Some vital tips to control cracking with Reinforce Concrete Slab on Ground

Steel reinforcing bars and welded wire reinforcement are used to check crack width in nonstructural slabs-on-ground.

Most slabs-on-ground are unreinforced or minimally reinforced for crack-width control. If steel reinforcement is arranged in the upper or top portion of the slab thickness, it restricts the widths of random cracks resulting from concrete shrinkage and temperature restraints, subbase settlement, applied loads or other issues. This type of reinforcement is normally defined as shrinkage and temperature reinforcement.

Shrinkage and temperature reinforcement is not same as structural reinforcement. Structural reinforcement is generally arranged in the bottom section of the slab thickness to enhance the load bearing strength of the slab. Most structural slabs-on-ground contain both top and bottom layers of reinforcement for managing crack-widths and improving load capacities. Due to constructability issues and costs regarding two layers of reinforcement, structural slabs-on-ground are not regularly used as nonstructural slabs.

The basics: Steel reinforcing bars and welded wire reinforcement can’t control cracking. Reinforcement mainly remains inoperative unless the concrete cracks. Once cracking happens, it gets activated and manages crack widths by limiting the expansion of crack.

When the slabs are provided on greater quality sub bases with uniform support and include low shrinkage concrete with joints perfectly installed at a gapping of 15 feet or less, reinforcement is normally is not required. Seemingly, there exist few random or out-of-joint cracking. In case of random cracks, they should remain moderately tight due to the restricted joint spacing and low concrete shrinkage thus future serviceability or maintenance issues will be reduced.

If slabs are arranged on difficult sub bases with risks of non-uniform support or comprise of medium to high shrinkage concrete or joint spacing surpassing 15 feet, then reinforcement should be provided to control the widths of cracks. Since crack widths expand and become about 35 mils (0.035 inches), the effectiveness of load transfer via aggregate interlock is reduced and differential vertical movements over cracks or slab "rocking" can happen.

Due to this, crack edges remain uncover and edge spalling takes place, particularly when the slab is uncovered to wheeled traffic and especially hard-wheeled lift trucks. As soon as spalling begins, crack widths at the surface get expanded and slab deterioration along cracks is raised considerably.

When contraction joints are inappropriate and not installed, shrinkage and temperature reinforcement is necessary. This design approach is sometimes called as continuously reinforced or joint-less slabs and produces several closely spaced (3 to 6 feet) fine cracks all through the slab.

Crack control options: Normally, the cracks in slabs-on-ground are controlled with the following ways -

1) check the location of cracking by installing contraction joints (does not control crack widths) or 2) Installation of reinforcement (does not control crack location).

With Option 1, we can know where to crack in the slab and widths of contraction joints or cracks in the joints are mostly managed by the joint spacing and concrete shrinkage. When joint spacings and concrete shrinkage are raised, joint widths also expand. Similar to cracks, when joint widths turn out to be about 35 mils, the effectiveness of the aggregate interlock to transmit loads and resist differential vertical movements across joints is considerably decreased. Because of this, several load-transfer devices like steel dowels, plates or continuous reinforcement through contraction joints are used to keep positive load transfer and control differential vertical movements across joints.

With Option 2, the slabs are allowed to crack indiscriminately but crack widths are controlled through steel reinforcing bars or welded wire reinforcement. Normally, contraction joints are not installed with this option rather cracking happens indiscriminately that develop several, tightly held together cracks.

Read more

~~~~~~~~~~~~~~~~~~~~~~~~~~
Published By
Rajib Dey
www.constructioncost.co
~~~~~~~~~~~~~~~~~~~~~~~~~~

Some useful tips to measure loads on column, beam and slab

In order to work out the total load on columns, Beam and Slab, there should be clear ideas on the types of loads enforcing on the column.

Different Loads operating on Column:

1) Column Self Weight X Number of floors
2) Beams Self Weight per running meter
3) Load of walls per running meter
4) Total load on Slab (Dead load + Live load + Self weight)

Apart from above loading, the columns are also susceptible to bending moments which should be taken into consideration in the final design.

For Colomn: The Self weight of Concrete remains approx 2400 kg/m3, that is similar to 240 kN and self weight of steel is approx 8000 kg/m3.

Therefore, if we consider a column size of 230 mm x 600 mm with 1% steel and 3 meters standard height, the self weight of column is approx 1000 kg per floor that is equivalent to 10 kN.

At the time of making calculation, self weight of columns is taken as 10 to 15 kN per floor.

For Beam: Similar method is also used for making calculations of beam. Suppose, each meter of beam contains dimensions of 230 mm x 450 mm without slab thickness. Therefore, the self weight should be approx 2.5 kN per running meter.

For Walls: The Density of bricks differs among 1500 to 2000 kg per cubic meter. For a brick wall with thickness 6 inch, height 3 meter a length 1 meter. The load / running meter should be equivalent to 0.150 x 1 x 3 x 2000 = 900 kg, that is identical to 9 kN/meter. This method is useful for working out the load of brick per running meter for any brick type.

For aerated concrete blocks and autoclaved concrete blocks similar to Aerocon or Siporex, the weight per cubic meter should remain 550 to 700 kg per cubic meter.

When these blocks are utilized for construction, the wall loads for each running meter should remain as low as 4 kN/meter, the cost of the project is decreased considerably with the use of this block.

For Slab: Suppose, the slab contains thickness of 125 mm.

Therefore, self weight of each square meter of slab should be = 0.125 x 1 x 2400 = 300 kg that is identical to 3 kN.

Now, If finishing load is taken to be 1 kN per meter and superimposed live load to be 2 kN per meter. Therefore, from above data, the load of slab can be calculated as 6 to 7 kN approximately per square meter.

Factor of Safety: At the end, once the total load on a column is computed, consider the factor of safety that is very crucial for any building design for safe and convenient performance of building during its design life cycle.

Read Continue

~~~~~~~~~~~~~~~~~~~~~~~~
Published By
Rajib Dey
www.bimoutsourcing.com
~~~~~~~~~~~~~~~~~~~~~~~~

10 Concrete Mix Thumb Rules Useful For Every Engineers

If 4 litres of water is added with 1 cubic meter of newly blended concrete, the following result is obtained :-

1. Expand slump roughly 25 mm.

2. Lessen compacted strength around 1.5 to 2 N/mm2.

3. Enhance shrinkage potential nearby 10%.

If temperature of newly mixed concrete is enhanced by 1 degree celcius, the result will be as follow :-

1. Around 4 litres of water per cubic meter sustains identical slump.

2. Air content drops around 1%.

3. Compacted strength reduces approximately 1.0 to 1.5 N/mm2.

In case the air content of newly mixed concrete :-

1. Enhances 1% then compacted strength reduces roughly 0.5%.

2. Lessens 1% then produced result will reduce nearby 0.03 cubic meter per 1 cubic meter.

3. Lessen 1% then slump reduces nearly 12.5mm.

4. Drops 1% then strength reduces almost 10%.

Ref. :www.cegyan.com

~~~~~~~~~~~~~~~~~~~~

Published By

Rajib Dey

www.constructioncost.co

~~~~~~~~~~~~~~~~~~~~

How To Set Up Wood Fence With Posts And Pickets

This construction video provides some useful construction tips for setting up wood fence along with posts and pickets.

At first dig holes for the component fence, then arrange the posts. Initially compute the posts and trim as per length. Prior to placing the posts, include gravel for drainage. Adheres to local building codes to get the exact amount required. Now organize the posts in proper position, insert concrete and water. Fill to roughly 2'' from the surface.

As soon as the concrete is dried, and the posts are securely in position, set the stringers or rails. Arrange one rail for about each 2' of post height. Affix the rails to the outer side of the posts on the bottom, middle and top by screws. Ensure that the rails are leveled with your posts.

Once the rails are installed, finally set up the pickets. Affix the boards through a nail gun to every rail. The pickets should remain right next to each other. Verify to ensure that the pickets are level. For edges, overlay the pickets for a clear and uniform corner.

~~~~~~~~~~~~~~~~~~~~~

Published By

Rajib Dey

www.constructioncost.co

~~~~~~~~~~~~~~~~~~~~~

How To Resolve Wood Fencing Damage Due To Rot

The following construction video offers some good construction tips on management of wood fence construction and how proper maintenance of wood fence can significantly reduces the costs for home repair. The video also highlights how wood fence construction is affected with rot due to moisture & water and collapsed easily. If you can sort out these problems easily, the life of your wood fence will be expanded greatly. Other reasons for damage may range from severe weather (like rainfall, snowstorms, heavy wind, or salt air), damage from vehicles or devastation, and insect incursion.

Some useful tips for installation

If you are going to set up a wood fence, ensure that the lumber is sufficiently dried to prevent warping. Apply pressure-treated wood (particularly for posts, which are uncovered to high levels of moisture in the soil) or naturally moisture-defiant hardwoods like cedar. A special finish, like stain or paint, will be essential to safeguard the above-ground structure. Refrain from using galvanized nails if you’re close on a body of salt water. Set up fence posts not less than 3 feet deep, with tops cut at an angle, to facilitate them to cast off precipitation.

~~~~~~~~~~~~~~~~~~~~~

Published By

Rajib Dey

www.constructioncost.co 

~~~~~~~~~~~~~~~~~~~~~

Usefulness Of Reinforced Concrete Over Plain Concrete

R.C.C. denotes Reinforced Cement Concrete, and in recent times it is considered as the primary Constructional Material to be utilized for developing various structural parts. Previously, high rise buildings were constructed with Pure Steel Structure but with the use of R.C.C., it becomes easier to obtain superior strength economically to resist any powerful imposing loads on structure.

R.C.C. is formed with two main diverse components like Concrete and Reinforcement involving Steel.

Why reinforcement is necessary?

Because of bending of a structure, one side of the structure has Neutral Axis Compresses and the other side is tensed. So both Compressive Stress and Tensile Stress grow in the structure from the Neutral Axis in the direction of the Extreme Fiber i.e. the external face of the structure in a cumulative manner according to magnitude of the stresses. So the Structure contains both Compressive and Tensile Stresses in it.

A Concrete can withstand Compressive Stresses efficiently but it can’t resist Tensile Stresses very well and it can only defy a very minute Tensile Stress. Therefore, to safeguard a structure and keep it in working condition to bear the Design Loads, the structural material should have the capability to resist both Tensile and Compressive Stresses. But the Concrete is not so powerful to combat the Tensile stress, so some steps should be initiated to provide something which can bear the Tensile Stresses established in Concrete. To make it possible, the Structural Member like Column, Beam, Slab etc are not constructed with Plain Concrete rather than Reinforced with Steel implanted in it to bear the Tensile Stresses developed.

Why Steel is applied as Reinforcement?

Because of the variation of temperature, the Concrete stretches and shrinks. Therefore, the material applied for Reinforcing will also stretch and shrink caused by the variation of temperature on the basis of their co-efficient of volumetric expansion. If the change of Volume of Concrete and implanted Reinforcement will not contain the similar amount at the surface of contact amid Concrete and Reinforcement, various types of stresses will form which can create cracking in concrete and lead to collapsing of the structure. After thorough research, It is established that the Co-efficient of Expansion of Concrete and Steel is nearly same. So with the variation of temperature, no differential Stresses will form and there will no cracks as well as collapsing of the structure.

Ref.: www.technogenome.com

~~~~~~~~~~~~~~~~~~~~~

Published By

Rajib Dey

www.constructioncost.co

~~~~~~~~~~~~~~~~~~~~~

Specification Of Concrete Cover For Concrete Slab Design

Concrete cover for reinforcement is essential for safeguarding the rebar against erosion as well as fire. The depth of the cover is based on ecological conditions and type of structural member. The slightest thickness of reinforcement cover is specified in the drawings, or achieved from the related code of practice. The specifications for reinforcement cover for various structural members in diverse conditions are explained below :-

a) To each end of reinforcing bar, not below 25 mm or below twice the diameter of the bar.

b) In case of a longitudinal reinforcing bar in a column, not under 40 mm not less than the diameter of such bar. For columns of least dimension of 20 cm or below, whose reinforcing bards should not surpass 12 mm, a cover of 25 mm should be applied.

c) For longitudinal reinforcing bars in a beam, not under 30 mm or below the diameter of the bar.

d) For tensile, compressive shear or other reinforcements in a slab or wall not below 15 mm, not less than the diameter of such bar.

e) For any other reinforcement not below 15 mm, not less than the diameter of such bar.

f) For footings and other major structural members in which the concrete is placed directly alongside the ground, cover to the bottom reinforcement must be 75 mm. If concrete is discharged on a layer of thin concrete, the bottom cover is curtailed to 50 mm.

g) For concrete surfaces uncovered to the weather or the ground once forms are separated - like retaining walls, grade beams, footing sides and top etc. below 50 mm.

h) Expanded cover thickness is offered as specified on the drawings, for surfaces uncovered to the action of injurious chemicals (or uncovered to earth polluted by such chemicals), acid, alkali, saline atmosphere, sulphorone, smoke etc.

For more information, visit this link.

~~~~~~~~~~~~~~~~~~~~~

Published By

Rajib Dey

www.constructioncost.co

~~~~~~~~~~~~~~~~~~~~~