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AKIS is featured in Internation Concrete Repair Institute’s Concrete Repair Bulletin – Hydrodemolition on Deteriorating Gunite

The International Concrete Repair Institute featured AK Industrial Services as a project profile in the May-June issue of the Concrete Repair Bulletin. Read the article text below or view a printer-friendly 4-page PDF of it as it appeared in the magazine.

Hydrodemolition on Deteriorating Gunite
Rehabilitation of the East Side Tunnel, Providence, Rhode Island
By Sarah McLellan

PROJECT OVERVIEW
The East Side Transit Tunnel in Providence, Rhode Island, was built in 1914 as an additional trolley route in the city, featuring a more gradual graded incline for trolley cars to take up College Hill¹. Currently, the tunnel is used exclusively for bus traffic and emergency services. The barrel vault section of the tunnel stretches approximately 1700 feet (518 m) long, with gunite from the 1990s throughout the entire length. The concrete liner on the tunnel is original concrete from 1914².

In spring 2023, it was discovered that the gunite and underlying concrete were deteriorating. In some sections of the tunnel, the gunite was failing due to quality, sandy composition, and age, whereas in other sections, the underlying concrete was weakening due to a smooth aggregate with sandy material². This resulted in an inadequate bond with the gunite. In some places, the gunite was sound, but it was either not bonded to the concrete liner or the concrete liner itself was deteriorating.

Investigations concluded that the gunite and concrete on the tunnel side walls and tunnel arch needed rehabilitation. All gunite needed to be removed from springline to springline, as well as some deteriorated patches on the tunnel side walls. The viability of single-lane closures was discussed to allow tunnel usage during rehabilitation, but it was eventually decided that closing both tunnel lanes was necessary, resulting in a full closure. This required a tight schedule, with different trades needing to work concurrently to be as efficient as possible to limit the duration of the closure².

CONSIDERATIONS FOR HYDRODEMOLITION
Hydrodemolition does not cause vibration to the structure, and it also does not leave cracks or microfractures in the surrounding area^3,4. Because no damage was done to the sound gunite, only the necessary repairs needed to be made⁵. The project required peak efficiency, with little room for setbacks or delays. Hydrodemolition is generally faster and more efficient than hand chipping, especially with overhead work. Since 100% of the gunite on the tunnel arches had to be removed, hydrodemolition presented an efficient and effective option. A hydrodemolition robot with a vertical tower was crucial to the task, as well as operators experienced in overhead work. Additionally, by using hydrodemolition, operators were able to adjust the pressure and the speed of the demo head to remove the required material to meet the project specifications.

Because the key component of hydrodemolition is water, silica dust is not a concern. When using conventional demolition methods, exposure to silica dust, which can cause silicosis, is a concern. Because hydrodemolition leaves surfaces and tools wet, it minimizes the chance of inhaling silica dust^3,4. This was especially beneficial for this project—where not only was it an enclosed space, but different trades were working on different areas of the tunnel simultaneously, with no cause for concern about silica dust exposure^3,4.

PROCESS
Due to logistics, only certain pieces of hydrodemolition equipment could be staged within the tunnel. The robot, as the essential piece, was staged inside, but the water pump and the water treatment system were staged at the tunnel portals.

The crew began with the tunnel arch. By using a robot with a vertical tower at 17,000 psi and 75 GPM (1172 bar and 284 LPM), operators were able to successfully remove the required gunite (Fig. 1, 2).

After completing the tunnel arch demolition (Fig. 3), the crew shifted focus to the side walls. Operators began the side wall demolition with the robot; however, in some areas, the gunite was stronger than the concrete liner. Using the robot proved to be too aggressive. Operators then used hand lances to surgically remove the remaining patches without removing the concrete liner (Fig. 4).

The slurry, the combination of hydrodemolition water (HDW) and the cement matrix, was pumped to the treatment system. To dispose of the HDW back into the environment, the crew sent the water through the treatment system at a rate of 75 GPM (284 LPM). By using CO₂, the treatment system can neutralize the pH. A series of baffles in addition to flocculant and coagulant reduced total suspended solids (TSS), making the HDW safe to discharge back into the city sanitary system. The filtered HDW was tested regularly and was deemed safe to discharge (Fig. 5).

CHALLENGES
All the tunnel arch gunite needed to be removed, but only certain areas of gunite on the side walls needed to be removed. The concrete liner was the original concrete from 1914, making it over 110 years old at the time of demolition. In some areas on the sidewalls, the significantly newer gunite was stronger than the underlying concrete. When the robot demoed some stronger gunite, it also hit the underlying concrete. This necessitated a switch to the hand lances for the sidewall patch demolition.

By using 34,000 psi and 5 GPM (2344 bar and 19 LPM), operators were able to remove the gunite patches without damaging the concrete liner (Fig. 6). By using hand lances, operators were able to achieve a more consistent product on the tunnel sidewalls (Fig. 6). Using hand lances also sped up the process, allowing the operators to adhere to the master schedule despite previous challenges.

Another challenge was water collection and the water treatment process. Because logistics did not allow for the treatment system to be staged inside, the HDW needed to be pumped out of the tunnel and to the system. A unique aspect of this project was the slope of the tunnel. The crew was able to use this to their advantage to establish a sump area.

While working on the upper half of the tunnel, the HDW flowed into a catch dam, where it was then pumped back up to the treatment system. Once the operators got to the halfway point of the tunnel, the treatment system was moved to the lower tunnel portal.

To account for a faster flow of water, operators built temporary weirs along the slope out of sandbags and plywood (Fig. 7). The weirs not only slowed the flow of the HDW, but they also acted as an initial treatment system to reduce TSS, as the HDW would pool at each weir, and the TSS would settle due to gravity.

RESULTS
The hydrodemolition began in late April 2024 and was completed by the middle of June. Any remaining gunite was inspected and found to be sound.

The East Side Transit Tunnel ultimately required 67,386 square feet (6,260 m2) of demolition along the side walls and arch. The hydrodemolition portion of the project cost about $1.7 million, with the overall rehabilitation of the tunnel costing about $25 million. The tunnel closed to traffic on March 25, 2024, and reopened on October 31, 2024¹.

By using hydrodemolition, the demolition process was efficient, targeted, and with minimal environmental risk.

REFERENCES
1. “East Side Tunnel Projects,” Rhode Island Public Transit Authority, accessed February 11, 2025, https://www.ripta.com/tunnel/
2. “East Side Tunnel Rehabilitation: Tunnel Lining Demolition and Lane Closure Investigation.” WSP USA Inc, Boston, MA, 2022, pp. 3-29.
3. “Rhode Island Public Transit Authority: Request for Proposals Number 23-24: Addendum 2.” WSP, July 19, 2023.
4. Nittinger, Bon, “Concrete Surface Preparation Using Hydrodemolition.” Concrete Repair Bulletin, January 2001, 6-7pp. https://www.icri.org/wp-content/ uploads/2024/04/CRBJanFeb01_Nittinger.pdf
5. ACI Committee E706, ACI RAP 14, Concrete Removal Using Hydrodemolition, American Concrete Institute, Farmington Mills, MI, 2013.
6. ICRI Guideline No. 310.3R-2014, Guide for the Preparation of Concrete Surfaces for Repair Using Hydrodemolition Methods, International Concrete Repair Institute, Minneapolis, MN, 2014, 1-7 pp.