PROPERTIES OF CONCRETE

Properties of concrete are influenced by many factors mainly due to mix proportion of cement, sand, aggregates and water. Ratio of these materials control the various concrete properties which are discussed below.

Different Properties of Concrete are as follows:

• Grades (M20, M25, M30 etc.)
• Compressive strength
• Characteristic Strength
• Tensile strength
• Durability
• Creep
• Shrinkage
• Unit weight
• Modular Ratio
• Poisson’s ratio

 

Grades of concrete :

Concrete is known by its grade which is designated as M15, M20 etc. in which letter M refers to concrete mix and number 15, 20 denotes the specified compressive strength (fck) of 150mm cube at 28 days, expressed in N/mm2. Thus, concrete is known by its compressive strength. M20 and M25 are the most common grades of concrete, and higher grades of concrete should be used for severe, very severe and extreme environments.

Compressive strength of concrete :

Like load, the strength of the concrete is also a quality which varies considerably for the same concrete mix. Therefore, a single representative value, known as characteristic strength is used.

 

Characteristic strength of concrete :

It is defined as the value of the strength below which not more then 5% of the test results are expected to fall (i.e. there is 95% probability of achieving this value only 5% of not achieving the same)

 

Characteristic strength of concrete in flexural member :

The characteristic strength of concrete in flexural member is taken as 0.67 times the strength of concrete cube.

 

Design strength (fd) and partial safety factor for material strength :

The strength to be taken for the purpose of design is known is known as design strength and is given by
Design strength (fd) = characteristic strength/ partial safety factor for material strength
The value of partial safety factor depends upon the type of material and upon the type of limit state. According to IS code, partial safety factor is taken as 1.5 for concrete and 1.15 for steel.
Design strength of concrete in member = 0.45fck

 

Tensile strength of concrete :

The estimate of flexural tensile strength or the modulus of rupture or the cracking strength of concrete from cube compressive strength is obtained by the relations
fcr = 0.7 fck N/mm²
The tensile strength of concrete in direct tension is obtained experimentally by split cylinder. It varies between 1/8 to 1/12 of cube compressive strength.

 

Creep in concrete :

Creep is defined as the plastic deformation under sustain load. Creep strain depends primarily on the duration of sustained loading. According to the code, the value of the ultimate creep coefficient is taken as 1.6 at 28 days of loading.

 

Shrinkage of Concrete :

The property of diminishing in volume during the process of drying and hardening is termed Shrinkage. It depends mainly on the duration of exposure. If this strain is prevented, it produces tensile stress in the concrete and hence concrete develops cracks.

 

Modular ratio :

Short term modular ratio is the modulus of elasticity of steel to the modulus of elasticity of concrete.

Short term modular ratio = Es / Ec

where,
Es = modulus of elasticity of steel (2×10^5  N/mm²)

Ec = modulus of elasticity of concrete (5000x√fck  N/mm²)

As the modulus of elasticity of concrete changes with time, age at loading etc the modular ratio also changes accordingly. Taking into account the effects of creep and shrinkage partially IS code gives the following expression for the long term modular ratio.

Long term modular ratio (m) = 280/ (3σ_cbc)

Where,  
σ_cbc = permissible compressive stress due to bending in concrete in N/mm².

Poisson’s ratio :

Poisson’s ratio varies between 0.1 for high strength concrete and 0.2 for weak mixes. It is normally taken as 0.15 for strength design and 0.2 for serviceability criteria.

 

Durability of concrete :

Durability of concrete is its ability to resist its disintegration and decay. One of the chief characteristics influencing durability of concrete is its permeability to increase of water and other potentially deleterious materials.
The desired low permeability in concrete is achieved by having adequate cement, sufficient low water/cement ratio, by ensuring full compaction of concrete and by adequate curing.

 

Unit weight of concrete :

The unit weight of concrete depends on percentage of reinforcement, type of aggregate, amount of voids and varies from 23 to 26KN/m². The unit weight of plain and reinforced concrete as specified by IS:456 are 24 and 25KN/m³ respectively.

TYPES OF CONCRETE AND ITS APPLICATIONS

 

High-strength concrete

High-strength concrete has a compressive strength greater than 40 MPa (5800 psi). High strength concrete is defined as concrete with a compressive strength class higher than C50/60. High-strength concrete is made by lowering the water-cement (W/C) ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond.
Low W/C ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used. To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate rather than in the matrix or at a void, as normally occurs in regular concrete.
In some applications of high-strength concrete the design criterion is the elastic modulus rather than the ultimate compressive strength.

Stamped concrete

Stamped concrete is an architectural concrete which has a superior surface finish. After a concrete floor has been laid, floor hardeners (can be pigmented) are impregnated on the surface and a mold which may be textured to replicate a stone / brick or even wood is stamped on to give an attractive textured surface finish. After sufficient hardening the surface is cleaned and generally sealed to give a protection. The wear resistance of stamped concrete is generally excellent and hence found in applications like parking lots, pavements, walkways etc.

High-performance concrete

High-performance concrete (HPC) is a relatively new term for concrete that conforms to a set of standards above those of the most common applications, but not limited to strength. While all high-strength concrete is also high-performance, not all high-performance concrete is high-strength. Some examples of such standards currently used in relation to HPC are:

• Ease of placement
• Compaction without segregation
• Early age strength
• Long-term mechanical properties
• Permeability
• Density
• Heat of hydration
• Toughness
• Volume stability
• Long life in severe environments
• Depending on its implementation, environmental

Ultra-high-performance concrete

Ultra-high-performance concrete is a new type of concrete that is being developed by agencies concerned with infrastructure protection. UHPC is characterized by being a steel fibre-reinforced cement composite material with compressive strengths in excess of 150 MPa, up to and possibly exceeding 250 MPa . UHPC is also characterized by its constituent material make-up: typically fine-grained sand, silica fume, small steel fibers, and special blends of high-strength Portland cement. Note that there is no large aggregate. The current types in production (Ductal, Taktl, etc.) differ from normal concrete in compression by their strain hardening, followed by sudden brittle failure. Ongoing research into UHPC failure via tensile and shear failure is being conducted by multiple government agencies and universities around the world.

Micro-reinforced ultra-high-performance concrete

Micro-reinforced ultra-high-performance concrete is the next generation of UHPC. In addition to high compressive strength, durability and abrasion resistance of UHPC, micro-reinforced UHPC is characterized by extreme ductility, energy absorption and resistance to chemicals, water and temperature. The continuous, multi-layered, three dimensional micro-steel mesh exceeds UHPC in durability, ductility and strength. The performance of the discontinuous and scattered fibers in UHPC is relatively unpredictable. Micro-reinforced UHPC is used in blast, ballistic and earthquake resistant construction, structural and architectural overlays, and complex facades.
Ducon was the early developer of micro-reinforced UHPC, which has been used in the construction of new World Trade Center in New York.

Self-consolidating concrete

The defects in concrete in Japan were found to be mainly due to high water-cement ratio to increase workability. Poor compaction occurred mostly because of the need for speedy construction in the 1960s and 1970s. Hajime Okamura envisioned the need for concrete which is highly workable and does not rely on the mechanical force for compaction. During the 1980s, Okamura and his Ph.D. student Kazamasa Ozawa at the University of Tokyo developed self-compacting concrete (SCC) which was cohesive, but flowable and took the shape of the formwork without use of any mechanical compaction. SCC is known as self-consolidating concrete in the United States.
SCC is characterized by the following:

• extreme fluidity as measured by flow, typically between 650–750 mm on a flow table, rather than slump (height)
• no need for vibrators to compact the concrete
• easier placement
• no bleeding, or aggregate segregation
• increased liquid head pressure, which can be detrimental to safety and workmanship

SCC can save up to 50% in labor costs due to 80% faster pouring and reduced wear and tear on formwork.
In 2005, self-consolidating concretes accounted for 10–15% of concrete sales in some European countries. In the precast concrete industry in the U.S., SCC represents over 75% of concrete production. 38 departments of transportation in the US accept the use of SCC for road and bridge projects.
This emerging technology is made possible by the use of polycarboxylates plasticizer instead of older naphthalene-based polymers, and viscosity modifiers to address aggregate segregation.
It is widely used in many countries around the world due to its various properties.

Shotcrete

Shotcrete (also known by the trade name Gunite) uses compressed air to shoot concrete onto (or into) a frame or structure. The greatest advantage of the process is that shotcrete can be applied overhead or on vertical surfaces without formwork. It is often used for concrete repairs or placement on bridges, dams, pools, and on other applications where forming is costly or material handling and installation is difficult.

There are two application methods for shotcrete.

• dry-mix – the dry mixture of cement and aggregates is filled into the machine and conveyed with compressed air through the hoses. The water needed for the hydration is added at the nozzle.
• wet-mix – the mixes are prepared with all necessary water for hydration. The mixes are pumped through the hoses. At the nozzle compressed air is added for spraying.

For both methods additives such as accelerators and fiber reinforcement may be used.

Limecrete

Limecrete or lime concrete is concrete where cement is replaced by lime.We know that lime has been used since Roman Times either as mass foundation concretes or as lightweight concretes using a variety of aggregates combined with a wide range of pozzolans (fired materials) that help to achieve increased strength and speed of set. This meant that lime could be used in a much wider variety of applications than previously such as floors, vaults or domes. Over the last decade, there has been a renewed interest in using lime for these applications again. This is because of environmental benefits and potential health benefits, when used with other lime products.

Environmental Benefits :

• Lime is burnt at a lower temperature than cement and so has an immediate energy saving of 20% (although kilns etc. are improving so figures do change). A standard lime mortar has about 60-70% of the embodied energy of a cement mortar. It is also considered to be more environmentally friendly because of its ability, through carbonation, to re-absorb its own weight in Carbon Dioxide (compensating for that given off during burning).
• Lime mortars allow other building components such as stone, wood and bricks to be reused and recycled because they can be easily cleaned of mortar/limewash.
• Lime enables other natural and sustainable products such as wood (including woodfibre, wood wool boards), hemp, straw etc. to be used because of its ability to control moisture (if cement were used, these buildings would compost!).

Health Benefits :

• Lime plaster is hygroscopic (literally means 'water seeking') which draws the moisture from the internal to the external environment, this helps to regulate humidity creating a more comfortable living environment as well as helping to control condensation and mould growth which have been shown to have links to allergies and asthmas.
• Lime plasters and limewash are non-toxic, therefore they do not contribute to indoor air pollution unlike some modern paints.

 

Pervious concrete

Pervious concrete, used in permeable paving, contains a network of holes or voids, to allow air or water to move through the concrete
This allows water to drain naturally through it, and can both remove the normal surface-water drainage infrastructure, and allow replenishment of groundwater when conventional concrete does not.
It is formed by leaving out some or all of the fine aggregate (fines). The remaining large aggregate then is bound by a relatively small amount of Portland cement. When set, typically between 15% and 25% of the concrete volume is voids, allowing water to drain at around 5 gal/ft²/ min (70 L/m²/min) through the concrete.

Installation :

Pervious concrete is installed by being poured into forms, then screeded off, to level (not smooth) the surface, then packed or tamped into place. Due to the low water content and air permeability, within 5–15 minutes of tamping, the concrete must be covered with a 6-mil poly plastic, or it will dry out prematurely and not properly hydrate and cure.

Characteristics :

Pervious concrete can significantly reduce noise, by allowing air to be squeezed between vehicle tires and the roadway to escape. Pervious concrete has been tested up to 4500 psi so far.

Roller-compacted concrete

Roller-compacted concrete, sometimes called rollcrete, is a low-cement-content stiff concrete placed using techniques borrowed from earthmoving and paving work. The concrete is placed on the surface to be covered, and is compacted in place using large heavy rollers typically used in earthwork. The concrete mix achieves a high density and cures over time into a strong monolithic block. Roller-compacted concrete is typically used for concrete pavement, but has also been used to build concrete dams, as the low cement content causes less heat to be generated while curing than typical for conventionally placed massive concrete pours.

 

Glass concrete

The use of recycled glass as aggregate in concrete has become popular in modern times, with large scale research being carried out at Columbia University in New York. This greatly enhances the aesthetic appeal of the concrete. Recent research findings have shown that concrete made with recycled glass aggregates have shown better long-term strength and better thermal insulation due to its better thermal properties of the glass aggregates.

 

Asphalt concrete

Strictly speaking, asphalt is a form of concrete as well, with bituminous materials replacing cement as the binder.

 

Rapid strength concrete

This type of concrete is able to develop high resistance within few hours after being manufactured. This feature has advantages such as removing the formwork early and to move forward in the building process at
record time, repair road surfaces that become fully operational in just a few hours.

 

Rubberized concrete

While "rubberized asphalt concrete" is common, rubberized Portland cement concrete ("rubberized PCC") is still undergoing experimental tests, as of 2009.

 

Polymer concrete

Polymer concrete is concrete which uses polymers to bind the aggregate. Polymer concrete can gain a lot of strength in a short amount of time. For example, a polymer mix may reach 5000 psi in only four hours. Polymer concrete is generally more expensive than conventional concretes.

Geopolymer concrete

Geopolymer cement is an alternative to ordinary Portland cement and is used to produce Geopolymer concrete by adding regular aggregates to a geopolymer cement slurry. It is made from inorganic aluminosilicate (Al-Si) polymer compounds that can utilise 100% recycled industrial waste (e.g. fly ash, copper slag) as the manufacturing inputs resulting in up to 80% lower carbon dioxide emissions. Greater chemical and thermal resistance, and better mechanical properties, are said to be achieved for geopolymer concrete at both atmospheric and extreme conditions.
Similar concretes have not only been used in Ancient Rome (see Roman cement), but also in the former Soviet Union in the 1950s and 1960s. Buildings in Ukraine are still standing after 45 years, so this kind of formulation has a sound track record.

 

Gypsum concrete

Gypsum concrete is a building material used as a floor underlayment used in wood-frame and concrete construction for fire ratings, sound reduction,radiant heating, and floor leveling. It is a mixture of gypsum, Portland cement, and sand.

Engineered cementitious composite

Engineered Cementitious Composite (ECC), also called bendable concrete, is an easily molded mortar-based composite reinforced with specially selected short random fibers, usually polymer fibers.Unlike regular concrete, ECC has a strain capacity in the range of 3–7%,compared to 0.1% for ordinary portland cement (OPC). ECC therefore acts more like a ductile metal than a brittle glass (as does OPC concrete), leading to a wide variety of applications.

Properties :

ECC has a variety of unique properties, including tensile properties superior to other fiber-reinforced composites, ease of processing on par with conventional cement, the use of only a small volume fraction of fibers (~ 2%), tight crack width, and a lack of anisotropically weak planes.These properties are due largely to the interaction between the fibers and cementing matrix, which can be custom-tailored through micromechanics design. Essentially, the fibers create many microcracks with a very specific width, rather than a few very large cracks (as in conventional concrete.) This allows ECC to deform without catastrophic failure.
This microcracking behavior leads to superior corrosion resistance (the cracks are so small and numerous that it is difficult for aggressive media to penetrate and attack the reinforcing steel) as well as to self-healing. In the presence of water (during a rainstorm, for instance) unreacted cement particles recently exposed due to cracking hydrate and form a number of products (Calcium Silicate Hydrate, calcite, etc.) that expand and fill in the crack. These products appear as a white ‘scar’ material filling in the crack. This self-healing behavior not only seals the crack to prevent transport of fluids, but mechanical properties are regained. This self-healing has been observed in a variety of conventional cement and concretes; however, above a certain crack width self healing becomes less effective. It is the tightly controlled crack widths seen in ECC that ensure all cracks thoroughly heal when exposed to the natural environment.
When combined with a more conductive material, all cement materials can increase and be used for damage-sensing. This is essentially based on the fact that conductivity will change as damage occurs; the addition of conductive material is meant to raise the conductivity to a level where such changes will be easily identified. Though not a material property of ECC itself, semi-conductive ECC for damage-sensing are being developed.

Types :

There are a number of different varieties of ECC, including:

• Lightweight (i.e. low density) ECC have been developed through the addition of air voids, glass bubbles, polymer spheres, and/or lightweight aggregate. Compared to other lightweight concretes, lightweight ECC has superior ductility. Applications include floating homes, barges, and canoes.
• ‘Self compacting concrete’ refers to a concrete that can flow under its own weight. For instance, a self-compacting material would be able to fill a mold containing elaborate pre-positioned steel reinforcement without the need of vibration or shaking to ensure even distribution. Self-compacting ECC was developed through the use of chemical admixtures to decrease viscosity and through controlling particle interactions with mix proportioning.
• Sprayable ECC, which can be pneumatically sprayed from a hose, have been developed by using various superplasticizing agents and viscosity-reducing admixtures. Compared to other sprayable fiber-reinforced composites, sprayable ECC has enhanced pumpability in addition to its unique mechanical properties. Sprayable ECC has been used for retrofitting/repair work and tunnel/sewer linings.
• An extrudable ECC for use in the extrusion of pipes was first developed in 1998. Extruded ECC pipes have both higher load capacity and higher deformability than any other extruded fiber-reinforced composite pipes.

Field Applications :

ECC have found use in a number of large-scale applications in Japan, Korea, Switzerland, Australia and the U.S. These include:

• The Mitaka Dam near Hiroshima was repaired using ECC in 2003. The surface of the then 60-year-old dam was severely damaged, showing evidence of cracks, spalling, and some water leakage. A 20 mm-thick layer of ECC was applied by spraying over the 600 m² surface.
• Also in 2003, an earth retaining wall in Gifu, Japan, was repaired using ECC.Ordinary portland cement could not be used due to the severity of the cracking in the original structure, which would have caused reflective cracking. ECC was intended to minimize this danger; after one year only microcracks of tolerable width were observed.
• The 95 m (312 ft.) Glorio Roppongi high-rise apartment building in Tokyo contains a total of 54 ECC coupling beams (two per story) intended to mitigate earthquake damage.The properties of ECC (high damage tolerance, high energy absorption, and ability to deform under shear) give it superior properties in seismic resistance applications when compared to ordinary portland cement. Similar structures include the 41-story Nabeaure Yokohama Tower (four coupling beams per floor.)
• The 1 km (0.62 mi) long Mihara Bridge in Hokkaido, Japan was opened to traffic in 2005. The steel-reinforced road bed contains nearly 800 m3 of ECC material. The tensile ductility and tight crack control behavior of ECC led to a 40% reduction in material used during construction.
• Similarly, a 225-mm thick ECC bridge deck on interstate 94 in Michigan was completed in 2005. 30 m³ of material was used, delivered on-site in standard mixing trucks. Due to the unique mechanical properties of ECC, this deck also used less material than a proposed deck made of ordinary portland cement. Both the University of Michigan and the Michigan Department of Transportation are monitoring the bridge in an attempt to verify the theoretical superior durability of ECC; after four years of monitoring, performance remained undiminished.
• The first self-consolidating and high-early-strength ECC patch repair was placed on Ellsworth Road Bridge over US-23 in November 2006. The high-early-strength ECC can achieve a compressive strength of 23.59 ± 1.40 MPa (3422.16 ± 203.33 psi) in four hours and 55.59 ± 2.17 MPa (8062.90 ± 315.03 psi) in 28 days, allowing for fast repair and re-opening the session to traffic. The high-early-strength ECC repair has shown superior long-term durability in field conditions compared to typical concrete repair materials.

DESIGN OF REINFORCED CONCRETE FOUNDATIONS

Reinforced concrete foundations are designed based on column loads and moments at base and the soil data. Following are the types of foundations in order of preference with a view to economy:

(i) Isolated footing

(ii) Combined footings (combination of isolated footings)

(iii) Strip footings with retaining wall acting as strip beam wherever applicable.

(iv) Raft foundations of the types (a) slab (b) beam-slab.

The brick wall footings can also be designed. Often plinth beams are provided to support brick walls and also to act as earthquake ties in each principal direction.

Important considerations in design of foundations:

Foundations are the structural elements which transfer loads from the building or individual columns to the earth. If these loads are to be properly transmitted, foundations must be designed to prevent excessive settlement or rotation, to minimize differential settlement and to provide adequate safety against sliding and overturning.

Depth of foundation:

Depth of foundation below ground level can be obtained by using Rankine’s formula:

Where,
h = minimum depth of foundation
 p= gross bearing capacity
 
= density of soil

= angle of repose or internal friction of soil.


Recommendations of IS456: 2000, Limit state design, bending, shear, cracking and development length:
To determine the area of foundation required for proper transfer of total load on the soil, the total load (combination of dead load, live load and any other load without multiplying it with any load factor) are considered.

 

 

Thickness of the edge of footing:
As per clause 34.1.3 of IS456: 2000, the thickness at the edge shall not be less than 15cm on soils.

Dimension of pedestal:

In the case of plain cement concrete pedestals, the angle between the plane passing through the bottom edge of the pedestal and the corresponding junction edge of the column with pedestal and the horizontal plane shall be governed by the expression.

Where q_o = calculated maximum bearing pressure at the base of the pedestal/footing in N/mm²
F_ck = characteristic strength of concrete at 28 days in N/mm²

 Fig: Dimensioning of pedestal

Maximum Bending moment in footings:

The bending moment will be considered at the face of column, pedestal or wall and shall be determined by passing through the section a vertical plane which extends completely across the footing, and over the entire area of the footing or one side of the said plane. The reference clause is 34.2.3.1 and 34.2.3.2 of IS456: 2000.

Shear capacity checks for footings:

The shear strength of footing is governed by the following two factors:
a) The footing acting essentially as a wide beam, with a potential diagonal crack intending in a plane across the entire width, the critical section for this condition shall be assumed as a vertical section located from the face of the column, pedestal or wall at a distance equal to the effective depth of the footing in case of footings on soils.
For one way bending action of footing (one way shear)
For one way shear action, the nominal shear stress in calculated as:

Where,

= shear stress


V_u = factored vertical shear force
b = breadth of critical section
d = effective depth


 

 = design shear strength of concrete based on % longitudinal tensile reinforcement. Refer table 61 of SP -16)

Fig: Critical section for one-way shear in foundation

Two way shear (or two way bending action or punching shear) of foundation:

For two way bending action, the following should be checked in punching shear. Punching shear shall be around the perimeter 0.5 times the effective depth away from the face of the column or pedestal.
For two way shear action, the nominal shear stress is calculated in accordance with clause 31.6.2 of IS456: 2000 as follows:

Where ,

= shear stress 

b_o = periphery of the critical section

d = effective depth

V_u = factored vertical shear force
When shear reinforcement is not provided, the nominal shear stress at the critical section should not exceed

Where, Ks = 0.5 + Bc (but not greater than 1)
Bc = (short dimension of column or pedestal / long dimension of column or pedestal)

N/mm


Note: It is general practice to make the base deep enough so that shear reinforcement is not required.

Development length of reinforcement bars in foundation:

The critical section for checking the development length in a footing shall be assumed at the same planes as those prescribed for bending moment in clause 34.2.3 of code and also at all other vertical planes where abrupt changes in section occur. Refer clause 34.2.4.3 of IS456: 2000.

Reinforcement in foundations:

The minimum reinforcement in footing slab specified by the code is 0.12% and maximum spacing specified is 3 times the effective depth or 450mm whichever is less. (clause 34.3).
Only tensile reinforcement is normally provided. The total reinforcement shall be laid down uniformly in case of square footings. For rectangular footings, there shall be a central band, equal to the width of the footing. The reinforcement in the central band shall be provided in accordance with the following equation.

Where,

Transfer of load at the base of column:

Clause: 34.4 of  IS456: 2000.

The compressive stress in concrete at the base of column or pedestal shall be transferred by bearing to the top of supporting pedestal or footing.
 The bearing pressure on the loaded area shall not exceed the permissible bearing stress in direct compression multiplied by a value equal to

but not greater than 2.


Where,

A₁ = supporting are for bearing of footing, which is sloped or stepped footing may be taken as the area of the lower base of the largest frustum of a pyramid or cone contained wholly within the footing and having its upper base, the area actually loaded and having side slope of one vertical to two horizontal.

A₂ = loaded area at the column base.

For limit state design, the permissible bearing stress specified is 45 f_ck.

If the permissible bearing stress is exceeded either in the column concrete or in footing concrete, reinforcement must be provided for developing the excess force. The reinforcement may be provided either extending the longitudinal bars into the footing or by providing dowels in accordance with the code as given by the following:

1. Minimum area of extended longitudinal bars or dowels must be 0.5% of cross-sectional area of the supported column or pedestal.

2. A minimum of four bars must be provided.

3. If dowels are used their diameter should not exceed the diameter of the column bars by more than 3mm.

4. Enough development length should be provided to transfer the compression or tension to the supporting member.

5. Column bars of diameter larger than 36mm, in compression only can be dowelled at the footing with bars of smaller diameters. The dowel must extend into the column a distance equal to the development length of the column bar. At the same time, the dowels must extend vertically into the footing a distance equal to the development length of the dowel.

Fig: Rigid and spread footings

Earth Below the Foot : Calculation of bearing capacity of soil.

What is bearing capacity of Soil?

The bearing capacity of soil is defined as the capacity of the soil to bear the loads coming from the foundation. The pressure which the soil can easily withstand against load is called allowable bearing pressure.

Following are some types of bearing capacity of soil:

Ultimate bearing capacity of soil (q_u)

The gross pressure at the base of the foundation at which soil fails is called ultimate bearing capacity.

Net ultimate bearing capacity (q_nu)

By neglecting the overburden pressure from ultimate bearing capacity we will get net ultimate bearing capacity.

qnu = qu - γ Df

Where
qu= unit weight of soil
D_f = depth of foundation

Net safe bearing capacity of soil (q_ns)

By considering only shear failure, net ultimate bearing capacity is divided by certain factor of safety will give the net safe bearing capacity.

q_ns = q_nu/ F

Where F = factor of safety = 3 (usual value)

Gross safe bearing capacity (q_s)

When ultimate bearing capacity is divided by factor of safety it will give gross safe bearing capacity.

q_s = q_u/F

Net safe settlement pressure (q_np)

The pressure with which the soil can carry without exceeding the allowable settlement is called net safe settlement pressure.

Net allowable bearing pressure (q_na)

This is the pressure we can used for the design of foundations. This is equal to net safe bearing pressure if q_np > q_ns. In the reverse case it is equal to net safe settlement pressure.

How to Calculate Bearing Capacity of Soil?

Calculation of bearing capacity of soil:

For the calculation of bearing capacity of soil, there are so many theories. But all the theories are superseded by Terzaghi’s bearing capacity theory.

Terzaghi’s bearing capacity theory

Terzaghi’s bearing capacity theory is useful to determine the bearing capacity of soils under a strip footing. This theory is only applicable to shallow foundations. He considered some assumptions which are as follows.

1. The base of the strip footing is rough.
2. The depth of footing is less than or equal to its breadth i.e., shallow footing.
3. He neglected the shear strength of soil above the base of footing and replaced it with uniform surcharge. ( D_f)
4. The load acting on the footing is uniformly distributed and is acting in vertical direction.
5. He assumed that the length of the footing is infinite.
6. He considered Mohr-coulomb equation as a governing factor for the shear strength of soil.

As shown in above figure, AB is base of the footing. He divided the shear zones into 3 categories. Zone -1 (ABC) which is under the base is acts as if it were a part of the footing itself. Zone -2 (CAF and CBD) acts as radial shear zones which is bear by the sloping edges AC and BC. Zone -3 (AFG and BDE) is named as Rankine’s passive zones which are taking surcharge (y D_f) coming from its top layer of soil.
From the equation of equilibrium,
Downward forces = upward forces

Load from footing x weight of wedge = passive pressure + cohesion x CB sinΦ

Where P_p = resultant passive pressure = (P_p)_y + (P_p)_c + (P_p)_q
(P_p)_{y is} derived by considering weight of wedge BCDE and by making cohesion and surcharge zero.
(P_p)_{c is} derived by considering cohesion and by neglecting weight and surcharge.
(P_p)_q is derived by considering surcharge and by neglecting weight and cohesion.
Therefore,

By substituting,

So, finally we get q_u = c’N_c + y D_f N_q + 0.5 y B N_y
The above equation is called as Terzaghi’s bearing capacity equation. Where q_u is the ultimate bearing capacity and N_c, N_q, N_y are the Terzaghi’s bearing capacity factors. These dimensionless factors are dependents of angle of shearing resistance ().
Equations to find the bearing capacity factors are:

Where

Kp = coefficient of passive earth pressure.
For different values of Φ , bearing capacity factors under general shear failure are arranged in the below table.

	Nc	Nq	Ny
0	5.7	1	0
5	7.3	1.6	0.5
10	9.6	2.7	1.2
15	12.9	4.4	2.5
20	17.7	7.4	5
25	25.1	12.7	9.7
30	37.2	22.5	19.7
35	57.8	41.4	42.4
40	95.7	81.3	100.4
45	172.3	173.3	297.5
50	347.5	415.1	1153.2

Finally, to determine bearing capacity under strip footing we can use

q_u = c’N_c + γD_f N_q + 0.5 γB N_y

By the modification of above equation, equations for square and circular footings are also given and they are.
For square footing

q_u = 1.2 c’N_c + γD_f N_q + 0.4 γ B N_y

For circular footing

q_u = 1.2 c’N_c +γD_f N_q + 0.3γ B N_y

Hansen’s bearing capacity theory

For cohesive soils, Values obtained by Terzaghi’s bearing capacity theory are more than the experimental values. But however it is showing same values for cohesion less soils. So Hansen modified the equation by considering shape, depth and inclination factors.
According to Hansen’s

q_u = c’N_c Sc dc ic + γD_f N_q Sq dq iq + 0.5 B γN_y Sy dy iy

Where Nc, Nq, Ny = Hansen’s bearing capacity factors
Sc, Sq, Sy = shape factors
dc, dq, dy = depth factors
ic, iq, iy = inclination factors
Bearing capacity factors are calculated by following equations.

For different values of Φ Hansen bearing capacity factors are calculated in the below table.

Φ	Nc	Nq	Ny
0	5.14	1	0
5	6.48	1.57	0.09
10	8.34	2.47	0.09
15	10.97	3.94	1.42
20	14.83	6.4	3.54
25	20.72	10.66	8.11
30	30.14	18.40	18.08
35	46.13	33.29	40.69
40	75.32	64.18	95.41
45	133.89	134.85	240.85
50	266.89	318.96	681.84

Shape factors for different shapes of footing are given in below table.

Shape of footing	Sc	Sq	Sy
Continuous	1	1	1
Rectangular	1+0.2B/L	1+0.2B/L	1-0.4B/L
Square	1.3	1.2	0.8
Circular	1.3	1.2	0.6

Depth factors are considered according to the following table.

Depth factors	Values
dc	1+0.35(D/B)
dq	1+0.35(D/B)
dy	1.0

Similarly inclination factors are considered from below table.

Inclination factors	Values
ic	1 – [H/(2 c B L)]
iq	1 – 1.5 (H/V)
iy	(iq)2

Where H = horizontal component of inclined load
B = width of footing
L = length of footing.

TYPES OF FOUNDATIONS

In this article we will discuss the common types of foundations in buildings.

Broadly speaking, all foundations are divided into two categories:

1.  Shallow foundations

2.  Deep foundations.

 The words shallow and deep refer to the depth of soil in which the foundation is made. Shallow foundations  can be made in depths of as little as 3ft (1m), while deep foundations can be made at depths of 60 - 200ft (20 - 65m). Shallow foundations are used for small, light buildings, while deep ones are for large, heavy buildings.

SHALLOW FOUNDATIONS :
 

Shallow foundations are also called spread footings or open footings. The 'open' refers to the fact that the foundations are made by first excavating all the earth till the bottom of the footing, and then constructing the footing. During the early stages of work, the entire footing is visible to the eye, and is therefore called an open foundation. The idea is that each footing takes the concentrated load of the column and spreads it out over a large area, so that the actual weight on the soil does not exceed the safe bearing capacity of the soil.

There are several kinds of shallow footings :

1. Spread or isolated or pad footing
2. Strap footing
3. Combined footing
4. Strip or continuous footing
5. Mat or raft footing

 In cold climates, shallow foundations must be protected from freezing. This is because water in the soil around the foundation can freeze and expand, thereby damaging the foundation. These foundations should be built below the frost line, which is the level in the ground above which freezing occurs. If they cannot be built below the frost line, they should be protected by insulation: normally a little heat from the building will permeate into the soil and prevent freezing.

 1. What is Spread Footing or Isolated or Pad Footing?

It is circular, square or rectangular slab of uniform thickness. Sometimes, it is stepped to spread the load over a larger area. When footing is provided to support an individual column, it is called “isolated footing”.

2. What is Strap Footing?

It consists of two isolated footings connected with a structural strap or a lever, as shown in figure below. The strap connects the footing such that they behave as one unit. The strap simply acts as a connecting beam. A strap footing is more economical than a combined footing when the allowable soil pressure is relatively high and distance between the columns is large.

 

 

3. What is Combined Footing?

It supports two columns as shown in figure below. It is used when the two column are so close to each other that their individual footings would overlap. A combined footing is also provided when the property line is so close to one column that a spread footing would be eccentrically loaded when kept entirely within the property line. By combining it with that of an interior column, the load is evenly distributed. A combine footing may be rectangular or trapezoidal in plan. Trapezoidal footing is provided when the load on one of the column is larger than the other column.

 

4. What is Strip Footing or Continuous Footing?

A strip footing is another type of spread footing which is provided for a load bearing wall. A strip footing can also be provided for a row of columns which are so closely spaced that their spread footings overlap or nearly touch each other. In such cases, it is more economical to provide a strip footing than to provide a number of spread footings in one line. A strip footing is also known as continuous footing.

5. What is Mat or Raft Footing?

It is a large slab supporting a number of columns and walls under entire structure or a large part of the structure. A mat is required when the allowable soil pressure is low or where the columns and walls are so close that individual footings would overlap or nearly touch each other. Mat foundations are useful in reducing the differential settlements on non-homogeneous soils or where there is large variation in the loads on the individual columns.They are most often used when basements are to be constructed. In a raft, the entire basement floor slab acts as the foundation; the weight of the building is spread evenly over the entire footprint of the building. It is called a raft because the building is like a vessel that 'floats' in a sea of soil.
 

 

Raft footing

DEEP FOUNDATIONS :

A pile is basically a long cylinder of a strong material such as concrete that is pushed into the ground so that structures can be supported on top of it.

Pile foundations are used in the following situations:

1. When there is a layer of weak soil at the surface. This layer cannot support the weight of the building, so the loads of the building have to bypass this layer and be transferred to the layer of stronger soil or rock that is below the weak layer.
2. When a building has very heavy, concentrated loads, such as in a high rise structure.

Pile foundations are capable of taking higher loads than spread footings.

There are two types of pile foundations, each of which works in its own way.

End Bearing Piles :

In end bearing piles, the bottom end of the pile rests on a layer of especially strong soil or rock. The load of the building is transferred through the pile onto the strong layer. In a sense, this pile acts like a column. The key principle is that the bottom end rests on the surface which is the intersection of a weak and strong layer. The load therefore bypasses the weak layer and is safely transferred to the strong layer.

Friction Piles :

Friction piles work on a different principle. The pile transfers the load of the building to the soil across the full height of the pile, by friction. In other words, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil. 

To visualise how this works, imagine you are pushing a solid metal rod of say 4mm diameter into a tub of frozen ice cream. Once you have pushed it in, it is strong enough to support some load. The greater the embedment depth in the ice cream, the more load it can support. This is very similar to how a friction pile works. In a friction pile, the amount of load a pile can support is directly proportionate to its length.


In practice, however, each pile resists load by a combination of end bearing and friction.

 

In the next article , i will show you the method of designing the foundations. 

 

Design of RCC Columns

 

RCC Column :

A column is a very important component in a structure. It is like the legs on which a structure stands. It is designed to resist axial and lateral forces and transfer them safely to the footings in the ground.

Columns support floors in a structure. Slabs and beams transfer the stresses to the columns. So, it is important to design strong columns.

A column is defined as a compression member, the effective length of which exceeds three times the least lateral dimension. Compression members whose lengths do not exceed three times the least lateral dimension, may be made of plain concrete.

The axial load carrying capacity of a column is deduced from the formula given below :

I would recommend using advanced structural design software like ETabs or Staad Pro for design of structures. Column design does not depend only on axial loads, but also on many other factors. There are bending moments and tortional forces induced due to beam spans, wind loads, seismic loads, point loads and many other factors.

In this article, we are going to discuss in detail the basics of classification of columns and different types of reinforcement required for a certain type of column.

A column may be classified based on different criteria such as:

1. Based on shape :

• Rectangle
• Square
• Circular
• Polygon

2. Based on slenderness ratio :

The ratio of the effective length of a column to the least radius of gyration of its cross section is called the slenderness ratio.

• Short RCC column, =< 10
• Long RCC column, > 10
• Short Steel column, =<50
• Intermediate Steel column >50 & <200
• Long Steel column >200

3. Based on type of loading :

• Axially loaded column
• A column subjected to axial load and unaxial bending
• A column subjected to axial load and biaxial bending

4. Based on pattern of lateral reinforcement :

• Tied RCC columns
• Spiral RCC columns

Minimum eccentricity :

E_min > l/500 + D/30 >20

Where,  l = unsupported length of column in ‘mm’

D = lateral dimensions of column

Types of Reinforcements for columns and their requirements :

Longitudinal Reinforcement :

• Minimum area of cross-section of longitudinal bars must be atleast 0.8% of gross section area of the column.

• Maximum area of cross-section of longitudinal bars must not exceed 6% of the gross cross-section area of the column.

• The bars should not be less than 12mm in diameter.

• Minimum number of longitudinal bars must be four in rectangular column and 6 in circular column.

• Spacing of longitudinal bars measures along the periphery of a column should not exceed 300mm.

Transverse reinforcement :

• It maybe in the form of lateral ties or spirals.

• The diameter of the lateral ties should not be less than 1/4^th of the diameter of the largest longitudinal bar and in no case less than 6mm.

The pitch of lateral ties should not exceed :

• Least lateral dimension

• 16 x diameter of longitudinal bars (small)

• 300mm

Helical Reinforcement :

The diameter of helical bars should not be less than 1/4^th the diameter of largest longitudinal and not less than 6mm.

The pitch should not exceed (if helical reinforcement is allowed);

• 75mm
• 1/6^th of the core diameter of the column

Pitch should not be less than,

• 25mm
• 3 x diameter of helical bar

Pitch should not exceed (if helical reinforcement is not allowed)

Least lateral dimension :

• 16 x diameter of longitudinal bar (smaller)
• 300mm

TABLE FOR

AXIAL LOAD CARRYING CAPACITY OF COLUMN  BASED ON PERCENTAGE OF REINFORCEMENT AND COLUMN SIZE FOR VARIOUS MIXES AND STEEL

Steel Grade Fy 415

CONCRETE  GRADE	Axial Load carrying Capacity in  KN (P)
M15	P=  (2.7205 p +  6) b D/1500
M20	P=  (2.7005 p + 8) b D/1500
M25	P=  (2.6805 p + 10) b D/1500
M30	P=  (2.6605 p + 12) b D/1500
M35	P=  (2.6405 p + 14) b D/1500
M40	P=  (2.6205 p + 16) b D/1500

Steel Grade Fy 500

CONCRETE  GRADE	Axial Load carrying Capacity in  KN (P)
M15	P=  (3.29 p +  6) b D/1500
M20	P=  (3.27 p + 8) b D/1500
M25	P=  (3.25 p + 10) b D/1500
M30	P=  (3.23 p + 12) b D/1500
M35	P=  (3.21 p + 14) b D/1500
M40	P=  (3.19 p + 16) b D/1500

Steel Grade Fy 550

CONCRETE  GRADE	Axial Load carrying Capacity in  KN (P)
M15	P=  (3.625 p +  6) b D/1500
M20	P=  (3.605 p + 8) b D/1500
M25	P=  (3.585 p + 10) b D/1500
M30	P=  (3.565 p + 12) b D/1500
M35	P=  (3.545 p + 14) b D/1500
M40	P=  (3.525 p + 16) b D/1500

Note :-

1. The axial Load carrying capacity column is arrived based on the formula

 P_u =0.4 f_ck A_c  + 0.67 f_y A_sc  as per IS 456-2000.

2.  Here  in the Table P is Axial Load Carrying capacity of column in KN.

p  = steel in  percentage say percentage as 1.

b  = breadth of column in mm.

D  = depth of column in mm.