Deleterious Substances in Aggregate- Alkali-Aggregate Reactions
- Organic impurities interfere with the hydration reaction
- Organic matter are mostly found in sand and consists usually of products of decay of vegetable matter (mainly tannic acid and its derivatives)
- Organic matter may removed from sand by washing
- To determine the organic content of aggregate, colorimetric test recommended by ASTM C 40-92.
- However, this test does not confirm the adverse effect of the organic impurity, because high organic content does necessarily mean that the aggregate is not fit for use in concrete
- For this reason, strength test on mortar with questionable sand as per ASTM C 87-90 is recommended.
- This strength has to be compared with the strength of mortar with washed sand
Clay and other Fine Materials
- Clay present on the surface of the aggregate particles in the coating form interfere with the bond between aggregate and the cement paste, adversely affecting the strength and durability of concrete
- Other fine materials which may be present in aggregate are silt (2 to 60 µm) and crusher dust.
- Silt and dust, owing to their fineness, increase the surface area and therefore increase the amount of water necessary to wet all the particles in the mix
- In view of above, it is necessary to control the amount of clay, silt and fine dust in aggregate
- Since no test is available to determine separately the clay content, silt and dust, the limits of fine materials are prescribed in terms of the percentage of material passing 75 ?m (No. 200) sieve
- ASTM C 33-93 limits the percentage of fines passing 75 ?m (No. 200) sieve as follows:
- FA: 3% when concrete is subjected to abrasion and 5% for other concrete
- CA: 1%
- Sand from seashore or dredged from the sea or a river estuary, as well as desert sand contains salt
- Coarse aggregate dredged from sea also contains salt
- Salts coming through aggregates cause reinforcement corrosion and also absorb moisture from the air and cause efflorescence
- The BS 882:1992 limits on the chloride ion content of aggregate by mass, expressed as a percentage of the mass of total aggregate, are as follows:
- For prestressed concrete: 0.01
- For R.C. made with sulfate resisting cement: 0.03
- For other reinforced concrete: 0.05
- Following are the two broad types of unsound particles found in aggregates:
- Materials fail to maintain their integrity
- Materials lead to disruptive expansion on freezing or even on exposure to water
· Unsound particles if present in large quantities (over 2 to 5% of the mass of the aggregate) these particles may adversely affect the strength of concrete and should certainly not be permitted in concrete which is exposed to abrasion
· Shale and other particles of low density are regarded as unsound
· Clay lumps, wood, and coal, included in aggregate, are also regarded as unsound
· Mica, gypsum, iron pyrites, etc. are also regarded as unsound. While mica is very effective in reducing strength (15% reduction in 28-d f’c with 5% mica), gypsum and iron pyrites are mainly responsible for expansion of concrete
· The permissible quantities of unsound particles laid down by ASTM C 33-93 are as follows:
Type of particles
Max. content (% of mass)
Friable particles1 and clay lumps
Chert2 that will readily disintegrate
1Easy to crumble/pulverize.
2A variety of silica that contains microcrystalline quartz
Alkali- Aggregate Reactions
- Reaction between alkali from cement and silica or carbonate from aggregate is called “alkali- aggregate reaction”
- The most common reaction is that between the active silica constituents of the aggregate and that alkalis in cement, called as “alkali-silica reaction”
- Another type of the alkali-aggregate reaction is that between dolomitic limestone aggregates, containing carbonate, and alkalis in cement, called as “alkali-carbonate reaction”
- Both types of the reactions cause deterioration of concrete, mainly cracking.
- Following are the reactive forms of silica:
- Opal (amorphous, i.e. shapeless)
- Chalcedony (cryptocrystalline fibrous)
- Tridymite (crystalline)
Sources of the above forms of reactive silica include: opaline or chalcedonic cherts, siliceous limestones, etc
- Na20 and K2O are the alkalis in cement which form alkaline hydroxide in pore water facilitating the alkali-silica reaction
- As a result of alkali-silica reaction, an alkali-silicate gel is formed either in pores of aggregate or on the surface of the aggregate particles
- The gel formation on the surface of aggregate particles destroys the bond between the aggregate and cement paste.
- The swelling nature of the gel exerts internal pressure and eventually lead to expansion, cracking and disruption of the hydrated cement paste
- In order to control the alkali-silica reaction, standard tests for aggregate reactivity should be conducted on the aggregate samples
Form of ASR deterioration in concrete
- The phenomenon of the alkali-carbonate reaction is different from that of alkali-silica reaction
- In case of alkali-carbonate reaction also, gel is formed, which upon swelling cause expansion of concrete
- Gel is formed around the active aggregate particles, causing cracking within rims and leads to a network of cracks and loss of bond between the aggregate and the cement paste
- The deterioration caused by ACR is similar to that caused by ASR
- However, ACR is relatively rare because aggregates susceptible to this phenomenon are less common and are usually unsuitable for use in concrete for other reasons.
- Aggregates susceptible to ACR tend to have a characteristic texture that can be identified by petrographers.
Petrography of Concrete with Aggregates susceptible to ACR.