QUALITY ASSURANCE IN

Civil WORKS

BY

S.N.PANDEY, CIVIL WORKS MANAGER, BIHAR EDUCATION PROJECT COUNCIL, PATNA

 

QUALITY CONSTRUCTIONQuality construction increases the lifespan,

stability and serviceability of the structure. It also combines all the resources into economical output and reduces unnecessary repairs and maintenance. To cap all it disseminates the reputation of the agency.

Quality of Civil Work depends on the use of correct structural design, specifications and proper material and workmanship.

Rational design is an effective tool for cost effective and quality construction. As such the design should be based on the latest relevant Indian Standards

SPECIFICATION AND PROPER MATERIALS :

The quality building material, like Cement, Brick,

Aggregate (Course & fine) should be procured at

competative rate. The community should be

trained about quality of each building material

and how to procure them. The aim is to procure

best possible quality material at competative

rate.

CEMENT CONCRETE

Followings are constituents of concrete :-

Aggregate

Sand

Cement

Plasticizers for workability.

CEMENT

Followings are the grades of cement being manufactured:-

33,43 & 53 MPa.

ISI / ISO marked cement of ACC, Ultra tech/Ambuja Birla Gold/Lafarge, Concreto etc to be procured.

CEMENT

Price increases with increase in grades

For good construction 33 & 43 grades are ideal.

Initial setting time is 30 minutes & final setting time is 600 minutes

With the passage of time cement loses strength due to hydration as follows:-

Period - % loss in strength of cement

Three months – 20

Six – 30

Twelve months – 40

As such fresh cement is to be used

AGGREGATES

Coarse aggregate- 20mm down, well graded is required for all the R.C.C. works in buildings.

Sand- Sand of I.S. Zone II and F.M. not less than 2.25Grading of coarse and fine aggregates shall follow the curve of Road

Note 4 which is attached. On the basis of large nos. of cube tests carried out combination of the aggregates is recommended for higher strength and quality concrete in concluding Para of this note. The method of test for aggregates for concrete is given in IS 5779.

The nominal maximum size of coarse aggregate should be as large as possible within the limits specified but in no case greater than one-fourth of the minimum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all reinforcement thoroughly and fill the corners of the form.

For heavily reinforced concrete members as in the case of ribs of main beams, the nominal maximum size of the aggregate should usually be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the minimum cover to the reinforcement whichever is smaller.

BRICKS

Bricks as per I.S. 1077 of class 125

preferably. The water absorption by the

brick should be maximum up to 20%.

WATER CEMENT RATIO : BULKAGE IN SAND

I.S. 456 clauses 9.3 and 9.3.1 deals with quantity of water of maximum amount to be added to per 50 Kg. of cement.

Water present in sand should be determined. The simplest method of determining the bulking of sand in the field is to fill a graduated jar with the sand being used in the work upto a mark. Add sufficient water. Sand will settle down to lower mark. Difference in the levels of sand gives the rough idea of bulking which has to be compensated by increase in the amount of sand. The moisture content and bulking of medium size sand is given below.

Moisture content Bulkage

0.4% 2%

0.75% 4%

1.45% 8%

3.1% 16%

4% 20%

WATER CENMENT RATIO : BULKAGE IN SAND

Moisture content Bulkage

5% 22%

8% 28%

10% 26%

12% 18%

15% 14%

20% %

There is no bulkage in sand if fully saturated or completely dry.

It is needless to say that very effort be made to make silt, clay, dust in the aggregates almost nil. The maximum percentage of fines in aggregate is limited to 3% of coarse and uncrushed fine aggregate according to I.S. 383

WORK FORCE

The work force consists of skilled & unskilled workers as

under :

(a)SKILLED : Masons, carpenter, plumber, electrician,

blacksmith, bar benders, etc. if these are very well trained,

this can make lot of dent in quality construction.

(b)UNSKILLED : Labour force, which assist the skilled

workers, should also be trained in order to achieve quality.

(examples)

QUALITY CONTROL MEASURES

Visual inspection.

Quantitative Quality measures.

Testing of building material like –

Sand

Bricks

Concrete Mix

Cement Water ratio

Cement mortar

QUALITY CONTROL OF CEMENT CONCRETE

1. Concrete Mix Design Concrete must be satisfactory in two stages, namely in the plastic stage and

the hardened stage. If the condition of the plastic concrete is not satisfactory it can not be properly compacted and its structural value is reduced. Thus, if there are 5% of air voids due to incomplete compaction, the strength will be reduced by 30% and 10% of air voids will cause a loss of strength of about 60%. The satisfactory compaction can be obtained only if the concrete is satisfactorily and sufficiently workable. The property of workability, therefore become of vital importance from structural point of view.

2. Workability of Concrete A Concrete is said to be workable, if it can be easily mixed, placed,

compacted and finishes at the surface. A workable concrete should not show any segregation or bleeding. Segregation occurs when coarse aggregate try to separate out from the

paste and get accumulated at one side. This results in large voids, less strength and less durability.

QUALITY CONTROL OF CEMENT CONCRETE

3. Tests for workability

Slump Test – I.S. 1199 para 5 deals with this test. The slump test apparatus is very simple and cheap and the test can be conducted during the progress of work in the filed. I.S. 456 para 7.1 prescribes the values of workability.

QUALITY CONTROL OF CEMENT CONCRETE

4. Durability

The durability of concrete depends on adequate cement content and

low water cement ratio. Cement and water is needed to bind together

the aggregates firmly. Thus, the suitable combination of the

aggregates is key to quality concrete. The proper grading of the

aggregates make the concrete dense. This also governs the quantity

of water required for workability

FACTOR GOVERNING QUALITY OF CONCRETE

Cement Aggregate ratio Grading of the aggregates Volume/ weight batching Mixing Compaction Curing Water / Cement Ratio Quality of Sand Quality of Water

FACTOR GOVERNING QUALITY OF CONCRETE

Machine Mixing : Sequence

First pour 25% of the total quantity of water in the drum.

Dry coarse and fine aggregates are to be discharged into drum

Full quantity of cement is to be deposited in the drum

Finally balance quantity of water to be poured. 25 to 30 revolutions are needed for mixing with drum rotating @ 15-20 revolutions per minute

(I.S. 456 Clauses 6.1.2, 8.2.4.1 & 9.1.2)Minimum Cement Content, Maximum Water-Cement Ratio and Minimum Grade of Concrete for Different Exposures with Normal Weight Aggregates

of 20 mm Nominal Maximum Size

Reinforced Concrete

Minimum Cement

Content Kg/m3Maximum Free Water- Cement

Ratio

Minimum Grade of Concrete

300 0.55 M 20

(I.S. 456 Clauses 9.3 & 9.3.1) PROPORTIONS FOR NOMINAL MIX CONCRETE

Grade of Concrete

Total Quantity of dry Aggregates by Mass per 50 kg of

Cement, to be Taken as the Sum of the Individual Masses of Fine

and Coarse Aggregates, Kg,

Max

Proportion of Fine

Aggregates to Coarse

Aggregates (by Mass)

Quantity of Water per 50

kg of Cement, Max

M20 250 kg

i.e. coarse aggregate - 166.66 Kg

Fine aggregate- 83.33 kg

Generally 1:2 but subject to an upper limit of 1:1½ and a lower limit of

1:2½

30

Volume batching per bag of cement i.e. 35 liters

50 kg cement – 250 aggregate (maximum)

volume = 0.0347 m3 – Volume - 0.1389 m3

i.e. cement aggregate ratio = ¼ by volume.

M20 (1; 1.5 :3)

i.e. Cement Aggregate ratio = 1:4.5

By volume (per bag cement) 1:1.5:3 = 35:52.5: 105 (all in litres)

Hence batching is to be done by mass of the aggregate.

REINFORCEMENT

The standard trade/ I.S. marks is to be taken into account. In case standard bars are not available the weight of piece bar is to be determined by weighing in the shop which should be compared with the following table to know whether the bar is standard one.

Diameter of the rod Weight in Kg/metre

6mm 0.222

8mm 0.395

10mm 0.617

12mm 0.888

16mm 1.578

TIMING OF CONCRETING IN SUMMER SEASON

In freshly laid slabs, sometimes cracks occur before concrete has set due to plastic shrinkage. This happens if concrete surface loses water faster than bleeding action brings it to top. Quick drying of concrete results in shrinkage and as concrete is plastic stage can not resist any tension, short cracks develop in the material. These cracks maybe 5 to 10 cm. in depth and the is width could be as much as 3 mm.

Rate of evaporation from the surface of the concrete depends on temperature of concrete, gain of heat from sun’s radiation, relative humidity of ambient air and velocity of wind playing over concrete surface. As such in winter days, there is no problem of timing of concreting. In summer concreting should be done during early hours of day when aggregates and mixing water is comparatively cool and sun rays are slanting. After concreting, top surface of concrete should be covered with plastic sheets/gunny bags, straw or any other available materials. This will minimise surface shrinkage cracking in the concrete.

FORMWORKS The shape, and surface finish of concrete depends upon the Form Work.

Suitable chamber in the form works shall be provided in horizontal members of the structures, specially in long span to counteract the effects of deflection. As a rough guide, it may be 1 to 250 for beams and 1 in 50 for cantilevers.

The inside surfaces of formwork shall be coasted with soap solutions, raw linseed oil or any other material, approved by the Engineer in-charge so as to prevent adhesion of concrete to formwork. Release agents shall be applied strictly in accordance with the instructions and shall not be allowed to come into contact with any reinforcement. Immediately before concreting, all formwork shall be thoroughly cleaned.

Propping and centering- Props used for centering shall be of steel or timber post, ballies or any other material, approved by the Engineer-in-charge.

FORMWORKS The shape, and surface finish of concrete depends upon the Form Work.

It should be strong to support wet concrete and have smooth surface to prevent loss of slurry from the concrete.

Thin, deteriorated or non-vertical ballies will not be permitted at site.

Timber ballies shall always be vertical and never be in inclined position.

Any scaffolding shall only be on hard ground and not soft soil.

STRIPPING TIME OF FORM (Ref: IS-456, Para 11.3.1)

Sl. No. Type of Formwork Minimum period before striking forms

1. Vertical formwork to columns, walls, beams

16-24 h

2. Slabs spanning up to 4.5 m

Slabs spanning over 4.5 m

7 days

14 days

3. Beams and Arches spanning up to 6.0 m/ over 6.0 m

14 days/21 days

CONCLUSION : MIX DESIGN Following mix design of 1:2:4 (Cement :Sand:

Aggregate) gives strength required for M20

Cement Aggregate ratio - 1:6 (i.e. 1:2:4)

Sand - 24%

Nominal size 10mm - 16%

Nominal size 20mm - 60%

This combination gives a minimum of 100% excess strength over nominal mix of 1:2:4. i.e. M15

WATER ABSORPTION OF BRICKS

Field Test : The Test specimen shall consist of five whole dry

bricks and shall be selected at random from the stack. The

apparatus shall consist of a balance sensitive of within 0.2 to 0.3

percent of the weight of the specimen.

The test specimen shall be weighed and shall then be

completely immersed in clean water at room temperature and

allowed to remain in this State for a period of 24 hours. The

specimen shall then be taken out wiped with a damp cloth and

then weighed immediately.

Percentage of water absorption

Weight of bricks after 24 hours immersion ________________________________ x 100 Weight of dry bricks

WATER ABSORPTION OF BRICKS

Laboratory Test : The Test specimen shall consist of five

whole selected at random from the lot of bricks obtained.

The apparatus shall consist of a balance sensitive to

within 0.1 percent of the weight of the specimen.

The test specimen shall be dried to constant weight in

ventilated oven at 1100 to 1150 . The specimen, shall then

be cooled approximately to room temperature and

weighed.

The dry specimens shall be completely immersed without

preliminary partial immersion, in clean water at 15.50 to

300 C for 24 hours. Each specimen shall then be removed,

the surface water wiped off with a damp cloth 2nd the

specimen weighed. Weighing any one specimen shall be

completed within three minutes after removing the

specimen from the water.

DETERMINATION OF COMPRESSIVE STRENGTH OF BRICKS

Five bricks shall be immersed in water at 250 to 290 C for 24

hours. They shall then be removed and allowed to drains at

room temperature for about five minutes and wiped free from

surplus moisture. Their frogs shall be filled with mortar

composed of one part portland cement and one and a half

parts clean coarse sand graded to 3 mm and down. The

bricks shall then be stored under damp sack for 24 hours.

After the expiry of this period, they shall be immersed in

water for seven days.

At the end of seven days, the samples of bricks shall be taken out, wiped dry and placed with the flat surfaces horizontal and the mortar filled face up wards between two-three plywood sheets each approximately 3 mm thick and carefully centered between the plates of the compression to testing machine. The load shall be applied axially at the uniform rate until failure occurs.

Arithmetic mean of the Maximum load at failure

Compressive Strength = Area of bricks

Thank you