Gintaris Kaklauskas

The modern construction industry has witnessed a marked shift towards the utilization of high-strength steel reinforcement, exhibiting yield strengths exceeding 600 MPa in reinforced concrete structures. Tension stiffening is a critical factor for accurate prediction of deflection and crack width. The current study evaluates the accuracy of state-of-the-art models in predicting curvature in Reinforced Concrete (RC) beams reinforced with high-strength steel (HSS) bars. This study employed three design code methods (Eurocode 2, ACI 318-14, and ACI 318-19) and two other models: the Bischoff model and Kaklauskas and Sokolov’s model. An RC beam with HSS bars was tested, and experimental data on another 63 RC beams reinforced with HSS rebars were collected from various published studies. The test data ranged in various geometrical and material characteristics and were evaluated across a wide range of steel stress intervals. An inverse analysis was carried out to calculate the resultant internal force of tensile concrete (tension stiffening) from the experimental moment–curvature diagram. The inverse analysis demonstrated that the fully cracked RC section reached stiffness at a bending moment of about 3M_cr, where M_cr is the cracking bending moment predicted according to the EC2 design code. Statistical analysis showed that the predicted mean normalized curvature (κ_th/κ_exp) across several reinforcement stress levels ranged from 0.99 to 0.81 for different models. The design codes tend to underestimate curvature. The coefficients of variation ranged between 17.8% and 24.9% for different models.

Accurate assessment of tension stiffening is important for predicting deflection and crack width in RC structures. Earlier studies by the authors have shown that an extended cover thickness increases tension stiffening in bending RC members. The current study experimentally and theoretically investigates curvature and tension stiffening in RC beams nominally having a 50 mm cover for 32 mm bars of tensile reinforcement. The four-point bending tests were carried out on square section (400×400 mm) RC beams. Mean experimental curvatures were obtained for the pure bending zone by different approaches, namely, from a mid-point deflection and from strains at several horizontal layers measured either by LVDT or DIC technique. The tension stiffening effect in the test beams was quantified by inversely calculating the resultant internal force of tensile concrete, Nct, using the test moment – curvature diagrams. Tension stiffening is characterized by parameter β0 indicating the ratio of β0 = M / Mcr at which the force Nct reaches zero. The condition Nct = 0 represents the bending stiffness of a fully cracked RC section. Earlier studies by the authors have shown that parameter β0 equals to 3 for the beams with a typical cover thickness (25–35 mm). The current study has demonstrated that for the beams having nominal cover thickness of 50 mm and bar diameter of 32 mm, parameter β0 reached rather high values indicating a little degradation of tension stiffening with increasing load.

Reinforcement corrosion in concrete structures with excessive crack width poses a high risk of reducing the structure's service life. The crack width behavior is one of the most complex aspects of the mechanics of reinforced concrete (RC). With most of the models used in practice being semi–empirical or empirical, very few analytical approaches have been proposed. However, the analytical models lack either accuracy or simplicity, or both. This paper presents a new analytical model, termed the Pure Shear Model, that predicts mean crack width by a simple formula. It is based on the partial interaction tension stiffening model considering a short RC tie subjected to short–term loading. The model assumes elastic material properties and neglects shrinkage, internal cracking, and slip at the interface. It presumes that the only deformations that occur in concrete are the shear strains due to shear lag that are taken constant across the cover thickness. Deplanation of concrete section due to shear lag results in crack width linearly increasing from zero at the bar to its maximum value on the surface of the RC member. Despite the simplicity of the proposed model, its accuracy in predicting mean crack width was shown to be comparable to that of the design code methods.

This study deals with experimental and numerical cracking analysis of full–scale reinforced concrete (RC) beams. Four RC beams of the rectangular section were tested under a four-point bending configuration. Crack development of test beams was recorded throughout the pure bending zone using an electronic microscope and digital cameras. Crack pattern has been obtained using digital image correlation (DIC) technique. Numerical analysis of experimental elements at macro level was performed using the finite element (FE) software package ATENA considering a plane stress state with non-linear constitutive laws for concrete and reinforcement. Comparison of experimental and numerical results has shown that FE modelling can be considered as a reliable tool for planning experimental research programs. Adequately developed finite element numerical models of RC members provide the possibilities to insight the tendencies and mechanism of possible crack propagation as well as changes of crack parameters.

In the modern construction industry, lightweight aggregate concrete (LWAC) is often used to produce load-bearing structural members. LWAC can be up to 40% lighter by volume than normal strength concrete. However, the lack of adequate numerical models often limits the practical application of innovative building materials such as lightweight concrete in real projects. The present study conducted a comparative numerical deformation analysis of a full-scale bridge deck slab and girder. Using the physical model proposed by the authors and the finite element software ATENA, the deformations of full-scale lightweight and traditional reinforced concrete elements under the short-term effects of permanent and variable loads were compared. Depending on the safety and serviceability limit requirements, it was found that the amount of longitudinal reinforcement in lightweight reinforced concrete elements could be reduced compared with that in standard reinforced concrete elements with the same parameters. The results of the numerical analysis showed that the deformation analysis model proposed by the authors could serve as an alternative tool for the design of lightweight concrete flexural members with the selection of optimum geometric and reinforcement parameters limited by the stiffness condition.

A bond mechanism at the reinforcement-concrete interface is one of the key sources of the comprehensive functioning of reinforced concrete (RC) structures. In order to apprehend the bond mechanism, the study on bond stress and slip relation (henceforth referred as bond-slip) is necessary. On this subject, experimental and numerical investigations were performed on short RC tensile specimens. A double pull-out test with pre-installed electrical strain gauge sensors inside the modified embedded rebar was performed in the experimental part. Numerically, a three dimensional rib scale model was designed and finite element analysis was performed. The compatibility and reliability of the numerical model was verified by comparing its strain result with an experimentally obtained one. Afterwards, based on stress transfer approach, the bond-slip relations were calculated from the extracted strain results. The maximum disparity between experimental and numerical investigation was found as 19.5% in case of strain data and 7% for the bond-slip relation at the highest load level (110 kN). Moreover, the bond-slip curves at different load levels were compared with the bond-slip model established in CEB-fib Model Code 2010 (MC2010). Overall, in the present study, strain monitoring through the experimental tool and finite element modelling have accomplished a broader picture of the bond mechanism at the reinforcement-concrete interface through their bond-slip relationship.