Quality Control For Concrete.

Quality Control

Specific quality control requirements for the work are indicated throughout the Contract Documents. The requirements of this section are primarily related to the performance of the work beyond furnishing of manufactured products.

 Quality is constantly checked by PDCA cycle i.e.,

   1. Plan

   2. Do

   3. Check

   4. Action

Quality control involves incorporating the following:

    1. Formulation & implementation of project specific quality plan

    2. Maintaining records for incoming material inspection

    3. Maintaining records for tests on construction materials

Material list of quality records

    1. Quality Plan

    2. Documents Control Sheet

    3. Internal & statistical quality control document

    4. Mix design of concrete

    5. Tests reports of bricks, cement, steel, concrete & aggregates

At QC , an Inspection Testing Plan is received at the foremost .It is then the purchase list of equipments is prepared according to the ITP and the acceptance of the raw materials is done.The basic raw materials used at site are listed below

1. Sand

2. Cement

3. Stone Chips

4. Admixture

5. Bricks

6. Adhesive

7. Waterproofing Components

8. Water Sealant

SAND:

In quality control, sand is checked for zone1, 2, 3 & 4 of which last two are of basic importance.

Zone 2 Sand: Used for Concreting

Zone 3 Sand: Used for Brick Masonry Work

·  Sieve Analysis

           Sieve analysis determines the gradation or distribution of aggregates particular sizes within a given sample.

 For zone 3

 sampling the percentage passing of sand through 600  micron sieve should be between 60-79.It is checked for silt contents.

          There are two methods of silt check

1.    By Volume Method

2.    By Weight Method

By volume method the silt content should be between 8% and by weight method it should be between 3%. IS code: 383-1970 & IS: 2386-1963.

 For zone 4

 sampling the percentage passing of sand through 300    micron sieve should be between 20-65.IS code: 1542-1992

CEMENT :

It is mainly divided in the following three parts

1. PPC - Portland Pozzolona Cement

2. OPC - Ordinary Portland Cement

3. PSC - Portland Slag Cement

N.B: Initital setting time of cement should not be less than 30minutes & final setting time should not be more than 10 hours

Some of the tests performed in QC for cement are:

1.    Consistency Test

2.    Compressive Test

3.    Initial Setting Time

4.    Final Setting Time

COARSE AGGREGATE:

 Stone chips are coarse aggregate used in concrete mix. Sieve analysis is performed in site laboratory for stone chips. Stone chips are passed through IS sieves of perforations 40 mm ,20 mm, 10 mm, 6mm , etc and graded according to the IS code ref 2386(Part 3) (Procedure) and 383(Actual Criteria).

ADMIXTURE:

IS: 9103-1999 covers the following types of admixtures

1. Accelerating admixtures

2. Retarding admixtures

3. Water reducing admixture

4. Super plasticizer admixture   and

5. Air entraining admixtures

BRICKS:

The bricks at site are divided into two main categories namely

  1. Traditional or Standard Bricks

  2. Modular Bricks

    As per CPWD specifications the size of traditional brick is (230x110x70) mm and the size of modular brick is (190x90x90) mm.

    The strength of traditional brick is 3 N/mm

·       Brick Testing :

1.    Size & Shape

2.    Sharpness

3.    Cold Crushing Strength

4.    Water Absorption

5.    Soundness

Concrete:

    Concrete is the homogeneous mixture of Sand, Cement, Coarse aggregate and Water. Concrete is used to take load in compressive zone.

·       Mixing :

All concrete, whether plain or reinforced, ordinary or controlled, shall be mixed in a standard type of concrete mixer not less than two minutes. Materials for concrete shall be deposited into the drum while it is in rotation. IS code reference for mix design: 10262-2009

Properties of Concrete that should be checked at QC are

1. Workability

2. Durability

3. Segregation

4. Bleeding

5. Curing

6. Slump Test

7. Cube test

    Workability

  The term workability is used to describe the ease of difficulty with which the concrete is handled, transported and placed between the forms with minimum loss of homogeneity.

  The workability, as a physical property of concrete alone irrespective of a particular type of construction, can be defined as the amount of useful internal work, necessary to produce full compaction.

   If the concrete mixture is too wet, the coarse aggregates settle at the bottom of concrete mass and the resulting concrete becomes of non uniform composition.On the other hand, if the concrete mixture is too dry, it will be difficult to handle and place it in position. Both these conflicting conditions should be correlated by proportioning carefully various components of concrete mixture. The important in correction withworkabilty are as follows:

1)    If more water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability.

2)    If the strength of concrete is not be affected ,the degree of workability can be abstained :

a.     by slightly changing the proportions of fine and coarse aggregates, in case the concrete mixture is too wet ; and

b.    By adding a small quantity of water cement paste in the proportion of original mix, in case the concrete mixture is too dry.

3)   A concrete mixture for one week may prove to be too stiff or too wet for another work. For instance, the stiff concrete mixture will be required in case of vibrated concrete work while wet concrete mixture will be required for this sections containing reinforcing bars.

4)    The workability of concrete is affected mainly by water content, water cement ratio and aggregate-cement ratio.

5)    The workability of concrete is also affected by the grading, shape, texture and minimum size of the coarse aggregates to be used in the mixture

Durability

The durability of concrete is defined as its ability to resist weathering action, chemical attack, abrasion or any other process of deteriorations. Durable concrete will retain its original form, quality and serviceability when exposed to its environment.

Generally, constructionindustry needs faster development of strength in concrete so that the projects can be completed in tirmof before time. The demand is cleared by high early strength cement, use of very low W/C ratio through the use of increased cement content and reduced water content.With higher quantity of cement content, the concrete exhibits greater cracking tendencies because of increased thermal and drying shrinkage. As the creep coefficient is low in such concrete, there will not be much scope for relaxation of stresses.Therefore; high early strength concretes are more prone to cracking than moderate or low strength concrete.

Field experience have also corroborated that high early strength concrete are more crack-prone. According to a recent report, the cracks in pier caps have been attributed to the use of high cement content in concrete. Contractors apparently though that a higher than the desired strength would speed up the construction time, and therefore used high cement content.

Similarly, report submitted by National Cooperative Highway Research Programme(NCHRP) of USA during 1995 , based on their survey ,showed that more than,100000 concrete bridge deck in USA showed full depth transverse cracks even before structures were less than one month old.The reasons given are that combination of thermal shrinkage and drying shrinkage caused most of the cracks.It is to be noted that deck concrete is made of high strength concrete .These concretes have a high elastic modulus at an early age.Therefore,they developed high stresses for a given temperature change or amount of dying shrinkage.The most important point is that such concrete creeps little to relieve the stresses.

Segregation

Segregation can be defined as the separation of the constituent materials of concrete. A good concrete is one in which all the ingredients are properly distributed to make a homo generous mixture.If a sample of concrete exhibits a tendency for separation of say ,coarse aggregate from the rest of the ingredients then, that sample is said to be showing the tendency for segregation. Such concrete is only going to be weak ; lack of homogeneity is also going to induce al undesirable properties in the hardened concrete.

Bleeding

Bleeding is sometimes referred as water gain. It is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete, being of the lowest specific gravity among all the ingredients of concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete.

Due to bleeding, water comes up and accumulates at the surface.Sometimes, along with the water; certain quantity of cement also comes to the surface. When the surface is worked up with in the towel and floats, the aggregates goes down and the cement and water come up to the top surface.

  Method of Test for Bleeding of Concrete

A cylindrical container of approximately 0.01meter cube capacity, having an inside diameter of 250 mm and height 280 mm used. A tamping bar similar to the one used for slump test is used. A pipette for drawing off free water from the surface, a graduated jar of 100 centimeter cube capacity is required for test.

A sample of freshly mixed concrete is obtained. The concrete is filled in 50 millimeter layer for a depth of 250±3 millimeter (5 layers) and each layer is tamped by giving stokes, and the top surface is made smooth by toweling.

The test specimen is weighed and the weight of the concrete is noted. Knowing the total water content in 1 meter cube of concrete quantity of water in the cylindrical container is also calculated.

The cylindrical container is kept in a level surface free from vibration at a temperature of 27⁰C ± 2⁰C; it is covered with a lid. Water accumulated at the top is dawn by means of pipette at 10 minutes interval for the first 40 minutes and at 30 minutes interval subsequently till bleeding ceases. To facilitate collection of bleeding water the container may be slightly titled. All the bleeding water collected in a jar.

Curing

The concrete surfaces are kept wet for a certain period after placing of concrete so as to promote the hardening of cement. It consists of a control of temperature and of the moisture movement from and into the concrete. The term curing of concrete is used to indicate all such procedures and process.

Period of concrete depend on the type of cement and nature of work. For ordinary Portland cement, the curing period is about 7 to 14 days. If rapid hardening cement is used, the curing period can be considerably reduced.

Following are the purposes of the curing of concrete:

1. The curing protects the concrete surfaces from sun and wind

2. The presence of water is essential to cause the chemical action which accompanies the setting of concrete.

3. The strength of concrete gradually increases with age, if curing is efficient.th increase in strength is sudden and rapid in early stages and it continues slowly for an indefinite period.

4. By proper curing, the durability and impermeability of concrete are increased and shrinkage is reduced.

5. The resistance of concrete to abrasion is considerably increased by proper curing.

Following are the basic factors on which the evaporation of water from the concrete surface depends:

1. Air temperature

2. Fresh Concrete temperature

3. Relative humidity; and

4. Wind velocity

Slump Test

Slum test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work. The apparatus for conducting the slump test essentially consists of a metallic mould in the form of a frustum of a cone having the internal dimensions as under:

         Bottom Diameter   :  20 cm

         Top diameter          :  10 cm

         Height                      :  30 cm

For tamping the concrete, a steel tamping rod 16 mm die, 0.6 meter along with bullet and is used. The mould is placed on a smooth,horizontal,rigid and non – absorbant surface.The mould is then filled in four layers,each approximates ¼ of the height of the mould.Each layer tamped 25 times by the tamping rod taking care to distribute the strokes evenly over the cross section.After the top layer has been rodded,the concrete is stuck off level with a trowel and tamping rod.The mould is removed from the concrete immediately by raising it slowly and carefully in a vertical direction.This allows the concrete to subside.Th subsidence is referred as SLUMP of concrete.The difference in level between the height of the mould and that of the highest point of the subsided concrete is measured. The difference in height in mm .is taken as Slump of Concrete.