Construction and Full-Scale Testing of Precast Concrete Piles made with Coal Combustion Products
Associate Professor, Dept. of Civil and Env. Engineering
Former Graduate Student, Dept. of Civil and Env. Engineering Southern Illinois University Carbondale, Carbondale, IL 62901
Bernard R. Schmitt
Former Undergraduate Student, Dept. of Civil and Env. Engineering Southern Illinois University Carbondale, Carbondale, IL 62901iversity Carbondale, Carbondale, Illinois 62901-6603
Regional Manager, Loadtest, Inc., 2631-D NW 41st St., Gainesville, FL 3260
The purpose of this investigation was to perform full-scale pile load tests on precast, reinforced concrete piles made with concrete composites having pulverized coal combustion (PCC) bottom ash to demonstrate the utilization of PCC bottom ash in the construction of precast concrete pile foundations. The goals of this investigation were accomplished by performing an extensive laboratory testing on various concrete composites to identify viable concrete composites, and field tests on full-size, reinforced concrete piles made with concrete composites containing PCC bottom ash. Field tests were also performed on piles made from an equivalent conventional concrete as the control mix. The piles were driven to refusal at depths between 18 to 20 feet using a double acting steam hammer. The piles were tested under field loading conditions. Test results presented show that the performance of precast concrete piles constructed using concrete composites was similar to that of piles constructed using conventional concrete.
KEYWORDS:Bottom ash, coal combustion, concrete composites, foundations, precast piles
Mankind has used pile foundations for more than 2000 years. The early piles were always made of wood and thus were limited in length, capacity, and durability. However, in last 200 years, precast concrete and steel piles have become a part of every other construction project in the world. Pile foundations are subjected to compression loads mainly due to dead and live loads from superstructure, and uplift and lateral loads due primarily to wind and earthquake. Pile foundations resist load due to friction between the sides of pile and end bearing at the tip of pile (Bowles, 1996 and Das, 1999). The allowable load that a pile or drilled pier can resist is smaller of the structural capacity of the foundation element, or resistance provided by the soil in which the foundation element is installed.
The most appropriate approach to determine the capacity of deep foundations is to install a full-size prototype pile at the site and load it to failure (Coduto, 1994). However, because of the cost to perform field tests on full-size piles, these tests are not performed routinely. Piles are designed using load test data and design methods available in literature. Therefore, if piles are to be constructed using a new material, availability of data demonstrating their performance, under actual field loading conditions is necessary to develop confidence in the engineering community.
It is relevant and important to note that many of the developing and developed countries use coal to generate energy. Due to readily available supply of coal, it continues to be the dominant fuel source for production of electricity in the United States. Use of coal in generation of electricity has resulted in production and accumulation of large quantities of coal combustion products (CCPs). During 2001, about 900 million metric tons (Mt) of coal was burned, and about 107 Mt of coal combustion products were generated by electric utilities and non-utilities (Kalyoncu 2003). According to Dube (1994), only 10% of the total ash produced was being used in early 90s. In 2001, approximately 33 percent of the total CCPs were used in various applications (Kalyoncu 2003) whereas in 2000 approximately 29 percent (28.59 Mt) of CCPs were used (Kelly and Kalyoncu 2002). The major utilization of CCPs has been in construction-related applications. Within the past 30 years, the concrete industry has given special attention to the safe and economical utilization of CCPs (Helmuth 1987). Current research on the beneficial use of CCPs as building and highway construction materials has identified several promising uses for these materials. In addition to use in concrete, CCPs have been successfully used in the agricultural industry, blasting grit and roofing material, cement clinker raw feed, flowable fill, grout, mineral filler, mining applications, snow and ice control, wallboard, roller compacted concrete, structural fill, embankments, and soil stabilization (Kalyoncu, 2003). Several case histories of utilization of coal combustion products in construction projects are available (ACAA, 2001; GAI, 1988; Golden, 1986; Hosin, 2001; Korcak, 1998; Kumar and Stewart, 2003a and b; Kumar et al., 2003; Kumar and Vaddu, 2003; Lovell et al. 1997; Naik et. al., 1997; Ng, 2001; Schroeder, 1994; Seals et al. 1972; Tikalsky and Carrasquillo, 1989).
It is a well known fact that concrete is very strong in compression compared to soil and therefore, under axial compression loads on piles, soils reach their ultimate capacity much before the concrete. However, due to weak tensile and flexural strength of concrete, uplift and lateral capacity of the concrete piles depends significantly on the material properties of the piles. As discussed earlier, the goal of this study was to construct and perform full-scale pile load tests on precast, reinforced concrete piles made with concrete composites having PCC bottom ash to demonstrate the utilization of PCC bottom ash in the construction of precast concrete pile foundations. The goal of this study was accomplished by conducting a series of laboratory and field tests. In order to identify viable concrete composites, three mixtures containing different matrix constituents and proportions were prepared and tested to determine strength, stiffness, and durability characteristics of the mixtures. Several reinforced concrete piles (precast) were constructed using two concrete composites and an equivalent conventional concrete, and tested to determine their performance when subjected to compression and uplift loads. Test results presented show that field performance of the precast concrete piles constructed using concrete composites containing PCC bottom ash was similar to that of shafts constructed using an equivalent conventional concrete.
SOIL AND GROUNDWATER CONDITIONS
Subsurface investigation at the site consisted of drilling two borings. One of the borings was drilled to a maximum depth of 25.5 feet and the other boring was drilled to a depth of 20.4 feet. The borings were drilled using a truck mounted CME 75 rotary drill. Standard Penetration Tests (SPT's) were performed using an automatic hammer. Split spoon samples and relatively undisturbed Shelby tube samples were obtained at various depths. The site used for conducting the tests is located in the Carterville campus of Southern Illinois University Carbondale (SIUC).
In general, the soil stratigraphy at the site consisted of medium stiff to stiff, brown silty clay to depths of approximately 21 feet. The silty clay is underlain by very stiff to hard, sandy clay shale to the maximum depths explored, i.e., 25.5 feet. Both the borings were terminated at spoon refusal (more than 50 blows required to penetrate first 6 inches of the split spoon). Laboratory testing was performed on the soil samples to estimate pertinent engineering and index properties of the soil. Moisture contents were determined for cohesive samples and Atterberg limit tests were accomplished on selected samples. Unconfined compression tests were performed on selected Shelby tube samples. The results of field and laboratory testing indicate that the compressive strength of the silty clay generally range between 0.75 to 1.75 ton per square foot. The moisture content of the silty clay generally varies between 15 and 29 percent.
Groundwater at the site was measured by installing a piezometer. The groundwater table was observed to be at depths between 4 and 6 ft below the ground surface. An artificial pond existed within 100 feet of the test site. The water level in the pond was observed to be 4 feet below the bank surface which generally corresponded with the groundwater level at the test site. Figure 1 shows the typical subsurface profile at the site.
Figure 1. Typical Soil Profile at the Test Site
CONSTRUCTION OF PRECAST CONCRETE PILES
Based on the results of an extensive laboratory testing, two concrete composites prepared by replacing 100 percent of natural fine aggregate with PCC bottom ash and by replacing 50 percent of natural fine aggregate with PCC bottom ash, and an equivalent conventional concrete (control mix) were selected to construct the piles. Table 1 shows the compressive strength of the concrete composites and an equivalent conventional concrete at various curing ages. Additional results from the laboratory investigation are presented elsewhere (Alarcon, 2002). The piles containing PCC bottom ash and conventional concrete were constructed using the forms available at Egyptian Concrete Co. in Salem, Illinois. All piles were 12 x 12 in. in cross-section and varied in length from 20 to 22 feet. To accomplish the goal of this study, the following full-scale tests were performed on the piles:
Table 1. Compressive Strength (fc’) of Concrete Composites and Control Mix
|Mixture||Curing Age (Days)|
|CM||% with respect to 28 Days||85||100||108||112||117|
|B100||% with respect to 28 Days||69||100||130||139||145|
|B50||% with respect to 28 Days||79||100||118||128||143|
The piles were appropriately reinforced as shown in Figure 2 and instrumented to obtain the performance data during testing. Reinforcement cages for all piles were first prepared and then instrumentation was attached to the reinforcement cages. For O-cell testing of piles, seven-inch diameter O-cells, boxed in steel cases of the same cross-section as that of piles, were used as shown in Figure 3. All piles were instrumented with vibrating wire rebar strain meters (Model 4911). The completed reinforcement cages were placed in the forms and concrete was placed using free fall method. Fresh concrete in the form was appropriately vibrated. After two days, piles were taken out of the forms and stacked. Piles were allowed to cure by placing plastic cover on them. All piles were then transported to the site in Carterville, Illinois.
Figure 2. Sketch Showing Reinforcement Details for Precast Concrete Piles
Figure 3. O-Cell Attached to the Reinforcement Cage
TEST RESULTS AND DISCUSSION
After allowing 90 days for curing of the piles under field environmental conditions similar to those in which commercial piles are cured, the piles were installed at a site in Carterville, Illinois and tested in general compliance with the pertinent ASTM Standards. The site was selected due to its proximity to the coal research laboratory. During testing each loading increment was held constant for eight minutes by manually adjusting the pressure of the loading device. Loads were removed in five decrements, each of which was held constant for four minutes. For conventional axial compression tests resistance was provided by two reaction shafts.
Conventional Axial Compression Tests
A 9-inch O-cell was used to apply the loading increments for the conventional axial compression tests. The applied load was determined from the cell’s pressure versus load calibration. A vibrating wire load cell was also used as a check on the applied load. Reaction was provided by a compound steel beam, set over the test shaft and anchored to adjacent reaction piles on either end. The load was increased until the ultimate strength of the reaction shaft Dywidag bars was reached. Figure 4 shows the axial compression test in progress. Figure 5 shows the top of shaft movements in conventional axial compression tests on piles constructed from concrete composites and conventional concrete. Based on the results presented in Figure 5, it was concluded that the piles constructed with concrete composites provided resistance similar to the piles constructed using conventional concrete, provided the piles made with concrete composites are constructed and installed in a manner similar to the conventional concrete piles.
Figure 4. Axial Compression Test on a Precast Concrete Pile in Progress
Figure 5. Load versus Downward Top of Shaft Movement
O-cell tests were started by pressurizing the O-cell in order to break the tack welds that hold the cell closed (for handling and construction of the shaft) and to form the fracture plane in the concrete surrounding the base of the O-cell. After the break occurred, O-cell was immediately depressurized. O-cell load testing was performed by again pressurizing the O-cell. The applied load was determined from the cell’s pressure versus load calibration. The load was increased until the ultimate capacity of the side shear resistance above the O-cell was reached and/or the maximum stroke of the O-cell was approached. Figure 6 shows the pile pushed out of the ground during an O-cell test.
In the O-cell tests, the upward shaft movement provides information about frictional resistance between the concrete and the surrounding soil whereas the downward O-cell movement only provides information on the capacity of the soil/rock to resist pressure applied. Therefore, only upward shaft movement results are provided. Figure 7 shows response of load versus upward movement at the O-cell level for three piles tested using O-cell. This figure shows that the performance of the piles constructed with concrete composites is similar to that of the piles constructed using conventional concrete
Figure 6. Pile Pushed out of Ground during an O-cell Test
Figure 7. Load versus Upward Movement at O-Cell Level
Conventional Axial Pull-out Tests
A center-hole jack was used to apply the loading increments for the conventional axial pull-out tests. Reaction was provided by a compound steel beam set over the test shaft and resting on wooden dunnage at each end. A Dywidag threadbar was coupled to the main reinforcing of the test shaft, passed through the reaction beam and the center-hole jack, and secured with a 2” thick steel plate and an anchor nut. The applied load was determined from the center hole jack’s pressure versus load calibration. A vibrating wire load cell was also used as a check on the applied load. The load was increased until the ultimate strength of the Dywidag bar was approached. Figure 8 shows the conventional axial pull-out test in progress.
Figure 8. Axial Pull-out Test in Progress
The results of axial pull-out tests on piles constructed from concrete composites and conventional concrete are presented in Figure 9. From this figure it appears that the piles constructed using conventional concrete, CM, and concrete composite, B50, provided higher resistance compared to the pile constructed using concrete composite, B50. It is important to note that piles constructed with concrete composites B100 and B50 could not be driven straight into the ground because of limitations of the driving equipment. This resulted in a loss of frictional resistance between the pile and the surrounding soil. Therefore, it was concluded that if all the piles are constructed and installed in a similar manner, performance of piles constructed with concrete composites and conventional concrete will be similar. The test results from the axial compression and O-cell tests support this argument since results from both of these tests show that the frictional resistance between all piles and surrounding soil was similar.
Figure 9. Load versus Upward Movement in the Conventional Axial Pull-Out Tests
Conventional axial compression and pull-out tests, along with O-cell tests, were performed on piles made from concrete composites containing PCC bottom ash. An equivalent conventional concrete (control mix) was tested in the field to evaluate the performance of concrete composites compared to that of conventional concrete when used to construct pile foundations. Tests performed on precast concrete piles under actual field loading conditions show that the response of piles constructed using the concrete composites was similar to the response of piles constructed using an equivalent conventional concrete, provided they are constructed, placed, and tested in the same manner. The concrete composites studied in this investigation may be used to construct precast reinforced concrete pile foundations without affecting the long-term performance of the foundations.
This project was completed with support, in part by grants made possible by the Illinois Department of Commerce and Community Affairs (IDCCA) through the Office of Coal Development (OCD) and the Illinois Clean Coal Institute (ICCI), Project Number 00-1/3.1B-2. Help received from Dr. Nader Ghafoori and Dr. Vijay K. Puri during testing phase is also acknowledged.
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