In 2001, Texas finished a five-year study of six crushed-fly-ash road sections; inspections, core strength and deflection tests found the pavements had performed well

The Texas Transportation Institute completed a five-year research project on six test pavements in the Atlanta District, submitting its final report in October 2001. Crushed fly ash, a byproduct of coal-fired power plants, served as the base for all of the pavements in the Atlanta District of northeast Texas. The conclusion was clear: the test pavements had done well based on visual surveys, deflection data, and core strengths.The work was performed for the Texas Department of Transportation under a research project from September 1996 to August 2001. Its title was awkward but correct: durability of surface treatments applied to crushed fly ash and long-term performance of crushed fly ash as a flexible base. What was behind that term was a practical question. Could a waste material from the power sector be used as road aggregate and stand up to actual traffic and real weather?The six pavement sections were built between 1993 and 1995, and the formal research project began in September 1996, with the first annual field evaluation conducted in spring 1997. In its first annual assessment, the Texas Transportation Institute reported that most of the test pavements were performing very well, although two showed minor distress that might have been related to the fly ash base.Crushed fly ash for base roadsThis material is no ordinary fly ash. Engineers employed something called hydrated fly ash, which is formed by curing a Class C fly ash, a self-cementing variety, with moisture. It sets hard and forms a hard, rock-like substance. This material can be fractured to form a synthetic aggregate. If the material is properly handled and compacted at its optimum moisture level, it can still build strength after being placed in the road. TTI’s final report particularly discusses this behaviour and states that the hydrated fly ash forms a stiff substance that may be crushed to make a synthetic aggregate.The Atlanta District created six pavement pieces using the material as a flexible base. Some parts had a fly ash base of approximately 6.5 to 13 inches, depending on the road, with the recycled material beneath the surface treatment or asphalt layers.What was measuredThe researchers evaluated the pavements through visual surveys, falling-weight deflectometer testing and laboratory analysis of pavement cores. Visual surveys recorded cracking, rutting and other surface deterioration, while the deflectometer measured pavement stiffness.Cores drilled out of the pavement were then evaluated in the laboratory for compressive strength. Researchers also employed ground-penetrating radar in parts of the study to look at the structure of the pavement without digging into it. The detailed testing also showed that the six sections were not behaving identically. TTI identified weak and stiff spots within individual pavement sections, and one pavement was considerably stiffer overall than the other five. Researchers observed the fluctuations and realised that the uniformity of the material could affect performance.Later assessments did not see those worries translating into widespread decline. The final assessment spanning the period from spring 1997 to spring 2001 found that the hydrated fly-ash test pavements had fared satisfactorily based on the combined evidence of visual inspection, deflection and core strength.The challenge of bondingThe researchers conducted another Texas Transportation Institute study on surface-treatment bonding. Laboratory testing investigated various prime materials and the degree of curing of the fly-ash base before the asphalt membrane was applied. They determined that the type of prime material was not the main reason for the poor bond and that the degree of curing prior to the surface treatment was more probably responsible. That difference was important because a pavement base could be solid, but the layer on top of it could still fail to connect properly. The problem wasn’t that the fly ash was unusable as a road basis; it proved that construction time and interaction between pavement layers were also critical.The importance of variabilityThe first-year evaluation showed differences in stiffness between sites within the same sections. So the researchers weren’t investigating some general substance that behaved the same everywhere. They were testing a particular processed fly ash in a controlled construction environment. That was also a limit of the 2001 study. The six test sections provided a small sample of pavements that performed well during the monitored period, but the research did not establish how the material would perform over several decades or under every combination of traffic and weather.So what do the results tell us?The Texas results show that a coal waste product, when properly cured, crushed and compacted, may be used as a flexible road base and continues to acquire strength after placement. Visual inspections, deflection tests and core-strength measurements were used to monitor the six test sections in the Atlanta District. The final assessment stated they performed well. The practical lessons are specific to road authorities: test the uniformity of the ash supply, control moisture during compaction, ensure the base is properly cured before applying a surface treatment, and monitor the pavement over several years rather than assuming an early result will continue. The Texas study did not show that other fly-ash bases will behave in the same way. It showed something more specific: when a suitable Class C ash was processed into a synthetic aggregate and used under controlled construction conditions, it could remain a functioning road base through at least five years of field monitoring.You use AI every day. Now get your AI Quotient. Take the AIQ test.