Verification of flexural stop criteria for proof load tests

Life-Cycle Analysis and Assessment in Civil Engineering: Towards an Integrated Vision – Caspeele, Taerwe & Frangopol (Eds) © 2019 Taylor & Francis Group, London, ISBN 978-1-138-62633-1

Verification of flexural stop criteria for proof load tests on concrete bridges based on beam experiments A. Rodriguez Burneo Universidad San Francisco de Quito, Quito, Pichincha, Ecuador

E.O.L. Lantsoght Delft University of Technology, Delft, The Netherlands Universidad San Francisco de Quito, Quito, Pichincha, Ecuador

ABSTRACT When performing proof load tests, irreversible damage may occur. Guidelines for performing the test have been developed, which establish stop criteria to terminate the test before this happens. The stop criteria prescribed in the currently available codes are mainly designed for buildings, but load tests are also performed on bridges. This investigation compares the results from beams tested in the laboratory with stop criteria and analyzes their applicability on reinforced concrete bridges. The stop criteria from ACI 437.2M-13, the German guideline of the DAfStB, and a proposal developed by Werner Vos from TU Delft were evaluated. It was found that the DAfStB concrete strain stop criterion provided the most consistent results. The ACI stop criteria should only be applied if the ACI loading protocol is being followed. The deflection proposal by Vos, seems to be a reliable option, but further investigation needs to be done before it can be applied.

1

INTRODUCTION

Existing civil structures deteriorate with time. They are subjected to continuous loading, environmental conditions, and damage caused by accidents or live loads larger than what these structures were designed for. This situation causes a loss of their initial material and structural properties, and consequently uncertainties on the structural behavior. Therefore, analyses should be carried out to confirm that the structure is still safe for use. If there are background data about the structure to be tested, simulations and computer analyses can be done. However, the level of assessment of these analyses may not be as close to reality as needed since the level of damage and deterioration is difficult to estimate. An option for analyzing deteriorated or damaged structures with or without background data is load testing, in which the actual structure is loaded and its behavior is measured. There are two types of load testing: the first type is diagnostic load testing (Olazek et al. 2014), in which the structure is loaded to a lower load level to obtain its mechanical properties and structural response to update analytical models (Lantsoght et al. 2017b). The second type is proof load testing (Faber et al. 2000), which is the subject of this research. Its purpose is to verify the safety of a structure by subjecting it to a specific maximum load known as target load, which represents the factored load combination the structure should be able to carry. If the structure withstands the target proof load, it passes the test and is still suitable for use. If a stop criterion is exceeded, a

structure may fulfill the code requirements for a lower load level. Proof loading is a common practice when there is not enough background information to perform a structural analysis, after the structure has been subjected to loads it was not designed to withstand, or when it has suffered severe damage material degradation (Lantsoght et al. 2017a) or when the analytical capacity is sufficient but additional sources of capacity are expected. Therefore, checking if the structure is able to bear a specific load allows to determine if the structure must be repaired or replaced, or if it remains safe for use. Three parameters must be established before the test is carried out: the loading protocol, the target load, and the stop criteria. The loading protocol establishes how the test is performed. This includes how and where the loads will be applied. Loads can be applied in cycles, increasing the maximum applied load, or in a monotonic way, increasing the load continuously after established periods of time. A single test may include different load cases with their respective parameters. The position of the load aims to recreate the most unfavorable condition. The stop criteria are parameters established to protect the integrity of the structure during the proof load test. During a test, stresses in the structure may increase to the point at which the structure suffers permanent damage, or in the worst case scenario, collapse. To avoid this, stop criteria must be established; these are defined as thresholds for the parameters that will be measured on the structure while

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the test is performed. Existing stop criteria include parameters such as: maximum deflection, crack width, and deviation of linearity index. If one of these parameters is exceeded during the test, it must be aborted immediately, whether the target load has been reached or not. Further loading after exceedance of a stop criterion is not permitted. 2 2.1

2.2.1 Monotonic load protocol deflection limits When a monotonic load protocol is used, the acceptance criterion for deflection is determined as:

STOP CRITERIA FROM THE LITERATURE DAfStB Deutscher Ausschuss für Stahlbeton

The German guideline for proof load testing (Deutscher Ausschuss für Stahlbeton 2000) applies to both plain concrete and reinforced concrete structures.The protocols established rely on a ductile failure mode. Load testing of shear-critical structures or elements is not permitted. For proof load tests, cyclic loading must be carried out with at least 3 steps of loading and unloading. 2.1.1 Concrete strain The first stop criterion from the German guideline introduces a limit strain as shown on equation (1) where εc is the measured strain during the proof load test, εc lim is the limit value for concrete strain based on concrete characteristic compressive strength defined by the German guideline as 600 µε, which can be increased to 800 µε if concrete compressive strength is greater than 25 MPa, and εc0 is the concrete strain caused by the permanent loads, determined analytically.

2.1.2 Crack width and increase in crack width The width of new cracks and the residual crack width formed during and after the test is limited to values of Equations (2) and (3). The increase in crack width and residual crack width of existing cracks is limited by Equations (4) and (5).

2.1.3 Deflection A proof load test must be stopped if more than 10% permanent deformation occurs after removing the load, or if there is a clear increase of the nonlinear part of the deformation.

r is the residual deflection measured 24 hours after the removal of the load and l represents the maximum deflection. L is the span length. 2.2.2 Cyclic load protocol When a cyclic load protocol is used, three acceptance criteria need to be verified after the test: the deviation from linearity index IDL , the permanency ratio IPR , and the deflection. Data obtained from the relation between deflections and the applied load are used to calculate the deviation from linearity index and the permanency ratio. These data depend on the loading protocol that is being applied. 2.3 Werner Vos proposal Werner Vos (Vos 2016) proposed two stop criteria. The first one is based on the relation stiffness-deflection, developed through a theoretical approach based on the moment-curvature relation developed by Monnier (Monnier 1970). The second proposal is based on the relation between crack width and deformation as developed by Van Leeuwen (Van Leeuwen 1962). 2.3.1 Stiffness deflection proposal Based on the moment-curvature relation established by Monnier (Monnier 1970), the deflection can be calculated in terms of the moment applied as shown in Equations (8) and (9). Using a semi-linear approximation of the moment-curvature diagram as seen in Figure 1, the stiffness from the unloading branch after yielding has occurred, EI te , is used to calculate the maximum allowable deflection as shown in Equation (10). From Figure 1, the first and second slope correspond to the stiffness of the structure before and after cracking has occurred. Based on Monnier’s moment-curvature research (Monnier, 1970), Vos’s work adapted the equation to use Eurocode notations. In Equation (10) where k is the curvature and δ the deflection, the yielding moment My is used to calculate the beam’s maximum deflection, which is used as a stop criterion. In this work, Thorenfeldt’s parabola (Wight & MacGregor 2012) was used to calculate My .

2.2 ACI 437.2M-13 acceptance criteria ACI 437.2M-13 (ACI Committee 437 2013) does not establish stop criteria, but defines quantitative rules to determine if the structure passes the load test, known as acceptance criteria. Acceptance criteria describe the acceptable limits of performance indicators. The acceptance criteria depend on the loading protocol.

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Figure 2. Experimental layout.

Table 1.

Specimen properties.

Specimen

b × h (mm)

rebar

f
c (MPa)

P804 P502

300 × 800 300 × 500

6φ20 mm 3φ20 mm

63.51 71.47

Figure 1. Semi-linear moment curvature approach.

2.3.2 Crack width and deformation proposal Equations (11) and (12) are based on Van Leeuwen’s (Van Leeuwen 1962) relation between crack width and deformation:

β is the ratio between the permanent or cyclic load and total load. The maximum value of β equals one. Therefore the maximum crack width can be found when β = 1. σ s1 is the steel stress at the crack, s is the space between cracks which depends on the type of bar: ribbed or plain, and fy is the yield strength of the steel. The value of s was not calculated using equations fromVos’s proposal but fromVan Leeuwen’s (Van Leeuwen, 1962) curve which establishes a relation between reinforcement ratio and crack spacing. Vos establishes the following threshold values as stop criteria:

where wsls is the maximum measured crack at the serviceability limit state. 3 3.1

EXPERIMENTS

Table 2.

Properties and results of experiments.

Experiment

a (mm)

h (mm)

l (mm)

Fmax (kN)

Failure mode

P804A1 P804A2 P804B P502A2

3000 2500 2500 1000

800 800 800 500

8000 8000 8000 5000

207 231 196 150

bending shear shear bending

in Figure 2 and Table 1 respectively. In Table 1b is the width, h is the height and fc
is the concrete compressive strength. Table 2 presents the conditions of the experiment where a is the distance from the edge of the beam to the position where load was applied, h is the height of the beam and l is the span length. The part of the beam in Figure 2 that is not located between supports was used for experiment P804B. The beams were reinforced with plain bars with a measured yielding stress of 296.8 MPa and ultimate stress of 425.9 MPa. Crack opening, horizontal and vertical deformations, deflection at location of the load and supports, acoustic emissions and strains were measured using LVDTs, laser distance finders, acoustic emission sensors and photogrammetric measurements. A cyclic loading protocol was used on experiments P804A1, P804A2 and P502A2, while P804B was subjected to a monotonic loading protocol. Even though two of the experiments presented shear failure, collected data during each load step allowed to compare the results to flexural stop criteria.

Description of specimen and properties

Three experiments were carried out on beam P804 (10 m long) cast in the laboratory to evaluate the stop criteria used during proof load testing. The material properties were designed to resemble concrete solid slab bridges (Lantsoght et al., 2017c). Additionally, one beam of 8 m, P502, was tested. Experimental layout and properties of the specimens are shown

3.2 Test results The results needed to analyze the considered stop criteria are deflection, crack width and concrete strain. The parameters are plotted as a function of the force applied. Figure 3 shows the results for experiment P804A1. Similar plots were obtained for the other experiments.

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Table 3.

Concrete strain stop criterion results.

Experiment

Load (kN)

εmeas (µε)

εmeas − εc0 (µε)

% of ultimate load

P804A1 P804A2 P804B P502A2

90 120 105 91

790 770 790 810

769 767 767 791

43% 51% 53% 61%

Table 4.

DAfStB residual deflection.

Experiment

Load (kN)

% of ultimate load

P804A1 P804A2 P502A2

84.98 195.31 73.28

41% 84% 49%

the stop criterion had the lowest margin of safety. For this experiment, the beam was cracked, the span length was smaller and the position of the load was closer to the support. Overall, this stop criterion shows consistent results for shear and flexure to apply on concrete bridges. However, even though 60% of the ultimate load is considered a good margin of safety, 40% which was the case for experiment P804A1 may be too conservative.

4.2 DAfStB deflection Table 4 shows, for each experiment, the maximum load applied during the cycle at which the stop criterion was exceeded, and the margin of safety in terms of a percentage of the ultimate load. The results for the stop criterion of deflection are not consistent.Therefore this stop criterion is not recommended for use in bridge proof load tests. Figure 3. P804A1 test results plots: (a) deflection vs. load, (b) strain vs. load, (c) crack width vs. load.

4.3 Crack width

4 ANALYSIS OF RESULTS 4.1

DAfStB concrete strain

Table 3 shows the concrete strain measurement ε closest to the German guideline threshold value, and the load at which the threshold value was exceeded. As can be seen, on all experiments, the stop criterion was reached between 40% and 60% of the ultimate load. For experiment P804A1, in which the stop criterion was exceeded with the largest margin of safety, the beam was uncracked. As for P804A2 and P804B, both reached the stop criterion at about half the ultimate load, and both presented a shear failure. Between these two experiments differences were the loading protocol, and for P804B the beam was uncracked. Differences between these two experiments and P804A1 was a variation of 500 mm of the position of the load and the conditions of the beam prior the test. For P502A2,

Table 5 shows the maximum crack width wmax prescribed by the German Guideline, the load at which the stop criteria is exceeded, and the measured crack width at this load level. The load at exceedance of the stop criterion is also expressed as a percentage of the ultimate load in the last column of Table 5. Table 6 shows the residual crack width wres prescribed by the German guideline, the load after which this stop criteria is exceeded, and the measured residual crack width after this load level. The load at exceedance of the stop criterion is also expressed as a percentage of the ultimate load in the last column ofTable 6. ForTable 6, data was analyzed after every loading cycle, when the load was minimum. This way it is possible to compare the results to see which of the two threshold values, wmax or wres , was exceeded first. The measured crack width data in both tables was the closest value to the threshold at the peak of the load cycle. Crack width data from experiment P804B was erratic, therefore, it could not

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Table 5.

Crack width results for which wmax was exceeded. DAfStB limits

Closest measurement

Experiment

wmax (mm)

Load (kN)

wmax (mm)

% of ultimate load

P804A1 P804A2 P502A2

0.5 0.3 0.3

178 159 123

0.56 0.31 0.26

86% 69% 81%

Table 6.

Crack width results for which wres was exceeded. DAfStB limits

Closest measurement

Experiment

wres (mm)

Load (kN)

wmax (mm)

wmax (mm)

P804A1 P804A2

0.083 –

139 –

0.28 –

0.082 –

P502A2

0.07

146

0.36

0.12

% of ultimate load 67% not exceeded 97%

be used to evaluate this stop criterion and is not presented on Tables 5 and 6. Calculations of the limit values were obtained using Equations (2) through (5) depending on the cracking conditions of the specimen. Looking at the results of the maximum crack width in Table 5, on the three experiments 69% or more of the ultimate load was applied before exceeding the threshold value. Thus, the safety margin is smaller than the stop criterion for concrete strain. Regardless of the conditions of the experiment or failure mode, the results are consistent even though this threshold value seems to be less conservative than the concrete strain stop criterion. The results for the residual crack width, on the other hand, were not as consistent. For experiment P804A2, the limit value was never exceeded and for P502A2, the threshold was surpassed at 97% of the ultimate load, which means there is no margin of safety. Comparing results between the two tables there is no consistency either. For experiment P80A1 the wres threshold was exceeded before wmax . Meanwhile, experiment P502A2 presented the opposite case: wmax was exceeded first. For P804A2, the wres threshold was never exceeded. When using these stop criteria, the maximum and residual crack width must be analyzed. Even though results for crack width, wmax , were consistent, the results for the residual crack width, wres , were not.Thus these stop criteria do not seem to be adequate to be applied. The issues when measuring cracks, and possible causes that prevent cracks from closing should be considered when using this stop criterion. 4.4 ACI deviation from linearity index Even though the ACI loading protocol was not applied in any of the experiments, the load-deflection graph

was analyzed to calculate the deviation of linearity index and the permanency ratio on the three experiments subjected to cyclic loading. The results where erratic for the three cases. P804A1 did not remain under the IDL ≤ 0.25 threshold value in any of the load steps. Meanwhile, P502A2 surpassed the threshold in the third load step, and P804A2 remained always under the maximum value. As for permanency ratio, for all three experiments the results are erratic. For one set of cycles, the permanency ratio is much smaller than the threshold value IPR ≤ 0.5 and for the next set of cycles it is much larger. Based on these results, calculated values are erratic and are not useful to compare with the threshold value. Consequently, these stop criteria should not be applied unless ACI’s loading protocol is being applied. For this loading protocol, a force-controlled load application is necessary, whereas for field testing, a deflection-controlled load application is to be preferred. 4.5 ACI maximum and residual deflection Table 7 shows the deflection for each experiment once permanent damage was achieved: yielding for experiments that failed under bending, and shear failure for the other cases. As can be seen in the results, the maximum allowable deflection is much larger than the maximum deflection measured before permanent damage or even failure was reached. The reason is that ACI establishes an acceptance criterion, not a stop criterion. For example on experiment P804B even after failure, the maximum allowable values were not reached. Under these circumstances P804B beam passed the proof load test under the maximum deflection acceptance criteria. The ACI residual deflection r from ACI 4372M.13 requires to be measured 24 hour after the load has been removed. This measurement was not done on any of the experiments since they were tested until failure. However, on experiment P502A2, on every cycle, the load was reduced to 0 kN. The deflection after every cycle was analyzed to compare its results to The ACI residual deflection threshold value. On load step 7, one loading cycle before reaching failure, a l = 5.2 mm deflection was measured, establishing a limit value of r < 1.3 mm. However, the measured residual deflection was around 0.4 mm. Similar to maximum deflection, the threshold value is far from being reached. Consequently, this acceptance criterion is not adequate to apply as a stop criterion on reinforced concrete bridges. 4.6 Werner Vos’s deflection To obtain a maximum allowable deflection according to Werner Vos proposal, the yielding moment My must be found. Using Thorefeldt’s parabola (Wight & MacGregor 2012) the results shown in Table 8 were obtained, where My calc is the calculated yield moment, FMy calc is the corresponding load to the yield moment,

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Table 7. damage.

Experiment

 (mm)

ACI (mm)

P804A1 P804A2 P804B P502A2

14.18 12.97 4.84 5.99

44.40 44.40 44.10 27.70

Table 8. load.

cracked, P804B was uncracked and both failed in shear. For both, the stop criterion was exceeded at around 85 % of the maximum load. However, P502A2 which was cracked as well, reached the limit right after yielding. This observation indicates that the stop criterion cannot correctly take into account the effect of existing cracks on the stiffness of the beam. P804A2 and P804B can be compared as the only difference was the state of cracking and the loading protocol. The results are similar for both experiments, which means that the uncracked condition of the beam, and the loading protocol do not seem to affect the stop criterion. Overall, Vos’s deflection proposal provided consistent results with a smaller margin of safety. With further investigation concerning the relation of the cracking state of the specimen and its stiffness, this could be a reliable stop criterion to be applied on concrete bridges. Moreover, this stop criteria relies on a flexural failure caused by yielding; therefore it should not be used for shear.

Deflection measured at point of permanent

Comparison between calculated and real ultimate

Experiment

My calc (kNm)

FMy calc (kN)

Fmax (k207N)

P804A1 P804A2 P804B P502A2

375 375 375 116

177 196 196 139

207 231 195 150

Table 9. Comparison of deflection values from Vos’s proposal.

P804A1 P804A2 P804B P502A2

Vos’s max (mm)

meas (mm)

Load (kN)

% of ultimate load

10.86 10.79 10.79 6.16

140 195 171 150

207 231 195 150

67% 84% 87% 100%

4.7 Werner Vos’s crack width

and Fmax is the ultimate load at failure of the experiments. Based on the experimental layout shown in Figure 2, a function of the moment at any position of the beam, M (x), is obtained. Using the value of FMy calc from Table 8, and Equation (10) with EI te , the threshold value for the maximum allowable deflection is obtained. Table 9 shows the stop criterion calculated from Vos’s proposal, max , the closest measured deflection meas , and the load at which meas is reached. Additionally, Table 9 shows the load at failure in the experiment Fmax , and the margin of safety obtained with the stop criterion. Experiment P804A1 reached the limit deflection at a lower percentage of the ultimate load, while P502A2 reached this limit right after yielding occurred. P804A2 and P804B presented good results where the maximum allowable deflection was not too conservative. Unlike P804A2, for P804A1 the beam was new, with no previous loads or cracks, had never been subjected to yielding and the failure mode was different. This may explain why the stop criterion was exceeded with a larger margin of safety. Based on the results from Table 9, there seems to be no explicit relation between the load at which threshold was reached and the state of cracking of the beam. P804A1 reached the limit at a 67% of the maximum load, which is a good fraction of the ultimate load. P804A2 was

Table 10 summarizes the results for Werner Vos’s crack width stop criterion. Limit values 0.9wmax and wres were calculated using Equations (11) through (14). The measurements closest to the stop criterion, wmax meas and wres meas , and the corresponding load in the experiment are also given in Table 10. The last column of Table 10 shows the margin of safety of Vos’s stop criterion for crack width. The crack width data for experiment P804B were erratic and could not be analyzed. On all experiments, the maximum crack width was exceeded before reaching permanent damage, either yielding or shear failure. The stop criterion for the residual crack width, on the other hand, was never exceeded. Therefore, the results are not consistent and a clear relation between the cracking state and the results cannot be established. For the failure mode, a relation cannot be established either. Experiments P804A1 and P502A2 presented flexural failure, but the fraction of the ultimate applied load at which the threshold value was exceeded is considerably different. This observation could be explained by the presence of cracks in experiment P502A2. As for P804A2, which failed in shear, the percentage of the ultimate load applied was even less than in the other two experiments. If a beam fails in shear before failing in flexure, a higher percentage for the margin of safety would be expected. Based on these results, the crack width stop criterion proposal, does not seem to be a good criteria to be applied on concrete bridges.

5

DISCUSSION

A comparison between the existing stop criteria based on the same data can be made. The German guideline

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Table 10.

Comparison of deflection values from Vos’s proposal. Limit values

Closest measurement

Experiment

0.9wmax (mm)

wres (mm)

Load (kN)

wmax meas (mm)

wres meas (mm)

% of ultimate load

P804A1 P804A2 P502A2

0.147 0.147 0.171

0.069 0.069 0.0075

118 115 122

0.17 0.21 0.27

0.043 0.018 0.02

57% 49% 81%

crack width stop criteria presents larger threshold values than Vos’s crack width proposal. Therefore, experiments reached the German guideline crack width stop criterion with a greater margin of safety. Additionally, limit values for wres on Vos’s proposal are much smaller than those from the German guideline. A crack smaller than 0.05 mm can be neglected since it is considered a microcrack, which is not structural, and wres values from Vos’s proposal are very close to or even smaller than 0.05 mm. Consequently, it does not seem to be an accurate stop criterion. However, it should be noted that these threshold values are calculated based on the crack spacing s. In this research, the value of s was obtained using Van Leeuwen’s (Van Leeuwen 1962) relation between reinforcement ratio with plain bars and average crack spacing; not the equations from Vos’s proposal. This may have caused the small values for wres . Additionally, when analyzing the residual crack width it should be considered that aggregate that spalls while concrete breaks could get stuck inside cracks and prevent its closure during unloading. This effect would cause a residual crack width larger than the allowable. In this case, the large value is not a signal of irreversible damage. Therefore, the 0.05 mm limit can be useful. The residual crack width also presented problems when collecting the data. Some measured values of residual crack width were negative, mostly because these are very small values and measurements combine elastic deformation and crack width which is difficult to isolate (Lantsoght et al. 2017b). This scenario should be considered if the crack width stop criterion is to be applied. The maximum crack width presented consistent results but several factors such as crack measurement and location of cracks make it difficult to apply this stop criterion on concrete bridges. Regarding the deviation from linearity index and the permanency ratio from ACI 437.2M-13, obtained values were erratic and the criteria could not be applied. These two criteria are based on taking data from the load-deflection diagram in the experiments and thus are influenced by the loading protocol.ACI 437.2M-13 (ACI Committee 437 2013) defines a loading protocol in order to apply this acceptance criteria, which was not followed in any of the experiments. Since the stop criteria are very sensitive to small changes in the values taken from the load-deflection plot, following ACI 437.2M-13 a force-controlled loading protocol would provide better data to apply these stop criteria. Additionally, the force-controlled loading protocol required by the ACI guide presents a problem for bridges:

displacement-controlled loading is safer when testing bridges because when large deformations occur no more force is applied. Therefore, the ACI deviation from linearity index and permanency ratio do not seem to be an adequate stop criteria for concrete bridges. Moreover, on what concerns Vos’s deflection proposal, a semi-linear approach seems to be an accurate approximation for the moment-curvature diagram. Experiments provided consistent results with an adequate margin of safety. However, the position of the load has an influence when calculating the maximum allowable deflection. For example, concentrated loads closer to the support would cause less deflection than the same load located at midspan. This was the case for experiment P502A2. The concentrated load was 1 m away from the support at a 5 m span; this reduction on deflection due to the position of the load may have led to reaching the maximum allowable deflection after yielding. Therefore, a loading protocol should be added to this proposal to obtain adequate threshold values. Finally, further investigation should address the following question: what percentage of the maximum allowable load must be applied at exceedance of a stop criterion, so that it is considered conservative or not? Limits too conservative will cause the proof load tests to be canceled long before the target load could be reached and will not provide any relevant information, and closing of a structure that performs adequately for the prescribed loads. On the other hand, threshold values at which the ultimate load is almost reached may be too risky to apply to real structures and, irreparable damage might be caused. 6

SUMMARY AND CONCLUSIONS

Proof load tests are used on structures to assure they fulfill the code requirements. High loads applied during these tests may cause irreversible damage on the structure. Therefore, guidelines have established stop criteria to end the test if irreversible damage is about to occur. Stop criteria evaluated in this research were taken from the German guideline of the DAfStB, ACI 437.2M-13 and a new proposal from Werner Vos (TU Delft). Only stop criteria based on flexure were analyzed. The measurements needed to apply these stop criteria are: load applied, deflection, concrete strain, and crack width. The results from four experiments of two castin-laboratory beams were used to analyze stop criteria. The conditions of the specimen vary between

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experiments: some beams were cracked before the experiment and other beams were not. Two experiments presented shear failure while the other two failed in bending. After analyzing and comparing the results of measured deflection, strains, and crack width between experiments and different stop criteria conclusions with regard to the existing stop criteria can be drawn. The German guideline concrete strain stop criterion is suitable for cracked and uncracked beams. Its results are consistent. For cracked beams, which resemble the conditions of concrete bridges, the stop criterion was reached with a good margin of safety. Therefore, the German guideline concrete strain seems to be a good criterion to be applied on concrete bridges. The German guideline crack width criteria andVos’s crack width proposal presented good results when the maximum crack width was analyzed. However, difficulties appeared when analyzing the residual crack width. Therefore, only the limit for the maximum crack width can be applied to concrete bridges. The German guideline residual deflection stop criterion does not seem to be a suitable criterion for both cracked and uncracked beams. The results are not consistent, and in two experiments the threshold value was surpassed during the first cycles. The ACI 437.2M-13 cyclic loading acceptance criteria must be used only if the ACI loading protocol is applied, otherwise the obtained results will be erratic and cannot be compared to the established threshold values. ACI maximum deflection is too permissive for use as a stop criterion. None of the specimens were close to reaching the threshold value. The residual deflection measurements must be done 24 hours after the test, and this procedure is not suitable for bridges. Moreover, force-controlled loading is required by the ACI loading protocol which is not as safe as displacement-controlled loading. Therefore, ACI 437.2M-13 acceptance criteria are not suitable to be applied on concrete bridges as stop criteria. Vos’s deflection proposal showed consistent results and seems reliable to be applied to concrete bridges. Nonetheless, further investigation should be done in what concerns stiffness from the unloading branch, so that it can take into account conditions of the structure prior to the test, and a relation to the prescribed loading protocol should be established.

ACKOWLEDGEMENTS The authors would like to acknowledge the support from the Dutch Ministry of Infrastructure and the Environment for the presented experiments, and from the USFQ program of Chancellor Grants for the research related to stop criteria for proof load tests. The participation of Dr. Yuguang Yang and Mr. Albert Bosman during the execution of the beam tests is also gratefully acknowledged. REFERENCES ACI Committee 437 2013. Code Requirements for Load Testing of Existing Concrete Structures (ACI 437.2M-13) and Commentary Farmington Hills, MA. Deutscher Ausschuss für Stahlbeton 2000. DAfStbGuideline: Load tests on concrete structures (in German). Deutscher Ausschuss fur Stahlbeton. Faber, M., Val, D. & Stewart, M. 2000. Proof load testing for bridge assessment and upgrading. Engineering Structures, 22, 1677–1689. Lantsoght, E.O.L., Koekkoek, R., Hordijk, D. & de Boer, A. 2017a. Towards Standarisation of proof load testing: pilot test on viaduct Zilweg. Structure and Infrastructure Engineering, 1744–8980, 1–16. Lantsoght, E.O.L., van der Veern, C., de Boer, A. & Hordijk, D., 2017b. State-of-the-art on load testing of concrete bridges. Engineering Structures, 150, 231–241. Lantsoght, E.O.L., Yang, Y., van der Veen, C., de Boer, A. & Hordijk, D., 2017c. Beam experiments on acceptance criteria for bridge load tests, ACI Structural Journal, 114-S84, 1031–1042. Monnier, T., 1970. The moment curvature relation of reinforced concrete, Heron, 17.2, 1–101. Olazek, P., Marek, L. & Casas, J.R., 2014. Diagnostic load testing and assessment of existing bridges: examples and application, Structure and Infrastructure Engineering, 10:6, 834–842. van Leeuwen Leeuwen, J., 1962. Over de scheurvorming in platen en balken, Heron, 10.1, 50–62. Vos, W. 2016. Stop criteria for proof loading – The use of stop criteria for a safe use of ‘Smart Proof loading’. M.Sc. Thesis, Delft University of Technology. Wight, J. & Macgregor, J., 2012. Reinforced Concrete: Mechanics and Design, Pearson Education, Inc., Upper Saddle River, NJ.

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