CMT - Midterms

AGGREGATE

A mass of crushed stone, gravel, sand, etc, predominantly composed of individual particles, but in some cases including clays and silts.

• Coarse Aggregate

• Fine Aggregate

• Maximum aggregate size

• Nominal maximum aggregate size

Types of Aggregate

Coarse Aggregate

particles that are retained on a 4.75mm sieve

Fine Aggregate

particles that pass a 4.75mm sieve

Maximum aggregate size

one sieve size larger than the nominal maximum aggregate size

Nominal maximum aggregate size

one sieve larger than the first sieve to retain more than 10% of the aggregates.

Natural sources

Manufactured aggregates

Where do you get these aggregates?

Cost and availability

are important when selecting an aggregate source.

Drilling cores and performing trial blasts (or shots)

to evaluate how the rock breaks and by crushing some materials in the laboratory to evaluate grading, particle shape, soundness, durability, and amount of fine material.

1 – As underlying material for foundations and pavements

2 – As riprap for erosion protection

3 – As ingredients in Portland cement and asphalt concretes

AGGREGATE USES

Portland cement concrete

Acts as a filler to reduce the amount of cement paste needed in the mix and have greater volume stability than cement paste

Asphalt cement

acts as a binder to hold the aggregates together, but does not have enough strength to lock the aggregate particles into position

Shape and surface texture of an aggregate

determine how the material will pack into a dense configuration and also determines the mobility of the stones within a mix.

Angular and rough-textured aggregates

produce bulk materials with higher stability than rounded, smooth-textured aggregates.

Flakiness

describes the relationship between the dimensions of the aggregate

• ASTM D4791 – flakiness test

• Only aggregates retained on the 9.5mm sieve are evaluated

How do you determine the flakiness of a coarse aggregate?

ASTM C1252

Test Method for Uncompacted Void Content of Fine Aggregate

ASTM C88

Simulates weathering by soaking the aggregates in either a sodium sulphate or a magnesium sulphate solution

a) Bone dry

b) Air dry condition

c) Saturated surface-dry (SSD) condition

d) Free moisture

Four moisture condition for an aggregate particle:

Specific gravity

mass of a material divided by the mass of an equal volume of distilled water

1) Bulk-dry

2) Bulk-saturated surface dry

3) Apparent specific gravity

4) Effective specific gravity

Four types of specific gravity are based on how voids in the aggregate particles are considered:

ASTM C127

specific gravity and absorption of coarse aggregates are determined in this test

ASTM C128

procedure for determining the specific gravity and absorption of fine aggregates

ASTM C29

procedure to determine the bulk unit weight of aggregate

ASTM C136, E11

process to evaluate the aggregates passing through a series of sieves

Gradation

Describes the particle size distribution of the aggregate.

Gradation specifications

define maximum and minimum cumulative percentages of material passing each sieve

Deleterious substance

any material that adversely affects the quality of Portland cement or asphalt concrete made with the aggregate

Toughness or abrasion resistance

ability of aggregates to resist the damaging effect of loads which is related to the hardness of the aggregate particle

Alkali-Aggregate Reactivity

Most common reaction, particularly in humid and warm climates, is between the active silica constituents of an aggregate and the alkalis in cement

• limiting the alkali content of the cement

• Keeping the concrete structure as dry as possible

How do we reduce the reactivity if alkali-reactive aggregate must be used?

Portland Cement

A fine powder made by heating limestone and other materials such as clay or shale in a kiln to form clinker, which is then ground into a fine powder

Portland Cement

Its strength, durability, and versatility make it suitable for wide range of construction applications, such as buildings, bridges, tunnels, dams, factories, pavements and roads

1 – to make Portland Cement Concrete (PCC)

2 – stabilising soils

3 – stabilising aggregate bases for highway construction

Main Purpose of Portland Cement

Particle size specifications

defined in terms of the surface area per unit weight

finer

The _____ the cement particles, the larger the surface area and the faster the hydration

3.15

Specific gravity of Portland cement (without voids between particles)

Capillary voids

result of the hydrated cement paste having a lower bulk specific gravity than the cement particles

setting, soundness, and compressive strength of mortar

Three properties of hydrated cement:

Setting

stiffening of the cement paste or the change from a plastic state to a solid state

False set

might occur in which the cement stiffens within a few minutes of being mixed, without the evolution of much heat

Soundness

ability to retain its volume after setting

reducing

Strength and other desirable properties are improved by ______ the weight of water used per unit weight of cement

Type I

most common and widely used type of cement in the Philippines

90%

Acceptance criteria for questionable water are specified in ASTM C94

After 7 days, the average compressive strength of mortar cubes made with questionable water should not be less than ____  of the average strength of cubes made with potable or distilled water

• To reduce the cost of concrete construction

• To achieve certain properties in concrete more effectively than by other means

Reasons for Using Admixtures

Air Entrainers

produce tiny air bubbles in the hardened concrete to provide space for water to expand upon freezing

Water Reducers

Increase the mobility of the cement particles in the plastic mix, allowing workability to achieved at lower water contents

water reducer

allows the use of a lower amount of mixing water whilst maintaining the same workability level.

Superplasticizers (plasticizers)

Either greatly increase the flow of the fresh concrete or reduce the amount of water required for a given consistency

Retarders

can be used to delay the initial set of concrete

Retarders

Increase the time required for the initial set but reduce the time between the initial and final set

Accelerators

Used to develop early strength of concrete at a faster rate than that developed in normal concrete

Fly Ash

Increases the workability of the fresh concrete and extends the hydration process

2.85 - 2.95

Specific gravity of slag cement:

Slag Cement

Non-metallic hydraulic cement consisting basically of silicates and aluminosilicates of calcium

Silica Fume

Can reduce concrete corrosion induced by deicing or marine salts

Pozzolan

is a siliceous and aluminous material that possesses little or no cementitious value but will react chemically with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties

1.Strength

2.Modulus of Elasticity

3.Durability

4.Porosity

Properties of Hardened Concrete important to the Materials Engineer:

1.acceptable workability of freshly mixed concrete

2.durability, strength, and uniform appearance of hardened concrete

3.economy

Three Qualities Required Of Properly Proportioned Concrete Mixtures

Air entrainment

is required whenever concrete is exposed to freeze–thaw conditions and deicing salts.

Mild exposure

Indoor or outdoor service in which concrete is not exposed to freezing and deicing salts. Air entrainment may be used to improve workability.

Moderate exposure

Some freezing exposure occurs, but concrete is not exposed to moisture or free water for long periods prior to freezing.

Severe exposure

Concrete is exposed to deicing salts, saturation, or free water. Examples include pavements, bridge decks, curbs, gutters, canal linings, etc.

Workability

defined as the ease of placing, consolidating, and finishing freshly mixed concrete

Ready-mixed concrete

is mixed in a central plant, and delivered to the job site in mixing trucks ready for placing.

1. Central-mixed concrete

2. Shrink-mixed concrete

3. Truck-mixed concrete

Three mixing methods can be used for ready-mixed concrete:

Curing

is the process of maintaining satisfactory moisture content and temperature in the concrete for a definite period of time.

1. Ponding or Immersion

2. Spraying or Fogging

3. Wet Coverings

4. Impervious Papers or Plastic Sheets

5. Membrane-Forming Compounds

6. Forms Left in place

7. Steam Curing

8. Insulating Blankets or Covers

9. Electrical , Hot Oil, and Infrared Curing

Different Curing of Concrete methods

Steel

One of the most widely used materials in the world due to its strength, durability and versatility

Steel

Alloy made primarily of iron and carbon, with small amounts of other elements such as manganese, chromium, nickel, or vanadium, depending on its specific properties

killed steels

Completely deoxidised steels are known as

Annealing

To refine the grain, soften the steel, remove internal stresses, remove gases, increase ductility and toughness, and change electrical and magnetic properties

Full annealing

Process annealing

Stress relief annealing

Spheroidisation

Annealing

Tempering

is performed to improved ductility and toughness

Heating

Holding

Cooling

Process of tempering:

Ensures the steel has enough toughness to withstand impact and stress whilst maintaining adequate hardness for wear resistance

Why is tempering important?

Structural Steel

Used for various types of structural members, such as columns, beams, bracings, frames, trusses, bridge girders, and other structural applications

Structural Steel Grades

Wide variety of systems for identifying or designated steel, based on grade, type and class

conventional or prestressed reinforcing

Structural concrete members must be reinforced either:

Torsion Test (ASTM E143)

To determine the shear modulus of structural materials

Shear modulus

ratio of maximum shear stress to the corresponding shear strain below the proportional limit of the material

Tension Test (ASTM E8/E8M)

To determine the yield strength, yield point, ultimate (tensile) strength, elongation and reduction of area

Semi-guided bend test (ASTM E290)

used to check the ductility to accommodate bending

Hardness

measure of a material’s resistance to localised plastic deformation

Rockwell Hardness Test (ASTM E18)

method used to measure hardness of steel and other materials

Ultrasonic Testing

Non-destructive method for detecting flaws in materials

Corrosion

defined as the destruction of a material by electrochemical reaction to the environment

Apply protective coatings

How to prevent corrosion?

Endogenous - not generally used for engineering applications

Exogenous - hardwood or softwood

Trees are classified either:

Growth Rings

concentric layers in the stem of exogenous trees

Wood

is an anisotropic material – has different and unique properties in each direction

fibre saturation point

level of saturation at which the cell walls are completely saturated, but no free water exists in the cell cavities

knots

checks

pitch pockets

shakes

stains

Common grade-reducing qualities of lumber:

1.5

Regardless of species, the substance composing the cell walls has a specific gravity of

Thermal Conductivity

measure of the rate at which heat flows through a material

Specific Heat

ratio of the quantity of heat required to raise the temperature of the material one degree to that required to raise the temperature of an equal mass of water one degree

Thermal Diffusivity

measure of the rate at which a material absorbs heat from its surroundings

Coefficient of Thermal Expansion

measure of dimensional changes caused by a temperature variance

Damping

phenomenon in which the amplitude of vibration in a material decreases with time