Executive Summary
Sustainable concrete design is often evaluated using embodied-carbon metrics such as Global Warming Potential (GWP) and Environmental Product Declarations (EPDs). These tools improve transparency of environmental impacts related to material production, but they do not predict how a concrete element will perform over time. They do not predict whether a concrete element will provide the durability and serviceability required to extend design life and minimize maintenance costs. For engineers tasked with designing more sustainable concrete structures, those long-term outcomes are vital.
A concrete element that cracks early or requires repeated maintenance or more extensive remediation can lose much of the environmental advantage suggested by its initial GWP or EPD values. Negative volume change, drying shrinkage cracking, moisture intrusion, deterioration, required repair materials, operational disruption and downtime all add cost and environmental burden over the life of a structure. These impacts are not captured by point-in-time GWP and EPD values.
Sustainable design should be evaluated more broadly to encompass durability-based design. In addition to GWP and EPD data, dimensional stability, cracking potential, service life, constructability, and lifecycle cost must be considered.
This article broadens the sustainability discussion beyond carbon metrics and provides insight on how Type K shrinkage-compensating cement concrete helps optimize sustainable designs that meet low GWP and EPD targets AND improve long-term performance, extend service life, reduce maintenance and prevent early deterioration.
1. Sustainability Metrics Must Be Evaluated Relative to Design Life
The engineering community is under increasing pressure to reduce embodied carbon in concrete construction. Owners, agencies, green-building frameworks, and procurement teams increasingly rely on GWP data and product-specific EPDs to compare materials during design and specification. These tools are useful and necessary, but they can skew decision-making if they are treated as the primary measures of sustainable design.
True sustainable design is inseparable from in-service performance. A structure that performs as intended for decades with limited maintenance during use, conserves raw materials, reduces replacement frequency, and prevents the environmental impacts associated with remediation delivers optimized sustainable design. Otherwise, the cumulative effects of repair, maintenance, remediation or early asset replacement often offset or exceed initial embodied-carbon savings.
To achieve global sustainability initiatives, metrics must be used in conjunction with durability and in-service performance requirements and support design life expectations. Material selections, their GWP and EPD metrics, long-term performance criteria (beyond 28-day lab testing), and their influence on structural design, in-service performance, and service life impacts must be evaluated holistically.
For structural engineers, this performance-based approach is undertaken during preliminary design to evaluate options and impacts on initial cost, schedule and carbon intensity. Savings can also be identified that can be realized in design, construction, and in-service. Evaluations at this early stage also provide adequate time for longer-term testing requirements to be established that more accurately reflect durability and long-term performance. Though the industry has historically accepted 28-day lab test results, sustainable design necessitates qualifying performance criteria well beyond that short snapshot of time.
2. Negative Volume Change Affects Long-Term Performance
An inherent limitation of portland cement concrete is the inevitable drying shrinkage related to negative volume change. This volume loss is a result of excess mix water that is not consumed during the hydration process. This excess mix water is necessary for transport, placing and finishing, but it results in drying shrinkage cracking in the long term.
Negative volume change is evidenced in early drying shrinkage cracking that can occur almost immediately and is often most notable within the first 12-18 months. Throughout its service life, it is evident in curling, warping, panel edge cracking, and dominant joints, and influences other structural behavior characteristics like creep. All of which result in remediation and repairs over time.
To overcome the challenges of negative volume change and the inevitable cracking to improve long-term performance, industry guidelines and engineering practices have accepted overdesign as a safety factor. Though it has been a reality for decades, this approach unnecessarily constrains and can negate optimized sustainable designs and drive costs.
Shrinkage and curling stresses related to negative volume change are key factors that influence concrete load capacity, design thickness, and reinforcement requirements. Increased slab thickness and increased reinforcement influence material consumption and cost and ultimately increase GWP impacts.
New engineering design tools are now available to more effectively address material influences and their performance. These tools take durability performance into account, incorporate longer-term drying shrinkage and curling stress data, and the influence of reinforcement types and quantities. This more comprehensive approach to design helps prevent unnecessary overdesign and optimize sustainable design.
These advanced finite element analysis programs provide an efficient method of evaluating various designs and allow engineers to optimize for both durability and sustainability. Design reviews and modifications that use to require days can now be completed in a matter of hours.
Total volume of concrete and amount of reinforcement required can now be qualified and quantified using technology that effectively integrates sound engineering methodologies and more effective material performance metrics. Ultimately, allowing engineers and owners to evaluate more sustainable design options that align with optimized GWP metrics and performance expectations. Owners can be more well-informed on the potential concrete design options, performance expectations, project timeline impacts, overall cost value and return on investment (ROI), and sustainable design impacts.
3. Materials Change the Sustainability Equation
Materials selected for use in concrete mix designs influence both short-term and long-term performance. They influence permeability, abrasion resistance, cracking potential, freeze-thaw performance, curling and warping, and overall structural behavior. The practical implications are readily seen in slabs-on-grade that commonly exhibit extensive cracking, joint failures, spalling and delamination. They are notably significant in bridge decks, post-tensioned structures, water and wastewater infrastructure, and marine environments, and in freeze-thaw regions and cold storage facilities where durability demands are high and access to perform repairs is costly.
The sustainability consequences of materials that meet 28-day testing requirements but fall short on long-term performance are cumulative. Each repair or remediation requires new material demands and creates additional carbon impacts for repair and remediation, in addition to costly downtime and inconveniences that further increase the real environmental and social costs.
Understanding the materials, their influence on durability and performance, impacts on construction efficiencies, and constraints or opportunities in design is paramount to sustainable engineering practices that must harmonize low carbon impact, extended asset life, project budgets, and construction timelines.
EPDs and GWP values are still essential, but they are point-in-time indicators that do not reflect in-service performance. They only depict carbon impacts associated with producing a product or concrete mixture, not the longer-term lifecycle impacts. The lowest initial GWP material is not an indicator of its durability and ultimate sustainable design performance. A concrete design that optimizes lower GWP targets while maximizing durability will more often produce a lower total carbon burden over the service life of the asset
4. Type K Shrinkage-Compensating Concrete as a Durable and Sustainable Design Strategy
Type K shrinkage-compensating concrete (ShCC) provides a material solution that improves durability and overall structural behavior and lowers the GWP of concrete designs. The dimensional stability it provides broadens the scope of design options, reduces the amount of reinforcement required, and increases overall concrete load capacity. This increased capacity helps reduce total concrete volume requirements or provides greater versatility of in-service use with variable load influences.
The Technology
Type K cement (American Society of Testing and Materials’ ASTM C845 – Standard Specification for Expansive Hydraulic Cement) is a hydraulic blended cement that combines an expansive calcium sulfoaluminate (CSA) cement-based additive with a portland cement source (ASTM C150, C595, and C1157). It is proportioned to achieve sufficient expansion to offset the shrinkage characteristics of regional materials used in concrete and grout mixes and achieve net zero shrinkage. The advanced hydration mechanism of the expansive CSA cement-based additive (marketed as Komponent®) drives the performance of Type K cement-based mixes.
During hydration, primary ettringite is formed that contributes to design strength, controlled set, and early expansion. The qualified dosage of the expansive cement additive needed to create adequate expansion is determined by ASTM standards designed for use with expansive cements (i.e., ASTM C806 – Standard Test Method for Restrained Expansion of Expansive Cement Mortar and ASTM C878 – Standard Test Method for Restrained Expansion of Shrinkage-Compensating Concrete). The goal is to create sufficient designed expansion to compensate for the shrinkage characteristics of the mix and ensure the concrete is kept in compression for the life of the placement.
By efficiently consuming excess mix water that is not used by portland cement during hydration, Type K shrinkage-compensating cement mixes result in dense, lower permeability concrete with substantially improved abrasion resistance without the use of densifiers or surface hardeners. The consumption of excess mix water prevents voids and capillaries that allow room for drying shrinkage and results in a more dimensionally stable concrete placement. By eliminating curling and shrinkage stresses, the load capacity of the placement is increased, allowing thinner sections to be placed and reducing the overall volume of concrete required. In addition, its 0% tricalcium aluminate (C3A) content means sulfate resistance is improved proportionately to the percentage replacement used with all portland cements, making it ideal for containment, marine, and other environmentally exposed structures.
From its earliest use in prestressed pipes and pavements to its use in shrinkage-compensated designs for post-tensioned structures, containment structures, dams, spillways, mat slab foundations, bridge decks, pavements, slabs-on-ground and more, Type K ShCC has proven to be a reliable solution for all types of critical structures. Its performance helps simplify designs and influence more efficient constructability while minimizing in-service maintenance costs and operational downtime for repairs.
Long-term durability and reductions in overall materials consumption minimize the total carbon impact of concrete designs from cradle-to-grave.
5. Implications for Structural Engineers and Specification Practice
As engineers and other industry professionals continue efforts to balance embodied-carbon reduction targets with long-term performance, materials, designs, and lifecycle impacts are paramount. The industry transition to new blended cements intended to reduce the carbon impact of concrete construction necessitates a fundamental shift away from prescriptive specifications to performance-based specifications and optimized designs.
Acceptance of testing requirements are moving beyond 28-day laboratory results to reasonable, but more representative timeframes for performance. For instance, 56 days for compressive strength acceptance and 56- or 90-day shrinkage testing results. Critical structures may require testing up to 365 days for qualified test results. Shrinkage specifications to improve dimensional stability are tighter (e.g., < -0.02% at 56-days or maximum 0.0% at 28-days).
With projects now more heavily influenced by supply chain dynamics, construction and maintenance budgets, and compressed project schedules, suppliers are harmonizing efforts with collaborative testing to optimize designs, improve construction efficiencies, and deliver on value and sustainability.
Performance specifications listing qualified materials now include supplier contact information to source the materials efficiently and prevent last-minute alternates that compromise design intent due to availability or inadequate planning. “Or equal” submittals are now requiring qualifying data to include EPD, GWP and longer-term performance test data. Earlier submittal dates are required to allow time for more effective evaluation and alignment with design, performance and sustainability requirements.
The lower GWP, design advantages, durability, and dimensional stability Type K ShCC affords is inspiring more side-by-side design comparisons. This equips engineers with insight and information to propose optimized Type K ShCC designs that deliver on performance, project schedules, project budgets, and environmental responsibility.
Performance specifications with more stringent and extensive testing requirements are becoming the new standard. Optimizing designs to balance GWP metrics with lifecycle impacts and structural reliability is the new charge. A race to the bottom for low GWP metrics will not deliver intended results.
Conclusion
The industry’s focus on GWP and EPDs has improved environmental accountability in concrete construction, but those tools should not be used as complete measures of sustainability. The use of concrete materials and optimized designs that deliver performance, lower GWP, and maximize investment dollars with lowest life-cycle cost is required to achieve truly sustainable designs.
Type K ShCC provides engineers and designers with a proven solution that delivers on all counts.
Its use not only lowers the overall carbon impact of any concrete design, but also significantly reduces overall carbon impact throughout its service life. By increasing capacity, reducing materials and labor requirements during construction, improving durability, and minimizing repair and maintenance, owners, designers and patrons benefit from an extended asset life two to three times that of a similar portland cement concrete structure.
With ever increasing demands to build more durable critical infrastructure efficiently, with more stringent sustainably targets and added value in-service, Type K ShCC will continue to deliver. Embracing innovative approaches to design, integration of new products, and the collaborative efforts of professionals throughout the industry in their use will make a difference in the race to carbon neutrality without sacrificing performance.
