Construction Method Of Concrete Slab On Grade

Slab on grade belongs to concrete floor slabs which are poured at grade or ground level to develop a foundation for a home. Prior to start the construction, contractors excavate the soil adjacent to the area where the foundation should be constructed and ensure that the ground has sufficient strength to support the structure.

Normally, the periphery of the slab is very dense as compared to the rest of the surface. This section has similarity with footer that facilitates to circulate the weight of the exterior walls uniformly over the soil underneath the structure. Slab on grade foundations are suitable for the areas where the soil is unusable for crawl spaces or basements.

Grade slabs are provided in areas where the ground is not solidified. This type of slabs may or may not include reinforcement in it. The inclusion of reinforcement depends on the floor loads and local building codes.

The thickness of Grade Slab should be at least 4 inches. If porosity exists in soil, the thickness of the slab should be raised. To maintain safety, a layer of gravel & bitumen should be placed on earth prior to arrange concrete slab to stop the penetration of moisture content into the slab.

Slab on Grade is categorized as follow:

1. Supported Slab on Grade
2. Monolithic Slab on Grade

Supported Slab on Grade / Grade Slab: Supported grade slab or slab on grade foundation should be used when the conventional footings are already set up on site to uplift the columns. Under this system, the wall is laid on a footing and the grade slab stands on a layer of gravel and moisture barrier.

The formwork applied for plinth beams operates as batter boards for slab mould. There is an expansion joint among concrete slab and wall to reduce the stress throughout high-temperature days. Control joints are set in a planned grid with chalk lines to manage occasional cracking on the slab.

Monolithic Slab on grade: There are no footings for monolithic grade slab, the concrete slab itself functions as a footing for the building; and the columns, walls are uplifted from the grade slab. This type of slab is structured with batter boards around the slab based on the plan and the concrete is poured inside batter boards. These batter boards operate as a mould to detect the slab corners.

Grade slabs normally stand on the layers of gravel and moisture barrier. These layers can resist penetration of water into the slab to develop surface cracks.

The periphery of the grade slab is very dense as compared to the rest of surface. This dense section operates as a mini footing and facilitates to circulate top loads more consistently over the adjacent soil.

Construction Method: Prior to cast slab on grade, the excavation of earth is done to the desired depth along with compaction to drive out air voids. Batters are leveled and provided in exact location based on the plan prior to concrete pour. These boards function as a concrete mould to facilitate detecting the slab corners.

After that, soil investigation is performed to design the thickness of the slab. After obtaining the results, the layers of gravel and moisture barriers (bitumen) is again poured on the ground. These layers are used as a sub-base for slab and resist the infiltration of moisture into the slab.

The thick concrete is poured at the edges to form an solid footing and reinforcement rods are placed to increase the strength of the edges.

To reduce random cracking on the surface, the concrete should be cured and dried for long times.

The expansion joint should be arranged among the wall and slab. The control joints on the slab are leveled with chalk lines prior to pouring which can check random cracking.

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Advantages of microsillica in concrete

Microsilica alias silica fume or condensed silica fume stands for is a mineral admixture that is formed with very fine solid glassy spheres of silicon dioxide. Most microsilica particles remain under 1 micron (0.00004 inch) in diameter, normally 50 to 100 times finer as compared to average cement or fly ash particles. Microsilica belongs to a by-product of the industrial manufacture of ferrosilicon and metallic silicon in high-temperature electric arc furnaces.

Microsilica may be suitable in managing heat formation in mass concrete. It can also be combined with fly ash to provide superior result.

If pozzolanic materials are integrated with concrete, the existent silica in these materials makes a reaction with the calcium hydroxide produced throughout the hydration of cement and develops supplementary calcium silicate hydrate (C – S – H) that enhances the strength and the mechanical properties of concrete.

Types of Microsilica: Microsilica is categorized as follow -

1. Powdered microsilica, 2. Condensed microsilica, 3. Slurry microsilica

Impacts of Microsilica on Concrete

1. Fresh Concrete

a. It decreases the scope of segregation, so it is applied as pumping aid.
b. It almost reduces bleeding, as a result finishing work commences before time.
c. Workability and uniformity of concrete reduces.

2. Hardened Concrete: The inclusion of microsilica enhances the following characteristics of hardened concrete -

a. Improves compressive strength that leads to improve flexural and tensile strength.
b. Bond strength
c. Abrasion resistance
d. Lessens permeability; consequently, it safeguards reinforcement steel against corrosion.
e. Impact and cavitations resistance.
f. Sulphate Resistance
g. Heat Reduction
h. Chemical Resistance

Properties of Microsilica:

1. Microsilica belongs to a grey; almost white to black powder.
2. Spherical particles remain under 1mm in diameter.
3. The mass density of microsilica is dependent on the degree of densification and differs from 130 to 600 kg/m3.
4. The specific gravity of microsilica differs among 2.2 to 2.3

Benefits:

a. Minimizes thermal cracking resulting from the heat of cement hydration.
b. Enhance the strength to resist against sulphate and acidic waters.
c. Minimizes the growth of temperature in preliminary stage.
d. Silica fume is cheap; therefore, it is inexpensive.
e. It minimizes the entire slab weight and cost.
f. Inclusion of microsilica reduces efflorescence caused by the refined pore structure and increased consumption of the calcium hydroxide.



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Characteristics of polymer-Modified Mortar

Mortar is considered as one of the vital components in masonry construction. It is normally formed by mixing water with portland cement, hydrated lime and sand. If the proportions of each of these ingredients are changed, strength and other characteristics may differ. Mortar with polymeric admixtures is applied extensively and inexpensively in definite situations.

Basics and function: Polymer-modified mortar is developed by interchanging a part of the conventional binders with polymers. Polymers are included with mortar to enhance the characteristics which may contribute to adhesion, toughness, flexural or tensile strength, and resistance to chemicals.

The purpose of the polymers is to make the capacity of work and adhesion of non hardened mortar better and often need fewer quantity of extra water as compared to conventional mortar. It leads to less pores and high capacity cements, consequently the immersion of water & and penetrability to salts are decreased.

Types of Polymers: Polymer-modified mortar is commercially obtainable with all constituents already provided in the mixture. Conversely, polymer additives separated into classes, are included with mortar mix. Redispersible polymer powders like ethylene vinyl acetate are normally included with dry mortar mix.

Water-soluble polymers like polyvinyl alcohol belong to powders but are added to wet mortar mix. Aqueous latex suspensions comprise of latex particles hanged up in water to coat hydrating cement particles. At the end, liquid polymers as epoxy resins or unsaturated polyesters are included throughout mixing to develop a network of cemented polymer hydrate and thus the strength of the mixture is raised significantly.

Application: Polymer-modified mortar is employed in a wide array of mortar and concrete repair and primary construction applications. Low water level and salt infiltrations transform polymer-modified mortar suitable for masonry prone to weathering and other exterior conditions. The main objective of polymer-modified thinset mortar is to bind tile to concrete and cement board substrates devoid of immersing the tiles earlier. Polymer-modified mortars are frequently applied for repairing purposes due to their low shrinkage and capacity to tie with even solid surfaces.

Supplementary Possible Admixtures: Besides, polymers, other types of materials can be included with mortars to attain required characteristics. Color pigments may be included with mortar to change the look of the mortar. If accelerators and retarders are included, these can decrease or raise the length of time necessary for the mortar to be cured, a vital characteristic to control in severely cold or warm, humid weather. Other mortar additives range from mineral additions, like silica fume, aggregates and inert fillers, plasticizing chemical admixtures and fibers to manage shrinkage efficiently.



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Design of Beam

1. Estimation of constants. -For the specified set of stresses, determine KC, JC and RC.

2. Kc= M~cbc/M~cbc+~st

3. Jc= 1-(Kc/3)

4. Rc= 0.5*L*Kc Jc

5. Estimation of bending moment. Suppose appropriate values of overall depth and breadth of beam, and find out the effective span. Work out the self-weight and total U.D.L. and maximum bending moment in the beam.

6. Design of the section. Workout the effective depth of the beam with the following expression:

7. d= [(M)/ (RC*b)] 0.5

8. Reinforcement. Workout the area with the formula.

9. Shear Reinforcement. Workout the maximum shear force in the beam.

10. Verify for Development length at the end.

11. Ld<= (M1/V) +L0.

Design of column:

1. Find out the allowable stresses in concrete, longitudinal bars and ties.
2. Determine the super impose load that should be borne by the column.
3. Find out the area from the following expression - P= ~cc *Ac+~sc*Asc.
4. After getting details about the area, find out the dimensions of column. If it is a square of side b, then b=Ag.
5. For the specified end conditions, find out the effective length of column. Measure lef/b ratio to determine whether the column is short or long.
6. If lef/b ratio<12 it will be designed as short column or else as long column; define the area of steel Asc.
7. Determine the diameters of bars utilized as ties and find out its pitch according to the rules.

DESIGN OF FOOTING: The width B of the footing will obviously be equivalent to [W+W‟]/qo. The thickness is measured based on the bending moment as well as punching shear.

1. Depth for bending moment.
d= [M/B*Rc] 0.5
2. Depth for shear.
3. tv=V/B*d
4. Steel Reinforcement. Ast=M/tjcd

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Common types of cracks in reinforced concrete column

Usually, Four types of cracks are found in reinforced concrete column which range from diagonal cracks, horizontal cracks, splitting cracks, corrosion cracks. The cracks happen in columns due to improper design, defective construction or overloading, erosion of reinforcement, isolated settlement of foundations, creep and shrinkage.

Cracking in reinforced concrete column may result in loosing of strength, consistency, longevity, and adversely impact aesthetics.

Types of cracks found in concrete columns:

1. Diagonal Cracks: If there are diagonal cracks in reinforced concrete columns, they encircle the whole face of column anywhere along its height. Because of insufficient load bearing strength of the columns, inadequate cross-section and inadequate reinforcement steel.

Diagonal Cracks significantly impact the structural soundness and therefore it should be monitored carefully.

2. Horizontal Cracks: Horizontal crack in reinforced concrete column is generally found at the beam-column joint, and on column face where tensile stress is massive. Columns with insufficient moment resistance strength, inadequate reinforcements, or disposition of installed reinforcement may lead to horizontal cracking owning to the effect of shear force and direct load and uniaxial bending.

Finally, horizontal cracks significantly decrease the shear strength of the column and as a result there is high risk of failure. So, it should be dealt with instantly and carefully.

3. Splitting Cracks: Splitting cracks in reinforced concrete column belong to short parallel vertical cracks having non-uniform width. These types of cracks may occur in columns with insufficient steel reinforcement, and low concrete quality. Splitting cracks in concrete columns originated because of attaining its maximum load bearing strength.

The ultimate Load bearing strength of the column is exceeded when concrete cross section is not enough or reinforcement ratio is inadequate or combination thereof.

4. Corrosion Cracks: Corrosion cracks in concrete columns are formed along the line of reinforcements. These types of cracks are usually homogeneous in width and expand as the column is ageing.

Probable reinforcement erosion and insufficient bond among the concrete and steel bars lead to erosion reinforcement in concrete columns. If these types of cracks are not dealt with properly, the erosion of reinforcement will be increased at a rapid rate.



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Benefits of post-tensioning concrete slabs in building

Post-tensioned concrete slabs in buildings provide various benefits over reinforced concrete slabs & other structural systems toward both single and multi-level structures. Described below, some of the advantages of the slabs :-

Longer Spans: Longer spans are utilized to lessen the number of columns. It leads to bigger, column free floor areas which significantly enhance the adaptability of application for the structure as well as leads to greater rental returns.

Entire structural cost: The complete cost of materials, labor and formwork which are essential to build up a floor is decreased for spans higher than 7 meters and consequently leads to huge cost savings.

Minimized floor to floor height: Thinner slabs are utilized for the similar imposed loads. The decreased section depths facilitate least building height together with consequent savings in facade costs. As a substitute, toward bigger buildings, it facilitates more floors to be developed inside the original building envelope.

Deflection Free Slabs: Unwanted deflections under service loads are virtually removed.

Water-resistant slabs: Post-tensioned slabs are designed to remain free from cracks and as a result water-resistant slabs should be formed with proper design, detailing and construction. The selection of concrete mix and curing method together with standard workmanship are also very important.

Early formwork stripping: The earlier stripping of formwork and curtailed backpropping requirements facilitate rapid construction cycles as well as fast reprocessing of formwork.

Materials Handling: The decreased material quantities in concrete and reinforcement significantly offer benefit to on-site carnage requirements. The stability of post-tensioning strand is roughly 4 times that of traditional reinforcement. So, the whole weight of reinforcing material is considerably minimized.

Column and footing design: The decreased floor dead loads are applied to create cost-effective design of the reinforcement concrete columns and footings. In multi-storied buildings, decreased column sizes may raise the floor net rentable area.



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Uses and benefits of concrete pumping

Concrete pumping is the latest technology applied in the modern construction industry to set the concrete in all the structures ranging from high skyscrapers to the slightest convenience store. With concrete pumping, it is possible to arrange the concrete where it requires to fit.

Concrete pumping is the most convenient method for pouring concrete through a machine to transmit the liquid concrete. Normally, two basic types of pumps are utilized in concrete pumping i.e. boom pump and line pump.

Boom pump – It is self-contained unit that comprises of the frame and a truck. It is mostly applied to pour concrete in high up or difficult to arrive at the location.

Line pump – It is a portable unit that can pump structural concrete along with mortar, shotcrete, grout, wet screeds and foamed concrete.

Given below, some useful benefits of concrete pumping :-

Huge savings in time - Time is significantly reduced as the time is only required for mixing concrete and delivering to proper location.

Besides, the crew doesn’t have to make way for concrete trucks to pour the concrete to all the various locations.

Greater Productivity – The concrete pumping allows the crew members to deal with other aspects of the project since they don’t require to utilize shovels and wheelbarrows for the purpose of shifting concrete where it requires to fit.

Higher Precision – As the concrete pump expands from the truck right to the proper location the concrete is required, the entire process is error-less and precise.

This precision helps with productivity, wasted concrete because of spillage and fewer times for cleaning up.

Curtailment in labor costs – Less workers are required to set up and utilize the concrete pump as compared to the standard method of executing things. As less workers are needed, the labor cost is also reduced considerably and a more gainful project is created for the builder.

Greater employee safety – The employees needed to drive the concrete pump remain safe as compared to the employees assigned to mix and move the concrete with the conventional method.

With concrete pumping, heavy lifting is not required and as concrete moves directly to the spot, it requires less possibility for mishaps that can lead to severe injury.

Applications of concrete pump:

Residential projects similar to walls, foundations and patios.
Agricultural projects similar to sheds, barns, silos and bunkers.
Civil projects like hydro stations, water treatment plants or bridges.
Commercial or industrial projects similar to high rise buildings, ice rinks, strip malls and factories.



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Various types of notations used in the construction of concrete culvert design

The following notations are used in the drawing of concrete culvert design.

With notation, it is possible to make clear communication among various project stakeholders. Notation helps in avoiding mistakes in any construction project.

A’ = The valid contact area of a footing, measuring unit is in square metres
A = A detailing dimension for culverts that contains skewed ends, measuring unit is in mm
B = Depth of the bottom slab of a box culvert, measuring unit is in mm
B = Gapping among adjoining bars with reference to detailing tables, measuring unit is in mm
C = A coefficient that is applied in finding out the quantities of reinforcing bar.
CANBAS = Canadian Bridge Analysis System
c’ = the valid cohesion among the base of the footing and the soil at the ULS, with reference to CHBDC. kPa
CHBDC = Canadian Highway Bridge Design Code, 2000 Edition
CGSB = Canadian General Standards Board
F = Width of footing for open footing culverts, measuring unit is in mm
F1 = A reinforcing bar spacing factor, measuring unit is in mm-1
HULS = maximum factored horizontal reaction at the level of the base of the footing at the ULS, kN
Lc = Culvert length that is calculated the longitudinal axis, measuring unit is in m
OCPA = Ontario Concrete Pipe Association
OMBAS = Ontario Modular Bridge Analysis System
OPSS = Ontario Provincial Standard Specifications
S = Culvert distance that is calculated perpendicular to the longitudinal axis of the culvert, measuring unit is in mm
SLS = Serviceability limit states, in accordance with CHBDC
T = Depth of top slab of culvert, mm tan φ’ effective friction coefficient for concrete cast against soil.
ULS = Ultimate limit states, with reference to CHBDC
V = Unfactored vertical reaction because of the dead load of cast-in-place concrete and soil fill, at the level of the base of the footing, kN
VSLS = Maximum vertical reaction at the level of the base of the footing at SLS, kN
VULS = Maximum factored vertical reaction at the level of the base of the footing at ULS, kN
W = Depth of wall of culvert, measuring unit is in mm
Γ = The extreme angle among the normal to the longitudinal axis and the end of the same culvert, degrees
φ’ = The valid angle of internal friction, with reference to CHBDC, measuring unit is in degrees
θ = Skew angle of culvert, degrees



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The Basic Requirements of Formwork

Formwork is a concrete construction that is used as either short or permanent molds into which fresh concrete or similar kind of materials are gushed to make it harden. The different types of concrete formwork construction depend on the material and the kind of structural element. They are given various names as per the type of structural member construction like slab framework, beam framework, column framework etc. to use in beams and columns.

The harden process requires time and includes an expenditure up to 20 to 25% of the cost of the structure or more than that; the temporary design structures are made of economic expenditure.

While the removing process of the framework is known as stripping that can also be used later; the unusable are called panel forms and the non-useable are called stationary forms. The most commonly used material for formwork is Timber.

This article is a short knowledge for people about formwork, the different types of formwork and the basic needs to complete a formwork properly. Formwork comes in several types such as:

1. Traditional timber framework
2. Engineering Formwork System
3. Re-usable plastic formwork
4. Permanent Insulated Formwork systems
5. Stay-In-Place structural formwork systems
6. Flexible formwork

Requirements of Formwork: It is stated above that framework varies as per material and material is the main ingredient for every framework, but for any kind of chosen material the three common principles of quality, safety and economy should be the same. While the quality of material ensures safety and also considerably helps to achieve the economy; any kind of failure in framework can cause of the loss of life and tragic financial loss. To avoid the loss the following guidelines should be followed from start to framing materials and for the associated components:

1. Strength: The strength of material must be sufficient to strong the forces expected and this is the important for both the structural design and safety aspect.
2. Stiffness: the structural movement under load should be small and sure; these deformities and separations are the necessary part of the whole deviations in the formed concrete surface. While planning the formwork system, a designer must take decisions upon the total acceptable variations and the extend workmanship errors and structural deformity. The material stiffness and the workmanship accuracy must be stable to ensure the stability of the total deviations to keep the tolerances.
3. Impact Resistance: The forms are made to make sure that the damaged form that is useless does not make falling debris and to make sure this important safety quality, materials displaying ductile failure are far above than those fail in a hasty and brittle manner.
4. Durability: The framework must be durable either it will affect the economy and the achievement quality concrete product at every reuse of the formwork; formwork is always pre made and used out in the open. When the matter of re-uses came the thing is dependable on its reaction of materials and components with weather and the framing, components and formface materials should be strong in any environment. Material durability is important for both the achievement of good quality concrete surface finishes and safe formwork structures.
5. Weight: While the assembling process in formwork, both the members and components are shifted into position by hand that will be applicable after the complete framework which are heavy and need a crane to do the shifting work. So the framing members, formwork components and formface materials must be keep in size so that they must be carried and lifted by the worker or the crane etc.
6. Accuracy: Every construction process is dependable on a lot of money so the work of lifting and cutting of materials must be done within a minimum amount of money and consistency of size of materials, plywood sheets and framing members is very important.
7. Compatibility: The framework materials should be fitted with either the fluid concrete or the strong concrete and at the formface, the elements of the form materials must not react with the wet cement or concrete.
8. Insulation: Some materials react with the environment if it’s become extra hot or cold, so the protection for the materials is must; if the mix becomes frozen and the chemical bondage damages then the concrete has to be placed at low temperatures, heat the mixing water etc. The placing of the fluid concrete for all forms can cause some damage with the crushed rock aggregate and the proper steps has to be taken to stop it.




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