Учебное пособие 1575

Many of today’s “high-performance” concrete structures, including many landmark bridges and buildings, employ some type of prestressing. Parking garages, high-rise residential towers, and many other kinds of structures also employ post-tensioning techniques.

TEXT 4

ROOF MATERIALS: NATURAL TILE

Tile is one of the oldest roof materials in the world used by people more than 4000 years. The beauty of the shape and colour has served a model of architectural image of a roof for thousands years. Even ancient Greeks and later Romans used marble and potter's tile. The recipe of tile was created by nature itself. Mixing limestone, sand and water, people got a particularly stable material.

Modern technology is also based on the usage of raw materials. Quartz sand and water are mixed with cement, which has substituted limestone and clay. The received mass is pressed under pressure and dries without the usage of firing and becomes firm due to the hydration of cement, which is present in the mixture. It's much cheaper than ceramic tile. To get a desirable shade of colour pigments are used which are received from oxides of the metal, the oldest due known to the mankind since the old times.

We must notice that clay (ceramic) and cement-and-sand rolled tile may be called "natural" as clay (to be more exact clay-carbonate marls) serves as a raw basis of cement.

Englishmen began to produce analogical roof covering in 1820. But cement-and-sand tile became widely spread in Europe in the middle of the XXth century when its appearance was polished so much that it was impossible to differ it from clay one.

Cement-and-sand tile has been in great demand during the last decades. Though natural ceramic (clay) tile has been preserved on the market, new types of false tile made of other materials: metal (metal tile) and on the basis of bitumen (soft tile) have appeared.

Today in the west in a new building and repairing natural tile occupies 86 %, metal tile – 3 %, other roof materials – 11 %. Tile may be used for stone, brick and

wooden buildings as in new construction and while being reconstructed. Modern tile (both ceramic and cement-and-sand) affords to fulfill pitched roofs of any difficulty from simple two-pitched roofs to the most complicatedly shaped roofs: hipped roofs with attics and attic windows - round-shaped towers with conic forms and many other roofs. It is possible to build angles or arched forms, eaves, gutters on the roof.

There is an opinion that the main limitation in the usage of tile is its big weight what requires the construction of high-capacity rafters. However, it is not really so. The share of the own weight of tile concerning the calculated load on the construction of

91

the roof is not so great in comparison with the load of snow for instance. A simple comparison shows that the calculated load for roofing made of cement-and-sand tile is 25-30% higher than for simple metal coverings. When using tile it is not necessary to increase a section of rafters, it will be enough to install the same rafters with a lesser step. For example, for metal roofing you need rafters 150 x 50 mm with a step 90 cm; for tile roofing - 150 x 50 mm with a step 70 cm.

As for "excessive", according to some mistaken viewpoint, weight of tile it may be considered as its advantage, realized necessity. Technologically the weight of tile can be lighter but it is not done for the sake of wind-steadiness. Thanks to its weight it protects the roof and the whole building from wind and outer forces. When the other kinds of roofing are carried away from roofs by the wind, a tile roof is always untouched by different strong winds. When engineers of the German concern "Braas" proudly demonstrated the models of rather thin and light cement-and-sand tile to German federal building committee, it turned out that they violated the norm of windsteadiness. They had to write off the expenses and readjust the equipment for the production of articles with normal thickness, weighing about 45 kg per 1m2, as it is required by the norm.

Technical Characteristics

Both ceramic and cement-and-sand tile are one of the most durable roofing, the exploitation period of which is more than 100 years. Tile is not sensitive to any influence: heat and frost, hail and hurricanes, oxygen of the air, acid rains, the activity of bacteria and the plants of the lowest development, ultra-violet radiation and infra-red radiation of the sun etc. In comparison with metal roofing, there is less possibility of a lightning getting into the roof, as the roofing does not collect static electricity, which is dangerous for a man and possible to cause fire.

Tile roofing easily adapts to inevitable deformations at a seasonal subsidence of buildings and fluctuation in temperature - the role of temperature joints is performed by tile joints. By the way, the joints of such type provide effective airing of under roof space from excess of vapor of every-day moisture, which is very active during a cold season and at a day overfall of temperatures. As we know, the condensate is a main enemy of wooden and metal constructions of a roof. It is rather cool under such roofing in summer and warm in winter because of low thermal conductivity. Low thermal conductivity combined with massive material provides the minimal formation of a condensate on the internal surface of a tile covering.

TEXT 5

DOES BENDABLE CONCRETE EXIST?

Since we use "concrete" colloquially to mean fixed, well defined, or inflexible, it may seem unbelievable that bendable concrete exists. However, in 2005, some civil engineers at the University of Michigan developed this innovative material, that will doubtless change the face of construction. Bendable concrete, with the help of interlocking fibers, actually flexes to absorb pressure, changes in temperature, and movement.

92

Ordinary concrete is made from a slurry of large aggregate rocks, smaller aggregate, and the inflexible mortar that bonds them together. Once it sets and dries, though, it becomes a stiff slab that will crack and buckle in cycles of freezing and thawing, earthquakes, or when a heavy truck drives over it. Therefore, it was a goal of civil engineers to figure out how to extend the life of roads, as well as make them better able to withstand environmental factors. Constantly replacing roads, bridges, and parking lots is very expensive, time-consuming, and inconvenient for commuters.

Amazingly, researchers have developed a versatile product called bendable concrete. The concrete is technically a composite because it mixes special fibers with regular concrete. The fibers account for some of the flexibility and strength because they distribute weight evenly throughout the slab. They also flex when met with a lot of pressure. The fibers move their position slightly to accommodate the added weight.

Not only is bendable concrete ductile, but recent tests have shown that it is much lighter. This is important for applications like bridges, where more weight means more expense in materials and construction. Furthermore, bendable concrete takes less energy and releases less carbon dioxide in its manufacturing.

Even before this new kind of concrete gets widely implemented, it is compatible with today's roads. Bendable concrete can be used as patching material for existing potholes or cracks. Since it is a relatively recent development, there is still much research and further improvement necessary before it is accepted as a feasible alternative to traditional pavement.

TEXT 6

DECORATIVE CONCRETE: EXPLORING THE OPTIONS

Want to permanently cover up surface imperfections in existing concrete? Or turn a plain-jane slab from drab to fab? With today’s decorative overlays, it’s easy to give almost any concrete surface, indoors or out, a complete face-lift – and at a much lower cost than removal and replacement. The real challenge is choosing from among the many resurfacing products available – and the diverse array of decorative finishes

possible.

Although cement-based overlays have been around for decades, many of today’s systems blend polymer resins with cement, sand, and other additives to improve performance, wear resistance, and aesthetic qualities. Polymermodified overlays can be applied in layers as thin as a credit card or up to several inches thick without

delamination or failure. They adhere well to existing concrete and resist damage from salt, chemicals, UV exposure, freeze-thaw conditions, and abrasion.

93

While most types of polymer-modified overlays offer similar performance benefits, each system has its own unique characteristics. Overlay manufacturers use different types of polymer resins, often blending them to produce proprietary products with distinct physical attributes. Many of today’s decorative overlays use acrylics or acrylic blends because these resins provide excellent bond strength and

UV resistance.

Here’s a primer of the various overlay types available and the decorative options with each. We also give you pointers on how to choose the best system for a particular application. With the many options available, you’re sure to find at least one system that meets your requirements for durability, appearance, maintenance, and cost.

Types of Overlays

Stampable overlays

Microtoppings and skim coats

Spray-down systems

Self-leveling overlays

WHEN TO USE A POLYMER OVERLAY

Breaking out and replacing an existing concrete slab can be expensive, messy, disruptive, and time-consuming. With an overlay, you can give worn, lackluster concrete a decorative makeover or restore it to like-new condition in as little as a day and at a fraction of the cost (from about $1 to $3 per square foot, depending on the type of system installed).

Polymer-modified overlays are ideal for:

Adding pattern, texture, and color to exterior concrete slabs, including patios, pool decks, driveways, and sidewalks. Using special techniques and tools, skilled installers can precisely replicate the look and feel of natural stone, brick, tile, or marl.

Resurfacing interior floors, including those previously covered with linoleum tile or carpeting (after removal of all residual mastic). With the use of stains or dyes, it’s possible to create colorful graphic designs, logos, or stenciled patterns. (Another highly versatile decorative topping for interior floors is epoxy terrazzo.)

Smoothing and leveling of concrete surfaces.

Restoring surfaces quickly with minimal downtime. Polymer overlays cure fast and

some systems can support foot traffic within a few hours.

Not all existing concrete is a suitable candidate for an overlay. The concrete must be structurally sound, without gapping cracks, severe delamination, or an unstable sub-

94

base. Proper surface preparation of the concrete is also imperative to ensure good bonding of the overlay. Any non-structural cracks wider than hairline should also be repaired.

TEXT 7

SEALERS

Despite its durability, concrete is a porous material, which means it can absorb water, chlorides, stains, and other wateror oil-based materials that it comes in contact with. By treating the concrete with a sealer, you can inhibit the penetration of these intruders while allowing the surface to breathe, so moisture within the concrete doesn’t become trapped.

Generally, there are two broad categories of sealers:

film formers block penetration of water and contaminants by forming a barrier on the concrete surface. Many also impart a gloss or sheen, so they often are used to enhance colored or exposed-aggregate concrete.

penetrants actually penetrate into the concrete surface to a depth of about 1 to 4 mils to increase water repellency and resist stains on absorbent concrete. Usually they provide invisible protection without changing the surface appearance.

For the ultimate in stain and water resistance without changing surface appearance, White Mountain’s Ultrapel and Invisoseal can be used.

Ultrapel is an invisible sealer that locks out oil, grease, and other organic stains from colored and stamped concrete, interlocking pavers, masonry, and other porous surfaces. Ultrapel is oleophobic, so it excels at blocking oil and grease stains, says Glessner. Commercial applications include heavy-use food preparation and serving areas, parking facilities, factory and warehouse floors, aircraft hangars, vehicle bays, and loading areas. Residential applications include driveways, garage floors, pool decks and patios, and entryways.

Invisoseal is a non-gloss, penetrating sealer that gives porous concrete and masonry surfaces up to 10 years of water-repellent protection. This hydrophobic dynamo is particularly effective at controlling efflorescence and mildew and algae stains.

Seal-Lock is a water-based acrylic sealer ideal for use on interlocking concrete pavers. It penetrates the concrete to protect against solvent-based stains, repel water, and retard the growth of mold and mildew. Another benefit: it also bonds and locks down the sand in the paving joints to seal the space between pavers and reduces weed growth. Seal-Lock’s low-gloss finish enhances the natural beauty and color of the pavers. It also makes the surface easier to clean and maintain, which is a big plus for large installations.

Recently, Seal-Lock was used by Earth Shelter Inc. to coat the largest installation of concrete pavers in North America, the Port of Oakland in Oakland, Calif. The project required the use of 20,000 gallons of Seal-Lock to protect 5 million square feet of pavers.

95

Seal-Lock was also used to enhance a concrete paver mural at Oakland’s Jack London Square. Depicting ocean waves splashing on a beach, the waterfront plaza covers about 31,000 square feet and features nine different colors of pavers.

White Mountain offers two barrier coatings that seal and protect while enhancing the natural beauty of concrete and masonry surfaces.

Wet Look Lacquer is a film-forming solvent-based acrylic and the only 20%-solids VOC-compliant sealer available in the U.S., says general manager Jim Glessner Jr., general manager of White Mountain Products.

It leaves a clear, high-gloss finish that deepens and enriches the color of exposedaggregate and stamped concrete surfaces while repelling oil, grease, water, and stains. When used on newly placed concrete, the sealer can help protect against spalling, dusting, efflorescence, freeze-thaw damage, and alkali degradation. In addition to enhancing decorative concrete, Wet Look can also protect floors in food-preparation areas from food and beverage stains and slabs in garages and parking facilities from grease and oil penetration.

Crystal Seal is a water-based acrylic barrier coating using a unique microemulsion technology. "Our water-based sealers are the only ones on the market using microemulsion technology, which gives them a much denser, finer-textured matrix so they penetrate better and last longer," says Glessner. Crystal Seal also deepens and enriches the color of decorative concrete but leaves a subtle satin finish. Applications include exposed aggregate, concrete pavers, and stamped or embossed concrete. Crystal Seal resists most stains for up to 72 hours and provides good protection against oil and solvents.

TEXT 8

BUILDING SYSTEMS

Today, home buyers expect to get more from their new home. They want beauty that’s more than skin deep. A home that fits their lifestyle – of course. But also a home with solid, high quality construction. Greater comfort and security. Lower energy bills. Lower maintenance. A home that’s healthier to live in, and easier on the environment.

It’s becoming harder and harder to meet their new expectations with the same old building technology – wood framing. So more and more builders and home buyers are turning to something new. A modern adaptation of a centuries-old technology using the most proven building material on earth.

There's more than one way to build your home with concrete.

96

Concrete Masonry Units

Today, new types of concrete block, advances in rigid foam insulation, and advances in construction methods have combined to provide concrete block building systems that cost-effectively outperform traditional frame building methods in every way.

Insulating Concrete Forms (ICF)

ICFs give you all the benefits that have made concrete the material of choice for home building worldwide: Solid, lasting construction that resists the ravages of fire, wind, and Father Time. But ICFs do plain concrete one better – or rather, two better – by giving you two built-in layers of foam insulation.

Removable Forms (Cast-in-Place)

This traditional concrete forming technique uses temporary forms, typically made of aluminum. Rigid foam insulation is placed inside the forms or between the forms and held in place with a system of nonconductive ties. Concrete is then poured on either side of or between the foam.

Panel Systems (Precast & Tilt-Up)

There are two main types of panel systems used for concrete walls: precast concrete and tilt-up concrete.

Autoclaved Aerated Concrete (AAC)

Technically, autoclaved aerated concrete systems consist of blocks held together by mortar. But that's where the similarity to conventional mortared block construction ends. AAC uses new, technologically advanced concrete and mortar to give builders and buyers a new range of advantages.

Shotcrete

Shotcrete is a process where concrete is projected or "shot" under pressure using a feeder or "gun" onto a surface to form structural shapes including walls, floors, and roofs.

97

TEXT 9

A NEW GENERATION OF REINFORCED CONCRETE OFFICES

IN NEW YORK CITY

505 5th is a modern reinforced concrete office building which conveys elegance in the form of a 30 story tower. Located at the corner of 42nd Street and 5th Avenue in Manhattan, the building gross square footage is 550,000 square feet which includes retail space in the lower floors and typical office space above the 7th floor.

Designed by Kohn Pederson Fox of New York City, the building architectural features offer stunning views across the street to the New York Public Library and Bryant Park. Early on in the design it was decided that the views from the building were the focus of the layout. The building columns were kept away from the perimeter and the main façade was angled to offer spectacular unobstructed views of 5th Avenue.

Stringent architectural requirements posed structural challenges and required innovative solutions by the structural engineering firm of Rosenwasser/Grossman of New York City. The floor layout required columns to be spaced at 30' on center. After many design iterations that the design team elected an 11" deep conventionally reinforced concrete flat-slab with 22" deep capitals as the most economical floor framing system. This floor system also provided an elegant solution for the large interior spans and eliminated the floor beams otherwise needed to support the slab. Beam supported slabs typically increase the floor to floor height, adds cost and complicates formwork.

A thickened 15” flat-slab was used to frame the 15’ cantilevers at the spandrel to meet the curtain wall design requirements while still allowing an impressive 15’ dual cantilevers at the south-west corner of the building. As is common with cantilevered structures, deflection at the building perimeter is of paramount concern. A maximum ¾” vertical displacement of the cantilever slab after the installation of the curtain wall was estimated. As a result, a ¾” upward camber will be included in the formwork so that after installation of the curtain wall system this camber is eliminated. The perimeter columns supported the cantilevers as well as a portion of the interior span. This structural design solution offered unrivaled architectural flexibility with spans reaching almost 45 feet from the curtain to the core.

A key advantage of conventional flat slab systems is the reduction of floor to floor heights which significantly reduced the cost of formwork and building frame.

98

The flat-slab lends itself to the use of conventional plywood construction while the lower floor to floor heights allows for the use of conventional stick shoring. Building each floor on a two-day cycle, is facilitated by of selecting this floor system with the simplified formwork it offers. While an aggressive construction schedule, the twoday cycle is the preferred method of construction in New York City. A trend set in the late sixties and made possible by the moderate spans and lower floor to floor heights common in residential hi-rise flat-plate construction. Unprecedented in office building construction, the two-day cycle will reduce the floor completion schedule by 50% compared to an equivalent structural steel floor system.

The lateral force resisting system efficiently incorporated the 11” flat-slab with a 12" thick concrete shear wall which completely encases the fire stair above the mezzanine floor. Located at the rear of the property, the shear wall maximizes the rentable space with views on 42nd St and 5th Ave. Along with gravity load support, the flat-slab serves as the horizontal diaphragm component of the lateral force resisting system. Managing the eccentricity created by the location of the shear wall elements was easily accommodated without the need for additional structural elements.

Higher strength concrete was implemented by the design team to reduce the size of the tower columns and increase rentable space. 12000-psi concrete is used up to the 7th floor for the 36-in. round columns that reduce to 28-in. above the 15th floor.

To further enhance the buildings safety, an 8-in. reinforced concrete curb used at the first floor and serves as a barrier against vehicular intrusion. The perimeter columns are encased in a steel shell

99

to maximize their robustness when subject to blast loads. In the event of local overloading and failure of a slab-column connection, sufficient steel is provided to arrest progressive collapse.

Cast-in-place concrete construction provided this building better acoustic properties, fireproofing at no additional cost and enhanced robustness in the event of terrorist attacks. The 12-in. concrete shear wall completely encasing the fire stairs is a significant safety feature of this structure. The design required two independent fire stairs separated by 12” concrete wall. Floor slab steel reinforcing detailing was modified in some instances and increased in certain areas to provide the added structural integrity mitigating the potential for progressive collapse. With minor additional engineering effort and cost to the owner, concrete structures have always been considered the natural solution to anti-terrorism, force protection and fire resistance.

TEXT 10

THE PANTHEON

This famous building stands in the business district of Rome – much as it was built some 18 centuries ago. Amazingly, it has withstood the ravages of both the elements and war permitting a firsthand view of a unique product constructed by Roman hands. Now, it is exposed to acid rain and fumes from passing automobiles, overshadowed by buildings of inferior taste; but, with trust in the future, the Pantheon will survive.

Unrecognized, the design of this ancient concrete building reveals unparalleled features not encountered in modern design standards. Recent studies reveal several major cracks in the dome, but it still functions unimpaired. This condition will surely excite the curiosity of our structural engineers. The building was built entirely without steel reinforcing rods to resist tensile cracking, so necessary in concrete members, and for this concrete dome with a long span to last centuries is incredible. Today, no engineer would dare build this structure without steel rods! Modern codes of engineering practice would not permit such mischief. No investor with knowledge of concrete design would provide the funding. We can learn from this activity. Workers can build from a plan and can successfully use their proven practices only if construction quality controls are maintained.

100