TG4.1 - Fibre-reinforced concrete

Model Code 2020 has completed the draft related to the homogenization of FRC to RC and PC design rules,starting from the principles introduced for the first time in Model Code 2010.

Even if the proposed equations are now better harmonized with those controlling the behaviour of the common concrete structures, many aspects, remained out of the code.

These aspects have been already investigated mainly in relation to steel fibres, but we need to extend them to any type of fibres and to hybrid concretes. Moreover, the market has been strongly oriented to sustainability and to the introduction of new matrixes to reduce CO2 emissions and therefore we have to understand which effectiveness can be guaranteed with the adoption of these eco-mixes.

After the publication of the Bulletin 105, we need a special bulletin able to propose other examples of real applications, aimed at checking the effectiveness of the equations introduced and the advantages correlated to sustainability. These examples should be also analysed in other Commissions like the number 1, 3 and 7. When a good proposal concerning the indicated aspects will be achieved, the suggestion is to introduce it, updating the actual draft of Model Code, without waiting for the next edition. To this aim a special role should be played by databases: the database already started by Albert De La Fuente has to be developed, because it could help the evolution of future proposals, making them much more reliable. It has to be enlarged to UHPC where a special need of data is required.

Convener
Marco Di Prisco

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TG4.3 - Structural design with flowable concrete

Flowable concrete (highly flowable, self-compacting and/or self-levelling) has evolved from a special type to a commonly applied building material. fib Task Group 4.3 (TG4.3) considers three aspects of flowable concrete (FC) for structural design: material properties, production effects and structural boundary conditions. The flow of concrete (initiated by some vibration and/or the weight of concrete) can affect the structural characteristics of hardening or hardened concrete. The mixture composition has to be adjusted and optimised in order to obtain a high flowability. TG4.3 aims at promoting the application of flowable concrete, improving and adapting the concrete design and the production technology and its implementation in guidelines and codes.

The technical work of TG4.3 considers the following aspects:

• mechanical/structural characteristics;
• local effects;
• effects of orientation/segregation due to the flow/vibration;
• mixture composition;
• production technique.

Research findings will be compiled and analysed in order to provide guidance for designers and users of concrete structures with FC. Areas of structural design where FC differs from traditional vibrated concrete (TC) have to be identified.

Convener
Steffen Grünewald

Co-Convener
Liberato Ferrara

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TG4.4 - Restoration of heritage in exposed concrete

The activity of Task Group 4.4 is focused in aesthetics of concrete surfaces. The topic is relevant for all exposed concrete structures but holds particular importance in the case of buildings designed by well-known architects. Previous work by this Task Group addressed the issues that need to be considered regarding concrete mix design and casting, aiming at obtaining exposed concrete surfaces with homogenous appearance. A state-of-the-art technical report was prepared with recommendations and guidelines.

Since existing exposed concrete structures are in direct contact with the environment, they are prone to experience degradation faster, and since appearance is a main key-issue, maintenance of this type of structures needs to be properly addressed. In addition, many of these structures have a unique cultural (historical, architectural, technical, other) value, being therefore classified as heritage. For this reason, the repair techniques adopted for current concrete structures may not be adequate in some situations and restoration methods must be adopted instead.

TG 4.4 future work will address the issues that need to be considered regarding conservation and restoration of the built heritage in exposed concrete. The main goal of the Task Group is to publish an fib guide of good practice, including recommendations and guidelines, as well as successful examples that can be assumed as reference case studies.

Convener
Eduardo Julio

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TG4.5 - Time-dependent Behavior of Concrete

The primary objective of the task group is to identify limiting aspects during the design of new or assessment of existing structures related to predicting the time-dependent (mechanical) behavior of “new” but also “traditional” concrete types. Based on the identified short-comings the task group will initiate literature reviews, compile/ update consistent databases and update existing model formulations. Where possible the TG will make use of the data already available in the scientific literature. Where this is not the case, the task group will strive to develop research strategies and coordinate research efforts by its members, supported by national or international research funds.

The task group plans to develop databases and calibrated prediction models for the time-dependent mechanical properties of cast concrete including:

• Maturity vs. time concepts, applicable to early age and multi-decade predictions
• Development of compressive and tensile strength as function of maturity/ time;
• Development of Young’s modulus as function of maturity/ time;
• Development of fracture energy as function of maturity/ time;
• Development of creep and shrinkage as function of maturity/ time;
• Empirical relationship between mechanical properties and compressive strength as function of maturity/ time;
• Development of stress-strain diagram as function of maturity/ time;
• Transport of liquids and gases;
• Guidance for the coupled hygro-thermal chemo-mechanical analysis of concrete with relevance to e.g. mass concrete or certain structural components prone to early-age cracking;
• Guidance for the time-dependent nonlinear (fracture mechanical) analysis of concrete including advanced constitutive models and strain rate effects;
• Time-dependent resistance of concrete subject to sustained load
• Time-dependent resistance of concrete subject to fatigue;

Convener
Roman Wan-Wendner

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TG4.7 - Structural Applications of Recycled Aggregate Concrete – Properties, Modeling, and Design

The main objective of the TG is to formulate design recommendations for the structural use of RAC. This will take the form of proposing new or adjusting existing expressions and models for mechanical and structural properties of reinforced and prestressed concrete structures.

To achieve this goal, the TG will first perform a comprehensive critical review of literature alongside a preparation of databases of experimental results regarding mechanical and structural properties of RAC. Where necessary and possible, identified gaps in existing results will be complemented by new studies of TG members within existing or new research projects. Based on this work, the TG will formulate expressions and models for the following:

• Physical properties of RAC – density, water absorption, permeability
• Mechanical properties – compressive strength, tensile strength, modulus of elasticity, stress–strain relationship, fracture energy, shrinkage, creep
• Durability-related properties – carbonation resistance, chloride ingress, freeze-thaw resistance, chemical attack
• Structural behavior – flexural strength, shear strength, axial strength, punching strength, seismic resistance, fire resistance, deformation, cracking, bond and anchorage
• Fire resistance of RAC and RAC structures – resistance under fire and residual resistance after exposure to elevated temperatures of RAC and reinforced and prestressed RAC members

Convener
Nikola Tošić

Co-Convener
Jean-Michel Torrenti

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TG4.8 - Low-carbon concrete structures

Decreasing the environmental impact of concrete structures is an objective put forward by almost all the actors involved in the domain of construction. Although cementitious materials intrinsically involve low embodied energy, their use in large volumes in worldwide construction lead to approximately 8% of global CO2 emissions. Portland cement is the main constituent responsible for the environmental impacts caused by the life cycle of concrete, as it generates on average more than 800 kg CO2/t of clinker.

The task group will have two main objectives:

1- Identify the different ways to obtain low-CO2 concretes among the different possible routes:

• Evaluate which ones are rapidly reachable and how far we are from an universal utilization of these concretes.
• Define the work to carry out to bring these concrete at an industrial level.
• Estimate the scientific, technical and economical obstacles and challenges that could retard the implementation and acceptances of such concretes.

2- Evaluate the consequences of these low-CO2 concretes on the design of concrete structures, in terms of:

• Durability, for instance the impact of these new concretes on carbonation and chloride ingress, the most widespread problems facing reinforced concrete worldwide.
• Structural design, with the verification of the applicability of the international codes (Eurocodes…). The part concerning creep and shrinkage will be developed in the new COM4/TG Time dependent behavior of concrete.

Convener
Martin Cyr

Co-Convener
Michael Haist

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