Topic No. 6

siennatearfulUrban and Civil

Nov 25, 2013 (3 years and 8 months ago)

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Topic No. 6

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.


ASR


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

Alkali carbonate reaction (ACR)


The aggregates [dolomitic (calcium
-
magnesium carbonate)] have specific
composition that is not very common.


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


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.


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.


Popouts

Alkali silica reaction (ASR)


List of most reactive substances:


Opal (SiO
2

nH
2
O)


Chalcedony (SiO
2
)


Certain forms of quartz (SiO
2
)


Cristobalite (SiO
2
)


Alkali silica reaction (ASR)


The most important harmful alkali reactive
aggregates:


Opaline cherts


Chalcedonic cherts


Siliceous limestones


Siliceous dolomite


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.


Alkali silica reaction (ASR)


Control of ASR


Use of low
-
alkali Portland cement (less than 0.6%
equivalent Na
2
O) 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.

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/m
3
.


Control the access of water to concrete.


Replacing 25%
-

30% of the reactive sand
gravel aggregate with crushed limestone
(known as
limestone sweetening
).

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.


Aggregate Processing

Consists of two stages:


Basic processing


This includes


crushing,


screening,


washing to obtain proper gradation and
cleanliness.

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.

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.

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.

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..



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.

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.


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.

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.

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.

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.

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.

Crushing concrete with a
beamcrsher

Recycled Concrete



(4) Grading and washing.




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

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.


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).


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.

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.


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.