Various tests conducted for Rebar

In rebar pieces, the following items are generally found :-
X stands for diameter of rebar in mm as for instance 20.
ABCD stands for brand of rebar ranging from TATA TISCON, SAIL, JSPL etc.

Y stands for yield stress of rebar as for example 500 signifies Fe500 having yield stress similar to 500 Mpa

Chemical composition test
LSA or ladle sample analysis is performed in LD and details are noted.
TPA or test piece analysis is performed at chemical lab.
The chemical composition analysis should be adhered to Amend 1 of Cl. 4.2 of IS 1786-2008

Main physical property tests for a specific rebar are provided below :

1. Tensile test : Numbers of rebar sample =4, length of rebar sample =about 600 mm
2. Bend test : Numbers of rebar sample =4, length of rebar sample =about 500 mm
3. Rebend test : Numbers of rebar sample =4, length of rebar sample =about 500 mm

Individual Sample: In order to work out the nominal mass of an individual sample, ascertain the mass of any individual sample selected arbitrarily as mentioned in 11.1 and divide the same with the actual length of the sample. The length of the sample should not be under 0. 5m

Batch: The nominal mass of a batch is computed from the mass of the test specimens selected as mentioned in 11.1 and divide it with the genuine total length of the specimens. The length of each specimen should not be under 0.5 m

Tensile test
Existing Inputs :
1. Length of sample = L
2. Weight of sample = w
A = w/0.00785L mm2
[According to Cl. 6.3.1 of IS 1786-2008 given at right]

Gauge length for a rebar of dia D mm = 5D mm
The rebar sample is marked at (5D/2) mm locations.

To get more clear information, go through the following article engineeringcivil.com

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Published By

Rajib Dey

www.constructioncost.co

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Properties and benefits of rebar cages

In drilled shaft construction, rebar cages are generally utilized to reinforce the shaft throughout excavation. To retain the strength of this cage as well as complete the entire construction project successfully, the design of the cage should be perfect.

Normally, a rebar cage for a drilled shaft comprises of longitudinal bars which are allocated with uniform spacing along the boundary of a cylinder.

In order to reinforce these bars, steel is arranged transverse to the bars and connected with ties, clamps or welds. Other elements of rebar cages may contain hoops for sizes, guides for centering the cages in the borehole and the premie inside of the cage, and stiffeners and pickup devices applied to facilitate lifting the cages.

Larger cages should contain temporary or permanent strengthening components to get rid of permanent distortion due to the stresses of lifting and placing.

As rebar cages are vital drilled shaft construction, it is essential that these cages should be properly constructed on the basis of a calculation of the stresses that it will endure.

The amount of reinforcing steel in a rebar cage should adhere to the structural requirements, taking into account combined stresses of axial load, lateral load and moment.

Properties of Steel Used for Rebar Cages

The American Society for Testing and Materials (ASTM) defines various steels which are employed for reinforcing drilling shafts.

The American Association of State Highway and Transportation Officials (AASHTO) approves most of these ASTM steels for being applied in building rebar cages for drilled shaft construction.

Normally, the steel accessible for these cages is AASHTO M 31 (ASTM A 615) in Grade 40 or Grade 60. If welding is required, then weldable steel, like ASTM A 706, should be utilized.

In case, there is risk for corrosion, galvanized or epoxy-coated steel are useful for longitudinal and transverse reinforcement. This is often stated for marine environments where the chloride content of ground or surface water is extreme.

As nicks and blemishes in the coating may happen at the time of the lifting and disposition of the rebar cases, rapid corrosion may occur. Under this situation, rebar without epoxy should be utilized and the drilled shaft should be filled with a low-permeability concrete to enhance the resistance strength against corrosion.

In unusual situations, high strength reinforcement is suitable. It may comprise of threaded couplers for splice connections and higher-strength rebar.

Contractors should compute the structural requirements of a drilled shaft cautiously while deciding the requirements of a rebar cage.

To get more details, go through the following articlewww.pilebuck.com



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Published By
Rajib Dey
www.constructioncost.co
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Different types of grades of concrete as per 456 : 2000 standard

This construction video tutorial from learning technology demonstrates the grades of concrete on the basis of IS 456 : 2000 standard. As per this standard the grades are categorized in three groups like ordinary concrete, standard concrete and high strength concrete. Here grades range from M10 to M80.

M10, M15 and M20 belong to Ordinary Concrete. M25, M30, M35, M40, M45, M50, M55 belong to Standard Concrete. M60, M65, M70, M75 and M80 belong to High Strength Concrete (HSC).

Here m denotes mix and number denotes characteristic compressive strength of 150 mm cube at 28 days in N/mm2 (Newton per millimeter square). It means if you take compressive test of a cube having dimensions 150 x 150 x 150 mm for 28 days, the value obtained is 10 N/mm2.

All these concrete grades are suitably for PCC work i.e. plain cement concrete work where rebar is not used.

Standard Concrete is generally used in 10 storied and higher buildings and super structures.

High strength concrete is mostly used for designing heavy structures.

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Published By

Rajib Dey

www.constructioncost.co

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The fundamentals of setting up Post-Tensioning Slabs

Construction of post-tensioned slabs on grade is equivalent to apply reinforcing steel, devoid of the tensioning step.

Cables are set up as per instructions of the engineer and placed to go over the center of the slab. For residential construction, tendons at 48 inches on center are generally accepted. Commercial foundations will contain much more steel. Tendons are routed around obstructions smoothly.

Generally, a residential post-tensioned concrete slab should have been 8 inches thick with 3000 psi concrete. As soon as the concrete achieves strength to 2000 psi, normally within the 3 to 10 days as suggested by PTI, the tendons are stressed.

Now-a-days, tendons are seven high-strength steel wires wound together and arranged inside a plastic duct. A PT anchor is situated at each end and these are found in pockets which are implanted into the slab edge. As soon as the strands are stressed, the wires are expanded —about 4 inches for a 50 foot strand—to employ 33,000 pounds of load.

The qualified workers should be appointed for doing stressing. Once the stressing is completed, the tendon is cut off and the pocket in which the anchors are situated is filled with grout to defend then against corrosion.

Bigger structural concrete members may also be post-tensioned, particularly in bridges and floors and beams in parking structures. The process is equivalent to that applied for slabs, with the exception of a bigger scale. The tendons will frequently be "draped" in order that they are low at the midpoint of a beam and high at the supports—this arranges the steel at the point of highest tension where it can retain the concrete to be remained together firmly.

With structural members the duct is frequently grouted full following stressing to tie the strand to the concrete along its entire length—these are known as bonded tendons. Unbonded tendons are mostly found in residential slabs and stay free to progress within the duct and are safeguarded from corrosion with grease.

The position of PT tendon and stressing is normally performed with companies with certified workers having expertise in this type of work.

For more information, read the following construction article concretenetwork.com



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Published By
Rajib Dey
www.constructioncost.co
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Some useful tips for making inspection of reinforcement in jobsite

Though the inspection of reinforcement involves lots of complexities and huge cost, but the problem should be properly resolved by appointing a standard consulting firm for accomplishing the inspection work.

The first step should be checking drawing. One must have adequate knowledge for analyzing a rebar drawing. The field engineers should have proper knowledge & capability to communicate with it in the jobsite.

To become an efficient rebar checker, practice for reading drawings on a regular basis. For this purpose one should gather proper knowledge with the steel rebar binding process. Given below, some useful tricks provided by some top peers who have lots of experience in this field :-

It is suggested to carry a measuring tape with yourself all the times for the purpose of examining the spacing and the splice or development length.

One should have clear conception on physical features of the construction component i.e. if some type of drainage pipe or electrical conduit should be set up or water stopper should be substituted or any other pipes / implanted item should be arranged.

Initially, the diameter of rebar should be examined properly through a vernier caliper. The other important things which should be examined thoroughly are rebar spacing, rebar development length, lap / splice length, alignment, no sag or buckling present in the bars, the couplers should be perfectly set and tightened, bars should be perfectly fixed, there should not be any rust on the bars, check for proper clear cover, no of bars should be computed properly and be similar with the drawings and should not be under or over 2 bars which are arranged there.

Always take suggestions from the contractor’s foreman or site engineer for the placement of the bars.

One should have sound knowledge with the steel quality tests like tensile strength check or torsion failure strength or coupler tension strength check etc. Make sure that the applied steel is taken from the verified lot and must not be a failed quality.

Though proper execution of design becomes very complicated because of inconsiderate field conditions and difficultly in installation / fixing of rebars, but always ensure the design of reinforcement is completely perfect as improper design can lead to failure of a building if fully neglected / violated.

Always keep in mind that the orientation of bars like main rebars should be under the distribution or temperature rebars. It can be verified from the drawings delivered to you.

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How concrete strength is impacted with different factors

The strength of concrete is impacted by various factors. The details are given below :-

Concrete porosity: Air and water are the useful substances to fill up voids in concrete. Air voids belong to pores in concrete. When concrete is blended it contains air trapped in the mix. The vibrators are used to clear out the air at the time of pouring walls.

If the concrete is less porous, it’s strength will be increased and calculated with compressive strength. The most crucial source of porosity in concrete refers to the proportion of water to cement in the mix, called the ‘water to cement ratio’.

Factors water/cement ratio: It is described as the mass of water divided by the mass of cement in a mix. As for instance, in a concrete mix if there are 400kg cement and 240litres(=240kg) of water, the water/cement ratio will be 240/400=0.6. The water cement ratio is shortened as ‘w/c ratio’ or just ‘w/c’. In mixes where the w/c is in excess of roughly 0.4, all the cement can, in theory, react with water to develop cement hydration products. If the w/c ratios are greater, it follows that the space occupied by the supplementary water over w/c=0.4 will persist as pore space filled with water, or with air when the concrete becomes dry.

As a result, when the w/c ratio become higher, the porosity of the cement paste in the concrete also upsurges. With the higher porosity, the compressive strength of the concrete will reduce.

Stability of aggregate: It is inevitable that when the aggregate in concrete is feeble, the concrete also becomes feeble. Rocks like chalk that contain low intrinsic strength, are not appropriate to be utilized as aggregate.

Aggregate-paste bond: The strength of the bond among the paste and the aggregate is vital. When no bond exists, the aggregate practically reproduces a void and the strength of concrete is decreased.

Cement-related parameters: Various parameters pertaining to the formation of the individual cement minerals and their ratios in the cement can impact the rate of strength growth and the final strengths gained.

To get more information, click on the following link www.onlinecivilforum.com

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