Concrete

Concrete is an artificial stone-like construction material. Most structures have concrete as their principal materials, used in one of the following forms:reinforced concrete,prestressed concrete and mass concrete. 

Concrete is primarily composed by aggregate,cement and water. The aggregate is generally a coarse gravel or crushed rocks such as limestone, or granite, along with a fine aggregate such as sand. Water is mixed with dry composites which allowed the chemical process that known as hydration to conduct. Hydration enables the water and the dry composites to be shaped and then solidified and hardened into rock-hard strength. 

The following video shows that how to cast a concrete. The water reacts with the cement which bonds the other components together, eventually creating a robust stone-like material.

Concrete has relatively high compressive strength, but much lower tensile strength. For this reason is usually reinforced with materials that are strong in tension (often steel). Cube test is usually used to test the compressive strength of concrete. After the steps of concrete casting that shown in Video 1, put the concrete with the cast faces in contact with the platens of the testing machine, the load on the cube is applied at a constant rate of stress of 0.2-0.4 MPa/sec, until failure of cube.

Concrete has good workability when it is fresh, high strength and hardness and adequate durability. The properties of concrete are affected by the amounts of the constituents, for example the mix proportions. Measuring exact volumes of the materials is difficult. Therefore the mix proportions are usually expressed as the weight of each material required in a unit volume of the concrete production., usually in kg/m3. The water/cement ratio is an important factor influencing many of the concrete properties.

The environmental impact of concrete is a complex mixture of not entirely negative effects; while concrete is a major contributor to greenhouse gas emissions, recycling of concrete is increasingly common in structures that have reached the end of their life. Structures made of concrete can have a long service life. As concrete has a high thermal mass and very low permeability, it can make for energy efficient housing.




Properties of concrete


The following are the important properties of fresh concrete


1. Setting
2. Workability
3. Bleeding and Segregation
a. Bleeding
b. Segregation
4. Hydration
5. Air Entertainment

1. Setting of Concrete

- The hardening of concrete before its hydration is known as setting of concrete.

- The hardening of concrete before it gains strength.

- 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.

In the case of concrete, consistence is sometimes taken to mean the degree of wetness; within limits, wet concretes are more workable than dry concrete, but concrete of same consistence may vary in workability.

Because the strength of concrete is adversely and significantly affected by the presence of voids in the compacted mass, it is vital to achieve a maximum possible density. This requires sufficient workability for virtually full compaction to be possible using a reasonable amount of work under the given conditions. Presence of voids in concrete reduces the density and greatly reduces the strength: 5% of voids can lower the strength by as much as 30%.


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

Factors affecting concrete workability:

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. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. In thin members like roof slab or road slabs and when concrete is placed in sunny weather show excessive bleeding.

Water while traversing from bottom to top, makes continuous channels. If the water cement ratio used is more than 0.7, the bleeding channels will remain continuous and un segmented. These continuous bleeding channels are often responsible for causing permeability of the concrete structures. While the mixing water is in the process of coming up, it may be intercepted by aggregates. 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.

Bleeding rate increases with time up to about one hour or so and thereafter the rate decreases but continues more or less till the final setting time of cement.

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.

The bleeding is not completely harmful if the rate of evaporation of water from the surface is equal to the rate of bleeding. Removal of water, after it had played its role in providing workability, from the body of concrete by way of bleeding will do good to the concrete.

Early bleeding when the concrete mass is fully plastic, may not cause much harm, because concrete being in a fully plastic condition at that stage, will get subsided and compacted. It is the delayed bleeding, when the concrete has lost its plasticity, which causes undue harm to the concrete. Controlled re vibration may be adopted to overcome the bad effect of bleeding.

3(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

1. Coarse aggregate separating out or settling down from the rest of the matrix.
2. Paste separating away from coarse aggregate.
3. Water separating out from the rest of the material being a material of lowest specific gravity.

A well made concrete, taking into consideration various parameters such as grading, size, shape and surface texture of aggregate with optimum quantity of waters makes a cohesive mix. Such concrete will not exhibit any tendency for segregation. The cohesive and fatty characteristics of matrix do not allow the aggregate to fall apart, at the same time; the matrix itself is sufficiently contained by the aggregate. Similarly, water also does not find it easy to move out freely from the rest of the ingredients.

The conditions favorable for segregation are:

1. Badly proportioned mix where sufficient matrix is not there to bind and contain the aggregates
2. Insufficiently mixed concrete with excess water content
3. Dropping of concrete from heights as in the case of placing concrete in column concreting
4. When concrete is discharged from a badly designed mixer, or from a mixer with worn out blades
5. 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

Vibration of concrete is one of the important methods of compaction. It should be remembered that only comparatively dry mix should be vibrated. It too wet a mix is excessively vibrated; it is likely that the concrete gets segregated. It should also be remembered that vibration is continued just for required time for optimum results. If the vibration is continued for a long time, particularly, in too wet a mix, it is likely to result in segregation of concrete due to settlement of coarse aggregate in matrix.

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.

If the hydration is to continue, extra water must be added to refill the loss of water on account of absorption and evaporation. Therefore, the curing can be considered as creation of a favorable environment during the early period for uninterrupted hydration. The desirable conditions are, a suitable temperature and ample moisture.

Concrete, while hydrating, releases high heat of hydration. This heat is harmful from the point of view of volume stability. Jeat of hydration of concrete may also shrinkage in concrete, thus producing cracks. If the heat generated is removed by some means, the adverse effect due to the generation of heat can be reduced. This can be done by a thorough water curing.

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:

1. Resistance to freeze–thaw action in the hardened concrete.
2. Increased cohesion, reducing the tendency to bleed and segregation in the plastic concrete.
3. Compaction of low workability mixes including semi-dry concrete.
4. Stability of extruded concrete.
5. Cohesion and handling properties in bedding mortars.

Why Concrete Is Important ?

Aside from the obvious factors of strength and rigidity, it is easy to make by just add some water and sand to the mix of crushed concrete, it has a typical 50 to 100 year life span. Besides, it is more economical in comparison with structural steel and can be molded into various shapes, unlike structural steel that is only fixed with certain shape. Most of the people prefer concrete to be used in constructions field is because the easy availability. Below is the further explanation of the importance of concrete. 


The strength of the concrete 

To understand compressive strength, here is some examples given, think about several packs of crackers sitting on the floor. If you are carefully stand on those packs of crackers, your weight will probably be supported, but you are putting those crackers in compression. Your weight tends towards crushing those crackers. If you jump up and land on those packs of crackers, you will increase the force applied and probably crush the crackers. You will have made the crackers fail in compression. 

Now try to jump on a concrete sidewalk. You would have to jump pretty high to make that sidewalk crush under your weight. In fact, you probably couldn’t make that sidewalk fail in compression. That’s the reason concrete gets used so much in construction.
Builders in the past understood these properties of concrete and stone and typically used those materials only in compression. So walls could be concrete and stone, as could foundations, since both primarily resisted downward compression loads.

     

This is how the strength of concrete been tested! 

   

An old concrete looks like! 

This is a concrete staircase!

                      

     

This is a bridge built by concrete too! 

Features of concrete may be also a factor in choosing concrete in your construction project

1. Prices of concrete.

Concrete makes up the majority of the cost of a concrete project. It costs $75 per cubic yard, which is around RM220-RM240 in Malaysia. This is more affordable and more economical compare to structural steel. Most of the construction firm will choose concrete to be used in foundation since it is cheaper compare to other material.

2. Concrete casting

Concrete is primarily composed by aggregate, cement and water. It is easy to made, save times and environment friendly. You can cast as many as you want according to the usage on the construction. Below are the video about how concrete is casting. 

 

3. Shape of concrete

Concrete can be made into various shapes by giving it a container to shape it; it can be design on strength, thickness, long and wide in according to the usage. Below is different type of concrete been casted. 

Slump Test:

Slump test is used to calculate the slump value and to determine the workability of fresh concrete. More specifically, it measures the consistency of the concrete in that specific batch. It is also used to determine consistency between individual batches. The consistency, or stiffness, indicates how much water has been used in the mix. The stiffness of the concrete mix should be matched to the requirements for the finished product. The slump test is based on American Society for Testing and Materials C 143 . The concrete slump test is known as "Standard Test Method for Slump of Hydraulic-Cement Concrete". The test is popular due to the simplicity of apparatus used and simple procedure.

Apparatus and Materials used for slump test:

●Trowel
●Slump cone
●Base plate
●Tamping rod
●Fine aggregate (Sand)
●Coarse aggregate (Gravel)
●Cement
●Water

Procedure for slump test : 

1. Weight cement, sand and aggregate (3/4" gravel) to get the concrete mixes, i.e. using

    800g cement for 1:3:6, 1100g cement for 1:2:4 and 1400g cement for 1:1 ½ :3

2. Mix the materials well to produce required samples
3. Clean and remove any superfluous moisture or set concrete on the inside of the slump

    cone and place it on a smooth, horizontal, vibration free and non-absorbent surface 

4. Fill cone 1/3 full by volume and rod 25 times with 5/8-inchdiameter x 24-inch-long

    hemispherical tip steel tamping rod. (This is a specification requirement which will

    produce nonstandard results unless followed exactly.) Distribute rodding evenly over

    the entire cross section of the sample.

5. Fill cone 2/3 full by volume. Rod this layer 25 times with rod penetrating into, but not

    through first layer. Distribute rodding evenly over the entire cross section of the layer.
6. Fill cone to overflowing. Rod this layer 25 times with rod penetrating into but not

    through, second layer. Distribute rodding evenly over the entire cross section of this

    layer.
7. Remove the excess concrete from the top of the cone, using tamping rod as a screed.

    Clean overflow from base of cone.
8. Immediately lift cone vertically with slow, even motion. Do not jar the concrete or tilt

    the cone during this process. Invert the withdrawn cone, and place next to, but not

    touching the slumped concrete. (Perform in 5-10 seconds with no lateral or torsional

    motion.)
9. Lay a straight edge across the top of the slump cone. Measure the amount of slump in

    inches from the bottom of the straight edge to the top of the slumped concrete at a

    point over the original center of the base. The slump operation shall be completed in a

    maximum elapsed time of 2 1/2 minutes. Discard concrete.

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Interpretation of Results 

The slumped concrete takes various shapes, and according to the profile of slumped concrete. 

There is three main type of slump : 

a) Collapse. 

 In a collapse slump the concrete collapses completely.

 ● Collapse slump indicates that concrete mix is too wet and the mix

    is regarded as harsh and lean.

b) Shear slump 

In a shear slump the top portion of the concrete shears off and slips sideways. Or if one-half of the cone slides down an inclined plane, the slump is said to be a shear slump.

● If a shear or collapse slump is achieved, a fresh sample should

   be taken and the test is repeated.
● If the shear slump persists, as may the case with harsh mixes,

   this is an indication of lack of cohesion of the mix.

C) True slump 

  In a true slump the concrete simply subsides, keeping more or less

  to shape

  ● This is the only slump which is used in various tests.
  ● Mixes of stiff consistence have a Zero slump, so that in the rather

     dry range no variation can be detected between mixes of

     different workability.

However , in a lean mix with a tendency to harshness, a true slump can easily change to the shear slump type or even to collapse, and widely different values of slump can be obtained in different samples from the same mix; thus, the slump test is unreliable for lean mixes.