In-Situ Evaluation of Two Concrete Slab Systems.
I: Load Determination and Loading Procedure
Nestore Galati, M.ASCE
1; Antonio Nanni, F.ASCE
2; J. Gustavo Tumialan, M.ASCE
3; and
Paul H. Ziehl, M.ASCE
4Abstract: The primary objective of in-situ load testing is to assess the safety and serviceability of an existing structural system with respect to a particular load effect. At this time, the most appropriate loading level and procedure, as well as the associated evaluation criteria are being reconsidered in light of technological advances in construction methods, analytical tools, and monitoring instrumenta-tion. The in-situ load test method for reinforced concrete systems described in the ACI Building Code Requirements for Structural Concrete, namely the 24–h load test method and its evaluation criteria, has been in use for several decades, but may no longer serve the needs of contemporary construction and engineering practices. As a result, other load test methodologies and associated evaluation criteria are under development. This paper and a companion paper describe the rationale and application of an alternative approach to the determination of load level, loading procedure, instrumentation requirements, evaluation criteria and outcomes for two field projects. The first case study is relative to a posttensioned concrete slab where many areas were characterized by tendon and reinforcement misplace-ment, resulting in inadequate flexural strength and inadequate shear/flexure transfer at column/slab intersections. The second case study is the structural evaluation of a typical floor bay of a two-way reinforced concrete slab system, presenting distributed cracking at the positive and negative moment regions. Finite-element-method models were created for both structures to aid the load test design. The numerical models validated the field observations.
DOI:10.1061/共ASCE兲0887-3828共2008兲22:4共207兲
CE Database subject headings:Concrete structures; Field tests; Instrumentation; Load tests; In situ tests; Concrete slabs.
Introduction
In-situ load testing is relevant for a variety of reasons including assessment of the effect of design and construction omissions and deficiencies; novel strengthening and retrofit methods; capability of an existing structure to carry loads different from the original design; and, safety of structures that have experienced corrosion and degradation. Presently, the default method for in-situ load testing of concrete structures is that prescribed in Chapter 20 of theBuilding Codepublished by the American Concrete Institute 共ACI兲Committee 318共2005兲. This load test method and its evalu-ation criteria are widely referred to as the 24 h load test because the test load is held in place on the structure for a period of 24 h.
There are two drawbacks associated with this approach.
• Recent work conducted by ACI Committee 437 共2007兲 has shown that the existing evaluation criteria of ACI Chapter 20 were developed for simply supported members using working stress design principles and material properties consistent with technology available in the 1920s 关concrete strength in the range of 2,000 psi共13.8 MPa兲and low yield-strength reinforc-ing steel兴. Such criteria are therefore not directly applicable to the majority of modern structural systems and may not be relevant to today’s construction and engineering practices. Further, the load factors and the resistance factors have changed with evolving building codes and minimum design load requirements, whereas a similar evolution has not been reflected in the test load levels and the corresponding evalua-tion criteria.
• The second drawback of the existing load test method is re-lated to feasibility and economics. This arbitrary length of loading time 共24 h兲 is only an apparent simulation of long-term effects and is more probably the consequence of using dead weight 共e.g., sand, cement bags, water兲 as the loading medium. In addition, because residual displacements are to be measured 24 h after the test load has been removed, the total test duration共not including setup兲requires 48 h to complete. Making use of modern equipment, instrumentation, and ana-lytical tools, researchers and practitioners have attempted to de-velop an alternative test method that is more economical and, more importantly, provides much improved information regarding the behavior of the structural system of interest. This alternative method is referred to as the cyclic load test共CLT兲method共Gold and Nanni 1998; Nanni and Gold 1998a,b; Mettemeyer and Nanni 1999; Galati et al. 2004; Casadei et al. 2005; ACI 2007兲. With the 1
Design Engineer, Strengthening Division, Structural Group, Inc., 7455 New Ridge Rd., Ste. T, Hanover, MD 21113. E-mail: ngalati@ structural.net
2
Professor and Chair, Dept. of Civil, Architectural, and Environmental Engineering, Univ. of Miami, 225 MacArthur Engineering Bldg., Coral Gables, FL 33124. E-mail: [email protected]
3
Senior Staff Engineer, Simpson Gumpertz & Heger, Inc., 41 Seyon St., Ste. 500, Waltham, MA 02453. E-mail: [email protected]
4
Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of South Carolina, 300 Main St., Columbia, SC 29208 共corresponding author兲. E-mail: [email protected]
Note. Discussion open until January 1, 2009. 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 pos-sible publication on September 10, 2007; approved on January 15, 2008. This paper is part of theJournal of Performance of Constructed Facili-ties, Vol. 22, No. 4, August 1, 2008. ©ASCE, ISSN 0887-3828/2008/4-207–216/$25.00.
CLT method, the load is typically applied with hydraulic jacks in stepped loading and unloading cycles that increase in magnitude as the test progresses. Deflection, strains and other parameters of interest are recorded continuously during the load test and the structural response is evaluated with criteria that compare the linearity, repeatability, and permanency of the structure. Apart from the use of more modern technology, the novelty of the CLT method is in its ability to query the structure with repeated load cycles rather than a constant load applied for a predetermined length of time. The CLT method also has drawbacks:
• Due to its recent introduction, there is a significant lack of historical data to provide confidence in the method.
• Only on rare occasions, this method has been used on struc-tures up to failure and, therefore, a calibration of the residual strength and evaluation criteria is not readily available. • Comparisons with the 24 h test method are scarce. This is due
to the fact that owners cannot justify the expenses of research and funding agencies perceive this topical area as not suffi-ciently fundamental.
This paper and its companion共Ziehl et al. 2008兲describe the CLT in-situ evaluation of two facilities共a parking garage facility and a library兲in order to introduce principles and outcomes of the load test method in the context of likely projects. These case studies represent an ideal test bed for the CLT procedure because the use of the traditional test method could have posed a safety threat. The first structure was deficient because of misplacement of posttensioning tendons and mild reinforcement, whereas the second facility presented diffuse cracking.
The first paper focuses on the determination of the load level and the loading procedure for each structure. Special consider-ations related to the design and conduct of this type of load test are presented and critically discussed. The companion paper fo-cuses on evaluation criteria and their significance, limitations and applicability.
Research Significance
There is a need for a safe and reliable in-situ evaluation method-ology of concrete structures. The CLT method, which consists of the use of stepped loading and unloading, such that changes in behavior of the structure become the basis for its evaluation, may possess these attributes. With the CLT procedure, there are no requirements for an arbitrary load hold period and measurement of deflection recovery long after the test load has been removed.
The CLT method is also inherently safe as the load is applied progressively and a sudden movement of the structure would cor-respond to an immediate drop in the load as applied by hydraulic jacks. Determination of the test load level and loading procedure are not entirely straightforward and are here exemplified using two case studies recently undertaken.
Background Research
Load testing of concrete structures in the United States is a cen-tury old tradition with one of the earliest well-documented cases to be found in the 1890s 共Birkmire 1894兲. In the early days, in-situ load testing was the most direct proof of performance of proprietary and novel, at the time, construction materials and methods. The American Concrete Institute began formalizing load test procedures for concrete structures in 1920 共ACI 1920兲. At that time, the evaluation criteria for passing the load test focused on maximum deflection under sustained load combined with the recovery of deflection after the test load was removed. Subse-quent codes 共ACI 1936兲 defined the deflection evaluation crite-rion as a function of the span length squared and divided by the total depth of the member cross section. This form of the deflec-tion criterion is still in effect共ACI 2005兲. Notable investigations into load testing of concrete structures documenting the practice of the last decades can be found in the literature共FitzSimons and Longinow 1975; RILEM 1984; Bungey 1989兲.
The cyclic load test method described in this paper is a rela-tively recent development and therefore only a limited number of reported case studies exist 共Gold and Nanni 1998; Nanni and Gold 1998a,b; Mettemeyer and Nanni 1999; Galati et al. 2004; Casadei et al. 2005兲. This method attempts to make use of ad-vances in technology共e.g., equipment, instrumentation and ana-lytical tools兲 to provide a safe and reliable procedure for structural evaluation consistent with contemporary construction and engineering practices and societal needs.
Among the technological developments that may allow a quantum leap in the use of load testing for structural evaluation is acoustic emission共AE兲 共ASTM 2006兲. AE evaluation is a stan-dard practice in other applications, such as composite vessels 共ASME 2004a,b兲. As a passive means of nondestructive evalua-tion, AE requires some form of loading to create a sudden release of energy, such as that caused by crack growth. The transient surface waves that are caused by this sudden release of energy are
Fig. 1.Loading profile for CLT method共ACI 2004兲 Fig. 2.共 Test area and location of loading points—two-way PT slab parking garage兲
detected and characterized with AE共piezoelectric兲 sensors. This monitoring technique happens to be a perfect match for the load-ing sequence of the CLT method. Evaluation of reinforced con-crete共RC兲with acoustic emission has been most widely applied in Japan in combination with a stepped loading procedure. Con-sequently, Japan has developed a recommended practice for AE evaluation of RC structures 关NDIS 2421 共2000兲兴 but without specifying a particular loading protocol.
One very important aspect of acoustic emission evaluation is the observation that upon reloading to a certain level the emission is generally much reduced from that which occurred during the original loading. This effect is known as the Kaiser effect共ASTM 2006兲. At higher levels of damage this effect begins to break down. When significant emission does begin during reloading at a lower level of load than was previously applied the Felicity effect is said to be present 共ASTM 2006兲. Because these effects are dependent upon both load history and load intensity most AE loading procedures use a pattern of loading, unloading, and then reloading either to the same level of load or a slightly lower level of load. This pattern enables a check for the Felicity effect.
Description of Loading Procedures
Two load test procedures are described in the following sections. The first is the 24 h monotonic uniform load test prescribed by ACI 318, whereas the second is the cyclic load test 共ACI 437.1R-07兲.
24-h Load Test
This method is based on a relatively long-term duration of load-ing and it is used to evaluate whether a structure or a portion of a structure satisfies the safety requirements of the code. The total test load is maintained for a period of 24 h. The test load is then removed and a set of final readings is made 24 h after the removal of the test load. As the test load is generally applied similarly to the design load pattern, i.e., in a uniformly distributed manner, certain characteristics of the structure, such as load sharing and fixity of supports, do not need to be fully investigated before the load test begins as the structure will behave as it would under design conditions, and its ability to hold the design load will be determined directly by the load test共ACI 437.1R-07兲. Preliminary calculations are typically done as a rough guide to correctly po-sition the instrumentation where the maximum responses are ex-pected.
The downsides of this method are related to the application of a uniformly distributed load, which can be time consuming and difficult, especially when testing large areas or performing mul-tiple tests within a structure. Also, the test duration is at least two days共24 h at maximum load and 24 h unloaded兲, assuming that retesting is not necessary.
Cyclic Load Test
The load is applied in cycles to discrete areas that have been selected to maximize specific responses investigated in the mem-Table 1.Moment and Shear Demand and Capacity of Structural Members—Two-Way PT Slab共Parking Garage兲
Test label Condition
Mu 关kN m共kip ft兲兴 Mn 关kN m共kip ft兲兴 Vu 关kN共kip兲兴 ⌽Vn 关kN共kip兲兴 Objective
CLT No. 1 Shear collar 140.9共103.6兲
at face of shear collar 171.2共125.9兲 at face of shear collar and 189.3共139.2兲 at face of exist. capital 744.2共167.3兲 at d/2 from face of shear collar 748.6共168.3兲 at d/2 from face of shear collar Evaluate performance of shear collar 共increase of punching
shear strength and reduction of flexural demand兲 Evaluate crack widths at service
CLT No. 1 – 24 h LT Shear collar Compare ACI 437 to
ACI 318 procedure CLT No. 2 CFRP strengthened 231.7共170.4兲 at face of exist. capital 278.1共204.5兲 at face of exist. capital 1,060.0共238.3兲 at d/2 from face of exist. capital 828.3共186.2兲 at d/2 from face of exist. capital Evaluate performance of CFRP strengthening 共increase of flexural and punching shear strengths兲
Table 2.Planned Point Load共PLL兲Values—Two-Way PT Slab共Parking Garage兲
Test label Criteria
Load combination
PLL 关kN共kip兲兴 CLT No. 1 Moment at the outer face of shoring posts共uniform direction兲 ACI 437 1.0DW+ 1.6L 51.2 共11.5兲
ACI 318b 1.15D + 1.5L 57.8 共13.0兲 CLT No. 1 – 24 h LTa Moment at the outer face of shoring posts共uniform direction兲 ACI 318b 1.15D + 1.5L 57.8 共13.0兲
CLT No. 2 Moment at the face of the capital共uniform direction兲 ACI 437 1.0DW+ 1.6L 66.7 共15.0兲
ACI 318b 1.15D + 1.5L 77.8 共17.5兲 ACI 318-Cc 1.2DW+ 1.6L 86.7 共19.5兲 a
Equivalent loadPLLwas kept constant on the structure for 24 h. b
Value under consideration by ACI 318. c
ber. In order to determine the required magnitude, quantity, and location of applied concentrated loads, a thorough understanding of the structure’s characteristics is necessary, including the effects of load sharing and end fixity共ACI 437.1R-07兲, which requires relatively complex modeling.
The CLT typically makes use of hydraulic jacks controlled by hand or electric pumps, assuring that the load can be removed in a matter of seconds. Increasing the loading–unloading cycles up to a predetermined maximum load level allows the engineer a real-time assessment of member characteristics共e.g., linearity and repeatability of response, as well as permanency of deforma-tions兲. The duration of the cyclic load test is a few hours. The procedure of a cyclic load test consists of the application of loads in a quasistatic manner in at least six loading/unloading cycles 共see Fig. 1兲 共ACI 437.1R-07兲. CLT has acceptance criteria to be checked during and after the load test: repeatability, permanency, and deviation from linearity. All are related to the response of the structure and they are described in detail in the companion paper 共Ziehl et al. 2008兲.
Case Histories
The following sections report two case studies on the application of the CLT method to assess the structural performance of two structures presenting different types of deficiencies. For both structures, numerical models based on the finite element method 共FEM兲were developed to predict the intensity of the concentrated forces that when applied to each structure would produce the same effect, in terms of bending moments and/or shear forces, resulting from the target factored, uniformly-distributed load combination.
Two-Way Posttensioned Concrete Slabs in a Parking Garage Structure
Description of the Structure
The garage decks are two-way, posttensioned共PT兲concrete slabs, supported by circular and square columns with capitals. The con-crete slab is mostly 165 mm共6.5 in.兲thick. The original drawings indicated a nominal concrete strength of 28 MPa共4,000 psi兲and minimum steel yield strength of 414 MPa 共60 ksi兲 for the mild reinforcement. The tendons consisted of low relaxation, 1,860 MPa共270 ksi兲, seven-wire strands subjected to an effective stress of 1,213 MPa共176 ksi兲after losses.
The slabs showed cracking on the slab topside, extending be-tween columns. A field investigation revealed that the tendon and mild reinforcement was misplaced at the negative moment re-gions, resulting in inadequate calculated flexural strength and
in-Fig. 3.Modification of the static scheme of the structure due to the installation of the shoring posts共CLT No. 1 and 24 h LT兲—two-way PT slab共parking garage兲
Fig. 4. Schematic of the load tests—two-way PT slab 共parking garage兲
Fig. 5. Loading and measuring equipment—two-way PT slab
adequate calculated shear/flexure transfer at the column/slab intersections. Two areas of a typical slab were load tested to de-termine the effectiveness of the two strengthening techniques se-lected as repair options. For brevity, only the results from one of the areas共shown in Fig. 2兲are described and further information can be found in Galati and Nanni共2006兲. At this location, the first test, labeled as CLT No. 1, was performed to evaluate the effec-tiveness of shear collars in increasing the punching shear strength and, at the same time, reducing the flexural demand by shifting the position of the critical section for bending analysis. Both the CLT and the 24 h techniques were used with this configuration. At the same location, a second test, coded as CLT No. 2, was performed with the purpose of evaluating the use of a carbon fiber reinforced polymer共CFRP兲material system to increase the flex-ural strength and, indirectly, the shear strength as the effective depth of the reinforcement could be increased.
Both load tests were intended for negative moment evaluation in correspondence of column F2 as shown in Fig. 2. The test procedure involved applying concentrated loads at predetermined locations of the floor slab and monitoring of its response in the vicinity of the applied loads.
Load Intensity
ACI 437.1R-07共2007兲recommends that the load intensity as pro-vided in Chapter 20 of 318-05 be redefined. In this instance, as only part of the structure is to be engaged and moment redistri-bution occurs, the test load magnitude, TLM共including dead load already in place兲is the largest of
TLM = 1.3共Dw+Ds兲 共1兲 or
TLM = 1.0Dw+ 1.1Ds+ 1.6L+ 0.5共LrorS兲 共2兲 or
TLM = 1.0Dw+ 1.1Ds+ 1.6共LrorS兲+ 1.0L 共3兲 where Dw⫽dead load due to the self-weight;Ds⫽superimposed dead load;L⫽live loads produced by the use and occupancy of the building not including construction or environmental loads, such as wind load, snow load, rain load, earthquake load, flood load, or superimposed dead loads; Lr⫽roof live load produced during maintenance by workers, equipment, and materials or dur-ing the life of the structure by movable objects such as planters and people; and S⫽snow load. For this building, the superim-posed dead load is equal to zero, as are the snow and roof live loads; therefore, the test load magnitude is given by
TLM = 1.0Dw+ 1.6L= 7.2 kPa共150 psf兲 共4兲 The load was applied at four points distributed around the column of interest as shown in Fig. 2. The intensity of the applied load at each point was determined to produce the same effect in terms of negative moment resulting from the factored, uniformly distributed load defined by Eq.共4兲.
Determination of Equivalent Loads
Numerical models were made to determine the magnitude of the concentrated point loads that would produce the similar bending moment due to the factored uniformly distributed loads 共UDL兲
Fig. 6. DCDT and strain gauge layout—two-way PT slab共parking
at the critical test section. A two-dimensional model was im-plemented using commercial FEM software 共Computers and Structures, Inc. 2000兲. The model consisted of one-dimensional “beam elements” representing columns and a fine mesh of “plate elements” to represent the floor systems. The mate-rial properties of concrete were assumed to be isotropic and li-near elastic using a concrete modulus of elasticity equal to Ec= 57,000
冑
fc⬘⬇24.8 GPa共3.6⫻106psi兲. The presence of a flex-ural crack along line F was modeled by reducing the flexural stiffness of the elements along this line. This assumption was validated with the experimental results collected in the field. The moment demands given in Table 1 were used as a reference for the test setup design. Shoring posts to simulate the presence of shear collars were used for CLT No. 1. After conducting CLT No. 1, the 24 h test was performed for comparison of the two meth-ods. Table 2 summarizes the findings in terms of point loads共PLL兲determined prior to testing. The equivalent point loads for Area 1 in its original configuration are significantly lower than when using the shoring posts. This is because their presence modified the static scheme of the structure共Fig. 3兲by acting as an elastic support next to the locations where the moments were to be cal-culated, resulting in much lower loads for those tests in which they were used.
Load Testing and Measurement Apparatus
Fig. 4 shows an overall schematic of the push-down test. Shoring was installed on one floor above the tested zone to provide con-trast for the hydraulic jacks. Wood bearing pads were used be-tween the spreader beams and the structural floor to protect the concrete from localized damage.
Table 3.Moment Capacity and Demand—Two-Way RC Slab共Building兲 Location Mn 关kN m共kip ft兲兴 Mu 关kN m共kip ft兲兴 Vn 关kN共kip兲兴 Vu 关kN共kip兲兴 Objective
Level B, column strip defined by Line 12 in correspondence of Column H
289.0共212.5兲 287.8共211.6兲 469.7共105.6兲 301.1共67.7兲 Evaluate performance of the slab to negative moments
Table 4.Point Load共PLL兲and Resulting Effects—Two-Way RC Slab共Building兲 Test label PLL 关kN共kip兲兴 Strip width 关m共ft兲兴 Mu,TLM 关kN m共k ft兲兴 Mu,test 关kN m共k ft兲兴 Vu,TLM 关kN共kip兲兴 Vu,test 关kN共kip兲兴 Load Line 1 126.3共28.4兲 3.20共10.5兲 259.2共190.6兲 260.3共191.4兲 285.6共64.2兲 244.2共54.9兲 Load Line 2 125.4共28.2兲 3.20共10.5兲 259.2共190.6兲 260.4共191.5兲 285.6共64.2兲 232.6共52.3兲
Fig. 8.Load cycles for the three tests—two-way PT slab 共parking garage兲
The equipment used consists of four 978.6 kN 共220 kip兲 hydraulic cylinder jacks and pump, direct current differential transducers 共DCDTs兲 for measuring deflections, two load cells 关900 and 1,800 kN 共100 and 200 kip兲兴, and eight R6I acoustic emission sensors 共resonant in the vicinity of 60 kHz兲 共see Fig. 5–7兲. A data acquisition system recorded values at a rate of 1 Hz, displaying the load versus deflection curves in real time. Acoustic emission data were recorded separately with an eight-channel, two-high-speed-board system. Deflection measurements were taken in 14 different locations. The layout of the DCDTs is shown in Fig. 6共a兲. For CLT No. 2, a total of eight strain gauges were also placed on the CFRP reinforcement distributed in corre-spondence with the repaired cracks关Fig. 6共b兲兴.
Because the slab was loaded in negative moment, the AE sensors were mounted on the compression face of the concrete slab 共underside兲 in a grid pattern with outside dimensions of 1.5⫻4.6 m共5⫻15 ft兲centered on Column F2关Figs. 7共a and b兲兴. High vacuum grease was used as couplant and contact was main-tained with specially fabricated magnetic hold-down devices. The evaluation threshold used was 45 dB. The wavespeed in the slab was determined on-site based on time-of-arrival with pencil lead breaks used as simulated acoustic emission sources.
CLT Loading Procedure
Figs. 8共a–c兲show the applied load cycles. The actual load cycles may vary slightly depending on the performance of the system as monitored during the test and the minimum load that has to be maintained to eliminate slack in the system. The applied load cycles do not start from zero to account for the weight of the testing equipment that was measured to be 360 kg 共800 lb兲 per loading point. The 24 h load test profile is shown in Fig. 8共b兲. Two-Way Reinforced Concrete Slab in a Building Structure
This section describes one of two load tests performed on Level “B2” of a library building. The aim of the load test was to assess the structural performance of the two-way RC slab system for the original design loads by monitoring the negative bending moment capacity. The selected area for the test is shown in Fig. 9. Description of the Structure
The structural floor is a two-way slab supported by rectangular columns. The concrete slab is mostly 265 mm 共10.5 in.兲 thick. The material characteristics indicated a nominal concrete strength of 20.7 MPa共3,000 psi兲and minimum steel yield strength of mild reinforcement of 275 MPa 共40 ksi兲. Further information can be found in Galati and Nanni共2007兲.
Fig. 10. Negative moments generated by uniform dead load and concentrated load—two-way RC slab共building兲
Fig. 11.Schematic of the load test—two-way RC slab共building兲
Fig. 12.DCVT layout—two-way RC slab共building兲
Load Intensity
The dead load was determined as the addition of the self-weight of the structure 193 Pa共130 psf兲and a super imposed dead load of 37 Pa共25 psf兲. The live load is 186 Pa共125 psf兲. The TLM as prescribed by ACI 318-05共2005兲was used
0.85共1.4D+ 1.7L兲= 543 MPa共365 psf兲 共5兲 Table 3 lists the values of bending moment and punching shear capacities of the existing slab and the factored forces due to the design loads. These values were determined for a column strip of 3.2 m 共10.5 ft兲 width. In Table 3, Mn and Vn⫽existing slab bending and punching shear capacities and Mu and Vu⫽factored bending and punching shear demands. The capaci-ties were calculated considering the material propercapaci-ties at the time of the construction, without accounting for degradation over time. The reason for the load test was to verify if this as-sumption would hold as diffuse cracking was observed in the entire structure.
Determination of Equivalent Loads
Numerical models were implemented in order to determine the magnitude of the concentrated point loads that would produce the same negative bending moments of the factored UDL at the criti-cal cross section as that caused by the design loads. A two-dimensional model consisting of beam elements representing columns and plate elements representing the slab was created and analyzed with SAP 2000. Table 4 summarizes the findings in terms of required point loads, PLL. The load PLLwas applied at
each of four locations共Fig. 9兲. At each point, the force was dis-tributed over a 300⫻300 mm共12⫻12 in.兲area to avoid punch-ing through the slab. It was not possible to apply the load symmetrically with respect to Column H12 due to the presence of piping. For this reason, different loads were applied at the Load-ing Lines 1 and 2 共Fig. 9兲. Fig. 10 shows the negative moment distribution corresponding to the application of the UDL and to the test loads, respectively.
Equipment and Measurement Apparatus
The load test was performed in a push-down method using a procedure similar to the one used in the parking garage共Fig. 11兲 with shoring installed on one floor above to provide contrast. Deflection measurements were taken with DCDTs mounted on tripods supported on the level below the one being tested 共Fig. 12兲. The wires from the data acquisition system were taken to the DCDTs through the air conditioning ducts. The testing equipment consisted of four 290 kN共66 kip兲hydraulic cylinder jacks and two hydraulic pumps, DCDTs for measuring deflec-tions, and two load cells 关450 and 900 kN共50 and 100 kip, re-spectively兲兴 共Fig. 13兲. A data acquisition system was set to record data at a rate of 3 Hz from all devices, providing real-time display of collected data as well as the load versus deflection curves. The acoustic emission data acquisition system was the same as that used for the parking garage. Six sensors were distributed on a grid Table 5.Experimental and Analytical Results CLT No. 1共DS10兲 —Two-Way PT Slab共Parking Garage兲
Load cycle ⌬max 关mm共in.兲兴 ⌬max 关mm共in.兲兴 共FEM兲 1 1.735 共0.0683兲 1.859 共0.0732兲 2 1.770 共0.0697兲 1.859 共0.0732兲 3 2.626 共0.1034兲 2.606 共0.1026兲 4 2.657 共0.1046兲 2.606 共0.1026兲 5 3.426 共0.1349兲 3.553 共0.1320兲 6 3.485 共0.1372兲 3.553 共0.1320兲 7 3.995 共0.1573兲 4.100 共0.1614兲 8 3.975 共0.1565兲 4.100 共0.1614兲
Fig. 14.AE sensor layout—two-way RC slab共building兲
with dimensions of 1.2⫻2.4 m共4⫻8 ft兲 centered on Column H-12 共Fig. 14兲. The sensor coupling devices and evaluation threshold were as described previously.
CLT Loading Procedure
The testing procedure was conceptually similar to the one used for the parking garage. Once all instruments were connected, a preload was applied to seat the components and to eliminate slack in the system. Following that, the slab was loaded in eight cycles. Four steps for loading followed by two steps for unloading were used. Each load step was maintained for at least 2 min. Fig. 15 shows the applied load cycles for the tested areas. The maximum load reached in the first two cycles corresponds to the service load level. The maximum load reached in Cycles 5 and 6 corresponds to the load combination suggested by ACI 437-06, whereas the last two cycles corresponded to the load level prescribed by ACI 318-05 Chapter 20共2005兲. The applied load cycles do not start from zero to account for the weight of the testing equipment that was measured to be 3.5 kN共800 lb兲per loading point.
Model Validation
Good agreement between experimental and analytical results at the service load level stages was observed in all cases.
Two-Way PT Concrete Slab
Table 5 shows a comparison between the experimental and the theoretical results at the most demanding location 共DS10兲 for CLT No. 1. The model fits well the experimental results for CLT No. 1 as the behavior of the structure was elastic with no residual deflection measured when the load was removed. In fact, no new cracks were observed when performing the cyclic load test; the effect of the applied loads was mostly to increase the size of the existing cracks.
For CLT No. 2 the preexisting crack was repaired 共filled by gravity feed of epoxy兲 before installing the FRP strengthening and, therefore, two models were considered: cracked and un-cracked. Table 6 shows a comparison between numerical and ex-perimental results at the location monitored by DCDT DS10 for CLT No. 2. These results indicate that deflection predictions for the uncracked slab condition matches well the first four cycles. The measured deflection is closer to the calculated deflection based on a cracked slab condition, accounting for permanency, for the later cycles共Fig. 16兲.
Two-Way RC Slab
Table 7 compares the FEM predictions with the experimental re-sults recorded during the load test. The cracking of the slab was introduced in the model as a smeared cracking as the location of the existing cracks and crack formation was not known. These results indicate that deflection prediction for the uncracked slab condition matches the first two cycles, whereas the measured de-flection is closer to the calculated dede-flection based on a cracked Table 6.Experimental and Analytical Results CLT No. 2共DS10兲—Two-Way PT Slab共Parking Garage兲
Load cycle ⌬max 关mm共in.兲兴 ⌬max关mm共in.兲兴 共FEM兲uncracked ⌬max关mm共in.兲兴 共FEM兲cracked ⌬max关mm共in.兲兴 共FEM兲cracked+ permanency
1 3.551 共0.1398兲 3.503 共0.1379兲 5.865 共0.2309兲 5.865 共0.2309兲 2 3.561 共0.1402兲 3.503 共0.1379兲 5.865 共0.2309兲 5.865 共0.2309兲 3 5.751 共0.2264兲 4.704 共0.1852兲 7.877 共0.3101兲 7.899 共0.3110兲 4 5.773 共0.2273兲 4.704 共0.1852兲 7.877 共0.3101兲 7.899 共0.3110兲 5 9.365 共0.3687兲 6.287 共0.2475兲 10.526 共0.4144兲 10.526 共0.4144兲 6 9.903 共0.3899兲 6.287 共0.2475兲 10.526 共0.4144兲 11.064 共0.4356兲 7 13.005 共0.5120兲 7.447 共0.2932兲 12.466 共0.4908兲 13.005 共0.5120兲 8 13.391 共0.5272兲 7.447 共0.2932兲 12.466 共0.4908兲 13.391 共0.5272兲 9 16.787 共0.6609兲 8.288 共0.3263兲 13.876 共0.5463兲 16.607 共0.6538兲 10 17.760 共0.6992兲 8.291 共0.3264兲 13.879 共0.54646兲 17.579 共0.6921兲
Fig. 16.Cycles deflection diagram for CLT No. 2—two-way PT slab
共parking garage兲
Table 7. Experimental and Analytical Results 共DS12兲—Two-Way RC Slab共Building兲 Load cycle ⌬max 关mm共in.兲兴 ⌬max关mm共in.兲兴 共FEM兲uncracked ⌬max关mm共in.兲兴 共FEM兲cracked 1 0.836 共0.0329兲 0.838 共0.0330兲 1.224 共0.0482兲 2 0.869 共0.0342兲 0.838 共0.0330兲 1.224 共0.0482兲 3 1.196 共0.0471兲 1.054 共0.0415兲 1.542 共0.0607兲 4 1.237 共0.0487兲 1.054 共0.0415兲 1.542 共0.0607兲 5 1.656 共0.0652兲 1.247 共0.0491兲 1.824 共0.0718兲 6 1.699 共0.0669兲 1.247 共0.0491兲 1.824 共0.0718兲 7 1.984 共0.0781兲 1.389 共0.0547兲 2.035 共0.0801兲 8 2.080 共0.0819兲 1.389 共0.0547兲 2.035 共0.0801兲
slab condition for the last two cycles共Fig. 17兲. The slab started to correlate better with the theoretical cracked slab behavior at the third cycle. The effect of the permanency was not considered as it was negligible compared to the recorded deflections.
Conclusions
This paper presented the loading procedure to conduct a CLT in-situ evaluation of two facilities: a posttensioned parking garage facility and a library. For both structures, the load test was con-ducted without disrupting the normal operation of the facilities.
FEM models were created for both structures and were well correlated with experimental observations. The selection of test load magnitude and the use of point loads to simulate distributed loading effects are described in detail.
Both case studies represented the ideal test bed for the CLT procedure due to its inherent safety. In fact, both structures, one deficient due to misplacement of posttensioning tendons and the other with diffuse cracking, could have presented safety hazards.
Acknowledgments
The ACI Concrete Research Council, the NSF Industry/ University Cooperative Research Center on Repair of Buildings and Bridges with Composites and the UMR–University Transpor-tation Center on Advanced Materials are gratefully acknowledged for their financial support to the research. Yizhuo Chen assisted with the load tests and data reduction and his assistance is greatly appreciated.
References
American Concrete Institute 共ACI兲. 共1920兲. “Standard building regula-tions for the use of reinforced concrete,”Standard Specification No. 23, Detroit.
American Concrete Institute共ACI兲.共1936兲. “Building code regulations for reinforced concrete.”ACI 501-36T, Detroit.
American Concrete Institute 共ACI兲. 共2004兲. “Strength evaluation of existing concrete buildings.” ACI 437R-03, ACI Committee 437, Farmington Hills, Mich.
American Concrete Institute共ACI兲.共2005兲. “Building code requirements for structural concrete.”ACI 318–05, Farmington Hills, Mich. American Concrete Institute共ACI兲.共2007兲. “Test load magnitude,
proto-col and acceptance criteria.” ACI 437.1R-07, ACI Committee 437, Farmington Hills, Mich.
ASME.共2004a兲. “Section V—Nondestructive examination.” Boiler and pressure vessel code, New York.
ASME.共2004b兲. “Section X—Fiber-reinforced plastic pressure vessels.” Boiler and Pressure Vessel Code, New York.
ASTM.共2006兲. “Standard terminology for nondestructive examinations.” ASTM E 1316 06a, West Conshohocken, Pa.
Birkmire, W. 共1894兲. Skeleton construction in buildings, Wiley, New York.
Bungey, J.共1989兲.The testing of concrete in structures, 2nd Ed., Chap-man and Hall, New York.
Casadei, P., Parretti, R. Nanni, A. and Heinze, T.共2005兲, “In-situ load testing of parking garage RC slabs: Comparison between 24-h and cyclic load testing.”Pract. Period. Struct. Des. Constr., 10共1兲, 40–48. Computers and Structures, Inc.共2000兲. SAP 2000, Version II, Berkeley,
Calif.
FitzSimons, N., and Longinow, A. 共1975兲. “Guidance for load tests of buildings.”J. Struct. Div., 101共7兲, 1367–1380.
Galati, N., Casadei, P., Lopez, A., and Nanni, A. 共2004兲. “Load test evaluation of Augspurger ramp parking garage, Buffalo, N.Y.”Rep. No. 04-50, Center for Infrastructure Engineering Studies, Univ. of Missouri-Rolla, Rolla, Mo.
Galati, N., and Nanni, A. 共2006兲. “Load testing of two post-tensioned concrete slabs at garage A. Windsor on the Plaza, Kansas City, Mo.” Final Rep., Prepared for Simpson Gumpertz & Heger Inc., Univ. of Missouri-Rolla, Rolla, Mo.
Galati, N., and Nanni, A.共2007兲. “In-situ structural evaluation of a two-way RC slab at the National Institute of Health, Bethesda, Maryland.” Final Rep., Prepared for Simpson Gumpertz & Heger Inc., Univ. of Missouri-Rolla, Rolla, Mo.
Gold, W. J., and Nanni, A.共1998兲. “In-situ load testing to evaluate new repair techniques.”Proc., NIST Workshop on Standards Development for the Use of FRP for the Rehabilitation of Concrete and Masonry Structures, Tucson, Ariz., NISTR 6288, D. Duthinh, ed., Feb. 1999, 3-102/112.
Mettemeyer, M., and Nanni, A.共1999兲. “Guidelines for rapid load testing of concrete structural members.”Rep. No. 99-5, Center for Infrastruc-ture Engineering Studies, Univ. of Missouri-Rolla, Rolla, Mo. Nanni, A., and Gold, W.共1998a兲. “Evaluating CFRP strengthening
sys-tems in-situ.”Concrete Repair Bull., Int. Concrete Repair Ins., 11共1兲, 12–14.
Nanni, A., and Gold, W. J.共1998b兲. “Strength assessment of external FRP reinforcement.”Concr. Int., 20共6兲, 39–42.
NDIS 2421.共2000兲. “Recommended practice for in situ monitoring of concrete structures by acoustic emission.” Japanese Society for Non-Destructive Inspection.
RILEM Technical Committee 20-TBS.共1984兲. “General recommendation for statical loading test of load-bearing concrete structures in situ 共TBS2兲.”RILEM technical recommendations for the testing and use of construction materials, E & FN Spon, London, 379–385. Ziehl, P. H., Galati, N., Nanni, A., and Tumialan, J. G.共2008兲. “In situ
evaluation of two concrete slab systems. II: Evaluation criteria and outcomes.”J. Perform. Constr. Facil., 22共4兲, 217–227.
