In-Situ Testing of a Carbon/Epoxy IsoTruss Reinforced Concrete Foundation Pile

Transcription

1 Brigham Young University BYU ScholarsArchive All Theses and Dissertations In-Situ Testing of a Carbon/Epoxy IsoTruss Reinforced Concrete Foundation Pile Sarah Richardson Brigham Young University - Provo Follow this and additional works at: Part of the Civil and Environmental Engineering Commons BYU ScholarsArchive Citation Richardson, Sarah, "In-Situ Testing of a Carbon/Epoxy IsoTruss Reinforced Concrete Foundation Pile" (26). All Theses and Dissertations This Thesis is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in All Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact scholarsarchive@byu.edu, ellen_amatangelo@byu.edu.

2 IN-SITU TESTING OF A CARBON/EPOXY ISOTRUSS REINFORCED CONCRETE FOUNDATION PILE by Sarah Richardson A thesis submitted to the faculty of Brigham Young University in partial fulfillment of the requirements for the degree of Master of Science Department of Civil and Environmental Engineering Brigham Young University April 26

3 BRIGHAM YOUNG UNIVERSITY GRADUATE COMMITTEE APPROVAL of a thesis submitted by Sarah Richardson This thesis has been read by each member of the following graduate committee and by majority vote has been found to be satisfactory. Date David W. Jensen, Chair Date Kyle M. Rollins Date Fernando S. Fonseca

4 BRIGHAM YOUNG UNIVERSITY As chair of the candidate s graduate committee, I have read the thesis of Sarah Richardson in its final form and have found that (1) its format, citations, and bibliographical style are consistent and acceptable and fulfill university and department style requirements; (2) its illustrative materials including figures, tables, and charts are in place; and (3) the final manuscript is satisfactory to the graduate committee and is ready for submission to the university library. Date David W. Jensen Chair, Graduate Committee Accepted for the Department E. James Nelson Graduate Coordinator Accepted for the College Alan R. Parkinson Dean, Ira A. Fulton College of Engineering and Technology

5 ABSTRACT IN-SITU TESTING OF A CARBON/EPOXY ISOTRUSS REINFORCED CONCRETE FOUNDATION PILE Sarah Richardson Department of Civil and Environmental Engineering Master of Science This thesis focuses on the field performance of IsoTruss R -reinforced concrete beam columns for use in driven piles. Experimental investigation included one instrumented carbon/epoxy IsoTruss R -reinforced concrete pile (IRC pile) and one instrumented steel-reinforced concrete pile (SRC pile) which were driven into a clay profile at a test site. These two piles, each 3 ft (9 m) in length and 14 in (36 cm) in diameter, were quasi-statically loaded laterally until failure. Behavior was predicted using three different methods: 1) a commercial finite difference-based computer program called Lpile; 2) a Winkler foundation model; and, 3) a simple analysis based on fundamental mechanics of materials principles.

6 Both Lpile and Winkler foundation model predictions concluded that the IRC pile should hold approximately twice the load of the SRC pile. Applying mechanics of materials principles found the predicted stiffness of the piles to be consistent with the laboratory results. Due to unresolveable errors, experimental field test data for the SRC pile is inconclusive. However, analysis predictions in conjunction with field test data for the IRC pile show that the IRC pile should perform similar to predictions and laboratory test results. Therefore, IsoTruss R grid-structures are a suitable alternative to steel as reinforcement in driven piles.

7 Table of Contents List of Tables List of Figures x xi 1 Introduction Brief History of Reinforced-Concrete Driven Piles Introduction to the IsoTruss R IsoTruss R Geometry Benefits of the IsoTruss R In Deep Foundation Piles Description of Research Summary of Pile Design and Fabrication Design of the Pile Reinforcement Fabrication of Reinforced Concrete Piles Pile Properties Summary of Pile Lab Tests Lab Test Description Pile Stiffness Pile Strength Pile Failure Mode vi

8 3.5 Pile Toughness Review of Results Recommendations and Conclusions Field Test Set-Up Test Site Pile Driving Accelerometer Installation Pile Cushions Pile Orientation Data Acquisition Equipment Strain Gages String Potentiometers Inclinometer Load Cell Test Preparation Hydraulic Jack and Extensions Hydraulic Jack Placement Equipment Check Experimental Procedure IsoTruss R Reinforced Concrete Pile Test Steel Reinforced Concrete Pile Test Inclinometer Data Reduction Strain and String Potentiometer Data Reduction vii

9 Data Consolidation Data Reversal Correction Experimental Results Loading Rate Deflection String Potentiometer Inclinometer Strain Analytical Procedure Lpile Program Analysis Soil Properties Input Pile Properties Input Winkler Foundation Model Analysis Application of Mechanics of Materials Cracked Moment of Inertia Pile Moment Capacity Analytical Results Lpile Deflection Predictions Winkler Foundation Model Deflection Predictions Discussion of Results Pile Stiffness Comparison to Lab Stiffness Results Verification of Lab Stiffness Results viii

10 9.2 Deflection Loading Rate Energy Energy-Modified Results Lpile Adjusted Soil Predictions Lpile SRC Pile Adjusted Reinforcement Predictions Error Evaluation Summary Conclusions and Recommendations Conclusions Recommendations References 121 ix

11 List of Tables 2.1 Pile Lengths [1] Pile Geometric Properties Reinforcement Weights [1] Summary of Lab Pile Strength Results for Lab Tests [2] Comparison of Stiffness, Moment, Curvature, Ductility and Toughness of the Lab Piles [2] Inclinometer Readings Taken During Field Testing Example of Inclinometer Data Material Properties Mechanics of Materials Analysis Results Pile Failure Loads Lpile Prediction Notation Comparison of Laboratory Test and Predicted Stiffness Values Original and Adjusted Soil Properties for the Top Two Layers in the Soil Profile x

12 List of Figures 1.1 Applications for Deep Foundation Piles Schematic of an 8-Node IsoTruss R Grid-Structure IsoTruss R End Views: (a) Standard IsoTruss R ; and,(b) IsoTruss R with Rounded Nodes [1] Cutting of First IsoTruss R Structure: (a) As Manufactured; and, (b) As Tested [1] Cutting of Second IsoTruss R Structure: (a) As Manufactured; and, (b) As Tested [1] Steel Extension to IsoTruss R Reinforcement [1] Steel Reinforcement Splice [1] Pile Cross Section Lab Test Pile with Strain Gage Locations Marked [2] SRC Pile Ready to Be Tested in the Laboratory [2] Lab Results for Average Deflections of All Piles [2] Lab Results for Moments vs. Curvature of All Piles [2] Lab Results for Average Moment vs. EI for the IRC and SRC Piles in the Center Region (gages 4-8) [2] Plan View of the Pile Testing Site [2] Pile Cushions Attached with Pieces of the Cardboard Concrete Forms [2] xi

13 4.3 Strain Gage Offset to Intended Line of Force [2] Drawing Showing a Plan View of the Beam, Loads, and Resisting Forces [2] Inclinometer Casing Photo of an Inclinometer Probe Hydraulic Jack, Load Cell, Swivel Head, and Pile Cradle Gap Between Piles and Reaction Load Points Jack Extension Layout Taking Inclinometer Readings for the IRC Pile Diagram of the Angle of Inclination and Related Lateral Deviation Slice of the Top of the Pile Showing the Angle Offset from Line of Load to Inclinometer Readings Comparison of Data: (a) Raw; (b) Consolidated Using the Consolidation Macro; and, (c) Both Comparison of Data: (a) Raw; (b) Adjusted Using the Reversal Macro, and; (c) Both Load vs. Time from Field Tests String Potentiometer Deflection from Field Tests Deflection at point of Load Application based on Inclinometer Readings from Field Tests Deflected Shape of the IRC Pile based on Inclinometer Readings from Field Tests Deflected Shape of the SRC Pile based on Inclinometer Readings from Field Tests Deflected Shape of the IRC and SRC Piles based on Inclinometer Readings from Field Tests Strain vs. Load of the IRC Pile from Field Tests xii

14 6.8 Strain vs. Load of the SRC Pile from Field Tests Strain vs. Load of the IRC and SRC Piles from Field Tests Soil Properties at the Test Site [3] Moment-Stiffness Generated by Lpile given SRC Pile Properties Lab Test Moment vs. Curvature Data Chauvenet s Criterion Envelope for Lab Test SRC Pile 2 Gage Moment vs Stiffness from Laboratory Testing Moment vs. Stiffness curve from Laboratory Testing with Simplified Curve for Lpile Input Elastic Foundation Model: (a) As Loaded; and, (b) Statically Adjusted Load for Winkler Foundation Model Three Displacement Components for Pile Deflection of the Beam due to Rotation at the Ground Surface Shifted Neutral Axis of Cracked Concrete Pile Area of a Circular Segment [4] Stress Distribution in Concrete Compression Region Lpile Prediction 1: Load vs. Deflection of the SRC Pile from the Field Tests Lpile Prediction 2: Load vs. Deflection of the IRC and SRC Piles from Field Tests Lpile Prediction 1 and 2: Load vs. Deflection of the IRC and SRC Piles from Field Tests Winkler Foundation Model Predicted Deflection at Point of Load Application of the IRC Pile from Field Tests Winkler Foundation Model Predicted Deflection at Point of Load Application of the SRC Pile from Field Tests xiii

15 9.1 Deflections of All Piles in Lab Tests Load vs. Deflection based on String Potentiometer Readings from Field Tests String Potentiometer and Inclinometer Tip Deflection Results from Field Tests Load vs. Time from Field Tests Adjusted Load vs. Time from Field Tests Soil Compaction Energy of the IRC and SRC Piles Energy-Modified Load vs. Deflection Data from Field Tests Lpile Deflection Prediction for the SRC Pile Compared to String Potentiometer Deflection Results for the IRC Pile in the Field Lpile Deflection Prediction for the SRC Pile Compared to String Potentiometer Deflection Results for the SRC Pile in the Field Lpile Deflection Prediction for the SRC Pile Compared to Adjusted String Potentiometer Deflection Results for the SRC Pile in the Field Actual Load vs. Deflection Behavior Compared to Lpile Predictions based on Adjusted Soil Properties Actual Deflected Shape of the SRC Pile Compared to Lpile Predictions Based on Original and Adjusted Soil Properties SRC Pile Adjusted Reinforcement Predictions Lpile Deflection Prediction for the SRC Pile Compared to String Potentiometer Deflection Results for the IRC Pile in the Field xiv

16 Chapter 1 Introduction This thesis focuses on the field performance of IsoTruss R grid-reinforced concrete beam columns for use in driven piles. Experimental investigation included one instrumented carbon/epoxy IsoTruss R grid-reinforced concrete (IRC) pile and one instrumented steel-reinforced concrete (SRC) pile which were driven into a clay profile at a test site. These two piles were quasi-statically loaded laterally until failure. Behavior was predicted using three different methods: 1) a commercial finite difference-based computer program called Lpile; 2) a Winkler foundation model; and, 3) a simple analysis based on fundamental mechanics of materials principles. This thesis is the concluding section of a three-part investigation of the suitability of IsoTruss R grid-reinforced concrete columns for use as driven piles. Part one, performed by David McCune [1], included the design and fabrication of the test piles. Part two, performed by Monica Ferrell [2], assessed the strength and stiffness of IsoTruss R grid-reinforced concrete piles through laboratory testing and preliminary field test design. Due to the significance of this research to the 1

17 investigation performed in this thesis, McCune s and Ferrell s work is summarized in Chapters 2 and 3, respectively with some of Ferrell s field test design in Chapter 4. This chapter includes a brief history of reinforced concrete which introduces the reader to previous research and the reasons for conducting further investigation in the area of reinforced concrete. An introduction to driven piles as well as a description of the IsoTruss R grid-structure used as reinforcement is also provided. A description of the research performed for this thesis concludes the chapter. 1.1 Brief History of Reinforced-Concrete In the mid seventeen hundreds, pebbles were added to a cement paste introducing the world to what would become a great power in structural materials, concrete. Concrete underwent another improvement when French gardener, Joseph Monier, added steel wire to his concrete pots. The use of steel in concrete was expanded to rail ties, pipes, floors, arches, and bridges [5]. Today this steel and concrete mixture, known as reinforced concrete, is used in almost every modern structure. Reinforced concrete has allowed engineers to design with the compressive strength of concrete combined with the tensile strength of steel thus making a strong, economic building material. Unfortunately, the addition of steel to concrete was not without flaws. Steel tends to corrode when exposed to water and chemical agents. As a result of this corrosion, the steel reinforcement looses strength and de-bonds from the concrete. 2

18 To increase the life of steel-reinforced concrete structures, fiber-reinforced polymer (FRP) wraps have been researched and implemented in many situations. Research indicates that FRP wraps increase the flexural and shear strength of existing steel-reinforced structures [6, 7]. These FRP wraps have also been found to increase the fatigue life of steel-reinforced concrete structures, which is important in cases of frequent freeze-thaw [8]. Not only are FRP being used for repair, they are also entering the concrete field as a primary reinforcement material that is lighter and more corrosion-resistant than steel with increased stiffness and tensile capacity [9]. However, with these advantages, FRP reinforcement generally has a lower bonding quality than steel and tends to be brittle [1]. Different shapes of FRP-reinforcement have proven to increase the strength and bond characteristics [1, 11]. An improvement to the one-dimensional FRP bars are FRP grids. FRP grids have shown to be both predictable and reliable [12]. The grid allows for a good transfer of load from the concrete to the reinforcement thus making a great alternative to steel as reinforcement in concrete [13, 12]. The IsoTruss R, which is discussed in further detail later in this chapter, is a superior type of FRP grid structure which could prove to be the most innovative improvement concrete has undergone since its invention over a century ago. 3

19 1.2 Driven Piles Pile foundations are long, slender structural elements driven into the soil profile to develop sufficient bearing resistance to support high-rise buildings and bridges. Piles typically consist of timber, steel pipe, or reinforced concrete columns. Piles are becoming more advantageous as America s infrastructure increases in size and diversity. Soils once considered unsuitable for building can be developed with the addition of piles. New buildings are taller and new bridges span greater distances than before and therefore require greater strength from the subsurface materials. Piles can play a key role in providing this strength. Figure 1.1 shows several applications for foundation piles. One application is to transfer loads from weak or active upper layers of soil to stronger, more stable layers of soil and rock found deep in the earth. Piles are also used to resist horizontal loads introduced by earthquakes or strong winds. They can reduce uplift or provide more bearing strength in cases of erosion. Piles therefore resist primarily high bending and compression forces [14]. 1.3 Introduction to the IsoTruss R The IsoTruss R is a composite structural grid built of strong fibers held together by polymer resin. The efficient shape and innovative material of the IsoTruss R make it a strong structure with several benefits for deep foundations. 4

20 Figure 1.1: Applications for Deep Foundation Piles IsoTruss R Geometry The unique geometry of the IsoTruss R gives it incredible strength at very low weights. Loads are carried in the IsoTruss R through two different sets of members. Longitudinal members run parallel to the length of the IsoTruss R and carry most of the compression and tension forces, as well as the bending forces in the structure. A second set of members wraps around the core of the IsoTruss R, crossing the longitudinal members at regular intervals between 3 and 6 degrees relative to the longitudinal axis of the IsoTruss R. These members, called helicals, resist the torsional and shear loads. When not placed in concrete, the helical members also play a critical role by bracing the longitudinal members to decrease their effective length and consequently reduce the onset of buckling. Load is transferred from one member to another through interweaving of the fibers at the intersections. Figure 1-2 shows these two types of members and how they form the 5

21 Figure 1.2: Schematic of an 8-Node IsoTruss R Grid-Structure IsoTruss R grid-structure. The longitudinal members are represented in black and the helical members are represented in gray Benefits of the IsoTruss R In Deep Foundation Piles Traditional foundation piles have been constructed of steel, concrete, and timber. Steel and concrete piles can be very strong but are limited to land applications due to their corrosive nature in water. Timber fares better in water but provides significantly less strength than concrete or steel piles. Even on land, the deterioration of steel reinforcement is a significant problem that has plagued the reinforced concrete industry for decades. This deterioration is becoming an even greater concern as our world s infrastructure is getting older. In 6

22 Corrosion of Steel in Concrete, the author states that: The economic loss and damage caused by the corrosion of steel in concrete makes it arguably the largest single infrastructure problem facing industrialized countries [15]. The IsoTruss R provides a nice solution to the corrosion problem encountered by foundation piles without sacrificing strength. Because of its non-metallic material, the IsoTruss R resists the chemical agents and water that rusts and weakens steel reinforcement. In addition to being non-corrosive, the IsoTruss R is significantly lighter than other building materials. Steel rebar is heavy and therefore more labor is required for its transport and installation. 1.4 Description of Research Research performed for this thesis focused on the field performance of an IsoTruss R reinforced concrete pile. Because the IsoTruss R is an alternative to steel reinforcement, the strength of an IsoTruss R reinforced concrete (IRC) pile was compared to that of a similar steel reinforced concrete (SRC) pile. Both experimental procedure as well as analysis were performed to understand the pile behavior. Experimental testing was performed on two reinforced concrete foundation piles: one with composite reinforcement and the other with similar steel reinforcement. Each pile was 3 ft (9.14m) long and 14 in (35.56 cm) in diameter. After the piles had been driven at the test site, a static lateral load test was 7

23 performed on each pile. The results of these tests were analyzed to compare the flexural strength and stiffness of the piles. Three different methods were used to predict the flexural strength and stiffness of the driven piles. The first method used a commercial software program called Lpile and the second method applied a Winkler elastic foundation model. These approaches were used to predict the flexural strength of the piles. The third method was based on mechanics of materials principles. The third approach included calculations to predict the cracked moment of inertia, stiffness, and bending strength of the pile. Both laboratory test data and material properties were used as input for these analyses. 8

24 Chapter 2 Summary of Pile Design and Fabrication This chapter provides an overview of the design and fabrication process McCune followed to construct the piles studied in this thesis. A more detailed description of the design and manufacturing process is provided in Reference Design of the Pile Reinforcement The process followed to design the IRC and SRC piles focused on creating two separate types of piles which would be comparable in application. Each pile was designed to have the same pile diameter, length, and stiffness. The IRC pile was designed such that it: (1) efficiently held the desired pile loading; (2) met typical pile form dimensions; and, (3) could be easily compared to the steel reinforcing cage. In order to meet these requirements, slight changes were made to the usual IsoTruss R geometry and corresponding equations that describe the modified geometry were developed. 9

25 The overall diameter of the IRC pile was determined by the size of a typical concrete form, 14 in (37 cm). Because composite materials are very corrosion resistant, a 1. in (2.5 cm) cover was used and therefore a 13 in (33 cm) outer diameter was chosen for the IsoTruss R reinforcement. The longitudinal members were designed to match the bending stiffness of the #4 grade 6 steel rebar used in the steel reinforced pile. The number of fibers in the longitudinal members determines the size and stiffness of the longitudinal members. Therefore the fiber number was adjusted until the longitudinal stiffness matched the rebar stiffness. The size of the longitudinal IsoTruss R members is expressed in tows, or bundles of 12, fibers. The final design was determined to be 8 longitudinal members consisting of 133 tows each, for a total member cross-sectional area of.15 in 2 (.97 cm 2 ). The helical IsoTruss R members were designed with respect to the longitudinal IsoTruss R members. Typically, a ratio of the longitudinal members to the helical members for an IsoTruss R of 1 to 2 has 2 3 been used. A ratio of 2 3 was chosen for the piles resulting in a helical design of 89 tows with a cross-sectional area of.1 in 2 (.65 cm 2 ). The most novel change made to the IsoTruss R geometry was the rounding of the usually pointed nodes of the helical members. The change in the IsoTruss R nodes was motivated by a desire to maximize the bending strength of the IsoTruss R reinforcement in the confined geometry. Bending strength is a function of the material properties and moment of inertia. To maximize the moment of inertia, the 1

26 longitudinal members were positioned as far away as possible from the center of the IsoTruss R within the constraints of the pile and IsoTruss R. This was achieved in a volume-constrained application by rounding the nodes of the helical members. Figure 2.1 shows a cross-section of a typical IsoTruss R and the comparative position of the longitudinal members with the new rounded nodes. By moving the longitudinal members further out, the moment of inertia was increased 7%, resulting in a corresponding increase in the bending strength of the IsoTruss R. ( a ) ( b ) Figure 2.1: IsoTruss R End Views: (a) Standard IsoTruss R ; and,(b) IsoTruss R with Rounded Nodes [1] Careful design of the steel reinforcement was important to ensure the SRC piles were comparable to the IRC piles. Eight #4 grade 6 steel bars were chosen for the longitudinal steel reinforcement for two reasons. First, eight bars is consistent with the 8-node design of the IsoTruss R structure. Second, #4 bars 11

27 permit testing with reasonable loads. The final step was to design the transverse reinforcement in the steel pile to be equivalent to the helical members of the IsoTruss R grid-reinforcement. The helical members spiral around the IsoTruss R. Therefore, comparing the composite helicals to the transverse steel reinforcement required estimation of the strength of the helical members in the direction of the transverse steel reinforcement based on the angles that the helical members form with a cross-section of the pile. 2.2 Fabrication of Reinforced Concrete Piles The two piles were fabricated using different processes. The IsoTruss R reinforcement was manufactured from T3C 2NT 12K tow carbon fiber pre-impregnated with TCR UF epoxy resin. Fabrication of the IsoTruss R reinforcement required three main steps. First, the pre-impregnated carbon fiber tows were wrapped around a collapsible form, called a mandrel. Layer upon layer of carbon fiber was wound onto the mandrel in bundles of 4 to 6 tows alternating between helical and longitudinal members in a predetermined pattern. This process formed interwoven joints and continued until the required amount of fiber was placed in each member. Second, the members were consolidated by wrapping Dunston Hi-shrink tape tightly around each member. Finally, the IsoTruss R was cured in a rudimentary plywood oven according to the curing instructions for Thiokol UF resin. 12

28 Figure 2.2: Cutting of First IsoTruss R Structure: (a) As Manufactured; and, (b) As Tested [1] Figure 2.3: Cutting of Second IsoTruss R Structure: (a) As Manufactured; and, (b) As Tested [1] Two 3 ft (9 m) long IsoTruss R structures were manufactured for testing purposes. The first IsoTruss R is shown in Figure 2.2. A short section measuring in (83 cm) was cut from each pile to be used in compression testing for quality control purposes. A longer section measuring 26.9 ft (8.2 m) was cut for the in-situ testing. The second IsoTruss R is shown in Figure 2.3. Two sections measuring ft (8.2 m) were cut for lab bending tests. In addition, small pieces from the second IsoTruss R were tested to assess the local member strength. 13

29 Figure 2.4: Steel Extension to IsoTruss R Reinforcement [1] The IRC pile to be tested in the field was designed to be 3 ft (9 m) long; however, Figure 2.2 shows that in (83 cm) was removed from the end of the pile for compression testing. To compensate for the lost length, a short section of steel cage reinforcement was attached to the end of the pile. Figure 2.4 shows the splice between the IsoTruss R and the steel rebar. The steel reinforcement was constructed according to industry methods. The longitudinal bars were attached to the transverse hoops in an 8-bar pattern. The #4 bars used for the longitudinal reinforcement came in lengths of 2 ft (6 m) and therefore splices were only necessary for the 3 ft (9 m) long in-situ SRC pile reinforcement. Figure 2.5 shows how the splices were alternated each bar so four of the splices were at one end of the pile and the other four were at the other end. Texas Measurements FLA LT strain gages were placed in several locations on the longitudinal members of the IsoTruss R and on the longitudinal 14

30 Figure 2.5: Steel Reinforcement Splice [1] steel reinforcement. A special pipe was inserted in each of the piles in order to take inclinometer readings. The pipe has an outer diameter of 2.75 in. (6.99 cm), and an inner diameter of 2.32 in (5.89 cm). To complete the pile construction, the reinforcements were placed in 14 in (36 cm) diameter Kolumn Forms forms purchased from Caraustar TM. The concrete was placed by Eagle Precast Company. 2.3 Pile Properties Four of the piles were for laboratory testing, two piles with IsoTruss R reinforcement and two with steel reinforcement. Each of the laboratory piles was 13 ft (4 m) in length. Two of the piles, 3 ft (9m) in length, were for field testing. Table 2.1 reports the lengths of each of the piles fabricated. A cross section of the pile is shown in Figure 2.6 and the specific measurements for each pile is shown in Table

31 Table 2.1: Pile Lengths [1] P i l e L e n g t h [ f t ( m ) ] S R C ( 2. 1 ) S R C ( 2. 1 ) I R C ( 2. 3 ) I R C ( 2. 3 ) R p Center Line d 2 d 1 Figure 2.6: Pile Cross Section Something interesting to note is the difference in weight between the IsoTruss R and steel reinforcements, as shown in Table 2.3. For approximately the same length and diameter, the IsoTruss R reinforcement is only about 37% as heavy as the steel reinforcement. 16

32 Table 2.2: Pile Geometric Properties Property IRC Pile SRC Pile Radius of the Pile, R p [in (cm)] Radius of the Reinforcement, r r [in (cm)] Cross Sectional Area of the Reinforcement, A r [in 2 (cm 2 )] Distance from Center to Bottom Layer of Reinforcement, d 1 [in (cm)] Distance from Center to Second Layer of Reinforcement, d 2 [in (cm)] Moment of Inertia of the Longitudinal Reinforcement, I m [in 4 (cm 4 )] 7 (17.8) 7 (17.8).22 (.56).25 (.64).15 (.38).2 (.51) 5.69 (14.5) 4.25 (1.8) 4.2 (1.2) 3.1 (7.6).184 (.77).31 (.12) Table 2.3: Reinforcement Weights [1] Sample Type Reinforcement Pile Lab Field Weight [lb (kg)] Steel 1 97 (44) 2 97 (44) 1 37 (17) IsoTruss 2 37 (17) Steel (14) IsoTruss w/o steel piece 1 76 (34) IsoTruss w/ steel piece 1 11 (5) 17

33 18

34 Chapter 3 Summary of Pile Lab Tests This chapter summarizes the basic testing procedure Ferrell followed with a summary of results obtained from the four pile sections tested in the laboratory. A more detailed description can be found in Reference Lab Test Description Four-point bending tests were performed in the laboratory on two instrumented carbon/epoxy IsoTruss R reinforced concrete piles (IRC piles) and two instrumented steel-reinforced concrete piles (SRC piles). The piles were were loaded to failure while monitoring load, deflection, and strain data. As shown in Figure 3.1, strain gages were located on opposite sides of the reinforcement at nine different locations on the test piles. Figure 3.2 shows one of the SRC piles in the test fixture, ready to be tested. Each of the four piles was tested to failure in the same manner. Lab testing revealed much about the stiffness, load capacity, failure mode, toughness, and ductility of the two piles. Each of these properties is addressed individually in the following sections. 19

35 i n ( c m ) 1 5. i n ( c m ) i n ( c m ) i n ( c m ) i n ( c m ) i n ( c m ) i n ( c m ) i n ( c m ) P i l e Figure 3.1: Lab Test Pile with Strain Gage Locations Marked [2] Figure 3.2: SRC Pile Ready to Be Tested in the Laboratory [2] 2

36 Deflection [cm] Total Transverse Load [kips] S1-SRC L4-SRC L3-SRC L2-SRC L1-SRC C-SRC R1-SRC R2-SRC R3-SRC R4-SRC S2-SRC S1 Load Cell 1 Load Cell 2 S1-IRC L4-IRC L3-IRC L2-IRC L1-IRC C-IRC R1-IRC R2-IRC R3-IRC R4-IRC S2-IRC S Total Transverse Load [kn] L4 L3 L2 L1 C R1 R2 R3 R Deflection [in] Figure 3.3: Lab Results for Average Deflections of All Piles [2] 3.2 Pile Stiffness The steel and IsoTruss R reinforcement were designed to have the same stiffness. Lab testing was useful in verifying the equality of stiffness in the two differently-reinforced piles. The stiffness is represented by the slope of the load vs. deflection curves, shown in Figure 3.3. Both types of piles exhibit similar displacements for the same load level until the steel in the SRC pile begins to yield, leading to eventual failure. Another verification of the pile stiffness was obtained from the strain data gathered. Stiffness can be related to moment, M, and curvature, κ, through the 21

37 following relationship: M = EIκ (3.1) where the product of E (modulus of elasticity) and I (moment of inertia) is stiffness. The moment was easily obtained from statics by multiplying the applied load by the distance to the strain gage locations marked in Figure 3.1. Assuming a linear strain distribution through the thickness (diameter) of the pile, the curvature is a function of the longitudinal strain: κ = ɛ l ɛ u h (3.2) where ɛ u and ɛ l are the strains on the upper and lower reinforcements, respectively, and h is the distance between the two strain gages. This distance was 9. in (23 cm) for the SRC piles and 12. in (31 cm) for the IRC piles. Moment curvature plots were developed for each of the nine locations on both piles. Two specimens of each pile type were tested and therefore averaged plots were made from the two moment vs. curvature plots. These plots are shown in Figure 3.4. As given in Equation 3.1, stiffness is the moment divided by the curvature, or the slope of the moment vs. curvature plot in Figure 3.4. These stiffness values are plotted as a function of moment in Figure

38 2 Curvature [microstrain/cm] Moment [kip-in] SRC 2-SRC 3-SRC 4-SRC 5-SRC 6-SRC 7-SRC 8-SRC 9-SRC 1-IRC 2-IRC 3-IRC 4-IRC 5-IRC 6-IRC 7-IRC 8-IRC 9-IRC Moment [kn-m] 5 S1 Load Cell 1 Load Cell 2 S Curvature [microstrain/in] Figure 3.4: Lab Results for Moments vs. Curvature of All Piles [2] Using a linear regression function in Excel, the average slope of the curves was calculated. The region between curvatures of 1 and 14 micro strain were chosen for these calculations because it is a region just after the initial noise and before yielding of the piles. These slope values were 3.8 kip-in 2 (19 kn-cm 2 ) for the SRC piles and 3.4 kip-in 2 (98 kn-cm 2 ) for the IRC piles. The closeness of the two stiffness values verifies the design objective of similar stiffness values for the two different reinforcement materials. 3.3 Pile Strength Laboratory testing of the IRC and SRC piles showed that the IRC piles held nearly twice the bending moment as the SRC piles at failure. The IRC piles failed 23

39 Moment [kn-cm] S1 Load Cell 1 Load Cell 2 S SRC IRC 4 EI x 1 6 [kip-in 2 ] EI x 1 6 [kn-cm 2 ] Moment [kip-in] Figure 3.5: Lab Results for Average Moment vs. SRC Piles in the Center Region (gages 4-8) [2] EI for the IRC and at an average moment of 1,719 kip-in (194 kn-m) while the SRC piles failed at an average moment of 895 kip-in (11 kn-m). Table 3.3 summarizes the ultimate load held by each of the four piles. Table 3.1: Summary of Lab Pile Strength Results for Lab Tests [2] Specimen SRC Piles Ultimate Load [kips (kn)] IRC Piles (161) 63 (28) (169) 65.4 (291) Average 37.1 (165) 64.2 (286) Standard Deviation 1.2 (5.66) 1.7 (7.78) 24

40 3.4 Pile Failure Mode The failure modes for the two different types of piles were very different. The failures of the SRC piles were ductile, as expected, while the failures of the IRC piles lacked ductility. Figure 3.3 shows that the deflection of the IRC pile increases linearly until failure while the SRC pile yields significantly prior to failure. statement: Ferrell explains the observed physical failure of the piles in the following From the very beginning of load application the IRC piles behaved differently than the SRC piles. At loads where the SRC piles had yielded and were heavily cracked throughout the region between load points, the IRC pile had much smaller deflections, and hence, much smaller hair-line cracks. The IRC pile seemed to be able to take the load much better and maintain its shape until loads much higher than the total capacity of the SRC piles [2]. 3.5 Pile Toughness The energy required to fracture a material is known as the toughness. Toughness is calculated by determining the area under the load vs. deflection curves. As a result of the brittle fracture of the IRC piles, the SRC piles absorbed approximately twice as much total energy as the IRC piles before failure. However, if toughness is calculated at the maximum loads rather than the maximum 25

41 Table 3.2: Comparison of Stiffness, Moment, Curvature, Ductility and Toughness of the Lab Piles [2] Property SRC IRC Flexural Stiffness [kip-in 2 (kn-cm 2 )] 3.8 (19) 3.4 (98) Maximum Moment [kip-in (kn-m)] 895 (11) 1719 (194) Maximum Curvature from Strain Gage [ E/in ( E/cm)] 149 (413) 55 (199) Maximum Curvature from Deflections [ E/in ( E/cm)] 12 (472) 12 (472) Maximum Strain in Reinforcement [ E] 54 (54) 72 (72) Toughness at Maximum Displacement [kip-in (kn-m)] 168 (19) 83 (94) Toughness at Maximum Loads [kip-in (kn-m)] 74 (836) 83 (94) deflections, the toughness of the IRC piles is 83 kip-in (94 kn-cm) while the toughness of the SRC piles is only 74 kip-in (836 kn-cm). This comparison seems more indicative of the pile capacity when considering that piles are designed for a specific load rather than a specific deflection. 3.6 Review of Results toughness. Table 3.6 displays the results for stiffness, moment, curvature, and 3.7 Recommendations and Conclusions Despite the lack of ductility observed in these tests, the IRC piles are nevertheless still suitable for use as pile foundations, due to their substantially greater strength than SRC piles. However, further investigations are recommended 26

42 to improve the ductility of the IRC piles, since ductility has been observed in other IsoTruss R grid-reinforced concrete piles. 27

43 28

44 Chapter 4 Field Test Set-Up Field test set-up, initiated by Ferrell [2] and completed as part of this thesis, included choosing a testing site, driving the piles, ensuring the data acquisition instruments were functioning properly and getting the load from the jack to the pile. Data acquisition tests were conducted before the field tests were performed and pile cradles and jack extensions were fabricated to ensure the load was distributed to the piles effectively. 4.1 Test Site The site chosen for testing the piles had a predominatly clay profile and was located near South Temple in Salt Lake City, Utah. Two freeways pass over the site and a railroad track is located several meters away from the test piles. The site was partially excavated in order to expose an old freeway concrete footing. This footing provided a surface against which the actuator could push to load the IRC pile. A pile made completely of steel was driven and provided a surface against which the actuator could push to test the SRC pile. Careful consideration was taken to ensure 29

45 ' ( 5 m ) L e g e n d S t e e l P i l e S R C P i l e I R C P i l e 7 ' ( 2 m ) 8 ' ( 2. 5 m ) 4 ' ( 1 m ) 7 ' ( 2 m ) 6 ' ( 2 m ) 7 ' 4 " ( 2. 2 m ) 2 4 ' ( 7 m ) 7 ' 1 1 " ( 2. 4 m ) 2 ' 1 1 " (. 1 m ) 1 7 ' ( 5 m ) D 1 4 " ( 3 6 c m ) 2 8 ' ( 9 m ) Figure 4.1: Plan View of the Pile Testing Site [2] that the testing of one pile did not disturb the soil surrounding the other piles. Figure 4.1 shows a plan view of the test site with the piles in place. 4.2 Pile Driving The piles were driven using an A IHC S-7 pile hammer on July 19, 24. The top 2. ft (.6 m) of both piles was left exposed above ground. Two concerns needed to be considered in the pile driving. First, the tops of the piles required protection from the force of the pile driver to avoid chipping the concrete. Second, 3

46 the strain gages in the piles needed to be oriented parallel to the actuator so that proper strain measurements could be recorded Accelerometer Installation An accelerometer and strain gage was attached to measure the acceleration and strain in the piles during driving. The data gathered from the accelerometer and strain gage can be used to estimate the axial capacity of the pile at the end of driving for the piles. Personnel from the Utah Department of Public Transportation performed the installation. The first attempt to install the accelerometer in the steel reinforced pile began at the same location as the strain gages. When this was discovered, the drilling was stopped, the column was rotated 9 degrees, and the installation resumed. Because the initial drilling was not deep, the wires and gages were not likely damaged Pile Cushions Cushions were made out of wooden disks to protect the ends of the piles from the driving hammer. Wedges were attached to hold the disks in place while the piles were being driven. This method proved to be ineffective when the disks shifted, exposing the concrete to the pile hammer. A portion of concrete was chipped from the top of the steel reinforced pile; however, this damage was not sufficient to influence the testing. The disks were better attached using pieces of the concrete forms as shown in Figure 4.2. This method proved to be effective. 31

47 Figure 4.2: Pile Cushions Attached with Pieces of the Cardboard Concrete Forms [2] Pile Orientation The orientation of the piles was critical in ensuring a direct line of action from the load point on the pile to the plane of the strain gages. The driving of the SRC pile was successful in orienting the pile parallel to the actuator s load. However, complications arose when the IRC pile rotated during the driving process leaving the strain gages 16.5 o out of alignment from the desired orientation. This rotation of the pile was large enough that the concrete foundation intended for use as a surface, on which the actuator could push, was no longer in the projected line of the strain gages. Figure 4.3 shows this offset from the projected line of the strain gages to the line of force intended. In order to solve the problem presented when the IRC pile rotated, a beam was connected to the existing concrete foundation thus providing an alternate 32

48 2 ' 1 1 " (. 8 9 m ) 1 3 " ( 3 3 c m ) Figure 4.3: Strain Gage Offset to Intended Line of Force [2] surface for the hydraulic jack to push against. The new surface needed to be oriented at the same angle as the strain gages, 16.5 o. The beam also needed to hold the large moment that would be created by the offset. The beam and connecting bolts were designed to hold the required loading and a small ramp was attached to the beam to provide the necessary angle. Figure 4.4 shows the beam and ramp that was installed at the test site. 4.3 Data Acquisition Equipment This section describes the instrumentation used during the field tests to acquire strain, deflection and load measurements. 33

49 F P P y w P x F s p F b s f P Figure 4.4: Drawing Showing a Plan View of the Beam, Loads, and Resisting Forces [2] Strain Gages Ten TML WFLA-6-11 strain gages were installed on the tension and compression sides of the pile reinforcement. Wires were run from the actual gages, along the reinforcement, and up through the top of the concrete. These bundles of wire were protected in a thick plastic wrapping after fabrication and were not exposed until the day of testing String Potentiometers String potentiometers were placed 6. in (15 cm) from the top of each pile to record tip deflection. The potentiometers were attached to an independent reference frame consisting of a wood beam which was supported outside of the heavily disturbed soil region. 34

50 4.3.3 Inclinometer A Slope Indicator Digitilt R Inclinometer Probe was used to take slope readings throughout the length of the pile. This inclinometer system is composed of four main components: Inclinometer Casing Inclinometer Probe Control Cable Inclinometer Readout Unit The inclinometer casing provides a shaft through which the probe may pass to take slope measurements. An inclinometer casing, shown in Figure 4.5, was placed in the center of both piles. Figure 4.5: Inclinometer Casing 35

51 The inclinometer probe was composed of an aluminum shaft with wheel assemblies at the top and bottom of the shaft. Figure 4.6 shows a photo of the probe. The upper and lower wheel assemblies are tilted to facilitate passage through the casing and to differentiate between positive and negative slope readings. Tilt is measured in the inclinometer probe by two force-balanced servo-accelerometers. One of the accelerometers measures tilt in the plane containing the wheels, the A axis. The other accelerometer measures tilt in the plane perpendicular to the wheels, the B axis. Figure 4.6: Photo of an Inclinometer Probe The control cable is connected to the top of the inclinometer probe to transmit readings to the inclinometer readout unit. Readings were taken at 2 ft (.6 m) intervals in each pile starting at 2 ft (.6 m) down from the top of the pile and ending 2 ft (.6 m) up from the bottom of the pile. Seven and nine sets of readings were taken for the SRC and IRC piles, respectively. Each set of readings includes two slope data readouts for each 2 ft (.6 m) interval. One of the readouts comes 36

52 from the first pass the inclinometer makes down the inclinometer casing. This process was repeated with the inclinometer rotated 18 degrees. In theory, the two passes should yield the same data, although the second data set will have the opposite sign. This practice provides redundancy in the data and eliminates bias in the probe Load Cell An RST Instruments model SG3 3-kip (13 kn) capacity load cell with a tolerance of +/-.1% was used to monitor the load applied to the pile. The load cell can be viewed in Figure 4.7. The center of the applied load was 18 in (46 cm) above the ground surface. Figure 4.7: Hydraulic Jack, Load Cell, Swivel Head, and Pile Cradle 37

53 4.4 Test Preparation Final test preparations included installation of pile cradles and jack extensions to effectively transfer the load to the piles. The data acquisition equipment also underwent final checks before beginning the field tests. In order to test the piles, a flat surface that the hydraulic jack could push against needed to be attached to the pile faces. As shown in Figure 4.7, a cradle was built using 3 4 in (1.9 cm) A36 steel to provide this flat surface for the IRC and SRC piles. An 8 in (2 cm) channel was tack welded onto the solid steel pile to provide its flat surface Hydraulic Jack and Extensions A Power Team 15-ton (13 kn) hydraulic jack, shown in Figure 4.7, was used to apply the load. However, the jack was not capable of extending the entire gap between the piles and their respective reaction load points, extensions were designed to shorten these gaps. The distances between the IRC pile and the SRC pile with their reaction load points measured 68 in (17 cm) and 82 in (21 cm), respectively. The jack itself is 22 in (56 cm) long with an additional 5 in (13 cm) attached load cell. Two extensions were constructed to shorten the rest of the distance shown in Figure 4.8 and provide a reaction for the compressive load. The material available to construct these extensions was 35 ksi (24 kn/cm 2 ), 6 in (15 cm) diameter standard steel pipe. Because the pipe was to be used in 38

54 ( a ) ( b ) Figure 4.8: Gap Between Piles and Reaction Load Points compression, it was analyzed as a column. The compressive strength was calculated for the pipe using a conservative K value of 1 and effective lengths of 29 in (74 cm) and 42 in (11 cm) for the extensions. Table 4-8 of the AISC Manuel of Steel Construction lists a factored compressive strength of 158 kips (73 kn) for the pipe at effective lengths under 6 ft (1.5 m). This capacity was well beyond the anticipated testing load of 5 kips (22 kn) to 6 kips (27 kn) [16]. The next step was to determine the required thickness for the end plates on the jack extensions. The AISC Manual of Steel Construction gives the following 39

55 equation for the minimum base plate thickness, t min : 2Pu t min = l.9f y BN (4.1) where F y is the yield strength; B and N represent the length and the width of the plate, respectively; P u is the ultimate required load; and l is the length of the pipe. A conservative value of 8 kips (936 kn) for P u and a 12 in (31 cm) x 12 in (31 cm) plate yielded a minimum thickness of.66 in (1.68 cm) for the 42 in (11 cm) pipe and.45 in (1.1 cm) for the 29 in (74 in) pipe. In order to accommodate both extensions, a.75 in (1.9 cm) base plate thickness was selected. Figure 4.9 shows the finished layout for the extensions Hydraulic Jack Placement The hydraulic jack could was carefully positioned to ensure a precise load was directed from the jack, through the extension and pile cradle, and onto the pile. The center line of the jack, extension, and cradle was aligned and held in place as the jack was extended enough to wedge all pieces between the pile and the reaction load points. This procedure was followed for each pile before testing began Equipment Check Equipment checks were performed on the strain gages, string potentiometers, and load cells. After the strain gages were connected to the computer input, several of the gages were either dysfunctional (showing very large strain without loading) or nonfunctional (no data entering the computer). To ensure the connection was not to 4

56 ( a ) ( b ) Figure 4.9: Jack Extension Layout blame for the output, each gage connection that did not function properly was rechecked several times and channels were changed until either the gage gave a reasonable readout or the gage was determined to be faulty. 41

57 42

58 Chapter 5 Experimental Procedure Experimenal procedure involved testing one IRC pile and one SRC pile in the field. This chapter includes a description of the field tests as well as the procedure followed for reducing data recorded during these tests. 5.1 IsoTruss R Reinforced Concrete Pile Test Testing of the IRC pile was performed October 4, 24, 77 days after pile driving. Once the loading devices were properly aligned and the strain gages and string potentiometer connected to the computer input, a lateral load was applied. The test was perfomed by applying a load sufficiant to achieve a given deflection target after which this load was held constant for five minutes. Target deflection levels were.5 in (1.3 cm). Inclinometer readings were taken at each target deflection. Figure 5.1 shows the inclinometer readings being taken with one operator lowering the probe and one operator at the readout unit. 43

59 Figure 5.1: Taking Inclinometer Readings for the IRC Pile The process of holding the load constant led to a gradual increase in deflection with time. Therefore, during the time that inclinometer readings were made the load was allowed to decrease somewhat although the pile head deflection remained essentially the same. Because the failure of the IRC pile was abrupt in the laboratory, inclinometer readings were discontinued after the load reached 21 kips (93 kn). This was done to avoid injury to those people right next to the pile taking inclinometer readings in case of sudden failure of the pile. 44