Structure III

Structure IIILecture 3Properties of Concrete

Main properties of fresh concreteWorkabilityConsistencySegregation Bleeding

Workability The workability is also defined as the ease with which a freshly mixed concrete can be properly compacted and also that it can be transported, placed, and finished properly.Significance of workability- A workable concrete allows full compaction using a reasonable amount of work. This helps in achieving maximum possible density (i.e. minimum possible voids) of concrete, which results in more strength and durability of concrete.

Measurement of workabilityThe test employed for workability in India are:-Slump testCompaction factor test

Slump testSlump test is used to determine the workability of fresh concrete. Slump test as per IS: 1199 1959 is followed.The apparatus used for doing slump test are Slump cone and Tamping rod.

Figure of slump cone

Procedure The internal surface of the mould is thoroughly cleaned and applied with a light coat of oil.The mould is placed on a smooth, horizontal, rigid and non absorbent surface.The mould is then filled in four layers with freshly mixed concrete, each approximately to one-fourth of the height of the mould.Each layer is tamped 25 times by the rounded end of the tamping rod (strokes are distributed evenly over the cross section).After the top layer is rodded, the concrete is struck off the level with a trowel.The mould is removed from the concrete immediately by raising it slowly in the vertical direction.The difference in level between the height of the mould and that of the highest point of the subsided concrete is measured.This difference in height in mm is the slump of the concrete.

Review questionsWhat is workability ?How is it measured from slump test ?

Compaction factor testCompacting factor of fresh concrete is done to determine the workability of fresh concrete by compacting factor test as per IS: 1199 1959.The apparatus used is Compacting factor apparatus.

Compaction factor apparatus

Procedure The sample of concrete is placed in the upper hopper up to the brim.The trap-door is opened so that the concrete falls into the lower hopper.The trap-door of the lower hopper is opened and the concrete is allowed to fall into the cylinder.The excess concrete remaining above the top level of the cylinder is then cut off with the help of plane blades.The concrete in the cylinder is weighed. This is known as weight of partially compacted concrete.The cylinder is filled with a fresh sample of concrete and vibrated to obtain full compaction. The concrete in the cylinder is weighed again. This weight is known as the weight of fully compacted concrete.

Compacting factor = (Weight of partially compacted concrete)/(Weight of fully compacted concrete)

Factor affecting workabilityIn general, the workability of concrete mixtures is controlled by the following factors:Water content-For a given maximum size of coarse aggregate, the slump or consistency of concrete is a direct function of the water content; i.e., within limits it is independent of other factors such as aggregate grading and cement content.At a constant water/cement ratio reduction in the aggregate/cement ratio causes increase in the water content, which consequently results into the increases in consistency of concrete. At a constant water content reduction in the aggregate/cement ratio decreases the water/cement ratio and consistency would not be affected

Contd.2. Cement content In normal concrete, at given water content, a considerable lowering of the cement content tends to produce harsh (i.e., low workable) mixtures with poor finish ability. Concretes containing a very high proportion of cement or a very fine cement show excellent cohesiveness but tend to be sticky.

Contd.3. Aggregate characteristics The particle size of coarse aggregate influences the water requirement for a given consistency. At same water content very fine sands or angular sands produce lesser workable concrete as compared to a coarser or a well-rounded sand.

Contd.4. Admixtures At given water content, air-entraining admixtures improve the consistency and cohesiveness of the concrete by increasing the volume of paste. Pozzolanic admixtures tend to improve the cohesiveness of concrete. Fly ash, when used as a partial replacement for fine aggregate, generally increases the consistency at a given water content. At a constant water content of a concrete mixture, the addition of a water-reducing admixture will increase the consistency.

Consistency Consistency is very closely related to workability. It is a term which describes the state of fresh concrete. It refers to the ease with which the concrete flows. It is used to indicate the degree of wetness.

Review questionHow compaction factor test is carried out ?What are the factors affecting workability ?How consistency is closely related to workability ?

Segregation The separation of coarse aggregate particles from the main mass of concrete in its plastic state is called segregation.

Precautions for freedom from segregationThe concrete mix should be properly designed. It should not be wetter than necessary.Every operation of handling, placing, and compaction should be carefully planned and controlled. The concrete should not be allowed to fall from greater heights. It should be placed as near its final position as possible.

Bleeding The appearance of water on the surface of freshly mixed concrete is known as bleeding.It is due to the inability of the concrete materials to hold all the mixing water.Bleeding disturbs the homogeneity of concrete and causes undesirable results i.e., rendering the concrete surface permeable.

Factors tending to minimize bleedingProperly graded and proportioned aggregates.Low water cement ratio.Relatively high cement content.Natural sand with sufficient fines.Air entraining agents.

Air entrainmentAir entrainment in concrete is defined as that air which is intentionally introduced in the concrete in the shape of uniformly distributed minute bubbles by a suitable agent.Entrained air is different from entrapped air which is accidently entrapped during batching and mixing operations and is random in amount and size.Air entrainment in fresh concrete:Reduces mixing waterIncreases workabilityReduces bleeding and segregation

Placing, compacting and curing of concreteGood concrete is concrete that has been placed, compacted and cured properly. In simple terms, poor compaction and poor curing produce poor concrete with lower durability.Placing- the concrete should be placed and compacted before setting commences. It should not be subsequently disturbed. The method of placing should be such as to prevent segregation.Compaction- All fresh concrete contains tiny bubbles of air. If nothing is done to get rid of them, the resultant blowholes or honeycombing weaken the finished concrete and make it less able to resist water penetration. They can also reduce the bond between the concrete and reinforcement

Curing Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration. It may be either after it has been placed in position (or during the manufacture of concrete products), thereby providing time for the hydration of the cement to occur.Methods of curing-Impermeable membrane (plastic sheeting)By water pondingWet covering (Jute covering)By sprinklingFog curing

Review questionsWhat is segregation ? What are the preventing measures for segregation ?What is bleeding ? What precautions should be taken to avoid bleeding ?what is curing ? Why curing is done ?

Compressive strength of concreteConcrete is strong in compression but it is weak in tension and has low strain at fracture. The low tensile strength of concrete is due to the presence of numerous micro cracks. These micro cracks further propagate under load and result in poor strength of concrete.

Determination of compressive strength of concreteCasting of moulds- For each mix 9 cube specimens of size 150 x 150 x 150mm were cast in C.I. moulds. 3 cubes was tested at 7 days, 3 cubes was tested at 14 days and the remaining 3 cubes were tested at 28 days of curing. Each compressive strength result is the average of 3 test results.

Contd.Testing of cubes for compressive strength- After 7,14 and 28 days of curing, the cubes were removed from the curing tank, weighed and tested for compressive strength in a 3000 KN digital compression testing machine with the cast face parallel to the axis of loading at the rate of 140Kg/cm2/minute as per IS: 516-1959114.The load at which the specimen fails is recorded. The experimental compressive strength was obtained by dividing the maximum load applied on the specimen during the test by its cross sectional area.

Durability of concreteThe durability of concrete is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration.

The common durability problems in concrete are as follows. 1) Sulphate and other chemical attacks of concrete. 2) Alkali-aggregate reaction. 3) Freezing and thawing damage in cold regions. 4) Corrosion of steel bars or tendons.

Contd.The durability of concrete is intrinsically related to its water tightness or permeability. Hence, the concrete should have low permeability and there should be adequate cover to reinforcing bars.The selection of proper materials and good quality control are essential for durability of concrete.

Nominal mix and design mixConcrete mix design- Concrete mix design is defined as the appropriate selection and proportioning of constituents to produce a concrete with pre-defined characteristics in the fresh and hardened states.Nominal mix design- it is a ordinary concrete design as per IS:456-1978. Nominal mix grade of concrete vary from M5, M10, M15 & M20.

Review questionsWhat do you understand by compressive strength of concrete ? How is it determined ?Discuss factors affecting durability of concrete.Differentiate between Nominal mix and Design mix.

Typical questionsQues 1. What is the ideal value of slump?Answer 1In case of a dry sample, slump will be in the range of 25-50 mm that is 1-2 inches. But in case of a wet concrete, the slump may vary from 150-175 mm or say 6-7 inches. So the value of slump is specifically mentioned along the mix design and thus it should be checked as per your location.Slump depends on many factors like properties of concrete ingredients aggregates etc. Also temperature has its effect on slump value. So all these parameters should be kept in mind when deciding the ideal slumpQues 2. How does a super plasticizer effect the slump of concrete?Answer 2Value of Slump can be increased by the addition of chemical admixtures like mid-range or high-range water reducing agents (super-plasticizers) without changing the water/cement ratio.Ques 3. How much time one should take to raise the cone?Answer 3Once the cone is filled and topped off [ excessive concrete from top is cleared ] raise the cone within 5-10 seconds.Ques 4. What are the dimensions of the cone used in Slump Test?Answer 3The dimensions areTop Diameter 10cmBottom Diameter 20cmHeight 30cm