BDCS Notes - Moisture Content of Wood

Moisture Content of Wood: The weight of water in the specimen expressed as a percentage of the oven-dry weight of the wood.

Oven-Dried Wood: A sample that has been dried in an oven at 100 C to 105 C until the wood attains a constant weight.

Moisture influences weight, shrinkage and strength of the wood. It exists in wood as either bound or free water. Bound water is held within the cell wall by adsorption forces, whereas free water exists as either condensed water or water vapor in the cell cavities.

The level of saturation at which the cell walls are completely saturated, but no free water exists in the cell cavities is called the fiber saturation point (FSP). The FSP ranges from 21% to 32% in most species. The FSP is important because the addition or removal of moisture below the FSP has a large effect on practically all physical and mechanical properties of wood, whereas above the FSP, the properties are largely independent of moisture content.

When moisture content of wood is above the FSP, wood is dimensionally stable. Anything below the FSP will result in dimensional changes. Also, moisture content in wood depends on air temperature and humidity.

The moisture content for the average atmospheric conditions is the equilibrium moisture content (EMC). The EMC ranges from less than 1%, at temperatures greater than 55C and 5% humidity, to over 20% at temperatures less than 27 C and 90% humidity.

BDCS Notes - Chemical Composition of Wood

Composition of Wood:

1. Cellulose

2. Lignin

3. Hemicelluloses

4. Extractives

5. Ash Producing Minerals

Wood is a linear polymer having a high molecular weight. The main building block of cellulose is sugar-glucose. As the tree grows, linear cellulose molecules arrange themselves into highly ordered strands, called fibrils. These ordered strands form the large structural elements that compose the cells walls of wood fibers.

Lignin: Accounts for 23% to 33% of softwood and 16% to 25% of hardwood by weight. It is mostly an intercellular material. Chemically lignin is an intractable, insoluble, material that is loosely bonded to the cellulose. It is the glue that holds the tubular cells together. It’s bonds affect the shear strength of wood.

Hemicelluloses: Are polymeric units made from sugar molecules. Differs from cellulose in that it has several sugars tied up in its cellular structure. Hardwood contains 20% to 30% hemicellulose and softwood averages 15% to 20%. The main sugar units include xylose and monnose.

Extractives: Compose 5% to 30% of the wood substance. Included in this group are tannins and other polphenolics, coloring matters, essential oils, fats, resins, waxes, gums, starches, and simple metabolic intermediates. These materials can be removes with inert neutral solvents (water, alcohol, acetone, and benzene).

Ash-Forming Materials: Account for 0.1% to 3% of the wood material and include calcium, potassium, phosphate and silica.

BDCS Notes - Structure of Wood

Growth Rings: The concentric layers in the stem of exogenous trees (also called tree rings or annual rings). One growth ring is produced per year.

Growth rings are composed of early wood, which is produced by rapid growth during the spring, and latewood from summer growth.

Latewood: dense, dark, and thick-walled cells producing a stronger structure than early wood.

Predominant physical features of wood:

1. Bark

2. Cambium

3. Wood

4. Pith

Bark: The exterior covering of the tree and has an outer and an inner layer. The outer layer is dead and corky and has great variability in thickness, dependent on species and age of tree. The inner bark layer is the growth layer for bark but is not part of the wood section of the tree.

Cambium: Thin layer of cells situated between the wood and the bark and is the location of all wood growth.

The Wood Section of the Tree: Composed of sapwood and heartwood. Sapwood functions as a storehouse for starches and as a pipeline to transport sap. Fast growing species have thick sapwood regions. Heartwood is not a living part of the tree. It is composed of cells that have been physically and chemically altered by mineral deposits. It provides strength for the tree. Since it doesn’t contain sap, it is resistant to decay.

Pith: Central core of the tree. Its size varies with the tree species ranging from barely distinguishable to large and conspicuous. Colors range from blacks to whitish, depending on species and locality. Pith can be solid, porous, chambered or hollow.

Anisotropic Nature of Wood: Wood is anisotropic. This means it has different and unique material properties in each direction. The three axis orientations in wood are longitudinal/parallel to the grain, radial/cross the growth rings, and tangential/tangent to the growth rings. These different orientations affect physical and mechanical properties such as shrinkage, stiffness and strength. The anisotropic behavior of wood is the result of the tubular geometry of the wood cells. Their growth in one direction, and their flexure upon moisture, causes the change in physical and material properties.

BDCS Notes - Wood

Pros of Using Wood:

1. Available

2. Low Cost

3. Ease of Use

4. Durability

5. Good for Use in Compression

Used extensively for buildings, bridges, utility poles, floors, roofs, trusses, and piles.

Engineered Wood Products: Laminates, plywood, and strand board.

Trees: A woody plant that attains a height of at least 20 ft., has a self-supporting trunk with no branches for about 4 ft. Over 600 species of trees in the United States.

Trees are classified as either endogenous or exogenous, based on type of growth. Endogenous trees, such as bamboo, grow with intertwined fibers. Wood from endogenous trees not used for engineering applications. Exogenous trees grow from the center out by adding concentric layers of wood around the central core. Structural applications for wood primarily concern the use of exogenous trees.

Exogenous trees are classified as either deciduous or conifers, producing hardwoods and softwoods, respectively.

Softwoods are softer, less dense, and easier to cut than hardwoods (although Balsa is a hardwood). They are also used more in construction than hardwoods. Hardwoods are typically used for furniture and decorative veneers (because of their nice grain pattern).

Deciduous trees shed their leaves at the end of each growing season. There are about 40 different kinds of deciduous trees used for commercial hardwood production.

Conifers (evergreens) have needlelike leaves and normally do not shed at the end of the growing season. They grow continuously through the crown to produce a uniform stem and homogenous characteristics. Softwood comes from 20 different kinds of conifers. The softwood is primarily used for structural purposes. The rapid maturing of conifers makes them a renewable resource.

BDCS Notes - Welding

Welding: A technique for joining two metal pieces by applying heat to fuse the pieces together.

Filler Metal: used to facilitate process of welding.

Arc Welding: Uses an arc between the electrode and the grounded base metal to bring both the base metal and the electrode to their melting points.

Shielded Metal Arc Welding (Stick Welding): Most common form of arc welding, limited to short welds in bridge construction. A consumable electrode, which is covered with flux, is used. The flux produces a shielding atmosphere at the arc to prevent oxidation of the molten metal. The flux also traps impurities in the molten weld pool.

Submerged Arc Welding: Semiautomatic or automatic arc welding process. A bare wire electrode is automatically fed by the welding machine while a granular flux is fed into the joint ahead of the electrode. The arc takes place in the molten flux, which completely shields the weld pool from the atmosphere. The molten flux concentrates the arc heat, resulting in deep penetration into the base metal.

Gas Welding: No flux is used. An external shielding gas is used, which shields the molten weld pool and provides the desired arc characteristics. Typically used for small welds.

Care must be taken during welding to consider the distortion that is the result of the non-uniform heating of the welding process.

When the molten weld metal cools, it shrinks, causing deformation of the material and introducing residual stresses into the structure.

Welding Zones – Determine the relative ease with which steel can be welded when compared to the hardness of the steel.

1. Zone I – Cracking unlikely, but may occur with high hydrogen or high restraint. Use hydrogen control method to determine preheat.

2. Zone II – The hardness control method and selected hardness used to determine minimum energy input for single-pass fillet welds without preheat.

3. Zone III – The hydrogen control method is used.

Hardness and Hydrogen Control Methods are means of determining the level of energy used to preheat the weld area before the weld is performed.

Whenever metal is welded, the base material adjacent to the weld is heated to a temperature that may be sufficient to affect its metallurgy. The material affect in this manner is termed the heat-affected zone, HAZ. This material is a high-risk area for failure, especially if proper preheating and cooling procedures are not followed.

BDCS Notes - Steel Corrosion

Corrosion: The destruction of a material by electrochemical reaction to the environment (or the destruction that can be detected by rust formation). Can result in lowering weight limits of structures, costly steel replacement, and/or collapse of a structure.

Corrosion requires four elements:

1. An Anode – The electrode where corrosion occurs.
2. A Cathode – The other electrode needed to form a corrosion cell.
3. A Conductor – A metallic pathway for electrons to flow.
4. An Electrolyte – A liquid that can support the flow of electrons.

Steel, being a heterogeneous material, contains anodes and cathodes. Steel is also an electrical conductor. Therefore, steel contains three of the four elements needed for corrosion, while moisture provides the fourth element (an electrolyte).

Salt, from deicing or a marine environment, accelerates corrosion of steel bridges and reinforcing steel in concrete.

Methods for Corrosion Resistance

Three mechanisms by which coatings provide corrosion protection:

1. Barrier Coatings – Isolate the steel from the moisture. Low water and oxygen permeability.

2. Inhabitive Primer Coatings - Contain passivating pigments. Low-solubility pigments that migrate to steel surface when moisture passes through the film to passivate steel surface.

3. Sacrificial Primers (Cathodic Protection) – Contain pigments (such as zinc), which gives up electrons to the steel, becomes the anode, and corrodes to protect the steel.

BDCS Notes - Reinforcing Steel

Structural Concrete: Concrete members subjected to tension reinforced with reinforcing steel bars. (Conventional or prestressed reinforcing)

Conventional Reinforcing: Stresses fluctuate with loads on the structure. No special requirements on the steel.

Prestressed Reinforcement: Steel is under continuous tension. Stress relaxation will reduce the effectiveness of the reinforcement, thus steel bars are required.

Manufactured Forms:

Plain Bars – Round, without surface deformations (not used for tension or bending)

Deformed Bars – Have protrusions (deformations) at the surface. Ensures a good bond between the bar and the concrete. Deformed surface of bond prevents slipping, allowing unit to work as one. (Used in concrete beams, slabs, columns, walls, footings, pavements, concrete structures, etc.)

Wire Fabrics (Plain and Deformed) – flat sheets in which wires pass each other at right angles, and one set of elements is parallel to the fabric axis. (Used in concrete slabs and pavements, to resist temperature and shrinkage stresses).

1. Plain Wire Fabrics – Develop anchorage in concrete at the welded intersections.

2. Deformed Wire Fabrics – Develop anchorage through deformations and at the welded intersections.

Kinds of Reinforcing Steel:

A615 – Billet (most widely used)

A616 – Rail

A617 – Axle

A706 – Low-Alloy

Four Grades of Reinforcing Steel:

40 – 276 MPa

50 – 345 MPa

60 – 414 MPa

75 – 517 MPa

Prestressed Concrete – Requires special wires, strands, cables and bars. Steel for prestressed concrete reinforcement must have high strength and low relaxation properties. High-carbon steels and high-strength alloy steels are used for this purpose.

BDCS Notes - Mechanical Testing of Steel

Tension Test: performed to determine the yield strength, yield point, ultimate (tensile) strength, elongation, and reduction of area.

Traditional way of calculating stress and strain uses the original cross-sectional area and gauge length. If the stress and strains are calculated based on the instantaneous cross-sectional area and gauge length, a true stress-strain curve is obtained (initial slope continuation), which is different than the engineering stress-strain curve (curve toward rupture point).

True Stress > Engineering Stress – Because of the reduced cross-sectional area at the neck. The specimen experiences the largest deformation at the regions closest to the neck.

Different carbon content steels have different stress-strain relations. Increasing the carbon content in steel increases the yield stress and reduces the ductility.

Steel is generally assumed to be a homogenous and isotropic material. However, in production of structural members, the final shape may be obtained by cold rolling. This causes the steel to undergo plastic deformations, with the degree of deformation varying throughout the member. Plastic deformation causes increase in yield strength and reduction in ductility. Therefore, it’s necessary to evaluate the sample of steel when it is collected for its material properties.

Torsion Test: Used to determine shear modulus of structural materials.

Shear Modulus: used in the design of members subjected to torsion, such as rotating shafts and helical compression springs.

In the test, a cylindrical, or tubular, specimen is loaded either incrementally or continually by applying an external torque to cause a uniform twist within the gauge length.

The amount of torque applied is measured against the responding angle of twist.

(tau)_max = Tr / J

(gamma) = (theta)r / L

&

G = (tau)_max / (gamma) = TL / J(theta)

Where: T = torque

r = radius

J = polar moment of inertia

(theta) = angle of twist in radians

L = gauge length

Charpy V Notch Impact Test: Used to measure the toughness of the material or the energy required to fracture a V-notched simply supported specimen.

Standard specimen is 55 X 10 X 10 mm with a V notch at the center. It is inserted into an impact-testing machine using centering tongs. A swinging arm of the machine has a striking tip that impacts the specimen on the side opposite the V notch. The striking head is released from the pretest position, striking and fracturing the specimen.

By measuring the height the strike head attains after striking the specimen, the energy required to fracture the specimen is computed (measured in ft-lb).

Fracture surface consists of a dull shear area (ductile) at the edges and a shiny cleavage area (brittle) at the center.

Bend Test: Used to check ductility needed to accommodate bending. Evaluates the ability of steel, or a weld, to resist cracking during bending.

Bend test is conducted by bending the specimen through a specified angle and to a specified inside radius of curvature. When fracture doesn’t occur, failure is measured by the number and size of cracks found on the tension surface.

Test is made by applying a transverse force to the specimen in the portion that is being bent, usually at midlength.

Three arrangements can be used:

1. Specimen fixed at one end and bent around a reaction pin or mandrel by applying a force near the free end.
2. Specimen held at one end and a rotating device is used to bend the specimen around the pin or mandrel.
3. Force applied in the middle of a specimen simply supported at both ends.

Hardness Test: Measured a material’s resistance to localized plastic deformation, such as a small dent or scratch on the surface of the material.

Tests include an indenter that is forced into the surface of the material with a specified load magnitude and rate of application. Depth, size and indentation is measured and related to hardness index number.

Rockwell Hardness Test: Depth of penetration of a diamond cone, or a steel ball, into the specimen is determined under fixed conditions. Preliminary load of 10 kg is applied first, followed by additional load. Rockwell number, which is proportional to difference in penetration between preliminary and total loads, is read from machine by means of a dial, display, pointer or other device.

For very thin steel, the Rockwell Superficial Hardness Test is used. Procedure is similar to test mentioned above, except that smaller preliminary and total loads are used.

The hardness number is reported as a number followed by the initials HR and another symbol representing the indenter and forces used. HRC, for instance, indicates a Rockwell hardness number of 68 on a Rockwell C scale.

Hardness tests are simple, inexpensive and nondestructive, and do not require special specimens.

Ultrasonic Testing: Nondestructive method for detecting flaws in materials. Useful for the evaluation of welds.

A sound wave is directed toward the weld joint and reflected back from a discontinuity. A sensor captures the energy of the reflected wave and the results are displayed on an oscilloscope.

Ultrasonic Testing is highly sensitive in detecting planar defects, such as incomplete weld fusion, delamination or cracks.

BDCS Notes - Steel Alloys

Steel Alloys

Alloys: used to alter the characteristics of steel. Around 250,000 different alloys of steel produced. As many as 200 used for civil engineering applications.

Alloys are added to improve:

1. Hardenability
2. Corrosion Resistance
3. Machineability
4. Ductility
5. Strength

By altering the carbon and alloy content and by using different heat treatments, steel can have many different characteristics.

Sectional Shapes:

Wide Flange: (W, HP and M shapes) (W used for beams/columns) (HP for bearing piles)

I-Beam: (S shape) (S used for beams/girders)

Channel: (C and MC shapes)

Equal Legs Angle: (L shape)

Unequal-legs Angle: (L shape)

Tee: (T shape)

Sheet Piling

Rail

W, M, S, HP, C, and MC shapes are designated by a letter, followed by two numbers separated by an “X.”

And example is W 44 X 335. The W indicates the W-shape, the 44 indicates nominal depth, and the 335 indicates the weight per lineal unit length.

Materials identified as “preferred” are available in the market place. Those identified as “other applicable materials” may or may not be readily available.

Specialty Steels in Structural Application

HPS: High Performance Steels (come as either 50W or 70W) (weathering steels)

(70W has higher tensile properties, and is used for bridge construction.)

Stainless Steels

Whereas structural steels have 0.3 to 0.4% chromium, stainless steel has in excess of 10%

Types of AISI Stainless Steels

304: The most readily available stainless steel, containing 18% chromium and 8% nickel.

316: Similar to 304, but with the addition of 3-4% molybdenum for greater corrosion resistance.

409: A straight chrome alloy, 11-12% chromium. Used for interiors.

410-3: A dual phase alloy with micro alloy element control that permits welding in up to 1.25 inches.

2205: A duplex structure with about equal parts of austenite and ferrite. Excellent corrosion resistance and about twice the yield strength of conventional grades.

Fastening Products

Conventional Bolts (snug-tightened, pre-tensioned, or slip critical)

Twist-off-type Tension Control Bolt Assemblies

Nuts

Washers

Compressible-Washer-Type Direction Tension Indicators

Anchor Rods

Threaded Rods

Forged Steel Structural Hardware

BDCS Notes - Steel

Steel

Classifications of steels:

1. Structural Steel: for use in plates, bars, pipes, structural shapes, etc.
2. Fastening Products: used for structural connections, including bolts, nuts and washers.
3. Reinforcing Steel: for use in concrete reinforcement.
4. Miscellaneous Products: forms and pans.

Steel Production

1. Reducing iron ore to pig iron
2. Refining pig iron to steel
3. Forming the steel into products

Materials used to produce pig iron – coal, limestone, and iron ore.

Coal – Supplies carbon used to reduce iron oxides in the ore.

Limestone – Helps remove impurities.

Iron – Magnetically extracted from the waste, and extra

cted material is formed into pellets and fired.

Blast Furnace – used to reduce the ore to pig iron. Ore is heated in presence of carbon.

Three types of furnaces used for refining pig iron to steel:

1. Open Hearth
2. Basic Oxygen
3. Electric Arc

Open Hearth / Basic Oxygen – Remove excess carbon by reacting the carbon with oxygen to form gases. Lances circulate oxygen through the molten material.

Electric Furnaces – Use an electric arc between carbon electrodes to melt and refine the steel. Require a tremendous amount of energy.

During the steel production process, oxygen becomes dissolved in the liquid metal. As steel solidifies, oxygen combines with carbon to form carbon monoxide bubbles that are trapped in the steel and act as points for failure. Deoxidizing agents, such as aluminum, ferrosilicon and manganese, eliminate the formation of the carbon monoxide bubbles.

Killed Steels –

- Carbon content greater than .25%

- All forging grades of steels

- Structural steels with carbon content between 0.15 and 0.25 %

- Some special steel in the lower carbon ranges

Molten steel with desired chemical composition is cast into ingots (large blocks of steel).

Iron-Carbon Phase Diagram

In refining steel from iron ore, quantity of carbon used must be carefully controlled in order for steel to have desired properties.

Figure below represents the iron-iron carbide phase diagram:

Abscissa extends to 6.67% out of convention. The left side of the figure demonstrates that pure iron goes through two transformations as temperature increases. Below 912 C there’s a BCC crystalline structure called ferrite. At 912 C there’s a polymorphic change to a FCC structure called austenite. At 1394 C another polymorphic change occurs, returning the iron to a BCC structure. At 1539 C the iron melts into a liquid. The high and low temperature ferrites are identified as (delta) and (alpha) ferrite, respectively.

At 0.77% carbon and 727 C a eutectoid reaction occurs. A eutectoid reaction is a solid phase change that occurs when the temperature or carbon content changes. Below 727 C ferrite and iron carbide form thin plates, a lamellae structure. This eutectoid material is called pearlite.

At carbon contents less than 0.77% carbon, hypoeutectoid alloys are formed.

Heat Treatment of Steel

Annealing: refines the grain, softens the steel, removes internal stresses and gases, increases ductility and toughness, and changes electrical and magnetic properties.

Full Annealing: (1) Heating the steel to about 50 C above the austenitic temperature line and holding the temperature until all the steel transforms into either austenite or austenite-cementite. (2) Cooling the steel at a rate of about 20 C per hour in a furnace to a temperature of about 680 C.

Process Annealing: Used to treat work-hardened parts made with low carbon steel (less than 0.25 percent carbon). The material is heated to 700 C and held long enough to allow recrystallization of the ferrite phase.

Stress Relief Annealing: Used to reduce residual stresses in cast, welded, and cold-worked parts and cold-formed parts. The material is heated to 600 to 650 C, held at temperature for about one hour, and then slowly cooled in still air.

Spheroidization: An annealing process used to improve the ability of high carbon steel to be machined or cold worked. Improves abrasion resistance.

Normalizing: similar to annealing, with difference in the temperature and the rate of cooling. Steel normalized by heating to about 60 C above the austenite line and then cooling under natural convection. The material is then air-cooled. Provides a uniform, fine-grained microstructure.

Hardening: Steel is hardened by heating it to a temperature above the transformation range and holding it until austenite is formed. The steel is then quenched by plunging it into, or spraying it with, water, brine, or oil.

Tempering: The predominance of martensite in quench-hardened steel results in an undesirable brittleness. Tempering improves ductility and toughness.

BDCS Notes - Water-Cementitious Materials Ratio

In 1918, Abrams discovered the ratio of weight of water to the weight of cement (the water-cement ratio) influences desirable qualities of concrete. This concept is referred to as Abram’s Law.

Supplementary cementitious material, such as fly ash, slag, silica fume, and natural pozzolans, have been used as admixtures in recent years to alter properties of portland cement concrete.

Hydration requires .22 kg to .25 kg of water per 1 kg of cement.

Excess water causes the development of capillary voids in concrete, which increases porosity and permeability, thus reducing strength.

Low W-C ratios increases resistance to weathering, provides a good bond between successive layers of concrete and steel reinforcement, and limits volume change due to temperature or moisture.

Air-Entrained Concrete – includes an air-entraining agent, an admixture, which is used to increase the concrete’s resistance to freezing and thawing.

BDCS Notes - Hydration

Hydration: chemical reaction between cement particles and water.

Hydration process occurs through two mechanisms:

1. Through-solution

2. Topochemical

Through-solution process involves:

1. Dissolution of anhydrous compounds into constituents

2. Formation of hydrates in solution

3. Precipitation of hydrates from the supersaturated solution

The through-solution process dominates the early part of hydration.

Topochemical hydration is a solid-state chemical reaction occurring at the surface of the cement particles.

The aluminates hydrate much faster than the silicates. Gypsum is used to slow down the rate of aluminate hydration. The balance of aluminate to sulfate determines the rate of setting.

The solidifying of paste begins 2 to 4 hours after the water to the cement. If there is an excess of both aluminate and sulfate ions, the workability stage may only last for 10 minutes and setting may occur in 1 to 2 hours. If the availability of aluminate ions is high, and sulfates low, either a quick set (10 to 45 minutes) or flash set (less than 10 minutes) can occur.

Chemical Reaction of Hydration:

Calcium silicates combine with water to form calcium-silicate-hydrate, C-S-H. The crystals begin to form a few hours after the water and cement are mixed. The calcium-to silicate ratio varies between 1.5 and 2.0.

Structure Development in Cement Paste

The sequential development of the structure in a cement paste is summarized by:

1. Initial C-S-H phase

2. Forming of gels

3. Initial set-development of weak skeleton

4. Final set-development of rigid skeleton

5. Hardening

In less than 10 minutes, the water becomes highly alkaline. As the cement particles hydrate, the volume of the cement particle reduces, increasing the space between the particles. During the early stages of hydration, weak bonds form. Further hydration stiffens the mix and begins locking the structure of the material in place. Final set occurs when the C-S-H phase has developed a rigid structure, all components of the paste lock into place and the spacing between grains increases as the grains are consumed by hydration. The cement paste continues hardening and gains strength as hydration continues. Hydration occurs as long as unhydrated cement particles and free water exists. The rate of hardening decreases with time.

Evaluation of Hydration Progress

Measure the following properties:

1. The heat of hydration

2. The amount of calcium hydroxide in the paste developed due to hydration

3. The specific gravity of the paste

4. The amount of chemically combined water

5. The amount of unhydrated cement paste using X-ray quantitative analysis

6. The strength of the hydrated paste, an indirect measurement

Voids in Hydrated Cement

Due to random growth in various crystals, voids are left in the paste structure (as cement hydrates). Concrete strength, durability, and volume stability are greatly influenced by voids.

Two types of voids are formed during hydration:

1. The interlayer hydration space

2. Capillary voids

Interlayer Hydration Space: Occurs between layers in the C-S-H. The space thickness is between 0.5 nm and 2.5 nm. This is too small to affect strength. It can affect the porosity of the paste. Water in the interparticle space is strongly held by hydrogen bonds, but can be removed when humidity is less than 11%, resulting in shrinkage.

Capillary Voids: The result of the hydrated cement paste having a lower bulk specific gravity than the cement particles. The amount and size of capillary voids depends on the initial separation of the cement particles, which is controlled by the ratio of water to cement paste. Voids should range from 10 nm to 50 nm. Anything greater than this will decrease strength and increase permeability.

Properties of Hydrated Cement

Evaluated with either cement paste (water and cement) or mortar (paste and sand).

Setting: Refers to stiffening of the cement paste or the change from a plastic state to a solid state. With setting comes strength, although this is not the same as hardening, which refers to the strength gain in a set cement paste.

Two levels describe setting: initial set and final set.

Vicat Test: Requires that a sample of cement paste be prepared, using the amount of water required for normal consistency according to a specified procedure. A needle is allowed to penetrate the paste for 30 seconds and the amount of pentration is measured. This process is repeated every 15 minutes until a penetration of 25 mm or less is obtained.

Gillmore Test: Requires a sample of cement paste of normal consistency be prepared. A Pat with a flat top is molded and the initial Gillmore needle is applied lightly to its surface. The process is repeated until the pat bears the force of the needle without appreciable indentation, and the elapsed time is recorded as the initial set time.

If the cement is exposed to humidity during storage, a false set might occur, in which the cement stiffens within a few minutes of being mixed. To solve this problem, the cement paste can be vigorously remixed without adding water in order to restore plasticity of the paste without losing strength.

Soundness: Refers to the ability of cement paste to retain its volume after setting.

Compressive Strength: The compressive strength of mortar is measured by preparing 50 mm cubes and subjecting them to compression. Mortar is prepared with cement, water, and standard sand. ASTM specifies minimum compressive strength standards.

BDCS Notes - Water and Admixtures

Water and Admixtures

Potable and non-potable waters are both suitable for mixing concrete. However, impurities in the mixing water can affect concrete set time, strength, and long-term durability. Chloride ions, for instance, can accelerate corrosion of reinforcing steel.

Acceptable Criteria

(Specified in ASTM C94)

After 7 days, the compressive strength of mortar cubes made with the questionable water should not be less than 90% of the strength of cubes made with potable or distilled water.

The set time of cement paste made with the questionable water should (by the Vicat Test) not be 1 hour less than 1-1/2 hours more than the set time of paste made with potable or distilled water.

Efflorescence: White stains forming on the concrete surface due to the formation of calcium carbonate. Caused by impurities in mixing water.

Disposal and Reuse of Concrete Wash Water

Wastewater is generally generated from truck wash systems, washing of central mixing plant, storm water runoff from the ready-mix plant yard, etc. Wastewater is considered a hazardous substance by the Water Quality Act (it contains caustic soda and potash). Also, the high pH makes concrete wash water hazardous under the EPA’s definition of corrosivity.

Admixtures for Concrete

Admixtures: ingredients other than portland cement, water and aggregates that may be added to concrete to impart a specific quality to either the plastic mix or the hardened concrete.

Classified by the following chemical and functional physical characteristics:

1. Air Entrainers

2. Water Reducers

3. Retarders

4. Hydration Controller Admixtures

5. Accelerators

6. Supplementary Cementitious Admixtures

7. Specialty Admixtures

Four Major Reasons for Using Admixtures:

1. Reduce the cost of concrete construction

2. Achieve certain properties in concrete more effectively than by other means

3. Ensure quality of concrete during the stages of mixing, transporting, placing and curing in adverse weather conditions

4. Overcome certain emergencies during concrete operations

Air Entrainers: Produce tiny air bubbles in the hardened concrete to provide space for water to expand upon freezing.

As moisture within the concrete pore structure freezes, 3 mechanisms contribute to the development of internal stresses:

1. Critical saturation – upon freezing, water expands in volume by 9%.

2. Hydraulic pressure – freezing water draws unfrozen water to it.

3. Osmotic pressure – Water moves from the gel to capillaries to satisfy thermodynamic equilibrium and to equalize alkali concentrations. Voids permit water to flow from the interlayer hydration space and capillaries into the air voids.

In addition to improving durability, air entrainment provides other important benefits to both freshly mixed and hardened concrete, including resistance to freeze-thaw cycles, deicers and salts, sulfates, and alkali-silica reactivity.

Air-entraining admixtures are available from several manufacturers and can be composed of a variety of materials, such as:

1. Salts of wood resins

2. Synthetic detergents

3. Salts of sulfonated lignin

4. Salts of petroleum acids

5. Salts of proteinaceous material

6. Fatty and resinous acids

7. Alkylbenzene sulfonates

8. Salts of sulfonated hydrocarbons

Air-Entrainers are usually liquid and should meet the specifications of ASTM C260. The agents enhance air entrainment by lowering the surface tension of the mixing water.

Water Reducers: increase the mobility of the cement particles in the plastic mix, allowing workability to be achieved at lower water contents.

Workability of fresh or plastic concrete requires more water than is needed for hydration. This is why water reducers are typically needed.

Water Reducers Mechanism: Cement grains develop small electric charges on their surface as a result of the cement-grinding process. Dissimilar charges attract, causing the grains to cluster or “flocculate” which in turn limits the workability. The chemicals in the water-reducing admixtures reduce the static attraction among cement particles.

Water reducing admixtures are used indirectly to gain strength, increase the slump of concrete, which in turn increases workability, while reducing the W-C materials ratio.

Superplasticizers: High-range water reducers that either greatly increase the flow of the fresh concrete or reduce the amount of water required for a given consistency. Used for the following cases:

1. Where a low W-C materials ratio is beneficial

2. When placing thin sections

3. When placing concrete around tightly spaced reinforcing steel

4. When placing cement underwater

5. When placing concrete by pumping

6. Where consolidating the concrete is difficult

Retarders: Used where conditions require that time between mixing and placing or finishing the concrete by increased. Also used for the following reasons:

1. Offsetting the effect of hot weather

2. Allowing the unusual placement of long haul distances

3. Providing time for special finishes (exposed aggregate)

Retarders can reduce the strength of concrete at early ages (one to three days). They can also entrain air and improve workability.

Hydration-Control Admixtures: Have the ability to stop and reactivate the hydration process of concrete. Consist of two parts: a stabilizer and an activator. Very useful admixtures in extending the use of ready-mixed concrete if the work at the job-site has been stalled for various reasons.

Accelerators: Used to develop early strength of concrete at a faster rate than that developed in normal concrete. Also used to:

1. Reduce the amount of time before finishing operations begin

2. Reduce curing time

3. Increase rate of strength gain

4. Plug leaks under hydraulic pressure efficiently

The first three reasons are particularly applicable for concrete work placed during cold temperatures.

Calcium chloride, CaCl₂, is the most widely used accelerator. It is used under the following conditions:

1. Concrete is prestressed

2. Concrete contains embedded aluminum such as conduits, particularly if the aluminum is in contact with steel

3. Concrete is subjected to alkali-aggregate reaction

4. Concrete is in contact with water or soils containing sulfates

5. Concrete is placed during hot weather

6. Mass applications of concrete

Supplementary Cementitious Admixtures (byproducts of other industries used for admixtures):

1. Fly Ash

2. Ground Granulated Blast Furnace Slag

3. Silica Fume

4. Natural Pozzolans

Fly Ash: Most commonly used pozzolans in civil engineering. By-product of the coal industry. Combusting pulverized coal in an electric power plant burns off the carbon and most volatile materials. Carbon contents of common coals range from 70% to 100%, while the non-carbon percentages are impurities (clay, feldspar, quartz, shale), which fuse as they pass through the combustion chamber. This fused material is fly ash.

Fly Ash Composition:

1. Silica (SiO₂)

2. Alumina (AlO₂)

3. Iron Oxide (Fe₂O₃)

4. Lime (CaO)

Fly Ash Classification:

1. Class N – Raw or calcined natural pozzolans.

2. Class F – Fly ash with pozzolans properties (less than 5% CaO, typically).

3. Class C – Fly ash with pozzolans and cementitious properties (15%-30% CaO).

The spherical shape of fly ash increases its workability with fresh concrete. Fly ash also extends the hydration process, allowing greater strength development and reduced porosity. Studies typically show 20% fly ash by weight is recommended for a smaller pore size distribution. A lower heat of hydration reduces early strength of the concrete, but gains strength through the extended reaction – more than what would be attained by plain portland cement.

Ground Granulated Blast Furnace Slag: GGBF slag is made from iron blast furnace slag. It is nonmetallic hydraulic cement consisting basically of silicates and aluminosilicates of calcium, which develop into a molten condition with iron in a blast furnace. It is then chilled by quenching it in water, which forms a glassy sandlike granulated material. This material is then ground to less than 45 microns, which produces the slag.

Used as a cementitious material in concrete since the beginning of the 1900s. Commonly constitutes between 30% and 45% of the cementing material in the mix.

Silica Fume: A byproduct of the production of silicon metal or ferrosilicon alloys. Used beneficially as a mineral admixture in concrete. A very reactive pozzolan. Concrete containing silica fume has high strength and is very durable. It also reduces concrete corrosion induced by deicing or marine salts.

Silica Fume Composition: Silicon metal and alloys are produced in electric furnaces. The raw materials are quartz, coal and woodchips. The smoke that results from furnace operation is collected and sold as silica fume. The composition contains primarily noncrystalline silicon dioxide.

Natural Pozzolans: A pozzolan is a siliceous and aluminous material that, although it possesses little or no cementitious value, will (in the presence of moisture) react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.

Some natural pozzolans include fine volcanic ash combined with burnt lime, which was used 2000 years ago for building construction. Up to 15% of the weight of portland cement is hydrated lime. Adding a pozzolan to portland cement generates an opportunity to convert this free lime to a cementitious material.

Specialty Admixtures: Other admixtures to be aware of include:

1. Workability agents

2. Corrosion inhibitors

3. Damp-proofing agents

4. Permeability-reducing agents

5. Fungicidal, germicidal and insecticidal admixtures

6. Pumping aids

7. Bonding agents

8. Grouting agents

9. Gas-forming agents

10. Coloring agents

11. Shrinkage reducing