By Michael Brennan, PE

Benjamin Franklin once wrote that “In this world, nothing is certain except death and taxes.” In the world of structural engineering, it may be argued that concrete cracking and deterioration over time also fits into Franklin’s idiom. Fortunately, there are ways to repair, rehabilitate, and extend the functional life of this important building material using shotcrete.

Shotcrete Repair and Rehabilitation

In situations where large-scale repair of concrete is necessary and installation of formwork is either cost prohibitive or difficult, shotcrete offers an economical and effective solution. Shotcrete is comprised of cementitious products with aggregates similar to concrete but is “shot” pneumatically onto prepared substrates. Shotcrete may be installed on vertical or horizontal surfaces. Bonding agents like grout or epoxies (to name a few) that are typically needed with conventional concrete cold joints are not required as adhesion is achieved during the shotcrete installation process.

Shotcrete may be used to protect exposed reinforcing, replace deteriorated or damaged concrete, restore structural integrity, or increase the structural capacity of existing concrete members.

Shotcrete Installation

Shotcrete may be installed using a wet-mix or dry-mix process, achieving the same result. A contractor familiar with shotcrete installation and the correct equipment is paramount to the success of the installation. Test panels should be required to verify not only the mix design but the ability of the contractor to provide uniform and quality shotcrete application.

The existing concrete surface must also be prepared correctly. Deteriorated or spalled concrete should be removed back to sound concrete. Exposed reinforcing bars should be undercut to provide the ability for a physical bond to develop to the shotcrete. Repair boundaries should be a sawcut to facilitate correct shotcrete placement. We generally recommend using abrasive or high-pressure water blasting to accomplish most surface preparation requirements. Finally, the existing concrete substrate should be thoroughly wetted before shotcrete application.

Holter Dam Spillway Rehabilitation

One project where we used shotcrete was to rehabilitate the spillway on Holter Dam, a NorthWestern Energy facility. The 100-year-old concrete had spalled and deteriorated after years of severe exposure and freeze-thaw conditions. Consequently, reinforcement was exposed in multiple locations. Since access to the spillway is limited, and repairs were needed in hard-to-reach overhead and vertical wall locations, we selected a shotcrete overlay to rehabilitate the structure. (See the photos below for before and after conditions of the existing concrete and shotcrete rehabilitation.)

Before and After Holter Dam Spillway

The photos in the top row show concrete before the rehabilitation process using shotcrete. The finished project photos are on the second row.