Existing bridges with large uncertainties can be assessed with a proof load test. In a proof load test, a load representative of the factored live load is applied to the bridge at the critical position. If the bridge can carry this load without distress, the proof load test shows experimentally that the bridge fulfills the requirements of the code. Because large loads are applied during proof load tests, the structure or element that is tested needs to be carefully monitored during the test. The monitored structural responses are interpreted in terms of stop criteria. Existing stop criteria for flexure in reinforced concrete can be extended with theoretical considerations. These proposed stop criteria are then verified with experimental results: reinforced concrete beams failing in flexure and tested in the laboratory, a collapse test on an existing reinforced concrete slab bridge that reached flexural distress, and the pilot proof load tests that were carried out in the Netherlands and in which no distress was observed. The tests in which failure was obtained are used to evaluate the margin of safety provided by the proposed stop criteria. The available pilot proof load tests are analyzed to see if the proposed stop criteria are not overly conservative. The result of this comparison is that the stop criteria are never exceeded. Therefore, the proposed stop criteria can be used for proof load tests for the failure mode of bending moment in reinforced concrete structures.@en

Most Dutch bridges were constructed around the middle of the twentieth century and considering the fact that traffic has increased exponentially since, it is important to find out if these bridges are still safe for use. Experiments on a 1:2 scale were carried out in the laboratory of the Delft University of Technology to investigate the bearing capacity of transversely prestressed concrete bridge decks subjected to concentrated wheel loads. All the tests showed failure in punching shear at loads much higher than expected from the current codes. This paper presents the results of the experimental parametric study including the effect of the transverse prestressing level (TPL), location of the load, number of loads, size of the loading area (wheel print/loading plate), and influence of previous damage to the deck slab panel, on the bearing capacity.@en

At the previous EURO-C conference, a guideline for nonlinear finite analysis and safety assessment of concrete structures was presented. This guideline is a results of a cooperation between the Delft University of Technology, the University of Parma and the Dutch Ministry of Infrastructure and the Environment. The nonlinear finite element results of a workshop around shear failure, held in Rotterdam in 2007, was the starting point for the development of a guideline for nonlinear finite element analysis. The scatter of the results presented in Rotterdam was considered as too large; knowledge of nonlinear finite element analysis of concrete structures should be controlled better in order to make nonlinear finite element analysis an accepted method. In 2012 the first version of the guideline that focussed on girder structures was published. In 2014, at the concrete structures laboratory of the Delft University of Technology, four almost similar pre-stressed girders were tested till the ultimate limit load level. This was considered as an opportunity to organise a contest to predict this ultimate load level at failure. The participants were asked to make use of the guideline. A workshop at the University of Parma gave the teams the opportunity to pitch their predictions. This paper presents the following topics: an overview of the content of the guideline, the gain of a controlled blind prediction contest and the main outcomes of the contest in Parma. @en

In a proof load test, a load corresponding to the factored live load is applied to a bridge, to directly demonstrate that a bridge fulfils the code requirements. The case of viaduct De Beek, a four-span reinforced concrete slab bridge that did not fulfil the requirements for bending moment after an inspection survey is studied. The middle spans, located above highway lanes, which could not be tested in the field, are the critical spans of this structure. Therefore, the observations from the field are used to check the finite element model, and improve the rating of the critical middle spans. The proposed method can be applied and further extended for proof load tests where the access to the site and the critical position of the viaduct is limited. This improvement will allow for an opti-mized combination of field testing and modelling, reducing the costs of field tests.@en

Reinforced concrete slab bridges are assessed for a combination of loads that include self-weight, superimposed loads, and distributed and concentrated live loads. The shear capacity of reinforced concrete slabs subjected to a combination of loads is thus an important topic for the assessment of existing bridges. Currently, a plastic model exists for the assessment of reinforced concrete solid slabs subjected to a concentrated load: the Extended Strip Model, based on the Strip Model for concentric punching shear. To apply this model to slabs subjected to a combination of loads, the model needs to be adapted based on theoretical principles. The results are then compared with the results from experiments on half-scale slab bridges subjected to a combination of a concentrated load close to the support and a line load. The result of this comparison is that the proposed method is suitable to find a safe estimate of the maximum concentrated load on the slab. The implication of this development is that an improved tool is available to estimate the maximum load of a truck that can be placed on a reinforced concrete bridge, thus improving the current assessment.@en

When designing a concrete structure, a structural engineer often assumes that it is initially free of stresses and cracks. Serviceability limit state analyses are therefore often carried out only taking into account the structure’s self weight and the other imposed loads. As a result, initial tensile stresses and (internal) cracking that can arise from, e.g., partially restrained shrinkage is not accounted for. This might imply that service life predictions can be too optimistic since micro-cracks can promote the penetration of chlorides, with the result of an accelerated corrosion of the reinforcement. The impact of restraint can be at both the structural level (plane sections must remain plane) and the material level (concrete is a non-homogenous material from components having different shrinkage behaviour and stiffness). In this contribution the results of the numerical analysis of eigenstresses and micro-cracking due to the shrinkage of the cement paste in concrete are highlighted. The two-dimensional Delft beam lattice model (BLM) is used to simulate concrete on meso-level as a three-phase material. Stress relaxation in the cement paste is calculated with the activation energy method. For the comparison of the calculated micro-cracking the results of laboratory tests on concrete are used by which the chloride penetration is measured under different restraint conditions with different levels of micro-cracking. The results of the numerical analyses with BLM specimens showed that micro-cracking can increase the diffusion coefficient of (uncracked) concrete with a factor 1.2–1.6, depending on the magnitude of stress relaxation. The residual tensile eigenstresses in concrete reduces due to micro-cracking and stress relaxation till values in the order of 0.5–1.0 MPa which are not taken into account in global calculations and can speed up (unexpected) cracking in the concrete.@en

In 1997 the Second Stichtse Bridge, a high strength concrete box girder bridge was built in the Netherlands using the balanced cantilever method. At that time, the long-term behaviour of this material (with a cube compressive strength of 75 MPa) was not known. Therefore, it was proposed to monitor the material behaviour and the deflections of the bridge for ten years, and a few properties have been monitored for twenty years. To evaluate the concrete material properties over time, concrete cubes were cast with the segments, and stored inside the bridge at the section locations. These samples have been tested periodically. Also shrinkage measurements were carried out on a concrete sample stored inside the bridge. The deflections of the bridge superstructure have been measured periodically along both edges of the bridge. Based on the available data, it is found that the concrete compressive and splitting tensile strength, as well as the shrinkage deformations, remain constant. The deflections are stabilizing as well.@en

In the Netherlands, approximately 150 prestressed T-beam bridges with cast-in-between decks and mainly built in the sixties are still in service. Upon assessment, the prestressed beams often do not fulfil the design codes requirements, whereas upon inspection, they show no signs of distress. For the assessment, additional load transfer mechanisms, which could significantly enhance their structural capacity, such as compressive membrane action and transverse redistribution, are not considered. The presented research aims at quantifying these effects. Therefore, an existing simply supported multi-span T-beam bridge was tested. In total seven experiments were carried out using a single point load placed on the T-beam. Three experiments were carried out with the original structural system unchanged. In four experiments, the castin- between deck was sawn in longitudinal direction, so that the individual behaviour of the beams could be tested. In both cases two load positions, i.e. a distance of 2.25 m and 4.00 m from the support were used. By analysing both the single beam and the connected beam tests a better understanding of the load carrying capacity of this type of bridge is achieved. @en

A large subset of the Dutch bridge stock consists of reinforced concrete slab bridges, for which assessment often results in low ratings. To prioritize the efforts of the bridge owner, more suitable assessment methods for slab bridges are necessary. Research efforts over the past years resulted in the development of several methods, at levels requiring increasing costs, time, and effort for increasing accuracy. The last option, when an analytical assessment is not possible due to uncertainties, is to use proof load testing to evaluate the bridge directly. To develop recommendations for the proof load testing of reinforced concrete slab bridges for the Netherlands, different methods are combined: pilot proof load tests on bridges with and without material damage, a collapse test, tests on beams taken from an existing bridge and new beams with similar dimensions cast in the laboratory, and an extensive literature review. The result of this study is a set of recommendations that describe how to prepare and execute a proof load test, and how to analyze the results. This paper summarizes the research program about proof load testing from the Netherlands and gives an overview of the currently developed recommendations and topics for further research.@en

Proof loading of existing bridges is an interesting option when
insufficient information about a bridge is available. To safely carry out a proof
loading test, high loads are placed on the bridge. To avoid permanent damage to
the structure, a controlled loading protocol needs to be described, and the
measurements need to be closely monitored to identify the onset of distress. The
criteria from existing codes and guidelines to evaluate the measurements, called
stop criteria, are not universally applicable. To develop recommendations for
proof loading of reinforced concrete solid slab bridges, beam experiments were
analysed. The beams were heavily instrumented to evaluate the existing stop
criteria, and possibly develop new stop criteria. The result of these experiments
is the development of a standard loading protocol for the proof loading of
reinforced concrete slab bridges. Recommendations for the use of the stop
criteria are also formulated. These insights are used to develop a new guideline
for the proof loading of reinforced concrete slab bridges in the Netherlands.@en

In the Netherlands a large number of concrete bridges with post-tensioned concrete decks are in service and are constructed in the 60s and 70s of the previous century. These decks need to be investigated regarding their structural safety because of increased traffic loads in both number and intensity. An important type of prestressed bridges present in the Netherlands is constructed using prestressed T-girders and transversely post-tensioned cast in-between slab panels.

During static tests in the Stevin laboratory of the Delft University of Technology (DUT) on this type of concrete bridge decks it was found that compressive membrane action has been generated which results in a higher static bearing capacity. On the other hand it was found that when a load is applied stepwise with only three load cycles per step in about 8 h, the load capacity reduces about 16% with respect to the static capacity.

The Dutch Ministry of Infrastructure and the Environment and DUT decided that further experimental research on the influence of cyclic loading on the fatigue strength for this type of bridge deck was necessary. In total 3 static and 9 fatigue tests were performed. Based on the experimental results a relation was found between the relative fatigue strength and the logarithm of the applied number of load cycles. For 1 and 100 million load cycles a relative fatigue strength of respectively 0.54 and 0.42 was found.@en

Proof load tests of bridges can be very useful for structures with a lack of information, or for structures of which the effect of material degradation is difficult to assess. Contrary to diagnostic load testing, proof load testing is not well-defined in current standards in terms of required load and analysis of measurements. The risk related to the high loads used in proof load testing requires standardisation for these tests. The paper highlights important considerations for proof load testing that may lead to the development of guidelines in the Netherlands, by illustrating a pilot study on the viaduct Zijlweg in the Netherlands. This reinforced concrete bridge rates too low in shear. Topics of interest are the required load that the bridge has to withstand to be approved by the load test and the interpretation of the measurements during the test to avoid permanent damage to the structure. These measurements were compared to the stop criteria from existing codes for buildings, to examine if recommendations for the use with bridges can be formulated. The final result of the test on this case study is that the capacity of the viaduct is proven to be sufficient for shear and bending moment.

@en

Many existing reinforced concrete bridges in The Netherlands are suspected of having insufficient shear strength, hence a methodical approach is necessary to assess them. The aim of this paper is to provide a structured approach for assessment of the reinforced concrete slab bridges, which make up a considerable proportion of the Dutch bridge stock. The proposed method uses the levels of approximation introduced in the fib Model Code for Concrete Structures 2010. The recommendations at all levels are linked to experimental research. The different levels include spreadsheet-based calculations (the “Quick Scan”), linear finite element models, nonlinear finite element models, the analysis of a structure with cracked concrete, and, for exceptional cases, proof loading of the structure. The result of this approach is a structured method of levels of assessment that can be used when many bridges need to be assessed. For most bridges, assessment at the lowest level will prove sufficient. For a limited number of cases, higher levels of assessment will offer options and guidelines for a more in-depth study of the structure being assessed.

@en

Typically, beam shear failure is studied by testing small, heavily reinforced beams subjected to concentrated loads, and punching shear failure by testing slab-column connections. Both cases are related to shear in slender, flexural members. For deep members, strut-and-tie methods provide solutions. One-way slabs subjected to concentrated loads close to supports, as occurs with truck loads on slab bridges, are much less studied. A theoretical solution to this problem is found by modifying the Strip Model for punching shear into an Extended Strip Model, taking into account the strut between the load and the support. Moreover, the Extended Strip Model takes into account the reduction in capacity resulting from unfavorable geometric circumstances. The resulting capacities based on the Extended Strip Model and on the shear provisions from NEN-EN 1992-1-1:2005 and ACI 318-14 are compared to experimental results, showing the improvement and uniformity of results by using the Extended Strip Model.@en

Most methods for the design and analysis of reinforced concrete slabs for punching are based on experiments on slab-column connections, reflecting the situation in building slabs. Slab-column connections with unbalanced moments have also been studied in the past. Experiments indicate that the accuracy of models for asymmetrically loaded slabs is lower than for symmetrically loaded slabs. In this paper, the difference in accuracy between test predictions for symmetrically and asymmetrically loaded slabs is tackled. A plastic model, the Extended Strip Model, is proposed. The results of maximum loads according to this model are compared to experimental results of symmetrically and asymmetrically loaded slabs. The comparison between the proposed Extended Strip Model and the experimental results shows that the model has a consistent performance for both symmetrically and asymmetrically loaded slabs. Moreover, the model has as an advantage that it combines the failure modes of flexure, shear and punching. The proposed model can be used for the analysis of slabs. In particular, it can be used for the assessment of existing slab bridges subjected to concentrated live loads.@en

As the bridge stock in the Netherlands and Europe is ageing, various methods to analyze existing bridges are being studied. Proof load testing of bridges is an option to experimentally demonstrate that a given bridge can carry the prescribed live loads. Based on extensive research on proof load testing of reinforced concrete slab bridges carried out in the Netherlands, recommendations for proof load testing of reinforced concrete slab bridges were developed. The recommendations for the preparation, execution, and post-processing of a proof load test are summarized in this paper. The novelty of the recommendations is that proof load testing for shear is studied, and that a proposal for stop criteria for shear and bending moment has been formulated. Further research on the shear behavior is necessary, after which the recommendations will be converted in guidelines for the industry.@en

Load testing of bridges is a practice that is as old as their construction. In the past, load testing gave the traveling public a feeling that a newly opened bridge is safe. Nowadays, the bridge stock in many countries is aging, and load testing is used for the assessment of existing bridges. This paper aims at giving an overview of the current state-of-the-art with regard to load testing of concrete bridges. The work is based on an extensive literature review, dealing with diagnostic and proof load testing, and looking at the current areas of research. Additional available information about load testing of steel, timber, and masonry bridges, buildings, and collapse testing is briefly cited. For the implementation of load testing to the aging bridge stock on a large scale, efficiency in procedures is required. The areas requiring future research are identified, based on the available body of knowledge.@en

Proof load testing can be a suitable method to show that a bridge can carry the required loads from the code without distress. This paper addresses the preparation, execution, and analysis of a proof load test on a four-span reinforced concrete solid slab bridge, viaduct de Beek. The bridge has one lane in each direction, but was restricted to a single lane, since an assessment showed that the capacity is not sufficient to allow both lanes. For this proof load test, the bridge was heavily equipped with sensors, so that early signs of distress can be seen. The difficulty in this test was that, for safety reasons, only the first span could be tested, but that the lowest ratings were found in the second span. A direct approval of the viaduct by proof loading was thus not possible, and an analysis was necessary after the field test. The result of this analysis is that only by allowing 6.7% of plastic redistribution in the second span, sufficient capacity can be demonstrated.@en

The bridges built during the development of the Dutch road network after the Second World War are reaching their originally devised service life. A large subset of the Dutch bridge stock consists of reinforced concrete slab bridges. This bridge type often rates insufficient according to the recently introduced Eurocodes. Therefore, more suitable methods are developed to assess reinforced concrete slab bridges to help transportation officials make informed decisions about the safety and remaining life of the existing bridges.
If information about a bridge is lacking, if the reduction in structural capacity caused by material degradation is unknown, or if an assessment shows insufficient capacity but additional capacity can be expected, a bridge might be suitable for a field test. A proof load test demonstrates that a given bridge can carry a certain load level. In the Netherlands, a number of existing reinforced concrete slab bridges have been proof loaded, and one bridge has been tested to collapse. Bridges with and without material damage were tested. These bridges were heavily instrumented, in order to closely monitor the behavior of the bridge. Critical positions for bending moment and shear were studied.
Based on the proof load tests that were carried out over the past years, a set of recommendations for the systematic preparation, execution, and analysis of proof load test results is compiled. These recommendations will ultimately form the basis of the guideline for proof load testing for the Netherlands, which is currently under development.
@en

Proof load testing is part of the engineering practice, and can be particularly useful for the rating of existing bridges. This paper addresses how reliability-based concepts can be used in combination with proof load testing, and discusses how this approach differs from the current practice for proof load testing. Whereas the calculation methods for determining the updated reliability index after a proof load test are available in the literature, this approach is now used to determine the proof load magnitude required to demonstrate a certain reliability level in a bridge, the viaduct De Beek. To determine the required proof load magnitude, the known integrals of the limit state function are solved. The method is applied to a case of a bridge that was proof load tested in the Netherlands, viaduct De Beek. The data of this bridge are used to determine the required proof load magnitude to fulfill a given reliability index. A sensitivity study is carried out to identify the effect of the assumptions with regard to the coefficient of variation on the resistance and load effects. The result of this approach is that large loads are necessary in proof load testing if a reliability index needs to be proven in a proof load test. In the current practice of proof load testing with vehicles, it can typically only be demonstrated that a certain vehicle type can cross the bridge safely. The results in this paper provide a new insight on the required proof load magnitudes to show that the reliability index of the tested bridge is sufficient. However, consensus on the coefficients of variation that need to be used on the resistance and load effects, is still missing, which significantly affects the results for the required proof load magnitudes.@en