Subject matter

Salt crystallization is a major cause of damage in porous building materials. Notwithstanding the extensive research in this field, the complexity of the problem hinders the use of theoretical (deterministic and probabilistic) models for forecasting decay due to salt crystallization. Nowadays, in the practice of construction and conservation, the durability of materials with respect to salt crystallization is mostly determined by accelerated ageing tests, carried out in laboratory. An effective ageing test should simulate in laboratory, in a reliable way and within a reasonable period of time, the performance and durability of a material in practice. However, existing (standard) crystallization test procedures are generally not realistically reproducing the transport and crystallisation process, resulting in damage types that are often different from those known from practice for the material concerned. Besides, many test procedures (e.g. NEN EN 12370) assess the durability of a material without considering its compatibility with the other materials present in a construction, leading often to misleading conclusions.

The literature on this subject shows the reluctance of researchers to use standard tests. This may be caused by the mentioned unrealistic aspects of existing standard tests as well as by the necessity to simulate situations which are not well-represented by those tests. The use of many different (and often also inadequate) test procedures hinders the comparison between the results of different studies, while relation with practical performance remains uncertain.

Moreover, none of the existing standard procedures prescribes an objective, quantitative and reliable method for monitoring the damage (apart from gathering the debris, method which is quite time consuming). No scale for the evaluation of the material durability with respect to salt decay is given and the assessment of durability with respect to salt decay is only based on comparison among the materials considered in the test.

Another limit of salt crystallization tests, common to many accelerated test procedures, consists in the scarce validation of the results from the laboratory accelerated test by comparison with field data, for what concerns e.g. the damage type and severity.

This RILEM TC aims at overcoming the above mentioned limitations in order to improve both research progress and improve conservation practice. The approach of this RILEM TC will consist in gathering the in-depth knowledge gained over the past years on this topic by research institutions, service providers and users.

Their experiences will be combined, assessed and applied to develop a new accelerated crystallization test, which will allow to assess in a reliable way the durability of building materials to salt crystallization. This approach requires multidisciplinary work between engineers, scientists, researchers, conservators and other persons responsible for building conservation.

A combination of literature study, collection of data from laboratory tests and in-situ surveys, and experimental research in laboratory will be used (see sections 3 and 4 for further details).

Focus will be on those porous building materials most affected by salt decay, such as fired-clay brick and natural stone, as well as (lime-based) bedding or plastering/rendering mortars. Concrete (blocks) and organic building materials will not be considered in this research, as decay due salt crystallization pressure is much less relevant to these materials.

Terms of reference

The main objectives of this RILEM TC are:

• Preparation of a state of the art report on existing accelerated crystallization tests, considering their advantages and limitations.
• Preparation of a state of the art report on salt damage forms found in the field for different materials and conditions.
• Definition of an effective (i.e. reliable and accelerated) salt crystallization test for the assessment in the laboratory of the durability of materials to salt crystallization decay. This will include:
  • Definition of different aggressiveness classes, i.e. classes of damage potential (based on relevant parameters like e.g. salt type and content, environmental conditions, etc.)
  • Identification of a combination of objective and reliable methods and techniques for the assessment and monitoring of different damage forms and severity, with a special emphasis on non-destructive and minimally destructive procedures. Techniques which are easily accessible to researchers and which can be used in laboratory as well as in the field should be preferred,
  • Definition of criteria to classify the durability of materials taking into account the severity of the damage and the class of aggressiveness of the test adopted. This classification should be put in relation also to specific intrinsic properties of the materials, such as porosity, pore size distribution, mineralogy (clay content), and physical mechanical properties. The assessment of the durability of a material towards salt crystallization damage should not neglect the assessment of its compatibility with other materials in the buildings, i.e. the consequences a material can have on the durability of others materials to salt decay.
• Validation of the test methods (including a round robin test and possible corrections to the initial procedure).
• Correlation of results from laboratory accelerated test with data from the field surveys.
• Dissemination of developed test among researchers and potential users.

These objectives will be achieved by reviewing and comparing research in the field of on-site monitoring of salt weathering, accelerated salt crystallization tests in laboratory and modelling of salt crystallization process. Based on the outcomes of this review, a salt crystallization test will be developed and validated by experimental research in laboratory, addressing the most relevant exposure conditions found in the field. Round robin tests will be performed. Additionally, data from field surveys (damage extension, type of salt, service life of material, etc.) available through the experts involved in the RILEM group, will be collected and analysed; the aim is to correlate laboratory and in-situ data, in order to assess the capacity of the laboratory test of reproducing the severity and type of decay observed in-situ.

Dissemination of the results will be obtained through publication in (inter)national journals (including Materials and Structures) and presentation at (inter)national conferences. Some of the participants are also member of CEN committees on related subjects, fact which may help in the dissemination and acceptance of the developed test.

The estimated time frame for achieving the proposed objectives is 4 to 5 years. A detailed working program and a time table with the main phases of the research is provided in section 4.

The regular and corresponding members to be recruited are well known experts with outstanding academic and/or practical experience in the field of study of salt crystallization decay of building materials. They cover all expertise fields necessary to the proposed research, from modelling of salt decay to in-situ and laboratory investigation, including expertise in non-destructive monitoring of material decay and environmental monitoring. Different countries are represented as well, therefore a large range of climates and materials is considered. Additionally, several of the members participated in former and current RILEM Committees.

Detailed working programme

The following research steps are foreseen:

1. Critical review of accelerated crystallization test procedures. Existing literature on both accelerated crystallization test and modeling of crystallization damage will be reviewed, and research results available at the involved researchers' labs will be collected and compared, with the aim of:

1. identifying advantages and limits of existing procedures;
2. defining different aggressiveness classes (based on relevant parameters such as salt type and content, environmental conditions, etc.) to be simulated in the salt crystallization test;
3. gauging key material properties that influence durability towards salt weathering.

2. Development of new standard laboratory test procedures for assessing durability of porous building materials with respect to salt crystallization. This step will necessarily include:

1. definition of a (combination of ) preferably non-destructive, objective and reliable methods or techniques for monitoring of the damage;
2. definition of criteria (e.g. severity, extension and type of damage) for a classification of durability of materials to salt decay;
3. validation of the developed test procedure, including round robin tests at the different laboratories involved.

3. Correlation of laboratory and in-situ data, comprising:

1. collection and analysis of data from in-situ surveys available from different laboratories;
2. comparison between laboratory and in-situ data, in order to assess the capacity of the developed procedure of reproducing the severity and type of decay observed in-situ.

4. Discussing and building up of consensus on the new test procedure among researchers and potential users. This will include publication of results in scientific and professional journals and at (inter)national conferences.

An indicative time schedule is provided in the attached document.