving freely. This restraint can lead to spalling and cracking at the joint face, and in severe cases, a full-depth, full-width crack may develop parallel to and near the locked joint, which then functions as an unplanned joint without proper load transfer.

• Subgrade and Subbase Settlement

  • Settlement of the subgrade and subbase can cause concrete pavement cracking. These cracks often appear diagonally and can extend continuously through multiple slabs, with their direction being variable.

• Rapid Drying (Plastic Shrinkage)

  • Plastic shrinkage cracks result from the rapid drying of the concrete pavement surface. This is influenced by factors such as high ambient temperature, high wind velocity, low humidity, or a combination thereof. These cracks are typically tight and form in parallel groups, perpendicular to the wind direction, shortly after concrete placement.

• Poor Drainage

  • Inadequate drainage can contribute to random cracking by allowing water accumulation under the slab, which leads to erosion and other types of damage. Unregulated cracks can also facilitate water infiltration into the base, subbase, and subgrade, potentially leading to further pavement distresses.

• Joint Infiltration and Restricted Movement

  • The intrusion of incompressible materials (like dirt, sand, and debris) into pavement joints, especially if joint seals have failed or are missing, can prevent the joints from closing normally during thermal expansion. This can lead to excessive compressive stresses and spalling. While spalling primarily occurs at joints, it signifies a failure in the pavement's ability to accommodate movement, which could indirectly contribute to stresses leading to cracking elsewhere.