UNIT II CONCRETE

PROPERTIES OF FRESH CONCRETE Following are the important properties of fresh concrete Setting Workability Bleeding and Segregation Bleeding Segregation Hydration Air Entrainment

1. SETTING OF CONCRETE The hardening of concrete before its hydration is known as setting of concrete. OR The hardening of concrete before it gains strength. OR The transition process of changing of concrete from plastic state to hardened state. Setting of concrete is based or related to the setting of cement paste. Thus cement properties greatly affect the setting time.

FACTORS AFFECTING SETTING: Following are the factors that affect the setting of concrete. 1. Water Cement ratio  2. Suitable Temperature  3. Cement content  4. Type of Cement  5. Fineness of Cement  6. Relative Humidity  7. Admixtures  8. Type and amount of Aggregate

2. WORKABILITY OF CONCRETE Workability is often referred to as the ease with which a concrete can be transported, placed and consolidated without excessive bleeding or segregation OR The internal work done required to overcome the frictional forces between concrete ingredients for full compaction. It is obvious that no single test can evaluate all these factors. In fact, most of these cannot be easily assessed even though some standard tests have been established to evaluate them under specific conditions

WORKABILITY OF CONCRETE Slump Test can be used to find out the workability of concrete.

FACTORS AFFECTING CONCRETE WORKABILITY Water-Cement ratio Amount and type of Aggregate Amount and type of Cement Weather conditions Temperature Wind Chemical Admixtures Sand to Aggregate ratio

I. WATER CONTENT OR WATER CEMENT RATIO More the water cement ratio more will be workability of concrete. Since by simply adding water the inter particle lubrication is increased. High water content results in a higher fluidity and greater workability. Increased water content also results in bleeding. another effect of increased water content can also be that cement slurry will escape through joints of formwork.

II. AMOUNT AND TYPE OF AGGREGATE More the amount of aggregate less will be workability. Using smooth and round aggregate increases the workability. Workability reduces if angular and rough aggregate is used. Greater size of Aggregate- less water is required to lubricate it, the extra water is available for workability Angular aggregates increases flakiness or elongation thus reduces workability. Round smooth aggregates require less water and less lubricationand gretaer workability in a given w/c ratio Porous aggregates require more water compared to non absorbent aggregates for achieving sam degree of workability.

iii. Aggregate Cement ratio More ratio, less workability. Since less cement mean less water, so the paste is stiff. iv. Weather Conditions 1. Temperature If temperature is high, evaporation increases, thus workability decreases 2. Wind: If wind is moving with greater velocity, the rate of evaporation also increase reduces the amount of water and ultimately reducing workability

v. Admixtures Chemical admixtures can be used to increase workability. Use of air entraining agent produces air bubbles which acts as a sort of ball bearing between particles and increases mobility, workability and decreases bleeding, segregation. The use of fine pozzolanic materials also have better lubricating effect and more workability

vi. Sand to Aggregate ratio If the amount of sand is more the workability will reduce because sand has more surface area and more contact area causing more resistance.

3(A). CONCRETE BLEEDING Bleeding in concrete 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 Due to bleeding, water comes up and accumulates at the surface. Sometimes, along with this water, certain quantity of cement also comes to the surface. When the surface is worked up with the trowel, the aggregate goes down and the cement and water come up to the top surface. This formation of cement paste at the surface is known as “Laitance”

The bleeding water is likely to accumulate below the aggregate The bleeding water is likely to accumulate below the aggregate. This accumulation of water creates water voids and reduces the bond between the aggregates and the paste. The above aspect is more pronounced in the case of flaky aggregate. Similarly, the water that accumulates below the reinforcing bars reduces the bond between the reinforcement and the concrete. The poor bond between the aggregate and the paste or the reinforcement and the paste due to bleeding can be remedied by re vibration of concrete. The formation of laitance and the consequent bad effect can be reduced by delayed finishing operations.

PREVENTION OF BLEEDING IN CONCRETE Bleeding can be reduced by proper proportioning and uniform and complete mixing. Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse. Air-entraining agent is very effective in reducing the bleeding. Bleeding can be reduced by the use of finer cement or cement with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes.

(B). SEGREGATION IN CONCRETE 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 homogeneous mixture. There are considerable differences in the sizes and specific gravities of the constituent ingredients of concrete. Therefore, it is natural that the materials show a tendency to fall apart.

Segregation may be of three types Coarse aggregate separating out or settling down from the rest of the matrix. Paste separating away from coarse aggregate. Water separating out from the rest of the material being a material of lowest specific gravity.

THE CONDITIONS FAVORABLE FOR SEGREGATION ARE: Badly proportioned mix where sufficient matrix is not there to bind and contain the aggregates Insufficiently mixed concrete with excess water content Dropping of concrete from heights as in the case of placing concrete in column concreting When concrete is discharged from a badly designed mixer, or from a mixer with worn out blades Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper, long lift by skip and hoist are the other situations promoting segregation of concrete

4. HYDRATION IN CONCRETE Concrete derives its strength by the hydration of cement particles. The hydration of cement is not a momentary action but a process continuing for long time. Of course, the rate of hydration is fast to start with, but continues over a very long time at a decreasing rate In the field and in actual work, even a higher water/cement ratio is used, since the concrete is open to atmosphere, the water used in the concrete evaporates and the water available in the concrete will not be sufficient for effective hydration to take place particularly in the top layer.

5. AIR ENTRAINMENT Air entrainment reduces the density of concrete and consequently reduces the strength. Air entrainment is used to produce a number of effects in both the plastic and the hardened concrete. These include: Resistance to freeze–thaw action in the hardened concrete. Increased cohesion, reducing the tendency to bleed and segregation in the plastic concrete. Compaction of low workability mixes including semi-dry concrete. Stability of extruded concrete. Cohesion and handling properties in bedding mortars

HARDENED CONCRETE Concrete that is in a solid state and has developed a certain strength.  reaction continues with time and produced hard, strong and durable solid material. Following are the properties of hardened concrete: Strength of concrete Concrete Creep Shrinkage Modulus Of Elasticity Water tightness (impermeability) Rate of Strength gain of Concrete  

1. STRENGTH: The strength of concrete is basically referred to compressive strength and it depends upon three factors. 1- Paste Strength  2- Interfacial Bonding  3- Aggregate Strength 1. Paste strength: It is mainly due to the binding properties of cement that the ingredients are compacted together. If the paste has higher binding strength, higher will be strength of concrete

2. Interfacial bonding: Interfacial bonding is very necessary regarding the strength. Clay hampers the bonding between paste and aggregate. The aggregate should be washed for a better bonding between paste and aggregate 3. Aggregate strength: It is mainly the aggregate that provide strength to concrete especially coarse aggregates which act just like bones in the body. Rough and angular aggregate provides better bonding and high strength.

FACTORS AFFECTING STRENGTH OF CONCRETE: Following are the factors that affect the strength of concrete: 1. Water-Cement ratio  2. Type of cementing material  3. Amount of cementing material  4. Type of aggregate  5. Air content 6. Admixtures

1. Water-Cement ratio: It is water cement ratio that basically governs the property of strength. Lesser the water cement ratio, greater will be strength. 2. Type of cement: Type of cement affect the hydration process and therefore strength of concrete. Amount of cementing material: it is the paste that holds or binds all the ingredients. Thus greater amount of cementing material greater will be strength.

3. Type of Aggregate: Rough and angular aggregates is preferable as they provide greater bonding. 4. Admixtures: Chemical admixtures like plasticizers reduce the water cement ratio and increase the strength of concrete at same water cement ratio. Mineral admixtures affect the strength at later stage and increase the strength by increasing the amount of cementing material.

STRENGTH Hardening is the process of strength growth and may continue for weeks or months after weighted needles penetrating into cement paste.the concrete has been mixed and placed. Hardening is due largely to the formation of calcium silicate hydrate as the cement continues to hydrate.

CREEP AND SHRINKAGE Concrete creep is defined as: deformation of structure under sustained load. SIMPTOMS: Basically, long term pressure or stress on concrete can make it change shape. This deformation usually occurs in the direction the force is being applied. Like a concrete column getting more compressed, or a beam bending.  Creep does not necessarily cause concrete to fail or break apart. Creep is factored in when concrete structures are designed

SHRINKAGE Shrinkage of concrete is the time-dependent strain measured in an unloaded and unrestrained specimen at constant temperature . It is important from the outset to distinguish between plastic shrinkage, chemical shrinkage and drying shrinkage

DRY SHRINKAGE Drying shrinkage is the reduction in volume caused principally by the loss of water during the drying process Chemical (or endogenous) shrinkage results from various chemical reactions within the cement paste and includes hydration shrinkage, which is related to the degree of hydration of the binder in a sealed specimen

ELASTIC PROPERTIES . But the elasticity is a way to characterize the mechanical response of the material body for applied stresses that stay within the linear regime. Very large stresses, which are high enough to fracture the material, cause non-linear deformations. These values vary greatly depending on the overall makeup of the concrete mixture. Many of the non-fracture-related mechanical properties of concrete are characterized by the elastic moduli.