Lecture No. 09-10. Subject: Alkali-Aggregate Reactivity Certain constituents in aggregates can react harmfully with alkali hydroxides in concrete and

Lecture No. 09-10 Lecture No. 09-10

Subject: Alkali-Aggregate Subject: Alkali-Aggregate ReactivityReactivity

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.

ASRASR

• 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)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


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


• 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.

PopoutsPopouts


• List of most reactive substances:– Opal (SiO2 nH2O)

– Chalcedony (SiO2)

– Certain forms of quartz (SiO2)

– Cristobalite (SiO2)


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


• 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.


• 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.


• 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).


• 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 ProcessingAggregate Processing

Consists of two stages:– Basic processing

• This includes –crushing, –screening, –washing to obtain proper gradation and

cleanliness.

Aggregate Processing: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: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 Handling and Storing Aggregates 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.


• 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..


• 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.


• 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 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.

saltssalts

• 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.

SeashellsSeashells

• 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 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 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 Crushing concrete with a beamcrsherbeamcrsher


– (4) Grading and washing.

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


• 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 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 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.


• 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.


• 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.

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Lecture No. 09-10. Subject: Alkali-Aggregate Reactivity Certain constituents in aggregates can react harmfully with alkali hydroxides in concrete and