In 2002, Houston built a runway for Boeing 747s using a concrete mix with 25% fly ash and 25% blast-furnace slag; test slabs reached 800 psi flexural strength in 90 days

Runway concrete must do more than hold up aircraft. It must hold up to repeated heavy loads, resist moisture and be structurally sound for years, which makes the choice of cementitious materials very critical. Engineers at Houston’s George Bush Intercontinental Airport sought to meet those criteria while substituting half of the traditional cement with industrial by-products. Runway 15R-33L opened to air traffic on June 13, 2002. The runway was 10,000 feet long and 150 feet wide, designed to take Boeing 747s, Antonov 124s and other big aircraft. Its most remarkable feature, however, was hidden in the concrete: half of the cementitious material was composed of 25% Class F fly ash and 25% blast-furnace slag.In a 2003 Concrete International article, Shondeep L. Sarkar and Adil Godiwalla described the material properties and early performance of the runway concrete. After about eight months of service, they reported high strength, low permeability and a dense concrete structure.Houston required a new runwayThe original Runway 15R-33L was not intended for the size of the aircraft Houston expected. It was just 6,038 feet long and 100 feet broad. There was approximately 15 inches of full-depth asphalt over six inches of lime- stabilised subgrade. It was originally designed based on a DC-9 weighing roughly 121,000 pounds. The renovation extended it to 10,000 feet and widened it to 150 feet, allowing the runway to accommodate Boeing 747-class aircraft with maximum weights of around 875,000 pounds. The $100 million job, regarded as the largest runway whitetopping project at a U.S. airport at the time, was covered in a comprehensive ASCE project paper on the rebuilding of Runway 15R-33L.Whitetopping involves placing a new concrete pavement over an existing pavement structure rather than removing the entire roadway. For the runway upgrade, engineers milled about 8 inches of asphalt in the keel section before placing a 19-inch concrete overlay, allowing the existing pavement structure to remain part of the rebuilt runway. The finished concrete road was about 18 inches thick and covered some 300,000 square yards. It had to be designed to withstand bending forces generated by very large aeroplanes moving across the slab time and again.The ternary mix explainedA ternary blend is a mixture of three cementitious materials. The concrete at Bush Intercontinental had 50% Type I portland cement, 25% Class F fly ash and 25% Grade 120 blast-furnace slag by mass of the cementitious material. Fly ash is fine waste from the combustion of coal and blast-furnace slag is a by-product of iron manufacture. Both can participate in chemical reactions within the concrete and contribute to later age strength and a denser internal structure.The pavement was developed by the engineers of Houston for a flexural strength of 650 pounds per square inch. Trial batches were above the design target, reaching 800 psi in flexure at 90 days. Flexural strength is of special concern for pavements, since concrete slabs flex under the passage of aeroplane loads. A pavement mixture can therefore be evaluated not only by the compressive force it can withstand but also by its resistance to bending and cracking under repeated aircraft loads.The early field results were encouraging. The 2003 study found compressive strengths of roughly 6,730 to 9,790 psi in cores sampled after about eight months of service. Electrical testing indicated low permeability, while microscopic analysis found a dense concrete structure with little microcracking.The caution from Houston's other runwayNot every batch of concrete acted the same in Houston. Engineers later explored trouble at a critical runway intersection at neighbouring William P. Hobby Airport, which used a high-early-strength concrete mixture. ASCE performed a different study on the Hobby Airport runway restoration, indicating the service life of the previous pavement was decreased and the deterioration was mostly caused by the production of secondary ettringite crystals. Given the great emphasis on minimising runway closure time, fast strength development was a crucial feature of the original design.That experience did not suggest that high early-strength concrete was intrinsically unfit. It demonstrated that a pavement mix must be tailored to project conditions and priorities. Maximising early strength solves one problem of building but raises other problems of durability. Bush Intercontinental Runway went a different route. The ternary concrete was meant to develop significant strength at later ages while also offering low permeability. The fly ash and slag lowered the percentage of ordinary portland cement while maintaining the ability of the concrete to meet the structural requirements of the runway.There were tradeoffs. Cementitious blends with high proportions of fly ash and slag produce strength differently than plain portland-cement mixes; therefore, curing, building and opening needs need to be considered in design. A runway closure to allow concrete to gain strength presents a different problem than an emergency repair that must be opened within hours.What the results showHouston’s Runway 15R-33L demonstrated at full scale that a heavy-duty airport pavement could use a cementitious blend containing 50% industrial by-products. Trial concrete achieved 800 psi flexural strength at 90 days vs. a design basis of 650 psi. Initial field cores indicated satisfactory compressive strength and minimal permeability. It was not just a case of replacing Portland cement with waste materials. Engineers had to find a blend that was right for the aircraft loads, the construction process and the long-term durability needs.Houston’s experience at Hobby Airport offers a contrasting lesson: concrete designed for very rapid strength gain still needs to be evaluated for long-term durability. At Bush Intercontinental, the engineers instead used substantial amounts of fly ash and slag and designed the mix around the aircraft loads, construction requirements and expected service conditions.You use AI every day. Now get your AI Quotient. Take the AIQ test.