The temperature that decides which binder goes into a road is not the air temperature, and it is not the surface temperature either. The Texas Department of Transportation states the depth outright in its binder selection procedure: the selection algorithm uses location and confidence inputs “and heat transfer calculations to determine a binder grade based on the pavement temperatures expected in the surface layer (i.e., 20 mm below the surface).” Every claim a specification makes about heat resistance is a claim about a band of asphalt three quarters of an inch under the tire.
That band is where both halves of a performance grade live. In the department’s April 2023 procedure, a PG 64-22 is expected “to perform at a high temperature of 64°C and a low temperature of -22°C,” and the grading system moves in increments of 6 °C. The high number is not a peak reading. It “represents the 7-day average high pavement temperature,” while the low number “represents a single occurrence low pavement temperature.” One is a week averaged, the other is a single bad night. The asymmetry is deliberate, because the two failure modes are not alike: the high figure is used to design for rutting resistance and the low figure for cracking resistance.
Two distresses, one softened binder
Federal practice defines the heat distresses without mentioning heat. In FHWA’s LTPP distress manual, rutting is “Longitudinal surface depressions in the wheel paths,” and bleeding is “Identified by a film of bituminous material on the pavement surface that creates a shiny, glass-like reflective surface that may be tacky to the touch in warm weather.” Those are field-identification entries written for a surveyor with a straightedge, and they describe a result rather than a cause.
The cause sits in the binder’s stiffness at temperature. Hot enough, and traffic moves the mix rather than merely loading it, which is why the summer signature of an under-graded asphalt road is a pair of troughs in the wheel paths and a slick, dark sheen between them. Note that the manual hedges even the tactile test with “may be tacky.” Bleeding is scored by appearance because the field condition on the day of the survey is not the condition that produced it. How binder gets its grade in the first place runs through how asphalt is made.
Slabs fail on a different clock
Concrete does not rut. It moves. FHWA’s 2023 review of pavement resilience practice tabulates a higher average temperature as producing “Increased concrete temperature-related curling and associated stresses,” plus “Increased concrete moisture-related warping if accompanied by lower relative humidity.” Higher extreme maximum temperatures add a distinct failure to that list: “Concrete pavement blow-ups due to excessive slab expansion.”
The LTPP manual defines a blowup as “The result of localized upward movement or shattering of a slab along a transverse joint or crack.” It is a compression failure at a joint that has run out of room, and the room was supposed to be there. The adaptations the same FHWA table lists follow from that: reduced joint spacing, modified joint design, smaller slabs with enhanced load transfer, more reinforcement in continuously reinforced pavement. Two of the listed responses are not design measures at all but operations: “Increased efforts to maintain (seal/clean) concrete joints,” and “Include concrete expansion joints in existing pavement if blow-ups become a recurring problem.”
The recommended fix for a road that keeps buckling is to saw expansion joints into a slab that has already been paved, after the problem has established itself. That is a maintenance program, and it competes for the same budget as everything else in the climate and resilience section.
A week that was outside the record
Late June 2021 supplied the documented case. NOAA’s event tracker recorded Portland reaching 108 °F on 26 June, 112 °F on 27 June and 116 °F on 28 June, an average high across those three days of 112 °F, “the hottest three-day period on record by an astonishing 6 degrees.” Quillayute, Washington set an all-time record of 110 °F, “beating the previous record by a mind-boggling 11 degrees.” Seattle hit 104 °F and then 108 °F, in a city that had reached 100 °F three times in the previous 126 years and then did it on three consecutive days. Lytton, British Columbia went to 116 °F, then 118 °F, then 121 °F. NOAA’s account of the infrastructure damage is one clause long: “roads buckled across the Northwest.”
The binder grades in those pavements had been chosen against a temperature record that did not contain that week. Neither did the design of the joints in the concrete.
The response is a traffic rule pressed into climate service
Superpave already contains a mechanism for raising the high side of a grade, and it was not built for warming. The department’s pavement manual states what the mechanism is for: “‘Bumping’ or increasing the binder high temperature rating by one or even two grades is predicated on building in stiffness to handle slow-moving or standing traffic or very high traffic volumes.” The selection procedure sets the increments, one grade for slow moving traffic and two for standing traffic, because longer loading times behave like higher temperatures in a viscoelastic material. Traffic volume gets its own rule, expressed in equivalent single axle loads: consider one grade between 10 million and 30 million ESALs, and above 30 million “increase the high temperature designation by minimum one grade.” Climate appears nowhere in that ladder. It is the initial climate grade that the ladder is applied to.
Using the same lever for a warmer future is a repurposing, and the federal literature is careful about it. FHWA’s 2023 review reports that “some basic actions can be used to mitigate the largest concerns (e.g., periodically choosing higher PG binder grades for overlays), but this conclusion is not present in all studies.” At two peer exchanges FHWA convened in October and December 2020, attended by 92 people from federal, state and local agencies, academia and industry, rising temperature ranked below flooding as a concern. Some participants said their agencies were considering a higher binder grade “especially if significant warming occurs or if rutting begins to impact performance,” and for concrete, blow-ups “were considered a risk, especially for older pavements that may not have been adequately maintained.” FHWA states plainly that these findings “do not constitute FHWA policy or transportation agency guidance.”
Bumping is also not free, and the price is not in the specification. TxDOT’s rule keys elastic property requirements to the useful temperature interval, the span between the two grade numbers: a PG 64-22 spans 86, a PG 76-28 spans 104, and any binder at 92 or above must demonstrate elastic recovery, a requirement that in practice points toward a polymer-modified binder, though the procedure itself does not spell out that connection. The procedure’s own warning about enthusiasm is worth quoting: bumping a base PG 64-22 three or four times to a PG 82-22 “would probably be overkill and would result in a very expensive binder, which also may be difficult to place.” The department’s ceiling is explicit. “A maximum two-grade increase to no higher than a PG 76 is usually sufficient in all but the most extreme conditions.”
Part of the published map is a supply chain
The statewide grade map that most designers actually use is not a pure output of the climate model. TxDOT compiled its 98 percent confidence map from the selection program “with modifications allowing for uniformity between local areas” and with “historical binder supplier production data (most suppliers produce standard materials that meet the low temperature designation of -22). For this reason, some specific location inputs in the computer program may not match the designation shown on the map.”
Confidence itself is a choice rather than a finding. The program accepts high and low temperature confidence levels anywhere from 50 percent to 99.99 percent, defined as “the percent chance that local climatic temperature variations will not exceed the design temperatures,” and the guidance puts the decision on the district: “The district should choose confidence levels it can support.” The sensitivity is lopsided. The high temperature portion “will not change unless one reduces the confidence significantly,” while the low temperature portion “might change with modest decreases in confidence levels.” A district arguing about its numbers is usually arguing about winter.
Every climate input in the toolchain looks backwards
The engine underneath grade selection is a temperature archive. FHWA’s LTPPBind carried climatic data from 7,928 weather stations across the United States and Canada, including “the average number of days with temperatures over 30 °C (86 °F).” Its 2017 online successor documents the algorithms: the high temperature equation “is function of target rut depth, yearly degree-days and latitude of site,” and the low temperature algorithm “relates surface low pavement temperature to air temperature, latitude, and depth.” The alternative data source it offers is NASA reanalysis, available globally since 1979. All of it is observed history.
FHWA’s 2023 review states the limitation for the whole discipline rather than for one tool: “Currently, all pavement design procedures and design assistance tools/methods (e.g., frost depth determination) that depend on environmental inputs base those inputs on historical climate data,” and given climate change “these data may not be accurate predictors of future environmental conditions.” Whether a longer design life makes that gap better or worse is the live argument inside long-life pavement design, and the same historical-input problem runs in the opposite direction through freeze-thaw and the spring load restriction calendar.
The confidence level a grade was drawn at, the depth it applies to, and whether anything above the base climate grade was bought for traffic or for weather explain more about how a road will behave in a heat wave than the two digits printed on the plans.