TB-0113 — Concrete Admixture Use with High Volume Fly Ash Technical Bulletin

The use of Fly Ash by Ready Mix producers continues to grow in the United States. In 1983, 31% of Ready Mix producers used Fly Ash in concrete mixes, with 46% of the concrete produced containing Fly Ash. By 1998 that number had grown to 94%, with 54% of concrete produced containing Fly Ash(1). Fly Ash contents in a concrete mix will vary but are normally 15% to 30% of total cementitious material. However amounts of 35% and greater have been used and are becoming more common. These concrete mixes have been reported by many sources in the literature(2)(3)(4) for some time, and are becoming increasingly more important to the building industry as a result of the advent of the US Green Building Council’s (USGBC) Leadership in Energy and Environmental Design (LEED) green building rating system.

Higher volumes of recycled materials in concrete can assist in a project achieving three (3) LEED credits, two in the Materials and Resources section and one in the Innovation section. The Recycled Content section is self-explanatory. The Innovation credit is based on a site-wide reduction in ordinary Portland cement (OPC) by 40% compared to the standard baseline mixes for a similar project. This reduction in OPC translates to an equal reduction in the amount of CO₂ produced for the cementitious portion of the concrete. Utilizing High Volume Fly Ash (HVFA) mixes in a project is a way to achieve the needed 40% reduction. ACI 232.2R defines HVFA as concrete containing higher amounts of Fly Ash than normal, usually 50% or greater by mass of cementitious material in a concrete mix. The use of HVFA also is not new, but it has been rarely used until recently. Due to a variety of reasons, the use of HVFA is starting to increase.(5)(6)(7) High performance HVFA mixtures are generally defined as concrete with a maximum cementitious amount of 680 lbs/yd³ (400 kg/m³), low water-to-cementitious ratio of 0.35 or less, strengths greater than 4,000 psi (28 Mpa) and incorporate high range water reducers to produce a 6 in. to 8 in. (150 mm to 200 mm) slump. 

The use of HVFA in concrete with strengths less than 4,000 psi (28 Mpa) is also growing. This Technical Bulletin will review some of the plastic and hardened concrete properties associated with HVFA concrete mixes, and the types and amounts of concrete admixtures needed for production of this type of concrete. Fly Ash is a supplementary cementitious material and is a by-product of the combustion of coal in power generating plants. Class F Fly Ash is common in the eastern portion of N. America, although some is produced in the West. Class C Fly Ash is predominately produced in the western and middle portions of N. America. Different sources of fly ash generally provide different performance in concrete due primarily to its chemistry, glass content, and fineness. For a good explanation of fly ash chemistry, the affects on plastic and hardened properties of concrete, and mixture proportioning, please see ACI 232.2R, Use of Fly Ash in Concrete.

When HVFA is used in concrete in the 3,000 to 4,000 psi (21 to 28 Mpa) range and in certain structural applications, these changes to the concrete properties may be acceptable, although construction practices will have to be altered to accommodate these effects.(7) However, when HVFA is used in applications that require more workability and high performance, the use of an admixture system of high range water reducers to produce lower water-to-cementitious materials ratios, and set accelerators to counter-act the slower set times and strength gain, are required. 

Multiple studies by GCP Applied Technologies have been performed on fly ash concrete over many decades. More recently these studies have been extended to HVFA concrete. A major objective has been to identify concrete admixtures that can minimize early set retardation and improve early strength gain. The following table of results shows that a combination of ADVA® and DARASET® 400 provided the best solution for similar time of set strength in HVFA concrete as compared to concrete without Fly Ash.