Supplementary Cementitious Materials (SCMs) in Modern Highway Construction

BasePDH  |  Course No. 006  |  3 PDH

Source: NCHRP Synthesis of Highway Practice

Series: NCHRP Synthesis of Highway Practice
Publisher: Transportation Research Board (TRB) / National Cooperative Highway Research Program
Sponsors: American Association of State Highway and Transportation Officials (AASHTO) in cooperation with the Federal Highway Administration (FHWA)
Authors: Jamshid Armaghani, Global Sustainable Solutions, Gainesville, FL; and Tara Cavalline, University of North Carolina at Charlotte, Charlotte, NC

A Synthesis of Highway Practice

Authors: Jamshid Armaghani, Global Sustainable Solutions, Gainesville, FL; and Tara Cavalline, University of North Carolina at Charlotte, Charlotte, NC

Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

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Summary

The use of supplementary cementitious materials (SCMs) in concrete mixtures has been a common practice of state departments of transportation (DOTs) since the early 1980s. SCMs are nonorganic additives used in concrete mixtures, either as a partial replacement of the Portland cement in the mixture or as a material preblended with the cement. The use of SCMs in concrete improves its fresh and hardened properties and provides environmental benefits by reducing the amount of cement in concrete mixtures. SCMs commonly used by state DOTs include fly ash, slag cement, silica fume (conventional SCMs), and, to a lesser extent, natural pozzolans (NPs), which are naturally occurring earth materials or materials evolved from volcano eruptions that can be processed to be used in concrete mixtures in a similar way as with conventional SCMs. With the reduced availability of fly ash in recent years in many regions, harvested and beneficiated ash from landfills and ponds meeting the ASTM C618-23 and AASHTO M 295 specifications has been used in some states. In addition, alternative supplementary cementitious materials (ASCMs) have also been developed and studied in recent years and are being used in some states.

The objective of this synthesis is to document state DOT practices for specifying and using SCMs in concrete. As part of this effort, information was collected on types of SCMs and NPs, types of highway applications and replacement rates, uses of SCMs to resist adverse chemical reactions, DOT specification requirements, impacts of weather and types of structures on SCM dosage rates, availability and supply issues of fly ash, harvested coal ash from landfills, and emergence of ASCMs and their use in concrete by state DOTs.

Information on SCMs was collected from a literature review, a survey of state DOTs, and case examples from follow-up interviews of selected state DOTs. A summary of findings, gaps identified in current knowledge, and suggestions for further exploration and research are presented. The literature review covered many aspects of conventional SCMs and NPs, availability of fly ash, utilization of harvested ash, and currently available and emerging ASCMs. Publications reviewed included American Concrete Institute (ACI) guides and reports, FHWA reports and technical briefs, NCHRP reports, TRB annual meeting papers and Transportation Research Record articles, peer-reviewed journal and conference papers, technical reports prepared for agencies, and international publications.

Information from the literature review suggests that the use of fly ash, slag, and silica fume in binary (a single SCM) or ternary (two SCMs) mixtures improves workability, reduces bleeding, and facilitates placement of concrete in vertical and flat structures; it also reduces heat development and temperature rise in mass concrete. For hardened concrete, using SCMs improves its strength at later ages. In ternary mixtures, when silica fume is added with fly ash or slag cement, significant strength gain and reductions in permeability are observed at ages earlier than the typically specified 28 days. Enhanced durability of SCM concrete increases resistance to adverse reactions such as sulfate attack, alkali-silica reactivity (ASR), and chloride-ion penetration from sea water and deicing salts.

The mechanism by which SCMs and ASCMs impart improved properties to concrete typically lies in the pozzolanic reactions of SCMs with calcium hydroxide (CH). The pozzolanic reaction of SCMs with CH produces additional calcium–silicate–hydrate (C-S-H) binder gel that increases the density of the paste by filling the large-capillary pores and reducing the void structure of the paste matrix. The result of the pozzolanic reaction is increased strength of the paste, reduced paste permeability, and increased bond strength at the paste–aggregate interface. With some exceptions, most pozzolanic reactions do not produce heat, which makes SCMs such as fly ash and slag cement excellent additives in mass-concrete mixtures to control rise in concrete temperatures and avoid its damaging effect in mass structural members.

According to ASTM C618-23, the source of the pozzolanic properties of SCMs is a combination of high silica (SiO₂), alumina (Al₂O₃), iron oxide (Fe₂O₃), and lime (CaO), as well as particle fineness at or below 45 μm, which energizes the pozzolanic reactions at early and later ages.

Beneficiated harvested and bottom ash offer similar performance benefits to concrete mixtures as those of conventional SCMs and NPs. Harvested ash is now included in ASTM C618-23, which refers to both fly ash and harvested ash as "coal ash." Also, ASCMs, which include a range of industrial by-products and waste materials, can exhibit desirable pozzolanic or latent hydraulic reactions after processing. These ASCMs are seeing increased use in some states. As the shortages already experienced by many states of conventional SCMs such as fly ash continue, the increased use of harvested ash and ASCM materials may help to fill state DOTs' needs.