Civil Engineering: Reinforced Concrete

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What Is Singly Beam & Doubly Beam

What Is Singly Beam?A singly beam is the beam which is provided with longitudinal reinforcement in the tension zone. Compressive forces are handled...

Guide to Design of RCC Columns

The columns in a structure carry the loads from the beams and slabs down to the foundations, and therefore they are primarily compression members,...

Basic Civil Engineering

As we all know that civil engineering can’t be isolated from other engineering fields. For the betterment of human life civil engineering require...

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,...

Why hook is provided in stirrups

Why hook is provided in stirrups- To prevent from buckling of column.- The main requirement for safety against bond failure is it provide a sufficient...

Excel Sheet Box Culvert Analysis and Design

A culvert is a structure that allows water to flow under a road, railroad, trail, or similar obstruction. Typically embedded so as to be surrounded by...

What is Deflection?

Deflection, in structural engineering terms, refers to the movement of a beam or node from its original position due to the forces and loads being applied to the member. Deflection, also known as displacement, can occur from external applied loads or from the weight of the structure itself, and the force of gravity in which this applies. It can occur in beams, trusses, frames and basically any other structure. To define deflection, let’s take a simple cantilevered beam deflection that has a person with weight (W) standing at the end:

In all practical engineering applications, when we use the different components, normally we have to operate them within the certain limits i.e. the constraints are placed on the performance and behavior of the components. For instance we say that the particular component is supposed to operate within this value of stress and the deflection of the component should not exceed beyond a particular value.

In some problems the maximum stress however, may not be a strict or severe condition but there may be the deflection which is the more rigid condition under operation. It is obvious therefore to study the methods by which we can predict the deflection of members under lateral loads or transverse loads, since it is this form of loading which will generally produce the greatest deflection of beams.

Assumption: The following assumptions are undertaken in order to derive a differential equation of elastic curve for the loaded beam

1. Stress is proportional to strain i.e. hooks law applies. Thus, the equation is valid only for beams that are not stressed beyond the elastic limit.

2. The curvature is always small.

3. Any deflection resulting from the shear deformation of the material or shear stresses is neglected.

It can be shown that the deflections due to shear deformations are usually small and hence can be ignored.

Because the design of beams is frequently governed by rigidity rather than strength. For example, building codes specify limits on deflections as well as stresses. Excessive deflection of a

beam not only is visually disturbing but also may cause damage to other parts of the building. For this reason, building codes limit the maximum deflection of a beam to about 1/360 th of its spans.

A number of analytical methods are available for determining the deflections of beams. Their common basis is the differential equation that relates the deflection to the bending moment. The solution of this equation is complicated because the bending moment is usually a discontinuous function, so that the equations must be integrated in a piecewise fashion.

Method of double integration The primary advantage of the double- integration method is that it produces the equation for the deflection everywhere along the beams.

􀂃 Moment-area method The moment- area method is a semigraphical procedure that utilizes the properties of the area

under the bending moment diagram. It is the quickest way to compute the deflection at a specific location if the bending moment diagram has a simple shape.

􀂃 The method of superposition, in which the applied loading is represented as a series of simple loads for which deflection formulas are available. Then the desired deflection is computed

by adding the contributions of the component loads (principle of superposition).

SNRN 5/06/2020 Admin Bandung Indonesia

What is Beam Deflection (Deflection Definition) ?

What is Deflection?Deflection, in structural engineering terms, refers to the movement of a beam or node from its original position due to the forces...

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

Type of Cracks in Concrete Slab

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...

Bar Bending Schedule (BBS)

Bar Bending Schedule (BBS) is basically the representation of bend shapes and cut length of bars as per structure drawings. BBS is prepared from construction...

What is an underground tank?
An underground tank (or sub-surface tank), is a water storage structure constructed below the
ground. The term also includes structures that are partially below ground. In most cases,
underground tanks collect and store runoff from ground catchments such as open grasslands,
hillsides, home compounds, roads, footpaths, paved and unpaved area . However, in
certain circumstances, roof catchments can also be channeled into underground tanks.

Underground tanks are especially suited to homesteads having thatched roofs and other traditional
structures or for areas where a roof catchment may not be feasible. However, it may be necessary
to pump (lift) water, except where the ground gradient permits and where gravity outlets are
constructed. Underground tanks can be designed as spherical or cylindrical and constructed using
bricks. Since underground tanks get support from the surrounding ground, thus they can be built
with less reinforcing material. Thus, underground tanks have lower construction costs and
therefore, are more suited for storing agricultural water than surface tanks.

Advantages and disadvantages of underground tanks

Advantages of sub-surface tanks:

 Underground tanks offer a cheaper to install due to its lower cost of reinforcements needed

during construction, as compared to surface tanks. This is due because ground support

provides the strength needed to hold the water

 They are appropriate in places where space above ground is limited; and they can be made

larger than surface tanks

 The water is sometimes cooler

 Larger volumes of water can be stored.

Disadvantages:

 Pump or some kind abstraction device (such as rope and bucket) is required to lift the water.

Except where the ground gradient permits and where gravity outlets are constructed.

 Higher possibility of contamination and sedimentation sediment inflow.

 They cannot be easily drained for cleaning.

 If not well managed e.g. properly covered, they pose danger to children and small animals.

 Leaks or failures are more difficult to detect

 Tree roots can damage the structure from beneath

 Flotation of the tank may occur if groundwater level is high, and

 Heavy vehicles driving over a tank or other weight can damage the structure.

SNRN 4/28/2020 Admin Bandung Indonesia

What is an underground tank?

What is an underground tank? An underground tank (or sub-surface tank), is a water storage structure constructed below the ground. The term also...

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