The cost of transporting granular materials for flexible bases can be a major factor in pavement construction costs. Sources of adequate construction aggregates are scarce in some areas of Texas, and pavement construction requires transporting significant amounts of aggregate to the construction site. High transportation costs create the need for alternative sources of material that are available locally.

Hydrated fly ash – a stiff material produced by allowing powdered fly ash from coal power plants to cure with moisture – may be used to make aggregate for flexible bases. Hydrated (cured) fly ash is so stiff that it can attain compressive strengths as high as 15,000 kPa. With its natural reactivity, fly ash will set in stockpiles, even in the absence of organized curing.


The Texas Tech University Department of Civil Engineering conducted study 0-1365, "Guidelines For Using Hydrated Fly Ash As A Flexible Base", for TxDOT, the Federal Highway Administration (FHWA) and the Texas Commission on Environmental Quality (TCEQ). The purpose of this study was to refine the existing specifications for using hydrated fly ash as flexible base.

Hydrated fly ash is a stiff material that can be crushed to form an aggregate. When properly processed, hydrated fly ash continues to gain strength after placement and can function satisfactorily as a road base for an extended period.

Available information on hydrated fly ash appears to indicate that it has great potential as a flexible-base material, easily meeting the TxDOT strength criterion for flexible-base materials.

Specifications need further development in several areas including:

  1. water demand,
  2. curing conditions,
  3. mechanism of bonding between the hydrated fly ash flexible base and asphalt surfaces such as seal coats and asphalt concrete, and
  4. the formation mechanism of crystalline products in locations where the hydrated fly ash base is exposed to moisture.

This study yielded laboratory characterization of fly ash available in the Amarillo district, draft specifications and guidance on the use of hydrated fly ash material as flexible base, and a cost-benefit analysis.

Based on the data and limited experience with fly ash in flexible-base construction, the following observations can be made:

  1. Fly ash is extremely strong when compared to TxDOT specification triaxial classes. It meets TxDOT's unconfined compressive strength criterion for Class I base material very easily.
  2. Observations indicate fly ash gets crushed during compaction and, as a result, the master grading criteria in the TxDOT specifications may not have been met in the construction layer. However, this appeared to have had little impact on achieving maximum dry density.
  3. Field observation found that fly ash undergoes further hydration after placement, forming a stiff, nearly homogeneous layer. Therefore, strict adherence to the gradation specification may not be needed.
  4. Laboratory compaction tests using hydrated fly ash with two different gradations (gap-graded and well-graded) revealed that both gradations achieved nearly the same maximum dry density values, but at different moisture contents.
  5. Powdered fly ash hydrated at lower moisture contents provides much higher strengths, resulting in better resistance of the aggregate to degradation. Also, the fly ash s should be thoroughly mixed with water during the hydration process.
  6. Aggregates produced using higher hydrating moisture contents have a lower unit weight and less strength.
  7. Care must be taken when fly ash is cured to ensure that it attains the required level of strength before it is milled. Otherwise, the fly ash may not meet specifications for degradation and durability.
  8. Care must be taken when fly ash is cured and during road construction to ensure that it is not allowed to dry excessively. If this happens, the fly ash will form undesirable compounds that may decrease the material's durability.
  9. Hydrated fly ash has a high water demand. Therefore, sufficient allowance should be made for subsequent wetting during curing and construction.
  10. Shrinkage cracks may appear if the fly ash has not reached an advanced stage of hydration in the curing ponds.

Available information on hydrated fly ash appears to indicate that, in regard to strength, it has great potential for use as a flexible-base material. Hydrated fly ash has the potential to perform as well as any other flexible-base material in use today. More research is needed to enhance understanding of the material, particularly with regard to its durability.

*The contents of this summary are reported in detail in Texas Tech University's College of Engineering Research Report 0-1365-1F, "Guidelines For Using Hydrated Fly Ash As A Flexible Base," Phillip T. Nash, Priyantha Jayawickrama, Sanjaya Senadherra, John Borrelli, and A.S.M. Ashek Rana, Preliminary Report Dated - August 1995. This summary does not necessarily reflect the official views of the FHWA, TCEQ or TxDOT.