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.

Glycerol and xanthan gum were employed to enhance the viscosity of the cementation solution in soil treated with enzyme-induced carbonate precipitation (EICP). The effect of increasing the viscosity of the solution was investigated because it may facilitate EICP treatment by retaining cementation solution as precipitation occurs. Columns 51 mm in diameter and 102 mm in height of Ottawa 20/30 and F-85 sands were treated with EICP solution containing no thickener, 50% (v/v) glycerol, and 0.25% (w/v) xanthan gum. Some columns had open bottom to allow drainage and some were capped to retain the cementation solution. Xanthan-enhanced EICP solution showed the highest level of fluid retention. Results showed that EICP-treated sand can be cemented with the addition of a viscosity enhancer but that cementation quality may be affected. The level of retention did not have a strong relationship with uniformity of cementation and did not result in enhanced unconfined compressive strength.

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 ultra-high strength concrete (UHSC) used in this study had a compressive strength greater than 145 MPa and was produced using materials commonly available in southern New Mexico. Specifically, locally available Type I/II cement, local sand with a top size of 600 μm, silica fume, steel fibers (13 mm long), and high-range water reducing admixture were used to produce the UHSC mixture with a water-to-cementitious materials ratio of 0.20. UHSC specimens were heat cured in a water bath at 50°C, and then removed from the water bath and dry cured at 200°C for two days prior to strength testing. This paper presents a combination of early age and longer term shrinkage measurements of UHSC that were used to characterize the total shrinkage that occurred during the curing process. Total shrinkage of fiber reinforced UHSC specimens that occurred during 30 days was 3,006 µ, with early age shrinkage (first 24 hours) contributing 58.5% of the total shrinkage.

Ultrahigh-performance concrete (UHPC) develops very high compressive strengths and exhibits improved tensile strength and durability properties that make UHPC a promising material for bridge applications. Through case studies on typical prestressed concrete girder bridges (simple and continuous), the potential impact of implementing UHPC in New Mexico was investigated. Two existing bridges with high-performance concrete girders were redesigned using UHPC with a compressive strength of 155.1 MPa (22,500 psi) and a modulus of rupture of 8.0 MPa (1,160 psi). The redesign used a modified load factor design procedure for the Service III flexure limit state and a modified load and resistance factor design procedure for the ultimate shear limit state that considered the compressive strength, modulus of rupture, and modulus of elasticity of UHPC. Additionally, 15- and 18-mm-diameter (0.6- and 0.7-in.) prestressing strands were investigated. The use of UHPC and 18-mm-diameter (0.7-in.) prestressing strands reduced the required volume of girder concrete by up to 40%. Additionally, the contribution of the steel fibers in the UHPC significantly reduced the required shear reinforcement.