Drainage Capabilities of a Nonwoven Fabric Interlayer in an Unbonded Concrete Overlay

Transcription

1 Drainage Capabilities of a Nonwoven Fabric Interlayer in an Unbonded Concrete Overlay Rita E. Lederle* University of Minnesota 500 Pillsbury Drive SE Minneapolis MN Tel: Fax: lede0038@umn.edu Kyle Hoegh University of Minnesota 500 Pillsbury Drive SE Minneapolis MN Tel: Fax: hoeg0021@umn.edu Tom Burnham Minnesota Department of Transportation 1400 Gervais Avenue Maplewood, MN Tel: Fax: tom.burnham@state.mn.us Lev Khazanovich Department of Civil Engineering University of Minnesota 160 Civil Engineering Building 500 Pillsbury Drive S.E. Minneapolis, MN Tel: Fax: khaza001@umn.edu *denotes corresponding author Word count: tables *(250 words) + 6 figures * (250 words) = 5868 Abstract word count: 164

2 Lederle, Hoegh, Burnham, and Khazanovich 1 ABSTRACT Fabric interlayers have been used for the last 30 years in Germany as a bond breaker between cement treated bases and jointed concrete pavements to prevent reflective cracking and increase drainage. Recently, research has begun in the US to determine if fabric interlayers can also be used to the same effect in unbonded concrete overlays. The goal of this research was to determine the drainage capabilities of a fabric interlayer in an unbonded concrete overlay. A test slab was built in the Minne-ALF accelerated load testing facility at the University of Minnesota consisting of a five inch concrete base slab, a fabric interlayer, and a five inch concrete overlay. The transmissivity (flow velocity) and flow rate of water running through the fabric was tested under both static and dynamic conditions. Actuators were used to simulate an 18 kip axel load in the dynamic load case. The fabric interlayer was found to drain well, and exceeded the required transmissivity under both static and dynamic loading.

3 Lederle, Hoegh, Burnham, and Khazanovich 2 INTRODUCTION Fabric interlayers have been used for almost 30 years in Germany to prevent bonding and water ingress between cement treated bases and concrete pavements. Following a scanning study of long-life concrete pavements in Europe by the Federal Highway Administration (FHWA), the American Association of State Highway and Transportation Officials (AASHTO), and the National Cooperative Highway Research Program (NCHRP), this technology was introduced to the United States. A test project using a fabric interlayer between cement treated base and a jointed plain concrete pavement, similar to the designs found in Germany, was constructed in Missouri (1, 2). Participants in the scanning study were inspired by the use of fabric interlayers between concrete pavements and cement treated bases to consider the technology for use in unbonded concrete overlays. A thin asphalt layer is traditionally used in unbonded concrete overlays as a bond breaker and to prevent reflective cracking. However, replacing this asphalt layer with a fabric interlayer could provide added drainage in addition to stress relief. To this end, a test project was constructed in Oklahoma to determine if fabric interlayers can be used as a bond breaker and to prevent reflective cracking in unbonded concrete overlays. While these tests are still ongoing, the initial results have been favorable (1, 2). Given the lack of data on the use of nonwoven fabric interlayers, it was determined that testing should be conducted in Minnesota to determine the feasibility of using nonwoven fabric interlayers in unbonded concrete overlays. To date, there had been no controlled lab studies using a fabric interlayer between a concrete slab and a concrete overlay. The objective of this experiment was to characterize the drainage provided by a non-woven fabric interlayer for unbonded concrete overlay systems. Drainage tests were conducted for both static and dynamic loading conditions. METHODS To test the drainage capabilities of the nonwoven fabric interlayer, a test slab consisting of a base slab and an overlay with a non-woven fabric interlayer were constructed. The slab was instrumented to collect drainage data. Testing Facility The slab was tested using the Minne-ALF testing apparatus, which is an accelerated load testing machine capable of simulating moving traffic loads (3, 4). The wheel load is simulated by the application of a 9 kip unidirectional load with adjacent actuators operating at a frequency of two hertz; an additional1 kip seat (constant) load is maintained throughout the wheel load simulation. The load is applied by two 9 in. by 9 in. plates centered approximately 6.5 inches from the slab edge and 12 in. from each other. A 90 degree phase lag between the actuators is used to simulate wheel movement across the pavement. Figure 1 shows an example wheel load application.

4 Lederle, Hoegh, Burnham, and Khazanovich 3 FIGURE 1 Minne-ALF loading scheme used to simulate an 18 kip single axel load Slab Construction The base slab was cast on the existing foundation of the Minne-ALF, which consists of a 1 inch layer of neoprene, topped by a 9 inch layer of clay loam, topped by a 3 inch layer of class 5 (dense graded aggregate material). The 15 foot by 6 foot base slab was five inches of a 3A41F mix, which is a standard mix used by the Minnesota Department of Transportation (MnDOT). An experienced local contractor formed and cast the slab in accordance with standard field practices. The slab was broom finished and covered with plastic to cure. After curing for 14 days, the slab was loaded with over 7 million ESALS, see TABLE 1 for the exact loading regime. Because the base slab did not break during the initial loading period, a joint was saw-cut in the base slab at mid-span to simulate a crack. The fabric interlayer was placed on top of the base slab before the overlay was cast. The overlay was five inches of a 3A41F mix, which is a standard mix used by MnDOT. The same local contractor formed and cast the slab in accordance with standard field practices. The slab was broom finished and covered with plastic to cure. A core sample taken from the slab shows the base slab, fabric interlayer and the overlay slab, see FIGURE 2. After curing for 14 days, the overlay slab was loaded with over 15 million ESALs, see TABLE 1 for the exact loading regime. TABLE 1 Applied Loads Target Loading Base Slab only Base slab + overlay Simulated 18 kip load 700,000 6,000,000 Simulated 24 kip load 100,000 0 Simulated 30 kip load 800,000 1,200,000 ESALS 7,188,889 15,381,330

5 Lederle, Hoegh, Burnham, and Khazanovich 4 FIGURE 2 Core sample, showing the base slab, fabric interlayer and overlay slab. Drainage Configuration Both slabs were constructed with a 1% slope to encourage drainage in the appropriate direction, and mimic typical installations on existing roadways in Minnesota. To allow the introduction of water for the drainage experiment, half inch inner-diameter schedule 40 PVC pipes were cast in the top slab, extending from the interlayer, through the upper slab, and several inches above the top surface of the slab. Casting the pipes in the slab rather than coring holes later eliminated any potential for damaging the fabric interlayer when coring and ensured that water reached the fabric interlayer easily. The pipes were placed at quarter points along the length of the slab at mid-width, see FIGURE 3 and FIGURE 4. At the end of each pipe in contact with the fabric interlay, a PVC tee connector was attached. The bottom of this tee was cut off to allow a larger area for water to be distributed and to reduce the probability of the water supply pipe clogging. A square of fabric interlayer material was placed on top of this setup, with a hole for the pipe to extend upwards through the overlay slab. A hose was connected to each pipe leading to a water supply tank. Each pipe had its own five gallon supply bucket. The water supply bucket was placed at a height required to provide the necessary head to ensure 2.9 psi of pressure under the static load of only the concrete overlay slab self-weight. Valves on each hose allowed water to be supplied to each pipe independently, and water was supplied to different pipe configurations to test how water flows through the interlayer. A water collection trough was placed at the lower edge of the slab to collect any water which flowed through the interlayer to daylight. FIGURE 4 shows an elevation view through the slab of the water delivery and collection systems, while FIGURE 5 shows the as built configuration.

6 Lederle, Hoegh, Burnham, and Khazanovich West Central East FIGURE 3 Plan view of the drainage test layout 5 PCC 5 PCC Water Inlet pipe Fabric Interlayer Fabric Interlayer Embedded flashing 12 base + subgrade 1% slope Collection gutter FIGURE 4 Elevation of the drainage test layout.

7 Lederle, Hoegh, Burnham, and Khazanovich 6 FIGURE 5 As built drainage test configuration Instrumentation Four moisture sensors were placed downstream of the water inlets. These sensors were in contact with the fabric interlayer and were used to determine when the fabric was saturated and how water flows through the system. When the bottom slab was cast, block-outs were provided for the sensors and the wire. This allowed the sensors to be flush with the surface of the slab and in contact with the fabric interlayer, but not extend above the slab surface, and thus potentially subject to damage. The moisture sensors were placed at the locations shown in FIGURE 3. These locations were selected to be far enough from the actuators so as not to be damaged, but spaced to show the distribution of the water flow. Because each drainage inlet can be supplied with water independently of the others, water can be supplied at only one location and the moisture sensors can be used to determine the lateral extent of the drainage though the fabric interlayer. The sensors used in this test were Decagon, ECH2O-5TE moisture sensors. These capacitance sensors use the electrical capacitance of a medium (the fabric) to suggest the presence of water. According to the manufacturer, moisture contents above 80% cannot be accurately determined with the sensors used. However, a moisture content of above 80%

8 Lederle, Hoegh, Burnham, and Khazanovich 7 indicates that the moisture content of the fabric is above 80%. Therefore, the sensor data can be used to determine when the water reached the sensors, which can in turn be used to determine the transmissivity of the fabric because the distance between the sensors is known. Testing Procedure Two tests were conducted, one with a static load and the other with a dynamic load. The static load consisted of the self-weight of the slab and the pressure head due to the height of the water supply buckets above the slab. The height of the buckets was adjusted so that the total static load on the fabric was 2.9 psi, which is the load at which the fabric specifications are provided. The simulated wheel loads were not used in the static test. The dynamic load included the static load and the simulated 18 kip wheel load described above. Wheel loads were applied at a frequency of two hertz for the duration of the dynamic test. For both the static and dynamic load cases, the testing procedure was the same. Water was introduced to the system through one inlet at a time, while the other two inlets were shut off. The west inlet was tested first, followed by the east inlet and final the central inlet; 30 minutes elapsed between testing of one inlet and the next. The rate of water entering the system was measured manually, while the moisture sensors recorded the movement of water in the fabric with time. RESULTS AND ANALYSIS The moisture sensors were used to determine the velocity of the water, which can be considered as the transmissivity, or permeability, of the fabric, as seen in TABLE 2. TABLE 2 Transmissivity of the Fabric Load case Static Test Dynamic Test Water Inlet west east west east Distance (ft) Time (sec) Velocity (ft/s) Average Velocity (ft/s) of water through the fabric Though there are no formal requirements for the drainage capabilities of the fabric, interim field specifications have been proposed by the scanning tour participants who brought this technology to the United States (2). From these specifications, the required permeability (transmissivity) of the fabric at 2.9 psi is 1.6*10-3 ft/s in the in-plane direction and 3.3*10-3 ft/s in the normal direction. The fabric exceeded the in-plane direction requirement for the static test, which had an induced stress of 2.9 psi from the self-weight of the concrete and the water head. The stress induced by the simulated axle load varies throughout the pavement, depending on the proximity to the actuators. The specification for the permeability of the fabric at a load of 29 psi is 6.6*10-4 ft/s in the in-plane direction and 3.3*10-3 ft/s in the normal direction. Though this load was exceeded during the dynamic test, the fabric was still found to meet the in-plane permeability requirements. By measuring the amount of water put into the system over time, the rate of drainage was calculated, see TABLE 3.

9 Lederle, Hoegh, Burnham, and Khazanovich 8 TABLE 3 Flow Rate of Water in the Fabric Load case Static Test Dynamic Test Water Inlet West East Center West East Center inlet inlet Inlet inlet inlet Inlet Water put though system (gal) Time for water to move through system (min) Flow rate (gal/min) Average flow rate (gal/min) From Table 3, it can be seen that the average flow rate was much faster during the dynamic test. This could be due to the action of the applied load pumping the water through the system faster. The fact that the flow rate was faster in the dynamic test, but the velocity was less is likely due to the fact that the water was flowing through a much larger area in the dynamic test, again likely due to pumping action. The water was introduced into the system in the order: west inlet, east inlet, central inlet, with a minimum time of 30 minutes elapsing between tests. By comparing the flow rate for the different inlets, it can be seen that the flow rate decreases as the fabric becomes more saturated in both the static and dynamic load cases. The outflow of the water from the system could not be accurately measured because the water left the system at all edges of the slab. The collection gutter was only placed downstream because it was not anticipated that the water would run against the slope of the pavement. The fact that the water ran against the slope in both the static and dynamic tests shows that the fabric has a great capacity to move water. More water ran out of the sides of the slab during the dynamic test than during the static test, which shows how the wicking effect of the fabric was exacerbated by the pumping of the load. After the static drainage testing was completed, the moisture sensors continued to collect data to determine how long the fabric remained saturated. The slab was under dynamic load during this time. This information is important because saturated fabric could potentially degrade faster than dry fabric. Alternately, the concrete could remain saturated for long periods of time, which reduces its durability. As was previously discussed, the moisture sensors were not capable of determining the exact moisture content of the fabric, especially at moisture contents above 80%. However, it is possible to determine a trend in the amount of time necessary for the moisture content to decrease substantially. TABLE 4 shows the variation in moisture content of the fabric over time as it dried out after the static load test. FIGURE 6 shows this information graphically. It should be noted that these values are for laboratory conditions, and results should further verified in a field test.

10 Lederle, Hoegh, Burnham, and Khazanovich 9 TABLE 4 Moisture Content of the Fabric Interlayer Over Time Time sensor 1 sensor 2 sensor 3 sensor 4 0 = baseline 21% 21% 38% 26% 6 hrs 64% 64% 74% 61% 12 hrs 61% 63% 73% 59% 1 day 60% 62% 73% 59% 2 days 57% 59% 73% 59% 3 days 53% 59% 72% 56% 5 days 47% 54% 68% 50% 7 days 41% 49% 63% 44% 10 days 36% 43% 59% 39% 15 days 32% 38% 55% 36% 20 days 27% 36% 53% 33% 80% 70% Moisture Content 60% 50% 40% 30% 20% 10% sensor 1 sensor 2 sensor 3 sensor 4 0% Time (days) FIGURE 6 Moisture content of the fabric interlayer with time after the static drainage test Time 0 is right before water was introduced into the system. At this time, the moisture content of the fabric is solely due to the moisture introduced from casting the overlay slab. These moisture contents should be used as a baseline to determine when the fabric interlayer has returned to a dry state. Unfortunately, the dynamic drainage test began 21 days after the static drainage test, which did not allow sufficient time for the moisture contents to return to their pretest baseline levels. However, from FIGURE 6, it can be seen that the rate of drying was progressing quite slowly and it is likely that considerable time would have been required before the moisture contents returned to their baseline levels. Moisture content data was not collected for more than 1 day after the dynamic test, so further analysis of drying time was not possible.

11 Lederle, Hoegh, Burnham, and Khazanovich 10 Forensic Investigation After the drainage test and all loading on the slab were completed, a forensic investigation was conducted. The overlay slab was saw-cut into pieces and carefully removed so that the fabric interlayer could be examined. It was found that there was no significant wear of the fabric in any location. Though it was not worn, the fabric was noticeably compressed towards the middle of the slab and near the actuators. In the areas away from the load, the fabric was approximately ¼ in thick, whereas, in the loaded area the fabric was generally permanently compressed to approximately 3 / 16 in thick in some areas, and to as little as 1 / 8 in thickness in other areas. Due to the large volume of water used in making the saw cuts, the fabric was fairly wet throughout. No determination could be made as to the moisture content of the fabric at the time of overlay slab removal because of the additional water used during the sawing process. The bond between the fabric and the overlay slab was quite strong, and the two could not be separated by pulling. The fabric did not bond to the underlying base slab. No debonding between the fabric and the overlay slab was found in any locations. At the water inlets, two layers of fabric had been placed, one above the water diffuser head and one below. In these locations, only the top layer of fabric bonded with the overlay slab. Once this top layer ended, the bottom layer was again bonded to the overlay slab. In an interesting aside on the performance of the overlay system, it should be noted that no reflective cracking occurred for the five inch overlay slab after more than 15 million ESALS. In further testing, the five inch overlay slab was replaced with a three inch thick overlay. This overlay also used a non-woven fabric interlayer, and again, no reflective cracking was observed. SUMMARY AND CONCLUSTIONS In this experiment, the drainage capabilities of a fabric interlayer were tested in an unbonded concrete overlay. The fabric interlayer was placed between a five inch thick jointed base slab and a five inch thick overlay slab. Testing was conducted to measure flow rate and velocity of water draining through the fabric under both static and dynamic loading conditions. Based on the results of the tests, the following conclusions were drawn: Drainage provided by the fabric interlayer exceeded the in-plane requirements for transmissivity during both static and dynamic loading. The fabric interlayer drained faster during the dynamic test and more slowly during the static test. The rate of drainage of the fabric interlayer decreases as the fabric becomes more saturated, regardless of the loading condition. ACKNOWLDEGEMENTS The authors would like to thank the Minnesota Department of Transportation for their financial support in this project. The technical assistance of Len Palek, John Pantelis, and Steve Olson was greatly appreciated. REFERENCES 1. Garber, S. and Rasmussen, R Nonwoven Geotextile Interlayers in Concrete Pavements. Transportation Research Record, No 2152, Transportation Research Board of the National Academies, Washington D.C., p

12 Lederle, Hoegh, Burnham, and Khazanovich Rasmussen, R. and S. Garber Nonwoven Geotextile Interlayers for Separating Cementitious Pavement Layers: German Practice and U.S. Field Trials. Federal Highway Administration. Washington D.C. 3. Embacher, R. A., M. B. Snyder, and T. D. Odden. Using the Minnesota Accelerated Loading Facility to Test Retrofit Dowel Load Transfer Systems. In Transportation Research Record: Journal of the Transportation Research Board, No. 1769, TRB, National Research Council, Washington, D.C., 2001, pp Khazanovic, L., Yut, I., Tompkins, D., and A. Schultz. Accelerated Loading Testing of Stainless Steel Hollow Tube Dowels. In Transportation Research Record: Journal of the Transportation Research Board, No. 1947, TRB, National Research Council, Washington, D.C., 2006, pp