In Situ Load Testing of Parking Garage Reinforced Concrete Slabs: Comparison between 24 h and Cyclic Load Testing

Transcription

1 In Situ Load Testing of Parking Garage Reinforced Concrete Slabs: Comparison between 24 h and Cyclic Load Testing Paolo Casadei, S.M.ASCE 1 ; Renato Parretti, A.M.ASCE 2 ; Antonio Nanni, F.ASCE 3 ; and Tom Heinze 4 Abstract: This paper reports on the results obtained on the applicability of the diagnostic cyclic load test method in comparison with the existing 24 h test procedure adopted in ACI-318. A parking garage, owned by St. Louis County, St. Louis, Missouri, scheduled for demolition during the summer of 2002 was used as a research test bed before demolition. This structure, a two-story steel and reinforced concrete (RC) frame with one-way RC slabs built in 1970 s, was ideal, in terms of size and construction system, for performing comparative field experimentation on load testing. Investigation and validity of acceptance criteria of existing and proposed testing methods were performed. Two identical RC slabs were tested, according to both the standard procedure (ACI ) and the proposed diagnostic load testing. In both instances, the applied total test load was such that the slab did not pass the load test. This allowed characterizing the critical test parameters that govern acceptability and draw conclusions on their values. After load testing, both slabs were loaded until partial collapse was reached. This allowed making observations on the margin of safety with respect to collapse, a determination that is not generally possible in a proof test. The analysis of the experimental data provides professionals with evidence on the validity of in situ assessment for the adequacy of structural members. DOI: /(ASCE) (2005)10:1(40) CE Database subject headings: Load tests; Cyclic loads; In situ tests; Slabs; Concrete, reinforced; Assessment; Parking facilities. Introduction A large number of old reinforced concrete (RC) structures in the United States are in a need of structural evaluation to determine if upgrade and renovation are needed for their specific use or for accommodating higher loads due to a change in use. The lack of records, such as as-built drawings, details of structural elements, and properties of materials, often implies assumptions to develop analytical models that may require validation with a proof test. Another field of application of the in situ load testing is the validation of the performance of structural members strengthened with emerging materials where the novelty of the upgrade technique raises doubts in the mind of owners, engineers, and building officials. Experiences in buildings (Bick 1998; Nanni and Gold 1998; Gold and Nanni 1998; Hogue et al. 1999; Nanni and Mettemeyer 2001) and bridges have already shown how a proof test can be helpful to determine the effectiveness of the strengthening. 1 PhD Candidate in Civil Engineering, 220 Engineering Research Laboratory, Univ. of Missouri-Rolla, Rolla, MO Structural Engineer, Co-Force America, Inc., 800 West 14th St., Rolla, MO V&M Jones Professor, Dept. of Civil Engineering, 224 Engineering Research Laboratory, Univ. of Missouri-Rolla, Rolla, MO Project Manager, St. Louis County, Division of Design and Construction, 41 South Central, Clayton, MO Note. Discussion open until July 1, Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on September 1, 2003; approved on December 2, This paper is part of the Practice Periodical on Structural Design and Construction, Vol. 10, No. 1, February 1, ASCE, ISSN / 2005/ /$ Several researchers (FitzSimons and Longinow 1975; RILEM Technical Recommendations 1984; Bungey 1989; Fling et al. 1989; Hall and Tsai 1989; Mettemeyer 1999) have investigated methods for applying test loads and measuring structural response parameters. These investigations have attempted to refine the testing procedures over the years, but the fundamental protocol remains unchanged: First, measurements of the structural response parameters are taken prior to any load application; and, second, the structure is loaded up to a certain level and the measurements are again recorded. There are several acceptance criteria for determining the outcome of the in situ load test. In this paper, the writers present and discuss a case study where two identical real-size RC slabs in a parking garage were tested according to the standard procedure [ACI (ACI 2002)] and the proposed diagnostic cyclic load testing [CIAS Rep. No (CIAS 2001)]. Test loads were set to simulate an increase in load of approximately 40 45% with respect to the original design. Once both the conventional and the diagnostic cyclic load tests were concluded, both members were loaded to failure to establish the margin of safety for the acceptance criteria. Load Test Protocols 24 h Load Test ACI 318 In situ load testing adopted by ACI Ch.20 (ACI 2002) is based on a relatively long-term duration of loading and it is used only to evaluate whether a structure or a portion of a structure satisfies the safety requirements of the code. The load must be arranged to maximize the deflection and stresses in the critical regions of the structural elements to be investigated. The total test load T TL, including the dead load already in place, shall not be less than 40 / PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION ASCE / FEBRUARY 2005

2 T TL = D + 1.7L 1 Once the structure is adequately instrumented, at the locations where the maximum response is expected, initial values of each instrument shall be recorded not more than 1 h before the application of the first load increment. After the test is started, the load must be applied in not less than four approximately equal increments. If the measurements are not recorded continuously, a set of response readings should be registered at each of the four load increments until reaching the total test load and after the test load has been applied on the structure for at least 24 h. Once the aforementioned readings have been taken, the test load must be removed immediately and a set of final readings must be made 24 more hours after the test load is removed. The evaluation of the structure is based on the two different sets of acceptance criteria to certify whether the member tested has passed the load test. On one side was a set of visual parameters, such as no spalling or crushing of compressed concrete and evidence of excessive deflections that would obviously be incompatible with the safety requirements of the structure. On the other hand, the measured maximum deflections shall satisfy one of the following two equations: max 2 t 20,000h rmax max 3 4 It has to be noted that Eq. (2) is especially relevant for very stiff structural members. If the measured maximum and residual deflections do not satisfy Eqs. (2) or (3), it shall be permitted to repeat the load test but not earlier than 72 h after the removal of the first test load as specified in ACI Section (ACI 2002). Even though this load test protocol has been used and become part of engineering practice, there appears to be no rationale or experimental evidence to substantiate its validity. Time constrains and costs can make the use of this test prohibitive. Diagnostic Cyclic Load Test CIAS Report 00-1 Among one of the first applications of the diagnostic cyclic load (DCL) testing was the proof testing of externally bonded fiber reinforced polymer (FRP) laminates to strengthen structural concrete members. From this experience, it becomes apparent that this testing technology could be used as a valid alternative to the 24 h load test protocol since it can overcome the disadvantages of this method. The main difference between the protocol of the DCL test and the 24 h load test is that the load is applied in cycles by the use of hydraulic jacks which are easily controlled by hand or electric pumps assuring that the load could be removed in matter of seconds. Utilizing both increasing loading and unloading cycles up to a predetermined maximum load allow the engineer a safer real-time assessment of member characteristics, such as linearity and repeatability of response, as well as permanency of deformations. The duration of the DCL is considerably reduced from that of the 24 h load test and, consequently, its overall cost is lower. The preliminary steps in planning the load test including preliminary investigation, structural analysis, and load definition are the same as for the 24 h load test. The main difference between these two protocols relies on the test setup configuration and on the procedure by which the load is applied. 2 Test Method There are several ways to provide the reaction to the hydraulic jacks [see CIAS Rep. No (CIAS 2001)]. Each method is different from the other and their individual differences can be seen in the setup time, minimum equipment required, load variation, source of reaction, and finally the limitations that comprise each method. Load Test Procedure The procedure of a cyclic load test consists of the application of concentrated loads in a quasi-static manner to the structural member, in at least six loading/unloading cycles. The number of cycles and the number of steps, as described, should be considered as a minimum requirement [for more details see CIAS Rep. No (CIAS 2001)]: Benchmark: The initial reading of the instrumentation is taken no more than 30 min before beginning the load test and any load being applied. It is shown in Fig. 1 as the constant line beginning at time zero and indicating no load. A: The first load cycle consists of five load steps, each increased by no more than 10% of the total test load expected in the cyclic load test. The load is increased in steps, until the service level of the member is reached, but no more than 50% of the total test load, as shown in Fig. 1. The maximum load level for the cycle should be maintained until the structural response parameters have stabilized. During each unloading phase, a minimum load P min of at least 10% of the total test load should be maintained to keep the test devices engaged. B: A repeat of A, that provides a check of the repeatability of the structural response parameters obtained in the first cycle. s C and D: Load s C and D are identical and achieve a maximum load level that is approximately half-way between the maximum load level achieved in s A and B and 100% of the total test load. The loading procedure is similar to that of Load s A and B. s E and F: The fifth and sixth load cycles, E and F respectively, should be identical, and they should reach the total test load, as shown in Fig. 1. Final : At the conclusion of F, the test load should be decreased to zero, as shown in Fig. 1. A final reading should be taken no sooner than two minutes after the total test load, not including the equipment used to apply the load, has been removed. Fig. 3 illustrates a schematic load versus deflection curve derived from the six cycles mentioned above. Acceptance Criteria Similar to the 24 h load test, for the diagnostic load test, there are acceptance criteria to be checked during and after the load test in order to establish whether the member tested has passed the proof test. The three parameters that have been established [see CIAS Rep. No (CIAS 2001)] to analyze the behavior of a tested structure are the following and all are related to the response of the structure in terms of displacement: Repeatability: It represents the behavior of the structure during two identical loading cycles and by measuring the repeatability of deflections, one is not only monitoring the structure s behavior, but also gaining assurance that the data collected during the test are consistent. PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION ASCE / FEBRUARY 2005 / 41

3 Fig. 1. Load steps and cycles for a cyclic load test It is calculated according to the following equation referring to Fig. 2: Repeatability = B B max r A A 100% max 4 r Experience (Mettemeyer 1999) has showed that a repeatability of greater than 95% is satisfactory. Permanency: It represents the amount of permanent change displayed by any structural response parameter during the second of two identical load cycles. It should be less than 10% (Mettemeyer 1999) and it is computed by the following equation referring to Fig. 2 (for example, during cycle B): Permanency = r B B 100% 5 max If the level of permanency of the second of two repeated cycles is higher than the aforementioned 10%, it may be an indication that the repeated loading has damaged the structural member further and nonlinear effects are taking place. Deviation from linearity: It represents the measure of the nonlinear behavior of a member being tested. As the member becomes increasingly more damaged, its behavior may become more nonlinear, and its deviation from linearity may increase. In order to define deviation from linearity we need first to define linearity. Linearity is the ratio of the slopes of two secant lines intersecting the load deflection envelope. The load deflection envelope is the curve constructed by connecting the points corresponding to only those loads, which are greater than or equal to any previously applied loads, as shown in Fig. 3. The linearity of any point i on the load deflection envelope is the percent ratio of the slope of that point s secant line, expressed by tan i, to the slope of the reference secant line, expressed by tan ref as expressed by Eq. (6): Linearity i = tan i 100% tan ref 6 The deviation from linearity of any point on the load deflection envelope is the compliment of the linearity of that point, as given in the following: Fig. 2. Schematic load versus deflection curve for two cycles Deviation from Linearity i = 100 % Linearity 7 Once the level of load corresponding to the reference load has been achieved, deviation from linearity should be monitored until the conclusion of the cyclic load test. Experience (Mettemeyer 1999) has shown that the values of deviation from linearity, as defined above, are less than 25%. Deviation from linearity may not be useful when testing a member that is expected to behave in an elastic manner and for such members repeatability and perma- 42 / PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION ASCE / FEBRUARY 2005

4 nency, as previously defined, may be better indicators of damage in the tested structure. Case Study Fig. 3. Schematic load versus deflection curve for six cycles Building Geometries and Material Characterization The parking garage was constructed in the early 1950 s, with masonry exterior walls around a concrete-encased steel frame supporting a RC floor system [see Fig. 4(a)]. The floor system consisted of one-way RC slabs supported by steel girders and joists [see Fig. 4(b)]. The parking garage was located in St. Louis, Missouri, and was scheduled for demolition in the summer of No construction or maintenance records were available from the St. Louis County, owner of the garage. A field investigation, based on visual inspection and the use of an electromagnetic rebar locator, showed that the typical RC slab was 14 cm 5.5 in. thick, 512 cm 16.8 ft long, and 255 cm 8.38 ft wide. The main reinforcement consisted of one layer of 12 mm diameter (No. 4) steel bars spaced 30 cm 12 in. center to center at midspan, and 12 mm diameter steel bars spaced 30 cm 12 in. center to center at the support, in correspondence of the steel joists, in the east west direction. In the north south direction, 12 mm diameter steel bars, spaced 45.7 cm 18 in. center to center were used as temperature and shrinkage reinforcement. All six steel bars tested showed an average yield strength of f y =415 N/mm 2 f y Fig. 4. Clayton parking garage PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION ASCE / FEBRUARY 2005 / 43

5 On-Site Safety Safety procedures were adopted for both load tests. The parking garage areas affected by each test were fenced and no one allowed within such areas. Shoring was provided and designed to carry the weight of the portion of slab tested (multiplied by a safety factor equal to 2.0 to account for impact) and the additional weight of the testing equipment. Shoring was not in direct contact with the slab soffitt to allow unobstructed deflection. Fig. 5. Yield lines pattern (U.S. units; 1 in.=2.54 cm.) =60 ksi. Concrete properties were evaluated using six cores taken from different locations in the slab prior to testing and an average concrete cylinder strength of f c =31 N/mm 2 f c =4,500 psi was found. Structural Analysis A detailed structural analysis was performed in order to determine the magnitude of the load to be applied to the structure. The objective was to find the ultimate factored distributed load for which the slab was designed and then back calculate an equivalent point load. A limit design was adopted by adopting the yield line method (MacGregor 1997). Based on the field boundary conditions, the yield line pattern chosen is as shown in Fig. 5. It was found that the new distributed design load for the slab was 5.13 kn/m ksf corresponding to a total concentrated load of 533 kn 120 kip to be applied during the in situ load test. Load Test Setup and Instrumentation The load test was conducted using a closed-loop loading configuration, where no external reaction is required, as shown in Fig. 6. The load was applied in cycles by two hydraulic jacks of 2,200 kn 500 kip each, connected simultaneously to a hydraulic hand pump, that transferred the load to the RC slab in eight points through four spreader steel beams (see Fig. 7), simulating an equivalent uniformaly distributed load. The steel joists, on which the slab rests, supplied the reaction. As the hydraulic jacks extended, they pulled on the high strength steel bars, which lifted the reaction beams below the slab: The reaction beams, two double C cm 10 ft long, were properly designed to carry the load necessary to resists the test load planned. Once the reaction beams came into contact with the steel joists, the resulting reactions were a downward force under each hydraulic jack transferred to the slab by the spreader steel beams. Plywood was placed under the spreader steel beams [see Fig. 7(b)] and between the steel joists and the reaction beams were used to protect the concrete from any localized damage and to avoid steelto-steel contact. The load was measured using only a 890 kn 200 kip load cell on top of one jack since the hydraulic system assured that both jacks applied the same amount of load contemporarily. For this phase, the only preparation work needed con- Fig. 6. Closed-loop test setup 44 / PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION ASCE / FEBRUARY 2005

6 Fig. 7. Test load distribution (U.S. units; 1 in.=2.54 cm.) sisted in drilling the two holes of small diameter [ 5 cm(2 in.)] necessary for letting the high-strength steel bars through the concrete slab. Since the diameter of the bar was small, the contribution of the drilled holes to the behavior of the slab was disregarded. Real-time measurement of structural response was achieved using an electronic data acquisition system acquiring data from ten linear variable differential transducers (LVDTs) all with an accuracy of mm in., used to measure deflection at chosen locations where maximum and critical displacements were expected [see Fig. 8(a)]. Four LVDTs with a ±5.08 cm 2 in. stroke were placed along midspan, where maximum deflections were expected; three LVDTs with a ±2.54 cm 1 in. stroke were placed under the load s patches and the two remaining ±1.27 cm 0.5 in. stroke LVDTs were placed under the steel joist and girder to verify that the no deformation was occurring, proving that all the load was being carried by the slab as carefully shown in Fig. 8(b). Analysis of Load Test Results 24 h Load Test After recording initial readings from each device, the test load was applied in four equal increments of 133 kn 30 kip each, and the structure inspected at each load step. Since the hydraulic jack was controlled by a hand pump, this assured the application of the load at increments of no more than a few hundreds pounds (less than 1 kn per load increment) at a time, allowing the structure enough time to come a steady loaded state after each load increment. Test measurements of the member s responses were recorded 10-min intervals apart. Once the maximum load was achieved, it was kept constant for 24 h and measurements were recorded (see Fig. 9). The structure was then unloaded without removing any of the loading equipment, and the final response was recorded after 24 h as specified by ACI Section 20.4 (ACI 2002). Maximum and residual deflections were obtained from test data as graphically shown in Fig. 10. The load reported in all graphs represents the total load applied by both hydraulic jacks and distributed in the eight patches on top of the slab. To evaluate acceptance criteria according to the 24 h protocol, Eqs. (2) and (3) are evaluated assuming for each variable the following values (U.S. units) as defined by ACI Section (ACI 2002): l t = cm in. h =14cm 5.5 in. obtained from in situ observations and from Fig. 10: max = 4.7 cm 1.85 in. Fig. 8. On site instrumentation (U.S. units; 1 in.=2.54 cm.) PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION ASCE / FEBRUARY 2005 / 45

7 Fig. 9. Load versus time plot for 24 h load test Fig. 11. Load versus time plot for cyclic load test rmax = 1.25 cm 0.49 in. It follows that: 2 l t 20,000 h = in. 2 = cm in. 20, in. 8 Since the maximum vertical deflection max exceeds the value expressed in Eq. (8), according to the protocol, it is necessary to check the recovery deflection and compare it with the following: max 4 = 1.85 in. 4 Also this check is not satisfactory, being = cm 0.87 in. 9 r,max max 10 4 It was opted not to repeat the load test after 72 h as allowed by the code and considering that the structure was not capable to resist the new test load. Diagnostic Cyclic Load Test The slab was tested in a total of six load cycles. The first load cycle consisted of five load steps, each increased by no more than 53.4 kn 12 kip, corresponding to the 10% of the total test load decided and without exceeding 50% of the total test load, kn 60 kip (see Fig. 11). At the end of each load step and once the maximum load level for the cycle was reached, the load was maintained until deflections have stabilized, and at least for 2 min as prescribed. During the unloading phase, the load was held constant at the same load levels as for the loading steps for at least 2 min (see Fig. 11). During the last unloading step of each cycle, the load was maintained to 53 kn 12 kip in order to leave the test devices engaged. The second load cycle was a repeat of the previous one. The third and fourth load cycles, indicated in Fig. 11 with C and D, were performed according to the protocol previously described and achieved a maximum load level of 400 kn 90 kip, following the same loading unloading procedure. The last two cycles, E and F, reached the total test load of 534 kn 120 kip following the same procedure as for the other cycles. The test was performed in a total time of 2 h and 30 min. All three acceptance criteria for the cyclic load test are reported for all cycles in Table 1. The only parameter that does not respect the limits set is Deviation from Linearity, leading to test failure (see Fig. 12). Fig. 13 shows a comparison of load deflection envelopes for the two load tests. When the test load is held for 24 h, the member experiences a larger deflection compared to the one shown in the cyclic load test. Ultimate Collapse Load Analysis Fig. 10. Load versus displacement plot for 24 h load test The slabs were loaded to failure in order to determine their ultimate capacity, and the safety margin between the ultimate load and the maximum test load. Fig. 14 shows the two load deflection envelopes and the last cycle till partial collapse of the member is clearly indicated. As expected both slabs reached the same ultimate load and collapsed with the same failure mode. A safety margin, defined as S % = 1+ P TLoad 18% cyclic P u-failure 100% 11 20% 24 h indicates that the strength reserve of a system that has failed a load test is still appreciable. 46 / PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION ASCE / FEBRUARY 2005

8 Table 1. Cyclic Load Test Acceptance Criteria A B C D E F Allowable values r in. a max in. a Repeatability (%) % 120% Permanency (%) % Linearity (%) % a U.S. units; 1 in. =2.54 cm. Fig. 12. Load versus deflection plot for cyclic load test Fig. 14. Comparison between 24 h and cyclic load test at collapse The analysis of the crack pattern in both members once failed, has also provided the opportunity to validate the yield line pattern chosen for the structural analysis and validated the choice made. Conclusions From comparison of these two protocols and the load test results that both the diagnostic cyclic load test and the 24 h load test yield the same outcome in that two identical RC slabs equally failed the acceptance criteria set under the two protocols when subjected to the same test load. This is very encouraging and it is hoped that similar evidence be gathered in the future. The test to collapse shows that the slabs that fail the load test have still about 20% reserve strength. This type of information is useful and needed to provide engineers a measure of the structures reserve capacity that cannot be determined during a proof test. Acknowledgements Fig. 13. Comparison between 24 h and cyclic load test The writers acknowledge the financial support from the National Science Foundation Industry/University Cooperative Research PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION ASCE / FEBRUARY 2005 / 47

9 Center at the Univ. of Missouri-Rolla and thank the St. Louis County for providing the opportunity for the testing of the structure. Notation The following symbols are used in this paper: D dead loads already in place; max measured maximum deflection; rmax measured maximum residual deflection; h overall thickness of the member (in.); L live loads applied; l t span of the member under load test (in.); P max maximum test load for one cycle; P min minimum test load for one cycle to maintain engaged the testing equipment; P TLoad total test load applied to the member; P u-failure total test load applied at partial collapse of the member; T TL total test load applied during the load test; B max maximum deflection in B under a load of P max ; B r residual deflection after B under a load of A max r A P min ; maximum deflection in A under a load of P max ; and residual deflection after A under a load of P min. References American Concrete Institute (ACI). (2002). Building code requirements for structural concrete and commentary. ACI 318R-02, Farmington Hills, Mich. Bick, R. R. (1998). Ocean Vista power generation station turbine deck rehabilitation project International Concrete Repair Institute Award for Outstanding Concrete Repair Recipient, Southern California Edison, 26. Bungey, J. H. (1989). The testing of concrete in structures, 2nd Ed., Chapman and Hall, New York. Concrete Innovation Appraisal Service (CIAS). (2001). Guidelines for the rapid load testing of concrete structural members. CIAS Rep. No. 00-1, ACI International, Farmington Hills, Mich. FitzSimons, N., and Longinow, A. (1975). Guidance for load tests of buildings J. Struct. Div. ASCE, 101(7), Fling, R. S., McCrate, T. E., and Doncaster, C. W. (1989). Load test compared to earlier structure failure, Concrete International, American Concrete Institute, Vol. 18, No. 11, Gold, W. J., and Nanni, A. (1998). In situ load testing to evaluate new repair techniques. Proc., NIST Workshop on Standards Development for the Use of Fiber Reinforced Polymers for the Rehabilitation of Concrete and Masonry Structures, National Institute of Standards and Technology, Gaithersburg, Md., Hall, W. B., and Tsai, M. (1989). Load testing, structural reliability and test evaluation. Structural safety, Elsevier Science, New York, Vol. 6, Hogue, T., Conforth, R. C., and Nanni, A. (1999). Myriad convention center floor system: Issues and needs. Proc., 4th Int. Symp. on FRP for Reinforcement of Concrete Structures (FRPRCS4). MacGregor, J. G. (1997). Reinforced concrete: Mechanics and design. 3rd Ed., Prentice Hall, Englewood, N.J. Mettemeyer, M. (1999). In situ rapid load testing of concrete structures. MS thesis, Dept. of Civil Engineering, Univ. of Missouri-Rolla, Rolla, Mo. Nanni, A., and Gold, W. J. (1998). Evaluating CFRP strengthening systems in situ. Concrete Repair Bull., International Concrete Repair Institute, 11(1), Nanni, A., and Mettemeyer, M. (2001). Diagnostic load testing of a two-way postensioned concrete slab. Pract. Period. Struct. Des. Constr. 6(2), RILEM Technical Committee 20-TBS. (1984). General recommendation for static loading test of load-bearing concrete structures in situ (TBS2). RILEM Technical Recommendations for the Testing and Use of Construction Materials, E& FNSpon, London, / PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION ASCE / FEBRUARY 2005