I have a project with a segmental retaining wall. The project is located in Maryland. The CMUs of the retaining wall have significantly deteriorated in several areas. The geo-grid fabric appears to be in good condition in the areas where it could be observed. I suspect the deterioration is caused by saltwater runoff during snow events combined with the freeze/thaw cycles. There are some localized areas where the masonry units have completely disintegrated. The largest area being approximately 4 ft2 (0.37 m2). Is it feasible to remove the deteriorated masonry material, down to a sound surface, and shotcrete the face to restore (or exceed) the structural integrity of masonry units? If so, will this restore the structural integrity of the segmental retaining wall?
Shotcrete is routinely used for the repair of deteriorated concrete masonry and brick. Shotcrete should easily have a minimum compressive strength of 4000 psi (28 MPa) when properly shot and cured. According to the National Concrete Masonry Association, current CMU units have a minimum 2000 psi (14 MPa) compressive strength. Older CMU had a lower 1500 psi (10 MPa) minimum. Thus, the shotcrete placement will be significantly stronger and less permeable than the in-place CMU. This should give the wall better resistance to saltwater and freeze-thaw exposures thus extending useful life.
Depending on the depth of the new shotcrete placement, you may consider mechanically tying the shotcreted layer back to the sound CMU with epoxy or mechanically embedded anchors or j-bolts. Thicker sections may also benefit from the use of a steel wire mesh or fibers.
When shooting onto existing CMU sections, the surface must be properly prepared and then shotcreted with proper shotcrete materials, equipment, and placement techniques. Shotcrete placed onto an existing CMU surface will provide an excellent bond IF the following conditions are met:
- Make sure the surface is roughened and clean.
- The amplitude of roughness should be +/- 1/8th in. (3 mm) or more.
- A high-pressure water blaster (5000 psi [34 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
- Bring the CMU surface to a saturated surface dry (SSD) condition. This means the surface feels damp, but water is not picked up on a hand.
- Make sure the shotcrete placement is properly executed with high-velocity placement and quality materials.
- The shotcrete should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
- Be sure the shotcrete contractor is using an air compressor able to produce at least 185 CFM (5.2m3/min) for wet-mix and 375 CFM (10.6 m3/min) for dry-mix (gunite) of air flow at 100 to 120 psi (0.7 to 0.8 MPa).
- Use of an ACI-certified shotcrete nozzleman is recommended.
- No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
Finally, without details on the loading conditions, and structural details of the original construction, we cannot comment on the structural integrity of the repaired wall. You should contact a professional engineer experienced in concrete repair to evaluate the structural integrity of the wall when repaired.
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ASA Outstanding Shotcrete Awards Program
The ASA Outstanding Shotcrete Project Awards Program exists to recognize excellence and innovation on projects in which the application of shotcrete has played a significant role.
ASA’s Annual Outstanding Shotcrete Project Awards Program provides an exciting real-world demonstration of the exceptional advantages of placing concrete via the shotcrete process. Many sustainability advantages are also inherent in the shotcrete process and play a significant role in winning projects as well as the project owner’s ultimate decision to use shotcrete as the method of concrete placement. Projects must be completed between January 1, 2021, through September 1, 2023, and can be submitted in the following areas: Architecture │ New Construction, Infrastructure, International Projects, Pool & Recreational, Rehabilitation & Repair, Underground.
To assist in your submission, we have provided submission resources to inform you of the submission guidelines, a list of questions, and a copy of the owner release form. Please email any questions to [email protected].
Award Archive
2025 – Twenty First Annual Outstanding Shotcrete Project Awardees
Enhancing Efficiency and Safety in Concrete Shaft Restoration: A Shotcrete Success Story
In September 2021, American Concrete Restorations Inc. was contracted to perform 60 yd³ (46 m³) of concrete repairs to an existing shaft that is used to access an underground tunnel system in a southern suburb of Chicago, IL. The job was for the Metropolitan Water Reclamation District of Greater Chicago, Thornton Construction Shaft. The dimensions of the shaft are 250 ft (76 m) deep and 30 ft (9 m) in diameter, and the only access is from the top using a crane basket. The project specifications called for the use of a polymer-modified, bagged product using the formand-pour method to replace the old concrete.
Sobre Productividad, Cálculo de Costos de Trabajo y Éxito en el Shotcrete
Como miembro de la Sociedad de Honor Internacional en Economía y con una doble licenciatura en Finanzas y Banca, ingresé al mundo del shotcrete preparado de manera un poco diferente que la mayoría de los que ingresan a esta industria. Eso fue hace más de 20 años, pero los principios de comprender el tiempo, el valor del dinero y los costos de oportunidad están indeleblemente grabados en mi psique. En la actualidad, junto con mi socio comercial, quien también proviene de un entorno de fabricación y negocios en un campo internacional, dirigimos una empresa de transporte, una empresa de concreto lanzado vía seca, una empresa de PebbleTec y una empresa de diseño, todo bajo un mismo techo. En conjunto, tenemos entre 55 y 60 empleados en cualquier día dado. En un negocio de este tamaño, la productividad y la comprensión de los costos laborales son clave para sobrevivir.
Advances in Shotcrete Technology for Ground Support in Tunnels and Mines in North America
In recent years, shotcrete has been widely used for ground support in civil tunnels and mines in North America. Shotcrete technologies have advanced with robust robotic sprayers, high-performance shotcrete mixture designs, and high-performance fiber reinforcement in conjunction with rigorous qualification of shotcrete nozzlemen and QC inspection and testing programs. Design engineers and contractors are using shotcrete more and more often for various underground applications including ground support and final linings in tunnels in soft ground and hard rock mines, as well as in repair and rehabilitation projects in railway tunnels and other underground openings. Large underground caverns have been constructed using shotcrete as the initial liner in San Francisco and Los Angeles, and for both the initial liner and final liner in New York and Washington D.C. This article focuses on recent underground shotcrete technology developments from project experience and provides lessons learned. It also demonstrates that proper quality control and shotcrete qualification programs are critical for successful shotcrete projects.
On Productivity, Job Costing, and Successful Shotcreting
As a member of the International Honor Society in Economics and with a double BS in Finance and Banking, I came into the world of shotcrete prepared a little differently than most entrants to the field. That was over 20 years ago, but the principles of understanding Time, the Value of Money, and Opportunity Costs are indelibly ingrained into my psyche.
Shotcrete is often the best alternative when repair and restoration are being contemplated and can be the ideal application method for both reinforced and nonreinforced construction. From tanks and pools to chemical and automotive to retaining walls and highway structures, the opportunities are endless.
The shotcrete process has also been used for repair and installation of new linings in industrial melting and firing facilities since 1915. The relining of blast furnaces, ladles, and casting facilities, as well as petroleum and cement producing plants, are some of the applications that employ both wet and dry process shotcreting of specialty, heat-resistant materials. The shotcrete process allows for new installations during downtime and “hot” installation of certain materials as a routine part of the production cycle—another advantage of this unique concrete construction method.
Proper surface preparation allows the shotcrete professional to be able to integrate new materials with in-place construction to form a composite that will stand up to exposure and use. As important as the materials are the coordination of efforts from a properly qualified contractor, materials supplier, engineer, and architect. When all of these pieces are in place, the exceptional benefits of the shotcrete process are realized.