Civil Engineering: Reinforced Concrete

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Dog legged stair case Design

Stairs provide access for the various floors of the building. The stair consists of series of steps(flight) with landings at appropriate intervals....

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.

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Concrete Curing Role

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

There are many texts on pile foundations. Generally, experience shows us that
undergraduates find most of these texts complicated and difficult to
understand.
This guide has extracted the main points and puts together the whole process of
pile foundation design in a student friendly manner.
The guide is presented in two versions: text-version (compendium from) and
this web-version that can be accessed via internet or intranet and can be used as
a supplementary self-assisting students guide.
Pile foundations are the part of a structure used to carry and transfer the load of
the structure to the bearing ground located at some depth below ground
surface. The main components of the foundation are the pile cap and the piles.
Piles are long and slender members which transfer the load to deeper soil or
rock of high bearing capacity avoiding shallow soil of low bearing capacity The
main types of materials used for piles are Wood, steel and concrete. Piles made
from these materials are driven, drilled or jacked into the ground and connected
to pile caps. Depending upon type of soil, pile material and load transmitting
characteristic piles are classified accordingly. In the following chapter we learn
about, classifications, functions and pros and cons of piles.

Function of piles

As with other types of foundations, the purpose of a pile foundations is:
to transmit a foundation load to a solid ground
to resist vertical, lateral and uplift load
A structure can be founded on piles if the soil immediately beneath its base
does not have adequate bearing capacity. If the results of site investigation
show that the shallow soil is unstable and weak or if the magnitude of the
estimated settlement is not acceptable a pile foundation may become
considered. Further, a cost estimate may indicate that a pile foundation may be
cheaper than any other compared ground improvement costs.
In the cases of heavy constructions, it is likely that the bearing capacity of the
shallow soil will not be satisfactory, and the construction should be built on
pile foundations. Piles can also be used in normal ground conditions to resist
horizontal loads. Piles are a convenient method of foundation for works over
water, such as jetties or bridge piers.

End bearing piles

These piles transfer their load on to a firm stratum located at a considerable
depth below the base of the structure and they derive most of their carrying
capacity from the penetration resistance of the soil at the toe of the pile (see
figure 1.1). The pile behaves as an ordinary column and should be designed as
such. Even in weak soil a pile will not fail by buckling and this effect need only
be considered if part of the pile is unsupported, i.e. if it is in either air or water.
Load is transmitted to the soil through friction or cohesion. But sometimes, the
soil surrounding the pile may adhere to the surface of the pile and causes
"Negative Skin Friction" on the pile. This, sometimes have considerable effect
on the capacity of the pile. Negative skin friction is caused by the drainage of
the ground water and consolidation of the soil. The founding depth of the pile is
influenced by the results of the site investigate on and soil test.

Please download attachment file to see details Pile Foundation Design Guide for student
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Pile Foundation Design Guide

There are many texts on pile foundations. Generally, experience shows us that undergraduates find most of these texts complicated and difficult to understand. This...

Isolated Foundation Design and Analysis Excel Sheet ACI 318M-95

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Strength Design Method Analysis and Design of T Beams

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There are five type of Portland cement used in construction
Type I – Ordinary Portland Cement
cement is made for use in general concrete construction. It should be regarded as a standard
material to be used on all work where no unusual conditions or requirements are likely to be encountered. Ready-mixed concrete is usually batched with Type I cement unless some other type is specified by the purchaser.

Type II – Moderate Heat Portland Cement
cement imparts to concrete all the essential characteristics obtainable with Type I cement, plus
improved resistance to sulfate attack,less generation of heat, somewhat better work a ability, lower permeability, and less tendency to bleed. Type II cement is ground some what finer than Type I, and it
has a somewhat different chemical composition. Concrete made with Type II cement will show lower early strength than concrete containing Type I cement, but at three months there is no important difference in strengths.

Type III – High Early Strength Portland Cement
cement is often referred to as high-early-strength cement. It is ground much finer than Types I
and II and its most important characteristic is rapid development of strength. It is used in emergency
construction, or under any conditions that require early discontinuation of curing and protection. As
the table indicates, the strength advantage of concrete made with Type III cement drops steadily with age, eventually equaling that of concretes made with Types I and II cement High-early strength can also be obtained at somewhat less cost by using an accelerator with either Type 1 or TvDe II cement.

Type IV – Sulfate Resisting Portland Cement
cement generates less total heat, and does it at a slower rate, than the other types. It is used mainly in massive concrete structures to prevent the severe cracking that may occur when high tempera-t u res are reached during hydration . Concrete made with Type IV cement is not generally suitable for ordinary structures because it requires extra care at early ages, and pro longed curing (21 days or more) is necessary to obtain adequate strength and weather resistance. As the accompanying table show s, Type IV cement develops comprehensive strength rather slowly, but with adequate protection and curing it reaches equality with concrete made with Type I cement.

Type V–Low-Heat Portland Cement
cement provides the high-est attainable resistance to alkali at-tack, and for this reason it is generally specified for structures which come in contact with water or soil having large concentrations of sulfates. Early strengths are low, but are somewhat higher than for Type IV cement. It also compares favorably with Type IV cement in respect to heat generation. When proper curing conditions are maintained, concrete made with Type V cement attains excellent strength .
Download this book to read more details.

Download from Google Drive
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File Size : 6.56 MB

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Types of Portland Cement (Khmer Language)

There are five type of Portland cement used in construction Type I – Ordinary Portland Cement cement is made for use in general concrete construction....

Reinforcement Development Lengths

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