The construction of an expanded stormwater storage facility in the low-lying section of the I-35W highway in Minneapolis, MN, was officially completed in August 2023. This project was the first Minnesota Department of Transportation project that specified a crystalline waterproofing solution. PENETRON ADMIX SB, a crystalline concrete waterproofing admixture, was specified to ensure a durable and waterproof concrete mix for the stormwater storage silos.
The Minnesota Department of Transportation (MnDOT) noted that the I-35W stormwater tunnel system, originally constructed in the 1960s to carry stormwater runoff from the highway and the surrounding neighborhoods, had been prone to increasingly frequent flooding during storm weather events. The new stormwater storage facility is located between 39th Street and 42nd Street in Minneapolis, a low point in the highway grade.
“Development in the surrounding area continued over the past decades, which has increased the volume of stormwater flowing into the tunnels,” notes Christopher Chen, Director of The Penetron Group. “In fact, wet weather storm events that flooded the I-35W would often create conditions in the stormwater tunnels that would blow manhole lids off through the force of the incoming stormwater!”
The MnDOT retained the team of Brierley Associates, Barr Engineering, and TKDA to develop and design a deep underground stormwater storage facility. Constructed immediately adjacent to the northbound I-35W interstate highway, the new facility can now accommodate a minimum of 14-acre-feet of water (4.8 million gallons) expected for a 6-year, 24-hour storm event. The design comprises six 45-ft diameter by 90-ft deep circular silos, which were built using a diaphragm wall construction method.
The site of this stormwater storage facility presented sizeable construction challenges, including a high groundwater table, a narrow construction area between a dense residential and business corridor and the highway, and maintaining the existing, active stormwater system during construction of the new replacement system.
Because of the many project requirements, MnDOT needed a reliable concrete waterproofing solution for the stormwater storage facility. Penetron provided the necessary test data to MnDOT to support a concrete mix design with a 1% dosage rate (by weight of total cementing materials) of PENETRON ADMIX SB, a crystalline concrete waterproofing admixture. This mix, supplied by Cemstone, the project’s ready-mix concrete supplier, ultimately met all specified requirements for durable and waterproof concrete when confirmed by laboratory tests.
“Notably, this project required a concrete mix with an above average amount of cementing materials contained in the standard 6.6 lbs. soluble bags of PENETRON ADMIX SB. As a result, Penetron provided larger, custom-sized soluble bags to maintain the 1% dosage rate – for simple ‘one bag per yard batching’ convenience. This allowed Cemstone to maintain a 1 bag/cubic yard dosage process to help prevent dosage-related mistakes during the batching phase,” adds Christopher Chen.
Cemstone, provided about 8,500 yds3 of PENETRON ADMIX SB-treated concrete specified for the inground stormwater retaining circular cells.
PENETRON ADMIX SB-treated concrete is impermeable, stopping water and harmful elements from entering the concrete, even under the sudden hydrostatic pressure within the stormwater storage facility. Its ability to permanently self-heal any microcracks minimizes concrete deterioration for the life of the concrete – an important aspect for an underground facility.