Improving Structural Behavior and Long-Term Performance of Critical Infrastructure with Type K Cement

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Improving overall structural behavior and long-term performance of critical concrete infrastructure is paramount to supporting today’s carbon neutral initiatives and ensuring good stewardship of both public and private owner investments. More stringent performance requirements combined with the challenging dynamics of change in type, quality, and availability of concreting materials is inspiring innovation, creativity, and collaboration. It is also renewing interest in materials with established, proven performance that offer solutions to today’s challenges. One of those experiencing an inspiring renaissance is Type K cement.

Developed in the late 1950s and brought to market in the early 1960s, Type K cement has proven to be a valuable innovation in cement technology for over 60 years. It is specifically engineered to eliminate negative volume change and improve durability by lowering permeability, increasing density, improving sulfate and abrasion resistance, and eliminating drying shrinkage cracking. It effectively addresses the fundamental shortcomings of Portland cements by improving the quality of the cement paste and inherently improving long-term performance and structural behavior of vital 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, and slabs-on-ground, Type K cement has proven to be a reliable solution for all types of critical structures. Its performance continues to aid designers in simplifying designs and influencing more efficient constructability while minimizing in-service costs and operational downtime for repairs.

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 overcome the shrinkage characteristics of concrete and grout mixes. The advanced hydration mechanism of the expansive CSA cement-based additive (marketed primarily as Komponent in the U.S. ) drives the performance of Type K cement-based mixes.

During hydration, primary ettringite (the mineral name for calcium sulfoaluminate) 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 (Figure 1).

By efficiently consuming the excess mix water that is not used by Portland cement during hydration and minimizing bleed water, Type K shrinkage-compensating cement mixes result in dense, low permeability concrete with substantially improved abrasion resistance without the use of admixtures 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.

When designing with shrinkage-compensating concrete, industry standards, guidelines, specifications, and other published resources are available to aid in design and constructability. The “Key Resources” sidebar provides a brief overview of commonly referenced standards, methods, codes and resources, from the American Concrete Institute (ACI) and ASTM, when designing with shrinkage-compensating concrete.

As engineers, owners, and project budgets are now more heavily influenced by supply chain dynamics, project and maintenance budgets, global warming potential (GWP) impacts, and commitments to sustainability, shrinkage-compensating concrete maximizes versatility. When designing with Type K shrinkage-compensating concrete, reinforcement options are no different than with other cement types required for concrete design—from traditional steel rebar and steel fibers to FRP, synthetic fibers and hybrids. Whichever is most suitable, available, and cost effective for the project can be used.

Type K cement was originally brought to market regionally as a finished cement. By the 1980s, effective ASTM standards and ACI guidelines had been developed to support consistent quality as a locally blended cement and made the use of the expansive cement additive economical and readily available nationwide. Type K cement is now prequalified per ASTM C878 and added at the local batch plant using normal bulk batching and mixing operations. This is a more practical and cost-effective way to deliver high-performance Type K shrinkage-compensating concrete (K-SCC) and grout (K-SCG), and it paved the way for use in a wide range of applications. Key critical structures like concrete containment, post-tensioned designs, bridges, and mass elements have been constructed optimizing design, durability, constructability, and sustainability.

For critical concrete containment structures, preventing contamination from external impurities and preventing seepage and leakage of contents into the environment is crucial. Since the early 1970s, K-SCC has been used successfully to create well designed, reliable concrete containment structures with extended joint wall panels, foundations and slabs that help prevent leakage, unscheduled downtime, and costly repairs.

The Bustamante Wastewater Treatment Plant in El Paso, TX, and the J.W. Rogers Water Treatment Plant demonstrate the design possibilities. With tanks including 110 feet (33.5 meters) diameter by 30 feet (9.1 meters) high digesters, 120 feet (36.6 meters) diameter by 20 feet (6 meters) high primary clarifiers, and 140 feet (42.7 meters) diameter by 16 feet (4.9 meters) high secondary clarifiers, all were constructed without joints by using K-SCC. More than ten years later they are still crack free and leak free.

The $255 million 69th Street Complex Wastewater and Sludge Treatment Plant in Houston, TX, used K-SCC for the construction of foundation slabs for the reactors, clarifiers, pump stations, thickener and digester complex, roof slabs, and reactor train beams. Key drivers for use on this project were eliminating drying shrinkage cracking and preventing leakage in critical areas, reducing shrinkage reinforcement, and placing large sections that minimized construction joints and waterstops. Other advantages included low permeability, higher abrasion and sulfate resistance, and its inherent cohesiveness that allowed for ease of placement of smooth, dense surfaces.

The reduction in steel reinforcement and waterstops combined with the reduction in formwork, mobilizations, man hours and pump days resulted in an overall savings to the project. As reported by the joint venture design team of Environmental Management Consultants (EMC) and the City of Houston, “The use of Type K concrete on the 69th Street Complex was certainly a valuable learning experience for both the consultants and the contractors and has demonstrated that the economies of shrinkage-compensating concrete construction and the resulting long-term, crack-free performance of those structures is worth the extra effort and care.”

The City of New York’s project for eight underground, four-million-gallon combined sewer overflow (CSO) tanks is another example of K-SCC delivering innovation, constructability and value. Hazen and Sawyer were selected to design these critical structures with a focus on minimizing the number of construction joints. With conventional concrete, ACI 350 – Environmental Engineering Concrete Structures recommends a maximum construction joint spacing of 30 feet to control shrinkage cracking. When using shrinkage-compensating concrete, ACI 223 – Shrinkage Compensating Concrete recommends spacing up to 150 feet (45.7m).
By using K-SCC a reduction in construction joints minimized points of water leakage and reduced total construction time. In a comparative analysis between the use of conventional concrete and shrinkage-compensating concrete, the net savings for the project was over two months of construction time saved on the slabs, four months on the walls, and approximately $900,000 in materials and labor. Additional value was realized in-service by minimizing maintenance costs and operational downtime.

In post-tensioned structures, the dimensional stability of K-SCC offers significant advantages. Post-tensioning provides control over flexural cracking while K-SCC eliminates other key challenges related to negative volume change, like drying shrinkage cracking and restraint-to-shortening (RTS). It also significantly reduces the effects of long-term creep and shrinkage helping to overcome the effects of differential displacement of supports and minimizing long-term relaxation of steel tendons. This improved dimensional stability eliminates labor intensive and time-consuming details like pour strips, slip joints, wrapped dowels and additional reinforcement for crack control.

Economic studies have shown that in properly designed structures, the savings in design element costs, traditional reinforcement and post-tensioned steel more than offset the moderate material cost impact of the cement additive. Of the three key influencers of prestress losses (i.e., prestressing steel relaxation, creep shortening, and shrinkage), shrinkage constitutes approximately 42% of the long-term losses and reduces the initial prestress force by ~7%. When shrinkage is eliminated, this reduction translates into a reduction in the quantity of prestressing steel required. Additionally, a K-SCC structure has less dimensional change resulting in substantial reduction in column moments and affects a reduction in total column steel.

Expansion joints can realistically be increased to approximately 500 feet (152.4 meters) without sacrificing performance.

The University of Alabama’s Ridgecrest Community Residence, an eight-story dormitory and parking structure, illustrates the advantages of using K-SCC in post-tensioned structures. Structural Design Group (SDG) of Birmingham, Alabama, used K-SCC to optimize design and improve structural behavior, meet budget requirements, and reduce time to completion. During onsite inspection two years after placement, critical areas where RTS cracking could be expected revealed the K-SCC placements to be crack-free. The structural designer stated: “The real proof is in the slab itself—there are virtually no cracks in more than 420,000 square feet (39,019 square meters ) of slab. Further, the concrete frame was bid and completed 42 days ahead of a very aggressive schedule.” Net savings by elimination of pour strips were estimated at $250,000 with a total realized savings of the redesign using K-SCC of ~$3 million.

The Dallas Municipal Center is another iconic structure that optimized design and performance of K-SCC. It exemplifies what can be achieved when innovation is combined with collaboration and communication. This structure is a monolith of 60,000 cubic yards (45,873 cubic meters) of buff-colored K-SCC integrated into a unique post-tensioned structure consisting of underground parking and office spaces. The use of K-SCC allowed structural engineers to address cracking concerns related to temperature and shrinkage and reduced the amount of reinforcement needed to address RTS. It also allowed the contractor to cast large individual sections of exposed concrete (up to 70 feet long by 14 feet high (21 meters long by 4 meters high)). This improved project efficiencies and time to completion. Final reports noted that the shrinkage-compensating concrete performed “very satisfactorily,” the contractor “did a commendable job in obtaining the designed architectural finish.”

The use of K-SCC in bridge decks has also been well documented through the years. The Ohio Turnpike Authority has been using it on their bridge decks since the 1980s with great success (Fig. 2). In a comparative study conducted from 1983 to 1990, results showed substantial improvement in bridge deck performance, resulting in zero instances of drying shrinkage cracking. When asked about maintenance cost impacts, Chief Engineer, Doug Hedrick noted “The answer is quite simple, it is very low cost to maintain the shrinkage-compensating concrete decks – no deck delamination’s, spalls, or steel corrosion. We don’t even think about cracks, crack maintenance or spalls on our shrinkage-compensating concrete decks.”

In cooperation with various states throughout the U.S., other studies of the use of K-SCC in bridge designs have been published demonstrating the performance advantages that can be achieved. The durability attributes that contribute to their extended service life include abrasion resistance, significantly reduced shrinkage cracking, reduced surface capillaries and porosity; superior compressive and splitting tensile strengths ease of placement, consolidation, and finishing, ultra-low permeability, and excellent freeze/thaw results with air entrainment.

Grouting is another common application of K-SCG. The Bob Kerrey Pedestrian Bridge in Omaha, Nebraska, is 3,000 linear feet. K-SCG was used for this cable-stayed superstructure to protect the post-tensioned (PT) tendons which are essential to the bridge’s long-term durability. Its low permeability, increased density and dimensional stability ensured the PT tendons were effectively protected from moisture, salts, air, and other elements that promote corrosion and deterioration. Adding the Type K expansive cement additive to a bulk shrinkage-compensating grout mix improved production efficiencies at the plant and during pumping operations on-site. It also achieved cost savings over the pre-blended bag material typically used.

Success stories continue as many transportation departments across the country embrace a proven solution to building safer, more sustainable infrastructure.

In reinforced pavement designs, eliminating joints in runways and taxiways is essential. Joints are the location of curling, spalling, and cracking that result in foreign object debris (FOD), creating safety hazards and costly maintenance and repair projects.

To identify the most durable pavement solution that would reduce the number of joints required on airport runways and taxiways and significantly reduce the costs associated with maintenance and repair, K-SCC was chosen for use on one of the most innovative concrete slabs ever constructed. In 1993, this post-tensioned, steel fiber reinforced pavement was placed at Rockford International Airport’s Runway Extension project in Rockford, Illinois.

In a side-by-side comparison of two K-SCC designs using steel fiber reinforcement the contractor was able to place two contiguous “Innovative Pavement Slabs” (IP1 and IP2, respectively) of taxiways paralleling a new runway extension. IP1 and IP2 were placed in 75-foot (23 meter) wide pavement sections. The additional flexural strength provided by using steel fibers allowed a reduction in pavement thickness from 15 inches (38.1 centimeters) to 10 inches (25.4 centimeters). Transverse joints were cut in the steel fiber reinforced IP1slab at varying span lengths from 85 feet to 200 feet (26 meters to 61 meters) to test how far apart natural cracking of the material would be with increased joint spacing. IP2 used steel fiber reinforced K-SCC for a 1,200-foot (365.8 meter) long placement with longitudinal post-tensioning and no control joints. The increased flexural strength provided by post-tensioning allowed further reduction in slab thickness to only 7 inches (0.18 meters). The pavement was inspected quarterly for five years. After 10 years of heavy use, the slabs were performing exceptionally well with minimal cracking and virtually no spalling. The FAA’s Pavement Condition Index (PCI) reported the steel fiber reinforced K-SCC (IP1) in Very Good condition (PCI 82), and the post-tensioned K-SCC slab (IP2) in Excellent condition (PCI 98).

Dams and spillways are other critical structures where K-SCC and K-SCG are used. The U.S. Army Corps of Engineers approved the use of K-SCC for their first full-size prototype at the Ririe Dam located about 25 miles (40 kilometers) from Idaho Falls, ID. The goal was to place larger, uniform surface slabs of concrete and minimize cracking. Using K-SCC, the spillway chute invert slab allowed the contractor to increase panel sizes to either 91 feet x 75 feet (27.7 meters x 22.9 meters) or 45.5 feet x 75 feet (13.9 meters x 22.9 meters) in area (vs. the conventional panel size of 22 feet x 25 feet (6.7 meters x 7.6 meters)). This allowed a reduction in total number of invert panels from 104 to either 18 or 9, depending upon the option selected. Minimum thickness was 12 inches (0.3 meters). Total spillway invert slab contained 5,584 cubic yards (4,269 cubic meters) of concrete. The reduction in linear feet of joints, percentage of steel reinforcement, linear f ootage of foundation anchors and foundation drain holes provided economic incentive for this project. Evaluations following the placement noted limited cracking at abrupt foundation irregularities and overbreak in the rock excavation, none deemed detrimental to the slabs, and many slabs that did not exhibit any signs of cracking.

In 2014, the U.S. Army Corps of Engineers, U.S. Department of the Interior, and US Bureau of Reclamation Cooperative used K-SCG for the $900M Folsom Dam Auxiliary Spillway Control Structure project (Folsom, CA). This bulk shrinkage-compensating grout project was batched on this remote site to achieve 7,000 pounds per square inch (psi) strength. Six submerged steel gate supports were successfully secured and protected using K-SCG with the two-year inspection indicating the grout was meeting dimensional stability design expectations.

With a GWP of 461, K-SCC provides a GWP 56% lower than Portland cements and 46% lower than Type 1L Portland-limestone cements (Fig. 3). The use of K-SCC not only lowers the overall carbon impact of any mix design, it also significantly reduces overall carbon impact throughout the concrete’s service life. By reducing materials and labor requirements during construction, improving durability, and minimizing repair and maintenance, owners, designers and patrons benefit from an 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, and with more sustainably and added value, Type K shrinkage-compensating cement will continue to deliver. Embracing innovative approaches to design, integration of new products, and the collaborative efforts of professionals throughout the industry in its use will make a difference in the race to carbon neutrality while improving performance.

About the Author

Susan Foster is Director – Strategic Initiatives & Komponent for CTS Cement. Her 30-plus years of experience includes CSA cements, exterior insulation and finish systems (EIFS), stucco, waterproofing, resinous floor, wall and lining systems, marine coatings, tile & stone setting materials, and decorative overlays.

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