REMOTE ENVIRONMENTAL DATA ACQUISITION

by PAT MCCORMICK, P.E., S.E.

This is a case study involving the use of a remote data acquisition system to monitor the environment in and around deteriorated precast concrete wall panels that enclose an industrial building to determine the current and potential effect of the environment on those panels.

When it is necessary to determine the effects of environmental conditions on concrete structures, obtaining data over a relatively long period of time can be of significant benefit to the both the Engineer and the building owner- to the engineer because it will be more likely that the design will account for all environmental conditions that effect the structure, making the design more effective – and to the owner because the increased effectiveness of the design by the engineer will result in a better more serviceable building.

BUILDING INVOLVED

The building involved in this case study is part of a paper mill facility. Brander Construction Technology, Inc. was asked to establish the existing condition of the deteriorated concrete wall panels and determine the most cost-effective way of addressing the deterioration.

The building is a four story, cast-in-place reinforced concrete framed building with cast-in-place floor slabs and enclosed with the precast concrete wall panels. The building covers approximately 21,000 square feet in plan. It was built in 1966/1967 to house the bleaching and washing operations at the paper mill. The wall panels are approximately 6′ 9″ high by 20′ 0″ long, 4′ thick and span between adjacent building columns.

When the building was constructed, the pulp washing operations inside the building associated with paper pulp production were uncovered, allowing large amounts of steam and moisture into the interior environment of the building. Since the washing operations involved the use of chlorine as a bleaching agent, this steam and moisture also contained chlorine. Some employees of the mill reported that it was not uncommon to see dense fog inside the building above the pulp washing operations and deterioration of the panels began soon after the building was constructed.

In 1977 hoods were installed over the washing operations and louvers were installed in the walls, which, according to reports from workers, dramatically reduced the amount of moisture in the interior environment of the building. While the hoods and louvers reduced the moisture in the building, because of the environment that existed before the hoods, it was not surprising to discover that the interior surface of the wall panels throughout the building had deteriorated.

At the time of our investigation, approximately 10% of the panels had already been replaced. The problem was, recent construction just outside the building meant the north side of the building was essentially inaccessible by crane, and removal of the north side wall panels from the exterior would be very difficult and costly.

Because of the difficulty and cost associated with panel removal, we were asked to determine if there was a reasonable way to extend the service life of the panels without sacrificing the safety of the personnel or disrupting operations within the building. We felt it was reasonable to see if we could find a way to repair or reinforce the existing wall panels in place, and to then control future deterioration of the panels to extend the service life of those panels.

INVESTIGATION

Observations

Our investigation began with observations of the deteriorated wall panels which revealed a number of interesting conditions:

1. The reinforcing bars at inside surface of the original panels had as little as 1/8″ of concrete cover.

2. Regular floor cleaning operations within the building sprayed water on the lower panels above each floor.

3. Up to 30% of the original reinforcing steel had been lost to corrosion in the most severely deteriorated panels.

4. The deterioration of the panels extended well above the floors of the building, indicating there had been more free moisture available than just that from the floor washing operations.

Core Samples

Core samples were removed from the wall panels and analyzed, with the analysis revealing that the chloride content of the concrete near the interior surfaces of the wall panels was as high as .6% by weight of cement in the concrete, three times the amount recognized as sufficient to initiate corrosion, indicating the corrosion of the reinforcing within the wall panels was due to the presence of chlorides. The analysis also revealed that the amount of chlorides in the concrete decreased significantly at 1″ to 2″ and 2″ to 3″ into the panel, clearly indicating that the source of the chlorides was the interior environment of the building, likely from the washing operations prior to the installation of the hoods.

Structural Analysis

Structural analyses were done to determine if the deteriorated wall panels had sufficient capacity to resist current design loads and to determine if the deteriorated panels could be effectively repaired or reinforced. The results of the analysis indicated that the panels did not have sufficient capacity to adequately resist the design loads, but that the wall panels could be effectively reinforced.

ENVIRONMENTAL TESTS

Mock-Ups

While it was determined that the panels could be adequately reinforced, it was still necessary to find a way to eliminate or significantly reduce the potential for deterioration of the existing panels. This reduction could be achieved by eliminating free moisture at reinforcing bar locations in the wall panels, which would essentially eliminate corrosion of those bars, the primary cause for the deterioration.

There were two potential sources of free moisture available in the building:

condensation

water that was sprayed on the walls during floor cleaning operations

The applied water was one source that we knew we had to deal with, but we had to determine if condensation was a source of free moisture that was causing the corrosion, and, if it was, we had to find a way to effectively prevent that free moisture from reaching the reinforcing bars. To determine if condensation was occurring and to determine the effectiveness of a number of proposed repairs, wall panel test mock-ups were developed in place in the plant to exposed to the same environment as the deteriorated wall panels.

To measure the potential for the presence of moisture at the level of the reinforcing bars in those test mock-ups, humidity and temperature sensors were installed at various locations in and around the mock-ups and a remote data acquisition system was designed and installed to record and transmit readings from those sensors back to our office for analysis.

Sensors were installed in and around a typical deteriorated wall panel to establish the existing conditions within a non-modified wall panel.

WALL SECTION 1

The first mock-up was a section of wall panel with the interior deteriorated surface repaired and with a vapor retarder applied to that interior surface.

WALL SECTION 2

For the second mock-up, an auxiliary reinforced concrete block wall was installed against the inside surface of an existing deteriorated wall panel, and a vapor retarder was then applied to the interior surface of the block wall.

WALL SECTION 3

For the third mock-up, we installed insulation and a metal skin on the outside surface of an existing wall panel.

Once the mock-ups and sensors were installed, the remote data acquisition system was installed and operated continuously to monitor the environmental conditions over a four month period of normal operations within the building. With the continuous recording of data over that four month period, we were able to detect any fluctuations in the environment that could have a bearing on the potential for condensation in the building.

MICROCOMPUTER INSTALLED IN THE FIELD

The remote data acquisition system consisted of a Microcomputer for storing the acquired data, A to D Signal Converters to convert the conditioned sensor signal to a digital format, a 16 character liquid crystal display to allow us to verify in the field that the system was operating correctly, and a standard telephone modem. As you can see here the system was mounted directly on the concrete block of our mock-up and was housed in a waterproof enclosure.

HUMIDITY TEMPERATURE SENSOR INSTALLED IN THE FIELD

The sensors used were common, readily available over-the-counter temperature and relative humidity sensors. The system recorded the Analog output from the sensors in the field at a preset interval and, when contacted by our office based computer, the system transmitted the current data in the memory of the microcomputer in the field to the office based computer. Once the data was transmitted to the office based computer, the data was deleted from the field microcomputer and the cycle was repeated. The sensors were generally read three times a day, and the data was transmitted back to my office once a day.

One point of interest:

Approximately one month into the testing period one temperature/humidity sensor failed and had to be replaced. One of the other engineers in our office replaced the sensor while he was in the area on another project. My description of the location of that sensor was less than ideal, so when he got to what he thought was the failed sensor, he deactivated it. Back at the office I adjusted the data retrieval time and interval, and the readings that were retrieved while he was working on the sensor showed that he was working on the wrong sensor. So with the other engineer on a cellular phone in the field and me on a phone back at my office 1500 miles away, I was able to direct him to the correct sensor and he was able to complete the replacement.

After that field replacement, all the sensors operated properly and data was compiled and graphed to give us representations of the temperature and humidity readings at the various locations in and around the facility and the mock-ups throughout the test period.

Graphs were prepared to show air temperature readings taken inside and outside the building. There were similar fluctuations in the exterior and interior air temperatures, with the interior air temperatures averaging approximately 12 o warmer than the exterior temperatures.

A second graph showed relative humidity readings taken inside and outside the building. Again there are similar fluctuations in the exterior and interior relative humidities, with the relative humidity of the outside air being generally about 15% higher than the relative humidity of the inside air.

A third graph was a typical graph of data collected at a mock-up wall panel. The graph showed the relative humidity of the outside environment, the relative humidity of the inside environment, and the relative humidity at the reinforcing bar level within the wall panel. The relative humidity was generally constant at the reinforcing bar level, never getting much over 50% at that location.

Interpretation of the temperature and relative humidity data revealed that the actual amount of moisture, or grains of moisture per pound of dry air, in the air was generally the same from interior to exterior. The equalization of moisture between the interior and exterior was likely related to the presence of the large open louver at each end of the building that allowed the free flow of air between the interior and the exterior. With the equalization of the moisture contents of the air and the higher temperatures inside the building, condensation on the inside surfaces of the wall panels was unlikely.

At no time in any of the wall panel systems did the relative humidity at the reinforcing bars go above 70%, indicating that condensation was not a major source of free moisture at those locations and probably hadnât been since the hoods were installed over the washing operations and the louvers were installed at each end of the building.

With the data we acquired using the remote data acquisition system to monitor the environment in and around the wall panels, we were able to eliminate condensation as a potential source of free moisture. That allowed us to design repairs and reinforcement for the deteriorated wall panels that addressed the specific environmental factors within the building, minimizing the potential for further deterioration and maximizing the effectiveness of our repairs to extend the service life of the wall panels.

For rehabilitation of existing concrete structures that have deteriorated because of environmental effects, monitoring the environment in and around those structures can be an effective way of determining the environmental design parameters on which to base your repair design. As I said earlier, this type of monitoring can be a benefit to both the engineer and owner by providing better data for more effective designs.