Although self-consolidating concrete (SCC) has many advantages for all types of concrete applications, its acceptance for cast-in-place construction has been slow in North America. The objective of the Industry Critical Technology Committee on Self-Consolidating Concrete is to change that to “15 by 15.” That is, the committee wants SCC to be 15% of all ready-mixed concrete by 2015.
SCC originated and gained initial acceptance in Japan in the late 1980s. It has been used successfully throughout the world in precast and cast-in-place concrete. Increasingly it is specified and used in North America because it greatly expands the possibilities of successful high-performance concrete placements with difficult and demanding requirements.
SCC is a highly flowable, nonsegregating concrete with a slump flow of 20 to 30 inches that can be easily placed and will fill forms completely under its own weight and without mechanical consolidation. SCC is produced using high-range water-reducing admixtures (HRWRA), viscosity-modifying admixtures (VMA), and well-graded aggregates. A polycarboxylate-based HRWRA is typical. The HRWRAs and VMAs provide the necessary fluidity and viscosity.
VMAs commonly are used in mixes with slump flows above 24 inches and in mixes with less than an optimum combined aggregate gradation. These mix designs are initially prepared in accordance with the project specifications, expected slump flow range, and with a setting time based on climatic conditions at the time of placement. The proposed mix must be verified by a successful onsite placement to confirm the slump flow, pumpability, finish, and setting time.
Advantages and challenges
Excessive form pressure can result from retarded concrete, too rapid placement of concrete, or both. Formwork must be designed with slump flow, rate of placement, and setting time established. There has been significant research on this issue for SCC which has shown SCC to have thixotropic properties that tend to minimize form pressures. Below, you can find a list of recent articles from the SCC 2010 conference that provide valuable information on slump flow, setting time, and the resulting form pressures.
Currently, SCC is used in precast concrete, architectural concrete, heavily reinforced concrete, and formed repairs. In the plastic state, SCC offers the benefits of speed of placement, ease of consolidation, deformability, and resistance to bleeding and segregation in both the dynamic and static states. During construction, SCC also can achieve:
- Faster placing, finishing, and stripping of forms
- Reduced equipment costs
- Faster turnaround time of concrete trucks
- Significant cost savings because of the elimination of vibration and increased rate of placement
- Reduction in patching and repair
- Safer for the workforce with the elimination of vibration
The benefits for hardened concrete include improved appearance and finish, higher early strength than conventional superplasticized concrete when a polycarboxylate admixture is used, and higher bond strengths because vibration causes some bleeding, which results in voids under reinforcement. These benefits are recognized by many owners, designers, and concrete producers throughout the country. Most structural engineering firms today include self-consolidating concrete in their master specifications.
Successful SCC projects
There have been many successful cast-in-place SCC concrete projects. In general, all have the following characteristics:
- The specification is clear as to usage, w/cm, air content, and the necessity for a preconcrete conference and a successful test placement onsite.
- The preconcrete conference agenda requires representatives from the designers, contractors, concrete producers, admixture manufacturers, and testing lab to thoroughly discuss the mix design requirements in both the plastic and hardened state, climatic conditions, form design, form release agent, schedule, rates of placement, test locations, and target slump flows.
The table Successful SCC Mix Designs shows mix designs—for details on these and other SCC projects. When planning and preparation are thorough and the QA/QC procedures are followed, successful projects are the result. Typical QA/QC requirements include:
- The acceptable slump flow range based on the onsite test placement
- Testing procedures at the concrete plant and in the field with respect to water and air content
- Acceptable architectural finish requirements regarding uniformity of finish, color, and bug hole limits on size and number
The entire concrete industry will benefit from more successful SCC projects. We can all look forward to 15 by 15.
Major Projects in the U.S.
A number of projects throughout the United States feature self-consolidating concrete. Here is a small sampling.
- The US Mission at the United Nations was the Grand Award Winner at the Concrete Industry Board awards dinner in 2009. Its 28 stories feature buff-colored, architectural SCC concrete, which achieved 8000 psi at 28 days.
- The Wharton Center for Performing Arts at Michigan State University is using SCC for walls.
- The Freedom Tower (1WTC) will top out at 1776 ft. The shear walls were 14,000 psi at 56 days (MOE 7.7 m) from the foundation to 70 ft. above street level; 12,000 psi concrete is being used for the next 330 ft.
- 301 Market Street is a 60-story, reinforced concrete building in San Francisco. The formed structural members were SCC with 10,000 psi at 56 days used for the first 20 floors of the building.
- The Trump Tower in Chicago was a major user of SCC. This 92-story reinforced concrete project required 4600 cu. yds. of SCC cast-in-place continuously for 22 hours to construct the mat foundation, which supports the finished structure. The mix had a seven-day compressive strength of 9950 psi and 28-day strength of 12,000 psi when a strength of 10,000 psi required at 56 days. This single pour is the largest ever recorded in North America using SCC.
- The 57-floor Comcast Center—the tallest building in Philadelphia—stands 975 ft. high and used 45,500 cu. yds. of SCC.
- In 2004, the Skyline Bridge in Omaha, Neb., was completed. The decking of this bridge is completely composed of SCC. Nebraska uses SCC for most bridge components, including long- and short-span girders, pilings, and Jersey barriers.
- In Virginia, VDOT is using SCC for applications such as prestressed, cast-in-place, and precast members. In 2005, VDOT used eight prestressed girders in the construction of the Pamunkey River Bridge located on Route 33.
- SCC is being placed in a 3-ft.-thick slab under the No. 1 subway line at the World Trade Center in New York City. The 50-ft.-wide slab runs the length of the World Trade Center.
Major Projects Around The World
- The Burj Dubai (Burj Khalifa) is the tallest building in the world. SCC was used throughout the structure, and pumped 166 stories from the bottom. The slump flow was 24 to 28 in.
- In 1997, the Jin Mao Building, Shanghai, China, used SCC to construct a 4-m thick, 64x64-m foundation (13-ft.-thick, 210x210-ft.) in 1997. This building is the fifth tallest building in the world standing at 421 m (1380 ft.) and 88 floors.
- The Mori Tower in Shanghai, China, used SCC for its structural frame. The foundation, composed of 37,000 cu. m (1,306,640 cu. ft.) of SCC, was cast-in-place in three phases; the last phase included 28,000 cu. m poured continuously for 40 hours using 19 pumps. This building, at 492 m (1614 ft.) tall and 101 floors, consumed more than 300,000 cu. m (390,500 cu. yds.) of concrete.
- The Beijing TV Centre Building, which was finished in 2005, used SCC poured in hollow steel tubes for structural support. This method is called concrete filled tube technique (CFT). The design called for a slump flow of 710 mm (28 in.) and 28-day strength of 96 MPa (13,924 psi). The project used 3000 cu. m (3900 cu. yds.) of SCC. The building, standing 239 m (784 ft.) high with 44 floors, was used to host the 2008 Olympic Games.
- Major renovations are taking place on the Sodra Lanken Project—the roadway infrastructure connecting East and West Stockholm. The $800 million (US) project is the largest development in Sweden. The project includes 6 km (3.7 miles) of four-lane highway and bridges, 16 km (10 miles) of total concrete-lined rock tunneling, and earth retention walls. SCC used in different areas of the project totaled 15,000 cu. m (19,500 cu. yds.).
- In Neuchatel, Switzerland, the La Maladiere Football Stadium was made up of 60,000 cu. m of SCC placed in 10 months. The required slump flow was 650 mm (26 in.) with a 28-day compressive strength of 44 MPa (6382 psi). This development, completed in 2007, contains a football stadium with 11,500 seats, a mall with 25,000 sq. m (269,098 sq. ft.) of retail space, and a 930-space parking structure.
- In Japan, the Yokohama Landmark Tower also used CFT with SCC for earthquake resistance. This is Japan’s tallest building at 296 m (971 ft.) and 74 stories.
- The Ritto Bridge in Japan, which was completed in April 2007, was constructed using precast SCC for its piers. The highest pier rose 65 m (213 ft.). The SCC used needed a compressive strength of 50 MPa (7252 psi) in order to assure safety for earthquake resistance. The slump flow was between 600 mm and 650 mm (24 in. and 26 in.).
- Another bridge in Japan, called the Kaikyo Bridge, is the longest suspension bridge in the world spanning 1991 m (6532 ft.). It used cast-in-place SCC for construction of its anchorages, and by using SCC, the estimated two-and-a-half-year project was completed in two. The bridge withstands earthquakes measuring 8.5 on the Richter Scale and winds up to 280 km/h (174 mph).