256-SPF : Spalling of concrete due to fire: testing and modelling
Technical Committee 256-SPF
Deputy Chair: Dr. Robert JANSSON MC NAMEE
Activity ending in: 2020
The subject dealt in this new TC is focused on the spalling of concrete due to fire. This phenomenon can be described as a sudden ejection of concrete on the exposed surface of structural elements which are exposed to fire. This topic has been studied for several decades, but due to the complexity of the physical mechanisms involved, it is still a very active topic of many research teams within the international scientific community.
Fire spalling may apply to all types of commonly used concrete (Ordinary Concrete) as well as more recently developed types of concrete with high mechanical and durability performances (High Performance Concrete, Self Compacting Concrete, Ultra High Performance Fibre Reinforced Concrete). It is now accepted that concrete spalling influences the fire resistance of concrete structures (reinforced or prestressed), due to a loss of resistant cross-section on the one hand and due to a loss of thermal protection of the reinforcement on the other hand. Many assumptions make it possible to better understand the possible causes of the concrete spalling. These assumptions are based on both experimental and numerical work. An important state-of-the-art on fire spalling of concrete has already been done in the context of the RILEM TC 200 - HTC (ordinary concrete) and of the RILEM TC 227 - HPB (which extended the work of the TC 200 - HTC to High Performance Concrete).
The state-of-the-art on fire spalling of concrete shows a significant heterogeneity of the experimental studies. Indeed, no true consensus is made with regard to critical test parameters such as the geometry of the specimen, the type of heating, the initial water content, the storage conditions of specimen or the mechanical boundary conditions. The differences between collected experimental data make the analysis of the causes of spalling not so evident. Similarly, no consensus appears regarding how to assess (or quantify), if existing, the concrete spalling.
In that context, and based on existing experimental and numerical studies, the main purpose of this new TC is therefore to establish recommendations on experimental methods for characterizing the fire spalling of concrete. In particular, the recommendations of the TC will concern the geometry of the specimen, the storage conditions, the type of mechanical boundary conditions, and the type of heating.
The new TC will also explore the possibility of using innovative techniques to characterize the fire spalling of concrete, as well during the fire as after the cooling phase. The contribution of techniques such as image correlation or acoustic emission will be studied. Similarly, recommendations will be established on how to measure the water content of specimen or the pore gas pressure. Finally, recommendations will be given on how quantifying concrete spalling during fire and after cooling.
On the other hand, the state-of-the-art done under the TC 227 – HPB, Task 4 (STAR on modelling), also shows heterogeneity of numerical works on the subject of fire spalling of concrete. The work of the new TC will aim to improve the prediction of fire spalling of concrete, in particular by proposing a study of sensitivity regarding to different parameters (type of concrete, heating, water content, initial mechanical boundary conditions, etc.). Furthermore, the simulations undertaken within the framework of the new TC will innovate by taking account of the stochastic properties of the concrete spalling. Finally a benchmark will be carried out in order to compare different models with experimental results.
The relevance of recommendations on testing methods for characterising the fire spalling of concrete will be assessed through round-robin tests. The establishment of these tests involves obtaining funding.
When considering fire safety, the fire resistance of the structure is one of the main goals to achieve. The new TC will consider more in detail the influence that can have the spalling of concrete on the fire resistance of concrete structures. Indeed, in some cases, concrete spalling can significantly influence the fire resistance of a structure element, whereas in other cases, the impact on fire resistance can be low. One of the first task of the new TC will then consist in studying the sensitivity of the fire resistance to the risk of concrete spalling. The conclusions will influence the following/organization/content of the other tasks.
Terms of reference
From experience with RILEM TC 227-HPB, it is estimated that the life time of the new TC should be 5 years.
The TC organization will be based on parallel working groups (WG). Members of the new TC can take part to more than one WG and some subjects of the new TC will be commonly discussed. Two annual meetings (2 days) will be organized.
Most of the available data on concrete fire spalling has been already collected by previous TC (200-HTC and 227-HPB). As a consequence for the new TC, one year should be enough to collect the most recent results dealing with concrete spalling (especially the experimental results underlining the influence of the experimental settings on the concrete spalling, including non-published results).
As said in §2, one of the first work of the new TC will be to analyse the real influence of concrete spalling on the fire resistance of concrete structures. It will be discussed at the beginning of the new TC life because the conclusions will influence the content of the other tasks. At least the 2 first years will be dedicated to this analysis.
The main objective of the new TC, as explained in the § 2, is to write recommendations on the better way of assessing the spalling of concrete when exposed to fire. At least 3 years will be necessary in order to build these recommendations. In parallel, 3 years will be dedicated to the study of the useful material properties and the associated testing methods.
On the other hand, the works undertaken in the field of simulations will take at least 3 years. This part of the TC activities will involve computer processing. The obtained results will be compared at each meeting of the group.
Finally, the last year of the TC will be dedicated to the formatting of the documents. As explained in § 2, the last year will be also dedicated to round-robin tests (provided that a funding will be obtained).
The TC group will be mainly composed by national laboratory and academic staff. Between twenty and thirty persons are expected to be part of the new TC. The expertise of the applicant members will be carefully examined (experimentation, modelling,…). The initial recruitment will be based on the active members of the RILEM TC 227-HPB.
Detailed working programme
The different tasks for the new TC will be as following:
Task 1: Experimental methods for assessing concrete fire spalling:
- Definition of spalling tests methods based on laboratory experience and modeling (geometry of samples, curing, temperature measurements, reported observations, spalling depths measurements, references, …)
- Innovative physical measuring procedures to quantify spalling during and after tests (for example digital image correlation, acoustic emission,...) as additional measurements
Task 2: Useful material properties and test methods:
- Thermal properties
- Water content profile before test, etc.
Task 3: Modelling of spalling phenomenon:
- Sensitivity to parameters
- Probabilistic behaviour
- Benchmark simulation vs. experiment
Task 4: Round-robin tests (provided funding).
Task 5: Influence of concrete spalling on fire resistance and residual capacity
This new TC should be part of the RILEM cluster C "Structural Performance and Design". It is closely related to the TC 227-HPB which will finalize its work in 2013.
Connection with the fib Task Group 4.3 (fire design of concrete structures) is also expected. In particular, several members of the new TC are already members of this fib group.
Expected members of the new TC are part of the revision group of the fire part of the Eurocode 2 (EN 1992 - 1.2).
Most of the expected members are part of national/regional groups for writing recommendations or guidelines on fire resistance assessing of concrete structures.
One of the direct benefits of the new TC will be a better consideration of concrete spalling in the existing (or future) international and/or national regulations. For instance works of the TC will be useful for the revision groups of the Eurocode 2 by proposing better rules and methods based on the scientific knowledge.
The recommendations from the new TC will be particularly useful for laboratories in charge of assessing concrete fire spalling and concrete elements fire resistance.
Recommendations should be published in "Materials and Structures" for an international broadcast of the results.In the same way, the report of innovative spalling modelling will be beneficial for the researches attempting to predict concrete spalling by simulations. This report should be edited by the RILEM.
A restitution of the new TC work will be done at the end of the TC life, probably included in the future editions of the "International workshop on Concrete Spalling due to Fire Exposure" (series of conference supported by RILEM).
Group of users
The work undertaken by the new TC will especially target a first group of users:
- Universities and testing laboratories
- Building material companies and industries
- Material scientists and experts
A second group of users should be targeted by the new TC:
- Building and infrastructure authorities
- Designers and civil engineers
- Standardisation bodies
Specific use of the results
Specific uses of the new TC results can be expected:
- For academic laboratories: concrete spalling is still a phenomenon that needs research works. Recommendations of the new TC on testing methods will allow a better comparison of international results. Report on innovative modelling will also constitute a real guideline for research works on that field.
- For testing laboratories: based on the recommendations of the new TC, the results from testing laboratories will be better harmonized, especially at an international level. The recommendations may also allow testing laboratories to extend their knowledge and their services/benefits on the field of concrete spalling.
- For industrialists: based on the recommendations of the TC, the development of industrial products (formulation of concrete, development of thermal protection, development of technical solutions limiting spalling…) will be more easily extensible to international markets. Efficiency of products, in comparison with other similar products, should be better proven.