Lecture #3: Aggregate Moisture and Physical Characteristics

Particle Shape and Surface Texture Results from processing Difficult to characterize Function of: Angularity: Sphericity: Described in terms of particle dimensions: a) angular b) rounded c) flaky d) elongated e) flaky and elongated Surface texture:

Moisture Content States Physical Properties: Porosity & Voids Content Moisture Content States

Moisture Content Equations The aggregate stockpile is ‘wet’ if MC% > AC%. Aggregate will ‘give up’ water. The aggregate stockpile is ‘air dry’ if MC% < AC%. Aggregate will ‘take up’ water. Absorption controls the water demand of portland cement concrete. Most normal weight aggregates have 0.5 to 2.0% AC%’s. Values higher than this are considered to be high-porosity and may present poor durability performance. Resistance to water penetration controls the capability of the aggregate to resist deterioration.

Moisture Analysis Water 500kg cement 250kg coarse agg 800kg (SSD) AC = 1.50% fine agg 450kg (SSD) AC = 0.75% If MCca = 1% and the MCfa = 1% what is the stock water amount to maintain the same water demand? Will the aggregate absorb or release water?

Specific Gravity Specific gravity is the ratio of the weight oa a unit volume of material to the Weight of the same volume of water at 20º to 25ºC.

Archimedes Principle

Apparent Specific Gravity Overall volume of the aggregate exclusive of the volume of the pores or Capillaries which become filled with water in 24 hrs of soaking

Bulk Specific Gravity

Specific Gravity Equations Bulk Specific Gravity Saturated Surface-Dry Apparent Specific Gravity Lab Formula

Specific Gravity Test for Sand Container with H2O OD Aggregate Container with H2O and with Aggregate

Gb < Gb(ssd)< Ga < Gtrue Specific Gravity Relationships Gb < Gb(ssd)< Ga < Gtrue

Moisture Content in Field The aggregate stockpile is ‘wet’ if MC% > AC%. Aggregate will ‘give up’ water. The aggregate stockpile is ‘air dry’ if MC% < AC%. Aggregate will ‘take up’ water. Absorption controls the water demand of portland cement concrete. Most normal weight aggregates have 0.5 to 2.0% AC%’s. Values higher than this are considered to be high-porosity and may present poor durability performance. Resistance to water penetration controls the capability of the aggregate to resist deterioration.

Aggregate Strength and Toughness Aggregate strength dependent upon: a) properties of constituent minerals b) bonding between grains c) porosity These properties are difficult to measure. LA Abrasion test measures aggregate degradation Affects particle breakdown LA abrasion test measures aggregate degradation due to grinding, impact, and attrition inside a rotating steel drum with a specified number of steel spheres. Aggregates are subjected to particle breakdown in asphalt mixtures due to manufacture, placement, and compaction during construction and under traffic. Breakdown more severe in open graded materials rather than dense graded. Use lower strength aggregates in dense mixtures.

Aggregate Surface Chemistry Aggregate surface chemistry affects bonding to cement Hydrophobic: water-repellent such as limestone and dolomites have a positive surface charge. Work well in asphalt concrete. Hydrophilic: water-attracting such as gravels and silicates (acidic) have a negative surface charge. Gravels may tend to create a weaker interfacial zone in concrete than lime- stone aggregates. Surface coating (dust of clay, silt, gypsum, etc.) tend to reduce bond strength.

Aggregate Durability Physical durability: resistance to physical processes 1. Unstable volume changes: due to freezing of absorbed water. 2. Mechanical breakdown during handling, stockpiling, or placement. Chemical durability: resistance to chemical processes 1. Alkali-silicate reaction

Alkali-Silica Reactivity

What is ASR? ASR only a concern if deleterious, e.g. causes cracking. Needs three factors to be deleterious : - Source of alkali - Internal and external - Reactive silica (aggregate) - Water ( humidity) > 80 %

What is ASR? Onset of ASR results in formation of expansive gels which produce internal stresses which may cause cracking of concrete. Problem is often misdiagnosed. Environmental factors such as freeze-thaw cycles, wetting/drying cycles, and traffic loading propagate cracking. Deicing salts, marine environments, can accelerate ASR expansion and deterioration processes. ASR can accelerate corrosion deterioration.

What is ASR?

Deleterious Substances Absorbent particles: shale, leached chert, or porous flint Clay lumps Coal or wood particles Organic impurities Flat or elongated particles

Clay Lumps