SUMMARY

Use of Supplementary Cementitious Materials for Concrete

The use of supplementary cementitious materials (SCMs) in concrete mixtures has been a common practice of state departments of transportations (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.

The other source of information used for this report was the survey results of state DOTs. A survey containing 25 questions was sent to all 50 state DOTs and those of Puerto Rico and the District of Columbia. Forty-three DOTs responded to the questionnaire (83% response rate). All 43 DOTs allow the use of SCMs. With respect to conventional SCMs, 40 DOTs use coal ash (fly ash) Class F, 37 use slag cement, 35 use silica fume, and 28 use fly ash Class C. Also, 19 DOTs allow the use of harvested ash. With respect to NPs, 16 DOTs use metakaolin, 15 use calcined clay, and 10 use both calcined shale and pumice; three DOTs also allow the use of rice husk ash. Many of the 41 responding DOTs use 20% fly ash Class F, 50% slag, and less than 10% silica fume in their concrete pavement projects, while 15 DOTs do not allow silica fume in pavement mixtures. For bridge structures, some DOTs use higher SCM dosages, ranging from 50% to 70% by mass of the total cementitious material in the concrete mixture, to achieve better durability from significant reduction in permeability as well as reduction of heat generation.

With respect to NPs, of the 41 DOTs that responded, 16 allow the use of one or more NPs. However, not many DOTs have developed specifications unique to NPs, and they instead use ASTM C618 or AASHTO M 295 to govern their use.

With regard to ternary mixtures, 29 of the 33 responding DOTs allow their use. The responding DOTs reported varying combinations of SCMs in ternary mixtures, such as fly ash and slag cement, fly ash and silica fume, slag cement and silica fume, and NPs and slag cement. Also, as related to the use of quaternary concrete mixtures (using three SCMs), seven out of 39 DOT respondents use such mixtures for both bridges and pavements.

Twenty-one DOT respondents indicated that their acceptance strength is 28 days for concrete mixtures with or without SCMs, while 12 DOTs have a later age, such as 56 days, for acceptance strength. With regard to SCM-specific tests, many DOTs do not require special

tests for concrete when using SCMs. Also, as related to concrete with Type IL cement, 16 DOTs reported no effects of SCMs on the properties of concrete with Type IL cement, eight noted minor effects, and two indicated positive effects.

On the availability issue of fly ash, 28 of the 33 responding DOTs indicated that they have experienced or expect to experience shortages of fly ash. To address these shortages, 13 DOTs rely on industry solutions; 11 modify specifications to allow the use of other conventional SCMs, NPs, and ASCMs; and seven have begun to import fly ash. Three DOTs adjust the ingredients of concrete mixtures, and three limit the use of fly ash to only when durability is required in concrete applications.

The survey results indicated that six of 33 responding state DOTs allow the use ASCMs, and seven plan to allow their use in the future. As an example, ground-glass pozzolan is an ASCM that is being specified by the DOTs of Vermont, New York, and Florida. The Utah, Washington, and Wisconsin DOTs are experimenting with other ASCMs. Two DOTs are conducting field trials on ASCMs, and seven others are planning trials on pavements and bridges.

For the purpose of developing case examples, the synthesis authors selected the five state DOTs of California, Colorado, Louisiana, Minnesota, and Utah for additional interviews. The use of SCMs and ASCMs in the case example DOTs began with the desire to mitigate ASR, improve concrete durability, and achieve cost savings. No unique specification governs the use of SCMs and ASCMs at these agencies; however, they are successfully applying conventional SCMs in concrete structures and capturing their benefits. Also, some of the case example DOTs have identified ways to approve, specify, and make use of emerging ASCMs.

The synthesis has identified gaps in information on ASCMs and NPs and in the use of quaternary mixtures. Approaches to evaluate and predict the reactivity of SCMs and ASCMs would support the use of a wider array of materials. As low-carbon materials become increasingly used, approaches for product category rules and environmental product declarations will need to be evaluated and identified. Field trials with these materials are needed in order to evaluate any changes in performance of fresh and hardened concrete. Research in these areas would provide the information needed for DOTs to develop unique specifications and guides for acceptance and wider use of these materials in binary, ternary, and quaternary concrete mixtures.