The Concrete Facts on an Emerging Technology

Self-Consolidating Concrete

Self-consolidating concrete (SCC) is one of the most innovative technologies in concrete construction today. SCC eliminates the use of vibrators and allows faster concrete placement while improving its consolidation. SCC's ability to reduce voids in the mixture leads to better concrete quality, durability and surface appearance. In addition, this technology reduces labor needs and work site noise. These benefits make SCC an improvement from traditional concrete that needs to be consolidated by vibration to remove air pockets to achieve optimum concrete properties and fill formwork.

This technology has the potential to be used in MTO applications where there have been problems achieving proper consolidation with conventional cast-in-place concrete, due to difficult access for vibrators. Potential areas for SCC use include: barrier and parapet walls, piers, columns, and re-facing. It is also suitable for pre-cast concrete products such as girders and culverts

After success in Japan and Europe, this technology is being introduced in North America. However, before SCC can be used widely, a number of practical issues must be addressed, including development of standards for its production and testing. Individual countries and producers tend to adopt their own SCC tests, making comparison of experiences impossible. Absence of standard tests has prevented SCC use in general construction practices because owners are reluctant to use a non-standard technology. To address this concern, North America is currently preparing guidelines and standard test methods: the American Concrete Institute will be publishing an Emerging Technology Series document regarding production and use of SCC; the Canadian Standards Association published a list of recommended test methods, including acceptance criteria, in the 2004 edition of the national concrete standard.

MTO prepared draft specifications for SCC and used it on a trial basis in pier rehabilitation at the Garden City Skyway, St. Catharines, in the summer of 2005. The contract called for concrete patches made by pumping concrete into forms, but MTO accepted the contractor's proposal to use SCC instead. MTO required their draft specifications for SCC be met in addition to the normal requirements for concrete strength and air void parameters, for durability. This was also the first contract with a linear shrinkage limit requirement for concrete to minimize drying shrinkage cracking.

SCC requires a new approach to testing that is unique from what the industry is accustomed to. Properties such as flow ability (ability to completely fill formwork without entrapped air pockets), passing ability (ability to pass between steel reinforcing bars and other narrow spaces without clogging) and segregation resistance (ability to prevent separation of stone and paste in the mix) are evaluated as part of MTO's specification for SCC. Concrete should be tested prior to placement to verify that SCC meets all three properties, otherwise, a good quality hardened concrete cannot be achieved.

In the case at Garden City Skyway, MTO relied on supplier-developed test methods and trial batches for evaluating the ability of concrete to meet the ministry's requirements. All the specified concrete properties, including compressive strength, air void system and linear shrinkage were met. The method was successful, although there were a few isolated problems with the bond of the patching/re facing to the parent concrete. SCC can cause high pressure on forms, due to its highly fluid nature, and create blowouts of formwork where bracing is insufficient. During trial contract SCC did not always fill out forms completely. However, use of additional air vents in the formwork may alleviate this problem.

To mitigate the challenges of SCC, MTO will continue working with the concrete industry to develop a common set of test methods to measure flow ability, passing ability and segregation resistance, as well as a performance-based concrete acceptance process. This technology's amazing potential to produce a high quality product, minimize the use of vibrators, quicken the construction process and fill formwork without segregation is sure to make the challenges of improving SCC worthwhile.