1 

Fatigue of concrete under compression: Database and proposal for high strength concrete
The compressive strength of concrete decreases as an element is subjected to cycles of loading. In a typical fatigue test for the concrete compressive strength, a concrete specimen (typically a cylinder) is loaded between a lower and upper stress limit. These limits are expressed as a fraction of the concrete compressive strength, and can be written as Sminfck and Smaxfck. The value of Smin and Smax are thus between 0 and 1. The upper limit for Smax in experiments is typically 0,95 and Smin can be as low as 0,02. Experiments in which alternating tensile and compressive stresses are applied can also be executed, but this loading case is not considered in the current study.
The result of fatigue tests on concrete cylinders in compression is the socalled Wöhlercurve, or SN curve. In this graph, a (linear) relation is found between the logarithm of the number of cycles N and the maximum fraction of the static compressive strength Smax. In the codes, different expressions are given for the relation between N and Smax. The codes that are studied in this report are the Dutch Code NEN 6723:2009 (Code Committee 351 001 "Technical Foundations for Structures", 2009), the Eurocode suite for concrete: NENEN 199211+C2:2011 (CEN, 2011a) with the Dutch National Annex NENEN 199211+C2:2011/NB:2011 (Code Committee 351 001, 2011a) and NENEN 19922+C1:2011 (CEN, 2011b) and the Dutch National Annex NENEN 1992 2+C1:2011/NB:2011 (Code Committee 351 001, 2011b), the new Model Code 2010 (fib, 2012). Some expressions from the literature are considered as well, such as the proposal by Hans Bongers (Snijders, 2013) and an expression suitable for higher strengths concrete (Kim and Kim, 1996).
The expression for concrete under compression subjected to cycles of loading from NENEN 199211+C2:2011 is more conservative than previously used expressions in the Netherlands. Therefore, different expressions are given in the National Annex NENEN 199211+C2:2011/NB:2011. The SN relationship given in the Dutch National Annex consists of two equations: the first branch is valid for N ≤ 106 cycles and the second branch for N > 106 cycles. The transition between these two expressions is not smooth, but instead causes a jump in the Wöhlercurve.
Because of this anomaly in the current code provisions, it is necessary to propose a new expression for concrete under cycles of compressive loading. Moreover, the proposed expression should be valid, yet not overly conservative, for high strength concrete. The current Eurocode NENEN 199211+C2:2011 is limited to concrete class C90/105. The fib Model Code goes up to C120. The goal of this report is to develop an expression that is valid up to C120. To check the quality of the proposed expression, it should be compared to experimental results. For this purpose, a database of experiments on (ultra) high strength concrete tested in compressive fatigue is developed first, and then used to validate the new proposal for concrete under cycles of compression.

[PDF]
[Abstract]

2 

Shear tests of reinforced concrete slabs and slab strips under concentrated loads
In slabs subjected to concentrated loads, the shear strength checks are conducted for two limit states:
1) shear over an effective width, and 2) punching shear on a perimeter around the point load. In current practice, the shear strength at the supports is determined with models that do not consider the transverse redistribution of load that occurs in slabs, which results in underpredictions for the actual slab shear capacity. Currently, an experimental program is being conducted at Delft University of Technology to determine the shear capacity of slabs under point loads near to the support. This paper presents the results of the tests conducted in continuous slabs and slab strips. In addition to studying the influence of the slab width, the specimens are tested with two types of reinforcement (ribbed and plain bars). The results of the experiments are compared to strength predictions from current design models. Also, recommendations for the support effective width and an enhancement factor for considering the effect of transverse load redistribution are given.

[PDF]
[Abstract]

3 

Shear Tests of Reinforced Concrete Slabs: Experimental Data of Undamaged Slabs. Concept v. 26102012

[PDF]

4 

Achtergrondrapport bij spreadsheet voor toetsing aan rand. Concept v. 16042012

[PDF]
[PDF]
[PDF]

5 

Voortgangsrapportage: Experimenten op platen in gewapend beton. Deel II: Analyse van de resultaten. Concept v. 16082012

[PDF]

6 

Background to Modified Bond Model. Concept v. 19102012

[PDF]

7 

Shear capacity of reinforced concrete slab bridges under a wheel load close to the support: Literature review. Concept v. 16082012

[PDF]

8 

Probabilistic approach to determine the increased shear capacity in reinforced concrete slabs under a concentrated load. Concept v. 12112012

[PDF]

9 

Tests of reinforced concrete slabs subjected to a line load and a concentrated load: Experimental data. Concept v. 25102012

[PDF]

10 

Shear In Reinforced Concrete Slabs under Concentrated Loads close to Supports
The capacity of existing solid slab bridges in the Netherlands is under discussion for two reasons: 1) the increased traffic loads and volumes and 2) the fact that the majority of the existing bridges were built before 1976, and are thus reaching the end of their original life span. Upon assessment according to the governing codes, a large number of slab bridges are found to be shearcritical. However, the shear capacity as prescribed by the codes is based on experiments on beams in shear. Slabs subjected to concentrated loads (such as truck wheel load) are assumed to have additional capacity as a result of transverse load redistribution. This thesis studies the capacity of slabs under concentrated loads close to supports. A literature review, resulting in a slab shear database with 215 experiments from the literature, is used to study the mechanisms at work in oneway and twoway shear. For this research, 156 experiments were carried out on 38 halfscale bridge deck specimens. The experimental results are studied by means of a parameter analysis. To determine the capacity of slabs in shear subjected to concentrated loads, two methods are proposed in the thesis: 1) the Modified Bond Model, a new theory to determine the capacity of slabs subjected to concentrated loads; and 2) by using a code extension proposal that results from probabilistic calculations following the safety philosophy of the Eurocodes. Finally, the link to the assessment practice is made by formulating recommendations, improving the Quick Scan assessment tool of the Ministry of Infrastructure and the Environment, and then applying this to cases of existing solid slab bridges.

[PDF]
[Abstract]

11 

Voortgangsrapportage: Experimenten op platen in gewapend beton onder combinatiebelasting. Deel II: Analyse van de resultaten. Concept v. 9112012

[PDF]

12 

Wide Beam Shear Behavior with Diverse Types of Reinforcement, Paper by S.E. MohammadyanYasouj, A.K. Marsono, R. Abdullah, and M. Moghadasi: Discussion by Eva O.L. Lantsoght

[PDF]

13 

Material properties: felt and reinforcement: for shear test of reinforced concrete slab

[PDF]

14 

FEM analyses: Shear tests of reinforced concrete slabs: experimental data of undamaged slabs
concept v. 29072011

[PDF]
[Abstract]

15 

Effective width in shear of reinforced concrete solid slab bridges under wheel loads

[PDF]

16 

Shear assessment of reinforced concrete slab bridges
The capacity of reinforced concrete solid slab bridges in shear is assessed by comparing the design beam shear resistance to the design value of the applied shear force due to the permanent actions and live loads. Results from experiments on halfscale continuous slab bridges are used to develop a set of recommendations for the assessment of slab bridges in shear. A method is proposed allowing to take the transverse force redistribution in slabs under concentrated loads into account, as well as a horizontal load spreading method for the concentrated loads. For selected cases of existing straight solid slab bridges, a comparison is made between the results based on the shear capacity according to the Dutch Code NEN 6720 and from the combination of the Eurocode (EN 19921 1:2005) with the recommendations, showing an improved agreement.

[PDF]
[Abstract]

17 

Concrete slabs under a combination of loads in shear
Previous experimental research at Delft University of Technology indicated an increased shear capacity of slabs under concentrated loads as a function of decreasing distance to the adjacent line support. Expressions have been derived for this increase, including the definition of an appropriate effective width. However, it is unknown if the uniformly distributed loads on solid slab bridges, e.g. due to dead loads, that act over the full width can be combined with the effects of concentrated loads acting only over the associated effective width at the support. To study this problem, additional experiments have been carried out at Delft University of Technology, in which a combination of loads consisting of a concentrated load close to the support and a line load over the full slab width are applied. The experimental results prove that the superposition principle applies to combinations of concentrated loads and distributed loads.

[PDF]
[Abstract]

18 

Practical application of transverse load redistribution in reinforced concrete solid slab bridges
For an initial design or assessment of a reinforced concrete solid slab bridge, spreadsheetbased or hand calculations are typically used. The shear stress is compared to the shear capacity as prescribed by the code. The distributed loads result in a uniform shear stress at the support. Concentrated loads are less straightforward to take into account. It is known that transverse load redistribution occurs in slabs. To explore the topic of transverse load redistribution, experiments on elements subjected to a concentrated load close to the support were carried out. These elements had an increasing width, starting at 0.5 m and increasing with steps of 0.5 m up to 2.5 m, so that the effect of transverse load redistribution could be studied. The threshold effective width resulting from the experiments was then compared to load spreading methods, in order to give recommendations for the practical use with concentrated loads. It was found that the load spreading method as used in French practice is to be preferred. As compared to load spreading methods that were used previously, the French load spreading method results in smaller shear stresses at the support. This result allows for more economic designs and provides a better assessment tool.

[PDF]
[Abstract]

19 

Predicting the shear capacity of reinforced concrete slabs subjected to concentrated loads close to supports with the modified bond model
The shear problem is typically studied by testing small, heavily reinforced, slender beams subjected to concentrated loads, resulting in a beam shear failure, or by testing slabcolumn connections, resulting in a punching shear failure. Slabs subjected to concentrated loads close to supports, as occurring when truck loads are placed on slab bridges, are much less studied. For this purpose, the Bond Model for concentric punching shear was studied at first. Then, modifications were made, resulting in the Modified Bond Model. The Modified Bond Model takes into account the enhanced capacity resulting from the direct strut that forms between the load and the support. Moreover, the Modified Bond Model is able to deal with moment changes between the support and the span, as occurs near continuous supports, and can take into account the reduction in capacity when the load is placed near to the edge. The resulting Modified Bond Model is compared to the results of experiments that were carried out at the Stevin laboratory. As compared to the Eurocodes (NENEN 199211:2005) and the ACI code (ACI 31811), the Modified Bond Model leads to a better prediction.

[PDF]
[Abstract]

20 

Shear Capacity of Existing Reinforced Concrete Slab Bridges under Traffic Loads
Poster. In the Netherlands, 60% of the existing bridges were built before 1975, while the traffic volumes and loads have increased over time. The results of a first assessment of the existing bridges showed that particularly the shear capacity of reinforced concrete solid slab bridges is often lower than the resulting shear stresses due to dead loads and traffic loads.

[PDF]
[Abstract]
