National Aquarium requires 20deg dewpoint for tank fix

After several years of wear, the coatings on two of the famous tanks at the National Aquarium failed, allowing seawater to penetrate the concrete. The project specified a coatings system that was tricky due to the harsh application conditions.

Polygon desiccant dehumidifiers were used to keep the dew point 20 degrees below the surface temperature while maintaining the tight curing schedule. Polygon’s custom drying solution created the specified conditions to ensure that the project was a success. To improve the replacement coating life, the engineering consultants selected a five-part lining system, including primer, epoxy undercoat, fiberglass matting, and two topcoats. To ensure adhesion between the concrete and the primer, the consultants specified a maximum moisture content for the concrete. Drying the concrete was left to the contractor, who used Polygon to ensure that specifications would be maintained at all times.

PROBLEM

Nearly all concrete coating systems have been known to fail prematurely. Coating manufacturers identify excess moisture as a contributor to these failures in two ways. First, surface moisture prevents the primer from bonding and curing properly. Then, as the concrete warms during hot weather, heat drives the concrete’s interior moisture into the coating from behind which makes the primer disbond.

Such problems can be avoided by drying the concrete before coating. To achieve the low moisture content required, the air above the surface must be dry. By doing so the air can absorb the moisture as it is released by the drying concrete.

During humid evenings and mornings, amine epoxy coatings will react with atmospheric CO2 when moisture is present to form a thin layer of “blush.” This surface contamination interferes with adhesion unless it is removed before the next coat is applied. Polygon’s all-season climate control proved to be the solution for both drying the concrete and avoiding the occurrence of blush.

SOLUTION

Steve Mainello, the project manager for SPS explained, “Polygon had the technical understanding of what equipment we would need, and they worked with us week-by-week as those needs changed. Polygon’s fast response and their flexibility made me feel like I was their most important customer.”

On-site logistics were a particular challenge because the aquarium is located on a short pier, with essentially no space around the building for climate control equipment or for generators. Polygon provided a solution with equipment small enough to use on-site power and light enough to be placed on the roof. Dry air was brought through a flexible hose down into the tanks. Ducts that ran more than 100 ft. were common as the requirement for dry air moved throughout the building.

The contractor accelerated drying time using heaters in combination with dehumidifiers. However, care was taken not to over-dry the concrete in the process of combining the technologies. The moisture content of the concrete was measured throughout the tank using the equilibrium moisture content method.

By quantifying and monitoring the concrete moisture content continuously, the drying process could be tracked section by section. Dry air and heaters were repositioned often because parts of each tank dried more quickly than others. This technique allowed the use of smaller, more cost-effective dehumidification equipment than had been used in the past for concrete drying.

After instruments read 75% RH in all the holes drilled for moisture measurement (about 5% moisture in the concrete), the coatings were applied. Desiccant dehumidifiers were then used to keep the dew point 20 degrees below the surface temperature, while each layer cured. Without that level of dryness, atmospheric humidity can exacerbate the coating’s reaction with CO2 to form the familiar amine “blush” that interferes with adhesion.

As cleaning, blasting, and coating operations moved from tank to tank, humidity control requirements changed. Polygon’s flex approach recognized this reality, helping to keep costs down without risking coating quality. Steve Mainello explained: “Polygon would bring in additional equipment and accessories as we needed it and allow the equipment to go off-invoice when we finished with it, even if it was still on the site. That trusting attitude and fast response really helped the job go easily. I could always count on Polygon— they were always there when I needed them. Our overhead on this job was $5,000 per day, and the project lasted about seven months. Polygon’s reliability has saved us at least a week over those months, which means we’ve saved tens of thousands of dollars.”

RESULTS

Fast Drying of Concrete

Excess moisture was removed from the 130,000-cu. ft. concrete tank in less than three weeks. The smaller pretreatment tanks were dried in four to six days. Fast concrete drying keeps projects on schedule, ensuring profits for the contractor and improving job quality for the owner.

High-Quality Coating Adhesion

With no excess moisture in the concrete, the primer adhered firmly to the tank surface, so the primary mechanism for lining failure was held in check.

Flexibility to Fit the Need

Polygon made sure the right equipment was always ready. Extensive inventory and the ability to modify equipment made sure that the job could proceed under humidity control despite the unusual site limitations.

Contact us to explore how Polygon temporary climate solutions can improve your drying process.