Substation in Building

Electrical substations shall normally be required in case of office buildings with a total plinth(covered) areas of 5000 sqr. m; even buildings with smaller plinth (covered) areas but with large loading may require a substation, the load limit being set by regulations in the Electricity Act or by the relevant electrical utilities.

To arrive at the of the substation required, a load factor of 70% shall be applied to the estimated load of the building, unless future expansion requirements dictate that a higher figure be considered.

Location

In a multi-storied building, the substation shall preferably be installed on the lowest floor level, but direct access from the street for installation or removal of the equipment shall be provided. The floor level of the substation or switch room shall be above the highest flood level of the locality. Suitable arrangements should exist to prevent the entrance of the storm or flood water into the substation area.

It is preferable to locate the electrical substation adjacent to the the air-conditioning plant room(if any) in such a way that the distance from the controlling switchboard of the air-conditioning plant rooms and corresponding switches in the electrical substation are kept minimum.

In case of a building complex, or a group of buildings belonging to the same organization, the substation should preferably be located in a separate building and should be adjacent to the generator room, if any. Location of substation in the basement floor should be avoided. In case the electric substation has to be located within the main building itself for unavoidable reasons, it should be located on ground floor with easy access from outside.
For transformers having large oil content ( more than 2,000 litres), soak pits are to be provided.

The minimum area required for substation and transformer room for different capacity are given in Table-1.

Table-1 : Area Required for Transformer Room and Substation for Different Capacities

Capacity of Transformer (Kva)	Transformer Room Area( m2)	Total  Substation Area (with HT, LT Panels &;Transformer Room but without Generators) (m2)
1 x 150	12	42
1 x 250	13	45
2 x 250	26	90
1 x 400	13	45
2 x 400	26	90
3 X 400	39	135
2 X 630	26	90
3 X 630	39	135
2 x 1000	26	90
3 X 1000	39	135

The minimum height of the substation room shall be 3.6 m.

Layout

In allocating the areas within a substation, it is to be noted that the flow of electric power is from supply company network to HT room, then to transformer and finally to the low voltage switchgear room. The layout of the rooms shall be in accordance with the flow.

The area given in Table-1 hold good if they are provided with windows and independent access doors in accordance with local regulations.

All the rooms shall be provided with partitions up to the ceiling and shall have proper ventilation. Special care should be taken to ventilate the transformer rooms and where necessary louvers at lower level and exhaust fans at higher level shall be provided at suitable locations in such a way that cross ventilation is maintained.

Arrangement shall be made to prevent storm water entering the transformer and switch rooms through the soak pits, if the floor level of the substation is low.

Provision for Standby Supply 

In buildings where interruption of electrical power supply would result in panic, hazard to life and property or major production loss, provision should be made for standby power supply.

The capacity of standby generating set shall be chosen on the basis of essential light load, essential air-conditioning load, essential equipment load and essential services load, such as one lift out of a bank of lifts, one or all water pumps, etc. Table-2 shows minimum generator room area requirements for different sizes of generators.

The generating set should be housed in the substation building to enable transfer of electrical load quickly as well as to avoid transfer of vibration and noise to the main building. The generator house should have proper ventilation and fire fighting equipment installed.

Individual Water Supply System

In the absence of a public water supply, the individual potable water source shall be used to supply water in a distribution system. The following water sources may be used for individual water supply purposes:

1. Drilled well
2. Dug well
3. Driven well
4. Spring
5. Infiltration galleryWater requirements

The capacity of source shall be sufficient to meet water supply requirements ( will be discussed in next blog).

Water quality

Water from developed well or cistern shall meet the potable water water quality standard requirements specified by WHO.

Chlorination

The well or cistern shall be chlorinated after their construction or repair.

Location of water source

The minimum distance of water source and pump suction line from potential sources of contamination shall be in accordance with table-1.

Well construction

Location of water table

The individual water supply shall not be developed from a water bearing stratum with water table at a depth less than 3 m below the ground surface.

Outside casing

The outside water tight casing shall have to be installed for each well up to a depth of at least 3 m below the ground surface and shall project at least 150 mm above the ground surface. The lower end of the casing shall be sealed in an impermeable stratum or extend into the water bearing stratum. The size of the casing shall be large enough to permit the installation of an independent drop pipe. The casing may be of concrete, tile, or galvanized or corrugated metal pipe. The annular space between the casing and the earth shall be filled with grout to a minimum depth of 3 m. For flood prone regions, top of the casing or pipe sleeve shall be at least 300 mm above the flood level.

Well Cover

All potable wells shall be equipped with a watertight cover overlapping the top of the casing or pipe sleeve. For dug or bored well, the overlap and downward extension of the cover shall be at least 50 mm outside the well casing or well. The annular space between the casing or pipe sleeve and the drop pipe shall have a watertight sealing.

Drainage from Well Platform or Pump House

The construction of well platform or pump house shall be such that this will drain away from the well by gravity.

Pumping Equipment

The design, installation and construction of pumps shall be such that they will not permit the entrance of any contaminating material into the well or water supply system. The pump shall be accessible for inspection, maintenance and repair.

Water Distribution in Tall Buildings

In tall buildings some of the fixtures at the lower level may be subjected to excessive pressure. The sanitary appliances and fittings in tall buildings shall not be subjected to greater than 350 kpa. This shall be achieved by one or combination of the following two methods:

a. Zoning floors by intermediate tanks:

An intermediate tank shall be provided on different floors so that plumbing fixtures are not subjected to excessive pressure.

b. Using pressure reducing valves:

The excessive pressure suffered by different fixtures shall be minimized by pressure reducing valves.

Recirculation of cooling water

Recirculation of cooling water and/or waste water from wash basin to the cistern of water closets and urinals in the lower floor may be provisioned through a separate tank. No connection between the potable water supply line and the recirculated waste water line shall be allowed with or without any nonreflex or nonreturn valves.

Treatment for Termite

Man can be affected in a multypilicity of ways by insects. Insects can attack his body, his food, crops, fruits, other aggricultural products including his stored possesssions and even the house in which he lives.

Insecets could be defined as ancient race of animals which have been in existance for million of years. They are capable of survival under most adverse conditions and environments.the type of insects known as termites cause maximum damage to the buildings. Termites are divided mainly into following two types:

1. Dry wood termites

2. Subterranean termites 

1. Dry wood termites

They live in wood and do not maintain contact with the ground. They normally build nests within the dry timber members like door window frames, wooden furniture etc. and destroy them gradually.

2. Subterranean termites

They on the other hand are mainly responsible for causing damage to the buildings and it cantents. Unlikely dry-wood termites they live in soil and require moisture for their existance. They build underground nests or colonies and form mud-wall tunnels or runways(tube) which serve as protected shelter for their movements. Sometimes they build nests near ground in stumps of dead trees or creat colonies in the form of dome shaped mounds on the ground. It is through these mud wall tubes that they maintain direct conctact with the soil for meeting their moisture requirements and conditions of darkness essentially needed for their survival. The termites enter the building through foundations or from adjacent to buildings and advance upward through floors destroying everything that comes eithin their reach.

Treatment

Pre-constructional Measures

1.For load bearing walls

For load bearing walls, treatments of soil shall be carried out at the bottom of the trenches and at the sides up to 300 mm above the bottom(fig-1 and fig-2). In such cases, 5 litre of the chemical shall be sprayed per sqr. M of the surface area. The backfill material in direct contact with the foundation shall be treated with 15 litre of chemical emulsion per sqr. m of the surface area of the foundation. If water is used for compaction operation, it shall be done applying the chemicals. Treatment shall follow the same layer wise sequence as that of the backfilling operation.

2. For frame structure

For frame structures, if the concrete mix of the foundation is 1:2:4 or richer, treatment of soil at the bottom of the trench is not needed. A layer of soil at a depth of 500 mm from the ground level shall be prepared. Details of the treatment is shown in fig-3. The density of the chemicals in such treatment shall be 15 litre per aqr.m.

The top surface of the plinth in any building having a floor at the ground level shall be treated with a chemical emulsion at the rate of 5 litre per sqr. m.

Fire Resistant Construction

It is encouraged to use non-combustible materials in construction to have a fire resistant structure. All the structural elements such as beams, columns, lintels, arches, floors and roofs, load-bearing walls or partition walls etc. should be constructed in such a way that they should continue to function as structural members at least for the period which may be sufficient for the occupants to escape. The following additional points should be kept in view while designing a fire-resistant structure:

a. The load-bearing walls or columns of masonry should be thicker in section so that they may successfully act as fire barrier for a considerable time.

b. As far as possible resisting material should be used in the construction of flooring. If the usage of material which is likely to be damaged cannot be avoided either due to financial or other considerations, the following precautions should be observed:

1. In case of wooden floor, thicker joists spaced at a greater distance apart should be used.

2. Fire stops should be provided in the wooden floors at suitable intervals.

3. Flooring made from materials like concrete, brick, ceramic tiles etc., is considered to be most suitable for fire-resistant construction. In case, cast iron, wrought iron or combustible materials like rubber, cork, carpet etc. have to used in flooring, such materials should be protected by covering of insulating material like ceramic tiles, terracotta, bricks etc.

c. Reinforced concrete framed structures should be preffered to steel structures. As steel is liable to twist, sag or distort under heavy fire, the metal shoild be protected by using concrete, hollow clay tiles, bricks, metal lath and plaster, etc. the treatment given to steel column is shown in fig-1. The cover of the protected material like tiles or bricks etc. all around the steel members should be at least 10 cm. from all sides. In any case, the metal flages should be protected with not less than 5 cm. layer of conrete. The cover of concrete for reinforced concrete members, like beams, or columns should be sufficient to enable the members function satisfactorily, under fire for maximum time. The concrete cover outside the main reinforcement should be at least 5 cm. for very important structural members, like columns, girder, trusses etc. 38 mm for ordinary beam, or long span slabs, arches etc. and 25 mm. for partition walls and short span slabs.

Classification of Prestressed-Concrete Structures

According to ACI Committee on Prestressed Concrete:

Concrete in which there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from given external loadings are counteracted to a desired degree. In reinforced-concrete members the prestress is commonly introduced by tensioning the steel reinfocement.The main difference between reinforced concrete and prestressed concrete is the fact that reinforced concrete combines concrete and steel bars by simply putting them togther and letting them act together as they may wish. Prestressed concrete, on the other hand, combines high-strength conrete with high-strength steel in “active” manner.

Classification

Prestressed-concrete structures can be classified in a number of ways, depending upon their features of design and construction. These are disscussed as follows:

1. Externally or internally prestressed

2. Pretensioning and post tensioning

3. End-Anchored or Non-End Anchored Tendons

4.Bonded or unbonded tendons

5. Precast, cast-in-place, composite construction

6. Partial or full prestressing

1. Externally or internally prestressed

Although the design of prestressed-concrete structures is mainly concerned with internally prestressed, presumably with high-tensile steel, it must be mentioned that it is sometimes possible to prestress a concrete structure by adjusting its external reactions. Theoretically, a simple concrete beam can be externally prestressed by jacking at the proper places to produce compression in the bottom of the fibre and tension in the top of fibres, fig -1, thus even dispensing with steel renforcement in the beam. Such an ideal arrangement, however, cannot be easily accomplished in practice, because, even if abutments favorable for such a layout are obtainable, shrinkage and creep in concrete may completely offset the artificial strains unless they can be readjusted. Besides, such a site would probabaly be better suited for an arch bridge.

For the statically indeteminate structure, like a continuous beam, it is possible to adjust the level of supports, by insurting jacks, for example, so as to produce the most desirable reactions, fig-2. This is sometimes practical, though it must be kept in mind that shrinkage and creep in conrete will modify the effects of such prestress so that they must be taken into account or else the prestress must be adjusted from time to time.

Geotextiles

Geosynthetics

Geosynthetics are polymeric products which are widely used in many geotechnical and environmental applications related to groundwater quality and control. It can be defined as planar products manufactured from polymeric material, which are used with soil, rock, or other geotechnical engineering-related material as an integral part of a man-made project, structure, or system (ASTM, 1995).

Among the different geosynthetic products, geotextiles are the ones that present the widest range of properties. They can be used to fulfill the different functions such as Separation, Reinforcement, Filtration, Drainage Infiltration barrier and protection for many different geotechnical, environmental, and groundwater applications. For example, Figure 1. shows the construction of a reinforced slope in which geotextiles were selected as multipurpose inclusions within the fill, because they can provide not only the required tensile strength (reinforcement function), but also the required transmissivity (drainage function) needed for that particular project (Zornberg et al., 1996).

FIGURE 1. Placement of a high-strength nonwoven geotextile to perform a dual function of reinforcement and in-plane drainage in a reinforced slope.

Geotextiles are manufactured from polymer fibers or filaments which are later formed to develop the final product. Approximately 75% of the geotextiles used today are based on polypropylene resin. An additional 20% are polyester, and the remaining 5% is a range of polymers including polyethylene, nylon, and other resins used for specialty purposes. As with all geosynthetics, however, the base resin has various additives, such as for ultraviolet light protection.

The most common types of fibers used in the manufacture of geotextiles are monofilament, staple, and slit-film. If fibers are twisted or spun together, they are known as a yarn. Monofilament fibers are created by extruding the molten polymer through an apparatus containing small-diameter holes. The extruded polymer strings are then cooled and stretched to give the fiber increased strength. Staple fibers are also manufactured by extruding the molten polymer; however, the extruded strings are cut into 25- to 100-mm portions. The staple fibers may then be spun into longer fibers known as staple yarns. Slitfilm fibers are manufactured by either extruding or blowing a film of a continuous sheet of polymer and cutting it into fibers by knives or lanced air jets. Slit-film fibers have a flat, rectangular cross-section instead of the circular cross-section shown by the monofilament and staple fibers.

The fibers or yarns are formed into geotextiles using either woven or nonwoven methods. Figure 2. shows a number of typical woven and nonwoven geotextiles. Woven geotextiles are manufactured using traditional weaving methods and a variety of weave types. Nonwoven geotextiles are manufactured by placing and orienting the fabrics on a conveyor belt and subsequently bonding them by needle punching or melt bonding. The needle-punching process consists of pushing numerous barbed needles through the fiber web. The fibers are thus mechanically interlocked into a stable configuration. As the name implies, the heat (or melt) bonding process consists of melting and pressurizing the fibers together.

Common terminology associated with geotextiles includes machine direction, cross machine direction, and selvage. Machine direction refers to the direction in the plane of fabric in line with the direction of manufacture. Conversely, cross machine direction refers to the direction in the plane of fabric perpendicular to the direction of manufacture. The selvage is the finished area on the sides

FIGURE 2. Typical woven and nonwoven geotextiles.

of the geotextile width that prevents the yarns from unraveling. Adjacent rolls of geotextiles are seamed in the field by either overlapping or sewing. Sewing is generally the case for geotextiles used as filters in landfill applications but may be waived for geotextiles used in separation. Heat bonding may also be used for joining geotextiles in filtration and separation applications.

Numerous tests have been developed to evaluate the properties of geotextiles. In developing geotextile specifications, it is important that the designer understands the material tests and that he or she specifies material properties important for the geotextiles’ intended use.

HEAT-RESISTANT CONCRETE

The behavior of concrete exposed to high temperatures is complex. Concrete pavement exposed to high temperatures from aircraft jet blast or from auxiliary power units can suffer damage. Typical concrete pavement damage resulting from high temperatures of jet blast includes spalling, aggregate popouts, scaling, cracking, and loss of joint sealant. The time that the concrete is exposed to the jet engine or auxiliary power unit exhaust is critical, since there is considerable thermal lag in concrete.

Behavior of concrete

If the concrete is wet when the heat is suddenly applied, the production of steam within the concrete can cause spalling. If the concrete is dry or the heat is applied slowly, relatively little permanent damage is done with concrete temperatures up to 400 to 500 degrees Fahrenheit (204 to 260 degrees Celsius). At concrete temperatures above this, water of hydration is lost, and the concrete strength decreases. At about 1,000 degrees Fahrenheit (538 degrees Celsius), compressive strength loss can be 55 to 80 percent of the original strength. At the time of heating, the degree of saturation of the concrete influences the severity of strength loss, and repetitions of heating and cooling cycles further degrade the concrete. At a temperature of around 1,060 degrees Fahrenheit (571 degrees Celsius), silica in the concrete aggregates undergoes a crystal change and expands, and in the range of 1,300 to 1,800 degrees Fahrenheit (704 to 982 degrees Celsius), carbonate aggregates undergo a chemical change. As the concrete surface is heated, a large temperature gradient develops between the surface concrete and the cooler slab depths that can lead to separation and spalling.

Properly designed pavements generally have not suffered heat damage from aircraft. Power check pads where extensive engine operations occur for maintenance are specially detailed to minimize the exhaust plume’s contact with the pavement surface. Where existing pavements, particularly if at a shallow slope, are converted to use as power check pads, extensive thermal damage can occur. Particular problems are posed by aircraft with vectored thrust such as the Navy’s Harrier or aircraft such as the B-1 or FA-18 with auxiliary power units that exhaust downward on the pavement for extended periods of time. AFCESA and TSMCX should be consulted for the most up-to-date guidance on how to deal with these problems.

Exposure Time and Temperature.

Concrete slabs exposed to an ASTM E119 standard fire for 2 hours indicated that after the temperature of the concrete at about 3/4 inch (19 millimeters) below the surface was 1,200 degrees Fahrenheit (649 degrees Celsius), at 1 1/2 to 2 inches (38 to 51 millimeters) it was 800 degrees Fahrenheit (427 degrees Celsius), and at about 3 1/2 inches (89 millimeters) it was 400 degrees Fahrenheit (204 degrees Celsius). (The atmosphere temperature for a standard ASTM E119 fire rises to 1,000 degrees Fahrenheit (538 degrees Celsius) at 5 minutes, 1,700 degrees Fahrenheit (927 degrees Celsius) at 1 hour, and 2,300 degrees Fahrenheit (1,260 degrees Celsius) at 8 hours.) Normally concrete would not be exposed to jet or auxiliary power unit exhaust for extended periods of time, and any thermal damage will be concentrated in the upper surface concrete. Concrete exposed to high temperatures must be of high quality. It should have a low water/cement ratio, and it must be properly cured. Leaner concrete mixes perform better than richer mixes. Construction must also be of high quality. Proper consolidation and proper finishing are critical. Finishing techniques that cause a paste on the surface will result in scaling. Selection of the proper materials in the concrete also has a dramatic effect on heat resistance. Aggregate selection probably is the most important single materials-related factor; however, no standard specification has been developed for heat-resistant aggregate.

An aggregate with a low coefficient of thermal expansion is generally considered to be desirable, and one rating system roughly groups aggregates as follows in descending order of desirability for heat-resistant concrete, as shown in Table 1.

Aggregate Performances.

Lightweight aggregates such as expanded shale tend to perform better than conventional natural concrete aggregates when exposed to high temperatures. Good results have also been reported for air-cooled slag aggregates. Hydrated Portland cement that has lower calcium hydroxide content appears to be preferable to those with higher contents for high-temperature applications. Therefore, some benefit may be obtained by using Portland cement blended with slag cement. For temperatures of 1,500 degrees Fahrenheit (816 degrees Celsius) or more, high alumina cement will provide superior performance over conventional Portland cement. Repair of concrete that has suffered thermal damage is a difficult problem. Proper patching procedures for spalls and popouts must be meticulously followed, and the repair material should have similar thermal characteristics to the original concrete. Even so, the repairs may only be temporary. Overlays using heat resistant concrete are a potential repair for scaled areas or for areas with concrete of poor heat resistance. If scaling is due to a paste on a concrete surface that is otherwise acceptable, grinding the surface may be adequate. Joint sealant used in concrete pavements exposed to high temperatures should conform to Federal Specification, SS-S-200E. This specification does require testing of the material at 500 degrees Fahrenheit (260 degrees Celsius) for 2 minutes so that some resistance to high temperatures can be achieved. However, when high temperatures are combined with jet blast, the sealant may still be damaged or blown out of the joint. Under these circumstances, increased periodic resealing must be accepted as routine maintenance. Conventional concrete and joint sealants should provide reasonable service up to concrete temperatures of about 500 degrees Fahrenheit(260 degrees Celsius)

Table 1. Aggregate Desirability

Above this temperature, deterioration of concrete and increased loss of sealant can be expected. High-quality concrete with selected aggregates can reduce the amount of damage. Above 1,000 degrees Fahrenheit (538 degrees Celsius), severe deterioration can be expected, and refractory materials such as high alumina may be needed. Where possible, blast shields, diverters, or increased slope of pavements should be used to allow the maximum dissipation of the exhaust plumes temperature before it impinges on the concrete. Use of continuously reinforced concrete for areas such as power check pads removes the need for joints and joint sealants. In one installation, refractory brick was used to surface a test facility where high-temperature engines were tested and evaluated.

Foundation Ties

One of the prerequisites of adequate performance of a building during an earthquake is the provision of a foundation that acts as a unit and does not permit one column or wall to move appreciably with respect to another. A common method used to attain this is to provide ties between footings and pile caps. This is especially necessary where the surface soils are soft enough to require the use of piles or caissons. Therefore, the pile caps or caissons are tied together with nominal ties capable of carrying, in tension or compression, a force equal to Ca/4 times the larger pile cap or column load.

A common practice in some multistory buildings is to have major columns that run the full height of the building adjacent to smaller columns in the basement that support only the first floor slab. The coefficient applies to the heaviest column load.

Alternate methods of tying foundations together are permitted (e.g., using a properly reinforced floor slab that can take both tension and compression). Lateral soil pressure on pile caps is not a recommended method because the motion is imparted from soil to structure (not inversely as is commonly assumed), and if the soil is soft enough to require piles, little reliance can be placed on soft-soil passive pressure to restrain relative displacement under dynamic conditions.

If piles are to support structures in the air or over water (e.g., in a wharf or pier), batter piles may be required to provide stability or the piles may be required to provide bending capacity for lateral stability. It is up to the foundation engineer to determine the fluidity or viscosity of the soil and the point where lateral buckling support to the pile can be provided (i.e., the point where the flow of the soil around the piles may be negligible).

Types of Bituminous Mixtures

Types of Bituminous Mixtures used in Pavement Construction

A bituminous mixture is a combination of bituminous materials (as binders), properly graded aggregates and additives. Since tar is rarely used in bituminous mixtures in recent years and asphalt is the predominant binder material used, the term “asphalt mixture” is now more commonly used to denote a combination of asphalt materials, aggregates and additives. Asphalt mixtures used in pavement applications are usually classified by

(1) Their methods of production, or

(2) Their composition and characteristics.

Classification by Method of Production

Hot-mix asphalt (HMA)

Hot-mix asphalt (HMA) is produced in a hot asphalt mixing plant (or hot-mix plant) by mixing a properly controlled amount of aggregate with a properly controlled amount of asphalt at an elevated temperature. The mixing temperature has to be sufficiently high such that the asphalt is fluidic enough for proper mixing with and coating the aggregate, but not too high as to avoid excessive aging of the asphalt. A HMA mixture must be laid and compacted when the mixture is still sufficiently hot so as to have proper workability. HMA mixtures are the most commonly used paving material in surface and binder courses in asphalt pavements.

Cold-laid plant mix

Cold-laid plant mix is produced in an asphalt mixing plant by mixing a controlled amount of aggregate with a controlled amount of liquid asphalt without the application of heat. It is laid and compacted at ambient temperature.

Mixed-in-place or road mix

Mixed-in-place or road mix is produced by mixing the aggregates with the asphalt binders in proper proportions on the road surface by means of special road mixing equipment. A medium setting (MS) asphalt emulsion is usually used for open-graded mixtures while a slow setting (SS) asphalt emulsion is usually used for dense-graded mixtures.

Penetration macadam

Penetration macadam is produced by a construction procedure in which layers of coarse and uniform size aggregate are spread on the road and rolled, and sprayed with appropriate amounts of asphalt to penetrate the aggregate. The asphalt material used may be hot asphalt cement or a rapid setting (RS) asphalt emulsion.

Classification by Composition and Characteristics

Dense-graded HMA mixtures, which use a dense-graded aggregate and have a relatively low air voids after placement and compaction, are commonly used as surface and binder courses in asphalt pavements. The term Asphalt Concrete is commonly used to refer to a high-quality, dense-graded HMA mixture.

A dense graded HMA mixture with maximum aggregate size of greater than 25 mm (1 in.) is called a large stone dense-grade HMA mix. A dense-grade HMA mix with 100% of the aggregate particles passing the 9.5 mm (3/8 in.) sieve is called a sand mix.

Open-graded asphalt mixtures, which use an open-graded aggregate and have a relatively high air void after placement and compaction, are used where high water permeability is desirable. Two primary types of open-graded mixes are

(1) open-graded base mix and

(2) open-graded friction course (OGFC).

Open-graded base mixes

Open-graded base mixes are used to provide a strong base for an asphalt pavement as well as rapid drainage for subsurface water. Open-graded base mixes usually use a relatively larger size aggregate that contains very little or no fines. Due to the lower aggregate surface area, these mixes have relatively lower asphalt content than that of a dense-graded HMA mix. Open-graded base mixes can be produced either hot or cold in an asphalt plant.

Open-graded friction courses (OGFC)

Open-graded friction courses (OGFC) are placed on top of surface courses to improve skid resistance and to reduce hydroplaning of the pavement surface. OGFC mixtures use aggregates with a small proportion of fines to produce high air voids and good drainage characteristics. Even though the voids content is higher, the asphalt film thickness is usually greater than that for a dense-graded HMA, and thus a typical OGFC mixture has about the same or higher asphalt content than that of a dense-graded HMA. A typical OGFC uses an aggregate of ½ in. (12.5mm) maximum size, and is placed at a thickness of ¾ in. (19 mm). An OGFC mixture is produced in a hot-mix plant in the same way as a dense-graded HMA mixture. Crumb rubber modified asphalt has been used in OGFC mixtures in recent years to improve their performance and durability. Due to the higher viscosity of the crumb rubber modified binder, thicker film thickness can be used. This results in a higher binder content and thus better durability for the crumb rubber modified OGFC mixtures.

Stone Matrix Asphalt (SMA), which was originally developed in Europe, was a special asphalt mixture of improved rutting resistance and increased durability. SMA mixtures are designed to have a high coarse aggregate content (typically 70–80%), a high binder content (typically over 6%) and high filler content (typically about 10%). Asphalts modified with polymers and/or fibers are typically used. The improved rutting resistance of the SMA mixture is attributed to the fact that it carries the load through the coarse aggregate matrix (or the stone matrix), as compared with a dense-graded HMA, which carries the load through the fine aggregate. The use of polymer and/or fiber modified asphalts, which have increased viscosity, and the use of high filler content, which increases the stiffness of the binder, allow the SMA mixtures to have a higher binder film thickness and higher binder content without the problem of draindown of asphalt during construction. The increased durability of the SMA mixtures can be attributed to the higher binder film thickness and the higher binder content. SMA mixtures require the use of strong and durable aggregates with a relatively lower L.A. Abrasion Loss. SMA mixtures can be produced in a hot-mix plant in a similar way as a dense-grade HMA mixture. The main disadvantage of using a SMA as compared with a dense-grade HMA is its relatively higher cost due to the requirement for the use of higher quality aggregates, polymer, fibers and fillers.

Self-Compacting Concrete

Self-compacting concrete (SCC) is a relatively new technology, in terms of construction. Since its introduction over 10 years ago in Japan, the concept of SCC has captured the imagination of researchers and practitioners around the world. This material can be considered as a high performance composite, which flows under its own weight over a long distance without segregation and without the use of vibrators. For the past decade, the focus on SCC has been on its fresh properties. Research and practical experience were well documented in the first symposium of self-compacting concrete held in Stockholm [RILEM 1999], and later in the state-of-the-art RILEM report [2000]. More information, particularly hardened properties, can be found in the second symposium held in Tokyo [SCC 2001].

The complete elimination of the consolidation process in SCC can lead to many benefits. Besides the obvious benefit of improved concrete quality in difficult sites relating to access and congested reinforcements, the use of SCC increases productivity, reduces the number of workers on site, and improves working environment. The reduction in overall construction cost could be around 2 to 5%. Depending on competition, the supply cost of SCC could be from 10 to about 50% higher than that of conventional concrete of similar grade. This leads to the low consumption of SCC in practice amounting to less than 5% of total concrete production. With improved quality control by suppliers and increased competitiveness in the market, the use of SCC is accelerating in many developed countries.

Fresh SCC must possess high fluidity and high segregation resistance. Fluidity or deformability means the ability of the flowing concrete to fill every corner of the mould as well as the ability to pass through small openings or gaps between reinforcing bars, often referred to as filling ability and passability of SCC respectively. To satisfy this high fluidity requirement, the maximum size of aggregate is generally limited to 25 mm. To improve flow properties, the amount of coarse aggregates is reduced and balanced by the increase in paste volume. Superplasticizer is needed to lower the water demand while achieving high fluidity. The common superplasticizer used is a new generation type based on polycarboxylated polyether, which is considerably more expensive than the traditional type used in conventional concrete. For SCC to have high segregation resistance, high powder content ranging from 450 to 600 kg per cubic meter of concrete should be specified. Powder generally refers to particles of sizes less than 0.125mm. Since cement content of 300 to 400kg/m3 is often available, SCC usually incorporates 150 to 250 kg/m3 of inert or cementitious fillers. Limestone powder is the common filler used, with fly ash and blast furnace slag enjoying increased popularity. Viscosity agent is sometimes incorporated to minimize the addition of fillers. This admixture is similar to that used in under-water concreting. It increases the viscosity of water, thereby increasing segregation resistance.

The rheology of fresh concrete is most often described by the Bingham model. According to this model, fresh concrete must overcome a limiting stress (yield stress, to) before it can flow. Once the concrete starts to flow, shear stress increases with increase in strain rate as defined by plastic viscosity, m. The target rheology of SCC is to reduce the yield stress to as low as possible so that it behaves closely to a Newtonian fluid. The other target property is “adequate” viscosity. The addition of water reduces both the yield stress and viscosity. Too much water can reduce the viscosity to such an extent that segregation occurs. The incorporation of superplasticizer reduces the yield stress but causes limited reduction in viscosity. The use of Bingham parameters is useful in describing the behavior of fresh concrete, but there is no consensus, at least at this stage, on their limiting values appropriate for SCC.

For site quality control, tests requiring simple equipment are often performed to indicate qualitatively or quantitatively the three basic properties of SCC: filling ability, passability, and segregation resistance.

Slump-flow test is the most popular test method used because of it simplicity. A representative sample of concrete is placed continuously into an ordinary slump cone with a jug without tampering. The cone is lift and the diameter of the concrete (i.e., slump flow value) after the concrete has stopped is measured.

The time to reach a flow diameter of 500 mm and final flow diameter are also noted. The degree of segregation can be judged to a certain extent by visual observation. This test reflects the filling ability, but the passability is not indicated. L-box, U-box, and V-funnel are other common tests available to assess one or more of the basic properties of SCC.