Lectures No. 09 & 10

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) –Develops by aggregates containing reactive silica materials. This form is more serious and common than ACR.

Subject: Alkali-Aggregate Reactivity 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- silca gel –Alkali-silca 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 alkali silca 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).

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 When the aggregates are not delivered by truck, acceptable and least-expensive results are obtained by forming the stockpile in layers with a clamshell bucket (cast-and-spread method). Spreading the material in thin layers will minimize segregation. 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.

Marine-Dredged Aggregate 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.

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. There is no maximum limit on the salt content of coarse or fine aggregate; however, generally accepted chloride limits should be followed.

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.

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.