Subject matter

For quite some time, and in the near future in particular, numerous important buildings and structures such as large dams, subway systems, tunnels, and wide span bridges are planned or are under construction in China and all over the world. More recently more and more big underground structures are planned and are being built. In developing countries in Asia, Africa and South America we can also observe a rapidly growing infrastructure, as well as planning and building of big harbours and airports. Early-age shrinkage cracking, however, is still a major problem. Cracks may be at the origin of leakage and aggressive salt solutions may penetrate through cracks in the concrete cover and initiate corrosion of steel at an early stage. The cost for maintenance and repair of existing structures has become so important in the meantime that even in rich countries damage of the infrastructure cannot be repaired in time (see for instance the yearly ACI report for the USA). For this reason there is an urgent need to reduce shrinkage cracking in order to increase repair free service life of reinforced concrete structures. One promising way to avoid, or at least to reduce noticeably, the risk of shrinkage cracking is the addition of a shrinkage compensating agent.

A suitable expansive agent usually is applied to produce shrinkage-compensating concrete. Well controlled applications in practice have shown that early shrinkage strain can be compensated in this way. It can be observed that formation of early shrinkage cracks can be avoided or at least considerably reduced.

Expansive agents can be subdivided, following their chemical composition, into the following four groups: (1) agents based on sulphoaluminate, (2) agents based on CaO, (3) composite agents based on sulphoaluminate and CaO, and (4) agents based on MgO. Several standardization and recommendation already exists for the use of expansive cement concrete or expansive agent such as recommendation given by JSCE Committee, specification proposed by AIJ Committee, standards like JIS A 6202, GB 23439, GB/J 50119, etc., in which almost all the above agents are included except for that based on MgO. In comparison with all other agents, MgO based agents have a number of clear advantages. The hydration product Mg(OH)₂  formed at an early age, is stable in young concrete and a small amount of water is required for hydration of MgO. Most important, however, is the fact that the expansion of young concrete after addition of MgO as expansive agent can be well controlled by regulating its reactivity and the micro-structure by well-designed calcination. In this way the necessary expansion can be designed to take place at any pre-determined age of the young hydrating concrete. In this way thermal shrinkage can be compensated during the construction process.

Expansion and early shrinkage of concrete obviously is a very complex process. Among the many influences the concrete composition plays a dominant role. In addition the size and geometry of the structural element have a strong influence. The temperature field, which develops in time, depends on all of these different parameters. Although MgO-based expansive agents are used in China for decades, so far no generally accepted model for prediction of the expansion of young concrete after addition of a MgO-based expansive agent exists.

In order to reach a better understanding of the expansion of concrete after addition of MgO-based expansive agents, this TC will take into consideration the concrete composition, the size and the geometry.

The objectives of this proposed TC include in particular the following topics:

• Development of a standardized test-method to characterize the expansion of concrete after addition of MgO-based expansive agents.
• Quantification of the influence of the reactivity of MgO-based expansive agents, the concrete composition, the size and the geometry on expansion of concrete by a standardized method.
• Establishment of a prediction model for concrete expansion, taking the concrete composition, the size and the geometry of structural elements into consideration.
• Reduction of the risk of crack formation by well-designed and controlled concrete expansion.

This will be a crucial contribution to increased service life and improved sustainability of reinforced concrete structures.

Terms of reference

• The necessary duration of this new RILEM TC is estimated to be five years. The period of work will range from 2018 to 2023. Regular meetings are planned at least once a year, whenever possible in conjunction with RILEM weeks.
• The members will be recruited from universities, research centers and industry, based on their interest and experience with early shrinkage and expansion of concrete. Research topics will be offered to young researchers and students. Representatives of governmental and private owners will also be encouraged to join the TC.
• The work will include a comprehensive literature review, publication of a state-of-the-art report, preparation and carrying out comparative test series (round robins) in at least five different laboratories in different countries, followed by a detailed evaluation of the test results, elaboration of recommendations.

The work of this TC will be relevant for design engineers, construction industry and owners of important structures.

Detailed working programme

1^st year:

• Literature review on concrete deformation after addition of MgO-based expansive agents, state-of-the-art report on existing test-methods, publications of a specific bibliography;
• Preparation of a detailed test program for comparative test series. Pilot tests shall be run in at least five different laboratories. First results shall be distributed for discussion among all members of this TC.

2^nd year:

• Carrying out the round-robin test series in at least five laboratories;
• Investigation of structures using concrete prepared with MgO-based expansive agents: successes, failures, causes;
• Preparation of a state-of-the-art report on properties of concrete prepared with MgO-based expansive agent and engineering application.

3^rd year:

• Evaluation of first results obtained by comparative test series and improvement of the test method if necessary, new run of tests with the revised test method;
• Development of a prediction model for the expansion of concrete prepared with the addition of MgO-based expansive agents for engineering-oriented crack resistance design.

4^th year:

• Discussion of results obtained so far, continuation of possibly modified test series, modification of prediction model based on results obtained.
• Comparative studies on calculations of crack-resistance of concrete structures built with MgO-added concrete (e.g., super-long underground structures) by using different types of software;
• Final revision and publication of the state-of-the-art report.

5^th year:

• Finalizing the test method and drafting of recommendations.
• Proposal of a standardized test method and a prediction model for concrete expansion under combined effect of concrete composition, size and geometry when concrete with MgO-based expansive agent is used.
• Preparation and publication of guidelines for practical applications of concrete prepared with MgO-based expansive agent.