1,721,073 research outputs found
Novel reliability-based design formulation for evaluating transmission and anchorage lengths in prestressed concrete elements
No full textThis work proposes two new formulations for designing both transmission and anchorage lengths in prestressed reinforced concrete members. Such two length values represent, respectively, the
distance required to transmit completely the prestressing force in the concrete, and the overall distance needed to fully anchor the tendon force in the concrete at the ultimate limit state. The
two formulations proposed here are based on a reliability-based design, starting from the equations discussed within the fib TG2.5 “Bond and Material Models” and from the current
approach present in the fib MC2010. For such scope, two datasets were collected, one for each investigated variable, which comprise more than 1000 experimental evidences from literature.
The collected results were used to carry out a probabilistic calibration of the two formulations, and to evaluate the model uncertainty for both the investigated cases. Lastly, suitable coefficients
are proposed to compute the two lengths, targeting to the required reliability level
Corrosion effects on bond between strand and concrete in pre-tensioned members: analytical modeling of transmission length
No full textCorrosion in pre-tensioned members causes loss of prestressing and worsening of strand mechanical characteristics. Moreover, it can lead to concrete cracking or wire breaking. These effects influence the bond between steel and cementitious matrix, which, in pre-tensioned concrete elements, plays a fundamental role in the whole performance of the structure. Bond is indeed responsible for the transmission of the initial prestress and, therefore, its variation directly reflects on the beam behavior. In this paper, a novel approach is proposed to model the bond strength in presence of corrosion. Particular attention is paid to the transmission length evaluation, through a proposal of an analytical model able to include the effects of corrosion. Thick-Walled Cylinder theory is applied to simulate the behavior of the corroded strand close to the beam ends. Simulation results show that corrosion has remarkable consequences on bond, inducing a strong increase in transmission length after a corrosion ratio of 7.00%. These outcomes are consistent with experimental evidence found in the literature
1st Workshop on The New Boundaries of Structural Concrete
No full textDuring the last 30 years, concrete technology has made considerable advances and provided new and high performance materials suitable for satisfying the requirements of designers and construction companies.
In the beginning, research achievements mainly concerned compression strength. The increase of the concrete strength was made possible due to a reduction of the water-cement (w/c) ratio up to a value of about 0.40. Below this value, it was not possible to assure the workability of the mixture for practical applications.
Later on, with the aim of reducing the placement costs, superplasticizers have been developed, which also allowed to further reduce the w/c ratio with the consequent benefits in terms of strength.
The development of new generations of superplasticizers has then allowed for the production of self-compacting concrete (SCC), which often represents the only solution suitable for the realization of earthquake-resistant RC joints, where the reinforcement density makes the placement of ordinary concrete very difficult, because of the reduced spacing among rebars.
Research efforts in last few years mainly focused the attention on material and structural durability which is a critical issue for the sustainability of the constructions. Actually, concrete durability represents one of the revolutionary novelties of the new Italian building code (D.M. 14-1-2008) that, for the first time, introduces the concept of “nominal life” of a structure in a mandatory document, defined as the “number of years in which the structure, although subjected to ordinary maintenance, has to accomplish the function to which is devoted”.
A durable structure requires the use of materials having an enhanced quality. The choice of durable materials is also a convenience’ choice as the increase of costs for improving the material performance is generally minimal if compared with the construction costs. In addition, it is further convenient when considering the maintenance costs not to say the lawyer expenses when the controversy goes into a court.
In order to increase the “nominal life” of RC structures (and so, to enhance their durability), the concrete porosity has to be reduced so that a better protection of steel reinforcement is assured. This implies the reduction of the w/c ratio which should be very low when the external environment is very aggressive.
The lower w/c ratio required for structural durability implies a higher concrete strength which, for the new Italian structural code, can have a class strength up to C70/85, characterized by a characteristic value of the compressive strength (measured on cubic specimens) not lower than 85 N/mm2.
Once a lower concrete porosity is assured, protection of steel reinforcement also requires a control of cracks occurring in the structure. This can be addressed by means of a correct placement of the rebars and, if necessary, also by adding fibers to the concrete matrix; special types and contents of fibers have to be chosen according to the required performance.
Fiber Reinforced Concrete (FRC) is one of the materials that, together with the alloys of aluminum, the alloys of copper, the fiber reinforced polymer materials, the panels with collaborating polyurethane or polystyrene, non-traditional wall materials, the structural glass and the non-metallic reinforcement, has been mentioned in the Italian structural code. For this reason, with the preliminary approval of Central Technical Service of the Infrastructure Department, FRC can be used for the structural members.
The Department of Infrastructure has under preparation specific Guidelines on the use of fiber-reinforced concrete for which the “Guide for the Design and Construction of Fiber-Reinforced Concrete Structures” - edited by the National Research Council - is already available (CNR DT 2004).
A new concrete performance which will have an increasing importance in the design regulations is represented by shrinkage, aimed at reducing the cracking of the structure. This need is already a priority in industrial concrete pavements and will become more relevant for other structural elements since the increase of concrete strength made the autogenous shrinkage significant.
The new Fib Model Code – reference document for several international structural codes – includes specific design regulations for the new materials.
In summary, it is evident that the research and technological development have allowed concrete to be keep up with time, by continuing to represent an optimal choice for the realization of constructions more and more demanding in terms of safety, durability and sustainability.
However, this modern know-how has not been completely understood by many structural designers that, often, make still use of “out-of-date” concretes.
This anomaly is mainly due to the lack of regular meetings between concrete technologists and structural engineers. This is one of the main motivations which encouraged to organize the first Workshop on “The new boundaries of structural concrete” held at the University of Salerno in April 22-23, 2010. The Workshop has been organized through a joint collaboration between the Department of Civil Engineering of the University of Salerno and the American Concrete Institute Italy Chapter.
Experts in concrete technology and structural engineering from universities, industry and construction companies joined this event.
This volume collects the papers presented at the workshop, divided in the following four sessions:
Session A – Performance and life-cycle costs of new concrete structures
Session B – Controlled-performance concrete
Session C - New scenarios for concrete
Session D – Concrete quality control on site
Four invited lectures opened the two workshop days, while each session was preceded by a General Lecture dealing with the specific topic of the session.
Two “Honorary Members” of ACI International, namely Professors Mario Collepardi and Surendra Shah, were present at the workshop and gave invited lectures.
The Workshop aimed to put together experts in concrete technology and structural engineering to discuss about the future of concrete, which is more and more devoted to the improvement of the performances, durability and care for the environment, without significantly affecting the costs.
Several scientific contributes on the use of recycled aggregates in concrete – obtained by demolition or industrial waste (such as pneumatics) – have been also presented at the workshop. Other papers have dealt with the problem of the structural rehabilitation by following the new earthquake-resistance’s requirements of the constructions
Lap splice connection of new-to-existing rebars: a numerical study based on literature data
Full text linkBond between steel reinforcement and concrete is the basic and fundamental mechanism which
assures load transfer between the two materials in reinforced concrete elements. Even though
bond has been widely investigated during the last decades, the number of parameters involved
in the mechanism, as well as the number of possible geometrical configurations of the final
element, are such that both models for the local bond-slip law and for anchorages or lap splices
still need further investigation. As evidence of this, design-oriented documents, such as the fib
Model Code, are constantly updated. Significant modifications of the bond models and the
methods for the calculation of the anchorage length are foreseen in the next generation of codes.
The use of post-installed reinforcement offers a reliable solution for the connections in concrete
structures, the strength and the stiffness being similar to traditional cast-in rebars. Many
applications may be found in the rehabilitation and strengthening of existing structures.
Nonetheless, post-installed rebars are becoming popular also in new constructions to make
easier building and flexibility in design. Within this framework, structural designers often face
the need of overlapping existing and new rebars, with a lack of design recommendations. This
paper aims to investigate the load transfer of the new-to-existing lap splices. Numerical
simulations were carried out with a commercial code, particularly an experiment from the
literature was reviewed and numerically revaluated, also simulating the effects of different
splice lengths, not counted in the original study. Inverse analysis was used to calibrate a local
bond-slip law, which was the input for the following simulations. Failure mode and crack
pattern are discussed showing that splitting is the dominant failure mode for short lap splices
CNR-DT 204/2006. Finalmente una normativa per la strutture in calcestruzzo fibrorinforzato
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An overview of enhancements to provisions for bond in the draft fib Model Code 2020
No full textfib Task Groups TG 4.5 (2000-2013) and TG 2.5 (2014 to date) are responsible for major revisions to fib Model Code provisions for bond related aspects of design and detailing of structural concrete. The Authors have served as either Convenor or Technical Secretary of these task groups since 2000. The paper describes enhancements to technical content and changes in the format of (draft) Chapter 20 of Model Code 2020. The revisions and additions to technical content include:
- Reformatting to determine design lap and anchorage lengths directly, removing bond strength from calculation.
- Provisions for laps and anchorages in areas of high stress
- Safety calibration for new design and for assessment
- The influence of casting position.
- Headed reinforcement
- Post-installed reinforcement
- Transfer lengths of pre-tensioned strand
- Issues related to repair and strengthening
- Consequences of deterioration, including fire and corrosion
- Extensions to provisions for local bond-slip modelling
- Standard method of test for bond
Adjustments in the format of provisions include:
- Restructuring to place ‘application rules’ ahead of more detailed models for behaviour. Introduction of Level 1 and Level 2 approximation
- Introduction of a new section on Assessment of existing construction, including older types of reinforcing materials
Bond in RC structures at high temperature and in fire: lessons from the past and hot issues still open to investigation
Full text linkHigh temperatures and fire are definitely among the various exceptional load situations RC
structures are required to resist, as demonstrated by the extensive research activity performed
so far, from the behavior of cementitious materials to that of single members and entire
structures. Reinforcement-concrete bond, however, has become a hot issue at a relatively late
stage, with an acceleration in the new millennium. The objective of this paper is to recall some
of the major issues treated in the literature since the last conference “Bond in Concrete”
(Brescia, Italy, 2012) and still open to investigation, such as: (1) bond micromechanics in fire;
(2) bond stress-slip law as a function of the temperature; (3) the role of polypropylene, steel
and hybrid fibers; (4) tension stiffening in fire; and (5) bonded fasteners in fire. This reexamination
takes advantage of the tests performed in Milan in the last decade on bond-stress
distribution along anchored bars in fire, pull-out vs. splitting failures, in-fire capacity of postinstalled
fasteners and tension stiffening at high temperature. Only by improving the knowledge
– and the modelling - of the basic resisting mechanisms, bond included, today’s refined FE
codes will provide rational structural responses based on clearly-recognizable contributions
Effect of confinement on the behavior of FRP straight anchors
Full text linkThe use of Fiber Reinforced Polymer (FRP) materials in combination with the Externally
Bonded Reinforcement (EBR) method is a strengthening system (EBR-FRP system) that is
commonly deployed to increase the load carrying and/or ductility capacity of structural
members. One of the main drawbacks of EBR-FRP systems is premature debonding, which
entails the debonding of the FRP reinforcement from the substrate at a strain level that is
typically a small fraction of the rupture strain. In order to prevent or delay premature debonding,
FRP anchors have proven to be a suitable solution to prevent the delamination of FRP materials
from the concrete substrate when the Externally Bonded Reinforcement (EBR) method is used
by ensuring continuity of the load path from the FRP sheets into the structure or improving the
FRP-to-concrete bond strength. On the other hands, one of the phenomena that could affect the
behavior of FRP anchors is the concrete confinement. Hence, the research here presented aims
to comparing test results for a single FRP anchor in confined and unconfined configurations to
establish a ratio between the confined and the unconfined bond strengths. Subsequently, Results
are discussed and compared with theorical studies
Hybrid Reinforcement (Rebars + Fibers) for elevated slabs
No full textWhen designing Fiber Reinforced Concrete (FRC) structures, one of the basic issues is represented by
the choice of a proper combination of fibers and conventional reinforcement that allows to obtain the
best structural performance with the minimum amount of materials. The combination of rebars and
fibers in the concrete matrix is generally known as Hybrid Reinforced Concrete (HRC). HRC represents
a feasible solution in many structures; among these, slabs are gaining an increasing interest among
practitioners. In fact, slabs are the most widespread structural elements in common practice since they
are typically used to construct industrial floors (slab on grade), foundations (slab on piles) or floors
(elevated slabs).
This paper focuses on the design of FRC elevated slabs by using the most recent design provisions
reported in the fib Model Code 2010. Emphasis will be given at the use of HRC for optimizing the slab
reinforcement. In more detail, the results of a parametric study performed to design the Hybrid
Reinforcement for elevated slabs will be presented and discussed and a procedure for designing the
Hybrid Reinforcement will be proposed and verified by nonlinear finite element analyses
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