How to Check Quality in Concrete Construction

Quality of the concrete plays an important role in the construction field as the concrete is the excessive element in any construction. The strength parameters such as durability, load bearing strength and resistance to environmental factors such as wind, snow and water depends on the quality of concrete.

So the quality of concrete is to be taken as a important factor in the construction of any building or other structure. Generally the quality of concrete is checked in the different stages of the concreting process.

Stages of Concrete Quality Checking

• Preconcrete quality checks
• Quality checks while concreting
• Post concrete quality checks

1.Preconcrete Quality Checks

This stage of quality checks consists of 2 steps.

1. Checking of specification requirements regarding excavation, forms, reinforcement and embedded fixtures etc.
2. Control test on concrete ingredients (i.e. on cement, aggregate & water)

Cement

Quality of cement is ascertained by making compressive strength tests on cement cubes. However for effective control cement:

• Should be tested initially once for each source and subsequently once for every two months
• Should be protected from moisture
• Should be retested after 3 months of storage, if long storage in unavoidable
• Should be rejected if large lump are found in cement bags.

Read More on quality checks on cement: Tests on Cement at Construction Site To Check Quality of Cement

Aggregate

Concrete aggregates should confirm to specified values as per standard specification.

The quality of concrete is affected by different physical and mechanical properties of aggregate, i.e. shape, grading, durability, specific gravity and water absorption etc. these properties of aggregated should be tested before using it for concrete production.

The quantity of deleterious materials and organic impurities should also be tested.

Bulking of sand is also an important property in several ways. It gives wrong results when volume batching is done. It increases water cement ratio which in turn reduces strength.

For effective control aggregates:

• Are required to be tested once initially for approval of source
• Should subsequently be tested once or twice daily for moisture content and allowance should be made for moisture content of aggregates.

Read More on quality checks on cement: Testing of Aggregate Quality at Construction Site for Concrete

Water

The quality of water should be checked for the requirements as specified in respective standard. Chemical analysis shall be conducted for approval of source. In case of suspended impurities, it is necessary to store water for some time to allow them to settle down. In case of doubt concrete cubes made with this water are tested.

Average 28 days compressive strength of at least three cubes or cylinders or specified size, prepared with water proposed to be used shall not be less than 90% of the average strength of three similar concrete cubes prepared with distilled water.

Read More on quality checks on cement: Tests for Water Quality for Concrete Construction and Recommended Limits

2.Checks while Concreting

Careful supervision during concrete manufacture is necessary for all concreting operations such as batching, mixing, transporting, laying, compacting and curing.

Following precautions should be taken during concreting operation.

1. The concrete mix should be designed in the laboratory with the materials to be used on site.
2. As far as possible concrete should be batched by weight. If weight batching is not possible, then volume batching may be permitted through proper supervision in the presence of engineer in charge.
3. During mixing the mixer should be charged to its full capacity. The materials should be fed in proper sequence. The speed of the mixer should be range from 15 to 20 revolutions per minute. The mixing time should not be less than 2 minutes in any case. Segregation should be avoided while unloading the concrete from the mixer.
4. Workability of concrete is an important property of concrete while concrete is in its fresh state. Therefore slump test or compaction factor test should be performed to check workability of concrete. About three tests should be carried out for every 25 m3 of concrete.
5. Care should be taken so that no segregation takes place during transportation of concrete.
6. Concrete should not be dropped from a height of more than 1 m. if the drop height exceeds 1 m chutes should be used.
7. To avoid rehandling of concrete it should be placed at its final position as far as possible.
8. Vibrators should be used for compacting concrete. The insertion spacing of internal vibrators should not be more than 0.6 m. It should be drawn out slowly so that no holes remain in the concrete. The frequency of vibrators should not be less than 7000 cycles/minutes.
9. Curing should be done for a specified period so that concrete develops requisite strength. Concrete should be covered with hessian as soon as it becomes hard.
10. The form work should correspond to final form of the structure. It should be checked before concreting is started. The inside of the forms should be cleaned and oiled. The forms should be removed after the specified period.
11. Concrete should be protected from hot and cold weather at early ages. Concreting should not be done at temperature below 4.50C and above 400 In very hot weather water and aggregates should be cooled. Retarders of approved quality can be used.
12. In very cold weather water and aggregates should be heated. Accelerators of approved quality can also be used.

3.Post Concreting Checks

Once the concrete is laid and compacted, compression tests are made on the cubes made out of this concrete. For ordinary concrete, cubes are made out of the concrete made at work site.

The hardened concrete has to be checked for trueness in dimensions, shape and sizes as per design specification. General surface appearance of concrete should also be checked.

Dimensions are ascertained by different measurements. Reinforcement should have adequate concrete cover and if the reinforcement is visible in part of a structure, the part should be rejected or necessary actions should be taken accordingly.

Concrete strength is normally to be ascertained from cube or cylinder samples tested at 28 days. In case the strength obtained is less than the specific minimum, one or more of following steps may be taken.

• Load test and measurement of deflection and / or strain (the quality of the structure can then be ascertained by calculating back the concrete strength)
• Cutting cores from the structures and testing them for strength
• Non destructive tests like Schmidt rebound hammer or ultrasonic pulse velocity test. These tests give only a very rough idea and are primarily used to ascertain the uniformity of construction.
• Chemical analysis of hardened concrete.

Important Quality Tests Of Concrete

there are many types of test that are done to assess the quality of concrete. The important tests that are generally performed are,

1. Slump test before leaving the batching plant and on arrival on site
2. Compressive strength test
3. Water Permeability test
4. Rapid Chloride Ion Penetration Test
5. Water Absorption Test
6. Initial Surface Absorption Test

Portland Pozzolana Cement – Manufacture, Properties and Uses

Portland Pozzolana cement is integrated cement which is formed by synthesising OPC cement with pozzolanic materials in a certain proportion. It is commonly known as PPC cement. In this article we discuss about the properties, manufacture, characteristics, advantages and disadvantages of Portland Pozzolana cement.

What is pozzolana or pozzolanic material?

Pozzolana is a volcanic powder found in Italy near Vesuvius. A pozzolanic material can be a natural or artificial which contains silica and aluminous in a reactive form. This materials usually doesnt posses any cementitious properties, but when it is mixed with water or moisture or lime to undergo reaction with calcium hydroxide to form compounds possessing cement properties.

Fig 1: Use of PPC in marine structure.

Types of pozzolana materials :

1. Artificial pozzolanas 

   Fly ash, silica fume, rice husk, blast furnace slag.

2. Natural pozzolanas 

   Burnt clay, pumicite, diatomaceous Earth.

Manufacture Of Portland pozzolana cement

• The primary raw materials used for this cement manufacture  are limestone (CaCO2) and clay (SiO2,AI2,O3,Fe2O3). Rocks are loaded into trucks and transported to the crushers, where the crushed into fine particles.
• Fine particles of clay and limestone are fed into the air-swept ball mills in desired proportions as per requirement and mixed very well before it is sent to silos for storing.
• This mixture is then pre-heated upto 800-1000c where calcinations of CACO3 to CaO takes place.
• The pre heated mixture is then sent into kiln where the mixture is heated to 1450C in rotary Kiln. The modules formed from the burning process called clinker. The clinker is cooled by a rotary cooler.
• This clinker is now mixed with gypsum and pozzolana materials in the required proportion and thus the Portland Pozzolana Cement is obtained.

Properties of Portland Pozzolana Cement

• Initial setting time = 30 min (minimum)
• Final setting time = 600 min (maximum).
• At 3 days 13MPa  (minimum)
• At 7 days 22 MPa (minimum)
• At 28 days 33 MPa (minimum)
• Drying shrinkage should not be more than 0.15%
• Fineness should not be less than 300 m2/kg
• Initial strength of PPC is less but final strength is equal to the 28 days strength of OPC
• PPC has lower rate of development of strength than OPC

Uses

1. Used in hydraulic structures, marine structures, construction near the sea shore, dam construction etc.
2. Used in pre-stressed and post-tensioned concrete members.
3. Used in masonry mortars and plastering.
4. As it gives better surface finish, it is used in decorative and art structures.
5. Used in manufacture of precast sewage pipes.
6. Used under harsh concreting conditions.

Advantages 

1. it is an eco-friendly cement as the material used in the manufacture are made of natural recycled waste.
2. It is very fine cement hence very good when used for plastering works.
3. Pozzolano consists of silica material which makes it cheap and hence reduces the cost of the cement making it economical to use.
4. Pozzolana cement has very good resistance against sulphate attack hence is used in hydraulic structures, marine structures, construction near the sea shore, dam construction etc.
5. PPC used in pre-stressed and post-tensioned concrete members.
6. It reduces the carbon monoxide emission from the concrete making it environmental friendly.
7. As the pozzolano materials are very fine, it can fill gaps between the reinforcement and aggregate , thus reducing the shrinkage, honeycomb formation and bleeding can be reduced, which in turn increases the strength and durability of concrete.

Disadvantages

1. The initial strength obtained is less, which effect the de-shuttering of supports early.
2. As it contains more fine material, handing of concrete is difficult.
3. When compared to the OPC setting time is less for PPC
4. Reduction in alkanity reduces the resistance to corrosion of steel reinforcement
5. As the strength of this concrete gains slowly, curing process is very important. Any error in this could cause durability problems.

What is Podium Slab, Its Functions, Advantages and Disadvantages

What is podium slab?

Podium slabs are special type of floor system that transfers loads from a steel or wood frame structure above the slab to walls and columns below.

Load bearing elements such as walls and columns of superstructure above the podium slab may not align with substructure load bearing elements below. Typically, the superstructure built from wood, metal studs or structural steel.

Generally, this type of slab constructed and placed at ground level parking with 3-4 levels of conventional residential construction above. The name is derived from the “podium” that separates the two occupancies.

This article presents podium slab along with its functions, advantages and disadvantages.

Fig.1: Podium slab

Functions of podium slab

• It works as a structural floor and transfer slab for loads from the above superstructure to the walls and columns below.
• As a horizontal separation, podium slab works as a fire separation between different building occupancy types.

Podium slab construction techniques

The following construction systems have been used for podium slabs:

• Post-tensions cast in place construction technique
• Precast hollow core; this construction system is preferred it provides immediate safe working platform and excellent construction speed.

Fig.2: Podium slab; precast hollow core units used for construction

Fig.3: podium slab construction using hollow core precast units

Advantages of podium slabs

• Provide solid separation between parking areas and living spaces and consequently offers sound and fire protection.
• The application of post-tensioned two-way podium slab permits the decrease of overall thickness of the frame. This will cause the reduction of excavation because the parking is below grade.
• Flat soffit does not need extra fire protection provided that adequate cover for rebars and post tensioning are provided. Additionally, the flat soffit permits efficient lighting, electrical, heating, ventilation, and air conditioning.
• Forms can be stripped two to three days after the post tensioning application.
• It is proven to be cost effective.
• It provides large open spaces at or below grade for example parking and retail.
• In single family and multi-family residential structures, podium slab provides high fire rating, decline sound transmission and floor vibration.

Disadvantages of podium slab

• In the case of precast hollow podium slab, provision of proper connections is not easy, the unit may damage in the case of improper handling and transportation, and need special equipment and machines to move and lift precast units.
• In the case of post-tensions cast in place podium slab, construction work is complex, low quality workmanship could cause accidents, and there is a possibility of tendon corrosion.