New Standards for UHPC Concrete Strength and Performance

New Standards for UHPC Concrete Strength and Performance

The American Concrete Institute (ACI) is the world’s preeminent authority on concrete research and industry standards. Based in Michigan and founded in 1904, ACI develops technical standards and certification criteria for the production, installation, and maintenance of concrete. Building and design codes at the local, state, federal, and international levels increasingly rely on ACI publications to establish specifications for concrete construction.

ACI recently hosted its first in-person conference since COVID-19 struck the world. In late March 2022, academic researchers, construction professionals, and policy experts gathered in Orlando, Florida to discuss the latest industry trends and preview new standards for the years ahead. Participants included the Federal Highway Administration (FHWA) and Federal Department of Transportation (DOT), among the key agencies implementing the nation’s construction standards.

ACI Seeks Standardization of the Compressive Strength of Ultra-High Performance Concrete (UHPC)

UHPC was a major topic at the convention, highlighting its continued momentum in becoming a more widely used replacement for traditional concrete. Three presentation sessions focused on UHPC. They touched on laboratory and field research, field-cast connections and precast applications, existing member repairs, and reinforcement case studies. These sessions discussed the current state of UHPC in relation to its real-world applications and establishing new standards that agencies and design engineers can rely on for future projects. 

UHPC’s strength properties were at the heart of the discussion. Minimum compressive strength, as measured by ASTM C39, has not yet been codified as part of more rigorous standardization. Prior ACI publications have commonly defined UHPC as concrete that has a minimum compressive strength of 22,000 psi (22ksi, or 150 MPa), whereas conventional concrete is often defined as having less than 8,000 psi (8ksi, or 55 MPa). 

When considering an industry standard, ACI uses theoretical findings and field test results to determine achievable baseline properties and to evaluate their efficacy. Standards should ultimately be set high to promote the safety and performance of structures built with UHPC. Current commercial and academic definitions for UHPC range from 17ksi to 25ksi minimum compressive strength. 

A minimum standard compressive strength of 21 to 22ksi for UHPC is emerging as the consensus for new publications, based on the research and case studies discussed at the conference. FWHA, DOT, and other stakeholders are noting and preparing for more standardized implementation.

PhD researchers and construction crews alike have worked with a variety of UHPC products on the market. ACI and other institutions are researching the most dependable ways to achieve long-lasting strength that can be implemented more broadly. 

Commercial Implications: Cor-Tuf UHPC Compressive Strength

Based on feedback at the conference and case studies from construction, we’ve observed that many UHPC producers would prefer that the community keep the bar low when it comes to strength requirements. They are resisting the minimum 21ksi to 22ksi standard. 

However, this approach is unfortunate and unnecessary, because we already have a commercial option that meets this standard at scale. Cor-Tuf UHPC strength is raising the bar. 

We want high standards because there’s no room for compromise in concrete safety and performance. Our UHPC’s compressive strength is approximately 25ksi when ambiently cured and can reach values in excess of 35ksi with advanced curing techniques.

Further, our products can be mixed, poured, and cured with traditional concrete techniques, achieving results that exceed the proposed compressive strength benchmark. 

This fact was not lost on the academic community, which is greatly interested in Cor-Tuf’s ready-mix success without a compromise in quality.

We expect new UHPC standards to emerge during the coming year based on the insights of regulatory bodies and researchers at ACI. A commercially viable product that meets theoretical expectations, backed by real-world testing and project success, offers the industry more confidence in standardization.

All testing results confirmed by independent laboratory CTL Group.

Beyond Compressive Strength: Tensile, Flexural, and Versatility Benefits

Compressive strength may be the “nameplate” measure of concrete performance, but UHPC achieves a much wider range of beneficial properties. Industry groups such as the Precast/Prestressed Concrete Institute (PCI) are adding to ACI’s knowledge bank. PCI research focusing on tensile and flexural strength, for example, proves that the fiber reinforcement of UHPC enhances performance and design versatility. 

The auxiliary benefits of UHPC will be codified to meet theoretical properties such as shear resistance, strain hardening, and ductility. ASTM C1609 may be the dominant test standard for tensile strength, with minimum stresses of 1.5ksi (10 MPa) to 2ksi (14 MPa) being realistic benchmarks. Flexural results from ASTM C78 may also approach 2ksi (14 MPa) in new standards. These baseline expectations far exceed those of conventional concrete, unlocking the potential for more unique, safe, and cost-efficient designs. 

Heat curing and other treatments have also been evaluated in literature and panel discussions. Decreasing cure times using heat is common, but it might not be needed in UHPC projects as frequently as traditional concrete installations because of the rapid realization of UHPC compressive strength with ambient heat. 

In a recent test, Cor-Tuf reached a compressive strength of 17,470 psi after curing for only 13 days, as compared to traditional concrete, which typically takes 28 days to reach a top compressive strength of about 5,000 psi. Applying unnecessary heat can introduce other points of weakness. Using Cor-Tuf UHPC avoids this completely. 

Exploration is ongoing into the benefits of tensile and flexural strength, along with water resistance, relative to various curing regimes, but the results are promising. A wide range of precast, field-cast, time-variable, and weather-variable applications are proving that UHPC performs well regardless of construction demands.

The result of such impressive performance presents new implementation opportunities, some of which may result in new standards of their own. For example, precast members with strategic voids are being designed, including standardized utility pipe entrances and other ways to simplify building construction. “Steel-less” designs with higher member strength have an exciting amount of potential applications. 

Such new types of concrete members made possible by the flexural properties of UHPC may become more widespread in bridge and building construction. Engineers may want to use lean UHPC where only steel beams or steel-reinforced concrete would have been allowable in legacy design codes.

Parity With Conventional Concrete, With Better Results

The real opportunity in UHPC is the ability to install high-performing structures using less material with more flexibility. Academic results indicate that structural design members can be optimized to shapes that have nearly 50% of the weight of conventional members while eliminating the need for steel reinforcement. UHPC can be less material-intensive, and, by extension, more cost-effective. Design guidelines from ACI, PCI, and regulatory agencies are honing in on these opportunities to set a new bar for performance.

A common concern in the community remains the installation technique. How does UHPC reach parity with traditional concrete in terms of time and labor? Fortunately, this question has real-world answers to reassure planners that UHPC can realize its ultimate potential. Our own experience with ready-mix UHPC demonstrates that theoretical standards can be met in the field, using traditional production and installation methods.

Another common concern in concrete industry circles involves the need for highly specialized components in commercial UHPC products. Many vendors only offer proprietary mixes that require that the vendor’s sand and cement be used. This significantly increases material and shipping costs. We believe that local sourcing and fair competition for materials should be more widely available. 

Cor-Tuf takes pride in providing UHPC products that are compatible with local sand and cements, and involve simple mixing procedures that can be applied anywhere in the world. The ability to mix UHPC in classic batch plants increases confidence across the sector, addressing concerns regarding supply chains and widespread viability.

Environmental impact is yet another hot topic at today’s technical conventions. UHPC reduces the overall material requirements compared to classic concrete, including lower demands for steel reinforcement. The result is a product with lower material requirements and logistical demands (i.e., emissions), longer service life, and a viable way for standards-makers to reduce the carbon footprint of the construction industry. Couple these benefits with Cor-Tuf UHPC’s compatibility with local materials, and the circular economy gets a bit closer to reality. 

The future for UHPC is bright within both academia and the broader concrete industry. As new design codes emerge, more engineers, planners, and policy makers will gain confidence in this material as the next phase in widespread concrete construction. Cor-Tuf takes pride in leading the industry in theoretical results, practical installation success, and cost-effective product offerings. 

Don’t settle for materials that make excuses for falling short of requirements. Cor-Tuf UHPC meets the next generation of concrete standards already, without the need for a new construction approach. Your sand, your cement, our UHPC.

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