254-CMS : Thermal cracking of massive concrete structures
Technical Committee 254-CMS
Deputy Chair: Dr. Miguel Ângelo Dias AZENHA
Activity ending in: 2019
- 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.
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
It should be linked to the following RILEM TCs:
- MDC: Multi-decade creep and shrinkage of concrete: material model and structural analysis.
- NUM: Numerical modelling of cement-based materials.
- 214-CCD: Concrete cracking and its relation to durability: Integrating material properties with structural performance.
- 238-SCM: Hydration and microstructure of concrete with supplementary cementitious materials.
The work of past TCs such as: TC119 (Avoidance of thermal cracking at early ages) and TC212 (Acoustic emission andrelated NDE techniques for crack detection and damage evaluation in concrete) is useful for the work of this TC-MCS.
A liaison to ICOLD should be established.
The work of the TC-MCS will produce as deliverables a state-of-the-art report and guidelines for the control of thermal cracks in mass concrete, based on existing information and experience. Two symposia are planned within the framework of thisCommittee.
Group of users
- Designers and consultants, that will apply new approaches, tools and criteria to more accurate assessment of thermal crack control of mass concrete.
- Owners of massive structures, e.g. governmental or private developers.
- Materials suppliers that will understand better their customers’ needs and find better ways of characterizing their products in view of application in mass concrete structures.
- Contractors that will optimize the construction of the structures.
- Laboratories in charge of materials testing, quality control and site monitoring.
Specific use of the results
The result of the TC work will provide:
- 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, that will allow faster construction withsignificant economic advantages
- A platform for experience and information exchange among specialists in Latam, that usually work in isolation from their neighbours
- The work of this TC could also be used for other applications, such as massive foundations
From the economical point of view it should be stressed that the optimization of the construction phase of a massive structure such as a hydropower plant can reduce:
- The construction time.
- The placing temperature of concrete.
This can correspond to an important reduction on the total costs of the project.
A better control of thermal cracking allows the optimization of the cementitious material. Besides the benefits of costs reduction it should also be stressed that this can contribute to the reduction of CO2 emissions regarding the large amount ofcementitious materials used in massive structures.