The Facts About Concrete

Concrete is a mixture of Portland cement, aggregates, sand, water, and in some cases which respect to the purpose, admixtures. It is known to the world of construction as the most widely used material for any build structure and often looked upon as “man made rock”. Furthermore, it is the most versatile construction material and adaptable to a wide variety of usage in building any desired structures for residential, commercial and industrial, and other sections. The way I see it, the usage often two times larger than other materials for any civil projects especially in bigger scale.

In General
Concrete can be placed or molded into virtually any imaginable shape and re-produce any surface texture. With proper materials, combination, and techniques, concrete can withstand many acids, silage, milk, manure, fertilizers, water, fire, and abrasion. It can be finished to produce surfaces ranging from glass-smooth to coarsely textured, and it can be colored with pigments or painted.

The Components

Portland Cement
The cement and water form a paste that hardens and bonds the aggregates together through hydration phase. It is a substance that hardens with time and holds or entraps objects or particles in a definite relationship to each other. Mostly and widely used product would be Portland cement.

Aggregates
Aggregates occupy largely of 60 to 80 percent of the volume of concrete. Sand, gravel and crushed stone are the primary aggregates used in the concrete mixer. All aggregates must be essentially free of silt and/or organic matter.

Water
Good water is essential for quality of concrete and it should be good enough for drinking purposes; free of trash, organic matter and excessive chemicals and/or minerals. The strength and other properties of concrete are highly dependent on the amount of water and the water-cement ratio used in the mixing of concrete.

Admixture
Admixtures are ingredients other than Portland cement, aggregates, and water. It would be added to the concrete mixture immediately before or during mixing, depending on its requirements.

The Concrete States
1) Plastic State
When the concrete is first mixed it is like ‘bread dough’. It is soft and can be worked or molded into different required shapes. In this state concrete is called plastic. Concrete is plastic during placing and compaction. The most important properties of plastic concrete are workability and cohesiveness.

2) Setting State
In the second state, concrete then begins to stiffen. The stiffening of concrete, when it is no longer soft, is called setting. Setting takes place after compaction and during finishing. Concrete that is sloppy or wet may be easy to place but will be more difficult to finish. A worker leaves footprints in setting concrete.

3) Hardening State
Finally, after concrete has set it begins to gain strength and harden. The properties of hardened concrete are strength and durability.

The Properties
Concrete has strength, durability, versatility, and economy. It has substantial strength in compression, but is weak in tension. Most structural uses, such as beams, columns, footings, slabs, staircase, and manure tank lids, involve reinforced concrete, which depends on concrete’s strength in compression and steel’s strength in tension. The compressive strengths of concrete generally range from 2,000 to 5,000 pounds per square inch (psi), but concrete can be made to withstand over 10,000 psi for special jobs.

Note: 1 psi = 6.894757293168361 kPa

The four main properties of concrete are:

Workability
Workability means how easy it is to: place, handle, compact, and finish a concrete mix. If not constructed properly, will not be as strong or durable when finally hardened. A ‘slump test’ can be used to measure the workability of concrete.

Workability is affected by:

• Amount of cement Paste: The cement paste is the soft or liquid part of the concrete mix. The more paste mixed with the coarse and fine aggregates, the more workable a mix.
• Grading of Aggregate: Well-graded, smooth, rounded aggregates improve the workability of a concrete mix.

Cohesiveness
Cohesiveness is how well concrete holds together when in plastic state.

Cohesiveness is affected by:

• Grading of Aggregate: Graded Aggregate means that there is a range of size of aggregates, from large rocks to small sands.Well-graded aggregates give a more cohesive mix, too much coarse aggregate gives a boney mix.
• Water Content: A mix that has too much water will not be cohesive and may separate and bleed. The single most important indicator of strength is the ratio of the water used compared to the amount of cement (water/cement ratio). Basically, the lower this ratio is, the higher the final concrete strength will be.

Strength and Durability

The well made concrete is a naturally strong and durable material whereby it is dense, reasonably watertight, able to resist the changes of temperature, as well as wear and tear from weathering process. Plus, it is very important to protect the steel in reinforced concrete. The strength of concrete in the hardened state is usually measured by the compressive strength using the ‘compression test’.

Strength and durability are affected by:

• Density: Denser concrete is more watertight (or less permeable).
• Compaction is removing the air from concrete. Proper compaction results in concrete with an increased density which is stronger and more durable.
• Curing is keeping concrete damp for a period, to allow it to reach maximum strength. Longer curing will give more durable concrete. It is critical to concrete’s long-term durability that it be kept as moist as possible for the first seven days after placement.

The Testing
In normal construction practice, concrete normally tested for 7 days and 28 days for determination of strength. There are two kinds of test category:

• Destructive Test: Compression test (Cube Crushing), Flexural Beam test, Tensile Strength test, Core test, etc.
• Non-destructive Test: Rebound Hammer test, Ultra Sonic Pulse test, Covermeter, etc.

The Different Types
There are many kinds of concrete produced depending on the usage:

• Cement Concrete
• Plain Mass Concrete
• Lean Concrete
• Structural Concrete
• Reinforced Concrete
• Prestressed Concrete
• Cast in Place / Cast In-Situ Concrete
• Precast Concrete
• Vacuum Concrete
• Pumped Concrete
• Spun Concrete
• Ready Mixed Concrete
• Water Resistant Concrete
• High Density Concrete
• Fibre Reinforced Concrete

The Typical Classes

• Normal Concretes: C8/10, C12/15, C16/20, C20/25, C25/30, C28/35, C30/37, C32/40, C40/50, C50/60, C55/67, C60/75, C70/85, C90/105, C100/115
• Lightweight Concretes: LC8/9, LC12/13, LC16/18, LC20/22, LC25/28, LC30/33, LC35/38, LC40/44, LC50/55, LC55/60, LC60/66, C70/77, LC80/88

Note: Take example, C20/25 means that the concrete compressive strength is 20 MPa with the usage of 25mm diameter aggregates.

In conclusion, these are the facts on concrete and may it be useful knowledge to you as well as for me. If there is any additional info you want to add-in, you are most welcome via comment…

Read More Constructive Posts:

• Steps of Concrete Mix Design

Tagged as: Concrete and Testing

Be the 1st Commenter...