Concrete is the mixture of Cement, Course Aggregate (stone crush), Fine Aggregate (sand), and water. It is one of the most widely used construction materials in the world. The cement, when mixed with water, forms a paste that binds the aggregates together, creating a long-lasting and strong material. Concrete has high compression strength but very weak in tension.
In order to meet the strength and durability requirements, the concrete is divided into the following categories:
Concrete made by using Ordinary Portland Cement (OPC) and other basic ingredients i.e. Coarse aggregate, Fine aggregate, and Water, having min 28 days Cylinder Compressive strength less than 28 MPa (4000psi). For ease of use, it is further classified into the following types:
The Concrete having min 28 days Cylinder Compressive strength 24 MPa (3500 psi). It shall be used for Structural members (RCC slabs, beams, columns, PCC works, etc.) or any other structural work where such strength is specified by the Designer.
The Concrete having min 28 days Cylinder Compressive strength 21 MPa (3000 psi). It shall be used for Structural members (RCC slabs, beams, columns, walls, partitions) and any other structural work where such strength is specified by the Designer.
The Concrete having min 28 days Cylinder Compressive strength 10.5 MPa (1500 psi). It shall be generally used for some structural members like foundations, hard standings, concrete blocks, etc., and any other works where such strength is specified.
The Concrete having 28 days Cylinder Compressive strength lesser than 10.5 MPa (1500 psi). It shall be used for nonstructural works like floor underlay, lean concrete, etc., requiring low strength, or where specified as 1:4:8 and 1:6:12, etc.
The Concrete requiring min 28 days estimated Cylinder Compressive strength, equal or more than 28 MPa (4000 psi) or requiring special features to meet higher strength, environmental, architectural and survivability requirements. Pre-stressed Concrete is the example of special concrete.
As per requirements and conditions, different types of concrete may be produced. Different types of concrete have different characteristics and qualities. Normally using concretes are as follows:
This is the basic type of concrete (also called lean concrete) which is produced by mixing normal concrete ingredients like Ordinary Portland cement, sand, aggregate, and water. This type of concrete is used under normal temperature conditions in foundations, at the bed of RCC, and drains, etc. This concrete has good compression strength but low tensile strength.
If steel bars are used as reinforced in concrete to increase the tensile strength of concrete, this type is called Reinforced Cement Concrete. This is the most widely used concrete in infrastructure (beams, columns, and slabs, etc.)
This type of concrete has low density and weight. This concrete is made by using lightweight aggregates and more water quantity. It is used to reduce the dead load of the structure.
If concrete is made in one place through plant mixture and poured at the site using trucks, it is called ready mix concrete.
In Polymer concrete, polymers are used as binding material along with cement. Polymer concrete has many advantages like improved durability, reduced permeability, flexibility, and rapid setting.
In this type of concrete, glass is used as aggregate fine, course or both. In glass concrete, wastage glass may be used. Glass concrete has many good qualities like good aesthetic appeal, unique colors and light-reflective qualities, Reduced Weight and Improved Crack Resistance.
Pervious concrete, also called permeable concrete, has the ability to allow water to pass through it. This penetrability makes it suitable for various applications where water controlling and drainage are vital.
This concrete shall consist of pre-stressing by furnishing, placing and tensioning of pre-stressing steel. Pre-stressed concrete includes high-strength steel cables to impart compressive stresses to the concrete before getting external loads. Pre-stressed concrete increased structural performance, allowing greater loads and longer span. This type of concrete is commonly used in bridges, buildings, and other huge infrastructure projects.
High-strength concrete should have a compressive strength of 6000 psi. This concrete is made by using strong and high-quality aggregates, extra cement content and low water content. The low water quantity is managed by using superplasticizer admixture.
In vacuum concrete, excessive water (which is not required for dehydration process) from concrete is extracted by using a pump with the help of Mats on filtering pads over the cement. This technique is used to get higher strength and durability level compared to normal concrete.
This type of concrete is normally used to make roads, parking lots, and other flexible pavement types. Asphalt concrete, also known as bituminous concrete, is a mixture of aggregates (i.e stone, sand, or gravel) and hot bitumen.
Rapid set concrete is made by using quick-setting admixture in the concrete repair and restoration. Rapid setting concrete is produced for applications where fast setting is required.
Self-compacting concrete is a type of concrete that is designed to flow and spread naturally without the need for vibration. Traditional concrete often requires vibration to ensure proper consolidation and removal of air voids; self-compacting concrete achieves this through its unique mix design.
If concrete members are separately made on-site or at a concrete pre-cast factory, with proper quality control procedures and then placed in the structure, it is called pre-cast concrete.
To get desired results from concrete work, the following material requirements must be followed:
Various types of Cement commonly used in Concrete works are given in Table 1. Cement shall conform to the specifications for Portland Cement ASTM C-150 / BS-12, unless otherwise specified to be of any particular quality, shall mean Ordinary Portland Cement.
Table 1 - Types of cement
Aggregate shall conform to the “Specifications for Concrete Aggregates” (ASTM - C 33). Aggregates must be clean, inert, hard, non-porous and free from dust, laminated particles, loam, clay, organic or other impurities and silt. Coarse aggregates shall be hard and durable broken / crushed stone, gravel or shingle. Alternatives, like “Hard broken bricks” and “Broken CC” may be used for plain concrete only under exceptional circumstances.
The Fine aggregate shall be non-plastic material and shall consist of sand, stone screenings or other inert materials with similar characteristics or a combination thereof and shall not contain more than 3% of material passing Sieve # 200 by washing and not more than 1% of clay lumps or shale. The Fine aggregate shall be uniformly graded and when tested as per ASTM C-117 & C-136, shall meet gradation requirements given in Table 2.
Table 2 - Fine Aggregate Grading
9.52 mm (3/8”)
100
No. 4
95-100
No. 8
80 - 100
No. 16
50-85
No. 30
25 - 60
No. 50
5 -30
No. 100
0 -10
It shall consist of crushed or broken stone, gravel or other inert material with similar characteristics or a combination. It shall be of uniform grading with max size as required for various types / classes of concrete meeting the grading requirements, Selection of Max Size of Aggregate is mentioned in Table 3.
Table 3 - Max Size of Course Aggregate
Plain Footings, caissons and Sub structure walls
50
2
Reinforced Foundation Walls and footings
37.5
1-1/2
Beams and Reinforced walls
25
1
Columns
Pavements and Slabs
19
3/4
Mass Concrete
The quality of mixing water shall be determined by ASTM-C 1602 and ASTM-C 1603. Water used in Concrete shall be clean and free from injurious amounts of acids, alkalis, salts, organic material or substances deleterious to concrete or reinforcement. Water for curing, washing aggregates & mixing shall be free from oil and shall not contain more than 1,000 ppm of Chlorides, nor more than 150 ppm of Sulphates (SO4).
An admixture can be defined as a chemical product which, except in special cases, is added to concrete mix in quantities not more than 5% by mass of cement during mixing or during an additional mixing operation prior to the placing of concrete, for the purpose of achieving a specific modification, to normal properties of concrete. Admixtures may be organic or inorganic in composition, but their chemical character as distinct from mineral, is their essential feature. Admixtures are commonly classified by their function in concrete. The classification as per ASTM C - 494 is as follows:
The max water cement ratio for Normal Concrete shall not exc 0.50 or as specified by the designer or as stated in the JMF prepared for the purpose. For Special Concrete, it shall be as specified in the design. Depending on the purpose of use, the recommended Slump for Normal Concrete is selected from Table 4.
Table 4 - Recommended slumps for various types of construction
Reinforced Foundation Walls and Footings
75
Plain Footings, Caissons and Substructure Walls
Beams and Reinforced Walls
Building Columns
It shall be worked out as per the Concrete Mix Design for different strength and survivability requirements, based on ACI specifications. The Min Cement Content shall not be less than the survivability requirement, corresponding to the purpose of use. As instance a concrete mix having ration of 1:2:4 should have 1 part of cement and compare to 2 part of sand and 4 part of aggregate.
Concrete shall not be poured in the forms until the Authorized Engineer has inspected the placing of the reinforcement, conduits, anchorages, and pre-stressing steel and has given his approval in writing.
Following be ensured during pouring or placing of Concrete:
Temperature of Concrete under hot weather shall not exceed 32°C at the time of pouring. Following actions may keep Concrete temperature below the above limit:
Concreting operations in Cold weather shall not be continued without the written approval of the Authorized Engineer, when a descending air temperature in the shade and away from artificial heat falls below 5oC, nor resumed until an ascending air temperature in the shade and away from artificial heat reaches 20oC. In such case, the mixing water and / or aggregates shall be heated to not less than 21oC nor more than 66oC prior to being placed in the mixer, so that temperature of 30–90 mm (12” to 36”) thick Concrete shall not be less than 10oC and not more than 27oC at the time of placing.
Compressive Strength of Concrete will be determined from test cylinders in accordance with ASTM C-31 & ASTM C-39:
Exposed Faces:
Channels:
Pointing:
Linear Labours (Small):
Incidental Labours:
Construction and Expansion Joints:
will be provided as required in concrete work or as shown on drawing. Following be adhered during construction.
• Construction joints shall be as few as possible, and shall be made on & vertical planes only or at places shown on the drawings or approved by the Authorized Engineer. Where concreting is stopped on a vertical plane, as in beams, provide lap joint with approved stop board. Make provisions to allow the reinforcement to pass through the joints without being temporarily bent or otherwise displaced.
• In case of slabs or walls, nail a 50mm (2”) x 25mm (1”) fillet slightly splayed (to permit easy removal) on the joint stop board to form a joggle running throughout the length of the joint. Remove any concrete flowing pass the joint as soon as initial set occur. When concreting against a hardened surface is resumed, well roughen, wet, clean surface and apply cement sand slurry of same ratio as of mortar used in the concrete.
• Provide approved water proofer in lieu of mortar at joint, in basement and water retaining walls more than 7.5M (30 ft) long, carry out concreting such that vertical gap of about 0.7M (2ft) width with vertical joggled ends at both sides is left out at about 7.5M (30 ft). Distances are filled in after 14 days of concreting the adjacent sections.
• Form expansion joints accurately in the position and to the dimensions and details shown on the drawing and as instructed by the Authorized Engineer.
such that it is strong to support the load, imposed on them, by fresh Concrete and stresses imposed by vibrating equipment and traffic. Prevent settlement of support. All joints be tight against the escape of cement and fines. Make due allowance (incl camber) for settlement and movement of forms under fresh Concrete. If metal ties are used in conjunction with bolts to pass through the Concrete, do not leave the metal closer than 50 mm (2”) from the face of Concrete.
Shortly before Concrete is placed, forms for exposed surfaces shall be coated with approved non-staining form oil, which shall not interfere with the setting of the Concrete nor be otherwise deleterious. After oiling, surplus oil on the form surfaces and any oil on the reinforcing steel or other surfaces requiring bond with the Concrete shall be removed. Forms for unexposed surfaces may be thoroughly wetted in lieu of oiling, immediately before placing the Concrete.
Removal of Formwork: Forms shall be removed with care so as to avoid injury to Concrete. Forms shall be removed as soon as practicable, keeping in view the min Concrete setting time requirements, to avoid delay in water curing, and to enable the earliest practicable repair of surface imperfections. In order to avoid excessive stresses in the Concrete that might result from swelling of the forms, wooden forms for wall openings shall be loosened, as soon as this can be accomplished without damage to the Concrete. Forms for the openings shall be constructed in such a manner as to be removed until the strength of the Concrete is such that form removal will not result in perceptible cracking, spalling, and breaking of edges of surfaces or other damage to the Concrete. In general, the approx. elapsed time before removal of forms shall be as per Table 5.
Table 5 - Min Period for Formwork Removal
Vertical or near vertical faces of mass concrete
24 hours
Vertical or near vertical faces of reinforced walls beams and columns
48 hours
Undersides of arches, beams and slabs (formwork only)
14 days
Supports to underside of arches, beams and slab upto 3M (10 ft) span
Supports and underside of beams, slabs larger than 3M (10 ft) span
21 days
Curing Methods for Concrete
Curing is essential to prevent the loss of moisture from concrete, due to sun, drying winds, and traffic, until the specified curing has been completed. Concrete shall be protected from heavy rains for 24 hours and shall be cured for 14 days. All galleries, conduits, and other formed openings through the Concrete shall be closed during the entire curing period. Concrete shall be kept wet for 15 days after laying. Following methods are used for effective curing:
