碱集料反应中的有害物质Deleterious Substances-AAR

1、Subjects:Deleterious Substances in AggregateAlkali-Aggregate ReactionsObjectives: · To explain the effects and need to limit the amount of following deleterious substances in aggregate: o Organic impurities o Clay and other fine materials o Salts o Unsound particles · To explain the alkali

2、-silica and alkali-carbonate reactions Organic Impurities · 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 re

3、moved 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

4、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 i

5、n 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, incr

6、ease 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

7、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: o FA: 3% when concrete is subjected to abrasion and 5% for other concrete o CA: 1% Salts &

8、#183; 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

9、83; 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: o For prestressed concrete: 0.01 o For R.C. made with sulfate resisting cement: 0.03 o For other reinforced concrete: 0.05 Unsound Particles · F

10、ollowing are the two broad types of unsound particles found in aggregates: o Materials fail to maintain their integrity o 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

11、) 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 unsoun

12、d · 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

13、 down by ASTM C 33-93 are as follows: Type of particlesMax. content (% of mass)In FAIn CAFriable particles1 and clay lumps3.02.0-10.0Coal0.5-1.00.5-1.0Chert2 that will readily disintegrate-3.0-8.0 1Easy to crumble/pulverize. 2A variety of silica that contains microcrystalline quartzAlkali- Aggregate

14、 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 reac

15、tion" · 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. Alkali-Sil

16、ica Reaction (to be covered in detail under durability) · Following are the reactive forms of silica: o Opal (amorphous, i.e. shapeless) o Chalcedony (cryptocrystalline fibrous) o Tridymite (crystalline) Sources of the above forms of reactive silica include: opaline or chalcedonic cherts, silic

17、eous 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 partic

18、les · 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 contro

19、l the alkali-silica reaction, standard tests for aggregate reactivity should be conducted on the aggregate samples Form of ASR deterioration in concreteAlkali-Carbonate Reaction (to be covered in detail under durability) · The phenomenon of the alkali-carbonate reaction is different from that o

20、f 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 a

21、nd 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 te

22、nd to have a characteristic texture that can be identified by petrographers. Petrography of Concrete with Aggregates susceptible to ACR.Local Aggregates SourcesEastern ProvinceFine aggregatesMost of the fine aggregate in the eastern province is dune sands with silica contents ranging from 79% to 98%

23、.Coarse AggregatesThe coarse aggregates are limestone and they contain high content of calcite and some quartz.Central ProvinceFine aggregatesGood quality fine aggregates are available throughout the central province; they contain quartz, feldspar, and calcite. In general, the fine aggregates contai

24、n 82% to 99% silica.Coarse AggregatesThe coarse aggregates are limestone, diorite, and amphibolites. These aggregates contain calcite, quartz, and dolomite.Western ProvinceFine aggregatesThe fine aggregates contain quartz, feldspar, calcite, and mica. In general, the fine aggregates in the western r

25、egion contain less silica (60% to 76%) compared to sands from eastern and central regions.Coarse AggregatesThe coarse aggregates are amphibolites, hornblende, diorite, etc. They contain about 50% SiO2.Unwashed local aggregate specially in the eastern province is the largest contributor of chlorides

26、in concrete in the Gulf region. The local aggregate in the eastern region of Saudi Arabia is composed of crushed limestone which is usually porous, absorptive, relatively soft and excessively dusty. The dust and fines are heavily contaminated with sulfate and chloride salts.The following Tables pres

27、ent some test results conducted at the RI/KFUPM on selected local coarse aggregates:Table 1.Mineralogical composition of the selected coarse aggregates determined by X-ray diffraction technique.Sample #Name of quarryLocationMineralogical composition, % by weightCalcium carbonate (CaCO3)Quartz (SiO2)

28、1Al-SuhaimiAbu-Hadriyah99.01.02Al-OsaisAbu-Hadriyah95.05.03Al-MoosaHofuf80.020.04Al-AflaqHofuf75.025.05Al-MuneerRiyadh road85.015.06Al-SummanRiyadh road75.025.0Table 2.Materials finer than ASTM # 200 sieve in the selected coarse aggregates.Sample #Name of quarryLocationMaterial finer than ASTM No. 2

29、00 sieve, %Acceptable value, % ASTM C 33, Saudi Aramco1Al-SuhaimiAbu-Hadriyah0.501.02Al-OsaisAbu-Hadriyah0.653Al-MoosaHofuf0.464Al-AflaqHofuf0.175Al-MuneerRiyadh road0.206Al-SummanRiyadh road0.44Table 3.Specific gravity and water absorption for the selected coarse aggregates.Sample #Name of quarryLo

30、cationSpecific gravityAbsorption, %Acceptable value, % Saudi Aramco1Al-SuhaimiAbu-Hadriyah2.522.322.52Al-OsaisAbu-Hadriyah2.532.43Al-MoosaHofuf2.431.804Al-AflaqHofuf2.451.25Al-MuneerRiyadh road2.591.066Al-SummanRiyadh road2.61.1Table 4.Loss on abrasion in the selected coarse aggregates.Sample #Name

31、of quarryLocationLoss on abrasion, %Acceptable value, % Saudi Aramco1Al-SuhaimiAbu-Hadriyah32.40402Al-OsaisAbu-Hadriyah33.203Al-MoosaHofuf35.054Al-AflaqHofuf25.895Al-MuneerRiyadh road23.666Al-SummanRiyadh road22.60Table 5.Chloride and sulfate concentrations in the selected coarse aggregates.Sample #

32、Name of quarryLocationChloride concen-tration, %Allowable chloride concentration, % Saudi AramcoSulfate concentration, %Allowable sulfate concen-tration, % Saudi Aramco1Al-SuhaimiAbu-Hadriyah0.0660.030.206 0.42Al-OsaisAbu-Hadriyah0.0280.0593Al-MoosaHofuf0.0260.0834Al-AflaqHofuf0.0110.0355Al-MuneerRi

33、yadh road0.0170.0596Al-SummanRiyadh road0.0220.067In conclusion· Tests conducted on the selected coarse aggregates indicated that the quality of coarse aggregates from quarries on the Riyadh road is relatively better than the coarse aggregates from quarries in Hofuf and Abu-Hadriyah. · The quanti