Brief overview of composite slabs and columns

Composite slabs contain reinforced concrete cast on top of profiled steel decking that functions as formwork all throughout construction and external reinforcement at the final stage.

Composite slabs:

• Comprise of profiled steel decking with an in-situ reinforced concrete topping.
• The decking (profiled steel sheeting) not only functions as permanent formwork to the concrete, but also arranges adequate shear bond with the concrete in order that when the concrete attains strength, the two materials function compositely side by side.
• Spacing should remain among 3 m and 4.5 m onto supporting beams or walls.
• When the slab is unpropped at the time of construction, the decking alone can withstand the self weight of the wet concrete and construction loads. Succeeding loads are implemented to the composite section.
• When the slab is propped, all of the loads should be countered by the composite section.
• The slabs are generally designed as simply supported members in the normal condition

Profiled steel sheeting:

• Depths vary from 45 mm to over 200 mm
• Yield strengths vary from 235 N/mm2 to minimum 460 N/mm2
• 0.8 mm and 1.5 mm thick
• The different shapes offer Interlock among steel and concrete
• Decking may also be applied to make the beams stable against lateral torsional buckling all through construction.
• Make the building stable entirely by performing as a diaphragm to transmit wind loads to the walls and columns.
• Temporary construction load generally governs the choice of decking profile.

Composite Columns: A steel-concrete composite column stands for a compression member that includes either a concrete encased hot-rolled steel section or a concrete filled tubular section of hot-rolled steel. The existence of the concrete is permitted in two ways.

• Protection from fire
• It is assumed to resist a small axial load
• Composite columns minimize the effective slenderness of the steel member that improves its resistance against axial load.

The bending stiffness of steel columns with H-or I-section is much higher in the plane of the web (‘major-axis bending’) than in a plane equivalent to the flanges (‘minor-axis bending’). The ductility performance of circular type of columns is considerably superior as compared to rectangular types. It is not necessary to arrange additional reinforcing steel for composite concrete filled tubular sections. Corrosion protection is arranged with concrete to steel sections in encased columns.

When local buckling of the steel sections may be detached, the cutback in the compression resistance of the composite column because of overall buckling should be permissible positively. The plastic compression resistance of a composite cross-section demonstrates the maximum load that can be implemented to a short composite column.



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Published By
Rajib Dey
www.constructioncost.co
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Details about R-value and U-value of Concrete Slab

The conduction of heat regarding the materials utilized in its construction is measured with the thermal conductivity coefficient alias k-value (W/m.K), of the materials used in its construction. It belongs to the rate at which heat moves through a material among points at dissimilar temperatures.

The thermal resistance alias R-value (m2K/W), is computed by dividing the thickness of the material (in metres) with the k-value. From this the thermal transmittance alias U-value (W/m2.K) concerning a building material is measured like the inverse of the sum of the R-values of the component parts and adjoining air layers.

R-value stands for the estimation of concrete slab (or other material) strength to counter heat flow i.e. it calculates the potency of insulation or thermal resistance. However, U-value stands for the estimation of heat transmission throughout concrete slab from ground into the closed space or oppositely.

Since, thermal insulation fluctuates contrariwise with density, lower density concrete offers superior insulation as compared to greater density concrete. With the purpose of determining the strength of the reinforced concrete slab to withstand heat transfer, it is necessary to estimate R-value and U-value of the reinforced concrete slab under consideration.

The method for estimating R-value for Concrete Slab: A perfect R-value for a normal concrete slab is computed with an R-value, the thermal resistance per inch of thickness, among 0.1 and 0.2 and multiplying it times the slab thickness. The value of R is determined with the following equation provided by ASTM C 168:

Here, the temperature variation (among exterior and interior of concrete slab) is provided in degrees Fahrenheit, the area is given in square feet, the time in hours, and the heat loss in Btus.

ASTM C 168 also offers two supplementary expressions to measure R-value and is available in ASTM C 168 document.

The R-value imperial unit and metric unit are given below:

Computation of u-value for Concrete Slab

The U-value for a concrete slab (and any other building material) stands for the inverse of its R-value and is computed with the formula given below:

The U-value unit is the inverse those of R-value:

Note: There is difference among the American Standard and European Standard for R-value and U-value. Therefore, to transform an American R-value into a European U-value, divide 1 with the R-value, then multiply the result by 5.682 whereas transforming a European U-value to an American R-value, multiply with 0.176, then divide 1 by the result.



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Published By
Rajib Dey
www.constructioncost.co
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Decorative concrete ideas on walls

With the emergence of advance technology, the color, texture and patterns can now be added to concrete to make it decorative and stylish product.

Given below, some useful tips on how to utilize aesthetic concrete in designs.

1. Stamped concrete: Emulating nature and reiterating the appearance of natural materials like wood grain textures or rock patterns, or even brick outlines and other creative designs, stamped concrete offers huge scopes for making decorative concrete surfaces. Now, advance technology makes it possible to provide decorative art effects on both walls and other vertical surfaces, both interior and exterior.

2. Lightly etched concrete: Now, lightly etched concrete surfaces are also becoming popular. These types of surfaces do not contain any joints and produce seamless surfaces. Usually, it is formed with sandblasting or acid wash. To develop a light surface with approx thickness of 0.25 mm, extra-light surface retarders are utilized. This dry-shake powder can is integrated with fresh cast-in-place concrete and new-generation stencils rather than stamps. Different types of designs are formed with diverse colors.

3. Shaped (precast) concrete: With shaped concrete, it is possible to develop attractive vertical surfaces. With the application of formwork or moulds to cast concrete panels, various types of shapes or designs can be produced which one can visualize with concrete from gently flowing structures, curves, edges and more.

4. Colored concrete: By applying an extensive range of color pigments, both ready-mix concrete and dry-cast products are colored to diverse intensities, shades and brightness. One can only select to color the surface or to indicate integral coloring, which permeates the concrete throughout in order that the color won’t wear away with the existence of abrasion.

It should be kept in mind that the same cement from the same plant should be employed to provide a persistent color because portland cement can change from light to dark grey. The color of pigmented concrete can also be changed with the cement content, aggregate color and texture, as well as the total water content.

In order to color or decorate existing concrete as well as repair faded colors, the concrete dyes, stains are used or it can be done with an overlay of pigmented concrete.

5. Metallic coatings: Metallic coatings are also used to produce unique dazzling textures and effects. They are suitable for industrial interiors where two surfaces do not appear exactly same. Metallic epoxy coatings are normally utilized with a coating knife and softened over the vertical surface or smeared with the help of creative blending methods. It is possible to apply in excess of one metallic color, with or without an accent background color, and can even create textured surfaces.

To get more detail, go through the following video tutorial www.buildinganddecor.co.za



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Published By
Rajib Dey
www.constructioncost.co
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