Joists

Posted by on 17/ 11/ 15

Rafters that are not supported at the top or somewhere along the span by a beam create an outward force on the wall they rest on. This force is frequently offset by ceiling joists joining the walls on the opposite side of the roof.

The chart below shows a common size of ceiling joist needed for different spans.

Ceiling Joist Span Chart

24” O. C. 20 PSF uninhabitable

Species & Grade Maximum Ceiling Joist Spans

2 x 4

2 x 6

2 x 8

Douglas Fir-Larch # 1

7′-8"

11′-2"

14′-2"

Douglas Fir-Larch # 2

7′-2"

10′-6"

13′-3"

Douglas Fir-Larch # 3

5′-5"

7′-11"

10′-0"

Hem-Fir # 1

7′-6"

10′-11"

13′-10"

Hem-Fir # 2

7′-1"

10′-4"

13′-1"

Hem-Fir # 3

5′-5"

7′-11"

10′-0"

Southern Pine # 1

8′-0"

12′-6"

15′-10"

Southern Pine # 2

7′-8"

11′-0"

14′-2"

Southern Pine # 3

5′-9"

8′-6"

10′-10"

Spruce-Pine-Fir # 1

7′-2"

10′-6"

13′-3"

Spruce-Pine-Fir # 2

7′-2"

10′-6"

13′-3"

Spruce-Pine-Fir # 3

5′-5"

7′-11"

10′-0"

The previous chart shows a common size of ceiling joist needed for different spans.

The bottom chord of roof trusses acts as a ceiling joist and provides support for the ceiling sheathing, and support to keep the walls from pushing out.

Attic areas above ceiling joists must be made accessible if there is a clear height of 30" or more.

This requires framing an attic opening. The opening must not be less than 20" by 30" and have at least 30" clear space above. The attic access should be framed similar to an opening in floor joists as illustrated on “Step 5-Frame Openings in Joists" in Chapter 3.

Step 1-Spread Trusses

Trusses spread ready to roll

Gable-end truss sitting on trusses.

When trusses are delivered in stacks, they should be set on the roof to allow for easy spreading. The gable ends should be on top because they go up first. The direction of the ridge is important so they can be spread and tilted up easily.

When spreading the trusses, place them on your layout marks so that when you roll them, you will have minimum moving of the trusses. They are easier to move lying down.

The Radioactive Countertop

Posted by on 17/ 11/ 15

John Banta was called to the home of a woman who was employed as a cook by the television in­dustry. Her task was to create samples of the same recipe in various stages of preparation, from raw ingredients to oven-ready mixtures to finished product. The prepared foods were then delivered to the television studio so a celebrity on a culinary arts program could demonstrate the recipe.

During the investigation, John discovered that his client was being exposed to an unex­pected occupational hazard. The orange-colored tile used for her counter was glazed with uranium

products for interior finishes and cabine­try (see Resource List for products)

• Medex or Medite II: A medium density fi – berboard manufactured without formal­dehyde

. Multi – core: A low-emissions plywood with a variety of hardwood veneers

• PureKor: Specify their formaldehyde-free board

• Roseburg SkyBlend Particleboard: Sci­entific Certification Systems certified, low emissions, certified recycled content, no urea-formaldehyde added during manu­facturing process

• Temstock-Free: No added urea-formalde­hyde particleboard underlayment and in­dustrial-grade particleboard

• Terramica: Scientific Certification Sys­tems certified no added urea-formalde­hyde particleboard underlayment and industrial-grade particleboard

• Tru-Spec: Line ofurea-formaldehyde-free, millwork-quality wood strand engineered

oxide, a highly radioactive substance that was making the numbers on the Geiger counter spin too fast to count. For over 30 years this woman had worked at a radioactive counter, slicing, dicing, mixing, and arranging her creations.

When she learned of the radioactivity, the cli­ent revealed to John that she had recently had a precancerous lesion removed from her intestines. Her surgical scar was located at the level where the counter pressed against her while she cooked. The client was advised to have her countertop re­placed. Her physician concurred.

wood products for interior wood finishes and cabinetry, GreenGuard certified

Alternatives to Hardwood

Because hardwoods take a long time to grow and are therefore not as sustainable as faster growing alternatives, you might consider the following healthy alternatives to hardwood for your finsh carpentry and cabinetry:

• Alio wood: Hardwood lumber substitute manufactured from faster growing soft­woods and agri-based materials

• Environ Biocomposites: Line of formal­dehyde-free, agri-based sustainable panel products for interior finishes and cabinetry

• Bamboo: Refer to Division 9 for bamboo sources

Countertops

The ideal countertop material for a healthy home would have a solid, nonporous surface that is stain – and scratch-proof. It could be attached by mechanical means directly to the cabinet boxes, thus avoiding the need for un- derlayment and adhesives. It would be beauti­ful, inexpensive, and manufactured in a variety of colors. Unfortunately, all of these character­istics are not found in combination in a single countertop option. Chart 6.1 reviews the most common countertop materials and outlines specification concerns for each.

Chart 6.1: Countertop Comparisons

Type

Relative cost

Advantages

Disadvantages

Comments

Specify

High pressure laminates (e. g., Formica, Wil­son art)

Lowest initial investment

•Wide variety of colors, patterns, textures, and sheens

• Low cost

• Seamless surface

• Glued to particle­board with toxic glues

• Particleboard outgasses formal­dehyde

• Not stain – or acid – resistant

• Will scratch

• Cannot be resur­faced

• Short life; deterio­rates quickly if the particleboard gets wet

• Not a good choice for a healthy home

• Fasten to cabinetry with mechanical fasteners

• Seal all exposed edges and surfaces of particleboard with foil or one of the vapor barrier sealants listed in Division 9

Solid-surface materials (e. g., Corian, Avonite, Swanstone, acrylic or plastic colored with resin)

Expensive

■ Nonporous

• Sanitary; integral sinks and rolled backsplashes are easy to clean

• No substrate needed for most

• Scratches and stains are easily sanded

• Attractive marble – and granite-like surfaces

• Can be mechani­cally fastened

* Can be more expensive than granite or marble

• Select a type that does not require substrate

• Fasten to cabinetry with mechanical fasteners

Type

Relative cost

Advantages

Disadvantages

Comments

Specify

Tile

Can be moder­ate

• Hard, scratch – resistant surfaces

• Large variety of sizes, colors, and textures to choose from

• Grout joints are subject to staining and mold and bacterial growth

• Glazes may contain heavy metals or be radioactive

•Tiles can crack or chip under heavy impact

• Choose large tiles to reduce the num­ber of grout joints

• Choose presealed tiles

• Choose commer­cially rated tiles

• Choose tiles requir­ing the narrowest grout joints

• Choosing porce­lains with integral color will disguise chips

• Follow recommen­dations in the tile section of Division 9 for underlay­ment, tile setting, and grout sealing

Butcher block

Moderate

• Warm, inviting aesthetics

• Natural material

• Can be refinished by sanding

• Does not require underlayment

• Can be mechani­cally fastened

• Porous surface stains easily and can harbor mold growth

• Seams might have been glued with formaldehyde – based adhesives

• Finish with odor­less, nontoxic oil such as walnut oil

■ Fasten to cabinetry with mechanical fasteners

• Use adhesives that are solvent – and formaldehyde-free

Solid sheet granite

Expensive

• Wide selection of very beautiful stones

• Hard, scratchproof, stain-resistant surface that will last forever

• Solid, seamless surface

• Can be mechani­cally fastened or glued with silicone

• May not require substrate

• May be cost pro­hibitive

• Surface must be finished with im­pregnating finish

• Oil or butter left on surface will stain it

• Must check for radioactivity

■ Examine MSDS of surface impreg­nating finish for toxicity

• Fasten mechani­cally or with 100% pure silicone caulk (aquarium/food grade)

Engineered stone quartz — 93% quartz, 7% resin (e. g., Sile – stone, Caesar – Stone, Cambria, Zodiaq)

Expensive

• Does not require sealing

• Stain-, scratch-, and heat-resistant

• Hygienic

• Can be mechani­cally fastened

* Heavy

• Requires profes­sional installation

• Good choice for durability and hygienic proper­ties and because it does not require use of impregnat­ing finish

– Fasten mechani­cally

Type

Relative cost

Advantages

Disadvantages

Comments

Specify

Granite tile

Moderate

• Can resemble granite but is less expensive than slab

• Very thin grout joints can be sealed with trans­parent silicone

• Mar- and scratch – resistant

• Requires epoxy­type glues to set

• Requires underlay – ment

• Oil or butter left on surface will stain it

• Must check for radioactivity

• Examine MSDS of surface impreg­nating finish for toxicity

• Refer to section on underlaymentfor tile in Division 9

Stainless steel

Expensive

• Nonporous, non­staining

• Easily cleaned con­tinuous surface

•Thinner gauges require underlay – ment

• Noisy

• Must be special – ordered

* Conducts electric­ity

• Proper ground fault interrupters are essential to prevent potential electrocution

• Use formaldehyde – free underlayment and mechanical fastening

Solid slate

Comparable to granite

• Nonporous, non­staining

• Softer than granite and can chip

• Can be mechani­cally fastened

• Does not require finish

CLEANING AND SEALING EXTERIORS

Posted by on 17/ 11/ 15

Use the gentlest, least damaging cleaning agents, chemicals, steam cleaner, or water pressure that works. Determine this by testing on an inconspic­uous area. If the gentlest method doesn’t work, move to the next stronger. If mortar joints are eroded, a pressure washer may make them leak. After cleaning, allow the brickwork to dry for two days or three days; then caulk gaps around doors and windows and replace worn flashing. Safety note: Whatever cleaning method you choose, wear a face shield, rubber gloves, protective clothing, and a respirator mask before you begin.

Cleaning brick surfaces. Get bids on hiring an authorized cleaning service. Because cleaning solvents can be hazardous and must be disposed of according to EPA and local environmental standards, hiring professionals will spare you those headaches. Moreover, the service will be responsible for achieving the desired results, however long it takes. If you’ve got a tight budget or an adventurous spirit, consider the DIY options described next; they’re listed more or less in order of gentleness. Wear safety glasses and rubber gloves for all procedures, and read operat­ing manuals carefully before using pressure washers, steam cleaners, and the like.

Use a garden hose to soak the surface, and then scrub with a nylon scrub brush. The warmer the water and the longer the soak, the more dirt you’ll remove.

If the hose wash isn’t sufficient, try a pressure washer on a low setting. Increase the pressure slightly—say, to 300 psi to 400 psi—and you’ll remove yet more. Note: If you see sand in the runoff water, lower the machine’s pressure set­tings immediately. Otherwise, you may be strip­ping the mortar joints. Likewise, monitor the inside of the building, especially around win­dows, for leaks; it’s easier to lower the pressure than to replace drywall.

Strippin

Steam cleaning is especially effective if sur­faces are mossy, or have ivy “trails” or built-up grime. Although somewhat slower than pressure washing, steam doesn’t generate the volume of runoff and won’t penetrate as deeply into brick surfaces or cracks.

If you’re in an urban area where soot and auto exhaust have soiled the building, try a nonionic detergent with a medium pressure (1,000 psi) next. Nonionic detergents such as GAF’s Igepal®, Union Carbide’s Tergitol®, and Rohm & Haas’ Triton® won’t leave visible residues, as household detergents and TSP (trisodium phosphate) will. Again, scrub with a synthetic-bristle brush, and rinse well.

If these methods don’t produce the results you want, proprietary chemical cleaners are the next step. They usually involve a three-step process— wetting surfaces, applying the cleaner and scrub­bing it in, and then rinsing—repeated as many times as needed. If you apply the cleaner, follow manufacturer’s instructions to the letter. Instructions will be quite specific about safety garb, dilution rates, dwell times (how long the chemical remains on), washer settings, tempera­ture ranges (most don’t work well below 50°F.), and so on. Before committing to a cleaning sys­tem, visit the manufacturer’s Web site and call its tech-support number.

Sealing exteriors. Are water-repellent or water­proof coatings necessary on exterior masonry walls above grade? Mostly, no. There may be a few 200-year-old buildings in every city whose porous brick would benefit from being coated, but most masonry exteriors won’t admit water if rain is directed away from the structure by gut­ters, downspouts, and other standard drainage details and if the masonry is properly flashed, caulked, and detailed.

Подпись: О PAINTED BRICKПодпись: If you intend to repoint a painted brick wall, strip the paint first—no small job. For this, rent a pressure washer: Have the rental company recommend a tip size and pressure—1,800 psi to 2,500 psi is common for stripping—and show you how to use the machine safely. Buy heavy-duty tarps, too. Caveat: Removing paint by any method makes a prodigious mess. At the job site, test the washer on an out-of-the-way area first. If the stripped brick doesn't look so great, rethink stripping it. The brick may have been painted in an attempt to protect crumbling mortar joints and eroding brick from further weathering. Finally, never sandblast old brick: Sandblasting equipment will destroy brick in a flash, and it's dangerous to use. Подпись: PRO"ГIP In spite of recommendations you may get from store clerks or old-time masons, don't use muriatic acid to clean masonry surfaces. Its fumes are noxious. It can burn your skin or, if you get it in your eyes, blind you. It also etches the surface of stone, brick, and mortar joints and so weakens their ability to shed water. And it stains. Подпись: 1111First of all, so-called exterior sealants, loosely divided into water-repellent and waterproofing

Подпись: TIPПодпись:Подпись: A professional chimney sweep needs a variety of brushes. The reel at the right contains a 50-ft. flexible rod that can push and pull brushes. coatings, don’t fully seal masonry surfaces, nor would you want them to. A perfect seal could trap water inside the walls. Moreover, masonry walls with water trapped inside and walls that are wicking moisture from the ground will, in time, exude soluble salts in the masonry as pow­dery white substances called efflorescence.

Water-repellent coatings, which are usually clear, penetrate masonry pores and so keep rain from penetrating to a large degree, while allowing water vapor from living areas to escape through the wall. Most water-repellent compounds are water-based, formulated from silanes, siloxanes, and silane/siloxane blends. Both premixed and concentrated coatings are available—typically applied in several coats. Some water-repellent coatings double as graffiti barriers, though they tend to be shiny.

Waterproof coatings come closer to being true sealers because they’re usually pigmented or opaque and form a thin elastomeric (flexible) film. Chemically, they run the gamut from water based to bituminous. Bituminous varieties are widely applied below grade on building founda­tions and, to a lesser extent, to interior basement walls where mild leaks have occurred.

Archimedes, and the first theoretical formulations of fluid mechanics

Posted by on 17/ 11/ 15

Archimedes (287 – 212 BC) was born at Syracuse, in Sicily. In all probability, he spent time in Alexandria where he studied geometry with the followers of Euclid. Though Archimedes belonged to the mathematical School, it would have been quite natural and possible for him to see the inventions of Ctesibios during his stay. Upon his return to Syracuse, he continued to correspond with the scholars of Alexandria, in particular with the mathematician Conon of Samos, and with the director of the Library, Eratosthene.11 This justifies the association of Archimedes’ work with the school of Alexandria.

First and foremost a mathematician, Archimedes was interested in the problem of buoyancy of bodies of arbitrary shape. Extreme rigor distinguishes his work – he first proposes axioms, then demonstrates their consequences. The initial postulate of his work on “floating bodies” introduces the notion of pressure:

“We take as a principle that liquid is of such a nature that, its parts being arranged in an equal and contiguous manner, the part that is the least compressed is displaced from its position by

Archimedes, and the first theoretical formulations of fluid mechanics

Figure 5.5 The two-body pump of Ctesibios of Alexandria, as it is described by Vitruvius: “It is appropriate now to describe the machine of Ctesibios, which lifts water. This machine must be made of bronze. At its base, and quite near to each other, are twin cylinders (A), to which are connected tubes which, forming a fork (B), converge symmetrically to an intermediate chamber (C). In this chamber are two flaps (D), precisely adjusted to cover the upper openings of the tubes; blocking the openings of these orifices, they prevent the return of fluid that air (itis notair, but the waterpressure!) has driven from the chamber. Above this chamber is a cover in the shape of an inverted funnel (E) that is held down by a slotted key to resist the raising effect of the water pressure. Another tube, called a trump, is vertically mounted above the cover. Under the lower openings of the tubes, the cylinders have additional flaps (H) covering openings in their bases. The pistons (I), nested in the upper portions of the cylinders, smoothed and lubricated with oil, are activated by rods (J) and levers (K); [….] thus, from a reservoir at a low location, water can be made to gush forth.”12

a more compressed part, and that each of these parts is compressed by the liquid above it,

I 3

unless the liquid is in a closed receptacle and is compressed by something else.”

As early as on the second page of his treatise, he reaches a remarkable conclusion – the surface of water at rest is not horizontal, but spherical:

“The surface of any liquid at rest will have the form of a sphere having the same center as that of the earth.”[174] [175] [176]

Later on, the pragmatic Roman engineers were perplexed by this proposition: was it

Archimedes, and the first theoretical formulations of fluid mechanics

Figure 5.6 A variation of the pump of Ctesibios: a fire pump from the 18th century (Belidor, 1737) ancient archives of ENPC

then not possible to rely on a water surface to represent a horizontal plane?[177]

Further on in this same treatise, laid out with the same rigor, are the various propo­sitions that comprise the theory of floating bodies and the well-known formulation of Archimedes’ Principle:

“Solid objects which have (for the same volume) the same weight as the liquid in which they are immersed, when released remain submerged in such a way as not to rise to the surface of the liquid or descend further down within it.

“Any solid object lighter than a liquid (for the same volume), released in this liquid, will be submerged to a level such that the liquid which occupies the volume of the submerged por­tion has the same weight as the entire solid body.

“Solid bodies lighter than a liquid (for the same volume), plunged into the liquid by force, are pushed upwards by a force equal to the excess of the weight of the body over the weight of the liquid which occupies the same volume as the solid body.

“Bodies heavier than a liquid (for the same volume), released in the liquid, descend toward the bottom until reaching it, and they are lightened in the liquid by the weight of the liquid contained in a volume equal to that of the solid body.

“If a body lighter than a liquid (for the same volume) is released in the liquid, the ratio of its weight to the weight of the same volume of liquid will be equal to the ratio of the submerged part to the total.”

In his treatise, Archimedes then determines the equilibrium of diverse solids in the form of spheres, hyperboloids of revolution, etc.

To get top plates straight

Posted by on 17/ 11/ 15

The bottom plates of your walls are straight because they’ve been nailed to chalklines. The top plates should be fairly straight if you used straight stock when plating. But walls can deviate from straight at the top plate, and that’s where we need to check them. The easi­est way to line a wall is to hop up on a ladder and look down the top plate to see if it’s straight. A lot of carpentry is done by eye.

Learn to trust vour eve. If something looks

* 4 C7

good, it is good. On long walls, it may be best to stretch a line (string) the length of the wall, as shown in the top photo at right. Nail a 2x block to each end of the wall and stretch the line tightly from one block to the other. Using another scrap of 2x as a gauge, slide it along the wall and check for a consistent 1 i^-in. space between the line and the wall.

To get top plates straightTo straighten a crooked walk you’ll need to move it in or out. To move a wall out, use two Шnails to secure a 2x brace to the nearest

Подпись:To get top plates straightПодпись: This pair of 8d nails clips one piece of wood next to another. Pairs of nails are frequently used to clip a trimmer stud plumb next to a king stud.

and bend the brace upward. As the brace Hexes upward, the top plate moves in. This works especially well on a wall that is badly out of line.

Make sure you use enough temporary – braces as you plumb and straighten the walls to keep every wall in place. Put braces on straight walls as well. Using plenty of braces ensures that the building will be held plumb and straight. Leave all the braces in place until the roof trusses and sheathing have been installed to prevent any frame movement.

Summary and Comments on Draindown Testing

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The methods of draindown testing described in this chapter differ in details. Table 8.4 shows the most important differences among them. Table 8.5 depicts commonly adopted assessment criteria of draindown testing results.

The following remarks deserve mention: [50]

TABLE 8.3

Drain-Off Test Parameters for Schellenberg’s Method according to EN 12697-18

Подпись: Number of samples Test temperature Подпись: Sample weight Подпись: Test time duration Test procedure Test three samples of the same mix with the same binder content.

Test temperatures depend on the binder type:

1. For road binder—at the production temperature of a mix defined according to EN 12697-35 and raised by 25°C

2. For modified binder—at the production temperature of a mix defined by the binder supplier and raised by 15°C

• Mass of an aggregate mix sample—1000 g for a mix with the density of 2.65-2.75 g/cm3

• If the density of an aggregate mix is different from the given reference density, a sample mass should be calculated to obtain the same test material volume

60 ± 1 minutes

1. Prepare three batches of aggregate (batches 1, 2, 3) and place batch in a metal container.

2. Put the beakers in the oven at the test temperature for 15 minutes minimum; then remove them, weigh them with an accuracy of 0.1 g, and return them to the oven.

3. Mix 1 kg of a bituminous mixture at the fixed temperature according to EN 12697-35.

4. Remove the beaker for batch 1 from the oven, quickly put the prepared mix in the beaker, weigh the beaker with the mix with an accuracy of 0.1 g, write down the time and the beaker number, and return the beaker to the oven (it should not be left outside the oven for longer than 60 seconds)

5. Prepare the two remaining batches of the mix in the same way and put them in the beakers.

6. Keep each beaker with the mix in the oven for 60 ± 1 minutes.

7. Remove the first beaker with the mix, measure its temperature, and put the mix aside.

8. Remove the remaining two beakers with the mix from the oven and empty them out by tilting them upside down and holding them in that position for 10 ±1 seconds.

9. After cooling beakers No. 2 and No. 3, weigh them together with the remaining binder with an accuracy of 0.1 g.

10. If more than 0.5% of the initial mass of the mix remains on the beaker walls (including aggregate grains and mastic), the material remaining in the beaker should be washed with the solvent and passed through a 1-mm sieve; next, the material remaining on the sieve should be dried and weighed with an accuracy of 0.1 g.

11. Determine draindown as the percentage of the binder mass remaining in the beaker compared with the mass of the mix.

12. Calculate the material draindown, D, and when appropriate, the material remaining on the 1-mm sieve, R:

Подпись:(W – W – W4)

(W2 – W1)

(Continued)

TABLE 8.3 (CONTINUED)

Drain-Off Test Parameters for Schellenberg’s Method according to EN 12697-18

Подпись: R = 100 xW,

(W – W)

where

D = Material draindown (% m/m)

R = Material remaining on the sieve 1.0 mm (% m/m)

W1 = Mass of the empty beaker (g)

W2 = Mass of the beaker with the mix (g)

W3 = Mass of the empty beaker together with remaining mastic (g)

W4 = Mass of the dry material remaining on the 1.0-mm sieve (g)

13. The average result of two measurements should be given with an accuracy of 0.1%.

14. Results for D and R (if applicable) should be reported.

• Подпись: RemarksNone of the three beakers containing the mix may be kept in the oven for longer than 60 ± 1 minutes.

• While mixing components of the mix, pay attention to the proper sequence of their dosages, particularly fibers, polymers, and so on.

• In the case of modified binder, a lot of mastic can stick to the walls of the beaker and remain there when emptied out (due to increased tackiness of a mix). In such a case, retesting should be conducted at a temperature 5°C higher. If the new result is lower than the previous one, it should recorded in a report.

• If the difference between the test results for two samples of the same mix with the same binder content exceeds 0.5%, a new pair of samples should be tested. [51] [52]

TABLE 8.4

A List of Differences among Methods of Draindown Testing

Test Time Duration

method

Test Temperature (°С)

(min)

Container

Original Schellenberg’s

170 ± 1

60 ± 1

Glass beaker

method

AASHTO T 305-97

Depends on the SMA production

60 ± 5

Wire basket

temperature and type of binder

or 70 ± 5

EN 12697-18 (Part 2)

Depends on the type of binder

60 ± 1

Glass beaker

TABLE 8.5

assessment criteria of draindown Testing Results

drain-off Testing result, % (m/m) assessment

>0.3 Risk of binder draindown

0.2-0.3 Acceptable value

<0.2 Recommended value [53]

8.2 FILLER TESTS

8.3.1 Tests of Specific Surface with the Use of Blaine’s Method

Blaine’s method is chiefly used when testing cement (grinding gradation control). It consists of the measurement of time necessary for air to flow through a compressed layer of tested material of a given size and porosity. At standard conditions, the specific surface is directly proportional to 4t (t = time of air flow). A master sample with a known specific surface is required to calibrate testing. The test can be carried out according to EN 196-6.*

Adsorption/Desorption

Posted by on 17/ 11/ 15

In the road context, adsorption/desorption phenomena greatly influence the fate of pollutants entering the road construction, present therein or transported through road-construction layers and further down. Sorption phenomena are also of importance regarding pollutants possibly leached (dissolved) from some road ma­terials (e. g. alternative materials) under the effect of infiltration, and adsorbed on a surface downstream. Sorption/desorption sequences (under the effect of surface characteristics and seepage pH, for example) can lead to a progressive downward transfer of substances.

Adsorption can be defined as the attachment or adhesion of a molecule or an ion in the gaseous or liquid phase to the surface of another substance (an adsorbent) in the solid phase or to the surface of a soil particle. Desorption describes the process by which molecules or ions move in the opposite direction. Adsorption/desorption is a universal surface phenomenon. It can occur at any surface, e. g. surfaces formed by any type of opening, capillary, crack, depression or other type of physical irregu­larity. The nature of the adsorbing surface plays an essential role in the process. The smaller the size of the soil particles, or the greater the porosity, the more efficiently the adsorption will occur because of the increase in surface area provided. Road pollutants are therefore leached much more quickly through a coarse-textured soil than through a clayey soil (Brencic, 2006). This feature is of particular relevance when traffic accidents involve cargoes of harmful or toxic compounds.

The adsorption/desorption of substances between the liquid form and the sur­face of solid-state materials, such as soil particles, is one of the processes of great­est importance for the behaviour of inorganic and organic substances in the soil. The degree of adsorption increases with the concentration of the substance in the solution outside the adsorbent until a maximum is gradually approached. As the reaction kinetics depend on temperature (adsorption decreases with higher tem­perature because the molecules are more energetic and less easily held by their potential sorbent), the quantitative assessment of adsorption is done by means of so – called isotherms. Various models can be used to interpret isotherms, e. g. Langmuir, Freundlich or Brunauer-Emmet-Teller (BET) (Fig. 6.4) a variant of which is given inEq. 6.16.

S = Qt в C/(1 + в C)

Fig. 6.4 Variation of the sorbed quantity (S) as a function of the concentration of sorbate (C) for different temperatures (T1 >T2>T3) – Langmuir isotherm (adapted from Bontoux, 1993 and Selim & Sparks, 2001)

where S = mass of sorbate sorbed per mass of sorbent (typically in units of mg/kg); QT = maximum sorption capacity of the sorbent at temperature, T(°), в = a variable that is only a function of the temperature, T, and C = aqueous concentra­tion of sorbate (typically in units of mg/l).

Desorption can occur when a “new” ion (or other chemical) arrives at a sorption site and is sorbed, preferentially, over a previously sorbed ion of a different type. Less readily, sorbed species can be desorbed if the concentration of that species decreases in the groundwater around the sorbent.

Time is required for sorption/desorption reactions to become complete. Therefore the approach adopted both in analysis and in testing is to allow sufficient time for equilibrium to develop. Often this will take hours, perhaps days, to complete. Care is required when the input or output condition is changing due, for example, to flow bringing more contaminant. Then, true equilibrium may not be possible. The use of an isotherm approach necessitates the assumption of equilibrium conditions.

More important, though, is that the adsorption is also pH dependent; cations such as most metal ions are more strongly adsorbed at increasing pH. The degree of adsorption rises sharply in a short interval of increasing pH. This is due to the fact that the charge of the particle surfaces is greatly pH dependent. The pH of the soil thus largely regulates the mobility of heavy metals occurring in the soil. With the exception of some amphoteric compounds (e. g. some metal hydroxides) and some oxyanions (e. g. MoO42-, AsO43-), the general rule is that many heavy metals are more mobile at lower pH (Berggren Kleja et al., 2006).

Channel Realignment

Posted by on 17/ 11/ 15

At times it will be advantageous or necessary to realign or change the hydraulic char­acteristics of the channel. Reasons for altering the channel include improving culvert alignment, protecting roadways from erosion damage, reducing maintenance require­ments, and eliminating hydraulic structures where the roadway recrosses the channel.

Plans for channel modifications must include a determination of what effect the change will have on the stream and the surrounding environment. Long – and short-term effects must be considered. The impact on the stream of the realignment or change in slope will vary from one site to another. At some sites, minor changes will have signifi­cant impacts, while at others the opposite may be true. Regardless of the magnitude of the effect on the stream and its environment that the change may have, plans should be developed to mitigate those effects.

Changes to a channel usually cause a decrease in the roughness and an increase in the slope. The resultant higher velocity may lead to increased scour and sedimentation buildup at the downstream end of the channel improvement, and may result in changes that affect the habitat in and around the stream. Any changes to existing streams that support fish or wildlife must be coordinated with the appropriate resource agencies early in the planning phase.

DESIGN OF OPEN CHANNELS

Posted by on 17/ 11/ 15

As the name implies, open-channel flow is concerned with the conveyance of water with a free surface. This article primarily concerns lined and unlined channels such as encountered along roadways in highway design.

5.2.1 General Considerations

The parameters to consider in choice of channel cross-section include hydraulics, safety, maintenance, economics, and the environment. These considerations are usually so inter­dependent that optimizing one can have detrimental effects on the others. The hydraulic engineer’s objective is to achieve a reasonable balance among the competing criteria.

Safety is always of primary concern to the highway engineer. If the channel is located far enough away from the traveled way, an adequate recovery zone may be available for vehicles accidentally leaving the roadway. Additionally, with regard to safety, a channel with flattened sideslopes and a curved transition to the bottom is pre­ferred to allow time for recovery for the errant vehicle. (See Chap. 6, Safety Systems.)

Periodic maintenance is required for hydraulic channels regardless of the cross-sectional design chosen. Access should be planned and provided for maintenance personnel and equipment. The proliferation of sediment and debris and the growth of vegetation can cause erosion or reduction of the capacity of the channel. The channel design should balance the cost of preventing these restrictions against the anticipated increased costs of removing them as they accumulate.

The proposed channel location and shape affect the economics of the project. A channel located away from the traveled way may be safer for the traveling public and more aesthetically pleasing; however, these considerations must be balanced against the potential increase in right-of-way costs as well as other associated costs. The shape also affects the cost of the channel. A channel with vertical sidewalls will typically be more expensive than one with sloping sides; the vertical walls must not only maintain flow within the channel but must also be designed to retain the earth outside the channel.

Proposed channel improvements must take into account the possible effects the project will have with regard to erosion, sedimentation, water quality, aesthetics, and fish and wildlife. Local, state, and federal resources and flood control agencies have an interest in drainage improvements and environmental impacts and should be con­tacted early in the planning process for input, cooperation, and assistance. A partial list of these agencies may be found in the AASHTO Highway Drainage Guidelines.

The necessary hydraulic parameters should be determined early in the design phase. As previously mentioned, the scope of the hydrologic study should be proportional to the importance of the hydraulic structure involved, the type of highway, the impacts on the local property, and potential risks involved. The hydraulic design of the channel involves selecting the cross-section and lining to maintain the flow predicted from the hydrologic study. The capacity of the channel is affected by its size, shape, roughness, and slope.

The slope is generally controlled by the existing terrain, and the engineer has little control over this. As much as is practical, however, the engineer should avoid sudden changes in the slope as well as the alignment of the channel. Abrupt changes in channel alignment can lead to unintentional channel changes by aggradation and avulsion. Abrupt changes in slope can cause either erosion, if the grade is steepened, or an accumu­lation of buildup, if it is flattened.

Erosion and deposition may also be limited by controlling the velocity of the flow. The velocity of the water is dependent upon the size, shape, roughness, and slope of the channel as well as the quantity of flow. Recommended flow velocities for unlined

TABLE 5.5 Recommended Permissible Velocities for Unlined Channels

Permissible velocity, ft/s

Permissible velocity, m/s

Type of material in excavation section

Intermittent

flow

Sustained

flow

Intermittent

flow

Sustained

flow

Fine sand (noncolloidal)

2.5

2.5

0.76

0.76

Sandy loam (noncolloidal)

2.5

2.5

0.76

0.76

Silt loam (noncolloidal)

3.0

3.0

0.91

0.91

Fine loam

3.5

3.5

1.07

1.07

Volcanic ash

4.0

3.5

1.22

1.07

Fine gravel

4.0

3.5

1.22

1.07

Stiff clay (colloidal)

Graded material (noncolloidal)

5.0

4.0

1.52

1.22

Loam to gravel

6.5

5.0

1.98

1.52

Silt to gravel

7.0

5.5

2.13

1.67

Gravel

7.5

6.0

2.29

1.83

Coarse gravel

8.0

6.5

2.44

1.98

Gravel to cobbles, under 6 in (150 mm)

9.0

7.0

2.74

2.13

Gravel and cobbles over 8 in (200 mm)

10.0

8.0

3.05

2.44

Source: From Highway Design Manual, California Department of Transportation, with permission.

channels are shown in Table 5.5. Velocities in lined channels can generally be much greater. To minimize deposition of sediment, the minimum gradient should be about 0.5 percent for earth-lined and grass-lined channels and 0.35 percent for paved channels. Also, decreasing gradients should be avoided.

Medium Density Fiberboards

Posted by on 17/ 11/ 15

Medium density fiberboard (MDF) is sheet – good material used for interior nonstruc­tural applications such as cabinetry boxes and shelving. Traditionally, MDF has been bound with urea-formaldehyde-based glues, making their use unacceptable in a healthy house. The following products are formaldehyde-free and may be available laminated with plastics or hardwood veneers:

• Arreis: Scientific Certification System cer­tified medium density fiberboard with no formaldehyde added in the manufacturing process

• Glacier Clear: Scientific Certification Sys­tem certified medium density fiberboard

with no formaldehyde added in the manu­facturing process

• Medex or Medite II: Scientific Certifica­tion System certified medium density fi­berboard manufactured without formal­dehyde

• Wheatboard or Wheatstraw: Medium density fiberboard made from straw fiber and containing no incremental formalde­hyde emissions

Cabinets

Although the drawers and doors on cabine­try are often made of solid wood, the boxes or cases are usually composed of particleboard, interior-grade plywood, or melamine, which has a particleboard core that is exposed where holes have been drilled for adjustable shelving. Cabinets are most often finished with solvent – based finishes that may outgas high levels of VOCs for several months.

Because standard cabinetry contributes significantly to poor indoor air quality, it is not acceptable in the healthy house. You will pay more for healthier cabinets, but in terms of indoor air quality, this is money well spent. If your budget is tight, we suggest you explore strategies that will reduce the amount of cabi­netry necessary. For example, you may choose to consolidate some of your kitchen storage in a pantry area, or to use attractive solid wood open shelving for dishes or cookware as a less expensive alternative replacing some of the upper cabinets.

Finishes on wood cabinet doors and draw­ers are commonly solvent-based applica­tions that will take many months to fully cure. Some of the lacquer finishes, although odor­ous when first applied, will completely cure before they are brought to the jobsite and will not require refinishing for many years. If these are applied in quality controlled, well – ventilated shops, they may be worth consid­ering. Ask for a recently applied, dated sample to help make your determination. Low-VOC, water-based finishes that are more suitable for jobsite or factory application are listed in Di­vision 9.

As cabinetmakers are becoming more fa­miliar with the need for healthier cabinetry, and as low-VOC finishes and materials be­come available, the price gap between stan­dard and healthy cabinets is decreasing. The following companies manufacture formalde­hyde free or low emissions cabinetry:

• Cervitor: Distributors of metal cabinetry with a baked-on enamel finish that may be used with metal or solid wood doors and drawers

• Core Home: Cabinets manufactured to European Ei emissions standards with FSC certified products

• Humabuilt Wheat Core Cabinets: Press ed wheatboard boxes, premium-grade woods and veneers assembled with ultra-low – VOC adhesives and finishes, free of syn­thetic formaldehyde

• Neff Cabinets: Manufactured cabinets with a 98% reduction in formaldehyde content

• Neil Kelly Cabinets: Cabinetry system designed to meet the needs of the chemi­cally sensitive, with cores of wheatboard or Medite II with wood veneers, and a wide variety of door and case veneers. Door and drawer fronts available with certified woods, prefinished with AFM clear sealers or paints.

For all options, specify the use of a solvent-free carpenter’s glue in the fabrication process.

The following products are formaldehyde – free sheetgoods that can be used for making cabinet boxes:

• Environ Biocomposites: Line of formal­dehyde-free, agri-based sustainable panel

CASE STUDY 6.2