Ultra-high performance concrete (UHPC) is a novel cement-based material with exceptional mechanical and durability properties. Silica fume, the primary supplementary cementitious material (SCM) in UHPC, is expensive in North America, so it is often substituted with inexpensive class F fly ash. However, future availability of fly ash is uncertain as the energy industry moves toward renewable energy, which creates an urgent need to find cost-effective and environmentally friendly alternatives to fly ash. This study investigated replacing cement, fly ash, and silica fume in UHPC mixtures with ground granulated blast-furnace slag (GGBFS), metakaolin, and a natural pozzolan (pumicite). To identify acceptable UHPC mixtures (28-day compressive strength greater than 120 MPa), workability, compression, and flexural tests were conducted on all mixtures. Then, durability properties including shrinkage, frost resistance, and chloride ion permeability (rapid chloride permeability and surface resistivity tests) were evaluated for the acceptable UHPC mixtures. Results showed that 75, 100, and 40% of fly ash in the control mixture could be replaced with pumicite, metakaolin, and GGBFS, respectively, while still producing acceptable strengths. Flexural strengths were greater than 14.20 MPa for all mixtures. For durability, UHPC mixtures had shrinkage strains no greater than 406 μstrain, durability factors of at least 105, and “very low” susceptibility to chloride ion penetration, indicating that these SCMs are suitable candidates to completely replace fly ash and partially replace silica fume in non-proprietary UHPC.

For several decades, class F fly ash has been an attractive supplementary cementitious material, at least in part, due to its ability to reduce Portland cement consumption and mitigate alkali-silica reactions in concrete. However, fly ash availability is becoming uncertain as the energy industry decommissions coal burning power plants as it transitions to renewable energy production. This situation creates a need to identify viable and sustainable alternative supplementary cementitious materials. There are several types of supplementary cementitious materials, such as natural pozzolans, metakaolin, or ground granulated blast-furnace slag, which appear to be potential alternatives to fly ash in concrete. In this research, a locally available natural pozzolan (pumicite) was selected to replace fly ash in concrete. After conducting alkali-silica reaction tests on mortar mixtures, rheological and strength properties, shrinkage, resistance to freezing and thawing, and chloride ion permeability of concrete mixtures containing different amounts of fly ash and natural pozzolan were evaluated. The results showed that pumicite was more effective than fly ash at mitigating the alkali-silica reaction, and a pumicite content of 20% was necessary to mitigate the alkali-silica reaction. Ternary mixtures containing both pumicite and fly ash were the most effective cementitious materials combinations for mitigating the alkali-silica reaction expansion. Additionally, pumicite provided acceptable compressive strength and modulus of rupture values (greater than 4.0 MPa) that exceeded the flexural strengths provided by established mixtures containing only fly ash. Shrinkage and durability factor values for all mixtures were less than 710 μstrain and greater than 75, which are generally considered acceptable. Additionally, all mixtures with acceptable alkali-silica reaction expansions had very low chloride permeability. These results indicate that pumicite can be a reliable alternative for fly ash.

Extensive research has been conducted on the sulfate attack of concrete structures; however, the need to adopt the use of more sustainable materials is driving a need for a quicker test method to assess sulfate resistance. This work presents accelerated methods that can reduce the time required for assessing the sulfate resistance of mixtures by 70%. Class F fly ash has historically been used in concrete mixtures to improve sulfate resistance. However, environmental considerations and the evolving energy industry have decreased its availability, requiring the identification of economically viable and environmentally friendly alternatives to fly ash. Another challenge in addressing sulfate attack durability issues in concrete is that the standard sulfate attack test (ASTM C1012) is time-consuming and designed for only standard mortars (not concrete mixtures). To expedite the testing process, accelerated testing methods for both mortar and concrete mixtures were adopted from previous work to further the development of the accelerated tests and to assess the feasibility of testing the sulfate resistance of mortar and concrete mixtures rapidly. This study also established criteria for interpreting sulfate resistance for each of the test methods used in this work. A total of 14 mortar mixtures and four concrete mixtures using two types of Portland cement (Type I and Type I/II) and various supplementary cementitious materials (SCMs) were evaluated in this study. The accelerated testing methods significantly reduced the evaluation time from 12 months to 21 days for mortar mixtures and from 6 months to 56 days for concrete mixtures. The proposed interpretation method for mortar accelerated test results showed acceptable consistency with the ACI 318-19 interpretations for ASTM C1012 results. The interpretation methods proposed for the two concrete sulfate attack tests demonstrated excellent consistency with the ASTM C1012 results from mortar mixtures with the same cementitious materials combinations. Metakaolin was shown to improve sulfate resistance for both mortar and concrete mixtures, while silica fume and natural pozzolan had a limited impact. Using 15% metakaolin in mortar or concrete mixtures with Type I/II cement provided the best sulfate resistance.

The work presented in this paper includes the construction methods and lessons learned from the placement of a non-proprietary ultra-high performance concrete (UHPC) overlay through the rehabilitation of a concrete bridge deck located in Socorro, New Mexico, USA. The selected bridge is a multi-cell, box girder bridge with four spans and a total length of 91.4 m and a width of 16.5 m with two traffic lanes. Rehabilitation of the bridge involved removing the top surface of the existing deck (deteriorated concrete), installing a high-performance deck (HPD) leveling course, and placing a 25 mm UHPC overlay. Sensors were installed in the bridge superstructure (multi-cell box girders, HPD, and overlay) for long-term monitoring. Overlay assessment included physical testing to evaluate the condition of the overlay–substrate bond by chain dragging and direct tension pull-off testing. Conclusions and lessons learned from this investigation serve as a fundamental list of best practices and recommendations for field construction of a non-proprietary UHPC overlay. Recommendations for preparatory tasks including material selection, substrate surface preparation, placement preparation, handling of materials, and UHPC mixing are provided. The recommendations also list best practices concerning the placement of the overlay, curing procedures, and quality assurance testing. Lastly, suggestions are presented for contracts pertaining to UHPC overlay projects.

Sulfate reaction is a damaging expansive process that deteriorates cement-based structures over time. Various methods have beenproposed to mitigate the effects of sulfate attack in concrete. Using Type II or Type V portland cement is an appealing approach tocontrol sulfate attack. Additionally, incorporating supplementary cementitious materials (SCMs) such as class F fly ash, a byproductof coal combustion for electricity generation, has proven effective at mitigating sulfate attack damage. However, futureavailability of fly ash is uncertain due to the energy industry transitioning towards more sustainable methods of energy productionrather than relying on coal combustion. Consequently, there is an incentive to seek alternative SCMs that can effectively mitigatesulfate attack while being environmentally sustainable and economically feasible. In this study, ASTM C1012, a globallyrecognized standard test method for sulfate evaluation, was employed to assess sulfate resistance of mortar specimens. In total, 14mortar mixtures containing various types and concentrations of alternative SCMs, including silica fume, metakaolin, and pumicitealong with two types of portland cement (Type I and Type I/II) were produced. The results indicated that Type I/II portland cementhad greater sulfate resistance compared to Type I cement in mortar mixtures, regardless of the type and concentration of SCMsused. Additionally, although metakaolin considerably improved sulfate resistance, silica fume and pumicite used in this study hadonly limited impact on sulfate resistance of the specimens. When evaluating ternary mixtures, using a combination of 22.5%metakaolin and 7.5% fly ash to replace 30% of Type I portland cement resulted in the greatest sulfate resistance among all mortarmixtures, with 0.054% expansion after nine months of testing. It is worth mentioning that when using Type I/II portland cementand only 15% metakaolin (as a cement replacement), sulfate resistance was comparable to the ternary mixture with 22.5%metakaolin, 7.5% fly ash, and with Type I portland cement. Overall, the results showed that metakaolin, fly ash, and pumicite canbe considered effective SCMs for improving sulfate resistance.

Sulfate reaction is a damaging expansive process that deteriorates cement-based structures over time. Various methods have been proposed to mitigate the effects of sulfate attack in concrete. Using Type II or Type V portland cement is an appealing approach to control sulfate attack. Additionally, incorporating supplementary cementitious materials (SCMs) such as class F fly ash, a by-product of coal combustion for electricity generation, has proven effective at mitigating sulfate attack damage. However, future availability of fly ash is uncertain due to the energy industry transitioning towards more sustainable methods of energy production rather than relying on coal combustion. Consequently, there is an incentive to seek alternative SCMs that can effectively mitigate sulfate attack while being environmentally sustainable and economically feasible. In this study, ASTM C1012, a globally recognized standard test method for sulfate evaluation, was employed to assess sulfate resistance of mortar specimens. In total, 14 mortar mixtures containing various types and concentrations of alternative SCMs, including silica fume, metakaolin, and pumicite along with two types of portland cement (Type I and Type I/II) were produced. The results indicated that Type I/II portland cement had greater sulfate resistance compared to Type I cement in mortar mixtures, regardless of the type and concentration of SCMs used. Additionally, although metakaolin considerably improved sulfate resistance, silica fume and pumicite used in this study had only limited impact on sulfate resistance of the specimens. When evaluating ternary mixtures, using a combination of 22.5% metakaolin and 7.5% fly ash to replace 30% of Type I portland cement resulted in the greatest sulfate resistance among all mortar mixtures, with 0.054% expansion after nine months of testing. It is worth mentioning that when using Type I/II portland cement and only 15% metakaolin (as a cement replacement), sulfate resistance was comparable to the ternary mixture with 22.5% metakaolin, 7.5% fly ash, and with Type I portland cement. Overall, the results showed that metakaolin, fly ash, and pumicite can be considered effective SCMs for improving sulfate resistance.