Forensic Analysis and Rehabilitation of a Failed Freezer Slab

Lee Durston BS, CBST BCRA – Tacoma, WA

ABSTRACT

In September of 2006, the Central Kitchen for Puyallup School District (henceforth, PSD), in Puyallup, Washington experienced a heaving failure with a concrete freezer slab. The slab had risen to a dome roughly four inches higher than constructed and was continuing to grow, rendering the freezer unusable. BCRA's Building Science division was retained to investigate the root cause, determine the extent of damages, and provide solutions for rehabilitation of the slab. This facility is in operation approximately 330 days a year and provides meals for over 10,000 students a day, so closing the facility or even a portion of the space would result in unacceptable costs. Because of this, it was in the school district's best interest to rehabilitate the slab while the kitchen continued at 100% operation.

BCRA developed a plan of action to eliminate moisture intrusion beneath the slab and to heat the slab from below, returning it to its original position. Also, BCRA designed a retro-fit glycol heating system to insure there were no further problems of this type. The ability to rehabilitate the slab without operational loss allowed for a projected savings of over $300,000. Infrared imaging was incorporated throughout the investigation to analyze the building envelope, measure temperature during slab heating and survey the air tightness of the component freezer system.

INTRODUCTION

BCRA was contacted by PSD in November of 2006 with a request to perform a site visit to its Central Kitchen. Upon inspection, it was evident that moisture had collected beneath the slab and upon freezing caused the slab to heave. Slab heave, in regards to a freezer space, takes place when moisture collects in or beneath a freezer slab and freezes. This phase change from water to ice causes a volumetric expansion that builds pressures strong enough to push the slab upward or footings outward. Over the years many tactics have been incorporated into the construction of the slab to offset this destructive and usually catastrophic failure. Some tactics include in-slab radiant heating lines and electric heating mats. In the case of the freezer at PSD’s Central Kitchen, a tactic using passive ventilation of ambient air through the sub slave drainage bed using vent lines was incorporated. This was a typical installation technique in the Pacific Northwest 10 – 15 years ago and it is failing in many cases due to vent pipe blockage. This common failure is routinely repaired by relocating the freezer’s contents, shutting it down and thawing out the slab. This is followed by the demolition and removal of the slab and subsequent replacement with a new slab. The newly poured slab must cure for a longer than normal period (up to 2 months) before the slab can be brought down to below freezing temperatures. Upon initial review of the case it became apparent that the costs associated with shut down and construction of this type would be unacceptable due to operational load. This case study details the process of diagnosing the freezer slab and the work completed to rehabilitate the slab while remaining fully operational.

FACILITY INFORMATION

Construction of PSD’s Central Kitchen Facility was completed in 1996. The facility was an addition to an existing 13,000 square foot warehouse. A portion of the warehouse was also renovated to include a large warehouse freezer built on a concrete slab that was originally intended for this with existing vent lines built in. The 17,000 square foot Central Kitchen addition included its own freezer in the northeast corner- positioned for the ability to articulate with the warehouse freezer and for consolidation of mechanicals. The kitchen freezer’s slab was prepared with 4 inch ventilation ADS pipe that ran the full length of the building, north to south, with a designed slope of ½% towards the north end over an insulated slab system. (Figures 2.1- 2.5)

Figure 2.1- As-built floor plan Figure 2.2- Wall / slab section

Figure 2.3- Insulated slab vent system and location of ice formation Figure 2.5- Vent components

INVESTIGATION

BCRA first documented the heave and damage to surrounding building components by taking detailed measurements and imagery, both visible and IR, of the freezer and building components. The freezer slab floor had risen to a dome 4” higher than constructed and was continuing to rise. To determine root cause of failure, BCRA identified all possible sources of moisture intrusion that may have contributed to the heave. This list included; sewer piping, roof details, condensation on refrigerant lines, ground water table, fire protection system failures, under-slab vent tubes, and possible user error through spills or mop-down techniques (Figures 3.1 – 3.10). In addition to identifying possible sources of moisture intrusion, BCRA conducted staff interviews, researched weather history, and conducted investigation work to determine the location and size of the ice formation causing the heave. Vent lines were video snaked to determine blockage and location, and a core sample of the slab was taken to determine at which layer of the insulated slab ice was present.

Figure 3.1- Existing conditions in freezer Figure 3.2- Temperature readings up vent lines

Figure 3.3- Tilt-up and freezer in warehouse Figure 3.4- Air leakage and temp. readings

Figure 3.5- Roof over freezer and adjoining tilt-up wall Figure 3.6- Thermograph of moisture in tilt-up wall

Staff interviews identified that there had always been cracks in the slab but never a heave and that the formation of the heave was a sudden event that had happened in early September of 2006. The kitchen staff noticed this immediately due to the fact that one day they entered the freezer and the product carts had all rolled away from the center of the room and continued to do so when replaced. Weather history data was applied to this timeframe and we noticed that the proceeding summer was irregularly warm and dry. Annual precipitation was ~60% below the 20 year average and there had been a significant increase in monthly mean temperatures, limiting the possibility of environmental precipitation load as a contributing factor to moisture intrusion.

With the established timeframe and the indication that the failure did not appear to be caused by environmental precipitation, BCRA focused investigative efforts on a unique situation occurring within the warehouse between the freezer wall and the tilt-up wall. Many seals of the component freezer had deteriorated allowing cold air leakage. Formation of ice between the warehouse freezer wall and the tilt-up concrete wall was discovered during site investigation and further investigated during staff interviews. Warehouse staff had, in past years, witnessed the growth of ice in this open cavity reaching to the full height (~20 ft) of the cavity which measure roughly 10 inches wide. Upon investigation of the site in November 2007, BCRA discovered the ice had receded and was present only near the bottom of the cavity. Infrared thermography revealed that the source of this moisture to be the tilt-up wall.

Figure 3.7- Leakage in freezer component seals Figure 3.8- Thermograph of leakage

Figure 3.9- Photograph of cavity space Figure 3.10- Thermograph overlay of cavity space

The ability of the low temperature cavity space to draw moisture through the concrete tilt up wall, storing it in the form of ice is easily documented using hygrothermal modeling software (WUFI). BCRA modeled this exact wall, incorporating specific weather history files from nearby Seattle, WA, showing evidence of how this unique circumstance can produce an unusually large moisture source- building ice during colder years and thawing to produce free water within the building envelope in warm years (Figure 3.11 – 3.12)

BCRA’s investigation yielded the opinion that the source of moisture causing the slab heave was within the warehouse. During the summer 2006 season, the ice that had formed in the cavity between the freezer wall and warehouse concrete tilt-up wall had melted and migrated under the freezer slab. Instead of draining through as intended, this moisture was trapped and frozen, eventually building up enough mass to force the slab to heave upwards in September 2006. It was speculated that for the free water to have frozen in the pea gravel a vent pipe must have become occluded in some way reducing its ability to use ambient air flow to hold the pea gravel above freezing temperatures.

REHABILITATION

In order to solve the slab upheaval while allowing the Central Kitchen to remain fully operational, BCRA recommended the following course of action:

BCRA recognized and conveyed to PSD that the methods recommended had never before been attempted and although extensive calculations supported the method, it could not be guaranteed. Most scrutinized was the proposal to heat and thaw the slab from beneath return it to a usable state, hoping that thaw consolidation would not render the slab unusable and necessitate demolition and removal. However, due to the fact that demolition of the slab would also require thawing of the slab and sub-slab materials PSD authorized BCRA to work to heat the slab from the underside in hopes that it would relax to a usable state.

Figure 4.1- DryAir heat exchange unit Figure 4.2- Monitoring progress of thaw in vent lines

BCRA incorporated the use of a DryAir heat exchanger and blower units to pipe 110°F air into the vent lines and thaw the slab system. BCRA monitored the progress with thermal reading from air as it exited the other end of the vent lines (Figures 4.1 – 4.2) By mid-March 2007, the slab had relaxed to a usable condition. This was easily observed by looking at the freezer double doors- as the heating of the slab progressed the doors began to seat properly in their frame (Figures 4.3 - 4.4). Next, work was initiated to install the permanent glycol system that would maintain the sub-slab materials at temperatures above freezing. Lastly, work was initiated to repair all noted sources of moisture intrusion.

Figure 4.3- Freezer doors out of alignment 1-5-07 Figure 4.4- Freezer doors re-aligned after slab thaw 2-23-07

SUMMARY

Due to the quick identification of the need for high level assessment incorporating building science, PSD was able to save substantial amounts of time and money. PSD is projecting a savings of over $300,000. BCRA was able to rehabilitate the slab without operational loss and provide solutions that prevent future failures of this type. Infrared thermography was incorporated throughout the investigation to assess the building envelope for moisture intrusion sources, survey the air tightness of the component freezer system, and measure temperature during slab heating.

ACKNOWLEDGEMENTS

The author wishes to thank -Puyallup School District staff, namely David Johnson, for providing the opportunity and being so helpful throughout the process -Jeff Schultz, of Schultz and Associates, for expertise on the glycol heating system, and -Larry Best of United Rentals, supplier of the rental equipment.

ABOUT THE AUTHOR

Mr. Durston has over six years of experience in the areas of building science and building forensics investigation. Investigations include expertise in the areas of building structure, building envelope, interior finishes, architectural design, and life safety. His investigation skills include invasive and non-invasive inspection techniques highlighted by a certification in building science thermography. Mr. Durston has had involvement in multiple construction defect litigation cases. His special approach utilizes his skills in science and engineering to define, analyze, and remediate problems or failures in the built environment.