Quantification of digital image correlation applicability related to in-situ proof load testing of bridges

I've been working closely together with my colleagues at the Danish University of Technology for quite a while, and am cosupervisor of Christian Christensen. The first paper of this collaboration and the PhD research of Christian was presented at SMAR 2019, the 5th international Conference on Smart Monitoring, Assessment and Rehabilitation of Civil Structures. The title of the paper is: "Quantification of digital image correlation applicability related to in-situ proof load testing of bridges".

The abstract of the paper is:
Advanced crack monitoring is crucial for high precision response- and threshold evaluation when performing proof- and diagnostic load tests on existing concrete structures. Mostly, crack monitoring techniques involve one monitoring method, which provide thresholds with regard to stop criteria and characterization information. Such thresholds and related precision uncertainties are expected to be of significant importance in identifying stop criteria as well as deliver input for probabilistic models. In the ongoing Danish bridge testing research program, it is hypothesized that several independent monitoring techniques are needed to reduce uncertainties related to crack detection and categorization. A number of novel monitoring methods are used in the research project. A special focus is however dedicated to two promising crack-monitoring techniques suited for combined use: a) Two-dimensional digital image correlation (2D-DIC) and b) Acoustic emission (AE). The output is expected to provide a unique crack evaluation, for which limitations and uncertainties of the techniques should be quantified individually as well as in combination. This paper presents initial research concerning evaluations related to digital image correlation based on sub-component beam tests performed in the DTU CasMat laboratory facility. The tested beams were prefabricated as TT-elements with a length of 6.4 m and cut into two T-beam elements. The test matrix consisted of ten beams strengthened with carbon fiber reinforced polymer (CFRP) in different configurations with and without post-tensioning of the CFRP, thus resulting in different crack initiation behavior. The investigations in this paper include: (1) time of crack detection compared to visual detection, (2) time of crack detection compared to time of crack width threshold values, and (3) crack width evaluation using 2D-DIC strain correction for out-of-plane deflection. The results show that cracks can be detected prior to both visual detection and significant stiffness change. After detection, crack development can be monitored for crack width stop criteria. Crack widths can also be successfully monitored for surfaces subjected to out-of-plane movement using a geometric correction method. The methodology is hypothesized to be of significant importance in future testing of full-scale concrete slab bridges in the Danish bridge testing project.

Optimizing Finite Element Models for Concrete Bridge Assessment With Proof Load Testing

My colleagues and I recently published a paper "Optimizing Finite Element Models for Concrete Bridge Assessment With Proof Load Testing" in Frontiers in the Built Environment - Bridge Engineering for the Research Topic Diagnostic and Proof Load Testing on Bridges.

The paper is Open Access and can be accessed here.

The abstract is
Proof load testing of existing reinforced concrete bridges is becoming increasingly important as the current bridge stock is aging. In a proof load test, a load that corresponds to the factored live load is applied to a bridge structure, to directly demonstrate that a bridge fulfills the code requirements. To optimize the procedures used in proof load tests, it can be interesting to combine field testing and finite element modeling. Finite element models can for example be used to assess a tested structure after the test when the critical position could not be loaded. In this paper, the case of viaduct De Beek, a four-span reinforced concrete slab bridge, is studied. Upon assessment, it was found that the requirements for bending moment are not fulfilled for this structure. This viaduct was proof load tested in the end span. However, the middle spans are the critical spans of this structure. The initial assessment of this viaduct was carried out with increasingly refined linear finite element models. To further study the behavior of this bridge, a non-linear finite element model is used. The data from the field test (measured strains on the bottom of the concrete cross-section, as well as measured deflection profiles) are used to update the non-linear finite element model for the end span, and to improve the modeling and assessment of the critical middle spans of the structure. Similarly, an improved assessment based on a linear finite element model is carried out. The approaches shown for viaduct De Beek should be applied for other case studies before recommendations for practice can be formulated. Eventually, an optimized combination of field testing and finite element modeling will result in an approach that potentially reduces the cost of field testing.

How do steel fibers improve the shear capacity of reinforced concrete beams without stirrups?

I've recently published a review paper in the journal Composites Part B: Engineering titled "How do steel fibers improve the shear capacity of reinforced concrete beams without stirrups?".

The paper addresses the different shear-carrying mechanisms, and the effect of adding steel fibers to a concrete mix on these shear-carrying mechanisms in specimens with steel tension reinforcement. This review paper is the result of my Poligrant 2017-2018, continued as a Poligrant 2018-2019. While originally I started the research to find a way to extend to Critical Shear Displacement Theory with an extra term that takes the contribution of the fibers into account, reading on the topic and understanding the mechanics further led me to the insight that the simple solution of adding an extra term is insufficient. This paper shows how all shear-carrying mechanisms are influenced by the presence of fibers.

The abstract is:
Even though the structural behavior steel fiber reinforced concrete (SFRC) has been extensively researched, structural applications are still limited. One barrier to its implementation is the lack of mechanical models that describe the behaviour of SFRC members failing in shear. This paper reviews the effect of steel fibers on the different mechanisms of shear transfer and combines the observations from the literature regarding the parameters that affect the shear capacity of SFRC. Additionally, a selection of currently available expressions for the shear capacity of SFRC is presented. This paper reviews the current state-of-the-art on the shear capacity of SFRC elements without shear reinforcement, shows the lacks in our current understanding on the shear behaviour of SFRC elements without shear reinforcement, and outlines the steps necessary to address these lacks. The presented work aims to be a framework for (experimental) efforts addressing the shear capacity of SFRC members without shear reinforcement.

You can access the article here.

Two papers from IABMAS 2018

My colleagues and I have published two papers in IABMAS 2018. I was supposed to travel to Melbourne to present these papers, but it was very shortly after my return from my annual research stay in Delft, and my baby girl did not take well to my absence and return, so I was adviced to invest time in restoring our bond. I was warned against not traveling after such a short time again, as it may leave her confused. So I canceled the conference (only second time ever I had to cancel a conference, and I did feel bad about it, but I also felt bad about my baby not being well because of my long absence...).

The two papers we published were part of a Special Session that we organized at the conference (it's a pity I couldn't travel and chair the session I spent so much time preparing on, but such is life...).

The first paper is "Monitoring crack width and strain during proof load testing" and the abstract is:
In a proof load test, the applied load is representative of the factored live load, to demonstrate experimentally that the bridge fulfils the code requirements. Signs of distress must be caught with the instrumentation by defining stop criteria. In the literature, several stop criteria for flexure are available. The German guidelines describe, amongst others, a limiting crack width and strain. However, the background of these limiting values is not clear. Therefore, a theoretical approach based on flexural theory is followed. The theoretically derived values are then compared to experimental results obtained from beam experiments. The result of this research work is a limiting value of crack widths and strains that can be used during proof load testing of concrete bridges. The arbitrary stop criteria that were used in the past can now be replaced by stop criteria that are based on the theory of concrete beams in flexure.

The second paper is "Twenty years monitoring of a high strength concrete cantilever bridge" and the abstract is:
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.

Two presentations at IABSE 2017

At IABSE 2017, my colleague presented two papers of our research. I didn't travel to IABSE, because the conference fell right in the middle of my maternity leave.

The first paper is titled "Proof load testing of the viaduct De Beek" and the abstract is as follows:

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.

You can find the slides of the presentation here:

Proof load testing of the viaduct De Beek from Eva Lantsoght

The second paper is titled "Recommendations for proof load testing of reinforced concrete slab bridges" with the following abstract:

Proof loading of existing bridges is an option to study the capacity when crucial information about the structure is lacking. To define the loading criteria for proof load testing, a review of the literature has been made, finite element models of existing viaducts have been made, and on these viaducts, proof loading tests have been carried out. These bridges were heavily instrumented, to learn as much as possible about the structural behaviour during proof loading. Additional laboratory experiments have been used to develop controlled loading protocols, and to identify which stop criteria can be used for which case. As a result of the analysis and experiments, recommendations are given for proof loading of bridges with respect to the required maximum load and the stop criteria. These recommendations have resulted in a guideline for proof loading of existing reinforced concrete slab bridges for The Netherlands.

This paper was presented in a poster session, with a short pitch. The pitch is as follows:

Recommendations for proof load testing of reinforced concrete slab bridges - presentation from Eva Lantsoght

The poster is:

Recommendations for proof load testing of reinforced concrete slab bridges - poster from Eva Lantsoght

Pilot Proof-Load Test on Viaduct De Beek: Case Study

My colleagues and myself recently published a paper in the Journal of Bridge Engineering, titled "Pilot Proof-Load Test on Viaduct De Beek: Case Study". The paper reports on the proof load test on viaduct De Beek which our research group carried out in November 2015. I couldn't join my colleagues in the field at that time, but in the summer of 2016, I helped finalizing the analysis report of the test.

The abstract is as follows:

For existing bridges, proof-load testing can be a suitable assessment method. This paper addresses the evaluation of a posted reinforced concrete slab bridge over a highway through proof-load testing, detailing the preparation, execution, and analysis of the test. As the target proof-load and the required measurements for proof-load testing currently are not well-defined in the existing codes, this pilot case was used to develop and evaluate proposed recommendations for proof-load testing for a future guideline on proof-load testing for the Netherlands. Moreover, the pilot proof-load test is used to study the feasibility of proof-load testing for both shear and flexure.

You can download the paper here:

Plastic model for asymmetrically loaded reinforced concrete slabs

My colleagues and I recently published a paper titled "Plastic model for asymmetrically loaded reinforced concrete slabs" in a Special Publication of the ACI, ACI-SP 321 "Recent Developments in Two-Way Slabs: Design, Analysis, Construction, and Evaluation". This SP is the result of a session held at the ACI Fall Convention in 2015.

You can purchase the SP through the ACI website.

The abstract of our paper is as follows:

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.

Towards standardisation of proof load testing: pilot test on viaduct Zijlweg

My co-authors and myself recently published a paper in Structure and Infrastructure Engineering about the load test on viaduct Zijlweg. The paper is titled "Towards standardisation of proof load testing: pilot test on viaduct Zijlweg". You can access the paper through this link.

The abstract is as follows:

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.

Determination of loading protocol and stop criteria for proof loading with beam tests

At the fib symposium 2017, I presented a paper titled "Determination of loading protocol and stop criteria for proof loading with beam tests". The abstract of the paper is as follows:

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.

Here you can find the slides of the presentation:

Determination of loading protocol and stop criteria for proof loading with beam tests from Eva Lantsoght

Extended Strip Model for slabs subjected to a combination of loads

I recently presented a paper titled "Extended Strip Model for slabs subjected to a combination of loads " at the fib symposium in Maastricht.

The abstract of the paper is:

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.

Here you can find the slides of the presentation:

Extended Strip Model for slabs subjected to a combination of loads from Eva Lantsoght

Development of recommendations for proof load testing of reinforced concrete slab bridges

My colleagues and I recently published a paper in Engineering Structures, titled "Development of recommendations for proof load testing of reinforced concrete slab bridges". You can download the paper for free for the next 50 days through this link.

This paper is the last journal paper in a series of papers based on the research I carried out with regard to proof load testing. I have a few more conference papers planned on smaller analyses that I did as part of the research, but my data are depleted by now. On to new research and/or more testing to deepen this topic (wherever the funding takes us)!

The abstract of the paper is as follows:

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.

State-of-the-art on load testing of concrete bridges

My coauthors and myself recently published a review paper on load testing, titled "State-of-the-art on load testing of concrete bridges" in Engineering Structures. You can download the article for free until September 13th 2017 by accessing this link.

The abstract of the paper is as follows:

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.

Modeling Concrete Material Structure: a Two-Phase Meso Finite Element Model

We recently published a paper titled "Modeling Concrete Material Structure: a Two-Phase Meso Finite Element Model". The work reported in this paper was a collaboration with the department of mechanical engineering of Universidad San Francisco de Quito. My colleague Dr. Bonifaz developed a coupling between Dream3D and Abaqus for modeling metal materials. In our collaboration, we looked at the possibility to apply these concepts to concrete.

You can find the full paper here.

The abstract reads as follows:

Concrete is a compound material where aggregates are randomly placed within the cement paste. To describe the behavior of concrete structures at the ultimate, it is necessary to use nonlinear finite element models, which for shear and torsion problems do not always give satisfactory results. The current study aims at improving the modeling of concrete at the meso-level, which eventually can result in an improved assessment of existing structures. Concrete as a heterogeneous material is modeled consisting of hydrated cement paste and aggregates. The stress–strain curves of the hydrated cement paste and aggregates are described with results from the literature. A three-dimensional (3D) finite element model was developed to determine the influence of individual phases on the inelastic stress–strain distribution of concrete structures. A random distribution and morphology of the cement and aggregate fractions are achieved by using DREAM.3D. Two affordable computational dual-phase representative volume elements (RVEs) are imported to ABAQUS to be studied in compression and tension. The virtual specimens (concrete mesh) subjected to continuous monotonic strain loading conditions were constrained with 3D boundary conditions. Results demonstrate differences in stress–strain mechanical behavior in both compression and tension test simulations. A strong dependency of flow stress and plastic strain on phase type, aggregate (andesite) size, shape and distribution upon the composite local response are clearly observed. It is noted that the resistance to flow is higher in concrete meshes composed of finer and homogeneous aggregate particles because the Misses stresses and effective plastic strains are better distributed. This study shows that at the meso-level, concrete can be modeled consisting of aggregates and hydrated cement paste.

Papers at SEMC 2016

This year, I was author and co-author of three papers at SEMC 2016, the conference on Structural Engineering, Mechanics and Computation in Cape Town, South Africa. While I did not have the chance to travel, my students Mr. Valdivieso and Mr. Mejia traveled to present their thesis work, and Dr. van der Veen traveled to present our co-authored paper.

The abstract of mr. Mejia's paper is:

Throughout history invasive methods for analyzing deflections and deformations have been used in concrete structures at the laboratory, but the advancement of technology has allowed the development of new non-invasive alternative methods such as digital image correlation (DIC). With this technique, it is possible to obtain information about the deflections, strains and strain fields in a structure. The current study consists of performing a flexural test on plain concrete beams and concrete arches reinforced with FRP rein-forcement. All tests were recorded with a cheap, small camera, then transferred into a series of images in or-der to apply the digital image correlation technique. The analysis with DIC results in the displacements, strains and strain fields of the surface under analysis. Finally, the percentage of error between the displace-ment derived from the DIC technique and the displacement measured by Linear Variable Differential Trans-formers (LVDTs) is calculated. In conclusion, the study shows that it was not possible to reach accuracy on the values of deflections and strains by the applied method and that a higher-speed camera is necessary to capture the moment of failure.

The abstract of mr. Valdivieso's paper is:

A large number of existing bridges in Europe and North-America are reaching the end of their devised service life. Therefore, it is necessary to improve the methods of assessment for existing bridges. One method, suitable for existing reinforced concrete slab bridges, is the Modified Bond Model. This method, however, currently only takes the effect of torsion for loads close to the edge into account in a simplified manner. In this study, finite element models are created of a slabs with two supports, three concentrated (pre-stressing) loads and a distributed load, representing a truck wheel print. The load is varied along the longitu-dinal and transverse directions of the slab to find the bending moments (mx and my) and torsional moments (mxy). The results is an expression for the effect of torsion in slabs, which can be used with the Modified Bond Model for assessment and design of slab bridges.

The abstract of the paper of which I am first author is:

For the assessment of existing structures and the design of new structures, it is important to have a good understanding of the flow of forces, here applied to reinforced concrete solid slabs. Two analyti-cal methods are used: finite element models with 3D solid elements and a plasticity-based model that is suita-ble for hand calculations, the Modified Bond Model. The slabs that are modeled are half-scale models of rein-forced concrete solid slab bridges. As the Eurocode live load model prescribes more heavily loaded trucks in the first lane, the load model is asymmetric. For the finite element models, limited use is made of the redistri-bution capacity of the slab. For the Modified Bond Model, the influence of torsion and the edge effect need to be taken into account. The results of these studies improve the current state-of-the-art for analysis and design of reinforced concrete slabs.

Here are the slides of my paper:

Modeling of symmetrically and asymmetrically loaded reinforced concrete slabs from Eva Lantsoght

Ruytenschildt Bridge: Field and laboratory testing

We recently published a paper on the field and laboratory testing of the Ruytenschildt Bridge. This bridge was tested to failure in two spans, and then beams sawn from the bridge were tested in the lab to study the behavior further.

The abstract of the paper is as follows:

A large number of existing reinforced concrete solid slab bridges in the Netherlands are found to be insufficient for shear upon assessment. However, research has shown additional sources of capacity in slab bridges, increasing their total capacity. Previous testing was limited to half-scale slab specimens cast in the laboratory. To study the full structural behavior of slab bridges, testing to failure of a bridge is necessary. In August 2014, a bridge was tested to failure in two spans. Afterwards, beams were sawn out of the bridge for experimental work in the laboratory and further study. Though calculations with current design provisions showed that the bridge could fail in shear, the field test showed failure in flexure before shear. The experiments on the beams study the transition from flexural to shear failure and the influence of the type of reinforcement on the capacity. The experimental results were compared to predictions of
the capacity for the bridge slab and the sawn beams. These comparisons show that the current methods for rating of existing reinforced concrete slab bridges, leading to a sharper assessment, are conservative. It was also found that the application of plain bars instead of deformed bars does not increase the shear
capacity of beams.

You can download the paper for free until November 12th 2016 via this link.

Using Eurocodes and AASHTO for assessing shear in slab bridges

We've recently published a paper in the Proceedings of the Institution of Civil Engineers - Bridge Engineering. You can find this paper online. The abstract is as follows:

Reinforced concrete short-span solid-slab bridges are used to compare Dutch and North American practices. As an assessment of existing solid-slab bridges in the Netherlands showed that the shear capacity is often governing, this paper provides a comparison between Aashto (American Association of State Highway and Transportation Officials) practice and a method based on the Eurocodes, and recommendations from experimental research for the shear capacity of slab bridges under live loads. The results from recent slab shear experiments conducted at Delft University of Technology indicate that slabs benefit from transverse force redistribution. For ten selected cases of straight solid-slab bridges, unity checks (the ratio between the design value of the applied shear force and the
design beam shear resistance) are calculated according to the Eurocode-based method and the Aashto method. The results show similar design shear forces but higher shear resistances in the North American practice, which is not surprising as the associated reliability index for Aashto is lower.

Case Study on Aggregate Interlock Capacity for the Shear Assessment of Cracked Reinforced-Concrete Bridge Cross Sections

We recently published a paper on a case study we did on an existing bridge in 2010/2011. The paper is published in the Journal of Bridge Engineering, and can be accessed through this link.

The abstract of the paper is as follows:

A 55-year-old bridge showed large cracking in the approach bridge caused by restraint of deformation and support settlement. After repair, it was uncertain at which crack width the traffic loads on the bridge should be further restricted. The shear capacity was calculated by counting on the aggregate interlock capacity of a supposedly fully cracked cross section. An aggregate interlock relationship between shear capacity and crack width based on an unreinforced section was used to find the maximum allowable crack width. Limits for crack widths at which load restrictions should be imposed were found. The large structural capacity of the cracked concrete section shows that the residual bearing resistance based on the aggregate interlock capacity of reinforced concrete slab bridges with existing cracks is higher than expected. This expected capacity could be calculated with the inclined cracking load from the code provisions. The procedure outlined in this paper can thus be used for the shear assessment of fully cracked cross sections of reinforced concrete bridges.

Proposal for the fatigue strength of concrete under cycles of compression

We've recently published a paper in Construction and Building Materials, titled "Proposal for the fatigue strength of concrete under cycles of compression". This paper is based on the research I did during summer 2014 in Delft, to develop a proposal for the Dutch National Annex to the Eurocode.

You can download the paper for free following this link until March 1, 2016.

The abstract of the paper is:

The Dutch National Annex to Eurocode 2 deviates from Eurocode 2 for the Wöhler curve for concrete in compression, but has a discontinuity in the S–N curve for 1 million load cycles. Therefore, a new expression for concrete subjected to repeated loading is sought, which should be valid, yet not overly conservative, for high strength concrete. A database of experiments on high strength concrete tested in compressive fatigue is developed, and used to derive new expressions. Two new formulas are proposed: (1) for the assessment of the fatigue strength of existing structures, and (2) a simplified method for
design.

Effective shear width of concrete slab bridges

My coauthors and I recently published a paper in ICE (Institute of Civil Engineering) - Bridge Engineering. This publication is our first in this journal. The work is based partially on the 7th chapter of my PhD thesis, and extended with some additional research that Dr. de Boer and I worked on.

The abstract is as follows:

For the shear assessment of existing concrete slab bridges in the Netherlands, different levels of approximation are used. Shear stress resulting from the dead loads and live loads is determined in a spreadsheet or from a linear finite element model. In a spreadsheet-based approach the effective width for the wheel prints needs to be assumed and in finite-element methods, the length over which the peak shear stress can be distributed needs to be assumed. To recommend a load-spreading method, experiments were executed on slab strips of increasing widths. The experiments, a statistical analysis and non-linear finite-element models support a distribution from the far side of the wheel print to the face of the support. To find the distribution width in a finite-element method, a numerical model is compared to an experiment in which the reaction forces were measured. These  measurements were compared to the stress profile at the support from the model, showing that the peak stress can be integrated and distributed over 4dl.

Experimental investigation on shear capacity of reinforced concrete slabs with plain bars and slabs on elastomeric bearings

My co-authors and I recently published a paper in Engineering Structures - this paper is our second paper in this journal. To better distribute the paper, Elsevier allows free access to the paper via this link until October 29th, 2015.

The abstract of the paper is the following:

One-way slabs supported by line supports and reinforced with deformed bars were shown previously to behave differently in (one-way) shear than beams. For the application to existing slab bridges, the influence on the shear capacity of using plain reinforcement bars and of supporting the slab by discrete bearings is investigated. To study these parameters and their influence on the shear capacity, a series of experiments was carried out on continuous one-way slabs (5 m x 2.5 m x 0.3 m), subjected to concentrated loads close to the support line. The results from these experiments are compared to code provisions and a method developed by Regan. These experiments confirm the findings that slabs subjected to concentrated loads close to supports have larger shear capacities than beams.