Lecture No. 09-10

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Lecture No. 09-10
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Subject Alkali-Aggregate Reactivity

• Certain constituents in aggregates can react
  harmfully with alkali hydroxides in concrete and
  cause significant expansion. There are two forms
  of this reaction
• Alkali silica reaction (ASR)
• Alkali-carbonate reaction (ACR)
• Alkali silica reaction (ASR)
• Develops by aggregates containing reactive silica
  minerals. This form is more serious and common
  than ACR.

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ASR

• ASR has been recognized as a potential source of
  distress in concrete since the late 1930s

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• Alkali carbonate reaction (ACR)
• The aggregates dolomitic (calcium-magnesium
  carbonate) have specific composition that is not
  very common.

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Alkali silica reaction (ASR)

• Mechanism
• The reaction can be visualized as a two-step
  process
• Alkali hydroxide reactive silica gel ?
  alkali-silica gel
• Alkali-silica gel moisture ? expansion

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Alkali silica reaction (ASR)

• The amount of gel formed in the concrete depends
  on
• Amount of and type of silica in aggregate.
• Alkali hydroxide concentration.
• Sufficient moisture.

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Alkali silica reaction (ASR)

• The ASR forms a gel that swells as it draws water
  from the surrounding cement paste (has great
  affinity to moisture). In absorbing water, these
  gels can induce pressure, expansion, and cracking
  of the aggregate and the surrounding paste.
• The alkali silica gels will fill the microcracked
  regions both within the aggregate and concrete.
  Continued availability of moisture to the
  concrete causes enlargement and extension of the
  microcracks which eventually reach the outer
  surface of the concrete. The crack pattern is
  irregular and referred to as map cracking (see
  Figure 5-20). Or fragments breaking out of the
  surface of the concrete (popouts) as in Figure
  5-21.

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Popouts
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Alkali silica reaction (ASR)

• List of most reactive substances
• Opal (SiO2 nH2O)
• Chalcedony (SiO2)
• Certain forms of quartz (SiO2)
• Cristobalite (SiO2)

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Alkali silica reaction (ASR)

• The most important harmful alkali reactive
  aggregates
• Opaline cherts
• Chalcedonic cherts
• Siliceous limestones
• Siliceous dolomite

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Alkali silica reaction (ASR)

• Identification of Potentially Reactive
  Aggregates
• Field performance history of structures in
  service for more than 15 years.
• Different tests can be conducted for initial
  screening and evaluating potential alkali-silica
  reactivity.

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Alkali silica reaction (ASR)

• Control of ASR
• Use of low-alkali Portland cement (less than 0.6
  equivalent Na2O) when alkali-silica reactive
  constituents are suspected to be present in the
  aggregate.
• If low-alkali cement is not available, the total
  alkali content can be reduced by replacing a part
  of high-alkali cement with supplementary
  cementitious materials such fly ash, ground blast
  furnace slag, and silica fume, or use blended
  cement.

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Alkali silica reaction (ASR)

• Control of ASR
• Wash beach sand and gravel with sweet water to
  insure that the total alkali content from the
  cement and aggregates in concrete does not exceed
  3 kg/m3.
• Control the access of water to concrete.
• Replacing 25 - 30 of the reactive sand gravel
  aggregate with crushed limestone (known as
  limestone sweetening).

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Alkali silica reaction (ASR)

• Utilization of silica fume, fly ash, and blast
  furnace slag as partial replacement of cement
  will reduce the expansion as shown in Figure
  5-23.

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Aggregate Processing

• Consists of two stages
• Basic processing
• This includes
• crushing,
• screening,
• washing to obtain proper gradation and
  cleanliness.

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Aggregate Processing

• Beneficiation (upgrading)
• Upgrading the quality of the aggregate by
  specific processing methods such as
• Media separation passing aggregates through a
  heavy liquid with specific gravity less than that
  of the desirable aggregate particles but greater
  than that of the harmful particles.
• Jigging a process to separate particles with
  small differences in density by pulsating water
  current. Upward pulsations of water through a
  jig (a box with a perforated bottom) move the
  lighter material into a layer on top and then
  removed.

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Aggregate Processing

• Rising-current classification separates
  particles with large differences in specific
  gravities. Light materials, such as wood and
  lignite, are floated away in a rapidly upward
  moving stream of water.
• Crushing used to remove soft and friable
  particles from coarse aggregates.

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Handling and Storing Aggregates

• Aggregates should be handled and stored in a way
  that minimizes segregation and degradation and
  prevents contamination by deleterious substances.
  Stockpiles should be built up in thin layers of
  uniform thickness to minimize segregation using
  the truck-dump method. The aggregate is then
  reclaimed with a front-end loader.

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Handling and Storing Aggregates

• Whether aggregates are handled by truck, bucket
  loader, clamshell, or conveyor belt, stockpiles
  should not be built up in high, cone-shaped piles
  since this results in segregation..

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Handling and Storing Aggregates

• Crushed aggregates segregate less than rounded
  (gravel) aggregates and larger-size aggregates
  segregate more than smaller sizes. To avoid
  segregation of coarse aggregates, size fractions
  can be stockpiled and batched separately.

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Handling and Storing Aggregates

• Washed aggregates should be stockpiled in
  sufficient time before use so that they can drain
  to a uniform moisture content. Damp fine material
  has less tendency to segregate than dry material.
  
• When dry fine aggregate is dropped from buckets
  or conveyors, the wind can blow out the fines.
  This should be avoided if possible.

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Marine-Dredged Aggregate

• When other aggregate sources are not available
  Marine-dredged aggregate, and sand, and gravel
  from the seashore can be used with caution in
  limited concrete applications. Aggregates
  obtained from seabeds have two problems
• Seashells.
• Salt.

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salts

• The presence of these chlorides may affect the
  concrete by
• Altering the time of set.
• Increasing drying shrinkage.
• Increasing the risk of corrosion of steel
  reinforcement.
• Causing efflorescence.

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Seashells

• The sea shells are hard materials that can
  produce good quality concrete, however, a cement
  content may be required due to angularity of the
  shells to obtain the desired workability.
• Aggregate containing complete shells should be
  avoided as their presence may result in voids in
  the concrete and lower the compressive strength.

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Marine-Dredged Aggregate

• Generally, marine aggregates containing large
  amounts of chloride should not be used in
  reinforced concrete.Marine-dredged aggregates
  can be washed with fresh water to reduce the salt
  content.

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Recycled Concrete

• Results in both material and energy savings.
• The procedure involves
• (1) Breaking up and removing the old concrete.
• (2) Crushing in primary and secondary crushers
  (see Figure 5-25).
• (3) Removing reinforcing steel and embedded items.

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Crushing concrete with a beamcrsher
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Recycled Concrete

• (4) Grading and washing.
• (5) Finally stockpiling the resulting coarse and
  fine aggregate (see Figure 5-26).

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Recycled Concrete

• Dirt, gypsum board, wood, and other foreign
  materials should be prevented from contaminating
  the final product.
• Recycled concrete is primarily used in pavement
  reconstruction.
• It has been satisfactorily used as an aggregate
  in granular subbases, lean-concrete subbases,
  soil-cement.

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Recycled Concrete

• Recycled concrete aggregate generally has a
  higher absorption (3 to 10) and a lower
  relative density than conventional aggregate. The
  absorption values increase as coarse particle
  size decreases (see Figure 5-27).

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Recycled Concrete

• Recycled concrete aggregate should be tested for
  durability, gradation, and other properties.
• New concrete made from recycled concrete
  aggregate generally has good durability.
  Carbonation, permeability, and resistance to
  freeze-thaw action have been found to be the same
  or even better than concrete with conventional
  aggregates.

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Recycled Concrete

• Drying shrinkage and creep of concrete made with
  recycled aggregates is up to 100 higher than
  concrete with a corresponding conventional
  aggregate. This is due to the large amount of old
  cement paste and mortar especially in the fine
  aggregate.

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Recycled Concrete

• Concrete trial mixtures should be made to check
  the new concrete's quality and to determine the
  proper mixture proportions.
• Frequent monitoring of the properties of recycled
  aggregates should be conducted due to the
  variability in the properties of the old concrete.