Research

Structural applications of conventional and innovative concrete structures

The research work mainly concerns the structural applications of conventional and innovative concrete structures. The research studies are developed through experimental campaigns, non-linear analytical approaches and non-linear finite element analyses. The latter are fundamental for taking into account the non-linear behavior of the construction materials considered as well as the actual behavior of concrete structural elements under investigation.

He has been working on precast tunnel segments and its structural optimization through the adoption of advanced reinforcement solutions based on innovative materials such as Fiber Reinforced Concrete (FRC) and its combination with conventional rebars. In order to properly take into account the post-cracking residual strengths exhibited by FRCs the adoption of opportune uni-axial-tensile constitutive law is fundamental. The latter can be retrieved by means of inverse analyses aiming at simulating the behavior of FRC small samples under bending or can be directly obtained from uni-axial direct tensile tests. Both methods have been adopted and the corresponding results have been compared by considering the effects of fiber orientation and distribution on post-cracking FRCs performances.

The reinforcement optimization generally leads to the adoption of a combination of fiber and traditional reinforcement, whose its effectiveness at Serviceability Limit State (SLS) has been investigated for different structural applications such as precast tunnel segments and pavements. The latter have been also investigated through advanced numerical simulations for evaluating the risk of cracking due to thermal and shrinkage effects.

Interests

The main research interests are the following:

• • infrastructures in Fiber Reinforced Concrete (FRC);

  • structural behavior at Serviceability Limit State (SLS) of concrete elements reinforced by a combination of fiber reinforcement and conventional rebars;

  • structural applications of Fiber Reinforced Concrete;

  • evaluation of the post-cracking response of Fiber Reinforced Concrete through uniaxial-tensile-tests and bending-tests.

Keywords

• • PE8_3 - Civil engineering, maritime/hydraulic engineering, geotechnics, waste treatment

  • Concrete Durability

  • Concrete Technologies

  • Concrete Durability

  • Fiber Reinforced Concrete

  • Fibers Orientation and Distribution

  • Finite Element Modelling

  • Non-Linear FE-Analysis

  • Pavement Engineering

  • Precast tunnel segments

  • Reinforced Concrete

  • Structural Design

  • Structural Optimization

  • Tunnelling

  • Concrete Technologies

Infrastructures in Fiber Reinforced Concrete (FRC)

In the scientific community, among practioners and precast industries there is a growing interest in Fiber Reinforced Concrete (FRC) for use in infrastructures (such as bridges, tunnel linings etc.) due to the high performance structural levels required at SLS and ULS for such important structures. Within this framework, the research work was focused to tunnel linings made by precast tunnel segments for evaluating the advantages due to FRC in terms of structural behavior, boost of the production process and reduction to time-consuming labor works (http://dx.doi.org/10.1016/j.tust.2015.08.013, http://dx.doi.org/10.1016/j.tust.2016.12.005, http://dx.doi.org/10.1016/j.engstruct.2019.109628).

The study was developed by considering the main loading conditions concerning precast tunnel segments (Tiberti, 2014, ISBN 978–88–548–7005–5); particular attention was devoted to the excavation temporary phase. In this stage the hydraulic jacks of Tunnel Boring Machine (TBM) exert high concentrated forces in order to provide the necessary normal compressive action at tunnel face for enabling the dig of the ground. The corresponding TBM thrust phase was deeply investigated through non-linear numerical simulations of a single segment or group of segments. In these research studies the noticeable post-cracking residual strengths provided by FRCs were included in numerical models adopted (http://doi.org/10.1016/j.engstruct.2021.112253).

The high concentrated forces introduced on tunnel segments generally by TBM rams determine tensile splitting transverse stresses, which can determine undesiderable cracking phenomena. The latter were deeply investigated through small-scale experimental tests for reproducing the local behavior under TBM hydraulic jacks (http://dx.doi.org/10.1016/j.tust.2015.08.013, http://dx.doi.org/10.1016/j.compositesb.2016.08.032, http://dx.doi.org/10.1680/jmacr.15.00430).

In order to better investigate the behavior of tunnel segments during the TBM thrust phase and in other important temporary phases (e.g.: de-moulding, storage and transportation) full scale tests were considered. The latter need special laboratory equipment for withstanding high forces involved in the aformentioned stages as well as steel moulds of precast tunnel segments. To this aim, Laboratorio Pietro Pisa of the University of Brescia was equipped with two steel moulds of precast tunnel segment in order to cast several full-scale tunnel segments with different reinforcement solutions (traditional rebars only, fibers only or opportune combinations of rebar and fiber reinforcement).

Full-scale test of precast tunnel segment

TBM thrust phase: numerical models adopted and typical numerical results

Structural Behavior at Serviceability Limit State (SLS) of concrete elements reinforced by a combination of fiber reinforcement and conventional rebars

Durability of reinforced concrete structures is becoming a topic of paramount interest in the scientific community. Concrete durability can be achieved by reducing the porosity of concrete matrix or through a reduction of crack widths exhibited by concrete structures at SLS. The latter can be obtained by a combination of conventional rebars and fiber reinforcement. In fact, the bridging effect provided by fibers at crack locations enable to exploit residual post-cracking strengths leading to the opportunity to re-introduce stresses into the concrete through steel-to-concrete bond with a shorter transmission length. This phenomenon was deeply investigated through tests on tension ties in order to evaluate the crack pattern development, reduction of mean crack spacing and lower crack widths due to FRCs (http://dx.doi.org/10.1016/j.cemconcomp.2013.10.004, http://dx.doi.org/10.1016/j.cemconres.2014.10.011, https://doi.org/10.1680/jmacr.17.00361). A simplified analytical model was also proposed for describing the mutual collaboration between conventional rebars and fiber reinforcement (Tiberti, 2014, ISBN 978–88–548–7005–5). The model describes the increment of tension-stiffening effect (reduction of the average member strain for a given applied load), the reduction of mean crack spacing and cracks widths due to fibers contribution.

Tension tie made by central conventional rebar and FRC: test set-up and typical results

Structural applications of Fiber Reinforced Concrete

Fiber Reinforced Concrete is currently adopted in numerous structural applications. Besides precast tunnel segments previously mentioned, FRC concrete pavements and FRC columns were investigated. An experimental program was defined to study the effects of the volume fraction and the type of fibers on the behavior of full-scale precast columns subjected to reversed cyclic horizontal loading under constant axial load. Furthermore, in a second phase of research, unidirectional (uniaxial) and bidirectional (biaxial) lateral displacement were imposed to RC and FRC columns in order to better simulate the actual response of a column during an earthquake. (http://dx.doi.org/10.1016/j.compositesb.2015.09.010, http://dx.doi.org/10.1617/s11527-015-0783-3).

The use of fibers as an alternative reinforcement to steel welded wire mesh and rebars is today an extensive practice for the reinforcement of concrete slabs-on-grade. Despite the widespread use of fiber reinforcement, the corresponding benefits in controlling cracking phenomena due to shrinkage are generally not considered in the design process of Fiber Reinforced Concrete (FRC) slabs-on-grade. Within this framework, in order to evaluate the effect of fiber reinforcement in slabs-on-grade with respect to shrinkage phenomena, a parametric study based on non-linear numerical analyses was carried out by considering different slab geometries and distributions of shrinkage deformations along the slab thickness; subgrade friction was investigated as well. Jointless pavements without contraction joints were considered in order to evaluate if fiber reinforcement can mitigate cracking phenomena even when rather large distances between construction joints are used (https://doi.org/10.3390/fib6030064).

Referring to FRC structural applications previously mentioned, most of these structures are subjected to cyclic loads whose effects should be considered by researchers and designers, since repeated loading may cause structural fatigue failure and, even if the load does not cause a fatigue collapse, several material properties (such as strength, toughness, stiffness and durability) can suffer detrimental effects under service conditions. Consequently, a further topic of research was the fatigue behavior of FRC. In this regard, post-peak fatigue three Point Bending Tests (3PBT) on notched beams were performed; hence, the tests concerned pre-cracked specimens, which means that a microcracked zone placed at the crack tip (known as Fracture Process Zone, FPZ) was already present when starting the cyclic stage (http://dx.doi.org/10.1617/s11527-015-0783-3).

FRC concrete pavements: numerical model adopted for evaluating the effects of shrinkage phenomena

Evaluation of the post-cracking response of Fiber Reinforced Concrete through uniaxial-tensile-tests and bending tests

Fiber reinforcement considerably improve the uni-axial post-cracking tensile response of concrete, providing noticeable post-cracking residual strengths in spite of a brittle behavior of plain concrete. Consequently, it is fundamental to include in non-linear analytical approaches as well as in non-linear numerical analyses the fiber resistant contribution after cracking through opportune stress vs. crack opening laws. In this regard, indirect tensile tests, such as three point bending tests (3PBTs), were carried out together with Uniaxial Tensile Tests (UTTs), which requires an accurate setup and a specialized equipment to avoid instability during the test. The latter were carried out in collaboration with Heidelberg Cement Group (http://dx.doi.org/10.1002/suco.201800224). Moreover, the influence of fiber orientation on flexural post-cracking residual strengths and on uniaxial constitutive law is currently under investigation. Within this framework, since 3PBTs on notched beams are generally used for determining the post-cracking performance of FRCs as well as the constitutive laws to be used for structural design, it is fundamental to study the fiber orientation detected in these tests. The latter can be different than that exhibited in the final structure and, hence, it is necessary to introduce a simplified parameter for increasing or reducing the post-cracking nominal strengths retrieved by 3PBTs, in order to better represent the actual structural behavior (https://doi.org/10.1016/j.cemconcomp.2018.07.012, http://dx.doi.org/10.1002/suco.201700068).

Major Research Project

National Italian Project, PRIN2006

Project name: “Ottimizzazione delle prestazioni strutturali, tecnologiche e funzionali, delle metodologie costruttive e dei materiali nei rivestimenti delle gallerie”, “Optimisation of the Structural, Technological and Functional Performance of Construction Methodologies and Materials in Tunnel Linings”, start period: March 2006, period (months): 36 months (end of project, March 2010)

International collaborations

Prof. dr. ir. C.B.M. Blom, Delft University of Technology

Em. prof. dr. ir. J.C. Walraven, Delft University of Technology

Prof. dr. Mohammad AlHamaydeh, American University of Sharjah

Prof. Dr.-Ing. habil. Peter Mark, Ruhr-University Bochum

Prof. Dr.-Ing. Klaus Holschemacher, Leipzig-University of Applied Sciences