CHAPTER 5

Summary of Findings

Introduction

The objective of this synthesis is to document state DOT practices for specifying and using SCMs in concrete. The synthesized information in the report includes that on conventional SCMs such as fly ash, slag cement, and silica fume, as well as on NPs, harvested and beneficiated coal and bottom ash, and ASCMs. The information collected for this report has been obtained from literature review, survey of state practices, and case examples of selected state DOTs. Presented in the following are a summary of findings, gaps in the available information, and research needs.

Summary of Findings

Literature Review

The use of SCMs as a replacement for a portion of the Portland cement content in well-proportioned concrete mixtures has been shown to improve the fresh and hardened properties of the concrete. SCMs also contribute to the reduction of concrete infrastructure’s carbon footprint, allowing agencies to achieve not only improved concrete strength and durability properties, but also sustainability benefits associated with the judicious use of cement in concrete mixtures used in structures and pavements.

SCMs engage in pozzolanic reactions with CH, an inactive by-product of Portland cement hydration. The pozzolanic reaction with CH produces additional hydrated C-S-H binder to fill the large-capillary pores in the paste, reducing its alkalinity, densifying its structure, and strengthening its bond with aggregates. This ultimately results in concrete with higher strength, lower permeability, and enhanced durability. The pozzolanic properties of SCMs are derived from their high amorphous silica content, fine particle sizes, and the presence of other oxides.

Since the 1980s, SCMs such as coal ash (fly ash), slag cement, and silica fume have been specified by most state DOTs for use in binary and ternary concrete mixtures. These SCMs are well established for improving the later-age strength of concrete and reducing its permeability to resist the damaging effects of adverse chemical reactions such as ASR, sulfate attack, and corrosion-inducing chlorides in seawater and from deicing salts.

Ternary concrete mixtures combine the advantages of two SCMs (while also offsetting the shortcomings of both) during the fresh and hardened phases of concrete. Capitalizing on the characteristics of two SCMs, ternary mixtures (such as those including coal ash and silica fume or slag and silica fume) not only improve later-age strength of concrete and reduce its permeability but can also achieve the same results at early ages too. These ternary blends also provide the advantage of reduced heat development to control concrete temperature, a critical need in construction of mass-concrete structures.

With the reduction in the availability of coal ash in many regions, NPs such as calcined clays (including metakaolin), calcined shale and slate, and volcanic pumice have been increasingly used to substitute for coal ash in concrete in the regions where they exist. NPs typically require processing to activate their amorphous form to enable their pozzolanic reaction capabilities. The processing required may include drying, calcination, and grinding to meet the requirements of ASTM C618-23/AASHTO M 295.

In response to the shortages of fly ash, many state DOTs have supported research to advance the use in concrete of harvested and beneficiated coal ash and bottom ash from landfills. The beneficiation process typically includes drying, removing contaminants, decarbonation by high-temperature heating, and finely grinding. It has also been suggested that an evaluation of the landfill or pond be performed prior to qualifying the ash for further processing and, also, that the beneficiated ash should meet the requirements of ASTM C618-23/AASHTO M 295.

Because of ASCMs’ potential performance and sustainability benefits as well as the decline in the availability of fly ash, DOTs have become increasingly interested in their use. ASCMs include a range of by-product and waste materials that can exhibit desirable pozzolanic or latent hydraulic reactions after processing and, sometimes, beneficiation. Most existing and emerging ASCMs contain amorphous silica, although others exhibit different mechanisms to support their reactivity. ASCMs of interest include waste materials such as recycled GGP, fines from recycling of concrete and ceramics, slag from non-ferrous metallurgy processes, ash from the combustion of biomass or municipal solid waste, and wastes from operations associated with mining, quarrying, and dredging. These materials each have unique and varying physical and chemical characteristics that must be evaluated prior to use. The reactivity and uniformity of these ASCMs is a key consideration, and contaminants or substances that could be deleterious to cement hydration or concrete properties must also be identified and filtered. Processing and beneficiation are often necessary to improve the reactivity of ASCM materials to support their use in concrete applications. Prior to ASCM approval and use, agencies would assess the characteristics and variability of the ASCM, evaluate the concrete produced with the ASCM, evaluate the performance of the ASCM or system containing the ASCM, assess the availability the ASCM, assess the uniformity of the ASCM’s production and the consistency of its properties, and ensure implementation of appropriate measures in concrete mixture design and construction practices to mitigate potential negative impacts from the ASCM’s use.

Survey Results

A survey of 25 questions was sent electronically to all 50 state DOTs and those of Puerto Rico and the District of Columbia. Forty-three DOTs responded to the survey, although not all answered all 25 questions. The survey responses revealed information about the DOTs’ practices related to the use of SCMs in concrete. All 43 responding DOTs allow the use of SCMs. The majority (41 responses) allow the use of the conventional SCMs, including Classes C and F fly ash, slag cement, and silica fume. In addition, 16 DOTs allow the use of NPs such as metakaolin, calcined clay, shale, and pumice. Three DOTs also allow the use of rice husk ash. Many of the 41 responding states use 20% Class F fly ash, 50% slag, and less than 10% silica fume in their concrete pavement projects, while 15 states do not allow silica fume in pavement mixtures. For bridge structures, the percentage of fly ash allowed by some states to support enhanced durability and reduced heat generation in mass concrete is 50% to 70%. Also, to reduce concrete permeability in bridge structures, higher amounts of slag cement and silica fume are often used as a replacement for Portland cement.

With respect to NPs, of the 41 state DOT responses, 18 allow the use of one or more NPs and reported on the percentage rate allowed in concrete mixtures. However, not many state DOTs

have developed specifications unique to NPs and instead use the current version of ASTM C618-23 or AASHTO M 295.

With regard to ternary mixtures in pavements and structures, 29 of the 33 responding DOTs allow the use ternary concrete mixtures, including three states that use them exclusively for bridges and mass structures. The responding DOTs reported allowing varying combinations of SCMs in ternary mixtures: fly ash and slag cement, fly ash and silica fume, slag cement and silica fume, and NPs and slag cement. Also, seven DOTs out of 39 that responded use quaternary concrete mixtures for both bridges and pavements, while two use these mixtures exclusively for bridges and other structures.

On the question of strength acceptance age for SCMs, 21 DOTs out of 33 that responded indicated that their acceptance strength is at 28 days for concrete mixtures with or without SCMs, while 12 DOTs accept strength at later ages such as 56 days. With regard to required tests, while many DOTs do not require specific tests for concrete with SCMs, they use similar tests to those used for non-SCM concrete. With regard to specific durability-related tests on concrete with SCMs, 16 DOTs use surface resistivity tests, and 12 use direct chloride permeability tests. Thirteen DOTs also test for shrinkage, and 10 DOTs perform tests for air voids.

Thirty-three states responded to the question on effects of SCMs on the performance of concrete mixtures with Type IL cement. Sixteen DOTs reported no effects, eight mentioned minor impacts that were addressed by producers and contractors, two indicated that there were positive effects, and the remaining seven had no opinion. Further comments were made in some responses on SCM impact on workability, air content, set time, finishing, shrinkage, and early strength gain.

On the availability issue of fly ash, 28 of the 33 DOTs that responded indicated that they have been, are currently, or expect to be experiencing shortages of fly ash. The remaining five indicated that they have had no problems with fly ash shortages. To address the shortages of fly ash, 13 DOTs resort to industry solutions; 11 modify specifications to allow the of other available SCMs, NPs, and ASCMs; seven import fly ash; three adjust concrete mixture designs; and three modify specifications the to limit the use fly ash for only durable concrete. Nine DOTs indicated other actions.

The survey results also indicated that of 33 DOTs that responded, six allow the use of ASCMs, while seven plan to allow their use in the future. GGP, as an example, is being specified by the DOTs of Vermont, New York, and Florida. The DOTs of Utah, Washington, and Wisconsin are experimenting with other ASCMs. Also, two DOTs are conducting field trials on ASCMs, and seven DOTs are planning field trial on pavements and bridges in the future.

Case Examples

Twenty-four DOT respondents agreed to be interviewed for case examples, and five were selected: Caltrans, CDOT, LaDOTD, MnDOT, and UDOT.

Findings from Each Case Example

• Caltrans has developed an approach that allows SCMs and ASCMs to be approved using its PEP. Data provided to support the PEP have been used by Caltrans to develop equations that use characteristics of the SCMs to compute allowable combinations of them and their replacement rates. These equations have been incorporated into Caltrans’s specifications. Caltrans has found that the use of these equations has been effective in supporting construction of concrete infrastructure with the desired performance qualities, and they also have allowed stakeholders the flexibility to use SCMs and ASCMs to safeguard against issues supply shortages.

• CDOT specifications allow SCMs to be used in concrete as additives and in blended cements. SCMs and ternary and quaternary blends of SCMs are allowed at replacement rates of up to 50% by mass of cement in pavements and structures. Allowable SCMs include Classes F and C fly ash, harvested or beneficiated coal ash meeting ASTM C618-23/AASHTO M 295, slag cement, silica fume, calcined clay, calcined shale, calcined pumice, metakaolin, and rice husk ash. The use of SCM mixtures has increased due to performance-based specification provisions that allow flexibility in material selection and mixture proportioning. CDOT is finding that its paving contractors view the use of SCMs an economic benefit and a driver of improved concrete quality.
• LaDOTD research has shown that high replacement rates of Portland cement with SCMs, particularly those in ternary blends, can provide economic benefits as well as increased durability and environmental benefits. Surface resistivity tests are used to assess concrete mixtures for the desired durability benefits. The DOT is seeing advantages to allowing the contractor or producer to design concrete mixtures using available materials. LaDOTD allows producers to supply ternary mixtures. This use of ternary blends of fly ash and slag is cheaper than cement and has resulted in very workable mixtures. This effort has led to increased SCM use.
• MnDOT’s specifications allow relatively high-volume replacements of conventional SCMs, which along with a low w/cm, have produced mixtures that provide the desired durability. MnDOT specifications ensure quality through low-permeability mixtures driven primarily from control of the w/cm ratio. There is no strength requirement for acceptance of paving mixtures. This approach helps accommodate the slower strength gain of many SCM mixtures while still achieving the benefits associated with lower concrete permeability. For mixture design approval, 28-day strength test results are still required in concrete mixtures with and without SCMs, MnDOT has occasionally allowed HPC bridge deck mixtures up to 56 days to meet strength requirements.
• UDOT is supporting increased use of SCMs in concrete infrastructure in order to lower the environmental impact of its infrastructure through reduction of cement content in its concrete mixtures. The Utah market is in proximity of several NP producers, and UDOT has several NPs that are used by concrete suppliers and contractors, including a natural volcanic ash, pumice, and expanded shale. Precast concrete companies tended to be the most affected by fly ash supply issues and, therefore, became the earliest users of NPs and blended cements containing NPs. The extensive mining industry present in Utah may be source of ASCMs in the future.

From these case examples, it is evident that state DOTs are finding ways to successfully capture the benefits of conventional SCMs and to develop processes for approval, specification, and use of NPs and emerging ASCMs.

Gaps in Information on SCMs and Research Needs

The benefits of designing concrete mixtures with conventional SCMs as a replacement for a portion of the cement content are well established. Additional information on the performance benefits of using ternary and quaternary concrete mixtures exists and is still emerging. The literature review indicated that research studies and implemented projects show that NPs show a strong potential to provide similar benefits in concrete. However, there seems to be notably less information on laboratory evaluation and field performance of concrete produced using individual NPs, since their properties may vary based on the material type and origin and their presence in different regions. The reluctance of some state DOTs to use harvested and beneficiated coal and bottom ash may stem from concerns about variability in the material. More information on product evaluation methodologies and reliable test methods would help expand the use of these ash materials in concrete. The information currently available on approval and potential

performance of ASCMs is not sufficient to prompt some state DOTs to allow their use. Research studies addressing the needs that follow will assist in closing these gaps:

• Laboratory and field trials with ternary blends, quaternary blends, NPs, harvested coal ash, and ASCMs are needed in order to evaluate any changes in performance of fresh and hardened concrete and to provide confidence in the use of these mixtures.
• Approaches are needed to evaluate and predict the reactivity (and any negative impacts on hydration, performance, and durability) of SCMs, NPs, harvested ash, and ASCMs given their composition and characteristics.
• An improved understanding is needed of the economic feasibility, variability, and environmental impacts and benefits of SCMs and ASCMs, along with companion tools to assist agencies in developing specifications to expand their use.
• Practical guidance is needed on approving existing and emerging ASCMs, including for their use in low-carbon concrete.
• The status of fly ash availability should be monitored, as should how the shortage in fly ash is changing the specifications of state DOTs and having an impact on developing guidelines, based on new and evolving ASTM tests and specifications, for approval of harvested ash, NPs, and ASCMs.