Summary of Amendments to Caltrans Standard Specification, Section 90 Portland Cement Concrete

Amendments to Section 90 “Portland Cement Concrete” of the 2006 Caltrans Standard Specifications were issued in June 2009. The section’s approach to concrete specifying is partially prescriptive and partially performance: the section prescribes certain minimums and maximums to ensure desirable characteristics and also requires that performance criteria be met (i.e. compressive strength, shrinkage). This type of specification allows the supplier and contractor to utilize a wide range of mixes, as long as these mixes meet Caltrans’ performance and prescriptive requirements.

The force behind this amendment is California’s Greenhouse Gas (GHG) reduction policy, outlined in Assembly Bill #32 (AB 32). Its intent is to reduce GHG emissions to 1990 levels by the year 2020, and to continue to reduce those levels by 2050. Cement manufacture produces roughly 7% of the GHG emissions in California; therefore, replacing a significant portion of cement with supplementary cementitious materials (SCMs) will reduce the amount of cement used per unit of concrete and reduce GHG emissions. Caltrans’ response to this initiative is to “green” their concrete ahead of the law by allowing unlimited amounts of SCMs and restricting the use of cement.

The SCMs allowed in Caltrans projects are Ground Granulated Blast Furnace Slag (GGBFS), Class F Fly Ash, Silica Fume, Ultra Fine Fly Ash, Metakaolin (a finely ground natural pozzolan), and Raw or Calcined Natural Pozzolans. The most commonly used SCMs in Graniterock mix designs are slag, fly ash, and silica fume. Ternary mixes, which are a combination of any three cementitious materials, are also common.

Caltrans has implemented two equations to determine the minimum amount of SCMs to be used. The first equation allows for virtually an infinite number of mix designs using any combination of SCMs, if at least one SCM is used and meets the minimum required. The second equation limits the percentage of portland cement. Because of this limitation, additional time is allowed to obtain strength (42 days instead of 28) for mixes specified for a compressive strength greater than 3,600 psi. This will allow Graniterock to use up to 50% fly ash mixes. Compared to straight-cement mixes, these high-volume-fly-ash mixes (HVFAs) have lower water requirements, higher durability, lower porosity, and better finish- ability. HVFAs also have lower heats of hydration, making them ideal mixes for mass concrete and/or projects with long haul times.

Another important addition to Section 90 is shrinkage limitations for paving and approach slab concrete and bridge deck concrete. Shrinkage testing data must now be submitted for concrete mix designs used for these applications. Graniterock is in the process of compiling shrinkage data for standard HVFA, slag, and ternary mixes that meet the new Caltrans criteria to assure a broad set of options for contractors.

For further details regarding the changes to Section 90, please contact a Research Technical Services concrete representative at 831.768.2330.