|FHWA > Engineering > Pavements > Concrete > Alkali-Silica Reactivity (ASR)|
What are SCMs, and how do they work?
SCMs (supplementary cementing materials) include fly ash, slag, silica fume, and other natural pozzolans. In mix design, SCMs are incorporated by replacing a percentage of the cement with the SCM. SCMs create a less permeable matrix by disconnecting the pore structure, making the pore solution less able to move freely. A less permeable pore structure does not promote the ingress of outside contaminants or moisture. Each SCM has its own advantages and disadvantages.
Picture by PCA: www.cement.org/Briefingkit/image_captions.asp
Fly ash is the result of the coal burning process and is considered to be a "man made" pozzolan that has been used in concrete construction for more than 50 years. Fly ash is categorized into sub groups according to its chemical composition. In the U.S. fly ash is categorized by two classes defined by ASTM: F and C. According to Canadian Standards Classification (CSA), fly ash is categorized by three types: F, CI, and CH.
Canadian Standards Classification - CSA A23.5
ASTM C 618 Classification
ASTM Class F generally equivalent to CSA Types F & CI
Figure A4a.F2. Classifications of Fly Ash.
The first work to demonstrate the effective use of fly ash was reported in 1952. There are hundreds of publications on fly ash and ASR that show the effect of fly ash depends on:
Typically, the amount of fly ash required to control ASR increases as:
Work done by Shehata and Thomas in 2000 based on ASTM C 1293 testing showed low calcium fly ash to be more efficient in controlling expansion than high calcium fly ash. This trend has been attributed to the fact that low calcium ashes reduce pore solution pH through a pozzolanic reaction that results in a diminished transition zone due to more C-S-H that binds significant amounts of alkalies (Na+ and K+).
Ground granulated blast furnace slag (GGBFS) is a by-product of the iron making industry. GGBFS is commonly referred to as slag or slag cement and has been used in concrete for more than 100 years. The first work to show slag having an effect on ASR was in 1952. There are many publications on slag and ASR that show the effect of slag depends on:
Silica fume is a by-product of the silicon metal and silicon-iron making industry. The use of silica fume for controlling ASR is less well-established as the use of fly ash and slag. In 1989, Oberholster showed increased alkali content of the concrete required greater percentage of cement replacement with Silica fume to effectively curb expansion due to ASR. In recent years, work by Thomas and Bleszynski (2000) proposed upper and lower bounds for the amount of silica fume required base on the alkali content of the concrete. However, their work indicated a need for more than the typical replacement by mass dosage in cases of extremely reactive aggregates. The problem with adding more than the typical dose of silica fume lies in the reduced workability and shrinkage problems that are created.
As an alternative, recent years have seen increased applications involving silica fume as part of ternary blends. Ternary cement blends, which contain three different cementing materials (portland cement plus two SCM) have been found to be very effective in controlling ASR, especially blends containing silica fume plus either Class F fly ash or slag. Relatively low levels of silica fume (3 to 5%) together with moderate levels of Class F fly ash (15 to 20%) or slag (25 to 35%) have been shown to be effective (Shehata and Thomas, 2002; Bleszynski et al., 2002).
Calcined shales and clays along with metakaolin are considered natural pozzolans. They are used to reduce permeability and increase strengths. Calcined clays and shales are used as cement replacements. Metakaolin is more often used as an additive to the cement content. ASR expansion is minimized due to the low permeability inhibiting the ingress of moisture.