Thermal Action and Mechanical Load

Paper category: Bibliography
Book title: Publications on Durability of Reinforced Concrete Structures under Combined Mechanical Loads and Environmental Actions: An Annotated Bibliography
Editor(s): Yao Yan, Wang Ling, Wittmann Folker
Print ISBN: 978-3-942052-03-0
Publisher: Aedificatio Publishers
Pages: 49 - 58
Total Pages: 10
Language: English

When concrete is exposed to high temperatures, its microstructure undergoes physico-chemical modifications throughout the heating resulting in dehydration of the cement gel (CSH). This dehydration induces changes in the microstructure of the material, and therefore a change in mechanical, thermal and transport properties. It also induces the creation of free water within the material and thus an increase in pore pressure.

Different experimental campaigns carried out so far have led to a better understanding of the behavior of the material with temperature and to study the influence of several parameters. The synthesis of observations and experiments carried out on the effect of high temperature on concrete showed that the temperature rise of concrete results in a number of physicochemical and microstructural transformations which will then lead to a change in mechanical properties. During a rise in temperature, the different categories of water within the concrete (free water, bound water) are successively eliminated, starting with the free water, gradually rising to the dehydration of CSH gel hydrates which begins at approximately 110 °C up to 600 °C. Following the evaporation of water, significant mass losses are recorded. Calcium hydroxide (Ca(OH)₂), which is the most important compound in the concrete, breaks into free lime CaO and water at about 500 °C. The second stage of hydrates’ decomposition is observed at 700 °C, calcium carbonate CaCO₃ decomposes into free lime and carbon dioxide CO₂. This decomposition can go up to 900 °C. In the cooling phase, the free lime CaO formed in the decarbonation reaction combines with atmospheric moisture with an increase in the volume of about 44 %. This phenomenon causes cracking and damage to concrete structures.

The main phenomen observed are :
- Dehydration and physico-chemical changes of concrete
- Cracking and deterioration of the paste-aggregate interface
- Evolution of porosity
- Evolution of the thermal and mass transport properties of concrete with temperature
- Evolution of mechanical properties of concrete with temperature
- Thermal deformation of the concrete at high temperatures
- Deformation of transient thermal creep.

5.1 – Thermal effect on permeability
Permeability defines the ability of a porous medium to be crossed by fluids under a gradient of pressure. Although the permeability of a porous medium is highly dependent on its porosity, it is also dependent on other parameters, such as: connectivity, tortuosity and constrictivity, related to the pore size. During the heating phase, thermal expansion of concrete constituents takes place and physico chemical changes as well as thermal cracking may strongly and irreversibly affect the porous structure of concrete. Therefore, after heatingcooling, it may be defined a thermo-chemical damage, caused by dehydration of the cement paste, and a thermo-mechanical damage, caused by cracking due to differential expansion of the paste and aggregates.

The increase in porosity due to the temperature has been shown experimentally on heated-cooled material. It has been also shown that the amount of pores of large sizes increases with heating temperature and is due to microcracking at pasteaggregate interfaces. With the creation of cracks, porous network connectivity increases. Cracks development around aggregates, especially when they are coarse, induces tortuosity increase. This effect is amplified for high performance concretes. As presented in this annotated bibliography, heat-induced microcracks investigation may be performed using MIP test or SEM, FIB/SEM or neutron imaging. When cracked concrete is crossed by fluids under a gradient of pressure, the flow occurs in the pores (volume cavities) as well as in the cracks (surface cavities). Generally, cracks increase and interconnect percolation pathways, thereby reduce the resistance of concrete to the flow, resulting in an increase in permeability. Among the limited number of experimental studies describing the effects of temperature on permeability of concrete to water or gas, some have highlighted the evolution of the permeability of the material after heating-cooling (residual permeability), while others have shown the evolution of permeability during heating.

Concerning the evolution of the permeability to liquid water during heating up to moderate temperature of 80 °C, an increase in hydraulic conductivity is noted and attributed partially to the decrease in viscosity of the water with the temperature. However, water may cause clogging of the pores by fine particles and possibly healing of concrete. Moreover, this phenomenon is accelerated by high temperature. It has been shown experimentally that the gas permeability, measured after heating-cooling or during heating, under temperature going up to 400 °C, whether apparent or intrinsic, increases with temperature. This phenomenon is associated with a decrease in the coefficient of Klinkenberg, considered as an indicator of the fineness of the pore network, resulting in an increase in pore size. Therefore, the increase in permeability with temperature is obviously due to to microcracking due to thermal dehydration and differential thermal deformations of concrete constituents, accompanied by an enlargement of pores, which may be accompanied by concrete spalling due to water vapour pressure inscrease. Above a temperature of 400 °C, and particularly when higher than 500 °C, permeability increase becomes tremendous due to important physico chemical modifications and microstructural degradations.

The increase of permeability with mechanical load (uniaxial compression) seems to be greater with temperature, inducing further alterations of concrete and expansion of the porous structure of the material. The experimental results presented in this annotated bibliography agree with the format of coupled evolution of the permeability, due to damage and temperature, assumed by some authors to model concrete durability. However, confinement applied during heating may reduce these phenomena due to irreversible closure of heat-induced micro-cracks. Nevertheless, if concrete structures after high temperature of fire hazard could be treated appropriately using, for example, silane water repellent agent, their tightness and therefore resistance to aggressive agents penetration can be improved effectively and their residual service life can be extended.

5.2 – Thermal effect on chloride diffusion
Diffusion is characterized by the mobility of species at molecular level. It depends on many parameters (temperature, porosity, etc. ..). However, the structures in contact with chlorides in outside environments are subject to temperature variations between summer and winter, day and night.

Regarding temperature, the work found in the literature deals rather with the influence of high or moderate curing temperature on mechanical behaviour of materials. There are few research studies which examine the influence of environment temperature on transport in cementitious material.

There are two main types of research on the influence of low temperature on the chloride transport:

- The influence of initial curing temperature (the temperature strongly influences the hydration and hardening of materials)

- The influence of the environment temperature on hardened concrete and properties of transport.

The physico-chemical transformations and decomposition of the CSH gel, and thermal cracking caused by thermal deformation, modify the geometry of the porous structure of the concrete. During a moderate drying, without changing the total porosity, the pore volume accessible to the gaseous phase increases. However, thermal cracking, accompanied by an extension of the pores, involves the increase of the total porosity. Moreover, with increasing porosity, mechanical strength decreases and diffusion coefficient increases.
The different phenomena that can be observed in the porous network are related to the maximum temperature reached. For temperatures up to about 300 °C, the variation of the pores is due to the decomposition of the cement paste after dehydration. Above this temperature, and particularly when higher than 500 ° C, the change in porosity is also related to the decay of calcareous aggregates. Porosity is also affected by the change of volume: in general, an expansion of the cement paste was observed between 20 and 200 °C. Above 200 °C, a limited expansion of the aggregate by the removal of the cement paste is often emphasized.

Some works deal with durability of concrete affected by high temperature, the authors show that if concrete structures could be treated appropriately using silane water repellent agent after high temperature of fire hazard, their capability of antichloride penetration can be improved effectively and their residual service life can be prolonged.

Online publication: 2014
Publication Type: full_text
Public price (Euros): 0.00