19.10. Failure criteria of rigid pavements

Fatigue cracking has long been considered as the major, or only criterion for rigid pavement design. The allowable number of load repetitions to cause fatigue cracking depends on the stress ratio between flexural tensile stress and concrete modulus of rupture.

Fatigue cracking occurs when the concrete in a rigid pavement experiences repeated loading, which causes it to weaken over time. The design criteria for such pavements strongly consider the number of times loads are applied. This is measured by examining the 'stress ratio', which compares the bending stress in the pavement layers to the strength of the concrete when it fractures. Understanding this ratio helps engineers predict how long the pavement will last before it starts to show signs of fatigue or cracking.

Think of fatigue cracking like a piece of cardboard being repeatedly bent back and forth. Initially, it holds its shape, but with enough repetitions, the cardboard weakens and eventually tears. Similarly, with rigid pavements, the material may initially withstand traffic loads, but over time and with frequent heavy loads, cracks may develop due to fatigue.

Of late, pumping is identified as an important failure criterion. Pumping is the ejection of soil slurry through the joints and cracks of cement concrete pavement, caused during the downward movement of slab under the heavy wheel loads.

Pumping happens when the weight of vehicle traffic compresses the pavement slab, causing a downward movement. This movement can force water and soil upwards through cracks and joints in the pavement, which is referred to as 'pumping'. This scene leads to damage in the pavement structure as the soil is expelled and small voids are created, making the pavement less stable.

Consider a sponge filled with water; if you press down on it, the water will shoot out from its surface. Similarly, when heavy vehicles drive over rigid pavements, they compress the concrete slab, and any trapped water and slurry can be forced out through the cracks, leading to problems in the pavement's integrity.

Other major types of distress in rigid pavements include faulting, spalling, and deterioration.

In addition to fatigue cracking and pumping, rigid pavements can suffer from various other issues. 'Faulting' refers to the uneven alignment of pavement slabs at joints, causing a bump or dip. 'Spalling' happens when pieces of concrete break away from the slab’s surface, often due to moisture infiltration and freeze-thaw cycles. 'Deterioration' indicates the overall degradation of the pavement’s condition due to environmental factors and vehicle loads. Recognizing these types of distress is crucial for maintenance and repair strategies.

Imagine a sidewalk that has uneven areas (faulting) where some tiles are raised compared to others. Over time, if water seeps into the cracks of these tiles and freezes, the edges may chip away and break off (spalling). As the years pass, the entire sidewalk begins to show wear and tear, making it less safe to walk on (deterioration). These examples illustrate similar issues that occur on rigid pavements.

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