J.C. Walraven

Parameter-study on the influence of steel fibers and coarse aggregate content on the fresh properties of self-compacting concrete

Self-compacting concrete (SCC) offers several economic and technical benefits; the use of steel fibers extends its possibilities. Steel fibers bridge cracks, retard their propagation, and improve several characteristics and properties of the concrete. Fibers are known to significantly affect the workability of concrete. Therefore, an investigation was performed to compare the properties of plain SCC and SCC reinforced with steel fibers. Two mixtures of SCC with different aggregate contents were used as reference. Each of the concretes was tested with four types of steel fibers at different contents in order to answer the question to what extent the workability of SCC is influenced. The slump flow, a fiber funnel and the J-ring test were used to evaluate the material characteristics of the fresh concrete. This paper discusses the suitability of the applied test methods and the effect of the coarse aggregate content, the content and type of steel fibers on the workability of SCC.

Shear in One-Way Slabs under Concentrated Load Close to Support

One-way slabs under concentrated loads are generally designed for shear by checking the beam shear resistance and the punching shear resistance over an effective width. Only a small number of test data regarding the shear resistance of one-way slabs subjected to concentrated loads is currently available. To be able to better evaluate the shear resistance of one-way slabs, a series of experiments was carried out on continuous one-way slabs (5 m x 2.5 m x 0.3 m [16.4 ft x 8.2 ft x 11.8 in.]) subjected to concentrated loads close to the supports, in which the load position, transverse reinforcement ratio, and concrete strength were varied. The test results are compared with code provisions and a method developed by Regan. The results show a different behavior in shear for slabs under concentrated loads than for beams.

Recommendations for the shear assessment of reinforced concrete slab bridges from experiments

Abstract Upon assessment of existing reinforced concrete short-span solid slab bridges according to the recently implemented Eurocodes that include more conservative shear capacity provisions and heavier axle loads, a number of these structures were found to be shear-critical. The results from recent experimental research on the shear capacity of slabs indicate that slabs benefit from transverse load distribution. Recommendations for the assessment of solid slab bridges in shear are developed on the basis of these experiments. A load spreading method for the concentrated loads is proposed and the applicability of superposition of loading is studied. The resulting most unfavourable position for the design trucks is provided and implemented in the so-called Dutch “Quick Scan” method (QSEC2). Cases of existing bridges are studied with the previously used QS-VBC as well as with the QS-EC2 that includes the recommendations. As a result of the assumed transverse load distribution, the shear stress to be considered at the support based on the recommendations becomes smaller.

Shear Capacity of Normal, Lightweight, and High-Strength Concrete Beams according to Model Code 2010. II: Experimental Results versus Nonlinear Finite Element Program Results

AbstractNonlinear finite element analysis (NLFEA) is a useful tool for the assessment and design of structures if the reliability of the prediction is known in advance. Therefore, it makes sense to tailor finite element programs to specific types of structural members by calibrating them against test results obtained on similar types of specimens. By increasing the accuracy of prediction for such specified types of structural members, assessment and design are possible with higher reliability than can be obtained with analytical models. This strategy is illustrated for the case of shear-reinforced flanged beams using the constitutive model PARC_CL implemented into ABAQUS code. A comparison with analytical models shows the power of tailored FEM calculations in combination with reliability considerations.

Improved Tensile Performance with Fiber Reinforced Self-compacting Concrete

The use of self-compacting concrete (SCC) eliminates the need for compaction, which has benefits related to economic production, the durability, the structural performance and working circumstances. SCC is able to transport fibers which can replace in some structures conventional reinforcement. By taking into account tailor-made concrete characteristics, new fields of structural application can be explored. This paper discusses the potential for an improved performance of fibers in self-compacting concrete. In flexural tests significant differences were observed between conventional and self-compacting concrete at a given fiber type and dosage concerning the variation of results and the flexural performance. Mechanical testing and image studies on concrete cross-sections indicate how the flow influences the performance, the orientation and the distribution of the orientation of fibers. Differences between traditionally compacted and flowable concrete are pointed out.

One-Way Slabs Subjected to Combination of Loads Failing in Shear

SUMMARY AND CONCLUSIONS For the assessment of existing solid slab bridges subjected to dead loads and live loads, it is necessary to analyze the combination of the shear stresses from concentrated and distributed loads. A first series of experiments on the shear capacity of slabs only considered concentrated loads close to supports. Recommendations for the effective width in shear resulted from these tests.The current series of experiments studied the combination of loads. The hypothesis of superposition of concentrated loads over their respective effective width and distributed loads over the full slab width was tested. A unique series of 26 experiments on eight slabs subjected to a combination of a concentrated load close to the support and a line load was conducted. It was found that the hypothesis of superposition is valid and conservative. Typically, larger shear capacities were found for the case of combined loading.The influence of the distance between the concentrated load and the support and the shear capacity was confirmed for the case of combined loading. Slabs supported by a line of elastomeric bearings showed a less brittle failure mode than slabs supported by a line of steel bearings. There is no experimental evidence for treating direct load transfer differ -ently depending on the support material (as prescribed by NEN-EN 1992-1-1:2005).Experiments were carried out with the concentrated load close to the simple and continuous support. A slightly larger shear capacity was found at the continuous support. This increase was smaller than predicted by α

Shear Capacity of Normal, Lightweight, and High-Strength Concrete Beams according to Model Code 2010. I: Experimental Results versus Analytical Model Results

Many models to determine the shear capacity of shear reinforced beams are based on the truss analogy. Various proposals have been formulated in recent years, all of which differ with regard to the limits of strut inclination. Remarkably, those limits do not depend on the type of concrete, which could be expected to be critical for the shear friction capacity of the cracks, which is supposed to be a major influencing factor with regard to the limit of strut rotation. Tests on beams with I-shaped cross sections have been carried out on beams made of normal, lightweight, and high-strength concrete. The experimental results are compared, in Part I of this paper, with those obtained by analytical models and, in Part II, with those obtained by nonlinear finite element programs tailored to this specific application. The result is that the type of concrete does not lead to significant changes in strut rotation capacity, so that the strut rotation limit values have general validity. The level of approximation approach, as presented in the Federation Internationale du Beton/International Federation for Structural Concrete (fib) Model Code of Concrete Structures 2010 is justified: more sophisticated calculation models lead indeed to more accuracy in the determination of the shear capacity.

Shear Capacity of Concrete Beams under Sustained Loading

Long-term tests on large-scale concrete beams without shear reinforcement, which are tested for more than two years under sustained loading close to the ultimate shear capacity (load ratio ranging from 87% to 95%) under climate controlled condition, show that sustained loading has no significant effect on the shear capacity. Although many flexural and shear cracks develop, the beams carry the load for a long time. The tests show that crack formation takes place only within some days after the load application, but after a week the cracks stabilize and become dormant.

Shear Behavior of Reinforced Concrete Beams without Transverse Reinforcement Based on Critical Shear Displacement

AbstractThis paper presents a new concept to evaluate the shear capacity of reinforced concrete beams without shear reinforcement having flexural shear failure. Based on experimental observations, it is proposed that the opening of the critical inclined crack can be considered as the lower bound for the shear capacity of a structural member. It is also proposed that the unstable opening of the critical inclined crack is triggered when the shear displacement in an existing flexural crack reaches a critical value Δcr. Thus the critical shear displacement is used as a failure criterion, and based on that a new shear evaluation method is proposed. The method shows agreeable accuracy when compared with test results in literature.

Case study on aggregate interlock capacity for the shear assessment of cracked reinforced-concrete bridge cross sections

AbstractA 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 assessm...