The Pavement That Breathes

How Rereading Thermal Concrete Stresses Rewrites the History of Urban Failure in the Contemporary Arab City

On a hot summer morning, as the street surface temperature rises beyond what concrete can endure, engineers observe what they call a “blowup”: an entire concrete slab detaches from the ground and lifts upward as though an invisible hand has pushed it from below. This scene, repeated across streets in numerous cities worldwide, is not a flaw in the concrete itself but the cry of a material forced to perform under conditions it was never designed for. What makes this all the more striking is that scientific research has been telling us for decades that the primary enemy of concrete pavements is neither heavy trucks nor traffic density — it is the sun itself.

The Street That Bends Under the Weight of the Sun

To understand what happens inside a concrete pavement slab, one must first abandon the intuitive assumption that heat affects only the surface. The reality, as Choubane and Tia demonstrated in their landmark study published in the Journal of Transportation Engineering in 1995, is far more complex. Their field measurements revealed that the temperature distribution through the depth of a concrete slab is not linear, as traditional design models had assumed for decades, but follows an irregular quadratic curve. This seemingly simple finding carries enormous consequences: the old linear assumption overestimated daytime tensile stresses by 11 to 15 percent while underestimating nighttime stresses by up to 19 percent. In other words, we had been designing pavements based on a misreading of how heat behaves within the material itself.

More than two decades after that study, Polish researchers Mackiewicz and Szydło confirmed these findings through continuous temperature monitoring of concrete pavements in Poland. They found that the thermal differential between the top and bottom of a slab can reach 10 degrees Celsius in summer under the relatively moderate climate of Central Europe, dropping to negative 5 degrees Celsius at night. When these data were fed into three-dimensional simulation models comprising roughly 230,000 elements, the resulting stresses in slabs longer than five meters reached approximately one megapascal, and could exceed 1.5 megapascals under incidental thermal differentials of 20 degrees Celsius — equivalent to 30 to 50 percent of the concrete’s total tensile strength depending on its class. The question then presses itself forward: if this is the case in Poland’s cold climate, what is happening on streets where surface temperatures exceed 50 degrees Celsius?

When the Pavement Explodes

A partial answer to this question was provided by Kerr and Dallis in their classic 1985 analysis of the pavement blowup phenomenon. A blowup is not ordinary cracking but a catastrophic structural failure that occurs when debris accumulated in joints prevents the natural thermal expansion of slabs, causing axial compressive forces to build until they exceed the bearing limit and the slab bursts upward. The two researchers defined what they called the “safe limit” for temperature rise above the neutral point, finding that this limit does not exceed 30 degrees Celsius in jointed pavements, while rising to roughly 50 degrees in continuously reinforced pavements. The critical difference is that joints — those lines visible on the surface of every street — reduce the safe limit by approximately 40 percent because they concentrate axial forces and introduce bending discontinuities.

These figures carry alarming implications for any hot city. If jointed pavement — the most common type on urban streets — cannot tolerate a thermal differential exceeding 30 degrees Celsius above the neutral temperature, then streets where surfaces are exposed to temperatures above 60 or 70 degrees Celsius exist in a permanent state of structural emergency. The study also demonstrated that increasing frictional resistance between the slab and the base layer widens the safe thermal range, while a higher coefficient of thermal expansion narrows it — which means that material selection is itself part of the climatic equation, not merely a structural decision.

Color and Shade: A Neglected Equation on Our Streets

The research, however, does not stop at diagnosing the problem; it points to solutions that have been in plain sight all along. In an extensive field study conducted by Kim and Nam in Texas and published in Road Materials and Pavement Design in 2010, surface color alone was shown to produce a radical difference in internal concrete temperatures. White surfaces reduce peak concrete temperatures by 6 to 8 degrees Celsius compared to gray surfaces, and by 12 to 18 degrees compared to black ones. The study also revealed that shading lowers daily peak temperatures by 6 to 10 degrees Celsius while slightly raising minimum temperatures, thereby reducing the overall daily thermal amplitude. Even covering concrete with polyethylene sheeting — a common practice to protect it from rain during pouring — raises surface temperatures by up to 14 degrees Celsius the day after placement, elevating the zero-stress temperature and exacerbating tensile stresses later on.

This is where structural engineering intersects with urban history in ways that cannot be ignored. The traditional Arab city, with its narrow alleys, covered passageways, and contiguous walls, was not merely an expression of culture or social convention but an integrated thermal system that minimized direct solar exposure on horizontal surfaces. When the contemporary Arab city replaced this fabric with wide, exposed streets designed on the Western model, it did not merely alter the urban form — it raised thermal stresses on its infrastructure to levels the pavements were never designed to withstand.

Seven Times Over: Heat Kills the Pavement Before Trucks Do

Perhaps the most striking finding in this entire field of research comes from Ryu and his colleagues, whose intensive field measurements on a jointed concrete pavement test road in South Korea were published in the Journal of Performance of Constructed Facilities in 2017. Their data demonstrated that thermal stress in a 30-centimeter-thick concrete slab is roughly seven times the stress produced by traffic loads. Seven times. Heat — not trucks — is the silent killer of concrete pavements. Yet most global design codes still give absolute priority to wheel loads in their calculations, treating thermal stresses as a secondary factor.

In the same vein, Maitra and his colleagues developed a generalized expression for estimating critical edge stress under the combined effect of axle loads and nonlinear thermal gradients, using a three-dimensional finite element model. When they compared their results with the Indian Roads Congress design method, they found that the latter overestimates critical stress by 11.6 percent due to its empirical treatment of curling stress. This means that current design codes — even when they attempt to account for heat — do so imprecisely, potentially leading either to overdesigned, costly pavements or to unexpected premature failure. Ryu’s measurements also revealed that the type of base layer makes a significant difference: lean concrete base produces the highest stresses under traffic loads, while asphalt-stabilized base generates the highest stresses under thermal loads due to greater frictional restraint against the underside of the slab.

The Thermal Memory of Concrete

Among the most thought-provoking concepts in this field is what is known as the “built-in temperature distribution” — the thermal profile that concrete assumes at the moment of final set during placement. Sok and his colleagues, in their numerical study published in Road Materials and Pavement Design in 2019, demonstrated that early-morning concrete placement produces a large positive built-in thermal gradient of up to 8.9 degrees Celsius, while afternoon placement yields a small positive gradient of no more than 3.6 degrees, and evening placement produces a slight negative gradient of roughly 1.8 degrees below zero. This built-in gradient determines the trajectory of stress development during the first three days of the pavement’s life, with morning-placed slabs recording the highest tensile stresses and therefore the greatest cracking risk.

What makes this particularly compelling is that the effect of this “thermal memory” fades over time due to the phenomenon of creep and stress relaxation in concrete — a viscoelastic property that allows the material to redistribute internal stresses. The researchers found that viscoelastic relaxation reduces stresses by up to 34 percent by the time the pavement reaches twenty days of age, meaning that purely elastic analysis significantly overestimates tensile stresses. Yet this compensatory mechanism does not erase the initial damage that occurs in the first hours and days, just as flawed urban planning decisions do not erase their cumulative damage over decades.

Rereading the Old Alley

When all this evidence is placed side by side, a picture emerges that is at once clear and deeply unsettling. Thermal stresses dominate concrete pavement behavior at a magnitude seven times greater than traffic loads. Dark, exposed surfaces raise concrete temperatures by up to 18 degrees Celsius compared to light-colored ones. The safe temperature rise limit for jointed pavements does not exceed 30 degrees Celsius. Slabs longer than five meters endure curling stresses that can consume half the material’s available tensile strength.

These figures are not merely dry engineering data but scientific testimony that the prevailing urban model in contemporary Arab cities — with its wide, exposed streets, dark pavements, and widely spaced joints — operates against the physics of the material itself. Conversely, the solutions the research points to — reflective surfaces, shading, reduced slab lengths, and appropriate placement timing — are not modern inventions but principles already embedded in the fabric of the old Arab city, which designed its narrow streets and covered passageways as an intuitive response to the very same sun that shatters today’s pavements.

The irony is that modern science requires three-dimensional simulation models comprising hundreds of thousands of elements to arrive at conclusions that were plainly visible in the alleys of old cities. And perhaps the deepest lesson of all this research is that the infrastructure crisis in our cities is not a crisis of materials or technologies but a crisis of reading: a misreading of climate, a misreading of history, and a misreading of the relationship between built space and the open sky above it.

✦ ArchUp Editorial Insight

The cracking of concrete pavements across Arab cities is not a materials failure but a procurement and planning failure operating at the scale of entire urban grids. The research is unambiguous: thermal stress exceeds traffic loading by a factor of seven, yet global design codes — calibrated for temperate climates where this ratio does not apply — continue to prioritize wheel loads in their calculations, a condition this archive identified in Physical Ground Footprint, where a temperate representational rule is exported as universal and the consequence is absorbed by those who never set the standard. The planner who approved the wide, exposed boulevard modeled on Western precedent exits the project before the first blowup; the municipality that inherits the repaving cycle every three to five years had no role in the original urban form decision. The old Arab city’s narrow, shaded alleys were not cultural ornament but calibrated thermal infrastructure — a grammar of solar avoidance that kept horizontal surface temperatures within the material’s safe range. Replacing that grammar with dark, unshaded asphalt on six-lane corridors did not modernize the city; it transferred the cost of a climatic misreading onto the public maintenance budget, where it compounds silently with every summer.

References

Choubane, B., and Tia, M. “Analysis and Verification of Thermal-Gradient Effects on Concrete Pavement.” Journal of Transportation Engineering, 1995.

Mackiewicz, P., and Szydło, A. “Thermal Stress Analysis in Concrete Pavements.” Journal of Transportation Engineering, Part B: Pavements, 2020.

Setyawan, A., Zoorob, S. E., and Hasan, K. E. “Investigating and Comparing Traffic-Induced and Restrained Temperature Stresses in a Conventional Rigid Pavement and Semi-Rigid Layers.” Procedia Engineering, 2013.

Sok, T., Kim, Y. K., and Lee, S. W. “Numerical Evaluation of Built-In Temperature Distribution Effects on Stress Development in Concrete Pavements.” Road Materials and Pavement Design, 2019.

Kim, S. M., and Nam, J. H. “Measurements and Experimental Analysis of Temperature Variations in Portland Cement Concrete Pavement Systems.” Road Materials and Pavement Design, 2010.

Kerr, A. D., and Dallis, W. A. Jr. “Blowup of Concrete Pavements.” Journal of Transportation Engineering, 1985.

Ryu, S., Lee, J., Kwon, S., and Cho, Y. H. “JCP Behavior with Traffic and Temperature Loadings: Analysis of a Test Road in Korea.” Journal of Performance of Constructed Facilities, 2017.

Lajčáková, G. “Moving Load Effect of a Truck on Concrete Pavements.” Applied Mechanics and Materials, 2014.

Maitra, S. R., Reddy, K. S., and Ramachandra, L. S. “Estimation of Critical Stress in Jointed Concrete Pavement.” Procedia – Social and Behavioral Sciences, 2013.