Civil Engineering: Prestressed Concrete

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Curing plays an important role on strength development and durability of concrete. Curing takes place immediately after concrete placing and finishing, and involves maintenance of desired moisture and temperature conditions, both at depth and near the surface, for extended periods of time. Properly cured concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability, resistance to freezing and thawing, and abrasion and scaling resistance.

The length of adequate curing time is dependent on the following factors:

- Mixture proportions

- Specified strength

- Size and shape of concrete member

- Ambient weather conditions

- Future exposure conditions

Slabs on ground (e.g. pavements, sidewalks, parking lots, driveways, floors, canal linings) and structural concrete (e.g. bridge decks, piers, columns, beams, slabs, small footings, cast-in-place walls, retaining walls) require a minimum curing period of seven days for ambient temperatures above 40 degrees Fahrenheit.

American Concrete Institute (ACI) Committee 301 recommends a minimum curing period corresponding to concrete attaining 70 percent of the specified compressive strength. The often specified seven-day curing commonly corresponds to approximately 70 percent of the specified compressive strengths. The 70 percent strength level can be reached sooner when concrete cures at higher temperatures or when certain cement/admixture combinations are used. Similarly, longer time may be needed for different material combinations and/or lower curing temperatures. For this reason, ACI Committee 308 recommends the following minimum curing periods:

ASTM C 150 Type I cement seven days

ASTM C 150 Type II cement ten days

ASTM C 150 Type III cement three days

ASTM C 150 Type IV or V cement 14 days

ASTM C 595, C 845, C 1157 cements variable

There are three main functions of curing:

1) Maintaining mixing water in concrete during the early hardening process

Ponding and immersion

Ponding is typically used to cure flat surfaces on smaller jobs. Care should be taken to maintain curing water temperature at not more than 20 degrees Fahrenheit cooler than the concrete to prevent cracking due to thermal stresses. Immersion is mainly used in the laboratory for curing concrete test specimens.

Spraying and fogging

Spraying and fogging are used when the ambient temperatures are well above freezing and the humidity is low. Fogging can minimize plastic shrinkage cracking until the concrete attains final set.

Saturated wet coverings

Wet coverings saturated with water should be used after concrete has hardened enough to prevent surface damage. They should be kept constantly wet.

Left in Place Forms

Left in place forms usually provide satisfactory protection against moisture loss for formed concrete surfaces. The forms are usually left in place as long as the construction schedule allows. If the forms are made of wood, they should be kept moist, especially during hot, dry weather.

2) Reducing the loss of mixing water from the surface of the concrete

Covering concrete with impervious paper or plastic sheets

Impervious paper and plastic sheets can be applied on thoroughly wetted concrete. The concrete surface should be hard enough to prevent surface damage from placement activities.

Applying membrane-forming curing compounds

Membrane-forming curing compounds are used to retard or reduce evaporation of moisture from concrete. They can be clear or translucent and white pigmented. White-pigmented compounds are recommended for hot and sunny weather conditions to reflect solar radiation. Curing compounds should be applied immediately after final finishing. Curing compound shall comply with ASTM C3094 or ASTM C13155.

3) Accelerating strength gain using heat and additional moisture

Live steam

Live steam at atmospheric pressure and high-pressure steam in autoclaves are the two methods of steam curing. Steam temperature for live steam at atmospheric pressure should be kept at about 140 degrees Fahrenheit or less until the desired concrete strength is achieved.

Heating coils

Heating coils are usually used as embedded elements near the surface of concrete elements. Their purpose is to protect concrete from freezing during cold weather concreting.

Electrical heated forms or pads

Electrical heated forms or pads are primarily used by precast concrete producers.

Concrete blankets

Concrete insulation blankets are used to cover and insulate concrete surfaces subjected to freezing temperatures during the curing period. The concrete should be hard enough to prevent surface damage when covering with concrete blankets.

Other forms of curing include internal moist curing with lightweight aggregates or absorbent polymer particles. For mass concrete elements (usually thicker than 3 feet), a thermal control plan is usually developed to help control thermal stresses. Additional information can be found in ACI Committee 308 report Guide to Curing Concrete. For specialty concretes, it is recommended to refer to other ACI reports as follows:

- Refractory concrete ACI 547.1R
- Refractory concrete ACI 547.1R
- Insulating concrete ACI 523.1R
- Expansive cement concrete ACI 223
- Roller-compacted concrete ACI 207.5R
- Architectural concrete ACI 303R
- Shotcrete ACI 506.2
- Fiber-reinforced concrete ACI 544.3R
- Vertical slipform construction ACI 313

Curing in either cold or hot weather requires additional attention. In cold weather, some of the procedures include heated enclosures, evaporation reducers, curing compounds, and insulating blankets. The temperature of fresh concrete shall be above 50 degrees Fahrenheit. The curing period for cold weather concrete is longer than the standard period due to reduced rate of strength gain. Compressive strength of concrete cured and maintained at 50 degrees Fahrenheit is expected to gain strength half as quickly as concrete cured at 73 degrees Fahrenheit. In hot weather, curing and protection are critical due to rapid moisture loss from fresh concrete. The curing actually starts before concrete is placed by wetting substrate surfaces with water. Sunscreens, windscreens, fogging, and evaporation retardants can be used for hot weather concrete placements. Since concrete strength gain in hot weather is faster, curing period may be reduced. Additional information can be found in ACI 306.1, Standard Specification for Cold Weather Concreting, ACI 306R, Cold Weather Concreting, ACI 305.1, Specification for Hot Weather Concreting, and ACI 305R, Hot Weather Concreting

Curing Concrete Test Specimens

Curing of concrete test specimens is usually different from concrete placed during construction. American Society for Testing and Materials (ASTM) has developed two standards for making and curing concrete specimens. ASTM C192 is intended for laboratory samples while ASTM C31 is intended for field samples. Both documents provide standardized requirements for making, curing, protecting, and transporting concrete test specimens under field or laboratory conditions, respectively.

ASTM C192 provides procedures for evaluation of different mixtures in laboratory conditions. It is usually used in the initial stage of the project, or for research purposes.

ASTM C31 is used for acceptance testing and can also be used as a decision tool for form or shoring removal. Depending on its intended purpose, the standard defines two curing regimes: standard curing for acceptance testing and field curing for form/shoring removal. Variation in standard curing of test specimens can dramatically affect measured concrete properties. According to the National Ready Mix Concrete Association (NRMCA), strength for concrete air cured for one day followed by 27 days moist cured will be approximately 8 percent lower than for concrete moist cured for the entire period. The strength reduction is 11 percent and 18 percent for concrete specimens initially cured in air for three days and seven days, respectively. For the same air/moist curing combinations, but 100 degrees Fahrenheit air curing temperature, the 28-day strength will be approximately 11 percent, 22 percent, and 26 percent lower, respectively.

SNRN 10/23/2020 Admin Bandung Indonesia

Role of Concrete Curing

Curing plays an important role on strength development and durability of concrete. Curing takes place immediately after concrete placing and finishing,...

1. Ducts

Thin sheet metal pipes with claw coupling or welded overlapped seam supplied in lengths of 5 and 6 m respectively are used as a standard. Ducts are connected to each other by an external screw coupling and sealed with PE tape. Plastic ducts are also available in the market these days which are water tight , frictionless and fatigue resistant

Ducts are embedded inside a concrete structure to form a void for installing and prestressing post-tensioning strands after the concrete attains desired strength. ducts are manufactured from different materials such as steel and plastic. ducts type and material can change according to the location and purpose of use.

There are two main reasons for grouting the ducts of post-tensioned concrete members. One is to provide efficient bond between the prestressing steel and the concrete member so as to control the spacing of cracks at heavy overload. This increases the ultimate strength of a structural member. The other is to prevent corrosion of the prestressing steel. Both of these objectives require complete filling of the void spaces within the duct.

2. Tendons

The basic element of a post-tensioning system is called a tendon. A post-tensioning tendon is made up of one or more pieces of prestressing steel, coated with a protective coating, and housed inside a duct or sheathing.

Post-tension steel tendons have an ultimate tensile strength of 270,000 pounds per square inch (PSI) compared to mild rebar steel with a typical tensile strength of 60,000 pounds per square inch. Post-tension tendons are fabricated to the proper lengths and delivered to the site where they are simply uncoiled and placed in the forms prior to concrete placement. Post-tension reinforcement is installed must faster than rebar steel which reduces construction time and cost. In short, post-tension concrete construction gives you more strength per dollar than conventionally reinforced concrete and when properly designed and constructed, less cracks.

3. Anchors

Anchors are used to anchor the tendons into the concrete while terminating or joining two tendons. Main function of anchorage is to transfer the stressing force to the concrete once the stressing process is completed.

A Post Tensioning Anchorages is a mechanical device,

consisting of all components required to transfer the post-tensioning force from the prestressing steel to the structure.in addition, Its including all accessories for grouting.

Furthermore, Stressing anchorage is the most common form of post-tensioning system.

It is widely used in concrete structure and soil / rock retaining walls.post tensioning anchorages consist of wedge, anchor head, bearing plate and spiral reinforcement.

SNRN 8/14/2020 Admin Bandung Indonesia

Components of Post Tensioning Slab

1. Ducts Thin sheet metal pipes with claw coupling or welded overlapped seam supplied in lengths of 5 and 6 m respectively are used as a standard....

There are three main types of cracks in concrete. Each has its own cause and strategies to prevent or minimize.

Plastic shrinkage cracks. These occur during the first few hours when the concrete is still in a “plastic” state. They are caused when the surface moisture evaporates too quickly, usually during hot or windy weather. Synthetic fiber additives can help reduce this type of cracking, but do little once the concrete has cured.

Drying shrinkage cracks. These occur as moisture leaves the concrete after the slab has hardened. The main cause is concrete that is too wet, referred to as a “high-slump” mix. The best solution is to use less water in the concrete mix. Concrete suppliers sometimes add water to make the concrete easier to work with, but this weakens the concrete.

Welded wire mesh can also help reduce shrinkage cracking, but only if it is placed in the middle or upper half of the slab, but at least 2 inches below the surface. Wire mesh also helps keep small cracks from growing. In too many cases, however, the wire mesh ends up on the bottom of the slab where it does nothing.

Shrinkage cracking can be managed by the use of control joints placed in the slab. Some contractors cut or form a grid of small grooves in the slab to keep the shrinkage cracks in an orderly grid, which looks better than random cracks, but functions the same way. If you are placing tile on the slab, it’s important the control joint line up with a control joint in the tile easier said than done. So random cracking might be a better approach for tile.

Structural cracks. Concrete can support a lot of weight in compression, but is weak in tension. For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab.

The best protection against structural cracking in residential structures is good compaction of the soil and gravel underneath the slab. In addition, rebar should be placed in the footings around the perimeter of the slab and at post bases within the slab.

SNRN 4/30/2020 Admin Bandung Indonesia

Civil Engineering

Role of Concrete Curing

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Advantages of Post Tension Slab

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Prestressed Concrete Design (Khmer Language)