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.