Subject matter

- Establish a state-of-the-art-report (STAR) on principles, criteria, methods and technologies applied in Latin America andother parts of the world to control thermal cracking in mass concrete, such that concrete dams, nuclear power plants,massive foundations, and massive members of concrete structures.

- Establish guidelines on how to analyse and to control the risk of thermal cracks in mass concrete.

Significance:

The need of energy generation and water management has led to a renewed wave of dam construction worldwide, butpredominantly in the developing countries. Concrete dams, that seemed to have lost preference to embankment dams, haveregained terrain due to the development of more efficient means to produce, deliver and place the concrete. On the other hand,concrete has always been used to the construction of spillways and power houses. Often, the rate of construction is limited bythe need to allow proper heat dissipation and by the creation of joints, both associated with control of thermal cracks. More accurate means of assessing the risk of thermal cracks and of managing the selection and proportioning of raw materials, as well as the execution and control of the construction process, will allow faster construction with significant economic advantages.
In some dams (particularly in China), the use of controlled expansive binders with higher than normal contents of periclase(MgO) have been used in an attempt to compensate the contraction associated with the cooling period of mass concretes. Latin America has a long, rich and successful history of construction of concrete dams, with considerable accumulatedexperience. Four of the 15 largest dams in the world are located in South America (Itaipú, Guri, Tucuruí and Yacyretá).Several projects are already under construction (Tocoma - Venezuela, Santo Antonio, Jirau and Belo Monte - Brazil, El Realitoand Zapotillo - México, Pirrís - Costa Rica), with many more planned for the next few years.

Regarding other structures, such as massive foundations and other massive structural members, the use of high performance concretes placed a new paradigm for thermal cracking in the early ages. A larger amount of cementitious material is usedincreasing the potential for onset of thermal gradients that may cause cracking. In this case, strains originated by autogenous shrinkage can also contribute to cracking and should be considered in the analysis.

Hence, it is considered appropriate to generate an exchange of experience among experts in the design, materials supply and construction of mass concrete structures and to consolidate the available knowledge in a STAR.The work will certainly uncover areas where knowledge is missing and will open a way to the creation of more specific TCswithin the environment of LatRILEM and/or generate regional R&D projects.

Terms of reference

The duration of the work of the committee is estimated in five years.

The membership should include designers, consultants, constructors, material suppliers, academics and testing laboratories involved in the design, construction and control of mass concrete structures. The participation of consultants directly involvedin projects, as well as representatives of governmental/private owners should also be encouraged.

A good deal of the work will be conducted by electronic correspondence, with one or two annual meetings.

The work will be conducted in Spanish/Portuguese and the meetings will take place exclusively in Latam countries. Therefore it is expected that most of the membership will come from Latam countries. However, experts from other regions of the worldwill be welcome to TC-MCS. Although the meetings will be conducted in Spanish/ Portuguese, all the documents will be written in English.

Detailed working programme

The work will concentrate on thermal cracking control of massive concrete structures, built with Conventional Vibrated Concrete (CVC), Roller Compacted Concrete (RCC), Pre-placed Aggregate Concrete (PAC), Fiber ReinforcedConcrete (FRC), Self-Compacted Concrete (SCC), and will cover:

A. Review of relevant cases in each country: successes, failures, causes and remedies.

B. Review of available data in the literature related to materials and concrete properties related to thermal cracking.

C. Criteria to define thermal cracking limit states (e.g. maximum restrained strains εr , maximum tensile stresses σ,fracture mechanics approaches).

D. Modelling of thermal evolution inside the concrete mass:

- Required meteorological data.

- Required concrete properties: Heat generation and thermal diffusivity (measurement test methods, predictionthrough data measured on individual components).

- Required characteristics/conditions of the exposed surfaces (formwork insulation, reflectance, etc.).

- Handling of variable geometry (by lifts for CVC, by layers for RCC).

- Models for the determination of concrete properties as a function of hydration (adiabatic temperature rise,mechanical properties, thermal properties).

- Models for the thermo-chemo-mechanical analysis of massive concrete.

- The role of reinforcement bars.

- Available software, characteristics and cost (free, license costs).

E. Development of εr / σ within the concrete mass:

- Required input of thermal evolution within the concrete mass.

- Consideration of restraint (internal and external).

- Required elastic properties of the concrete, evolution and measurement techniques.

- Required visco-elastic properties of the concrete, evolution and measurement techniques.

- Required mechanical properties of the concrete, evolution and measurement techniques.

- Available software, characteristics and cost (free, license costs).

F. Explicit description of cracking limit state, safety factors, maximum allowed temperature drop / gradients.

G. Implementation of practical cracking control measures:

- Selection of cement and supplementary cementitious materials.

- Selection of aggregates.· Selection of admixtures, including eventual expansive agents.

- Recommendations on Mix Design.

- Pre-cooling: cooling of water (ice) and aggregates (cool water or air), liquid nitrogen, estimate of temperature ofthe concrete based on that of components, mixing and transit time, weather conditions.

- Post-cooling: layout of pipes, temperature of the cooling water, estimate of heat removed, etc.

H. Site monitoring and control:

- Production and quality control of relevant variables during construction (type of tests and frequency).

- Monitoring of thermal evolution inside the mass (type of sensors, layout, frequency of readings, interpretation).

- Monitoring of εr / σ within the concrete mass (type of sensors, layout, frequency of readings, interpretation).

- Techniques for crack detection and monitoring.

Suggested time frame for major activities:

- 2013, February: potential members contacted and proposal to TAC submitted

- 2013, March: Approval of TC-MCS- 2013, June/July (possibly in Foz do Iguaçu/Itaipu a triple frontier city in the south west of Brazil) Kick-off Meeting, assignment of Sub-Committees, work programme, etc.

- 2014, March/April: Meeting, discussions about the preliminary works of the STAR.

- 2014, September (Sao Paulo, Rilem Week): TC Monitoring Meeting, Revision of preliminary drafts.

- 2015, March/April (tbd): Meeting: evolution of STAR; early proposals of Guidelines.

- 2015, September (Rio de Janeiro the same period of SSCS 2015 – RILEM conference - Numerical Modeling Strategies for Sustainable Concrete Structures): Discussion of Final Drafts + Symposium.

- 2016, March (tbd): Final STAR + Draft of Guidelines- 2016, November (tbd): Discussion draft of Guidelines

- 2016, November (tbd): Discussion draft of Guidelines

- 2017, April (tbd) Discussion draft of Guidelines- 2017, September (tbd) Final Guidelines + Symposium