Durability of concrete Flashcards
What is durability of concrete?
Defined as concrete’s ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration, so it retains its original shape, dimension, quality and serviceability.
What are the common causes of concrete
deterioration affecting its durability?
The causes of deterioration of concrete can be divided into four categories:
physical causes, chemical causes, thermal causes, and biological causes.
Common physical causes include crazing and surface abrasion.
Chemical causes sulphates, chlorides, carbon dioxide, alkalis and acids.
The most often thermal cause to concrete deterioration is freeze-thaw attack.
Biological causes include plants, fungus, and micro-organisms.
What is permeability and diffusivity?
When water is flowing through a piece of concrete or from one part to another under a hydraulic gradient, permeability is the governing parameter.
When gases (e.g. oxygen) move through concrete (either dry or wet) or ions (e.g. chloride) move through the pore solution, the process is governed by the diffusion coefficient (or diffusivity).
Generally speaking, since both the permeability and diffusivity are related to the pore structure of concrete, concrete with low permeability will also possess low diffusivity.
What are the factors affecting permeability of concrete?
a) Permeability of cement paste
Permeability of concrete increases significantly when
w/c ratio increases. In general, the higher the w/c ratio, volume, connection, diameter of capillary pores, the higher the permeability.
b) Permeability of aggregate
Lightweight aggregate would have a higher permeability due to a higher porosity.
c) Air voids and cracks
Lack of compaction and bad curing, cracking occurs.
What is crazing?
It’s the development of a network of fine random cracks on the surface of concrete mortar caused by shrinkage of the surface layer.
What are the causes of crazing?
Poor or inadequate curing or the delayed application of curing will permit rapid drying of the surface and promote crazing.
An excessive concentration of cement paste and fines at the surface caused by an overly wet mix, which allows coarse aggregate to settle.
Bull-floating or finishing while there is bleed water on the surface or using a steel trowel sealing the surface and diluting the cement paste.
Sprinkling cement on the surface to dry up the bleed water is a frequent cause of crazing surfaces. This concentrates fines on the surface.
Crazing may occur when water sprinkling is employed as a curing method. Care must be taken to insure that a continuous spray rather than an intermittent spray are used. Curing water that is much colder (20 degrees or lower) can also contribute to crazing.
What are the associated problems of crazing?
Crazing does not affect the structural integrity ofthe concrete and rarely affect the durability orwear resistance.
But bring disgrace on the cosmetic appearance.
What are the preventive measures of crazing?
Use moderate slump 80 to 100 mm, air-entrained concrete to limit bleeding.
Start curing as soon as possible. A high-quality cure and seal is the best measure.
Use the right amount of finishing at the right time. Do not perform finishing operations while there is excess moisture or bleed water on the surface.
On outside work, use a broom finish whenever practical.
Dampen the subgrade just prior to concrete placement to prevent it from absorbing too much water from the concrete.
What are the corrective measures of crazing?
Resurfacing concrete with overlaying systems is a viable way to improve existing concrete surfaces.
Plastering
Polymer resurfacing
What is surface abrasion?
The movements of people and traffic on concrete surfaces cause abrasive wear.
What are the associated problems of surface abrasion?
Surface discolouration, cracks or surface imperfections after a prolonged abrasive wear would reduce the wear resistance and durability of concrete.
What are the preventive measures of surface abrasion?
Good curing provides a hardened and durable concrete surface to resist abrasive wear.
Incorporate pozzolans (silica fume and fly ash) into concrete mix to densify the microstructure of concrete by pozzolanic reaction.
What are the corrective measures of surface abrasion?
Resurfacing concrete with overlaying systems is a viable way to improve existing concrete surfaces.
Asphalt resurfacing
Polymer resurfacing
What are chemical attacks?
They govern the rate of decomposition of concrete, thus its durability.
It involves movement of reactive substances within concrete or from atmosphere to concrete (internal attack).
The process depends on the nature of chemicals, pore structure, ambient temperature and characteristics of concrete.
What are some freeze-thaw tests?
Visual inspection:
Locate areas of damaged concrete and make an initial determination that the freeze-thaw mechanism is the source of the damage by investigation of moisture pathways, and the pattern of cracking.
Freeze-thaw cycle tests:
Extracting a core of the concrete under investigation and subject the specimen to cycles of wetting drying and freezing. This test does not give any absolute meaning, but does give a relative measure for comparison with other cases.
Petrographic analysis:
The presence or absence of entrained air can be detected by examination of the concrete.
Surface scaling and D-crack scaling
