In-Situ Evaluation of Two Concrete Slab Systems.II: Evaluation Criteria and Outcomes

Paul H. Ziehl, M.ASCE1; Nestore Galati, M.ASCE2; Antonio Nanni, F.ASCE3; andJ. Gustavo Tumialan, M.ASCE4

Abstract: The primary objective of in-situ load testing is to evaluate the safety and serviceability of an existing structural system withrespect to a particular load condition and effect. In light of technological advances in construction methods, analytical tools and moni-toring instrumentation, new different evaluation criteria are being proposed in addition to different in-situ load test methods. Some criteriamay be more appropriate than others based on the expected damage and failure mechanisms of the structure being considered. Thecompanion paper describes the rationale and application of both a consolidated and an alternative approach to the determination of loadlevel, loading procedure and instrumentation requirements for two case studies. This paper discusses in detail the evaluation criteria andoutcomes of these two field projects consisting of a posttensioned concrete slab with structural deficiencies due to tendon and mildreinforcement misplacement and a floor bay of a two-way reinforced concrete slab showing cracking at the positive and negative momentregions. After discussing the relative merits of the evaluation methodologies and the significance of their respective acceptance thresholds,concepts for the development of a new global criterion are discussed.

DOI: 10.1061/�ASCE�0887-3828�2008�22:4�217�

CE Database subject headings: Concrete structures; Field tests; Instrumentation; Load tests; In situ tests; Concrete slabs;Serviceability; Safety.

Introduction

There is a clear need for the in-situ strength evaluation of rein-forced, prestressed, and posttensioned concrete �RC, PC, and PT,respectively� structures. In-situ load test evaluation may be theonly tool available to the structural engineer in many instancesincluding the assessment of the effects of construction and designdeficiencies, the efficiency of strengthening or retrofit methods,and the capabilities of an existing structure to carry loads notconsidered during its design.

This paper presents the application and interpretation of theevaluation criteria adopted for the assessment of in-situ load testsperformed on two structures �a parking garage and a building�described in the companion paper �Galati et al. 2008�. For eachcase study, the evaluation criteria of interest were those associated

1Associate Professor, Dept. of Civil and Environmental Engineering,Univ. of South Carolina, 300 Main St., Columbia, SC 29208�corresponding author�. E-mail: ziehl@engr.sc.edu

2Designer Engineer, Strengthening Division, Structural Group, Inc.,7455 New Ridge Rd., Ste. T, Hanover, MD 21113. E-mail: ngalati@structural.net

3Professor and Chair, Dept. of Civil, Architectural, and EnvironmentalEngineering, Univ. of Miami, 225 MacArthur Engineering Blvd., CoralGables, FL 33124. E-mail: nanni@miami.edu

4Senior Staff Engineer, Simpson Gumpertz & Heger, Inc., 41 SeyonSt., Ste. 500, Waltham, MA 02453. E-mail: gtumialan@sgh.com

Note. Discussion open until January 1, 2009. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be filed with the ASCE ManagingEditor. 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 the Journal of Performance of Constructed Facili-ties, Vol. 22, No. 4, August 1, 2008. ©ASCE, ISSN 0887-3828/2008/4-

217–227/$25.00.

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with the recently proposed cyclic load test �CLT� method andacoustic emission �AE� criteria. The results obtained in the twoprojects are described and the relative merits of the evaluationmethodologies are compared. Finally, a new global criterion isproposed and its significance demonstrated.

Research Significance

Evaluation criteria based on principles of structural behavior suchas repeatability, linearity, and permanency as recently introducedwith the development of the cyclic load test method as well asevaluation criteria based on principles of acoustic emission ap-pear of significant relevance in the assessment of concrete struc-tures. These criteria are similar in the sense that they focus onchanges in behavior of the structural system due to a pattern ofloading and unloading. The primary difference between the CLTand AE evaluation criteria are in the nature of the data collected.Whereas the CLT evaluation criteria typically relate to load andassociated displacements or strains, AE evaluation criteria arebased on the energy released as the result of crack growth �fromloading and load holds� or crack closure during unloading. CLTand AE evaluation criteria have the potential to be used in acomplimentary fashion. This paper presents the results obtainedfor two field projects. Such results from load tests performed onactual structures are vital for calibration and understanding of theevaluation criteria.

Background Research

The requirements of Chapter 20 of the ACI 318 Building Code

ACI 318 �2005� include provisions for load testing of RC and PC

F CONSTRUCTED FACILITIES © ASCE / JULY/AUGUST 2008 / 217

structures. The general procedure required by ACI 318-05 �2005�involves gradually applying the test load until a maximum load isreached and maintaining such load level for 24 h. Measurementsare recorded before any load is applied, after each load increment,when the maximum load is achieved, after 24 h of sustained load-ing, and 24 hours subsequent to the removal of the test load. Itshould be noted that the recovery deflection measurements areonly needed if the limit on maximum deflection is exceeded. Nocommentary, however, is offered regarding the purpose of theintermediate deflection readings. These measurements couldallow verification of the linear response of the structure and pro-vide an opportunity to terminate loading if a large increase indeflection is observed after a loading increment. This concept of“deviation from linearity,” could provide an explicit guideline forinterpretation of deflection readings taken during the sequence ofload application steps �ACI 2007�. Evaluation of the structure isbased on the maximum recorded deflection and the amount ofdeflection recovery. No acceptance criteria for establishing satis-factory behavior at service load level are given.

Alternative methods of analyzing displacement data to estab-lish new evaluation criteria have been introduced recently to ac-company the cyclic load test method �ACI 2007�. Deflectionevaluation criteria should be based on the following principles ofengineering mechanics �ACI 2007�:• Maximum deflection under full test load compared with calcu-

lated theoretical maximum deflection at that load level,• Recovery of deflection upon full removal of load, and• Linearity of deflection response during loading and unloading.

A recent literature search conducted by ACI Committee 437�ACI 2007� has shown that historical load test practice suggeststhat deflection recovery can be properly used as an evaluationcriterion for load testing of concrete structures. Arnan et al.�1950� showed that deflection recovery can be monitored with theapplication of the test load in cycles of loading and unloadingwith increasing peaks until the full test load is in place. ACI318-05 �2005� suggests that the deflection recovery after 24 h in astatic load test, without incremental loading and unloading of thestructure, should be at least 75% of the maximum recorded.Arnan et al. �1950� and more recently ACI 437.1R-07 �2007�suggest that the deflection recovery requirement should be signifi-cantly higher, on the order of 90%, when using the cyclic load testmethod or when retesting a structure using the static load testmethod.

Evaluation of RC components and structures with AE hasbeen in existence for over 20 years with notable advances inJapan in terms of its acceptance and standardization �NDIS 2421�2000��. Notable investigations include that of Hearn and Shield�1997�, wherein a pattern was recognized that is consistent withother studies for the “Felicity Ratio” �see the Appendix� to de-crease as loading is increased. Acoustic emission during unload-ing was included in evaluation criteria in the form of a plot of“calm ratio” versus “load ratio” as described in Ohtsu et al.�2002�. Colombo et al. �2005�, also focused on AE during unload-ing. A study by Ridge and Ziehl �2006�, proposed an evaluationcriterion that is based on a ratio of energy released during the holdportion of load and reload cycles. Other published AE investiga-tions of RC structures include Yuyama et al. �1999, 2001�. Evalu-ation of deteriorated RC bridges with AE is described in Golaski

et al. �2002�.

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Evaluation Criteria

ACI 318-05

Section 20.5 of ACI 318-05 �2005� defines the evaluation criteriafor interpreting the results of the 24 h monotonic load test basedon two different sets of values. The measured maximum or re-sidual deflections must satisfy one of the following two equations:

�max �lt2

20,000 h�1�

�rmax ��max

4�2�

If the measured maximum and residual deflections do not sat-isfy these equations, the load test may be repeated, but not earlierthan 72 h after the removal of the first test load. It should benoted that if Eq. �1� is met then Eq. �2� need not to be met.

ACI 437-07

Service Load LevelIrrespective of the loading procedure �monotonic or cyclic load�and type of load �uniformly distributed load over the entire tribu-tary area, strip load, or patch load�s��, measurements of flexuraldeflection and crack spacing and width under the test load equiva-lent to the service condition should be recorded and checkedagainst limiting values given in Table 9.5�b� of ACI 318-05 Chap.9 �2005�. These limits are only applicable if the load distributionpattern reflects the one used for design. The maximum width ofnew flexural cracks formed during the course of the load test orthe change in width of existing flexural cracks is limited to thesuggested tolerable crack widths as reported by ACI Committee224 �2001� for an aggressive environment.

All Load LevelsThe three parameters that have been established to analyze thebehavior of a tested structure when loading is performed accord-ing to the CLT method are repeatability, permanency �related todeflection recovery�, and deviation from linearity. The three cri-teria may be related to any response �e.g., deflection, rotation, andstrain�; however, deflection appears to be the most convenient andfor this reason, deflection is used herein.1. Repeatability represents the behavior of the structure during

two identical loading cycles. Repeatability is calculated ac-cording to Eq. �3� referring to Fig. 1

IR = repeatability index =�max

B − �rB

�maxA − �r

A � 100% �3�

Mettemeyer �1999� has shown that a repeatability index inthe range 95–105% can be considered as satisfactory.

2. Permanency represents the amount of permanent change dis-played by any structural response parameter during the sec-ond of two identical load cycles. The permanency indexshould be less than 10% �Mettemeyer 1999� and is computedby the following equation referring to Fig. 1 �e.g., duringCycle B�

IP = permanency index =�r

B

�B � 100% �4�

max

CE / JULY/AUGUST 2008

3. Deviation from Linearity represents the measure of the non-linear behavior of a member being tested. As the memberbecomes increasingly more damaged, its behavior may be-come more nonlinear, and its deviation from linearity mayincrease.

To define deviation from linearity, linearity itself must first bedefined. Linearity is the ratio of the slopes of two secant linesintersecting the load-deflection envelope. The load deflection en-velope is the curve constructed by connecting the points corre-sponding to only those loads greater than or equal to anypreviously applied loads, as shown in Fig. 1. The linearity of anypoint i on the load-deflection envelope is the percent ratio of theslope of that point’s secant line �tan �i� to the slope of the refer-ence secant line �tan �ref�, as expressed by

linearity =tan �i

tan �ref� 100% �5�

The deviation from linearity index of any point on the load–deflection envelope is the compliment of the linearity of thatpoint, as given in the following:

IDL = deviation from linearity index = 100 % − linearity �6�

Once the level of load corresponding to the reference load hasbeen achieved, deviation from linearity should be continuouslymonitored. Experience �Mettemeyer 1999� has shown that accept-able values of deviation from linearity, as defined earlier, are lessthan 25%. Deviation from linearity may not be useful when test-ing a member that is expected to behave in a nonlinear elasticmanner; for such a member, its repeatability and permanency maybe better indicators of damage.

Acoustic Emission

Terminology related to AE evaluation is given in the Appendix.Two evaluation criteria based on AE measurements are describedin the following.

Calm Ratio versus Load RatioOne method that has been suggested for quantifying AE data dueto loading and unloading cycles is referred to as calm ratio versusload ratio �Ohtsu et al. 2002�. With this approach the data areplotted and then separated into four quadrants identified as minor,intermediate, and heavy damage. For the investigations described,

Fig. 1. Schematic load versus deflection for computation ofevaluation criteria �ACI 437-07� �Reprinted with permission fromACI �2007��

the load ratio is defined as

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load ratio = �Load at onset of significant AE activity/

previous maximum load� �7�

The onset of significant AE activity was determined from thechange in slope of the cumulative signal strength curve. Loadratio is similar to the Felicity ratio �ASTM 2006�.

The calm ratio compares the AE activity during loading to thatduring unloading. Cumulative signal strength �CSS� was used toquantify the amount of AE activity and calm ratio was defined as

calm ratio = �CSS during unloading of the reload cycle/

CSS during loading of the previous load cycle�

�8�

Although the calm ratio versus load ratio criterion has usuallybeen applied on a qualitative basis, for purposes of developing amore quantitative evaluation the distance from the point of nodamage �load ratio�1.0, calm ratio�0� is proposed as an addi-tional evaluation criterion. To give equal weight to the calm andload ratio the calm ratio is first normalized to the load ratio andthen the distance from the point of no damage to the load setunder consideration is calculated. This method does not fullyagree with the qualitative assessment plot; however, it is believedto provide a reasonable interpretation of the damage state and isnecessary for the development of the global performance indexdescribed later in the paper. Similar to the indices for the CLTmethod, and index is proposed for plots of calm ratio �CR� versusload ratio �LR� as follows:

ICRvsLR = ��normalizedCR�2 + �1 − LR�2 �9�

Cumulative Signal Strength RatioRecent studies indicate that the amount of AE activity �defined bycumulative signal strength� generated during reloading whencompared to that generated during initial loading may be a usefulindicator of damage. An index is proposed for CSS ratio as fol-lows:

ICSSR = �CSS during initial loading/CSS during reloading�

� 100% �10�

Limited previous studies �Ridge and Ziehl 2006� indicated thatCSS ratios in excess of 40% were indicative of a significant level

Fig. 2. Load versus deflection at DS10 �CLT No. 1�

of damage.

F CONSTRUCTED FACILITIES © ASCE / JULY/AUGUST 2008 / 219

Results of Case Studies and Discussion

Two-Way PT Concrete Slab „Parking Garage…

CLT Evaluation CriteriaIn this section the results of the three tests introduced in thecompanion paper �Galati et al. 2008� �i.e., CLT No. 1 and 24 hNo. 1 for the simulated shear collar, and CLT No. 2 with CFRPstrengthening applied to increase negative moment and punchingshear capacity� are described. Only the measurements from thesensors at the most demanding locations are reported here.

CLT No. 1 �simulated shear collar�. The structure was loadedusing the procedure shown in Fig. 8�a� of the companion paper�Galati et al. 2008�. Fig. 2 shows the measured deflections atDS10 �see Fig. 6 in the companion paper� during the eight cyclesdepicting an elastic behavior with no residual deflection. Thestructure passed the test as repeatability, permanency, and devia-tion from linearity were within the limits prescribed by ACI 437�see Table 1�.

No new cracks formed when performing the cyclic load test:the effect of the applied load was mostly to increase the size ofthe existing cracks �Fig. 3�. The maximum change in width ofexisting flexural cracks at the service load condition �1.0 D+1.0 L� was 0.14 mm �0.0045 in.�, and did not exceed the sug-gested tolerable crack width of 0.18 mm �0.007 in� �ACI 2001�for this type of structure and environmental condition.

24-h No. 1 �simulated shear collar�. After concluding the cy-clic load test, the maximum load was kept on the structure for24 h, as shown in Fig. 8�b� of the companion paper. Fig. 4 showsthe corresponding time–deflection diagram. The plot shows thatdeflection increases 20% from the instant value at maximum load.

Table 1. Experimental Results—Two-Way PT Slab �Parking Garage� CL

Load setnumber

Loadcombination

Loadlevel

�kN �kip��

Repeatability�95–105%�

�%�

P

1 0.5�1.0D+1.6L� 27.1 �6.1� 103.9

2 0.75�1.0D+1.6L� 39.1 �8.8� 102.1

3 1.0D+1.6L 51.2 �11.5� 101.7

4 1.15D+1.5L 57.8 �13.0� 101.2

Fig. 3. Crack opening measured at DS4 �CLT No. 1�

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This significant increase in deflection is explained as the result ofcracks opening as demonstrated by AE activity during the entireduration of the test. The crack growth could not be confirmed bythe horizontal direct current variable transformer �DCVT� mea-surements because the increments in crack size were comparableto the resolution of the sensor type.

The structure met the acceptance criteria of Chap. 20 of ACI318-05 �2005�, as at the completion of the 24 h sustained loadingand upon unloading, the two criteria listed in the following wereverified:

�max = �max,24 h + �r,cyclic = 4.57 mm �0.18 in.�

+ 0.25 mm �0.01 in.� = 4.82 mm �0.19 in.�

�lt2

20,000 h= 20.1 mm �0.79 in.� �11�

and

�rmax = �rmax,24 h + �r,cyclic = 0.48 mm �0.019 in.�

+ 0.25 mm �0.01 in.� = 0.7 mm �0.029 in.�

��max

4= 1.1 mm �0.045 in.� �12�

where �r,cyclic�residual deflection after the CLT and �max,24 h and�rmax,24 h represent the maximum and the residual deflectionsafter the 24 h load test, respectively. Eqs. �11� and �12� accountfor the fact that the 24 h test was conducted following the CLTand, therefore, both maximum and residual deflections includedthe values recorded during the cyclic load test.

CLT No. 2 �after CFRP strengthening�. The load levels forCLT No. 2 were higher than those for CLT No. 1 �see Table 2�because the simulated shear collar shifts the location at which themoment capacity is to be evaluated. This results in lower concen-trated loads for CLT No. 1 when compared to CLT No. 2. Fig. 5

1: Simulated Shear Collar

ency% ��

Deviationfrom linearity

��25% ��%�

Performanceaccording toACI 437-07

Calmversus

load ratio��0.45�

CSSratio

��30% ��%�

0.3 Satisfactory NA NA

0.5 Satisfactory NA NA

1.0 Satisfactory 0.15 4

1.5 Satisfactory 0.11 9

Fig. 4. Time versus deflection at DS10 �24 h No. 1�

T No.

erman��10

�%

2.1

3.3

4.0

7.2

CE / JULY/AUGUST 2008

shows the measured deflections at DS10 during all cycles. At44.5 kN �10 kips�, the repaired flexural crack �filled by gravity-feed of epoxy� reopened causing significant permanent deflection.For this reason, the slab did not meet the permanency criteriadescribed in ACI 437-07 �2007� as shown in Table 2. Eventhough the structure did not meet the permanency criteria at thesecond load set the load test was continued. In the last two cycles,the structure did not meet either the permanency or the deviationfrom linearity acceptance criteria. At this load level the structurewas severely damaged and the crack propagated to the two adja-cent columns.

Once the crack opened, the maximum change in width of theflexural cracks at the service load condition was 0.11 mm�0.0042 in.�, which does not exceed the tolerable crack width of0.18 mm �0.0070 in.� �Fig. 6�. Fig. 7 shows the distribution ofstrain at the same location along the carbon fiber reinforced poly-mer �CFRP� strips as a function of applied load. Fig. 7 shows thatthe crack likely started opening from the side of strain gauge SS1and propagated to the other side of the column. A significantresidual strain was recorded after unloading because of the inabil-ity of the crack to completely close. No signs of CFRP delamina-tion were observed during the test.

AE Evaluation CriteriaPlots of calm ratio versus load ratio were developed and dividedinto quadrants similar to those reported in Ohtsu et al. �2002�. Theresults of the AE evaluation are shown in Figs. 8�a and b� �CLTNo. 1 and CLT No. 2�. Evaluation may be misleading when littleAE activity is present and in this case should not be applied. Theminimum value for AE evaluation was set to 4�106 pV s �basedon cumulative signal strength recorded during the initial loading

Fig. 5. Load versus deflection at DS10 �CLT No. 2�

Table 2. Experimental Results—Two-Way PT Slab �Parking Garage� CL

Load setnumber

Loadcombination

Loadlevel

�kN �kip��

Repeatability�95–105%�

�%�

P

1 0.5�1.0D+1.6L� 34.7 �7.8� 104.7

2 0.75�1.0D+1.6L� 46.7 �10.5� 104.4

3 1.0D+1.6L 62.3 �14.0� 103.3

4 1.15D+1.5L 73.8 �16.6� 103.2

5 1.2D+1.6L 82.3 �18.5� 98.6

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cycle�. In Fig. 8�a�, the results for Load Sets 3 and 4 fall withinthe range of minor and intermediate damage. Load Sets 1 and 2were not evaluated due to an insufficient amount of AE activity.In Fig. 8�b�, the results fall within the quadrants related to minor�Load Set 1� and intermediate �Load Sets 2, 3, and 4� damagewith the exception of Load Set 5. This load set falls well withinthe quadrant related to heavy damage and is notable for its sig-nificant increase in the calm ratio value.

The distance from the point of no damage to Load Set 5 inFig. 8�b� is calculated as 0.90. To establish a quantitative measureof damage a threshold value must be determined. Based on thedata from CLT No. 1 and CLT No. 2 a threshold value of 0.45 forthe distance from the point of no damage was chosen.

The AE results are plotted in terms of CSS ratio in Figs. 9�aand b� �for CLT No. 1 and CLT No. 2�. The numerical value ofcumulative signal strength is plotted for each load cycle. Theimportant aspect of the plots is the value of the reloading cycle ofeach load set �e.g., Load Cycle 8 in Load Set 4� when comparedto the initial load cycle of that same load set �Load Cycle 7 inLoad Set 4�. In Fig. 9, the results for the CSS ratio are shownnumerically as a percentage value above the reloading cycle ofeach load set. In Fig. 9�a�, all values are well below the 40%threshold reported in the literature �Ridge and Ziehl 2006�. In Fig.9�b�, the value for Load Set 5 does approach 40%. Given theinformation from the plot of calm ratio versus load ratio where itis clear that Load Set 5 of CLT No. 2 plots in the quadrant in-dicative of heavy damage, a threshold equal to 30% may be moreappropriate and is adopted for purposes of the evaluations de-scribed here. The value of 40% as reported in the literature refersto passively reinforced beam specimens as opposed to the post-

2: after CFRP Strengthening

ncy�

Deviationfrom linearity

��25% ��%�

Performanceaccording toACI 437-07

Calmversus

load ratio��0.45�

CSSratio

��30% ��%�

3.5 Satisfactory 0.00 2

10.4 Unsatisfactory 0.13 8

15.6 Unsatisfactory 0.24 25

21.5 Unsatisfactory 0.33 18

26.7 Unsatisfactory 0.90 39

Fig. 6. Crack opening measured at DS4 �CLT No. 2�

T No.

ermane��10%

�%�

9.6

11.9

16.4

17.5

17.5

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tensioned slab system described and therefore some variationfrom the literature seems justified. The results of the AE evalua-tion for calm ratio versus load ratio and CSS ratio are shownalong with the CLT evaluation results in Tables 1 and 2.

AE activity was monitored continuously during the 24 h loadtest and it was present throughout the load test period as shown

Fig. 7. Strain distribution in CFRP strips �CLT No. 2�

Fig. 8. Calm versus load ratio

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for one 4 h period �Fig. 10�a��. Source location was determinedbased on time of arrival and triangulation. A plot of AE activityshowing active cracking �determined by located AE events� dur-ing the same 4 h period is shown in Fig. 10�b�. Much of theemission shown in Fig. 10�a� was not located. This is because the

Fig. 9. CSS ratio

Fig. 10. AE Activity �24 h No. 1, 4.0 h period�

CE / JULY/AUGUST 2008

activity was generally of low amplitude and therefore in manycases only one or two sensors detected a given hit. For two-dimensional source location a hit must be detected by a minimumof three sensors.

Two-Way RC Slab „Library…

CLT Evaluation CriteriaThe structure was loaded using the protocol described in Fig. 15of the companion paper �Galani et al. 2008�. Fig. 11 shows theload–deflection diagram with deflections measured at DS12 �seeFig. 12 in the companion paper�. As seen in Fig. 11, the structurehad a nonlinear behavior with a change in slope at a load ofapproximately 45 kN �10 kip�. This is attributed to the formationof new cracks at the first load set. In the first two load sets re-peatability, permanency, and deviation from linearity were withinthe limits prescribed by ACI 437 �2007� �Table 3�. The structureexceeded the maximum permanency at Load Set 3 with deviationfrom linearity also very close to the threshold. In the last load set,both deviation from linearity and permanency exceeded accep-tance thresholds.

AE Evaluation CriteriaThe calm ratio versus load ratio plot is shown in Fig. 12�a� andthe CSS ratio plot is shown in Fig. 12�b�. Load Sets 1 and 2 werenot meaningful for evaluation due to the low amount of AE ac-tivity recorded. Load Set 3 falls within the range of intermediatedamage, whereas Load Set 4 falls on the boundary of minor and

Fig. 11. Load deflection diagram—two-way RC slab �building�

Table 3. Experimental Results—Two-Way RC Slab �Building� CLT: Ne

Load setnumber

Loadcombination

Loadlevel

�kN �kip��

Repeatability�95–105%�

�%�

1 D+Ds+L 75.6 �17.0� 104.3

2 0.75�1.0D+1.1Ds+1.6L� 95.2 �21.4� 103.9

3 1.0D+1.1Ds+1.6L 112.5 �25.3� 101.7

4 0.85�1.4�D+Ds�+1.7L� 125.4 �28.2� 104.1

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intermediate damage. In terms of the quantitative approach de-scribed previously, both Load Sets 3 and 4 had a distance of lessthat 0.45 from the point of no damage.

In regard to the CSS ratio criterion, Load Set 4 had a CSS ratioof 34% and exceeded the 30% threshold proposed in this paper.The results of the AE evaluation are shown in Table 3 along withthe CLT evaluation results.

New Concept for Acceptance Criteria: GlobalPerformance Indices

For both structures, the AE activity indicated a significant in-crease in damage when both the permanency and deviation fromlinearity criteria were failed at the same time. Repeatability hadvery small variations during the loading and unloading cycles,indicating that the limits posed on repeatability for statically in-determinate structures may not be appropriate due to load redis-tribution. This behavior was not observed in statically determinate

Moment

manency10% ��%�

Deviationfrom linearity

��25% ��%�

Performanceaccording toACI 437-07

Calmversus

load ratio��0.45�

CSSratio

��30% ��%�

3.9 2.8 Satisfactory NA NA

3.3 12.3 Satisfactory NA NA

10.5 24.6 Unsatisfactory 0.30 21

13.5 26.5 Unsatisfactory 0.18 34

Fig. 12. Acoustic emission results—two-way RC slab �building�

gative

Per��

F CONSTRUCTED FACILITIES © ASCE / JULY/AUGUST 2008 / 223

structures for which the failure of only one of the three CLTcriteria indicates significant damage in the structure �Galati et al.2004�.

The two load tests described show that, for statically indeter-minate structures, overall performance should not be evaluatedbased on the load level at which one acceptance criterion is failed.Rather, it should be based on a combination of all acceptancecriteria �i.e., permanency, repeatability, and deviation from linear-ity� and on how far the computed value falls from the global limit.An equation that reflects such an approach is described in thefollowing equation:

IG =1

3��RiR + �P

IP

10+ �DL

IDL

25��G � 1 �13�

For an even more robust structural evaluation, the CLT and AEcriteria could be combined in the proposed global limit given in

IG =1

5��rir + �P

IP

10+ �DL

IDL

25+ �CRvsLR

ICRvsLR

0.45+ �CSSR

ICSSR

30�G

� 1.0 �14�

where iR�index of repeatability defined as

iR =2 −

IR

95if IR � 95

1

5�IR − 100� if 95 � IR � 95

IR

105if IR � 105

The coefficients �R, �P, �DL, �CRvsLR, and �CSSR in Eqs. �13�and �14� are variables to account for the importance of each in-dividual index given the type of structure under investigation. Thedenominators in the equation are chosen such that the globalindex threshold corresponds to unity. The multiplier �G takes intoaccount the knowledge of the structure by the load test team andthe number of load tested members with respect to the total num-ber of similar members in the building structure. For instance, alower �G factor can be used in Eqs. �13� and �14� if the as-built

Table 4. Evaluation Criteria Results and Ultimate Capacity Margins

Load testdescription Evaluation method

Two-way PT slab�parking garage, CLT No. 2,after CFRP strengthening�

Permanency �%� ��10�Repeatability �%� �95–105�

Dev. from linearity �%� ��25�Calm versus load ratio ��0.45�

CSS Ratio ��30�Global performance index IG ��1.0�

Two-way RC slab�building, CLT,negative moment�

Permanency �%� ��10�Repeatability �%� �95–105�

Dev. from linearity �%� ��25�Calm versus load ratio ��0.45�

CSS ratio �%� ��30�Global performance index IG ��1.0�

Note: Italics denotes criterion results that are unsatisfactory.

conditions �i.e., depth of reinforcement, thickness of slab, con-

224 / JOURNAL OF PERFORMANCE OF CONSTRUCTED FACILITIES © AS

crete compressive strength, etc.� are known. Conversely, if theonly information available is that shown on the original structuraldrawings, then a higher �G should be used.

For the two structures described �both continuous two-wayslabs and therefore statically indeterminate�, the repeatability maynot be critical and therefore the coefficient �R could be taken asless than 1.0, whereas permanency and deviation from linearitycould have coefficients �P and �DL greater than 1.0. Additionally,for the parking garage, the tested area was selected by the engi-neer of record after conducting a comprehensive investigation ofthe entire structure, which included computation of structural ca-pacities based on as-built conditions and selection of the mostcritical bay. For the building, the test area was selected at randombased on original drawings and space available for load testing.The coefficient �G is intended to capture the level of knowledgeof the structure by the load testing team, resulting in lower valuesfor the parking garage and in a higher, statistically dependentvalue for the building.

Due to the scarcity of research data on load testing, both in thelaboratory and in the field, numerical values for the coefficients�R, �P, �DL, �CRvsLR, �CSSR, and �G cannot be proposed at thistime. The calibration of these coefficients should be based onreliability analysis after conducting a statistically significant num-ber of tests to failure for various classes of structural members.

Table 4 reports the corresponding values of the proposed glo-bal performance index according to Eq. �14� assuming coeffi-cients �R, �P, �DL, �CRvsLR, �CSSR, and �G to be equal to 1.0 forCLT No. 2 of the parking garage and the CLT for the building.For both load tests, the increase in AE activity in the structure isaccompanied by a jump in the global performance index. Theresults for CLT No. 1 �simulated shear collars� as well as 24 hNo. 1 of the parking garage are not shown as the structure passedthose tests.

Ultimate Capacity Margin

Because the load tests were conducted on structures in use, localor global failure had to be avoided; therefore, the true ultimatecapacity of the slabs could not be determined. However, an ex-pected ultimate capacity margin �UCM� can be calculated based

Load set number

1 2 3 4 5UCM�%�

9.6 11.9 16.4 17.5 19.5 54

104.7 104.4 103.3 103.2 98.6 �25

a3.5 10.4 15.6 25.5 26.7 33

0.00 0.13 0.24 0.33 0.90 25

2 8 25 18 39 25

0.43 0.63 0.92 0.98 1.41 25

3.9 3.3 10.5 13.5 — 29

104 104 102 104 — �19

2.8 12.3 24.6 26.5 — 19

NA NA 0.30 0.18 — �19

NA NA 21 34 — 19

NA NA 0.88 0.99 — �19

on the finite element method analysis performed on the tested

CE / JULY/AUGUST 2008

component. This UCM index when experimentally validated andavailable would allow engineers to know how much reserve is leftin the structure once it fails the load test. The availability of UCMwould transform a “proof” test into a “prognostic” test.

The expected UCM may be expressed in terms of the load �ormoment� at which any of the individual or global evaluation cri-teria reaches a threshold level as given in the following:

UCM�%� = �1 −Mcf

Mult� � 100% �15�

where UCM�%��ultimate capacity margin expressed �%�Mcf�load or moment at which an evaluation criterion reaches thethreshold value �cf�criterion failure�; and Mult�theoretical ulti-mate load or moment capacity.

UCM values for CLT No. 2 of the parking garage and for thebuilding are shown in Table 4. The comparison of the variousUCM’s based on individual CLT and AE indices shows that theAE method corresponds to the lowest UCMs and such valuesgenerally agree with those corresponding to the global perfor-mance indices. For the parking garage, the UCM was 54% as thepermanency did not meet the ACI 437 criteria at the second Load-set. This ultimate capacity margin appears to be overly conserva-

Fig. 13. Global performance indices

tive and this is attributed to the fact that the acceptance criteria

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defined in ACI 437 are based on limited experience �Chap. 3 ofACI 437� and do not account for structural redundancies.

The proposed global performance indices led to lower ultimatecapacity margins than the ACI 437 approach. The results of ap-plying the global performance indices are shown in Fig. 13 forCLT No. 1 and CLT No. 2 of the parking garage and the CLT ofthe building. In Fig. 13, it is clear that Load Set 5 for CLT No. 2is well beyond the threshold value of 1.0. All other load sets arebelow 1.0 and, as expected, they tend to increase with increasingload set number. It should be noted that Load Set 4 of the CLT forthe building practically reaches unity. Further calibration is re-quired to establish more appropriate values for the coefficients�R, �P, �DL, �CRvsLR, �CSSR, and �G even though the proposedapproach appears promising.

Summary and Conclusions

This paper presents the evaluation of two two-way concrete slabs�PT and RC�. For each case the structural assessment based onCLT and AE evaluation criteria is critically discussed.

For both structures redundancies resulted in very conservativeACI 437 criteria values when compared with the AE evaluation.In fact, significant AE activity was recorded when two of the ACI437 criteria failed at the same time. This observation alloweddefining, for statically indeterminate structures, a global perfor-mance index accounting of permanency, repeatability, deviationfrom linearity, and the AE evaluation criteria of load ratio versuscalm ratio and CSS ratio. An equation was also defined for usewhen AE data is not available.

The proposed global performance index was used to evaluatethe ultimate capacity margins for the load tests performed on bothstructures. The general approach appears very promising, how-ever, further extensive experimental and analytical work isneeded for the calibration of the parameters utilized.

Acknowledgments

The ACI Concrete Research Council, the NSF Industry/University Cooperative Research Center on Repair of Buildingsand Bridges with Composites and the UMR–University Transpor-tation Center on Advanced Materials are gratefully acknowledgedfor their financial support to the research. Yizhuo Chen assistedwith the load tests and data reduction and his assistance is greatlyappreciated.

Appendix. Terminology Related to AcousticEmission

Acoustic emission �AE��class of phenomena whereby transientelastic waves are generated by the rapid release of energy fromlocalized sources within a material, or the transient waves so gen-erated �ASTM 2006�.

Amplitude�largest voltage peak in the AE signal wave form;customarily expressed in decibels relative to 1 V at the preamp-lifier input �dBae� assuming a 40 dB preamp �Physical 2001�.

Calm ratio�AE activity during unloading/AE activity duringprevious maximum loading �as described in Ohtsu et al. 2002�.

dB�unit of measurement for AE signal amplitude A, definedby A �dB�=20 log Vp, where Vp is the peak signal voltage in mi-

crovolts referred to the preamplifier input �Physical 2001�.

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Felicity effect�presence of acoustic emission, detectable at afixed predetermined sensitivity level at stress levels below thosepreviously applied �ASTM 2006�.

Felicity ratio�ratio of the stress at which the Felicity effectoccurs to the previously applied maximum stress �ASTM 2006�;because stress varies with location and direction, load is used inplace of stress for the evaluation described.

Kaiser effect. The Kaiser effect represents the irreversibility ofAE events that are not generated in a material until it is stressedbeyond its prior stress state �Ohtsu et al. 2002�; it is defined inASTM �2006� as “the absence of detectable emission at a fixedsensitivity level, until previously applied stress levels are ex-ceeded.”

Signal strength�measured area of the rectified AE signal withunits proportional to volt-seconds �ASTM 2006� �the proportion-ality constant is specified by the AE instrument manufacturer�.

Notation

The following symbols are used in this paper:CR � numerical value of calm ratio;

D � dead load; units depend on structuralmember considered;

Ds � superimposed dead load; units depend onstructural member considered;

h � overall thickness of member �mm �in.��;ICRvsLR � calm ratio versus load ratio index

�dimensionless�;ICSSR � cumulative signal strength index

�dimensionless�;IDL � deviation from linearity index

�dimensionless�;IG � global performance factor �dimensionless�;IP � permanency index �dimensionless�;IR � repeatability index �dimensionless�;L � live load; units depend on structural

member considered;LR � numerical value of load ratio;

lt � span of member under load test; unitsdepend on structural member considered�ACI 318�;

Mcf � load or moment at which an evaluationcriterion reaches the threshold value�cf�criterion failure�;

Mult � theoretical ultimate load or momentcapacity;

normalized CR � numerical value of calm ratio normalizedto load ratio;

P � applied load during load test;Pi � load of point i in load–deflection envelope

for computation of IDL acceptance criterion;Pmin � minimum load to be maintained during

load test �typically 10% of total testload�;

Pref � reference load for computation of IDLacceptance criterion;

TLM � test load magnitude �including dead loadalready in place�; units depend on structuralmember considered;

UCM � ultimate capacity margin expressed aspercent;

� � strength-reduction factor as per ACI 318;

226 / JOURNAL OF PERFORMANCE OF CONSTRUCTED FACILITIES © AS

�DL � importance coefficient applied to thedeviation from linearity;

�i � slope of secant line of point i in load–deflection envelope �deg�;

�CRvsLR � coefficient to be applied to calm ratioversus load ratio criterion;

�CSSR � coefficient to be applied to CSS Ratiocriterion;

�P � importance coefficient applied to thepermanency;

�R � importance coefficient applied to therepeatability;

�ref � slope of reference secant line inload-deflection envelope, degrees;

�G � global coefficient accounting for the levelof knowledge of the structure by the loadtesting team;

�Amax � maximum deflection in Cycle A undermaximum test load �mm �in.��;

�Ar � residual deflection after Cycle A underminimum test load �mm �in.��;

�Bmax � maximum deflection in Cycle B undermaximum load �mm �in.��;

�Br � residual deflection after Cycle B underminimum test load �mm �in.��;

�i � deflection of point i in load-deflectionenvelope for computation of IDL

acceptance criterion;�max � measured maximum deflection �mm �in.��;�ref � reference deflection for computation of IDL

acceptance criterion; and�rmax � measured residual maximum deflection

�mm �in.��;

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