CGN3501C Test 1 Study Guide

Test 1 Study Guide

Lecture 2

• Aggregate fills 60-80% of volume in concrete 

• Functions of aggregate in concrete

  • Economy (used as space filler since it’s less expensive than portland cement)

  • Strength

  • Reduction in shrinkage and expansion

• Desirable characteristics of aggregates 

  • Hard, strong, and durable

  • Free of organic impurities

  • Low alkali reactivity with cement 

  • Proper gradation (for good workability and packing of voids)

    • Gradation = size distribution (want to pack in as much as possible)

    • Better packing = less portland cement, more aggregate 

• Classifications of Aggregate

  • By Size

    • Coarse aggregate: particles retained on No. 4 sieve (4.75 mm or 3/16 in) 

    • Fine aggregate: particles passing No. 4 sieve

  • By Source

    • Natural mineral aggregate: sand, gravel, crushed stone 

      • Majority of aggregates

    • Artificial or synthetic aggregate: blast furnace slag, expanded clay, expanded shale 

      • Slag = waste product when manufacturing iron 

      • Expanded clay = heat up clay until it melts, let it absorb air and cool. Used to make lightweight concrete. 

  • By Types of Rock

    • Igneous rock: formed on cooling of magma 

      • Intrusive igneous: formed by slow cooling beneath earth’s surface

        • Characteristics: completely crystalline minerals, coarser grain (ex: granite, trap rock)

      • Extrusive igneous: formed by more rapid cooling at or near earth’s surface

        • Characteristics: finer grain, minerals with smaller crystals or glassy structures (ex: basalt, perlite)

    • Sedimentary rock: formed from disintegration of other rocks and deposited as sediments (ex: limestone, sandstone, shale)

      • Florida limestone: lighter color compared to other limestone, lower density more moisture, not as strong

        • Is it a good aggregate? Ok for normal concrete (ex: housing, pavement, low-rise buildings), but have to adjust for absorption. Good for compressive strengths up to 6000 psi. 

          • Can NOT be used for high-rise buildings, long-spanning bridges 

    • Metamorphic rock: igneous or sedimentary rocks that have changed its structure due to heat and pressure. Harder and denser (ex: marble, slate).

      • Which type would make better concrete? Igneous, sedimentary, or metamorphic? Metamorphic rock used for higher strength, but cannot say one is better than another. 

• Prediction of Behavior of Aggregates in Service

  • From past performance record: best basis for prediction of performance

  • From mineral composition: for example, the minerals amorphous opal, chalcedony and tridymite are known to cause alkali-silica reaction

  • From results of tests: most commonly used method 

• L.A. Abrasion Test

  • Purpose: to determine hardness or resistance to abrasion of an aggregate 

  • FDOT specs: LA loss should be less than 45% (lower = better, higher = weaker, not suitable)

    • Example:

      • Weight of sample retained on #12 before test: 5000 g

      • Weight of sample retained on #12 after test: 3254 g

      • LA loss = (5000 - 3254) / 5000 x 100% = 34.92%

• Aggregate Soundness Test 

  • Purpose: to measure resistance of an aggregate to weathering through cycles of soaking in sodium or magnesium sulfate and oven drying 

  • Soundness = durability, resistance to weathering 

  • FDOT specs: soundness loss should be less than 12%

    • Good to have low weight loss. If it’s more durable, less material will be broken down. 

• Test for Potential Alkali Reactivity (Mortar Bar Method) 

  • Purpose: to determine the potential alkali reactivity of an aggregate 

• Organic Impurities Test (for sand)

  • Purpose: to determine the presence of injurious organic compounds in natural sands 

• Strength ratio should be greater than 95%

• Materials Finer than No. 200 Sieve (smallest sieve, 75 micrometers)

  • Desirable to have low percentage of material passing No. 200 sieve

  • Typical maximum allowable: 1% for coarse aggregate, 3-5% for fine aggregate

  • Determine % passing No. 200 sieve by wet sieving and drying

    • Method A - uses only water

    • Method B - uses water and a wetting agent 

• Lightweight Particles in Aggregate

  • Purpose: to determine the percentage of lightweight particles in aggregates 

  • Usually don’t want many lightweight particles (not as strong)

    • Specific gravity: density as related to water’s density (ex: specific gravity of 2.0 is twice as dense as water) 

      • If specific gravity is less than 2.0, it will float to the surface

      • Heavy liquid with specific gravity of 2.0 is used to separate coal and lignite. Liquid with specific gravity of 2.4 is used to separate chert and shale. 

  • Typical maximum allowed: 0.5–1% for coal and lignite, 3–8%  for chert and shale

• Clay Lumps and Friable Particles in Aggregate

  • Don’t want material that breaks easily (clay lumps)

  • Typical allowable limits: 3% for fine aggregate, 2–10% for coarse aggregate

• Sieve Analysis 

  • Purpose: to determine gradation (size distribution) of aggregates 

  • Standard Sieves: 6”, , 3”, 1.5”, 3/4”, 3/8”, #4, #8, #16, #30, #50, #100, #200

  • The next standard sieve is half the size of the preceding standard sieve.

• Fineness Modulus of Sand

  • A number to quantify the fineness of an aggregate

    • Small number = very fine

    • Big number = very coarse

  • The sum of the cumulative percentages retained (cum. % retained) on the standard sieves 6”, 3”, 1.5”, 3/4”, 3/8”, #4, #8, #16, #30, #50 and #100 sieves, divided by 100.

    • Exclude non-standard sieves and anything over No. 100 sieve 

  • Usually used only for fine aggregate

• Terminology on Aggregate Gradation 

  • Maximum Size - The smallest sieve that 100% of the aggregate must pass

  • Nominal Maximum size - The smallest sieve which the major portion of the aggregate must pass. It may retain 5% to 15% of the aggregate, depending on the size number of the aggregate.

    • Definition in Superpave: One size larger than the first sieve to retain more than 10%

  • Well-Graded or Dense-Graded - Well distributed in various sizes, resulting in low air voids and high density when compacted

  • Uniform Gradation - mostly one size

  • Gap-graded - missing a few sizes

  • Both uniform and gap-graded aggregates are open-graded (high air voids and low density).

• Maximum Density Gradation

  • P = % passing the sieve

  • d = Size of the sieve

  • D = Maximum aggregate size

  • n = 0.5 in Fuller’s Maximum Density Curve, n = 0.45 in FHWA Maximum Density Curve

• Densities of Aggregate

  • Bulk density = Mass of aggregate / (volume of solids + voids)

  • Apparent density = Mass of aggregate / (volume of solids + impermeable voids)

  • True density = Mass of aggregate / volume of solids

• Measurement of Dry Bulk Specific Gravity

  • For an impermeable solid: (Wt. in air) - (Wt. in water) = Wt. of water of same volume (or water displaced)

  • Specific Gravity = Wt. in air / (wt. in air - wt. in water)

  • For a permeable aggregate: (Saturated-Surface-Dry Wt. in air) - (Wt. in water) = Wt. of water of volume of (solid + voids)

  • Dry bulk specific gravity = dry weight in air / (SSD weight in air - weight in water)

• FDOT Specifications

  • L.A. Abrasion Loss 45% max

  • Soundness Loss (Sodium sulfate, 5 cycles) 12% max

  • Flat or elongated pieces 10% max

    • (A flat or elongated particle is one having a ratio between the maximum and minimum dimensions exceeding 5 to 1)

• Lightweight Aggregate

  • Bulk unit weight of less than 70 pcf. (Normal natural aggregate has a unit weight of 95 to 105 pcf.)

    • Ex: pumice (natural agg.), expanded clays, expanded shale, expanded perlite.

  • Used to produce structural lightweight concrete or nonstructural insulating concrete.

• Heavyweight Aggregate

  • Bulk unit weight of over 130 pcf.

  • Used to produce heavyweight concretes for use as nuclear radiation shields.

  • Ex: iron ore, titanium ore, steel punchings.

• Blast-Furnace Slag

  • Waste product from the blast-furnace process for

  • manufacturing of steel and iron.

  • Bulk unit weight of 70 to 85 pcf.

  • Used in making precast concrete products, such as masonry blocks, where high strength is not required.

  • Sulfur content in slag may cause durability problem in concrete. FDOT spec. limits sulfur content to a maximum of 1.5%

• Recycled Concrete as Aggregate

  • The strength and durability of the concrete produced are limited by those of the old concrete.

  • Generally has a higher absorption, a lower specific gravity, and a lower strength than a normal natural aggregate.

  • FDOT spec. allows a maximum L.A. Abrasion Loss of 50% for recycled concrete aggregates.

Lecture 4

• Hydraulic cement - hardens by reacting with water to form a water-resistant product. The presence of air is not required for the hardening process. 

  • ex: Portland cement.

• Nonhydraulic cement - reacts with water to form a product which is not stable in water. The hydration product may then react with air to form a water-resistant product. 

  • ex: quick lime

• Physical Properties of Portland Cement

  • Particle Size: Finer than No. 200 sieve (75  micrometers)

  • Typical Specific Gravity : 3.15

  • Typical Unit Weight: 94 pcf

  • Commercial bag of Portland cement in the U.S. weighs 94 lbs

• Chemical Composition of Portland Cement

  • Consists of various compounds of calcium 

    • (1) CaO - Calcium Oxide - C

    • (2) SiO2 - Silicon Oxide - S

    • (3) Al2O3 - Aluminum Oxide - A

    • (4) Fe2O3 - Iron Oxide - F

    • (5) MgO - Magnesium Oxide - M

    • (6) SO3 - Sulfur Oxide - Ŝ

    • (7) H2O - Water - H

  • 4 main compounds: C3S, C2S, C3A, C4AF

  • Other components of Portland cement are (1) Gypsum, which is added to control the rate of setting of cement, and (2) impurities, such as magnesium oxide (MgO).

• Types of Portland Cement

  • Type I - For general use. No limits are placed on any of the four principal compounds.

  • Type II - Moderate sulfate resistance & moderate heat of hydration. Specification limits the C3A content to a maximum of 8%.

  • Type III - High early strength. C3A content is limited to a maximum of 15%.

  • Type IV - Low heat of hydration. Maximum limits of 35% and 7% on C3S and C3A, respectively. Minimum of 40% C2S.

  • Type V - High sulfate resistance. Maximum limit of 5% on C3A.

  • Type IA, IIA, IIIA - Air-entraining

• Pozzolanic Reaction

  • Pozzolan - a siliceous or siliceous and aluminous material which in itself possesses no cementing property but will, in the presence of water, react with calcium hydroxide to form a cementitious product which is stable in water.

    • Ex: volcanic ash (or pumicite) and fly ash.

  • Characteristics of pozzolanic reaction:

    • (1) Reaction is slow

    • (2) Consumes calcium hydroxide instead of producing it

    • (3) The large capillary spaces are filled up by the reaction products, making the concrete less permeable and more durable.

• Effects of Pozzolans and Blast-Furnace Slags on Strength of Concrete

  • Lower early strength & lower heat of hydration as compared with normal concrete.

  • Permeability is reduced 

    • More pores are filled by the products of pozzolanic reaction

    • Since the rate of sulfate attack depends on the permeability and the amount of calcium hydroxide, the sulfate resistance of the concrete is increased (when permeability and calcium hydroxide ↓) 

    • Type IS cement containing 60 to 70% slag is highly resistant to sulfate attack irrespective of C3A content 

  • Alkali-aggregate expansion is reduced

  • Ultimate strength can be higher than that of normal concrete

  • Portland blast-furnace slag cement has a more rapid strength gain than Portland pozzolan cement

  • Water resistance is increased since calcium hydroxide is consumed

• Granulated Blast-Furnace Slag

  • Blast-furnace slag: nonmetallic waste product from the blast-furnace process in the manufacturing of iron and steel

  • Granulated slag: glassy product formed by rapid cooling of the molten blast-furnace slag

    • Unlike pozzolans, granulated blast-furnace slag is self-cementing. However, when it hydrates by itself, the amount of cementitious products formed and the rate of formation are insufficient to give adequate strengths for structural applications.

  • When used with Portland cement, the hydration of the slag is accelerated in the presence of calcium hydroxide and gypsum. The calcium hydroxide is also consumed by the slag in a pozzolanic reaction.

• Special Hydraulic Cements

  • Portland blast-furnace slag cement (Types IS)

    • Blend of Portland cement with 25 to 70% by weight of granulated blast-furnace slag

  • Portland pozzolan cement (Type IP & P) 

    • Blend of Portland cement with 15 to 40% fine pozzolan by weight

    • Type P has a lower early strength than Type IP, and is used when high early strength is not required.

• White Cement

  • Used to produce architectural concrete where white color is desired.

  • White color is achieved by reducing the iron content of the cement.

  • Similar to normal Portland cement in properties.

• Colored Cements

  • Produced by (1) adding pigments to white cement, or (2) using clinkers having the corresponding colors.

  • Similar to normal Portland cement in properties

Lecture 5

• Loss on Ignition Test 

  • Purpose: To measure the level of pre-hydration of a cement

  • Limits the maximum Loss on Ignition at 3.0% for Types I, II, III and V and 2.5% for Type IV Portland cement

• Tests for Fineness 

  • Fineness is characterized by specific surface, usually in units of m²/kg.

  • The greater the fineness, the more rapid the rate of hydration (will react with water faster)

  • Two commonly used methods are (1) Wagner Turbidimeter test and (2) Blaine Air Permeability test

    • Blaine test gives a higher number than the turbidimeter test

    • Minimum fineness of 160 m²/kg by the turbidimeter test and a minimum of 280 m²/kg by the air permeability test for Type I, II, IV and V cements

• Test for Soundness of Portland Cement

  • Soundness of cement is the ability to retain its volume after setting without undue expansion

  • Most commonly used test is Autoclave Expansion of Portland Cement

    • Limits the maximum autoclave expansion to 0.80%

      • High percentage means it has expanded too much

• Tests for Time of Setting

  • Time of setting of a cement is to determine the quality of a cement with regards to its rate of setting.

  • (1) the Gillmore needle test and (2) the Vicat needle test

    • Gillmore test produces higher values than the Vicat test

    • Minimum Initial Setting Time: 

      • Gillmore: 60 minutes, Vicat: 45 minutes

    • Maximum Final Setting Time: 

      • Gillmore: 600 minutes, Vicat: 375 minutes

Lecture 6

• Batching of ingredients → mixing → fresh concrete → placing and curing → hardened concrete 

• Properties of Fresh Concrete

  • Desirable characteristics:

    • ease of transport and placement

    • resistance to bleeding and segregation

  • Consistency - describes the ease of flow of the fresh concrete

  • Cohesiveness - describes the water holding capacity (or resistance to bleeding) and the coarse-aggregate holding capacity (or resistance to segregation) of the fresh concrete

  • Workability - describes the composite property of both consistency and cohesiveness of the fresh concrete

• Slump Test

  • A consistency test for fresh concrete

• Ball Penetration Test 

  • Purpose: Measures consistency of fresh concrete

• Factors Affecting Workability of Fresh Concrete

  • Water Content: most important factor affecting the consistency of fresh concrete. Higher water content, higher slump. 

  • Cement Content: decreasing the cement content = harsher mixture = harder to finish. Increasing = gives better cohesiveness but higher stickiness

  • Aggregate Size: increasing the maximum aggregate size increases the slump of the fresh concrete

  • Aggregate angularity and roughness: higher angularity and roughness produce fresh concrete with lower slump

  • Admixture: water reducing, air-entraining and fly ash admixtures increase the slump of the fresh concrete

• Bleeding of fresh concrete is manifested by the appearance of water on the surface after a concrete has been placed and compacted.

  • Effects of bleeding:

    • Concrete is weaker near the top than at the bottom.

    • Only some of the bleeding water reached the surface; a large amount of it gets trapped under larger pieces of aggregate. The concrete is weaker at the locations of the trapped water.

    • Laitance - bleeding water tends to carry with it very fine particles of cement, sand and clay and deposit them. This results in a softer surface which is prone to dusting.

  • Methods to reduce bleeding:

    • Use air entrainment & reduce water content

    • Increase the proportion of sand and/or cement in the concrete mix

    • Use mineral admixtures such as fly ash or ground blast furnace slag

• Factors affecting strength

  • Water Cement Ratio: Lower water/cement ➔ higher strength

  • Age (or curing time): Strength generally increases with curing time

  • Curing Condition: Longer moist curing & higher temperature ➔higher strength

  • Type of cement & admixture: Affects rate of strength gain & ultimate strength

  • Strength of aggregate: Strength of concrete is limited by the strength of aggregate

  • Moisture Content of Concrete: higher moisture ➔ lower strength

• Properties of Hardened Concrete

  • Strength

    • Compressive strength at 28 days cure: 15-40 MPa (2000-6000 psi) for normal concrete

    • Tensile Strength = 10% of compressive strength

    • Flexural strength = 15% - 20% of compressive strength

  • Elastic modulus: the ratio between stress and strain

    • Measure of the stiffness of the concrete

    • 15-40 GPa for normal concrete

  • Creep: the continued yielding under sustained stress.

    • May cause reduction of prestress in prestressed concrete structures

  • Durability: resistance to freezing & thawing, chemical corrosion & mechanical wear

  • Permeability: ease of flow of fluid or gas through the concrete

    • Low permeability of concrete is needed to prevent:

      • Disintegration caused by freezing of saturated porous concrete

      • Dissolving of slowly soluble components in concrete

      • Chloride intrusion causes corrosion of rebars.

  • Density

    • Normal concrete:  2.4 Mg/m3 (150 pcf)

    • Concrete made with Florida limestone:  2.24 Mg/m3 (140 pcf)

Lecture 8

• Concrete Finishing

  • (1) Strikeoff or Screeding - Striking off excess concrete to bring the top surface to proper grade.

  • (2) Bullfloating - Leveling to eliminate high and low spots.

  • (3) Floating - purpose is to firmly embed the aggregate, to compact the surface and to remove any surface imperfections

    • To be performed when the concrete is able to sustain foot pressure with only slight indentation

  • (4) Troweling - Finishing to obtain a smooth, dense surface

  • (5) Texturing - Finishing to obtain desired surface texture

    • e.g. Brooming to obtain slip-resistant surface