Concrete Slab Construction Methods

 

What is Concrete Slab.

The concrete slab is a Horizontal structural element to provide a flat surface for arranging floors and ceilings . Concrete Slabs are reinforced with mild steel bars having different diameters and surfaces like deformed steel bars and Twisted/Tor Bars to produce good bonding with concretes.

Steel Types used in Concrete.

According to strength, there are two types of Steel used in construction.

1, Low carbon Steel /Mild steel.

2, High carbon steel.

In construction Mild steel is usually used in all types of Civil engineering structures but High carbon steel is preferred in Precast structures only.

Mild steel or low carbon steel is mostly used in construction because of its more protection against corrosion.

In most structures Grade 60 is preferred because of economic suitability and its minimum Yield strength is used in designing RCC structures. 

STRENGTH in (PSI)	Grade 40	Grade 60	Grade75
Minimum Yield Strength	40000	60000	75000
Maximum Yield Strength	60000	90000	100000

Concrete mix for RCC/PCC slabs.

DETAILS OF CONCRETES
TYPE OF CONCRETE	MIX RATIO	COMPRESSIVE STRENGTH in (PSI) AFTER 28 DAYS
PCC	(1:5:10)	1000
PCC	(1:4:8)	1500
PCC	(1:3:6)	2000
RCC	(1:2:4)	3000
RCC	(1:1.5:3)	4000
RCC	(1:1:2)	4500

Generally, the above mixtures are used in the production of Plain cement concretes and Reinforced cement concretes but special concretes may be produced by special measuring and testing in labs. These special concretes are made for high loading demands using moderated tools and materials and specialists are needed to produce such high-strength concretes.

Some of these special concretes are as follows

M25   (25MPa)= 3625 PSI

M30    (30MPa)= 3625 PSI

M35    (35MPa)= 5075 PSI

M40    (40MPa)= 5800 PSI

M50    (50MPa)= 6525 PSI

M55    (55MPa)= 7250 PSI

M60    (60MPa)= 8700 PSI

M65    (65MPa)= 9425 PSI

M70    (70MPa)= 10150 PSI

Methods of Construction.

There are some methods involved in the Concreting process.

1- Preparation of Flooring for Scaffolding.

Scaffolding can be done on firm ground but there may be chances of settlement so it should be better to lay Plain cement concrete which may be part of the floor in later stages. Arrangement and fixing of scaffolding should be according to the weight of Concrete which may be calculated from the following formula.

Weight of plain cement concrete = Volume of concrete x Density of Concrete

Density of Plain cement concrete=140 Pound/ Cft

Density of Reinforced cement concrete=150 Pound/ Cft

One scaffolding pipe can bear 2240 Pound Safely if braced at vertical interval or 3Ft.

So the number of vertical pipes under PCC /RCC = Weight /2240= No of Pipes

Horizontal pipes should be calculated according to the height of pipes and according to loading of slabs.

2- Fixing of Shuttering

It is a very important stage as shuttering provides a firm platform for placing concrete and steel, so shuttering should be water-tight so the cement paste may not go out from the concrete mix as it will decrease the strength of cement concrete. Shuttering may be made of wooden ply sheets supported by wooden beams, and may also, be made of steel sheets supported by steel girders.

Shuttering should be leveled as per drawing and design and also according to the elevation of the building.

3- Fixing of Steel Reinforcement.

Steel reinforcement should be placed and fixed at shuttering platform according to drawing and design of slab. There are different sizes of steel bars as 1.25" ,1" ,7/8",6/8",5/8",4/8"3/8"2/8" used in building construction.In Oneway slabs Main bars are placed along short spans whereas distribution bars are placed along the longer sides of slabs. In TwoWay slabs both longer and shorter bars contribute equally so up or down does not matter as per design aspects. Negative bars control negative moments that apply on supports and up to the distance of L/4.Supporting bars should be provided beneath the Negative bars usually at intervals of 12 " c/c. Chairs should be provided to control the deflection of steel bars and to keep steel in position during the concreting process.

Concrete Spacers should be provided under the steel reinforcement mesh and also on the sides of slabs, normally concrete cover is equal to the larger dia of steel used in slab but the minimum cover should be up to 1/2".

4- Fixing of Electric and Plumbing Conduits.

After steel electric and plumbing conduits should be placed as per the location of electric fixtures like Fans, Lights, Fan Boxes Etc. Plumbing pipes are kept vertically according to the required layout of lines. The position of lines should not be disturbed during the concrete, Care should be taken to avoid the breaking of pipes from the movement of labor and wheel borrows because broken pipes get choked and hinder the passage of electric cables in later stages which causes problems of rerouting by doing more wall trenching and fixing and inserting new cabling. 

5- Mixing and Placing of Concrete.

Concrete maybe mixed by hand or in the mechanical mixer, it should be methodically mixed and the concrete placed in its final position with the minimum delay. Normal RCC Works are constructed using (1:2:4) mix with concrete incidents as Cement, Sand, Crush, Water, Steel Reinforcement, and Admixtures. Three mixing practices can be adopted for concreting 1, By Hand Mixing, 2, By Machine Mixing 3, By Batching Plant.

The most prominent mixing method is mixing by batching plant as quantities can be measured with controlled mechanism but such mixings are only required to avail high strength concretes which requires more than 4000 PSI compressive strength. Batching plants prepare the concrete using weights of ingredients that are mentioned in the job mix formula. Transportation of concrete Transit mixer trucks is used whose minimum volume may be 5 cum. In Some places, the batching plants are available at the construction site and concrete is transported to the workplace using tower cranes and pumps along with pipelines.

Batching plants are used for massive construction projects and for normal works such a the method is not affordable at all.

Mixing by hand may be adopted for small works in which all ingredients are mixed roughly for two minutes on cleaned and water-tight platforms. An additional quantity of cement up to 10 % should be added in such mixes to ensure the quality of concrete.

In any case, water should be added only according to suitably designed Water Cement Ratios because excessive water reduces the strength of concrete.

On the Construction of slabs for small houses, mixture machines are used along with a tower lift to transporting the concrete on the heights OF slabs, if the quantity of concrete is massive than more than one machine and tower lifts can be fixed to avoid delays in the pouring of slabs.

Compaction of concrete is very important as it provides dense concrete expelling the air voids from the concrete volume and also increases the strength of concretes.

Compacting can be efficiently done using Vibrator machines, the vibration should be 1 minute for 1 cum of concrete, Over vibration should be avoided as it segregates the ingredients of concrete.

Concrete should be smooth and well leveled using wooden and steel floats, Level points may be maintained by the surveyor to ensure the exact thickness of the slab on all portions.

6- Curing of Concrete slabs.

Curing is an important activity as this saves concrete from hydration which causes cracks and strength losses in concrete structures so curing should be executed at least for 14 days for slab structures because in 14 days concrete maintains 90 % of its ultimate strength.

7-Removing of shuttering and scaffolding.

Removing of shuttering may be started after 14 days for medium spans of the building but larger spans should not be suspended before 28 days as concrete avails its 100 % strength in 28-days. 

Wind Load on Building Structures.

 Wind is a mass of air that travels from high-pressure zones to low-pressure zones having low and high velocities which exert pressures on buildings and causes resultant loads.

                                 Figure 1.Showing Wind Pressure on Building       

An instrument for measuring wind speed is called Anemometer.       

Formula for wind-pressure is as follows

P =K.V^2

Here

P= Wind Pressure

K= Coefficient

V= Velocity of wind

Value of "K" depends on the speed of the wind and shape of the structure bearing wind pressure,

Using wind velocity in Miles per Hour (mph) wind pressures may be calculated using Coefficient of 0.00256 which will result in Pressure in Pounds per Square (psf).

Now we will calculate the intensities of pressure using various wind velocities.

World Record for Fastest Wind Speed

The fastest wind speed ever recorded comes from a hurricane gust. On April 10, 1996, Tropical Cyclone Olivia (a hurricane) passed by Barrow Island, Australia. It was the equivalent of a Category 4 hurricane at the time, 254 mph (408 km/h). So we will calculate pressures up to the wind velocity of 254 mph.

Wind applies three types of forces structures.   Description of wind is different on different velocities with units of Km/hour or Mile/Hour .

Uplift load - Wind flow pressures exerts uplift pressure as in buildings wind flow under the roof pushes upward.

Shear force load – Mostly wind flow direction remains horizontal having different angles which exert pressure and creates Shear-Forces on structures.

Lateral load – Wind Pressure results in a push and pull forces which can Slide the structure or overturn the structure , these forces are called Lateral Forces. on:

Designing Factor for against Wind Load.

The wind is a natural force that can destroy and uplift any structure which causes high losses so always consider few designing factors for the construction of structures.

1- Calculate wind loads and apply on a minimum area of 100 Sft where width will be 9 ft this will give better pressure impacts in the case of building designing.

2-Resultant pressure acts at the height of h/2 or h/3 so reinforce the building accordingly.

3-Weight of the building should be greater than wind loads a factor of safety of 2 should be adopted only for wind pressure dealing.

4-weak points in building structures are door and windows which can be sustained up to 170 mph, but if the shingle pieces or high moisture content exists in the wind air it can easily break windows even on lower velocity.

5-Buildings having steel roofs faces large intensities of uplifting pressures so the building should be designed having proper structural framing properly braced in footings, vertical members, and horizontal members.

6- In no case Resultant line created from the Center of gravity of any structure should go outside the foundation because it will overturn the building.

7-Light structures get damaged at wind velocity of 47 mph, trees are uprooted on wind velocity of 55 mph and widespread structural damages accrue on 64 mph.       

8-Building structures should be design calculating  pressures on outer sides and also inner side of buildings, make sure the weight of the building is enough to bear wind loads also ensure the stiffness of all structural elements in buildings.

9-Design outer exposed walls monolithic and with homogeneous materials to create uniform stiffness and strengthening the structure.

10-Lighting poles should be designed after calculating horizontal pressure using wind velocity of that area where light poles exist. Provide weight in foundations greater than the horizontal pressures. A Factor of safety of 1.5 to 2 may be adopted comparing wind pressure and weight of foundations.

 

CLASSIFICATION OF WIND VELOCITIES
CLASS OF WIND	WIND VELOCITIES in MPH
Strong Breez	25	28
Moderate Gale	34	49
Strong Gale	50	53
Whole Gale	54	62
Strom	63	72
Hurricane	73	78
Violent Hurricane	79	199
Tornadoes	200	311

Tornadoes may have velocities more than 200 mph.

Tornadoes are ferociously revolving columns of air that extend from a thunderstorm to the ground. Tornadoes can destroy buildings, turn over cars, and create deadly flying debris. A tornado can happen anytime and anywhere. carry strong winds, over 200 miles per Hour.

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Repairing of corrosion damage of reinforced concrete.

 Corrosion in RCC Element is a natural process that takes place when the steel bars within reinforced concrete structures become rusty. Scientifically concrete corrosion definition is  the “ruining of metal by chemical, electrochemical, and electrolytic reactions within its location.” It usually forms as the concrete ages.

Corrosion of reinforcing steel and other entrenched metals is the leading cause of deterioration in concrete. When steel corrodes, the resulting rust occupies a greater volume than the steel. This expansion creates tensile stresses in the concrete, which can eventually cause cracking, delaminating, and spalling of RCC structures.

                                    Corrosion in Reinforced cement concretes.

Concrete corrosion is initiated when the harmful materials to steel-like Co2 and chloride from de-icing salts start to penetrate concrete and finally reach the steel reinforcement.

This will lead to a potential difference between the anodic and cathodic areas at the surface of the steel reinforcement, which makes rust. When rust occupies a core volume than steel, it exerts inner stress which causes adjacent concrete to crack and become damaged. RCC structure must be inspected and tested on regular basis to notice and avoid corrosion, particularly when structures get older. When corrosion is found suggest appropriate treatment for the affected structure.

Repairing procedure.

The entire affected surface must be carefully cleaned and prepared. All loose particles, laitance, dust, curing compounds, oil, grease, fat, bitumen, and paint must be removed if good bond strength is to be achieved.  All laitance friable concrete should be removed by chipping, grit blasting, or scabbling until a sound base is obtained., If there is any requirement to dismantle cracked flaky portions with Hammering safety must be made by supporting and bracing of structure from places where maximum shear and bending exists, and special safety measures should be arranged for labor and supervisors. There are three steps for repairing affected structures.

    Application of Epoxy Bonding agent for the bond of old and new concretes.

1- Use of  Epoxy bonding agent for concrete repairs, bonding concrete to concrete, steel, and granolithic toppings.

Epoxy bonding agent should be applied evenly across the whole surface with a clean using a short-haired paintbrush or maybe by laying-on trowel.

Coverage of epoxy bonding materials are 1.9 to 2.9 m²/kg dependent on surface levels of porosity.

Advantages of these materials are High Strength, Nonshrink, Moisture Tolerant, Durability, and resistance against chemical attacks.

2- Second Phase includes the use of a Dual-phase Corrosion Inhibitor for Reinforced Concrete Structures, used to protect steel and concrete from corrosion. This liquid is applied at the surface with a rate of 200 Sft /Gallons using 2 to 3 coats.

The application can be done with a temperature of -17 °C to 50 °C, after the first coat second coat can be applied after 15 minutes.

3- The third Phase is Use of High performance, styrene-butadiene (SBR), latex emulsion for improving cement-based mortars. The latex consists of microscopic particles of rubber dispersed in an aqueous solution. synthetic It is used as an admixture in a mortar and concretes to improve compressive and flexural strengths, improve bond increase resistance to water penetration, improve abrasion resistance and durability. It is used with Portland cement as a reliable water-resistant bonding agent. There are achievable advantages by use of this product like Earlier hardening, improved flexibility, greatly reduces shrinkages, lower water-cement ratio, produces excellent adhesion to steel and concrete, prolong rust protection, and saves from salts.

It may be used 1 Liter /Bag of cement and 5 Liters per Bag of cement for making cement sand mortars or for cement Grouts to fill the cracks.

                                                                           SBR 

4-Last and Final stage is the application of cement sand plaster having rich ratio like (1:1), (1:2) using an admixture of SBR with a Quantity of 5 Liters /  50 Kgs of cement.

Precautions during Hammering for removal of loose concrete.

 Before the start of demolition work by hammering or by use of Hilti Drill Machine /Jack Hammer special measures should be done at the site which is as follows.

                                                              Bending and Shear Diagrams.

 1- Calculate bending moment and shear force and draw a diagram to find maximum load and stress positions and then brace the structure using scaffolding and Steel Girders.

Jacks may be used to lift the weight from affected parts. Do not start breaking from the locations where maximum shear and bending are coming at the affected structure.

Start work by gradual dismantling and repairing patterns but do not break more than 10 % of structural depth as it will increase stress in tension and compression zones of RCC sections. 

            

Safety measures should be ensured at the site because safety is always first.

 

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Calculations for Water Usage in Construction Industry

 

Water is used for the production of many elements in the construction industry as water is the key ingredient being mixed with cement to form a paste that binds fine and coarse aggregates to produce concretes and mortars. Hydration of cement cannot be completed without the use of water in concrete mortar, Hydration is a chemical reaction that starts after mixing all ingredients of concrete including water, and then due to setting and hardening of concrete an increase in temperature occurs and a considerable quantity of heat is evolved.

The term the water-cement ratio is very famous among people who work in construction industries, WCR (Water cement ratio ) is the ratio between the weight of water used in concretes and mortars and the weight of cement used in concretes and mortars.

WCR = Weight of water /Weight of cement

This formula is used to calculate the required quantity of water for concretes and mortars, Approximately the Water quantity required for 50 kgs (One Bag) of cement is 35 Liters for Hand compacted concretes.

 In case of vibrated concretes following quantities of water are required in concretes, and also submitting water quantities for different mortars in civil works.

 

These quantities may be used to calculate water quantity required to complete different engineering projects.

I am submitting some natural loose materials which are used in different pavements and also mentioning their required optimum moisture content so that the Quantity of water may be calculated within the ranges of OMC.

1-Clayey Sand , OMC=   =   10-11%

2-Sand Silt, OMC=   11-15%

3-Inorganic Silt ,OMC=12-24%

 4-Organic Silt, OMC= 21-33%

 5-Highly plastic clay, OMC= 19-36% 

 6-Organic Clay, OMC=21-45% 

7-Sand, OMC=   4-7% 

8-River bed stone or Shingles, OMC=   5-8% 

9-Water Bound Macadam Layers, OMC=   4-9%

We can use the following formula to calculated water in compaction of above-listed materials from       S no.1-9.

Quantity of water in Liters  = Volume of Compacted Layer x (% of OMC) x 28.32 =

Answer will be in Liters of Water.

                             Compaction Process Using OMC

Using this information on water usage may be organized with regular checks on construction teams which will result to save water.

Drinkable water is used in construction works so this should be the duty of every member working on construction sites to save water for the survival of this world.

 Use of water should be done according to designed Water cement ratios because extra water in concrete mortars will result in loss of strength and durability of concretes. 

Water should be added in concrete with measurable volumes so that water use may be ensured according to the estimated amount of water.