What Is The Wiss And Parmelee Rating Factor?

Wiss and Parmelee also conducted research to refine the findings of Lenzen’s research. In particular, they attempted to quantify, in a more scientifically rigorous manner, human perception to transient floor motion.

They subjected 40 persons, standing on a vibrating platform, to transient vibration episodes with different combinations of frequency (2.5 to 25 Hz), peak displacements (0.0001 to 0.10 in), and damping (0.1 to 0.16, expressed as a ratio of critical).

After each episode, the subject was asked to rate the vibration on a scale of 1 to 5 with the following definitions: (1) imperceptible, (2) barely perceptible, (3) distinctly perceptible, (4) strongly perceptible, and (5) severe. Using regression analysis, an equation was perception ratings.

This equation is presented below. Wiss and Parmelee rating factor:

R= 5.08 (FA/ D^0.217)^0.265

R= response rating; 1= imperceptible; 2= barely perceptible; 3= distinctly perceptible; 4= strongly perceptible; 5= severe.
F= frequency of the vibration episode, Hz
A= maximum displacement amplitude, in
D= damping ratio, expressed as a ratio of critical

A graph of this subjective rating system is shown in Fig. 5.115. It should be noted that the lines represent a mean for that particular rating. The authors suggest that the boundaries for each rating lie halfway between the mean lines.

The boundaries defining R= 1 and R= 5 are not identified by the authors. These ratings are unbounded; therefore, a mean line cannot be computed.


Effects Of Repeated Loadings

In the preceding sections we have considered the behavior of a test specimen subjected to an axial loading. We recall that, if the maximum stress in the specimen does not exceed the elastic limit of the material, the specimen returns to its initial condition when the load is removed.

You might conclude that a given loading may be repeated many times, provided that the stresses remain in the elastic range. Such a conclusion is correct for loadings repeated a few dozen or even a few hundred times.

However, as you will see, it is not correct when loadings are repeated thousands or millions of times. In such cases, rupture will occur at a stress much lower than the static breaking strength; this phenomenon is known as fatigue. A fatigue failure is of a brittle nature, even for materials that are normally ductile.

Fatigue must be considered in the design of all structural and machine components that are subjected to repeated or to fluctuating loads. The number of loading cycles that may be expected during the useful life of a component varies greatly.

For example, a beam supporting an industrial crane may be loaded as many as two million times in 25 years (about 300 loadings per working day), an automobile crankshaft will be loaded about half a billion times if the automobile is driven 200,000 miles, and an individual turbine blade may be loaded several hundred billion times during its lifetime.

Some loadings are of a fluctuating nature. For example, the passage of traffic over a bridge will cause stress levels that will fluctuate about the stress level due to the weight of the bridge. A more severe condition occurs when a complete reversal of the load occurs during the loading cycle.

The stresses in the axle of a railroad car, for example, are completely reversed after each half-revolution of the wheel. The number of loading cycles required to cause the failure of a specimen through repeated successive loadings and reverse loadings may be determined experimentally for any given maximum stress level.

If a series of tests is conducted, using different maximum stress levels, the resulting data may be plotted as a s-n curve. For each test, the maximum stress s is plotted as an ordinate and the number of cycles n as an abscissa; because of the large number of cycles required for rupture, the cycles n are plotted on a logarithmic scale.

A typical s-n curve for steel is shown in Fig. 2.16. We note that, if the applied maximum stress is high, relatively few cycles are required to cause rupture. As the magnitude of the maximum stress is reduced, the number of cycles required to cause rupture increases, until a stress, known as the endurance limit, is reached.

The endurance limit is the stress for which failure does not occur, even for an indefinitely large number of loading cycles. For a low-carbon steel, such as structural steel, the endurance limdecrease as the number of loading cycles is increased. For such metals, one defines the fatigue limit as the stress corresponding to failure after a specified number of loading cycles, such as 500 million.

Examination of test specimens, of shafts, of springs, and of other components that have failed in fatigue shows that the failure was initiated at a microscopic crack or at some similar imperfection. At each loading, the crack was very slightly enlarged.

During successive loading cycles, the crack propagated through the material until the amount of undamaged material was insufficient to carry the maximum load, and an abrupt, brittle failure occurred.

Because fatigue failure may be initiated at any crack or imperfection, the surface condition of a specimen has an important effect on the value of the endurance limit obtained in testing. The endurance limit for machined and polished specimens is higher than for rolled or forged components, or for components that are corroded.

In applications in or near seawater, or in other applications where corrosion is expected, a reduction of up to 50% in the endurance limit can be expected.is about one-half of the ultimate strength of the steel. For nonferrous metals, such as aluminum and copper, a typical s-n curve (Fig. 2.16) shows that the stress at failure continues to decrease as the number of loading cycles is increased. For such metals, one defines the fatigue limit as the stress corresponding to failure after a specified number of loading cycles, such as 500 million.


What Is Axial Loading? What Is Stress?

The deformation caused in a body by external forces or other actions generally varies from one point to another, i.e., it is not homogeneous. In fact, a homogeneous deformation is rare. It occurs, for example, in a body with isostatic supports under a uniform temperature variation or in a slender member under constant axial force.

Rod BC of the example considered in the preceding section is a two-force member and, therefore, the forces FBC and F'BC acting on its ends B and C (Fig. 1.5) are directed along the axis of the rod. We say that the rod is under axial loading.

An actual example of structural members under axial loading is provided by the members of the bridge truss shown in Photo 1.1.

Returning to rod BC of Fig. 1.5, we recall that the section we passed through the rod to determine the internal force in the rod and the corresponding stress was perpendicular to the axis of the rod; the internal force was therefore normal to the plane of the section (Fig. 1.7) and the corresponding stress is described as a normal stress.

Thus, formula (1.5) gives us the normal stress in a member under axial loading:

σ =P/A 

We should also note that, in formula (1.5), s is obtained by dividing the magnitude P of the resultant of the internal forces distributed over the cross section by the area A of the cross section; it represents, therefore, the average value of the stress over the cross section, rather than the stress at a specific point of the cross section.

To define the stress at a given point Q of the cross section, we should consider a small area DA. Dividing the magnitude of DF by DA, we obtain the average value of the stress over DA. Letting DA approach zero, we obtain the stress at point Q:

σ = lim dF/dA      as dA approaches infinity (1.6)

In general, the value obtained for the stress s at a given point Q of the section is different from the value of the average stress given by formula (1.5), and s is found to vary across the section. In a slender rod subjected to equal and opposite concentrated loads P and P' , this variation is small in a section away from the points of application of the concentrated loads, but it is quite noticeable in the neighborhood of these 

It follows from Eq. (1.6) that the magnitude of the resultant of the distributed internal forces is

∫dF = ∫σ dA     lower limit = A

But the conditions of equilibrium of each of the portions of rod require that this magnitude be equal to the magnitude P of the concentrated loads. We have, therefore,
P = ∫dF = ∫σ dA    lower limit = A

which means that the volume under each of the stress surfaces must be equal to the magnitude P of the loads. This, however, is the only information that we can derive from our knowledge of statics, regarding the distribution of normal stresses in the various sections of the rod. 

The actual distribution of stresses in any given section is statically indeterminate. To learn more about this distribution, it is necessary to consider the deformations resulting from the particular mode of application of the loads at the ends of the rod.

In practice, it will be assumed that the distribution of normal stresses in an axially loaded member is uniform, except in the immediate vicinity of the points of application of the loads. The value s of the stress is then equal to save and can be obtained from formula (1.5). 

However, we should realize that, when we assume a uniform distribution of stresses in the section, i.e., when we assume that the internal forces are uniformly distributed across the section, it follows from elementary statics† that the resultant P of the internal forces must be applied at the centroid C of the section. 

This means that a uniform distribution of stress is possible only if the line of action of the concentrated loads P and P' passes through the centroid of the section considered. This type of loading is called centric loading and will be assumed to take place in all straight two-force members found in trusses and pin-connected structures, such as the one considered in Fig. 1.1. 

However, if a two-force member is loaded axially, but eccentrically we find from the conditions of equilibrium of the portion of member that the internal forces in a given section must be equivalent to a force P applied at the centroid of the section and a couple M of moment M = Pd. The distribution of forces—and, thus, the corresponding distribution of stresses—cannot be uniform. Nor can the distribution of stresses be symmetric.


How To Drill Tiles Without Cracking It?

This article is a step by step process in drilling tiles, without cracking it. Many installations in kitchens involve drilling through a tiled surface.

It is essential to use the correct technique for drilling through tiles so they do not crack. The dust created from drilling ceramic tiles can discolor grout and sealant so you may want to vacuum dust from holes as you drill them.

Tools and materials
Felt-tip pen
masking tape
drill and bits,
vacuum cleaner
wall plug


1. Mark the point for the hole using a felt-tip pen. Apply some masking tape over the mark—it should still be visible.

2. Fit a tile drill bit. Remember to switch off any hammer action.

Selecting a tile bit: Tile bits differ in shape based on material. The spear-shaped tip penetrates a tile, then enlarges the hole to the diameter of the tip’s base.

Caution: Take care when changing a bit after operating a drill: the bit may be hot. Wear gloves to avoid a burn.

3.  Position a vacuum cleaner below the mark and switch it on. Start up the drill on a low speed, and slowly increase the speed.

4. Once through the tile, change the bit for a masonry bit or wood bit, depending on the surface below. Drill to the required depth.

5. Remove the masking tape from the tile, then plug the hole with the appropriate wall plug, and insert the fastener as required.

Selecting wall plugs

Unless you are using masonry screws, a wall plug is required to secure a screw that is
inserted into masonry. The plugs shown here are masonry plugs, and the different colors relate to their width, or gauge.

Wall plugs are also needed to make strong connections in hollow walls such as stud walls; these are of a different design from those used in masonry.


Free E-Book Download Link Of The Book Schaums Outline Of Engineering Mechanics Dynamics

Modified to conform to the current curriculum, Schaum's Outline of Engineering Mechanics: Dynamics complements these courses in scope and sequence to help you understand its basic concepts.

The book offers extra practice on topics such as rectilinear motion, curvilinear motion, rectangular components, tangential and normal components, and radial and transverse components. You’ll also get coverage on acceleration, D'Alembert's Principle, plane of a rigid body, and rotation.

Appropriate for the following courses: Engineering Mechanics; Introduction to Mechanics; Dynamics; Fundamentals of Engineering.


765 solved problems
Additional material on instantaneous axis of rotation and Coriolis' Acceleration
Support for all the major textbooks for dynamics courses
Topics include: Kinematics of a Particle, Kinetics of a Particle, Kinematics of a Rigid Body, Kinetics of a Rigid Body, Work and Energy, Impulse and Momentum, Mechanical Vibrations

About the Author
E. W. Nelson taught Mechanical Engineering at Lafayette College and later joined the engineering organization of the Western Electric Company (now Lucent Technologies).

Charles L. Best is Emeritus Professor of Engineering at Lafayette College. W. G. McLean (Easton, PA) is Emeritus Director of Engineering at Lafayette College.

Merle Potter is professor emeritus of Mechanical Engineering at Michigan State University.



Free E-Book Download Link Of Steel-Reinforced Concrete Structures: Assessment and Repair of Corrosion

A Practical Guide to Maintenance
Carrying a billion-dollar price tag, corrosion of reinforced concrete is the enemy of every country’s investment in real estate. The widespread and long-term use of reinforced concrete makes its correct and proper examination, maintenance, and repair paramount.

Steel-Reinforced Concrete Structures: Assessment and Repair of Corrosion explains the corrosion of reinforced concrete from a practical point of view, highlighting protective design and repair procedures.

The book begins with a discussion of the corrosion phenomena, the effect of concrete properties on corrosion, and the precautions available in the construction stage to mitigate corrosion. It covers the theoretical and practical methods in evaluating the concrete structures and new, practical methods to protect steel reinforcement.

The book also includes methods established in the last decade that provide new ways of protecting steel-reinforced bars and the traditional and advanced repairing methods. The author explains the importance of implementing an integrity management system to provide a comprehensive maintenance strategy and concludes with coverage of the traditional, time-tested, and advanced repair techniques.

A special feature is a chapter focusing on the advance maintenance plan philosophy and risk-based maintenance for reinforced concrete structures.

The author examines economic analysis procedures and the probability of structural failures to define structure risk assessment. He covers precautions and recommendations for protecting the reinforced concrete structures from corrosion based on codes and specifications. He uses case histories from all over the world to demonstrate the widespread application and range of advanced repair techniques and presents a practical guide to the maintenance of concrete structures. The book provides procedures for corrosion diagnosis and determining the appropriate methods for repair, as well as economic models for on-site decision making.

The Author
Mohamed A. El-Reedy's background in structural engineering. His main area of researches is reliability of concrete and steel structure. He has provided consulting to different engineering companies and oil and gas industries in Egypt and to international companies as the International Egyptian Oil Company (IEOC) and British Petroleum (BP). Moreover, he provides different concrete and steel structure design package for residential buildings, warehouses and telecommunication towers and electrical projects with WorleyParsons Egypt. He has participated in Liquified Natural Gas (LNG) and Natural Gas Liquid (NGL) projects with international engineering firms. Curently, Dr. El-Reedy is responsible for reliability, Inspection and maintenance strategy for onshore concrete structures and offshore steel structure platforms. He has performed these tasks for hundred structures in Gulf of Suez in the red sea.

Dr. El-Reedy has consulted with and trained executives at many organizations, including the Arabian American Oil Compnay (ARAMCO), bp, Apachi, Abu Dhabi Marine Operating Company (ADMA), the Abu Dhabi National Oil Company and King Saudi's Interior ministry, Qatar Telecom, EGPC , (SAPIC) , the Kuwait Petroleum Corporation, and Qatar petrochemical Company (QAPCO). He has taught technical courses about repair and maintenance for reinforced concrete structures and the advanced materials in concrete industry worldwide , especially in the Middle East, malaysia and singapore.

Dr. El-Reedy has written numerous publications and presented many papers at local and international conferences sponsored by the ASCE, CSCE, ACI, API and in technical committe for OMAE conference sponsor by ASME.He has published many research papers in international technical journals and has authored four books about total quality management, quality management and quality assurance, economic management for engineering projects, and repair and protection of reinforced concrete structures. He received his bachelor's degree from Cairo University in 1990, his master's degree in 1995, and his Ph.D from Cairo University in 2000.



What Are Some Causes Of Unsatisfactory Concrete Test Results

The two most common kinds of failure are:
• failure to get the required strength, the concrete being otherwise apparently good;
• structural failures, such as honeycombing, sandy patches, and cracking.

Failure to get the right strength in cubes taken from a concrete pour can sometimes have a very simple cause. Among such causes are the following:
• the cube was not compacted properly;
• it was left out all night in hard frost or dried out in hot sun;
• there was a mix-up of cubes and a 7-day old cube was tested on the assumption it was 28 days old;
• the cube was taken from the wrong mix.

Such simple errors are not unusual and must be guarded against because they cause much perplexity and waste of time trying to discover the cause of a bad test result.

The concrete must be fully compacted in the mould, which is kept under damp sacking until the next day when the mould can be removed and the cube marked for identity.

It is then best stored in water at ‘room temperature’ for curing until sent to the test laboratory. If poor cube test results appear on consecutive batches, an error in the cement content of batches may be suspected, or else the quality of the cement itself.

Honeycombing is most usually caused by inadequate vibration or rodding of the concrete adjacent to the face of formwork.

Sometimes too harsh a mix is used so there are insufficient fines to fill the trapped interstices between coarse aggregate and formwork, or the larger stones cause local arching.

Sand runs – patches of sandy concrete on a wall surface which can be scraped away with a knife – can be due to over-vibration near a leaking joint in the formwork which allows cement and water to pass out of the mix.

One simple, and not infrequent, cause of poor concrete is use of the wrong mix due to a ‘failure of communication’ with the batching plant operator or ready-mix supplier. An experienced concreting foreman should be able to detect a ‘wrong mix’ the moment it is discharged.


ICE Conditions of Contract for Works of Civil Engineering Construction

These are generally known as the ICE conditions and have for many years been the most widely used conditions for UK civil engineering works. They have a long history of satisfactory usage and have been tested in the courts and in arbitration so that the parties to a contract can be confident as to the meaning and interpretation to be placed on these conditions.

The latest edition is the 7th, published in 1999 together with guidance notes, reprinted with amendments in 2003. This edition is known as the Measurement Version to distinguish it from other ICE types of contract based on this established standard.

The principal provisions of the Measurement Version are as follows:
• The contractor constructs the works according to the designs and details given in drawings and specifications provided by the employer.

• The contractor does not design any major permanent works, but may be required to design special items (such as bearing piles whose choice may depend on the equipment he owns) and building services systems, etc.

• An independent engineer, designated ‘the Engineer’ is appointed by the employer to supervise construction, ensure compliance with the contract, authorize variations, and decide payments due; but his decisions can be taken by the employer or contractor to conciliation procedures, adjudication and/or arbitration.

• The contractor can claim extra payment and/or extension of time for overcoming unforeseen physical conditions, other than weather, which ‘could not…reasonably have been foreseen by an experienced contractor’ (Clause 12) and for other delays for which the employer is responsible.

• Payment is normally made by re-measurement of work done at rates tendered against items listed in bills of quantities, which can also include lump sums.

A particular advantage of the ICE conditions is that interpretation of the provisions of the contract lies in the hands of an independent Engineer, who is not a party to the contract, but is required to ‘act impartially within the terms of the contract having regard to all the circumstances’ (Clause 2(7)).

This gives assurance to both employer and contractor that their interests and obligations under the contract will be fairly dealt with. Also the contractor is paid for overcoming difficulties he could not reasonably have foreseen.

Both these matters reduce the contractor’s risks, making it possible for him to bid his lowest economic price. This benefits the employer, since the initial price is low and he does not pay out to cover risks which may not occur.

The ICE conditions contain many other provisions that have stood the test of time. These include requirements for early notice of potential delays and problems such as adverse ground conditions and provisions for submission and assessment of claims and valuation of variations.

Properly drawn up and administered, a contract under these conditions appears fair to both parties, and the percentage of contracts ending in a dispute which goes to arbitration is very small.


What Are Welded Wire Fabric?

Welded-wire fabric is an orthogonal grid made with two kinds of cold-drawn wire: plain or deformed. The wires can be spaced in each direction of the grid as desired, but for buildings, usually at 12 in maximum.

Sizes of wires available in each type, with standard and former designations, are shown in Table 9.6.

Welded-wire fabric usually is designated WWF on drawings. Sizes of WWF are designated by spacing followed by wire sizes; for example, WWF 6 12, W12/ W8, which indicates plain wires, size W12, spaced at 6 in, and size W8, spaced at 12 in. WWF 6 12, D-12/D-8 indicated deformed wires of the same nominal size and spacing.

All WWF can be designed for Grade 60 material. Wire and welded-wire fabric are produced to conform with the following ASTM standard specifications:

ASTM A82, Plain Wire

ASTM A496, Deformed Wire

ASTM A185, Plain Wire, WWF

ASTM A497, Deformed Wire, WWF

Epoxy-coated wire and welded wire fabric are covered by the ASTM specification A884/A884M. Applications of epoxy-coated wire and WWF include use as corrosion-protection systems in reinforced concrete structures and reinforcement in reinforced-earth construction, such as mechanically-stabilized embankments.


What Are The Different Types Of Laminated Rubber?

Rubber is often combined with various textiles, fabrics, filaments, and metal wire to obtain strength, stability, abrasion resistance, and flexibility. Among the laminated materials are the following:

V Belts. These consist of a combination of fabric and rubber, frequently combined with reinforcing grommets of cotton, rayon, steel, or other high-strength material extending around the central portion.

Flat Rubber Belting. This laminate is a combination of several plies of cotton fabric or cord, all bonded together by a soft-rubber compound.

Conveyor Belts. These, in effect, are moving highways used for transporting such material as crushed rock, dirt, sand, gravel, slag, and similar materials. When the belt operates at a steep angle, it is equipped with buckets or similar devices and becomes an elevator belt.

A typical conveyor belt consists of cotton duct plies alternated with thin rubber plies; the assembly is wrapped in a rubber cover, and all elements are united into a single structure by vulcanization. A conveyor belt to withstand extreme conditions is made with some textile or metal cords instead of the woven fabric. Some conveyor belts are especially arranged to assume a trough form and made to stretch less than similar all-fabric belts.

Rubber-Lined Pipes, Tanks, and Similar Equipment. The lining materials include all the natural and synthetic rubbers in various degrees of hardness, depending on the application. Frequently, latex rubber is deposited directly from the latex solution onto the metal surface to be covered.

The deposited layer is subsequently vulcanized. Rubber linings can be bonded to ordinary steel, stainless steel, brass, aluminum, concrete, and wood. Adhesion to aluminum is inferior to adhesion to steel. Covering for brass must be compounded according to the composition of the metal.

Rubber Hose. Nearly all rubber hose is laminated and composed of layers of rubber combined with reinforcing materials like cotton duck, textile cords, and metal wire. Typical hose consists of an inner rubber lining, a number of intermediate layers consisting of braided cord or cotton duck impregnated with rubber, and outside that, several more layers of fabric, spirally wound cord, spirally wound metal, or in some cases, spirally wound flat steel ribbon.

Outside of all this is another layer of rubber to provide resistance to abrasion. Hose for transporting oil, water, wet concrete under pressure, and for dredging purposes is made of heavy duty laminated rubber.

Vibration Insulators. These usually consist of a layer of soft rubber bonded between two layers of metal. Another type of insulated consists of a rubber tube or cylinder vulcanized to two concentric metal tubes, the rubber being deflected in shear.

A variant of this consists of a cylinder of soft rubber vulcanized to a tubular or solid steel core and a steel outer shell, the entire combination being placed in torsion to act as a spring. Heavy-duty mounts of this type are employed on trucks, buses, and other applications calling for rugged construction.


What Are Elastomers Or Synthetic Rubbers?

Rubber for construction purposes is both natural and synthetic. Natural rubber, often called crude rubber in its unvulcanized form, is composed of large complex molecules of isoprene.

Synthetic rubbers, also known as elastomers, are generally rubber-like only in their high elasticity. The principal synthetic rubbers are the following:

GR-S is the one most nearly like crude rubber and is the product of styrene and butadiene copolymerization. It is the most widely used of the synthetic rubbers. It is not oil-resistant but is widely used for tires and similar applications.

Nitril is a copolymer of acrylonitrile and butadiene. Its excellent resistance to oils and solvents makes it useful for fuel and solvent hoses, hydraulic-equipment parts, and similar applications.

Butyl is made by the copolymerization of isobutylene with a small proportion of isoprene or butadiene. It has the lowest gas permeability of all the rubbers and consequently is widely used for making inner tubes for tires and other applications in which gases must be held with a minimum of diffusion. It is used for gaskets in buildings.

Neoprene is made by the polymerization of chloroprene. It has very good mechanical properties and is particularly resistant to sunlight, heat, aging, and oil; it is therefore used for making machine belts, gaskets, oil hose, insulation on wire cable, and other applications to be used for outdoor exposure, such as roofing, and gaskets for building and glazing.

Sulfide rubbers—the polysulfides of high molecular weight—have rubbery properties, and articles made from them, such as hose and tank linings and glazing compounds, exhibit good resistance to solvents, oils, ozone, low temperature, and outdoor exposure.

Silicone rubber, when made in rubbery consistency forms a material exhibiting exceptional inertness and temperature resistance. It is therefore used in making gaskets, electrical insulation, and similar products that maintain their properties at both high and low temperatures.

Additional elastomers include polyethylene, cyclized rubber, plasticized polyvinyl chloride, and polybutene. A great variety of materials enters into various rubber compounds and therefore provide a wide range of properties.

In addition, many elastomeric products are laminated structures of rubber-like compounds combined with materials like fabric and metals.


What Are The Different Types Of Plastic Resins?

Materials under this heading in general can be softened by heating and hardened by cooling.

Acrylics. In the form of large transparent sheets, these are used in aircraft enclosures and building construction. Although not so hard as glass, they have perfect clarity and transparency.

Among the most resistant of the transparent plastics to sunlight and outdoor weathering, they possess an optimum combination of flexibility and sufficient rigidity with resistance to shattering. A wide variety of transparent, translucent, and opaque colors can be produced.

The sheets are readily formed to complex shapes. They are used for such applications as transparent windows, outdoor and indoor signs, parts of lighting equipment, decorative and functional automotive parts, reflectors, household-appliance parts, and similar applications. They can be used as large sheets, molded from molding powders, or cast from the liquid monomer.

Acrylonitrile-Butadiene-Styrene (ABS). This three-way copolymer provides a family of tough, hard, chemically resistant resins with many grades and varieties, depending on variations in constituents. The greatest use is for pipes and fittings, especially drain-waste-vent (DWV). Other uses include buried sewer and water lines, mine pipe, well casings, conduit, and appliance housings.

Polyethylene. In its unmodified form, this is a flexible, waxy, translucent plastic. It maintain flexibility at very low temperatures, in contrast with many other thermoplastic materials.

Polyethylene may be provided as low-density, or standard, or as high-density or linear material. High-density polyethylene has greater strength and stiffness, withstands somewhat higher temperatures, and has a more sharply defined softening temperature range.

The heat-distortion point of the low-density polyethylenes is low; these plastics are not recommended for uses above 150 F. Unlike most plastics, polyethylene is partly crystalline. It is highly inert to solvents and corrosive chemicals of all kinds at ordinary temperatures.

Usually low moisture permeability and absorption are combined with excellent electrical properties. Its density is lower than that of any other commercially available nonporous plastic.

It is widely used as a primary insulating material on wire and cable and has been used as a replacement for the lead jacket in communication cables and other cables. It is widely used also in geogrids, geonets, and geomembrane and as corrosionproof lining for tanks and other chemical equipment.

Polypropylene. This polyolefin is similar in many ways to its counterpart, polyethylene, but is generally harder, stronger, and more temperature-resistant. It finds a great many uses, among them piping, geotextiles, and geogrids, and complete water cisterns for water closets in plumbing systems.

Polycarbonate. Excellent transparency, high impact resistance, and good resistance to weathering combine to recommend this plastic for safety glazing and for general illumination and shatter-resistant fixtures. It is available in large, clear, tinted, and opaque sheets that can be formed into shells, domes, globes, and other forms. It can be processed by standard molding methods.

Polytetrafluorethylene. This is a highly crystalline liner-type polymer, unique among organic compounds in its chemical inertness and resistance to change at high and low temperatures. Its electrical properties are excellent. Its outstanding property is extreme resistance to attack by corrosive agents and solvents of all kinds.

Waxy and self-lubricating, polytetrafluoroethylene is used in buildings where resistance to extreme conditions or low friction is desired. In steam lines, for example, supporting pads of this plastic permit the lines to slide easily over the pads.

The temperatures involved have little or no effect. Other low-friction applications include, for example, bearings for girders and trusses. Mechanical properties are only moderately high, and reinforcement may be necessary to prevent creep and squeezeout under heavy loads. These fluorocarbons are difficult to wet; consequently, they are often used as parting agents, or where sticky materials must be handled.

Polyvinylfluoride. This has much of the superior inertness to chemical and weathering attack typical of the fluorocarbons. Among other uses, it is used as thin-film overlays for building boards to be exposed outdoors.

Polyvinyl Formal and Polyvinyl Butyral. Polyvinyl formal resins are principally used as a base for tough, water-resistant insulating enamel for electric wire. Polyvinyl butyral is the tough interlayer in safety glass. In its cross-linked and plasticized form, polyvinyl butyral is extensively used in coating fabrics for raincoats, upholstery, and other heavy-duty moisture-resistant applications.

Vinyl Chloride Polymers and Copolymers. Polyvinyl chloride is naturally hard and rigid but can be plasticized to any required degree of flexibility as in raincoats and shower curtains. Copolymers, including vinyl chloride plus vinyl acetate, are naturally flexible without plasticizers.

Nonrigid vinyl plastics are widely used as insulation and jacketing for electric wire and cable because of their electrical properties and their resistance to oil and water. Thin films are used in geomembranes.

Vinyl chlorides also are used for floor coverings in the form of tile and sheet because of their abrasion resistance and relatively low water absorption. The rigid materials are used for tubing, pipe, and many other applications where their resistance to corrosion and action of many chemicals, especially acids and alkalies,
recommends them.

They are attacked by a variety of organic solvents, however. Like all thermoplastics, they soften at elevated temperatures.

Vinylidene Chloride. This material is highly resistant to most inorganic chemicals and to organic solvents generally. It is impervious to water on prolonged immersion, and its films are highly resistant to moisture-vapor transmission. It can be sterilized, if not under load, in boiling water. It is used as pipe for transporting chemicals and geomembranes

Nylon. Molded nylon is used in increasing quantities for impact and high resistance to abrasion. It is employed in small gears, cams, and other machine parts, because even when unlubricated they are highly resistant to wear.

Its chemical resistance, except to phenols and mineral acids, is excellent. Extruded nylon is coated onto electric wire, cable, and rope for abrasion resistance. Applications like hammerheads indicate its impact resistance.

Polystyrene. This is one of the lightest of the presently available commercial plastics. It is relatively inexpensive, easily molded, has good dimensional stability, and good stability at low temperatures; it is brilliantly clear when transparent and has an infinite range of colors.

Water absorption is negligible even after long immersion. Electrical characteristics are excellent. It is resistant to most corrosive chemicals, such as acids, and to a variety of organic solvents, although it is attacked by others.

Polystyrenes as a class are considerably more brittle and less extensible than many other thermoplastic materials, but these properties are markedly improved in copolymers. Under some conditions, they have a tendency to develop fine cracks, known as craze marks, on exposure, particularly outdoors. This is true of many other thermoplastics, especially when highly stressed. It is widely used in synthetic rubbers.


What Are Thermosetting Plastics?

Phenol Formaldehyde. These materials provide the greatest variety of thermosetting molded plastic articles. They are used for chemical, decorative, electrical, mechanical, and thermal applications of all kinds.

Hard and rigid, they change slightly, if at all, on aging indoors but, on outdoor exposure, lose their bright surface gloss. However, the outdoor-exposure characteristics of the more durable formulations are otherwise generally good.

Phenol formaldehydes have good electrical properties, do not burn readily, and do not support combustion. They are strong, light in weight, and generally pleasant to the eye and touch, although light colors by and large are not obtainable because of the fairly dark-brown basic color of the resin. They have low water absorption and good resistance to attack by most commonly found chemicals.

Epoxy and Polyester Casting Resins. These are used for a large variety of purposes. For example, electronic parts with delicate components are sometimes cast completely in these materials to give them complete and continuous support, and resistance to thermal and mechanical shock.

Some varieties must be cured at elevated temperatures; others can be formulated to be cured at room temperatures. One of the outstanding attributes of the epoxies is their excellent adhesion to a variety of materials, including such metals as copper, brass, steel, and aluminum.

Polyester Molding Materials. When compounded with fibers, particularly glass fibers, or with various mineral fillers, including clay, the polyesters can be formulated into putties or premixes that are easily compression- or transfer-molded into parts having high impact resistance. Polyesters are often used in geotextiles.

Melamine Formaldehyde. These materials are unaffected by common organic solvents, greases, and oils, as well as most weak acids and alkalies. Their water absorption is low. They are insensitive to heat and are highly flame-resistant, depending on the filler.

Electrical properties are particularly good, especially resistance to arcing. Unfilled materials are highly translucent and have unlimited color possibilities. Principal fillers are alpha cellulose for general-purpose compounding; minerals to improve electrical properties, particularly at elevated temperatures; chopped fabric to afford high shock resistance and flexural strength; and cellulose, mainly for electrical purposes.

Cellulose Acetate Butyrate. The butyrate copolymer is inherently softer and more flexible than cellulose acetate and consequently requires less plasticizer to achieve a given degree of softness and flexibility. It is made in the form of clear transparent sheet and film, or in the form of molding powders, which can be molded by standard injection-molding procedures into a wide variety of applications.

Like the other cellulosics, this material is inherently tough and has good impact resistance. It has infinite colorability, like the other cellulosics. Cellulose acetate butyrate tubing is used for such applications as irrigation and gas lines.

Cellulose Nitrate. One of the toughest of the plastics, cellulose nitrate is widely used for tool handles and similar applications requiring high impact strength. The high flammability requires great caution, particularly in the form of film. Most commercial photographic film is cellulose nitrate as opposed to safety film.

Polyurethane. This plastic is used in several ways in building. As thermal insulation, it is used in the form of foam, either prefoamed or foamed in place. The latter is particularly useful in irregular spaces. When blown with fluorocarbons, the foam has an exceptionally low K-factor and is, therefore, widely used in thin-walled refrigerators.

Other uses include field-applied or baked-on clear or colored coatings and finishes for floors, walls, furniture, and casework generally. The rubbery form is employed for sprayed or troweled-on roofing, and for gaskets and calking compounds.

Urea Formaldehyde. Like the melamines, these offer unlimited translucent to opaque color possibilities, light-fastness, good mechanical and electrical properties, and resistance to organic solvents as well as mild acids and alkalies.

Although there is no swelling or change in appearance, the water absorption of urea formaldehyde is relatively high, and it is therefore not recommended for applications involving long exposure to water. Occasional exposure to water is without deleterious effect. Strength properties are good, although special shock-resistant grades are not made.

Silicones. Unlike other plastics, silicones are based on silicon rather than carbon. As a consequence, their inertness and durability under a wide variety of conditions are outstanding. As compared with the phenolics, their mechanical properties are poor, and consequently glass fibers are added.

Molding is more difficult than with other thermosetting materials. Unlike most other resins, they may be used in continuous operations at 400 F; they have very low water absorption; their dielectric properties are excellent over an extremely wide variety of chemical attack; and under outdoor conditions their durability is particularly outstanding.

In liquid solutions, silicones are used to impart moisture resistance to masonry walls and to fabrics. They also form the basis for a variety of paints and other coatings capable of maintaining flexibility and inertness to attack at high temperatures in the presence of ultraviolet sunlight and ozone. Silicone rubbers maintain their flexibility at much lower temperatures than other rubbers.


How To Prevent Aluminum Corrosion?

Although aluminum ranks high in the electromotive series of the metals, it is highly corrosion resistant because of the tough, transparent, tenacious film of aluminum oxide that rapidly forms on any exposed surface.

It is this corrosion resistance that recommends aluminum for building applications. For most exposures, including industrial and seacoast atmospheres, the alloys normally recommended are adequate, particularly if used in usual thicknesses and if mild pitting is not objectionable.

Pure aluminum is the most corrosion resistant of all and is used alone or as cladding on strong-alloy cores where maximum resistance is wanted. Of the alloys, those containing magnesium, manganese, chromium, or magnesium and silicon in the form of MgSi2 are highly resistant to corrosion.

The alloys containing substantial proportions of copper are more susceptible to corrosion, depending markedly on the heat treatment.

Certain precautions should be taken in building. Aluminum is subject to attack by alkalies, and it should therefore be protected from contact with wet concrete, mortar, and plaster.

Clear methacrylate lacquers or strippable plastic coatings are recommended for interiors and methacrylate lacquer for exterior protection during construction. Strong alkaline and acid cleaners should be avoided and muriatic acid should not be used on masonry surfaces adjacent to aluminum.

If aluminum must be contiguous to concrete and mortar outdoors, or where it will be wet, it should be insulated from direct contact by asphalts, bitumens, felts, or other means. As is true of other metals, atmospheric-deposited dirt must be removed to maintain good appearance.

Electrolytic action between aluminum and less active metals should be avoided, because the aluminum then becomes anodic. If aluminum must be in touch with other metals, the faying surfaces should be insulated by painting with asphaltic or similar paints, or by gasketing. Steel rivets and bolts, for example, should be insulated.

Drainage from copper-alloy surfaces onto aluminum must be avoided. Frequently, steel surfaces can be galvanized or cadmium-coated where contact is expected with aluminum. The zinc or cadmium coating is anodic to the aluminum and helps to protect it.


How Welding and Brazing Of Aluminum Works?

Weldability and brazing properties of aluminum alloys depend heavily on their composition and heat treatment. Most of the wrought alloys can be brazed and welded, but sometimes only by special processes.

Finishes for Aluminum and Aluminum Alloys
Types of finish Designation*
Mechanical finishes:
As fabricated M1Y
Buffed M2Y
Directional textured M3Y
Nondirectional textured M4Y
Chemical finishes:
Nonetched cleaned C1Y
Etched C2Y
Brightened C3Y
Chemical conversion coatings C4Y
General A1Y
Protective and decorative (less than 0.4 mil thick) A2Y
Architectural Class II (0.4–0.7 mil thick) A3Y
Architectural Class I (0.7 mil or more thick) A4Y
Resinous and other organic coatings R1Y
Vitreous coatings V1Y
Electroplated and other metallic coatings E1Y
Laminated coatings L1Y
*Y represents digits (0, 1, 2, . . . 9) or X (to be specified) that describe the
surface, such as specular, satin, matte, degreased, clear anodizing or type of coating.

The strength of some alloys depends on heat treatment after welding. Alloys heat treated and artificially aged are susceptible to loss of strength at the weld, because weld is essentially cast.

For this reason, high-strength structural alloys are commonly fabricated by riveting or bolting, rather than by welding. Brazing is done by furnace, torch, or dip methods. Successful brazing is done with special fluxes.

Inert-gas shielded-arc welding is usually used for welding aluminum alloys. The inert gas, argon or helium, inhibits oxide formation during welding.

The electrode used may be consumable metal or tungsten. The gas metal arc is generally preferred
for structural welding, because of the higher speeds that can be used.

The gas tungsten arc is preferred for thicknesses less than 1⁄2 in. Butt-welded joints of annealed aluminum alloys and non-heat-treatable alloys have nearly the same strength as the parent metal.

This is not true for strainhardened or heat-tempered alloys. In these conditions, the heat of welding weakens the metal in the vicinity of the weld. The tensile strength of a butt weld of alloy 6061-T6 may be reduced to 24 ksi, about two-thirds that of the parent metal.

Tensile yield strength of such butt welds may be only 15 to 20 ksi, depending on metal thickness and type of filler wire used in welding.

Fillet welds similarly weaken heat-treated alloys. The shear strength of alloy 6061-T6 decreases from about 27 ksi to 17 ksi or less for a fillet weld.

Welds should be made to meet the requirements of the American Welding Society, ‘‘Structural Welding Code—Aluminum,’’ AWS D1.2.


What Is Iron-Carbon Equilibrium Diagram?

The iron-carbon equilibrium diagram in Figure below shows that, under equilibrium conditions (slow cooling) if not more than 2.0% carbon is present, a solid solution of carbon in gamma iron exists at elevated temperatures.

This is called austenite. If the carbon content is less than 0.8%, cooling below the A3 temperature line causes transformation of some of the austenite to ferrite, which is substantially pure alpha iron (containing less than 0.01% carbon in solution).

Still further cooling to below the A1 line causes the remaining austenite to transform to pearlite—the eutectoid mixture of fine plates, or lamellas, of ferrite and cementite (iron carbide) whose iridescent appearance under the microscope gives it its name.

If the carbon content is 0.8%, no transformation on cooling the austenite occurs until the A1 temperature is reached.

At that point, all the austenite transforms to pearlite, with its typical ‘‘thumbprint’’ microstructure.

At carbon contents between 0.80 and 2.0%, cooling below the Acm temperature line causes iron carbide, or cementite, to form in the temperature range between Acm and A1,3. Below A1,3, the remaining austenite transforms to pearlite.


What Are The Different Commercial Grade Of Woods?

Lumber is graded by the various associations of lumber manufacturers having jurisdiction over various species. Two principal sets of grading rules are employed: (1) for softwoods, and (2) for hardwoods.

Softwood lumber is classified as dry, moisture content 19% or less; and green, moisture content above 19%. According to the American Softwood Lumber Standard, softwoods are classified according to use as:

Yard Lumber. Lumber of grades, sizes, and patterns generally intended for ordinary construction and general building purposes.

Structural Lumber. Lumber 2 in or more nominal thickness and width for use where working stresses are required.

Factory and Shop Lumber. Lumber produced or selected primarily for manufacturing purposes.

Softwoods are classified according to extent of manufacture as:

Rough Lumber. Lumber that has not been dressed (surfaced) but has been sawed, edged, and trimmed.

Dressed (Surfaced) Lumber. Lumber that has been dressed by a planning machine (for the purpose of attaining smoothness of surface and uniformity of size) on one side (S1S), two sides (S2S), one edge (S1E), two edges (S2E), or a combination of sides and edges (S1S1E, S1S2, S2S1E, S4S).

Worked Lumber. Lumber that, in addition to being dressed, has been matched, shiplapped or patterned:

Matched Lumber. Lumber that has been worked with a tongue on one edge of each piece and a groove on the opposite edge.

Shiplapped Lumber. Lumber that has been worked or rabbeted on both edges, to permit formation of a close-lapped joint.

Patterned Lumber. Lumber that is shaped to a pattern or to a molded form.

Softwoods are also classified according to nominal size:
Boards. Lumber less than 2 in in nominal thickness and 2 in or more in nominal width. Boards less than 6 in in nominal width may be classified as strips.

Dimension. Lumber from 2 in to, but not including, 5 in in nominal thickness, and 2 in or more in nominal width. Dimension may be classified as framing, joists, planks, rafters, studs, small timbers, etc.

Timbers. Lumber 5 in or more nominally in least dimension. Timber may be classified as beams, stringers, posts, caps, sills, girders, purlins, etc.

Actual sizes of lumber are less than the nominal sizes, because of shrinkage and dressing. In general, dimensions of dry boards, dimension lumber, and timber less than 2 in wide or thick are 1⁄4 in less than nominal; from 2 to 7 in wide or thick, 1⁄2 in less, and above 6 in wide or thick, 3⁄4 in less.

Green-lumber less than 2 in wide or thick is 1⁄32 in more than dry; from 2 to 4 in wide or thick, 1⁄16 in more, 5 and 6 in wide or thick, 1⁄8 in more, and 8 in or above in width and thickness, 1⁄4 in more than dry lumber.

There are exceptions, however. Yard lumber is classified on the basis of quality as:

Appearance. Lumber is good appearance and finishing qualities, often called select.

Suitable for natural finishes
Practically clear
Generally clear and of high quality
Suitable for paint finishes
Adapted to high-quality paint finishes
Intermediate between high-finishing grades and common grades, and partaking somewhat of the nature of both

Common. Lumber suitable for general construction and utility purposes, often given various commercial designations.

For standard construction use
Suitable for better-type construction purposes
Well adapted for good standard construction
Designed for low-cost temporary construction
For less exacting purposes
Low quality, but usable

Structural lumber is assigned modulus of elasticity values and working stresses in bending, compression parallel to grain, compression perpendicular to grain, and horizontal shear in accordance with ASTM procedures.

These values take into account such factors as sizes and locations of knots, slope of grain, wane, and shakes or checks, as well as such other pertinent features as rate of growth and proportions of summerwood.

Factory and shop lumber is graded with reference to its use for doors and sash, or on the basis of characteristics affecting its use for general cut-up purposes, or on the basis of size of cutting. The grade of factory and shop lumber is determined by the percentage of the area of each board or plank available in cuttings of specis determined from the poor face, although the quality of both sides of each cutting must be considered.

Because of the great diversity of applications for hardwood both in and outside the construction industry, hardwood grading rules are based on the proportion of a given piece that can be cut into smaller pieces of material clear on one or both sides and not less than a specified size.

Grade classifications are therefore based on the amount of clear usable lumber in a piece. Special grading rules of interest in the construction industry cover hardwood interior trim and moldings, in which one face must be practically free of imperfections and in which Grade A may further limit the amount of sapwood as well as stain.

Hardwood dimension rules, in addition, cover clears, which must be clear both faces; clear one face; paint quality, which can be covered with pain; core, which must be sound on both faces and suitable for cores of glued-up panels; and sound, which is a general-utility grade.

Hardwood flooring is graded under two separate sets of rules: (1) for maple, birch, and beech; and (2) for red and white oak and pecan. In both sets of rules, color and quality classifications range from top-quality to the lower utility grades.

Oak may be further subclassified as quarter-sawed and plain-sawed. In all grades, top-quality material must be uniformed in color, whereas other grades place no limitation on color.

Shingles are graded under special rules, usually into three classes: Number 1, 2, and 3. Number 1 must be all edge grain and strictly clear, containing no sapwood. Numbers 2 and 3 must be clear to a distance far enough away from the butt to be well covered by the next course of shingles.


What Are The Type Of Glass Used In Civil Construction?

Clear Window Glass.
This is the most extensively used type for windows in all classes of buildings. A range of grades, as established by Federal Government Standard DD-G-451c, classifies quality according to defects.

The more commonly used grades are A and B. A is used for the better class of buildings where appearance is important, and B is used for industrial buildings, some low-cost residences, basements, etc.

With respect to thickness, clear window glass is classified as ‘‘single-strength’’ about 3⁄32 in thick; ‘‘double-strength,’’ about 1⁄8 in thick; and ‘‘heavy-sheet,’’ up to 7⁄32 in thick.

Maximum sizes are as follows: single-strength, 40 50 in; doublestrength, 60 80 in; and heavy sheet, 76 120 in. Because of flexibility, single strength and double strength should never be used in areas exceeding 12 ft2, and for appearance’s sake areas should not exceed 7 ft2.

Plate and Float Glass. 
These have, in general, the same performance characteristics. They are of superior quality, more expensive, and have better appearance, with no distortion of vision at any angle.

Showcase windows, picture windows, and exposed windows in offices and commercial buildings are usually glazed with polished plate or float glass. Thicknesses range from 1⁄8 to 7⁄8 in. There are two standard qualities, silvering and glazing, the latter being employed for quality glazing.

Processed Glass and Rolled Figured Sheet.
These are general classifications of obscure glass. There are many patterns and varying characteristics. Some provide true obscurity with a uniform diffusion and pleasing appearance, while others may give a maximum transmission of light or a smoother surface for greater cleanliness. The more popular types include a clear, polished surface on one side with a pattern for obscurity on the other side.

Obscure Wired Glass.
This usually is specified for its fire-retarding properties, although it is also used in doors or windows where breakage is a problem. It should not be used in pieces over 720 in2 in area (check local building code).

Polished Wired Glass.
More expensive than obscure wired glass, polished wired glass is used where clear vision is desired, such as in school or institutional doors. There are also many special glasses for specific purposes:

Heat-Absorbing Glass.
This reduces heat, glare, and a large percentage of ultraviolet rays, which bleach colored fabrics. It often is used for comfort and reduction of air-conditioning loads where large areas of glass have a severe sun exposure.

Because of differential temperature stresses and expansion induced by heat absorption under severe sun exposure, special attention should be given to edge conditions. Glass having clean-cut edges is particularly desirable, because these affect the edge strength, which, in turn must resist the central-area expansion. A resilient glazing material should be used.

Corrugated Glass, Wired Glass, and Plastic Panels. These are used for decorative treatments, diffusing light, or as translucent structural panels with color.

Laminated Glass. 
This consists of two or more layers of glass laminated together by one or more coatings or a transparent plastic. This construction adds strength.

Some types of laminated glass also provide a degree of security, sound isolation, heat absorption, and glare reduction. Where color and privacy are desired, fadeproof opaque colors can be included.

When fractured, a laminated glass tends to adhere to the inner layer of plastic and, therefore, shatters into small splinters, thus minimizing the hazard of flying glass.

Bullet-Resisting Glass.
This is made of three or more layers of plate glass laminated under heat and pressure. Thicknesses of this glass vary from 3⁄4 to 3 in. The more common thicknesses are 13⁄16 in, to resist medium-powered small arms: 11⁄2 in, to resist high-powered small arms; and 2 in, to resist rifles and submachine guns. (Underwriters Laboratories lists materials having the required properties for various degrees of protection.) Greater thicknesses are used for protection against armorpiercing projectiles.

Uses of bullet-resisting glass include cashier windows, bank teller cages, toll-bridge booths, peepholes, and many industrial and military applications. Transparent plastics also are used as bullet-resistant materials, and some of these materials have been tested by the Underwriters Laboratories. Thicknesses of 11⁄4 in or more have met UL standards for resisting medium-powered small arms.

Tempered Glass.
This is produced by a process of reheating and sudden cooling that greatly increases strength. All cutting and fabricating must be done before tempering. Doors of 1⁄2- and 3⁄4-in-thick tempered glass are commonly used for commercial building.

Other uses, with thicknesses from 1⁄8 to 7⁄8 in, include backboards for basketball, showcases, balustrades, sterilizing ovens, and windows, doors, and mirrors in institutions. Although tempered glass is 41⁄2 to 5 times as strong as annealed glass of the same thickness, it is breakable, and when broken, disrupts into innumerable small fragments of more or less cubical shape.

Tinted and Coated Glasses.
These are available in several types and for varieduses. As well as decor, these uses can provide for light and heat reflection, lower light transmission, greater safety, sound reduction, reduced glare, and increased privacy.

Transparent Mirror Glass.
This appears as a mirror when viewed from a brightly lighted side, and is transparent to a viewer on the darker opposite side. This oneway- vision glass is available as a laminate, plate or float, tinted, and in tempered quality.

Plastic Window Glazing. 
Made of such plastics as acrylic or polycarbonate, plastic glazing is used for urban school buildings and in areas where high vandalism might be anticipated. These plastics have substantially higher impact strength than glass or tempered glass.

Allowance should be made in the framing and installation for expansion and contraction of plastics, which may be about 8 times as much as that of glass. Note also that the modulus of elasticity (stiffness) of plastics is about one-twentieth that of glass.

Standard sash, however, usually will accommodate the additional thickness of plastic and have sufficient rabbet depth.

Suspended Glazing.
This utilizes metal clamps bonded to tempered plate glass at the top edge, with vertical glass supports at right angles for resistance to wind pressur. These vertical supports, called stabilizers, have their exposed edges polished.

The joints between the large plates and the stabilizers are sealed with a bonding cement. The bottom edge or sill is held in position by a metal channel, and sealed with resilient waterproofing. Suspended glazing offers much greater latitude in use of glass and virtually eliminates visual barriers.

Safety Glazing. 
A governmental specification Z-97, adopted by many states, requires entrance-way doors and appurtenances glazed with tempered, laminated, or plastic material.


What Are Concrete Mixes Fibers?

As used in concrete, fibers are discontinuous, discrete units. They may be described by their aspect ratio, the ratio of length to equivalent diameter. Fibers find their greatest use in crack control of concrete flatwork, especially slabs on grade.

The most commonly used types of fibers in concrete are synthetics, which include polypropylene, nylon, polyester, and polyethylene materials. Specialty synthetics include aramid, carbon, and acrylic fibers.

Glass-fiber-reinforced concrete is made using E-glass and alkali-resistant (AR) glass fibers. Steel fibers are chopped high-tensile or stainless steel.

Fibers should be dispersed uniformly throughout a mix. Orientation of the fibers in concrete generally is random. Conventional reinforcement, in contrast, typically is oriented in one or two directions, generally in planes parallel to the surface.

Further, welded-wire fabric or reinforcing steel bars must be held in position as concrete is placed. Regardless of the type, fibers are effective in crack control because they provide omnidirectional reinforcement to the concrete matrix.

With steel fibers, impact strength and toughness of concrete may be greatly improved and flexural and fatigue strengths enhanced.

Synthetic fibers are typically used to replace welded-wire fabric as secondary reinforcing for crack control in concrete flatwork. Depending on the fiber length, the fiber can limit the size and spread of plastic shrinkage cracks or both plastic and drying shrinkage cracks.

Although synthetic fibers are not designed to provide structural properties, slabs tested in accordance with ASTM E72, ‘‘Standard Methods of Conducting Strength Tests of Panels for Building Construction,’’ showed that test slabs reinforced with synthetic fibers carried greater uniform loads than slabs containing welded wire fabric.

While much of the research for synthetic fibers has used reinforcement ratios greater than 2%, the common field practice is to use 0.1% (1.5 lb /yd3). This dosage provides more cross-sectional area than 10-gage weldedwire fabric.

The empirical results indicate that cracking is significantly reduced and is controlled. A further benefit of fibers is that after the initial cracking, the fibers tend to hold the concrete together.

Aramid, carbon, and acrylic fibers have been studied for structural applications, such as wrapping concrete columns to provide additional strength. Other possible uses are for corrosion-resistance structures. The higher costs of the specialty synthetics limit their use in general construction.

Glass-fiber-reinforced concrete (GFRC) is used to construct many types of building elements, including architectural wall panels, roofing tiles, and water tanks. The full potential of GFRC has not been attained because the E-glass fibers are alkali reactive and the AR-glass fibers are subject to embrittlement, possibly from infiltration of calcium-hydroxide particles.

Steel fibers can be used as a structural material and replace conventional reinforcing steel. The volume of steel fiber in a mix ranges from 0.5 to 2%. Much work has been done to develop rapid repair methods using thin panels of densely packed steel fibers and a cement paste squeegeed into the steel matrix.

American Concrete Institute Committee 544 states in ‘‘Guide for Specifying, Mixing, Placing, and Finishing Steel Fiber Reinforced Concrete,’’ ACI 544.3R, that, in structural members such as beams, columns, and floors not on grade, reinforcing steel should be provided to support the total tensile load. In other cases, fibers can be used to reduce section thickness or improve performance. See also ACI 344.1R and 344.2R.


What Are Corrosion Inhibitors Admixtures For Concrete?

Reinforcing steel in concrete usually is protected against corrosion by the high alkalinity of the concrete, which creates a passivating layer at the steel surface.

This layer is composed of ferric oxide, a stable compound. Within and at the surface of the ferric oxide, however, are ferrous-oxide compounds, which are more reactive.

When the ferrous-oxide compounds come into contact with aggressive substances, such as chloride ions, they react with oxygen to form solid, iron-oxide corrosion products.

These produce a fourfold increase in volume and create an expansion force greater than the concrete tensile strength. The result is deterioration of the concrete.

For corrosion to occur, chloride in the range of 1.0 to 1.5 lb /yd3 must be present. If there is a possibility that chlorides may be introduced from outside the concrete matrix, for example, by deicing salts, the concrete can be protected by lowering the water-cement ratio, or increasing the amount of cover over the reinforcing steel, or entraining air in the concrete, or adding a calcium-nitrate admixture, or adding an internal-barrier admixture, or cathodic protection, or a combination of these methods.

To inhibit corrosion, calcium-nitrate admixtures are added to the concrete at the time of batching. They do not create a physical barrier to chloride ion ingress. Rather, they modify the concrete chemistry near the steel surface.

The nitrite ions oxidize ferrous oxide present, converting it to ferric oxide. The nitrite is also absorbed at the steel surface and fortifies the ferric-oxide passivating layer.

For a calcium-nitrite admixture to be effective, the dosage should be adjusted in accordance with the exposure condition of the concrete to corrosive agents. The greater the exposure, the larger should be the dosage.

The correct dosage can only be determined on a project-by-project basis with data for the specific admixture proposed. Internal-barrier admixtures come in two groups. One comprises waterproofing
and dampproofing compounds.

The second consists of agents that create an organic film around the reinforcing steel, supplementing the passivating layer. This type of admixture is promoted for addition at a fixed rate regardless of expected
chloride exposure.


What Are Water Reducing Concrete Admixtures?

Water-Reducing Admixtures
These decrease water requirements for a concrete mix by chemically reacting with early hydration products to form a monomolecular layer of admixture at the cementwater interface.

This layer isolates individual particles of cement and reduces the energy required to cause the mix to flow. Thus, the mix is ‘‘lubricated’’ and exposes more cement particles for hydration.

The Type A admixture allows the amount of mixing water to be reduced while maintaining the same mix slump. Or at a constant water-cement ratio, this admixture allows the cement content to be decreased without loss of strength.

If the amount of water is not reduced, slump of the mix will be increased and also strength will be increased because more of the cement surface area will be exposed for hydration. Similar effects occur for Type D and E admixtures. Typically, a reduction in mixing water of 5 to 10% can be expected.

Type F and G admixtures are used where there is a need for high-workability concrete. A concrete without an admixture typically has a slump of 2 to 3 in. After the admixture is added, the slump may be in the range of 8 to 10 in without segregation of mix components.

These admixtures are especially useful for mixes with a low water-cement ratio. Their 12 to 30% reduction in water allows a corresponding reduction in cementitious material.

The water-reducing admixtures are commonly manufactured from lignosulfonic acids and their salts, hydroxylated carboxylic acids and their salts, or polymers of derivatives of melamines or naphthalenes or sulfonated hydrocarbons. The combination of admixtures used in a concrete mix should be carefully evaluated and tested to ensure that the desired properties are achieved.

For example, depending on the dosage of admixture and chemistry of the cement, it is possible that a retarding admixture will accelerate the set. Note also that all normal-set admixtures will retard the set if the dosage is excessive.

Furthermore, because of differences in percentage of solids between products from different companies, there is not always a direct correspondence in dosage between admixtures of the same class. Therefore, it is
important to consider the chemical composition carefully when evaluating competing admixtures.

Superplasticizers are high-range water-reducing admixtures that meet the requirements of ASTM C494 Type F or G. They are often used to achieve highstrength concrete by use of a low water-cement ratio with good workability and low segregation.

They also may be used to produce concrete of specified strengths with less cement at constant water cement ratio. And they may be used to produce self-compacting, self-leveling flowing concretes, for such applications as longdistance pumping of concrete from mixer to formwork or placing concrete in forms congested with reinforcing steel.

For these concretes, the cement content or watercement ratio is not reduced, but the slump is increased substantially without causing segregation. For example, an initial slump of 3 to 4 in for an ordinary concrete mix may be increased to 7 to 8 in without addition of water and decrease in strength.

Superplasticizers may be classified as sulfonated melamine-formaldehyde condensates, sulfonated naphthaline-formaldehyde condensates, modified lignosulfonates, or synthetic polymers.


What Are The Fire And Smoke Barriers In Building Construction?

A major consideration in building design is safety of the community. Hence, buildings should be designed to control fires and smoke so that they will not spread from building to building. One way that building codes try to achieve this objective is to establish fire zones or fire limits that restrict types of construction or occupancy that can be used.

Additional zoning regulations establish minimum distances between buildings. Another way to achieve the objective is to specify the types of construction that can be used for enclosing the exterior of buildings.

The distance between adjoining buildings, fire rating, and stability when exposed to fire of exterior walls, windows, and doors, and percent of window area are some of the factors taken into account in building codes for determination of the construction classification of a building.

To prevent spread of fire from roof to roof, building codes also often require that exterior walls extend as a parapet at least 3 ft above the roof level. Parapets also are useful in shielding fire fighters who may be hosing a fire from roofs of buildings adjoining the one on fire. In addition, buildings should be topped with roof coverings that are fire-resistant.

Fire Divisions.
To prevent spread of fire vertically in building interiors, building codes generally require that floor-ceiling and roof-ceiling assemblies be fireresistant. The fire rating of such assemblies is one of the factors considered in determination of the construction classification of a building.

Also, openings in floors and roofs should be fire-protected, although building codes do not usually require this for one-story or two-story dwellings. For the purpose, an opening, such as that for a stairway, may be protected with a fire-resistant enclosure and fire doors.

In particular, stairways and escalator and elevator shafts should be enclosed, not only to prevent spread of fire and smoke but also to provide a protected means of egress from the building for occupants and of approach to the fire source by fire fighters.

To prevent spread of fire and smoke horizontally in building interiors, it is desirable to partition interiors with fire divisions. A fire division is any construction with the fire-resistance rating and structural stability under fire conditions required for the type of occupancy and construction of the building to bar the spread of fire between adjoining buildings or between parts of the same building on opposite sides of the division. A fire division may be an exterior wall, fire window, fire door, fire wall, ceiling, or firestop.

A fire wall should be built of incombustible material, have a fire rating of at least 4 hr, and extend continuously from foundations to roof. Also, the wall should have sufficient structural stability in a fire to allow collapse of construction on either side without the wall collapsing. Building codes restrict the size of openings that may be provided in a fire wall and require the openings to be fire-protected (Art. 11.55).

To prevent spread of fire through hollow spaces, such spaces should be firestopped. A firestop is a solid or compact, tight closure set in a hollow, concealed space in a building to retard spread of flames, smoke, or hot gases.

All partitions and walls should be firestopped at every floor level, at the top-story ceiling level, and at the level of support for roofs. Also, very large unoccupied attics should be subdivided by firestops into areas of 3000 ft2 or less.

Similarly, any large concealed space between a ceiling and floor or roof should be subdivided. For the purpose, firestops extending the full depth of the space should be placed along the line of supports of structural members and elsewhere, if necessary, to enclose areas not exceeding 1000 ft2 when situated between a floor and ceiling or 3000 ft2 when located between a ceiling and roof.

Openings between floors for pipes, ducts, wiring, and other services should be sealed with the equal of positive firestops. Partitions between each floor and a suspended ceiling above are not generally required to be extended to the slab above unless this is necessary for required compartmentation. But smoke stops should be provided at reasonable intervals to prevent passage of smoke to noninvolved areas.


Importance Of Building Fire Protection

Although fires in buildings can be avoided, they nevertheless occur. Some of the reasons for this are human error, arson, faulty electrical equipment, poor maintenance of heating equipment, and natural causes, such as lightning.

Consequently, buildings should be designed to minimize the probability of a fire and to protect life and limit property damage if a fire should occur. The minimum steps that should be taken for the purpose are as follows:

1. Limit potential fire loads, with respect to both combustibility and ability to generate smoke and toxic gases.

2. Provide means for prompt detection of fires, with warnings to occupants who may be affected and notification of the presence of fire to fire fighters.

3. Communication of instructions to occupants as to procedures to adopt for safety, such as to staying in place, proceeding to a designated refuge area, or evacuating the building.

4. Provide means for early extinguishment of any fire that may occur, primarily by automatic sprinklers but also by trained fire fighters.

5. Make available also for fire fighting an adequate water supply, appropriate chemicals, adequate-size piping, conveniently located valves on the piping, hoses, pumps, and other equipment necessary.

6. Prevent spread of fire from building to building, either through adequate separation or by enclosure of the building with incombustible materials.

7. Partition the interior of the building with fire barriers, or divisions, to confine a fire to a limited space.

8. Enclose with protective materials structural components that may be damaged by fire (fireproofing).

9. Provide refuge areas for occupants and safe evacuation routes to outdoors.

10. Provide means for removal of heat and smoke from the building as rapidly as possible without exposing occupants to these hazards, with the air-conditioning system, if one is present, assisting the removal by venting the building and by pressurizing smokeproof towers, elevator shafts, and other exits.

11. For large buildings, install standby equipment for operation in emergencies of electrical systems and elevators.