Construction Technology -B 2010 1 Project (A) A two storey commercial building is proposed for super market and the building is decided to construct in the heart of the city Kandy, which a steel structural frame work and precast concrete panels. Task- 01 1.1, 1.2 Steel has become a universal building product due to its strength, versatility, durability and economic value. Among its most popular uses today are standing seam metal roofs. Standin g seam metal roo fs are fast becoming the material of cho ice f or countless structur es. In fact, according to the American Iron and Steel Institute, the roofing system has been used in nearly 50% of all low-rise commercial, industrial and institutional buildings erected in the last several years. Standing seam metal r oofing can now be seen on virtually every type of building, from shopping centers and schools to churches and libraries. The system currently accounts for well over one billion square feet. This acceptance has carried over to the re -roofing market where standing seam roofs have been used successfully as replacements for built-up and single ply systems. In retrofit pro jects where costly tear -offs want to be avoi ded, a sub -framing system is attache d to the existing roof surface to pro vide a minimum ¼:12 pitc h for the new metal roof. Steel Provides Aesthetic Appeal, Long Service Life The use of metal roofing is growing rapidly because steel offers a variety of benefits. One is its aesthetic appeal. The standing seam roof is one of the most attractive roofing systems foralmost any building. It is available in a wide range of finishes, color and pr ofiles, providing building owners and architects with extensive design flexibility. Another of metal roofing¶s benefits is its long service life. As a result of the zinc, aluminum, oraluminum-zinc alloy metallic coating ap plied to the base st eel, toda y¶s metal roof panels resist corrosion and pr ovide a service life of 20 years or more of troub le -free per formance, considera bly longer tha n the standard pr otection fo r built -up and single ply systems.
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A two storey commercial building is proposed for super market and the building is decided to
construct in the heart of the city Kandy, which a steel structural frame work and precast
concrete panels.
Task- 01
1.1, 1.2
Steel has become a universal building product due to its strength, versatility, durability andeconomic value. Among its most popular uses today are standing seam metal roofs.
Standing seam metal roofs are fast becoming the material of choice f or countless structures. Infact, according to the American Iron and Steel Institute, the roofing system has been used innearly 50% of all low-rise commercial, industrial and institutional buildings erected in the lastseveral years. Standing seam metal r oofing can now be seen on virtually every type of building,from shopping centers and schools to churches and libraries. The system currently accounts
for well over one billion square feet.
This acceptance has carried over to the re-roofing market where standing seam roofs have
been used successfully as replacements for built -up and single ply systems.
In retrofit projects where costly tear -offs want to be avoided, a sub -framing system is attachedto the existing roof surface to provide a minimum ¼:12 pitc h for the new metal roof.
Steel Provides Aesthetic Appeal, Long Service Life
The use of metal roofing is growing rapidly because steel offers a variety of benefits. One is its
aesthetic appeal. The standing seam roof is one of the most attractive roofing systems for almost any building. It is available in a wide range of finishes, color and profiles, providing
building owners and architects with extensive design flexibility.
Another of metal roofing¶s benefits is its long service life. As a result of the zinc, aluminum, or aluminum-zinc alloy metallic coating applied to the base st eel, today¶s metal roof panels resistcorrosion and provide a service life of 20 years or more of trouble -free performance,considerably longer than the standard protection for built -up and single ply systems.
Metal roofs are also very cost effective. Standing seam steel roofs pay for themselves from theday they are installed. Their life expectancy is long, they require little or no maintenance, andtheir life cycle costs are low, especially compared to non-metal alternatives in low slopeapplications.
Metal roofs are also energy efficient, especially when used in ³cool roof´ applications. Coolmetal roofs feature heat-deflecting coatings that decrease unwanted heat build -up inside abuilding, thereby reducing cooling loads. Based on research on cool metal roofing, thereflectivity and emissivity of steel roofs have been proven to provide significant savings in
energy consumption.
Environmental Benefits of Steel Are Many
While building owners and archi tects have long recognized steel for its strength, durability andfunctionality, they are now increasingly recognizing another of steel¶s important attributes ± its
environmental benefits.
The recycled content for steel used in metal roofs and walls, for e xample, is at least 25%. Thislevel of recycled content reduces both the cost and environmental impact of making new steel,as it conserves energy and other natural raw materials.
The fact that the recycled content of steel is at least 25% by weight helps earn points in theU.S. Green Building Council¶s Leadership in Environmental and Energy Desgin (LEED)program. Steel¶s recycled content is especially important when it is compared to other materialssuch as concrete, which has a recycled content of only 3 % (fly ash) and even less when theweight of the recycled material is factored in.
Steel Is 100% Recyclable at End of Useful Life
Steel is also 100% recyclable at the end of its long, useful life. In fact, of the metals used inroofs and walls, steel is the most recycled. Easily separated from other materials via magnetic,
steel is reclaimed through a vast collection and processing n etwork.
Every ton of steel recycled saves over 4,000 pounds of raw materials, including 2,500 poundsof iron ore, 1,400 pounds of coal and 120 pounds of limestone. And, according to the EPA,new steel made with recycled material uses as little as 26% of t he amount of energy that wouldbe required to make steel from iron and other materials extracted from nature. In addition, the
original embodied energy of steel products is amortized as steel is recycled again and againinto new steel products.
Main frame
The frame includes the following components: Inner frames, End walls, Anchor bolts Wind
members on roof and in walls; Generally, the inner frame is a rigid frame made of weldedmembers and columns jointed by means of high -strength bolts (10.9 Class).The joints between
column base plates and foundations are pinned. Joint may be fixed, Extra painting is optional.
Structures are treated using shot -blasting (SA 2.5 degree) and painted with a primer (of grey or
red color).
Secondary frame
The frame is used for supporting wall cladding and roof systems. The secondary frame
comprises a system of grits, purling & eave struts Z- or C-shaped hot deep galvanized cold
formed structural sections and secured to the main frame by bolts (4.6 Class).Th is system
increases frame capability when using together with flange braces. The steel used is of 390
MPa yield strength.
Roofing
The roof system is composed of: Outer steel sheet with Super polyester protective coating.
Waterproofing lay. Insulation of thickness required. Barrier against vapor penetration. Inner
steel sheet - as required. Set of self -tapping screws with heads of the panel -matching color
and all insulating and sealing elements required.
PVC Membrane
PVC membrane is used for covering roof of 15% slope. The PVC membrane comprises a
solid welded homogenous coating made of material strips secured to the base and
overlapped. The PVC membrane is vapor -proof: it lets vapor out but holds water. The PVC
membrane roof consists of: The base coated with vapor insulant. Insulant. Geotextile (when
polystyrene is used in the insulant layer). PVC membrane.
Wall cladding systems
The standard wall cladding system is structurally similar to the SRR 1 roofing system: Outer
steel sheet with super polyester protective coating. Waterproofing lay. Insulation of
thickness required. Barrier against vapor penetration. Inner steel sheet - as required. Set of
self-tapping screws with heads of the panel -matching color and all insulating and sealing
In the types above the frame spacing is in -between 4-12 m
Application:
Buildings with specific column spacing
Multi-storeybuiding Application:
Multi-tier parking facilities and shopping centers.
y Precast Concrete work in the construction
Precast concrete consists of concrete (a mixture of cement, water, aggregates and admixtures)
that is cast into a specific shape at a location other than its in-service position. The concrete is
placed into a form, typically wood or steel, andcured before being stripped from the form,
usually the following day. These components are then transported to the construction site for
erection into place.
Precast concrete components are reinforced with either conventional reinforcing bars, strands
with high tensile strength, or a combination of both. The strands are pre-tensioned in the form
before the concrete is poured. Once the concrete has cured to a specific strength, the strandsare cut (de-tensioned). As the strands, having bonded to the concrete, attempt to regain their
original un-tensioned length, they bond to the concrete and apply a compressive force. This
³pre-compression´ increases load-carrying capacity to the components and helps control
cracking to specified limits allowed by building codes.
Precast components are used in various applications and projects of all types.
y Beams
Beams are typically considered structural components and are made in one of three key
shapes:
1. Rectangular
2. Inverted Tee Beams
3. L Beams
Beams are horizontal components that su pport deck members like double tees, hollow-core,
and solid slabs, and sometimes other beams. They can be reinforced with either pre -stressing
strand or conventional reinforcing bars. This will depend on the spans, loading conditions, and
the producer¶s preferred production methods.
Columns
Columns are typically used to support beams and spandrels in applications such as parking
structures and total-precast concrete structural systems of all types. They typically are
designed as multilevel components rangin g from a single story to six levels or more.
Mullions are thin, often -decorative pieces that fill open space in a building facade. They are
often isolated elements forming a long vertical line, requiring them to be cast perfectly straight
to avoid any visual deformities. To some degree, these variations can be handled by precast
concrete connections with adjustability.
Piles
Precast, pre-stressed concrete pilings are often the preferred choice for permanent , durable,
and economical foundations, espec ially in marine or bridge environments, due to their excellent
adaptability and resistance to corrosion.
Piles can be spliced together to create longer piles. They are used primarily where longer piles
are required but transportation needs make the longer lengths more difficult or costly to handle
due to escort needs and the need for specialized rigs.
Shear Walls
Shear walls act as vertical cantilever beams, transferring lateral forces acting parallel to the
face of the wall from the superstructure to the f oundation. Typically, there are two shear walls
oriented to resist lateral loads along each principal axis of the building. They should be
designed as load bearing panels.
Solid Slabs
Solid slabs are used as structural deck components similar to hollow -core slabs. They can be
made in a long-line pre-tensioning facility and reinforced with pre -stressing strand or cast in
individual forms with either pre -stressing strand or conventional reinforcing bars.
They are typically cast in the same position as used i n the structure.
y Raker Beams-Raker beams are angled, notched beams that support stadium riser
units. They are used universally in outdoor stadiums and arenas and in many indoor
arenas and performing -arts theaters.
y Stadium Risers -Stadium risers are used to support seating in stadiums, arenas,
theaters, and other types of grandstands.
Typically, they are made as single, double, or triple risers with heights cast to satisfy site lines
in the venue. Specifying single, double, or triple risers will depend on the layout and may be
dictated by weights and crane access during construction
y
Stairs-Precast concrete stairs are used in any application where a stair tower or individualsteps are required. These modules c an provide fast erection and durable access in buildings
or parking structures.
Wall Panels - Wall panels can be strictly architectural, strictly structural, or a combination of
both. They can be placed in either a horizontal position, as in a multifamily housing application
or in a vertical position, as in the exterior of a warehouse. Wall panels can be load bearing and
support floor and roof components or they can be no load bearing to complete a façade.
The faster the fire service can respond, enter ,locate the incident, and safety operate in a
building, the sooner they can mitigate an incident in a safe manner for themselves as well as
occupants.
Fire Apparatus access.
Properly fire Apparatus can be critical at a fire scene. In particular, placing aerial Apparatus is
critical for positioning of the aerial ladder or elevating platform, which is mounted on top of
these vehicles. Pumper Apparatus also need to get close enou gh to the building to facilitate
hose line use. The location of other specialized Apparatus, or small vehicles, such as chief¶s
cars or ambulances, should only be of particular concern to the designer of unusual facilities.
For instance, a sports arena may need to be designed for entry of ambulances but not fire
Apparatus.
There are many considerations for both public roads and fire lanes: clear width, clear height,length, turn radius, arrangement, distance from the building, and paving materials.
Extent of Access
Minimum building access for fire Apparatus is a function of the access road reaching to within a
certain distance of all portions of the building¶s first floor exterior walls.
Perimeter Access
The options available for attacking a fire increase as more of a building¶s perimeter becomes
accessible to fire Apparatus.
Clear width
The basic clear width requirement for Apparatus access in the IFC and NFPA 1 is 20feet.
NFPA 1141 calls for one way fire lanes that are 16 feet wide; however, this applies t o roads
that do not about buildings. A clear width of 20ft will allow most aerial Apparatus to extend the
outriggers necessary to support the aerial ladder or elevating platform while in operation.
However, some recently manufactured aerial apparatus require 24 ft of clear width for outrigger
extension.
Lanes wide enough for apparatus to pass one another will facilitative developing and
expanding operations. NFPA 1141contains a 24 ±foot clear width requirement for two -way fire
lanes. Appendix D of the IBC calls for a 26 ±foot clear width at fire hydrant locations, extending
for distance of 20ft in both directions. As well as a 26- foot width in the vicinity of buildings that
are 30feet or more in height (for aerial operations.)Rolled or rounded curbs adjacent to properly
designed sidewalks can effectively increase access width. These allow apparatus to easily
In areas with aerial apparatus that may respond to an emergency, the road or fire lane should
be positioned at a distance from the building that will accommodate aerial ladder operation.
Access too close or too far from the building will limit aerial ladder use. Where a fire lane is
parallel to a building that is more than 30ft high.
Loads ± All access roads or lanes should be built to withstand the loads presented by modern,
heavy fire apparatus as well as potential weather conditions. Paved surface, bridges, and other
elevated surfaces (such as piers or boardwalks) should be designed to handle the weight of a ll
apparatus that may use them. The IFC Appendix D has a load design requirements of 75000
pounds.
Materials ± All weather paved access s the best surface some jurisdictions permit the use of
paved blocks or subsurface construction for fire lanes. These permit an area to be partially or
fully landscaped, while being strong enough to allow fire apparatus to negotiate the area.
Manual gates cause inherent delays because personnel must dismount to unlock them or cut
through chains. However, they can also hel p keep the fire access lane clear by preventing
vehicle parking.
Fire Hydrants
Optimal positioning, spacing, location, and marking of fire hydrants can aid the fire service
during emergency operations. Public fire hydrants are often under the purview of a local water
authority, standards for fire flow and other criteria.
Spacing ± Maximum distance between hydrants differs greatly, depending on various local
standards.
Location ± Pumpers may utilize hydrants in different ways. If the fire is close enough, apumper can be positioned at a hydrant and use a large diameter suction hose.
Marking ± A number of methods are used to enable firefighters to rapidly identify hydrant
locations. The color used for hydrants should contrast as much as possible with the
predominating surroundings. Some localities place reflective tape around the hydrant body .
Other jurisdictions mount reflectors (usually blue) in the roadway in front of each hydrant:
however, in cold weather climates these refle ctors are often obstructed by snow. The best way
to identify hydrants in areas subject to snowy weather is a locator pole which is visible above
the highest expected snowfall. These are reflective or contrasting in color, and some have a
flag, sign, or reflector mounted on top.
Firefighter Access
Once firefighter have arrived and positioned their apparatus, they must go to work. Some
factors affecting their efficiency include: the distance and terrain between the apparatus access
and the building: how easily they can enter the building: the buildin g¶s interior layout and
vertical access (stairs / elevators/ roof access): and, how quickly firefighters can locate fire
Bonded post-tensioned concrete is the descriptive term for a method of applying compression
after pouring concrete and the curing process (in situ). The concrete is cast around plastic,
steel or aluminum curved duct, to follow the area where otherwise tens ion would occur in the
concrete element. A set of tendons are fished through the duct and the concrete is poured.
Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react
against the concrete member itself. When the tendons have stretched sufficiently, according to
the design specifications (Hooke s law), they are wedged in position and maintain tension after
the jacks are removed, transferring pressure to the concrete. The duct is then grouted to
protect the tendons from corrosion.
This method is commonly used to create monolithic slabs for house construction in locations
where expansive soils (such as adobe clay) create problems for the typical perimeter
foundation. All stresses from seasonal expansion and contraction of the underlying soil are
taken into the entire tensioned slab, which supports the building without significant flexure.
Post-tensioning is also used in the construction of various bridges, both after concrete is cured
after support by false work and by the assembly of prefabricated sections, as in the segmental
bridge .The advantages of this system over unbounded post-tensioning are: Large reduction in
traditional reinforcement requirements as tendons cannot distress in accidents.
Tendons can be easily weaved allowing a more efficient design approach. Higher ultimate
strength due to bond generated between the strand and concrete. No long term issues with
maintaining the integrity of the anchor/dead end.
Hooke's law
In mechanics, and physics, Hookes law of elasticity is an approximation that states that the
extension of a spring is in direct proportion with the load added to it as long as this load does
not exceed the elastic limit.
Wedge (mechanics)
A wedge is a triangular shaped tool, a compound and porta ble inclined plane, and one of the
six classical simple machines. It can be used to separate two objects or portions of an object,
lift an object, or hold an object in place. It functions by converting a force applied to its blunt
end into...
Grout
Grout is a construction material used to embed rebars in masonry walls, connect sections of pre-cast concrete, fill voids, and seal joints. Grout is generally composed of a mixture of water,
cement, sand and sometimes fine gravel.
Corrosion
Corrosion is the dis integration of a material into its constituent atoms due to chemical reactions
with its surroundings. In the most common use of the word, this means a loss of electrons of
metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron.
False-work consists of temporary structures used in construction to support spanning or arched
structures in order to hold the component in place until its construction is sufficiently advanced
to support itself.
Segmental bridge
As its name implies, a segmental bridge is a bridge built in short sections , i.e., one piece at a
time, as opposed to traditional methods that build a bridge in very large sections. Un -bonded
flat slab construction , Un-bonded anchors in banded layout
Un-bonded Mono-strand System
Supply of mono strand system using 15.7mm and 15.2mm Dyform/Compact strand Installation
of mono strand post tensioning system Un -bonded live anchors installed Sloping un -bonded
post-tensioned on slab edge form -work, roof awaiting concrete pour
Un-bonded post-tensioned concrete
Un-bonded post-tensioned concrete differs from bonded post -tensioning by providing each
individual cable permanent freedom of movement relative to the concrete. To achieve this,
each individual tendon is coat ed with a grease (generally lithium based) and covered by a
plastic sheathing formed in an extrusion process. The transfer of tension to the concrete is
achieved by the steel cable acting against steel anchors embedded in the perimeter of the
slab.
The main disadvantage over bonded post -tensioning is the fact that a cable can destress itself
and burst out of the slab if damaged (such as during repair on the slab). The advantages of this
system over bonded post-tensioning are: The ability to individually ad just cables based on poor
field conditions (For example: shifting a group of 4 cables around an opening by placing 2 toeither side). The procedure of post -stress grouting is eliminated. The ability to de -stress the
tendons before attempting repair work.
Extrusion - Extrusion is a process used to create objects of a fixed cross -sectional profile. A
material is pushed or drawn through a die of the desired cross -section.
Lithium - Lithium is a soft, silver -white metal that belongs to the alkali metal group of
chemical elements. It is represented by the symbol Li, and it has the atomic number three.
Under standard conditions it is the lightest metal and the least dense solid element.
Applications
Pre-stressed concrete is the predominating material for floors in high-rise buildings and
concrete chambers in nuclear reactors.Un -bonded post -tensioning tendons are commonly used
in parking garages as barrier cable. Also, due to its ability to be stressed and then de -stressed,
it can be used to temporarily repair a damaged building by holding up a damaged wall or floor
Water is the most important factor for survival of man and animals. A person can do withoutfood for five weeks or more, but without water he can survive for only a few days.
y Your water and your health gives a listing of the ten basic kinds of water,
1. Hard water. 6. Soft water.
2. Boiled water. 7. Filtered water.
3. Raw water. 8. Reverse osmosis.
4. Rain water. 9. De- ionized water.
5. Snow water. 10. Distilled water.
First I would like to explain bit of think about water such as,
y Per capital demand as per IS ± 1172- 1963
Purpose Requirements in litters per head per day
Domestic 185
Public use 15
Firefighting 15
Industrial use 50
Wastage & Losses 50
Total 270Lits/H/D
Water supply in buildings
Many administrative authorities controlling water supply have their own set of bye - laws, rule
and regulation. While Laying pipe lines or plumbing systems these bye-laws should be strictly
confirmed. For intermittent supplies a minimum storage of half days supply is needed for
overhead tanks. IS-2065-1072 deals with water supply of building and covers general
requirements and regulations for water supply, Plumbing, connected to public water supply,
inspection and maintained water fittings and appliances.
joist with minimal disturbance to the decorations. There is a tendency to feel that pull switches
are only suitable for bathrooms etc, however this limits the opp ortunities and should be
avoided. When a new light is to be positioned over a work surface or even an external light
fitted, there is no reason why a pull switch should not be mounted in any convenient position.
2 WIRING METHODS
There are 2 basic wiring methods of wiring lights- by ceiling rose and by junction box. Systems
using the ceiling roses make all the connections at the ceiling rose. While this removes the
need for one junction box per lamp, it is often more awkward for the average diy¶er.
RING CIRCUIT
The ordinary wall sockets around the house are normally connected to a ring circuit (also
referred to as a ring main). The ring circuit of a domestic property supplies the socket outlets
and fixed appliances in the premises.
The µring¶ is formed by the cable going from the consumer unit to the first socket, than on to the
second socket and than the next socket etc. Unit the cable returns to the consumer unit. Thismeans (in simple terms) that every socket on the ring circuit consist of a red (live) wire, a block
( neutral) wire and a bars copper earth wire, all three being enclosed by an outer PVC
sheathing. The cable used in domestic ring circuits is either 2.5 sq mm or 4.0sq mm twis core
and earth, these are rated (in free air) at 24am p or 32 amps respectively. Each ring circuit is
protected by a 32amp fuse or trip fitted in the consumer unit. Modern installations incorporate a
residual current device (RCD) before the consumer unit which trips the whole system off if a
fault is detected.
A ring circuit is considered to be rated at 30amps (7200 walls). A ring may serve up to 100m sq
of floor area and, in theory, may have any number of sockets outlets or fused connection units
connected to it. Wire each socket outlet is normally rated at 1 3 amps, as a µrule of thumb¶ they
are limited to under twenty outlet; it is unlikely that the variety of domestic appliances being
used at any one time will exceed 30amps. The length of cable used in a ring circuit is limited to
50 m for circuits protected by an MCB. The sockets are normally mounted flush with the wall
although surface mounted boxes are often easier to fit when sockets are added to the circuit. It
is advisable to have two ring circuits in all premises, in multi floor houses, one for each fl oor.
RING CIRCUIT FUSED OUTLET UNIS
Where connection to a fixed appliance is required, a fused outlet unit may be fitted to the wall
(rather then a plug socket) and connected into the ring main. These u\outlets require the
correct fuse rating for the appliance and are connected to the appliance by a cable or flex. The
outlet may be switched or unswitched and may be fitted with an indicator light to show whenthe supply is connected. Where a flex is taken to heater of any sort (e.g. night store heater) the
flex must be of a special µhigh temperature¶ type suitable for the elevated temperatures
ine of communication travelled way) using a stabili ed base other than rails or air str ips open
to pub lic traffic, pr imar ily f or the use of road motor vehicles running on their own wheels.Context:
Included are br idges, tunnels, suppor ting structures, junctions, crossings and interchanges. oll
roads are also included. Excluded are dedicated cycle paths.
A road is an identifiable route, way or path between places. oads are typically smoothed,
paved, or otherwise prepared to allow easy travel; Road surface Br itish English) or pavement
Amer ican English) is the durable sur f ace mater ial laid down on an area intended to sustain traffic veh icular or f oot traffic). Such sur f aces are f requently marked to guide traffic. he most
common modern paving methods are asphalt and concrete. In the past, br ick was extensively
used, as was metalling. oday, permeable paving methods are beginning to be used more f or low-impact roadways and walkways.
The most common modern paving methods are asphalt and concrete.
Histor ically, pavements have been divided into two broad categor ies, r igidand flexible. hese
classical definitions, in some cases, are an over -simplification. However , the terms r igid and
flexible provide a good descr iption of ho w the pavements react to loads and the environment.
Sub-bases and Sub-grades
A reasonably unif orm sub-grade or sub-base, with no abrupt changes in suppor t, is ideal f or
an y concrete pavement. Most native soils are not too unif orm and thus require some
improvement or additiona l layers to compensate.A sub-base is a thin layer of mater ial placed
on top of the prepared sub-grade.Sub-bases provide unif orm suppor t to the pavement and a
stable platf orm f or construction equipment. Sub-ba ses also help prevent movement of sub-
grade soils at transverse pavement joints in roads sub ject to a large volume of truck traffic.
Sub-bases may be gravel, stone, cement-modified soil, asphalt, or econocrete low-strength
concrete). he sub-base and base courses may be constructed f rom more than one individual
layer of the specified mater ial, but f or our purposes, we will consider each layer as being a
single entity. Sometimes, , levels permitting, an existing bitmac or concrete sur f ace can be
over laid with a new sur f ace course. his is acceptable provided that the new sur f acing is
Surface drainage is concerned with removing all water that is present on the pavement surface,
shoulder surface or any other surface from which it may flow onto the pavement. If not
systematically removed, this water can accumulate underneath and weaken the pavement
structure. There are three primary means used to prevent water infiltration and accumulation:
Impermeable HM A. HM A tends to be impermeable below about 8 percent air voids, therefore
proper compaction practices should be followed to ensure an impermeable pavement. Also,
minor cracks in the HM A should be promptly sealed.
Slope. The pavement section should be sloped to allow rainwater to sheet flow quickly to the
edge where it is typically collected in a curb and gutter system or a roadside ditch. A generally
accepted standard is a 2 percent cross slope.
Grade. The curb and gutter or roadside ditch must be properly graded to allow flow to central
collection points such as catch basins or detention ponds. A generally accepted standard is a
grade of 0.5 percent or more although lesser grades have been used effectively.
Subsurface Drainage
Subsurface drainage is concerned with removing water that percolates through or is contained
in the underlying subgrade. This water, typically the result of a hig h water table or
exceptionally wet weather, can accumulate under the pavement structure by two chief means:
Gravity flow . Water from surrounding areas can be absorbed by the soil then flow by gravity to
areas underneath the pavement structure. In pavement with high air voids (above 8 ± 9
percent), water can percolate down through the pavement structure itself.
Capillary rise. Capillary rise is the rise in a liquid above the level of zero pressure due to a net
upward force produced by the attraction of the water molecules to a solid surface (e.g., soil).
Capillary rise can be substantial, up to 20 ft. or more. In general, the smaller the soil grain size,
the greater the potential for capillary rise. Often, capillary rise is a problem in areas of high
groundwater tables.
Most pavements have performed adequately without considering these effects. However, HM A
pavements can fail because of subgrade support deterioration as a result of excessive
moisture or other water-related problems. These issues can be addressed in two manners:
1. Minimize water infiltration into the pavement structure . In most cases, the accumulated water
in the underlying subgrade that causes the damage comes from surface infiltration. Thisinfiltration can be minimized by providing proper roadside drainage and minimizing air voids
within the HM A.
2. Provide subsurface drainage . This needs to be done judiciously, because it may be somewhat
akin to treating the symptom rather than the problem. Subsurface drainage consists of three
basic elements A permeable base to provide for rapid removal of water which enters the
pavement structure. Based on recent research from California, permeable base layers ma y
strip and become clogged with fines thus weakening the overall pavement structure. A method
of conveying the removed water away from the pavement s tructure.
Retaining walls provide lateral support to vertical slopes of soil. They retain soil which
would otherwise collapse into a more natural shape. The retained soil is sometimesreferred to as backfill.Retaining walls can be constructed of many different materials and
with a variety of
Various types of retaining walls
Gravity walls depend on the weight of their mass (stone, concrete or other heavy material)
to resist pressures from behind and will often have a slight batter setback, to improve
stability by leaning back into the retained soil. For short landscaping wal ls, they are often
made from mortarless stone or segmental concrete units (masonry units)
Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing in frost
areas.Prior to the introduction of modern reinforced -soil gravity walls, cantilevered walls
were the most common type of taller retaining wall. Cantilevered walls are made from a
relatively thin stem of steel -reinforced, cast-in-place concrete or mortared masonry (often in
the shape of an inverted T).
These walls cantilever loads (like a beam ) to a large, structural footing, converting
horizontal pressures from behind the wall to vertical pressures on the ground below.
Sometimes cantilevered walls are butressed on the front, or include a counterfort on the
back, to improve their strength resisting high loads. Buttresses are short wing walls at rightangles to the main trend of the wall. These walls require rigid concrete footings below
seasonal frost depth. This type of wall uses much less material than a traditional gravity