Period of removal of formwork

STRENGTHENING OF R.C.COLUMNS

Strengthening of reinforced concrete columns is needed when:
1. The load carried by the column is increased due to either increasing the number of floors or due to mistakes in the design.
2. The compressive strength of the concrete or the percent and type of reinforcement are not according to the codes’ requirements.
3. The inclination of the column is more than the allowable.
4. The settlement in the foundation is more than the allowable.
There are two major techniques for strengthening reinforced concrete columns:

1 REINFORCED CONCRETE JACKET
The size of the jacket and the number and diameter of the steel bars used in the jacketing process depend on the structural analysis that was made to the column.
In some cases, before this technique is carried out, we need to reduce or even eliminate temporarily the loads applied to the column; this is done by the following steps:

• Putting mechanical jacks between floors.
• Putting additional props between floors.

Moreover, in some cases, where corrosion in the reinforcement steel bars was found, the following steps should be carried out:

• Remove the concrete cover.
• Clean the steel bars using a wire brush or sand compressor.
• Coat the steel bars with an epoxy material that would prevent corrosion.
• If there was no need for the previous steps, the jacketing process could start by the following steps:

1. Adding steel connectors into the existing column in order to fasten the new stirrups of the jacket in both the vertical and horizontal directions at spaces not more than 50cm.Those connectors are added into the column by making holes 3-4mm larger than the diameter of the used steel connectors and 10-15cm depth.
2. Filling the holes with an appropriate epoxy material then inserting the connectors into the holes.
3. Adding vertical steel connectors to fasten the vertical steel bars of the jacket following the same procedure in step 1 and 2.
4. Installing the new vertical steel bars and stirrups of the jacket according to the designed dimensions and diameters.
5. Coating the existing column with an appropriate epoxy material that would guarantee the bond between the old and new concrete.
6. Pouring the concrete of the jacket before the epoxy material dries. The concrete used should be of low shrinkage and consists of small aggregates, sand, cement and additional materials to prevent shrinkage.

The previous steps are illustrated in Fig 1.

 

Fig:1. Increasing the cross-sectional area of column by RC jacketing.

2 STEEL JACKET
This technique is chosen when the loads applied to the column will be increased, and at the same time, increasing the cross sectional area of the column is not permitted.
This technique is implemented by the following steps as shown in Fig 2:
1. Removing the concrete cover.
2. Cleaning the reinforcement steel bars using a wire brush or a sand compressor.
3. Coating the steel bars with an epoxy material that would prevent corrosion.
4. Installing the steel jacket with the required size and thickness, according to the design, and making openings to pour through them the epoxy material that would guarantee the needed bond between the concrete column and the steel jacket.
5. Filling the space between the concrete column and the steel jacket with an appropriate epoxy material.

 

Fig:2. Increasing the cross-sectional area of column by steel jacketing. 

In some cases, where the column is needed to carry bending moment and transfer it successfully through the floors, one should install a steel collar at the neck of the column by means of bolts or a suitable bonding material.

 Fig:3. shows a column which was strengthened with steel angles.

FINENESS MODULUS METHOD OF CONCRETE MIX DESIGN

Fineness Modulus Method of Concrete Mix Design

The index number, which describes the relative sizes of coarse and fine aggregates, is called as fineness modulus (FM). Fineness modulus is determined separately by sieving coarse and fine aggregates through the following set of sieve.
IS sieves for coarse aggregates are 80 mm, 40 mm, 20 mm, 10 mm and 4.75 mm.
IS sieves for fine aggregates are 1 mm, 600 , 300 , 212 , 150 , and 75 .

Where, X= Fineness modulus of coarse aggregate,
Y= Fineness modulus of fine aggregates, and
Z= Economical value of fineness modulus for combined aggregate.
The above formula will fix the proportion of fine aggregate to coarse aggregate. The percentage of the coarse aggregate is equal to 100 minus the percentage of fine aggregate.
Table below gives the economical value of F.M. for combined aggregate.

Table: Economical Value of F.M. for Combined Aggregate

S. No.	Particle size (mm)	Fineness modulus		Average F.M.
		Minimum	Maximum
1	20	4.7	5.1	4.90
2	25.4	5.0	5.5	5.25
3	31.5	5.2	5.7	5.45
4	38.1	5.4	5.9	5.65
5	40	5.7	6.3	6.00
6	80	6.5	7.0	6.75

With high water-cement ratio, lower value of fineness modulus should be adopted and with low water-cement ratio, higher value of fineness modulus should be adopted.

The next step is to find out the quantity of water-cement paste to be added to fine and coarse aggregates in order to obtain the required workability. Slump test is used to confirm the quantity of water-cement paste. The method consists of preparing a number of batches of combined aggregate in the fixed proportions. In each batch, varying quantities of water-cement paste having fixed water-cement ratio are added to get the required slump. In order to start with, the minimum quantity of water is added in the first batch. It is determined from the following formula:

where, P= Quantity of cement by weight,
Y= Quantity of fine aggregate by weight,
Z= Quantity of coarse aggregate by weight, and
= Water cement ratio

ADVERSE EFFECT OF CONCRETE ADMIXTURE

Adverse effects due to improper application :
Admixtures are used extensively to produce high workable, high strength high performance and highly durable concrete with minimum cost. However, these admixtures are not used judiciously and with the poor knowledge of admixture among engineers at site results in the following adverse effect on concrete.
Rapid slum loss : This effect general observed in rich mixes with higher cement content and it can be reduced by adding booster dosages at different intervals.
Severe segregation/bleeding : This is generally observed in lean mixes with low cement content and depends on dosage of admixture. This can be minimized either by reducing admixture dosage or by increasing content of fine in the concrete.
Over retardation : This effect a noticed when the admixture is added beyond the specified dosage and it would effect the construction schedule, result in low strength development at early age. However ultimate strength of the concrete remains same.
Plastic shrinkage : This is general observed in large floor slabs of this sections and due to excess evaporator of water from the surface of the concrete at high temperatures are continuous breezing. However the plastic shrinkage cracks are determental to structures.

Effect of CaCl₂ on setting time of OPC

PLUMBING WORKS (Drainage lines)

Its a general information as well as to remind the peoples to pay their attention specially on the toilet and kitchen drainage line works,
It has been noticed the pipes @ turning points were provided with short and narrow bend,
will block the smooth flow as well as will collect the durts, and waste, soil stack at that point.

So proper 45degree bend must be provide on the drainage system specially for the houses.
Please see the attached pics.
Engr.Noushad Ali

Bridge types

Truss analysis

Design Methods

Bamboo bridge is still the compulsion for the people of rural areas.

Pile Load Test

: 7500 MT Compression load test on 1800 mm dia. Pile..

Geotechnical Engineering Laboratory Experiments

SURCHARGE.

SURCHARGE.

Warren’s drain collection system could experience
- “surcharge conditions” – that result in basement flooding.

The original etymology comes from French language:
surcharger: with sur = “over” + chargier = “to load”

Understanding where words come from, what they truly mean,
is crucial to the English language used in government documents.

Surcharge:
means to overload the sewer system, literally.
(more water than the system can convey per unit of time)
Result: Water level in the system raises;
causing water to flow backwards from normal flow
and causing flooding in areas that are at a lower elevation.
Water flows backwards in a sewer lateral,
continues to rise, comes out of the basement floor drain
and begins to flood the basement

By Engr.Noushad A

Rebar Tieing machine

Minimum Thickness Of structural members

slump test,

The test has been typically used in precast operations and at large construction sites. Some of the work imparted into the concrete is lost in friction between the hoppers and the concrete. The magnitude of this friction varies between different concrete mixtures and may not reflect field conditions.The compaction factor test gives more information (about compactability) than the slump test,

Effect of water addition on slump and strength of concrete

Typical cross section of a flexible pavement

CANTILEVER RETAINING WALL

• Cantilever wall are usually of reinforced concrete and work on the principles of leverage.
• Have much thinner stem, and utilize the weight of the backfill soil to provide most of the resistance to sliding and overturning.
• Most common type of earth- retaining structure.
• The cantilever retaining wall (“cantilever wall”) constructed of reinforced Portland-cement concrete (PCC) was the predominant type of rigid retaining wall used from about the 1920s to the 1970s
• Earth slopes and earth retaining structures are used to maintain two different ground surface elevations.

FUNCTION
To retain the soil at a slope that is greater than it would naturally assume, usually at a vertical or near vertical position.

Design Consideration
In order to calculate the pressure exerted at
any point on the wall, the following must be
taken in account:

• height of water table
• nature & type of soil
• subsoil water movements
• type of wall
• material used in the construction of wall

The effect of two forms of earth pressure need to be considered during the process of designing the retaining wall that is:
a)Active Earth Pressure
“ It is the pressure that at all times are tending to move or overturn the retaining wall”
b)Passive Earth Pressure
“It is reactionary pressures that will react in the form of a resistance to movement of the wall.

Two Basic Form of Cantilever Wall
1 ) A base with a large heel so that the mass of earth above can be added to the wall for design purposes.

Figure 1 :Typical reinforced concrete cantilever walls.

2 ) If form 1 is not practicable, a cantilever wall with a large toe must be used.

Figure 2 : Typical reinforced concrete cantilever retaining walls

From figure 1 and 2 :

• The drawing show typical section and pattern of reinforcement encountered with these basic forms of cantilever retaining walls.
• The main steel occurs on the tension face of the wall and nominal steel (0.15% of the cross-sectional area of the wall) is very often included in the opposite face to control the shrinkage which occurs in in-situ concrete work.
• Reinforcement requirements, bending, fabricating and placing are dealt with in the section on reinforced concrete.

Reinforcement of cantilever retaining wall.


Advantages and details about cantilever wall
Reinforced cantilever walls have an economic height range of 1.200 to 6.000 m; walls in excess of this height have been economically constructed using prestressing techniques. Any durable facing material may be applied to the surface to improve the appearance of the wall but it must be remembered that such finishes are decorative and add nothing to the structural strength of the wall.

Cantilever Wall Failure

• Effect of water: Ground water behind a retaining wall, whether static or percolating through a subsoil, can have adverse effects upon the design and stability.
• Slip circle failure: sometimes encountered wit cantilever wall in clay soils particularly if there is a heavy surcharge.
• Low quality of material that use in cantilever construction
• Low design reinforcement in cantilever wall.
• Mistake in calculate height of water table, nature & type of soil.
• Subsoil water movements.

Identifying Failure of Cantilever Wall

• Cantilever wall be in sloping position.
• Cantilever wall had curve on its surface/wall.
• Crack on wall structure.
• Cantilever wall awashed.

FORMWORK– TECHNICAL, FUNCTIONAL & ECONOMIC REQUIREMENTS

Formwork is a temporary mould into which fresh concrete and reinforcement are placed to form a particular reinforced concrete element.
A typical breakdown of total construction percentage costs shows that formwork material and labour alone consists of 35% of the total concrete construction cost. In the construction of a structural element, the cost distribution can be found approximately as:

• Concrete Cost – Materials 28%; Labour 12% = 40%
• Reinforcement Steel Cost -Materials 18%; Labour 7% = 25%
• Formwork Cost – Materials 15%; Labour 20% = 35%.

To ensure that the formwork is economical and practical to build, the following basic technical, economical and functional requirements that should be kept in mind when designing and constructing formwork.

Technical requirements of formwork:

• Formwork should be of the desired shape, size and and fit at the location of the member in structure according to the drawings.
• Formwork shall be carefully selected for required finish surface and linings to produce the desired concrete surface.
• Formwork should withstand the pressure of fresh concrete and working loads and should not distort or deflect from their position during the concrete placing operation.
• Formwork should support the designed loads any other applied loads during the construction period.
• The formwork must not damage the concrete or themselves during  removal from structure.
• Panels  of the formwork should be tightly connected to minimize gap at the formwork connection to prevent leakage of cement paste.

Functional requirements of formwork:

• Form sections should be of the size that can be lifted and transported easily from one job site to another.
• Formwork should be dismantled and moved as easily as possible so that construction of the building advances.
• Formwork Units must be interchangeable so that they can be used for forming different members.
• Formwork shall be designed such that it fits and fastens together with reasonable ease.
• Forms should be simple to build.
• Formwork should be as lightweight as possible without any strength reduction.
• Forms should be made such that workers can handle them without any safety issue, respecting the Health, Safety, and Hygiene Regulation in effect.

Economic requirements of formwork:

• Formwork shall be made of low cost materials, energy and labour if possible.
• Formwork should be manufactured such that it can be repetitively used and shall be as adaptable as possible. They must be able to withstand a good number of reuses without losing their shape.
• Formwork must be designed so that the whole formwork can be assembled and dismantled with unskilled or semi-skilled labour.
• Formwork care and maintenance should be done according to specifications.

Foundation construction for building tower block | Pile caps under preparation

Expected modes of failure of cracked non-ductile beams retrofitted with one strip,,,,,,,,

Expected modes of failure of cracked non-ductile beams retrofitted with one strip,,,,,,,,

(a) pure shear failure,
(b) shear failure with debonding of retrofitting strip, 
(c) shear failure with determination of concrete cover, 
(d) combined shear and flextural failures and 
(e) pure flextural failure.

CRACKS IN FOUNDATION

Serious structural problems in houses are not very common, but when they occur they are expensive to repair. Some can’t be fixed at all. This report won’t turn you into an expert, but it will give you some of the common indicators.

Cracks in Foundation

Uneven Floors. Uneven floors are typical, particularly in older homes. Here is a trick to help distinguish between a typical home with character and a structural problem. If the floor sags to the middle of the home, it’s probably just a charming old home. Houses are like people, they sag in the middle when they get older. On the other hand, if the floor slopes towards an outside wall, there is a good chance that the house has significant structural problems. While no house is perfect, this is one area where you should be very careful. Take a look at the house from across the street. If the house appears to be leaning one way or the other, there may be a structural problem. It may help to line up a front corner of the house with the back corner of an adjacent house just for reference. The corners should be parallel. Stepping back from the house to take a look is always a good idea. It is easy to miss something major by standing too close to it! If there is a lean that is detectable by eye, don’t take any chances, get it checked out.

Horizontal Foundation Cracks are Bad.
It is not uncommon to find cracks in the foundation, especially poured concrete foundations. This goes for new houses as well as old ones. While there is a great deal of engineering that goes into “reading” these cracks, there is one rule that you should never forget. “Horizontal cracks are a problem”. Of course not all vertical cracks are acceptable, but they are generally not as serious as a horizontal crack.

Leaning Walls.
A leaning foundation wall is not ideal, but may not be a significant defect if movement does not appear to be recent. Check with your home inspector to be sure.

Harmless Cracks.
Poured concrete shrinks as it cures. Shrinkage cracks in a new house are common and can be small vertical cracks or small 45 degree cracks at the basement windows. These cracks are about 1 /8 inch wide or less. They don’t affect the structure. The only concern is leakage. If you see small cracks in a new foundation, don’t panic. In fact, in a new home, some builders will pre-crack the foundation and fill the crack with flexible material.

Plaster or Drywall Cracks.
Few things are more misunderstood than plaster or drywall cracks on the inside of the house.

The following crack types are not generally related to structural movement: We call these “stress cracks” or “surface cracks”.

• a small crack (less than 1 /4 inch) that follows the corner of the room where two walls meet
• small cracks that extend up from the upper corner of a door opening

The following cracks may be related to structural movement

• large cracks (larger than 1 /4 inch in width) or cracks that have deflection (a lip, where one side of the crack is beyond the other side of the crack).
• cracks that run diagonally across the wall, or in a stair step fashion.
• cracks on the interior finish that is in the same vicinity as cracks on the exterior of the house.

Structural movement or structural damage cracks can be repaired in a number of ways, such as; building buttresses, pilasters, steel tie-backs, steel channel columns, sister walls, etc. A good inspector can describe these methods to you should the need arise.