Construction Technology::All about civil construction.: ESTIMATING LIFE OF RESIDENTIAL BUILDING

Construction Technology::All about civil construction.: ESTIMATING LIFE OF RESIDENTIAL BUILDING: There are different methods of estimating the life of a building. This can be done by carrying out health check up of the building. Depe...

ESTIMATING LIFE OF RESIDENTIAL BUILDING

There are different methods of estimating the life of a building. This can be done by carrying out health check up of the building. Depending on the degree of sophistication and desired accuracy level vis-a-vis the problem in hand ,there are many equipment and methodologies available to evaluate the probable life of a structure. Many times a thorough visual inspection reveals the distress and the causes.

Equipments used for HEALTH CHECK UP of the building are:

REBOUND HAMMER: It senses the soundness of concrete up to a marginal depth.

IMPACT ECHO TESTER: Finds out defects in the interior of concrete.

ULTRASONIC TESTERS: Scans through the concrete for the full depth/thickness.

COVER METER: Finds out the cover to the reinforcement (steel bars inside the concrete).

PROFOMETER: Establishes configuration and quantum of interior reinforcements (steel bars inside the concrete)

REBAR PHOTOGRAPHER: Displays interior reinforcements (steel bars) skeleton.

ENDOSCOPIC DEVICE: To examine the void in the concrete.

THERMOGRAPHIC CAMERA: To locate possible defect in a new building.

CRACK MEASUREMENT DEVICE: It measures surface cracks.

PERMEABILITY TESTER: Tests for water penetration in concrete.

THICKNESS GAUGE: Measures the thickness from the surface.

LEAK SEEKER: It locates source of leakages.

COROSION ANALYZER: It measures the extent of corrosion in reinforcements. Corrosion is regarded as equivalent of cancer in the concrete.

X-RAY: Scans inside the concrete.

Carbonation tests

Ground penetrating radar (GPR)

Vibration characteristics

FIELD TESTS ON CEMENT

Field tests on cements are carried to know the quality of cement supplied at site. It gives some idea about cement quality based on colour, touch and feel and other tests.

The following are the field tests on cement:

(a) The colour of the cement should be uniform. It should be grey colour with a light greenish shade.
(b) The cement should be free from any hard lumps. Such lumps are formed by the absorption of moisture from the atmosphere. Any bag of cement containing such lumps should be rejected.

(c) The cement should feel smooth when touched or rubbed in between fingers. If it is felt rough, it indicates adulteration with sand.
(d) If hand is inserted in a bag of cement or heap of cement, it should feel cool and not warm.
(e) If a small quantity of cement is thrown in a bucket of water, the particles should float for some time before it sink.
(f) A thick paste of cement with water is made on a piece of glass plate and it is kept under water for 24 hours. It should set and not crack.
(g) A block of cement 25 mm ×25 mm and 200 mm long is prepared and it is immersed for 7 days in water. It is then placed on supports 15cm apart and it is loaded with a weight of about 34 kg. The block should not show signs of failure.
(h) The briquettes of a lean mortar (1:6) are made. The size of briquette may be about 75 mm ×25 mm ×12 mm. They are immersed in water for a period of 3 days after drying. If cement is of sound quality such briquettes will not be broken easily.

HOW TO MEASURE FIELD DENSITY OF SOIL USING NUCLEAR DENSITY GAUGE?

Measurement of Density of Soil in place by Nuclear Density Gauge

Purpose

This is a quick method of determining the in-situ density of soil which is based on the radiation.

Equipment

For this test special equipment which measures in place density using gamma radiation is used. Gauge usually contains a small gamma source (about 10 mCi) such as Cesium – 137 on the end of a retractable rod.

NUCLEAR DENSITY GAUGE – used for in situ density measurement of soil

Procedure

1. Make the surface even by using a guide plate or any other suitable equipment.
2. Make a hole by pounding a steel rod with a similar diameter to that of gauges retractable rod. The hole should be at least 50mm deeper than the intended depth of measurement.
3. Nuclear Density Gauges normally operate in two modes.
   1. Direct Transmission
   2. Back Scatter
4. For measuring the density of soil, set the equipment to ‘Direct Transmission Mode’.
5. Lower the source rod into the hole. Set the handle to the depth position required.
6. Read the detector count on the panel. Use the calibration chart provided by the manufacturer to obtain density of material.
7. It may be noted that the detector count is inversely proportional to the density of the surrounding material.

Note

The Nuclear Density Gauges are calibrated at the factory. Since the source material undergoes decay, it needs to be calibrated from time to time in accordance with the procedure given by the manufacturer

HOW HOT & COLD WEATHER CAN AFFECT CONCRETE?

Effect of Extreme Weather on Concrete

Concrete is not recommended to be placed at a temperature above 40⁰C and below 5⁰C without proper precaution as laid down in IS: 7861 (Part-1 or part-2 as the case may be).
IS:7861 part-1 deals with hot weather concreting and Part-2 deals with cold weather concreting.

Hot Weather Concreting

Special problems are encountered in the preparation, placement and curing of concrete in hot weather. High temperatures result in rapid hydration of cement, increased evaporation of-mixing water, greater mixing water demand, and large volume changes resulting in cracks. The problems of hot weather on concrete are further aggravated by a number of factors, such as use of rapid-hardening cements, handling of larger batches of concrete, etc.
Any operation of concreting done at atmospheric temperature above 40⁰C may be put under hot weather concreting. In the absence of special precautions as laid down under IS: 7861 (Part-1), the effect of hot weather may be as follows:

A) Accelerated Setting

A higher temperature of fresh concrete results in a more rapid hydration of cement and leads to reduced workability/ accelerated setting. This reduces the handling time of concrete.

B) Reduction in Strength

Concrete mixed, placed and cured at higher temperature normally develops higher early strength than concrete produced and cured at normal temperature but at 28 days or later the strength are generally lower.

C) Increased Tendency to Crack

Rapid evaporation may cause plastic shrinkage and cracking and subsequent cooling of hardened concrete would introduce tensile stresses.

D) Rapid Evaporation of Water During Curing Period

It is difficult to retain moisture for hydration and maintain reasonably uniform temperature conditions during the curing period.

E) Difficulty in Control of Air Content in Air-Entrained Concrete

It is more difficult to control air content in air-entrained concrete. This adds to the difficulty of controlling workability. For a given amount of air-entraining agent, hot concrete will entrain less air than concrete at normal temperatures.
In order to avoid harmful effect of hot weather concreting IS: 7861 (Pt.1) recommends that temperature of ingredients should be controlled so that the temperature of produced concrete is lower. Mixing water has the greatest effect on lowering of temperature of concrete. The use of chilled water/ flaked ice in mixing produces adequate reduction in concrete temperature.
In order to control the temperature of concrete and to avoid adverse effect of hot weather, it is desirable to limit the maximum temperature of concrete as 35⁰C to keep margin for increase in temperature during transit.

Cold Weather Concreting

The production of concrete in cold weather introduces special and peculiar problems which do not arise while concreting at normal temperatures. Quite apart from the problems associated with setting and hardening of cement concrete, severe damage may occur if concrete, which is still in the plastic state, is exposed to low temperature thus causing ice lenses to form and expansion to occur within the pore structure. Hence it is essential to keep the temperature of the concrete above a minimum value before it is placed in the form-work. After placing, concrete may be kept above a certain temperature with the help of proper insulating methods before the protection is removed. During periods of low ambient temperature, special techniques are to be adopted to cure the concrete while it is in the form-work or after its removal.
Any concreting operation done at a temperature below 5⁰C is termed as cold weather concreting.
IS: 7861 (Part-2) recommends special precautions to be taken during cold weather concreting.
In the absence of special precautions, the effect of cold weather concreting may be as follows:

A) Delayed Setting

When the temperature is falling to about 5⁰C or below, the development of strength of concrete is retarded compared with development at normal temperature. Thus, the time period for removal of form work has to be increased.

B) Freezing Of Concrete At Early Stage

The permanent damage may occur when the concrete in fresh stage is exposed to freeze before certain pre-hardening period. Concrete may suffer irreparable loss in its properties to an extent that compressive strength may get reduced to 50% of what could be expected for normal temperature concrete.

C) Stresses Due To Temperature Differentials

Large temperature differentials within the concrete member may promote cracking and affect its durability adversely. It is a general experience that large temperature differentials within the concrete member may promote cracking and have a harmful effect on the durability. Such differentials are likely to occur in cold weather at the time of removal of form insulation.

D) Repeated Freezing and Thawing Of Concrete

If concrete is exposed to repeated freezing and -thawing after final set and during the hardening period, the final qualities of the concrete may also be impaired.
In view of above, it is desirable to limit the lowest temperature of concrete as 5⁰C

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CONTRACTION JOINT IN CONCRETE – WHAT, WHY & HOW?

What is Contraction Joint?

A contraction joint is one in which the two concrete surfaces are free to move away from one another as a result of shrinkage or thermal movement. Relative movement in the plane of the joint is prevented.

Why to Provide Contraction Joint?

As concrete hardens and dries out, it shrinks. Unless this shrinkage is unrestrained, it creates tensile stresses in the concrete which may cause it to crack.
Whilst reinforcement will resist these tensile stresses and help prevent the formation of large cracks, it does not completely prevent cracking. It merely ensures that the cracks, as they occur, are more closely spaced and of smaller width. In properly designed reinforced concrete, they will not be obvious or of concern when seen from normal viewing distances.
Unreinforced concrete, on the other hand, will tend to develop somewhat larger cracks at more irregular intervals; wherever the tensile strength of the concrete is exceeded by the shrinkage stresses.
To prevent such cracks, contraction joints must be installed at appropriate intervals. It may also be advisable to install contraction joints in reinforced concrete rather than relying solely on reinforcement to control shrinkage stresses.
Contraction joints may also be required in mass concrete or very large members, to allow for the shrinkage or reduction in volume which occurs as concrete cools or loses temperature after it has been placed.

How to Locate Contraction Joint?

The location of contraction joints is a matter for the designer or supervising engineer to decide. For example, their location will often be defined on the drawings for pavements, industrial floors and similar applications, while in other cases they will be in a regular pattern or be an integral part of the architectural features.
Generally they will be situated where the greatest concentration of tensile stresses resulting from shrinkage are to be expected:

• At abrupt changes of cross-section; and
• In long walls, slabs.

Contraction joints are most common in large areas of concrete pavement where they are used to divide the concrete into bays. Ideally, these should be approximately square. They may also be necessary in long walls, particularly where an unplanned crack would be undesirable.
Contraction joints form a convenient point at which to stop concrete work at the end of the day.
Construction joints should never be formed in the middle of a bay.

Construction

Fig-1 Vertical Contraction Joint

Contraction joints are formed by creating a vertical plane of weakness in the slab or wall. Movement is allowed at this point to accommodate that due to shrinkage. On the other hand, it is usually necessary to prevent movement in other directions, i.e. in directions parallel to the plane of the joint Fig-1. These twin requirements have the following consequences:

• The bond between abutting concrete surfaces in the joint must be broken.
• Reinforcement is terminated on both sides of the joint.
• Dowel bars if used must be unbonded on one side of the joint.

Control Joints

Fig-2 Sawn Joint in Concrete Pavement

A control joint is a form of contraction joint which is formed by building a plane of weakness into either a vertical or horizontal member. As the concrete shrinks, tensile stress is concentrated on this plane causing the concrete to crack there rather than elsewhere.
Normally, mechanical interlock across the two faces of the joint is expected to prevent other movement in the joint.
Control joints are, therefore, a relatively simple alternative to a fully formed contraction joint. They are placed wherever a formed joint would have been placed and are most widely used in unreinforced floors and pavements. Joint spacing in these applications, range from 1 m for thin pedestrian pathways and driveways to, say, 5 m for road pavements.
Control joints can be made at any one of three stages during construction, viz:

• A premoulded strip may be inserted into the concrete, as it is being placed, to create a plane of weakness. Metal strips inserted into terrazzo or preformed plastic strips inserted into concrete pavements to form the centre line of the pavement are examples.
• A joint can be formed in the surface of the concrete with a suitable jointing or grooving tool. Upon hardening, the concrete cracks at this point, creating a joint.
• After the concrete has hardened sufficiently to prevent ravelling of the edges, a sawn joint may be formed. The joint should be made as early as possible and prior to drying shrinkage starting to occur. Delay can result in unplanned cracking of the pavement. The sawn joint is then filled with a joint sealant to prevent dirt and other debris entering it Fig-2 as unsealed joints tend to fill with dirt and become ineffective.

WHY STEEL IS USED IN CONCRETE?

              Role of Steel in Concrete

Reinforced concrete is a material that combines concrete and some form of reinforcement into a composite whole. Whilst steel bars, wires and mesh are by far the most widely used forms of reinforcement, other materials are used in special applications, e.g. carbon-filament reinforcement and steel fibres.
Concrete has a high compressive strength but a low tensile strength. Steel, on the other hand, has a very high tensile strength (as well as a high compressive strength) but is much more expensive than concrete relative to its load-carrying ability. By combining steel and concrete into a composite material, we are able to make use of both the high tensile strength of steel and the relatively low-cost compressive strength of concrete.
There are some other advantages to combining steel and concrete in this way which are derived from the characteristics of the materials. (These characteristics are summarised in Table-1).

Table-1 Characteristics of steel and concrete
Characteristics of Concrete	Characteristics of Steel
High compressive strength	High compressive strength
Low tensile strength	High tensile strength
Relatively high fire resistance	Relatively low fire resistance
Plastic and mouldable when fresh	Difficult to mould and shape except at high temperatures
Relatively inexpensive	Relatively expensive

For example, the plasticity of concrete enables it to be moulded readily into different shapes, whilst its relatively high fire resistance enables it to protect the steel reinforcement embedded in it.
The aim of the reinforced concrete designer is to combine the reinforcement with the concrete in such a manner that sufficient of the relatively expensive reinforcement is incorporated to resist the tensile and shear forces which may occur, whilst utilising the comparatively inexpensive concrete to resist the compressive forces.
To achieve this aim, the designer needs to determine not only the amount of reinforcement to be used, but how it is to be distributed and where it is to be positioned. These latter decisions are critical to the successful performance of reinforced concrete and it is imperative that, during construction, reinforcement be positioned exactly as specified by the designer.
It is important, therefore, that both those who supervise the fixing of reinforcement on the jobsite, and those who fix it, have a basic appreciation of the principles of reinforced concrete as well as the principles and practices of fixing reinforcement.
Like reinforced concrete, prestressed concrete is a composite material in which the weakness of concrete in tension is compensated by the tensile strength of steel – in this case, steel wires, strands, or bars.
The compressive strength of the concrete is used to advantage by applying an external compressive force to it which either keeps it permanently in compression even when loads are applied to it during its service life (fully-prestressed) or limits the value of any tensile stress which arises under load (partial prestressing).

Fig-1

The pre-compressing or prestressing of concrete can be likened to picking up a row of books by pressing the books together Fig-1. The greater the number of books (the longer the span) the greater the force that has to be applied at either end of the row to prevent the row (the beam) collapsing under its own weight. A load applied to the top of the books would require an even greater force to be applied to prevent collapse.
In reinforced concrete, the steel reinforcement carries all of the tensile stresses and, in some cases, even some of the compressive stresses. In prestressed concrete, the tendons are used primarily to keep the concrete in compression. The tendons are stretched (placing them in tension) and then bonded to the hardened concrete before releasing them. The force in the tendons is transferred to the concrete, compressing it.
A fully prestressed concrete member is designed to be permanently under compression, effectively eliminating most cracking. In this case, if the member is slightly overloaded, some tension cracks may form but these should close up and disappear once the overload is removed, provided always that the steel has not been overstrained beyond its elastic limit. In partially prestressed members, some tensile stresses, and therefore some cracking, is accepted at the design ultimate load.
In reinforced concrete, the steel is not designed to operate at a high level of stress, as elongation of the steel will lead to cracking of the concrete. In prestressed concrete, the steel does carry very high levels of tensile stress. Whilst it is well able to do this, there are some penalties attached. Firstly, because of the forces involved, considerable care must be exercised in stretching the tendons and securing them. Stressing operations should always be carried out, or at least supervised, by skilled personnel. Secondly, the structure must be able to compress, otherwise the beneficial prestressing forces cannot act on the concrete. The designer must detail the structure so that the necessary movements can occur.