Concrete - Geomaterials: Under the Microscope - page 108

However, it is important to remember that the relative
intensity of evidence for each mechanism does not
necessarily correspond with the importance of the
process as a causal factor in any damage or deterioration.
In diagnosis, therefore, petrographers must analyse the
observations as well as simply recording the descriptive
details (Sims et al ., 2004).

strength reduces rapidly and concrete that has been
heated in excess of 600°C will be of no use structurally.
300ºC is normally taken to be the critical temperature
above which concrete is deemed to have been
significantly damaged. Normally concrete exposed to
temperatures above 300°C is replaced if possible.
Otherwise the dimensions are increased (for example, by
reinforcing columns), depending upon the design loads.
Visually apparent damage induced by heating includes
spalling, cracking, surface crazing, deflection, colour
changes, and smoke damage. Visual survey of reinforced
concrete structure can be performed using a classification
scheme from Concrete Society Technical Report No. 68
(2008). This system uses visual indications of the degree
of damage to assign each structural member a class of
damage from 1 to 5. Each damage classification number
has a corresponding category of repair, ranging from
decoration to major repair. The Concrete Society
classification system is summarized in Table 26 .
Spalling of the surface layers is a common effect of
fires and may be grouped into two main types. Explosive
spalling is erratic and generally occurs in the first 30
minutes of the fire. A slower spalling (referred to as
'sloughing off') occurs as cracks form parallel to the fire-
affected surfaces, leading to a gradual separation of

F IRE - DAMAGED CONCRETE
Concrete structures most likely to be subjected to fire
include buildings such as offices, warehouses, and
schools. Other common scenarios involve vehicle fires in
car packs or concrete-lined tunnels. Fortunately, even
after a severe fire, concrete structures are generally
capable of being repaired rather than demolished.
Guidance for the assessment, design, and repair of fire-
damaged concrete structures is provided in Concrete
Society Technical Report No. 68 (Concrete Society, 2008).
The strength of concrete after cooling varies
depending on temperature attained, the heating rate, mix
proportions, applied loading, and any external sealing
that may influence moisture loss from the surface. For
temperatures up to 300°C the residual strength of
structural quality concrete is not severely reduced.
Generally, between 300°C and 500°C the compressive

Table 26 Simplified visual concrete fire damage classification (after Concrete Society TR68; 2008)

Features observed

Class of Reinforcement

damage Finishes Colour Crazing Spalling bars Cracks Deflection

0 (Decoration Unaffected Normal None None None None None

required) exposed

1 (Superficial Some Normal Slight Minor None None None

repair required) peeling exposed

2 (General Sunstantial Pink/red Moderate Localized Up to 25% None None

repair required) loss exposed

3 (Principal Total loss Pink/red or Extensive Considerable Up to 50% Minor None

repair required) whitish grey exposed

4 (Major repair Destroyed Whitish Surface Almost Up to 50% Major Distorted

required) grey lost total exposed