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Wet-sprayed Cellulose Improves Bridge Repairs

The results of a field trial conducted in July 2003 by MTO's Materials Engineering and Research Office in collaboration with Ryerson University are breaking new ground for an innovative curing method that facilitates more durable bridge repairs.

MTO uses silica-fume shotcrete for bridge soffit repairs. Shotcrete is a concrete like material that is sprayed onto a surface at high pressure. Silica-fume is a mineral admixture that improves concrete properties such as bonding strength, freeze/thaw resistance and permeability.

Shotcrete must be properly cured to develop into a strong and impermeable layer to protect bridge reinforcement and a moist environment must be created around the freshly applied shotcrete layer to ensure proper hydration of its components in the first few days after placement.

Silica-fume shotcrete demands more curing care than traditional concretes, as it may shrink and crack if not properly treated. Surface cracks lead to the accelerated corrosion of the bridge's steel reinforcement, as both water and salt can penetrate the protective shotcrete layer. Silica-fume shotcrete must be cured by adding water. This is best accomplished on overhead bridge structures through the application of a fog-mist over a seven day period. Misting produces optimal curing, but is generally not done on bridge soffits over busy 400-series highways due to strict lane-closure restrictions. A spray-applied curing compound is used instead, which prevents moisture loss but fails to replenish water in the hydrating mix.

MTO recently investigated a new method to achieve proper moist curing that does not entail lane closures or ongoing access to the concrete surface: the application of wet sprayed cellulose. Wet-sprayed cellulose is essentially the same product widely used for thermal and acoustic insulation, consisting of layers of paper-based fibres saturated with glue and water, sprayed as an adhesive coating on freshly applied shotcrete. Prior to the field trial, it was theorized that the ability of cellulose to retain moisture would make it an effective curing agent, as it would share moisture with the shotcrete layer upon application. This concept was originated by Concrete Section's Senior Concrete Engineer Jana Konecny, and a trial project was launched under the direction of Tom Klement, Senior Research Engineer.

Enthusiastic private sector involvement contributed to the success of the trial. Two cellulose companies provided their services at no cost to MTO, while Underground Services (1983) Ltd. agreed to facilitate the trial at their company compound in Thorold at reduced cost.

The trial was conducted in cooperation with a civil engineering research team from Ryerson University, led by Professor Medhat Shehata, under MTO's Highway Infrastructure Research and Innovation Grant Program.

"The trial allowed our students to gain practical field experience and make a real contribution to the transportation industry, while MTO received the benefit of their determined research efforts," commented Professor Shehata.

The purpose of the field trial, initiated on July 10th, 2003, near the Thorold Tunnel in Niagara Falls, was to determine if cellulose curing could function as an effective alternative to curing compounds and fog misting. The trial simulated bridge repair conditions and involved the creation of nine test panels suspended on a scaffold system and sprayed with silica-fume shotcrete. Panel one was air-cured, panel two was sealed with a curing compound and panel three was mist-cured. Panels four through six were sprayed with a cellulose mix provided by an Ontario-based company, while panels seven through nine were sprayed with a cellulose product produced by a U.S company. The panels were suspended for 28 days, after which the cellulose layers (still in a moist state) were scraped off and cores taken from each panel for testing at Ryerson labs.

As expected, the panels cured with air and curing compound exhibited significant shrinkage and cracking around the panel perimeter. The mist-cured panel had no visible cracks, reaffirming the fact that mist curing is the most desirable, yet the least practical, curing method. Ontario cellulose material, engineered for home insulation, required onsite experimentation with different adhesive content to achieve an optimum mix. Only one out of three such panels fully retained the cellulose and showed no evidence of shrinkage cracking. All three panels cured with the U.S. cellulose material retained all of the applied cellulose and experienced no visible cracking.

The Ryerson team assembled a conclusive body of results by April 2004. Various tests measured the strength, permeability and porosity of the core samples. Results indicate that cellulose curing is as effective as mist curing, and is considered superior to conventional curing compounds.

Cellulose curing offers a cost effective alternative to mist curing, and allows for the re-opening of traffic lanes almost immediately after repair completion.

MTO's Concrete Section plans to conduct pilot projects in which cellulose curing will be applied to active bridge structures. Specifications for optimum performance will be developed from these trials. Once specifications are finalized, the curing of silica-fume shotcrete with wet-sprayed cellulose can be implemented in stages throughout Ontario.

MTO's shotcrete repair program is expected to benefit from the use of cellulose curing, as the long-term implementation of this method offers the possibility of improving the cost-savings and effectiveness of bridge soffit repairs.