Concrete

Concrete is an artificial stone that is made by casting in place in plastic condition a mixture of cement, fine aggregates, coarse aggregates and water. The hardening occurs due to a chemical reaction between the cement and the water. Concrete is used for the construction of foundation, slab, beams, columns etc…at site. It has very good compressive strength but very weak in tension. Concrete this design to provide following requirements,

1. Strength
2. Durability
3. Protection against fire
4. Thermal and sound insulation

Concrete is composite material consisting cement, aggregates, water and admixtures. The concrete should possess the strength, durability, impermeability and resistance to abrasion required for the proposed structure. It should be free of such defects as cracking, honeycombing, undue shrinkage etc. The concrete should be of good quality and should be produced with careful control over the batching of materials, water cement ratio etc. Necessary care and attention should be given to the design and preparation of formwork.

In its fresh state, concrete should,

• Be composed of accurately batched proportion
• Have the required W/C ratio
• Be well mixed
• Not segregate during transportation and placing
• Be fully compacted
• Be provided with specified finish
• Quality of cement
• Type of aggregate
• The grading of aggregate
• Mixing method and mixing time of concrete
• Method of transportation
• Curing

These quantities are depended on the material selected the proportion in which they are mixed, the method of mixing, placing, transportation and compaction.

MATERIALS OF CONCRETE

Cement

Cement is a material with adhesive and cohesive properties which make it capable of bonding mineral fragments. There are many types of cements, ordinary Portland cement, rapid hardening Portland cement, low heat Portland cement, sulfate resisting cement, white cement and pigments. Generally ordinary Portland cement is widely used.

Aggregates

Aggregate is commonly considered inert filler, which accounts for 60 to 80 percent of the volume and 70 to 85 percent of the weight of concrete. Although aggregates are most commonly known to be inert filler in concrete, the different properties of aggregates have a great impact on the strength, durability, workability, and economy of concrete.

Aggregates are classified in to two types, coarse and fine aggregates. Aggregates smaller than 5mm are known as fine aggregates while lager than 5mm are known as coarse aggregates. Sand is used as fine aggregates and crushed metal is used as coarse aggregates widely in Sri Lanka. Usually river sand is used as fine aggregates. Generally ¾" metal is used as coarse aggregate for normal concrete mix. 1 ½" metal used for mass concrete. Metal used for concrete is to be free from clay or any other foreign matters. Because otherwise it can be affect to the bonding.

Water

All water using for mixing concrete mortar grout shall be perfectly fresh, clean and free from acid, salt and all impurities, which may adversely effected the setting, hardening and durability of the concrete. This requirement is usually satisfied by using water which is suitable for drinking .Water is used to effect the chemical action in cement, which is known a hydration. Therefore the water/cement ratio is a most important factor when concrete qualities are concern because it affects the setting, Harding and strength of cement. Normally water /cement ratios are in the range of 0.4 to 0.6 Water also acts as a lubricant for the cement and the aggregates in the compaction of the concrete.

Admixtures

An admixture is a chemical product that is added to concrete mix during mixing or during an additional mixing operation prior to the placing of concrete, for the purpose of achieving a specific modification. There are water-reducing, retarding, accelerating admixtures.

• Accelerating Admixtures

Their function is primarily to accelerate the early strength development of concrete that is hardening although they may also coincidentally accelerate the setting of concrete.

• Retarding Admixtures

A delay in the setting of the cement paste can be achieved by the addition of a retarding admixture.

MIXING OF CONCRETE

Concrete must be thoroughly mixed until a uniform color is attained. In our site we used ready mixed concrete except in some instances where a small quantity was required.

Hand mixing

In small jobs of concrete was mixed by hand. The ingredients were first assembled on a water tight-mixing platform. First of all in hand mixing work, the required amount of sand was spread out on the flat surface. The proper number of bags of cement was spread out on top, and the two materials were turned together until a uniform color is obtained. The required volume of coarse aggregate wetted, measured, and spread in a layer on top of the cement and sand mixer. All the materials were again mixed and depression was formed in the center of the heap.

Finally, the required amount of water was added and mixed until the all materials are thoroughly mixed.

Ready mixed

Instead of mixing in site, concrete is delivered from a central plant (called batching plant), it is referred to as ready mixed or pre mixed concrete. This type of concrete is extensively used as it offers numerous advantages in comparison with other methods.

• Close quality control of batching which reduces the variability of the desired properties of hardened concrete
• Use on congested sites where there is little space for a mixing plant and aggregate stockpiles.
• Use agitator trucks to ensure care in transportation, thus preventing segregation and mentally work ability.

BATCHING PLANT

Process in a batching plant

For Grade 25, 0.75m³   ------------->  208 – Sand                (Unit – Bucket)

                                                            248 – Metal

                                                              88 – Cement

Capacity of silo = 3 bulk (33,000kg)

Before poring the concrete there have some concrete test to carry out. Such as slump cone and cube test.

PLACING AND COMPACTION OF CONCRETE

The concrete should be placed in its final position rapidly so that it is not too stiff to work. Water should not be added after concrete has left the mixture. A heap of concrete, which will have to be moved to some other part of the form, should not be allowed to accumulate in one place. Normally concrete was placed in even layers and each of layers was compacted before the next layer was placed. Each layer was placed before the previous layer has got set. But for the slab it was placed as single layer.

Compaction of concrete is the process adopted to expel the entrapped air from the concrete. If the air is not removed fully the concrete loses its strength considerably. Compaction can be manual or mechanical compaction. The internal vibrator or poker was used in our site, which is the most common type of vibrator. This is a vibrating tube at the end of a flexible drive. Pokers vary in size, usually from 25mm to 75mm in diameter. A poker vibrator should not be dragged through the concrete, nor used to help heaps of concrete to spread out. It should place vertically in the concrete, held in position until air bubbles cease to come to the surface, then slowly with drawn so that concrete can flow into the space previously occupied by the poker. This should be repeated at about 0.5m centers. The concrete should be placed in layers never more than 600mm thick, and the vibrator should be lowered at least 100mm in to layer beneath 

PLACING OF CONCRETE

The method of placing concrete is very important and needs proper planning and careful study if the structure to be concreted is to be durable, strong and having a good appearance. All formwork should be check clean and oiled before concrete is placed on it. Place the concrete as soon as possible in uniform layers placing height should be less than 4ft.                                                                       

VIBRATION OF CONCRETE

The progress of compacting concrete by vibration consists essentially of the elimination of entrapped air and forcing the particles in to a closer configuration.

MANUALLY VIBRATING

Using hammer

This was done in places where vibrator can’t insert such as column concrete. This concrete was compacted by ramping. Pushing up and down a rammer on the concrete did ramping. Wide wood piece was used to make the rammer.

Using a Steel Rod

Steel rode uses when compacting at the thinner areas where vibrators or rammers can’t be inserting. This method is very effective and efficient in small concreting like manhole construction.

COMPACTION BY VIBRATOR

Best and quickest method was vibration. Of several types of vibrators, this is perhaps the most common one. It is consists of porker, housing eccentric shaft driven through a flexible drive from a motor. The poker is immersed in concrete and thus applies approximately harmonic forces to it; hence, the alternative names of Poker vibrator or immersion vibrator.

The correct method of insetting Poker to the concrete as follows; Compaction process consolidates fresh concrete with in the formwork and around reinforcement. Compaction is necessary to remove entrapped air which is present in concrete after it is mixed.

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Curing

Why curing is needed

Curing is designed primarily to keep the concrete moist, by preventing the loss of moisture from concrete during the period in which it is gaining strength. Protection from premature drying is necessary so that the strength development of concrete is not affected by water removal.

The consequences of too early water losses are:

• Lower strengths in part near the surface.
• Tendency to dusting.
• Higher water permeability.
• Reduced weather resistance.low resistance to chemical attacks.
• Occurrence of early age shrinkage cracks.
• Increased risk of all forms shrinkage cracking.

The curing period may depend on,

• The properties required of the concrete.
• The purpose for which it is to be used.
• The ambient conditions, i.e. the temperature and relative humidity of the surrounding atmosphere.

Compressive strength at 180 days as % of continuously moist sample

Effect of duration of curing on properties of concrete

As may be seen in this example, concrete allowed to dry out immediately achieves only 40% of the strength of the same concrete water cured for the full period of 180 days. Even 3 days water curing increases this figure to 60%, while 28 days water curing increases it to 95%. Keeping concrete moist, therefore, is a most effective way of increasing its ultimate strength.

Methods of curing

There are two distinct ways of curing.

1.By retaining water in the concrete, in the early hardening period

Methods used in this process are:

• Ponding, which is done by a dammed wall of sand around the concrete formation and then flooding with water.
• Sprinkling water on to the formation. This method is achieved by continuous spraying of water on to the slab with garden sprinklers or hand held hose.
• Use a layer of sand of gunny bags that holds and retains sprayed on water. The sand cover or gunny bags need to be kept wet at all times for the duration of the curing period.

2.By preventing evaporation of water (Sealing the exposed surface)

• Use a clear polythene sheet laid on top of the slab to stop the evaporation process. This is one of the most efficient methods to cure concrete, especially in sloping areas and horizontal surfaces.
• Leave form-work used to create the concrete formation. The framework itself, if left in place, underneath a suspended slab, or around a concrete column will stop concrete from drying out too quickly which helps the curing process.
• Use concrete curing compounds. These compounds now come in a variety of types. Use water soluble waxy emulsions that can be sprayed on the fresh concrete with a hand pump.

When to start ‘Curing’

It is always better to start curing as soon as the concrete is hardened to a point that the concrete is not damaged when the water / curing agent is added. Curing can be started after 2-3 hours of concrete placement.

Effects of not having good curing conditions

• Plastic shrinkage cracks occur when the surface of the concrete dries rapidly and shrinks before it can gain sufficient strength to resist cracking.
• When ambient conditions and concrete temperatures combine to produce conditions that create a high evaporation rate, it creates increased chances of surface drying and shrinkage cracks forming prematurely.
• Formation of plastic shrinkage cracks could be reduced by adopting the curing process early.

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What is durability of concrete?

It is the ability to resist weathering action, chemical attack, abrasion, or any process of deterioration.

Durability and sustainability

Durability of concrete structure of paramount importance for the following reason

• Financial investment especially in infrastructure projects is very high. Due to this, building durable structures should be sustained and practiced by the construction industry.
• It is the responsibility of the designer, consultant, engineer to design and build the structure keeping the environmental condition, usage etc in mind. This ensures the long life of the building and adds to durability of concrete.
• Also the selection of right material and construction practices by the contractor are vital practices to be followed.

How do we ensure ‘durable concrete’?

The following is an important list of control factors that need attention in the production of durable concrete.

• Structural design of concrete including the cover to reinforcement.
• The concrete mix design and specification (Water to Cement ratio, maximum cement content, type of cement).
• Workability and cohesiveness.
• Batching/ mixing/ transportation/ placing and compacting.
• The curing process.
• Maintenance and usage during service life.

Reasons for deterioration

Deterioration of concrete can take place basically due to porosity.

Concrete has porosity of several types:

• Capillary pores: Depending on the water cement ratio, the number of capillary pores can increase. The table shows the age at which capillary pores get blocked with different water cement ratios. It clearly shows that if W/C ratio is more than 0.7, the capillary pores will never be blocked and remain for the entire lifetime of the structure. Meaning, the durability of the structure/ concrete is in equation.

Water Cement Ratio	Age at which capillary pores become blocked
0.40	3 days
0.45	7 days
0.50	14 days
0.55	14 days
0.60	3 months
0.70	Over 1 year
Over 0.70	Infinity

Coefficient of Permeability represents the level of permeability of concrete. The table below gives the coefficient of permeability of concrete made using same materials but with different W/C ratios:

Coefficient of Permeability for different W/C   

Sr. No.	W/C	Coefficient of Permeability (Valenta)
1	0.35	1.05 × 10-3
2	0.50	10.30 × 10-3
3	0.65	1000 × 10-3

When the w/c ratio is increased two-fold, the coefficient of permeability increases thousand-fold. This clearly explains how important w/c ratio is to durability and strength of concrete.

• Entrapped air: Proper compaction methods should be followed to ensure removal of all entrapped air within the concrete.
• Honey combs: Proper concrete mix and good concrete practice are the key tools to eliminate honeycombs in concrete.
• Cracks: Good concrete practice, right choice of materials, right design and maintenance – all contribute to minimize cracks in concrete.

Concrete in Aggressive Environments.

Deterioration of concrete takes place in stages. The speed of deterioration and damage to concrete will greatly depend on the following:

Physical Factors

• Continuous or discountinuous cracks.4weathering effects.
• Cyclic loading of impact loading.

Chemical Factors

• Environmental actions or aggressive chemicals, iron and their penetration.

Entry of Chemicals into Concrete and their impact

Trigger	Impact
Chloride	Enters through capillary pores, corrodes reinforcement, causes swelling of concrete
Water	Enters through capillary pores, corrodes reinforcement
Carbon Dioxide	Reacts with concrete and deteriorates it
Sulphates	Reacts with concrete and deteriorates it

For the design purpose of concrete, aggressive environment is defined depending on the severity of presence of chemicals stated above.

Preventive measures for corrosion of reinforcement

• Use minimum water cement ratio.
• Avoid entrapped air and segregation during construction.
• Avoid plastic and drying shrinkage cracking of concrete by taking adequate care in designing concrete mix and by proper construction practices especially curing.
• The concrete mix should have good workability and cohesiveness, so that it can be placed and compacted properly.
• Protective coating on steel.
• Another very important preventing measure is to use blended cement containing fly ash or slag.

Type of Cement	Chloride Diffusion Sq.cm / S × 108
OPC	4.47
Pozzolana Cement (70% OPC & 30% Fly ash)	1.47
Slag Cement (35% OPC & 65% slag)	0.41
Sulphate Resistant Cement	10.00

The table shows clearly that Pozzolana or Fly Ash cement Prevent chloride entering into concrete.

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Water Cement ratio

Role of water in cement

When cement is mixed with water, hydrated cement paste is formed. It consists of 3 parts:

• Hydration products
• Anhydrous cement
• Capillary pores

As cement hydration progresses, the amount of capillary pores decree. However, when the hydration process is completed. Anhydrous cement will disappear and hydrous cement and capillary pores will remain. Therefore, higher the water content added to cement, higher the amount of capillary pores in the cement.

As the capillary pores in cement paste reduces, the strength increases and permeability of the concrete also decreases. Therefore, in order to achieve higher strength, and less permeability, it is very important the amount of water used in cement, i.e. water should be proportionate with cement content.

Water to cement ratio

Water-cement ratio of weight of water to the weight of cement used in a concrete mix. It has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability. The water-cement ratio is independent of the total cement content (and the total water content) of a concrete mix.

The outcome of not maintaining a proper water to cement ratio would be:

• Not achieving required compressive strength of concrete.
• Reduction of concrete durability due to higher permeability.
• Loss of fresh concrete properties.

The relationship between the strength and water cement ratio was established by Duff Abrams in 1918 as a result of extensive testing at the Lewis Institute, University of Illinois. Popularly known as Abram’s water/cement ratio rule, this inverse relation is represented as follows:

f_c = k₁/k₂^w/c

W/C represent the water/cement ratio of the concrete mixture and k1 and k2 are empirical constants.

Understand the role or Water Cement

The water cement ratio helps us determine the strength and durability of the concrete.

Relationship of water/cement ratio with other elements in concreting.

In design a concrete mix, there has to be a reasonable balance between workability, strength, durability and cost consideration. How do we achieve this?

• Workability of concrete can be expressed in terms of consistency and cohesiveness.
• Consistency of the mix is measured in terms or the slump of the mix (i.e. wetness of the mix).
• For given slump, the water requirement generally decreases when.
• The maximum size of a well graded aggregate in increased.
• The content of angular and rough textured particles in aggregates is reduced.
• The amount of entrained air in the concrete mixture is increased,

• Cohesiveness is a measure of compactability which is generally evaluated by trowelability and visual judgment of resistance to segregation.
• If the cohesiveness is poor, the sand/ coarse aggregate proportion or partial replacement of coarse sand with a finer and increase of cement/ aggregate ratio at the given water cement ratio may be done.
• However, past experience and visual judgment supports in deciding the correct water content in the mix.

The W/C ratio and durability

The W/C ratio affects the porosity and thereby the durability of the concrete, the higher the porosity, permeability to many external chemicals and substances is increased. This results in faster deterioration of concrete. 


SUMMARY

• W/C ratio is one of the most important factors in making concrete and plays an important role with cement in concrete. 
• When excess water to cement ratio is used it affects to the concrete strength and durability. 
• As a rule of thumb, the lower the water content, the better the strength and durability. Although water is required for the workability of concrete and for cement hydration reaction, it is should be noted that achieve-ment of workability by adding more water should be avoided as much as possible. Use of correct water to cement ratio will ensure enhanced performance of concrete. 

• In designing a concrete mix, there has to be a reasonable balance between workability, strength, durability and cost consideration. 

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