What is Concrete?
The concrete word comes from the Latin word which means compact. The common name of concrete is plain cement concrete (PCC). It is a composite mixture of binding materials such as cement, coarse and fine aggregates, with water and sometimes with the addition of admixtures. When the concrete is fresh or in the plastic state, we can easily change it into our desired shape.
It is the most important and useful material for construction works. The compressive strength of concrete is very high but tensile strength is too low. It is the basic engineering material and used in almost every civil engineering structures because it is economical, durable, and easy to make and can be manufactured at the site. In the plastic state, it has the ability to mold it into any desired shape and size when it in a plastic state.
Many types of materials are used to get the good quality of concrete. The main ingredient’s used in concrete are:
Cement is the main raw material (work as the binder) which used in concrete. It is a fine grey powder. The 1.5 billion tons of cement are produced in the world per year. There are different types of cement, which are used in the different grades of concrete. These all grades are depending on the compressive strength which was achieved after 28 days (With proper curing).
Aggregates are the main ingredient in PCC, which can cover up to 70 – 75% of the volume of concrete. Two kinds of aggregates which are,
- Fine Aggregates (Below 4.75 mm)
- Coarse Aggregates (Above 4.75 mm)
The solid PCC strength depends on the sizes and texture of aggregates. Fine aggregates (sand) is used to fill up the gaps(Work as a filler) in PCC. The fine and coarse aggregates used in concrete are must free from grass, roots of plants and other organic matters.
Water is the less expensive material as compared to the other material used in the manufacturing of PCC. The main purpose of water is to achieve the workability in PCC and also to distributes the cement particles evenly.
They are used to increase the workability or (Increase or decrease) setting time of PCC. Concrete Admixtures can change the properties of concrete such as strength workability, setting time etc.
Types of concrete:
This PCC is mixed in a central plant and then transported to the site for concreting. There are three mixing methods are used for ready mixed concrete:
- Central-mixed: It is mixed completely in a stationary mixer and delivered in an agitator truck (2 rpm to 6 rpm). Rpm means revolution per minute.
- Shrink-mixed: It is partially mixed in a stationary mixer and completed in a mixer truck (4 rpm to 16 rpm).
- Truck-mixed: concrete is mixed completely in a mixer truck (4 rpm to 16 rpm).
2: Normal weight concrete or PCC:
The normal mixture of cement, aggregates, and water is called normal-weight concrete.
- The setting time of normal weight concrete is 30 to 90 minutes.
- Its compressive strength varies from 10 Mpa (1400psi) to 40 Mpa (5800 psi) after the 28 days with adequate curing.
- It is strong in compression and weak in tension.
- This type is not durable against the freezing and thawing action.
- The slump varies from 25-100 mm.
3: High-Strength concrete:
- The compressive strength of high strength concrete is greater than 6000 lb/in2.
- This is made at the lower water-cement ratio to 0.35 or below.
- The use of admixture is done to make workability.
- Mostly superplasticizer admixture is used in high strength concrete.
- Lower size of aggregates should be selected for high strength concrete.
- The compressive strength is more than normal-weight concrete.
4: High-Performance concrete:
- This has high strength and high workability and can be ease in placement.
- The H-P is made by usually kept the lower water-cement ratio to 0.25 or below.
- The strength of the high performance is ranging between the 10,000 psi to 15,000 psi.
- Superplasticizer admixture is used in H-P.
5: Self-compacting concrete:
It is also known as self-consolidating concrete and it highly flows able, no segregating concrete that can be spread into place.
- The vibrator is not used in it.
- Self-compacting concrete can be placed at a faster rate, with no use of a vibrator and less screening.
- This type also improves and makes more uniform the architectural surface finish with little or no remedial surface work.
- This also improves the consolidation around the reinforcement and its bond.
- It can also improve the pumpability.
6: Light-weight concrete:
Concrete which has the lower mass per unit volume and the concrete made of ordinary materials is called light-weight concrete.
- We use Lightweight aggregates.
- Lightweight concrete requires 28 days compressive strength of 17 Mpa (2500 psi) and a unit weight of less than 1850 kg/m3.
- It cost more but also reduces the dead load of the structure.
7: Pumpe concrete:
Pumpe concrete works in huge construction projects.
- Special pumps deliver the PCC directly into the forms.
- Careful ensured well-mixed PCC with proper workability.
- The slump should vary from 400 to 100 mm before pumping the concrete.
- During pumping slump decreases by 0.5″ to 1″.
Shotcrete is mortar or small aggregates concrete which is sprayed at high velocity on to the desired surface. It is also known as gunite or sprayed concrete.
- Shotcrete is a relatively dry mixture that should be placed on vertical or horizontal surfaces without sagging.
- It is applied by either the dry or wet process.
- Accelerating admixtures should be used to reduce the initial setting time and to allow buildup the thicker layer of shotcrete.
- Steel mesh or fibers helps to improve strength, ductility, and toughness.
Strength of concrete
The compressive strength of PCC is the ability of the structure to withstand the loads on its surface without any crack or without any deformation. The alone PCC is good in compression but it is weak in tension.
- The strength of PCC is determining with the cylinder test which made of fresh concrete at the site.
- In reality, long term strength is more as compared to the strength at 28 days.
- The density of normal strength PCC is 2400 kg/m3 and strength varies from 3000 to 6000 psi.
- 2500 psi (17 Mpa) strength we use in residential construction. The 4000 psi (28 Mpa) in commercial construction and high strength 10,000 psi (70 Mpa) for certain applications.
- High strength concrete strength is greater than 6000 psi (42 Mpa) after the 28 days cylinder compressive strength.
Different Grades of PCC at 7 & 28 Days’ Compressive Strength
Grade at 7 days(Mpa) at 28 days(Mpa)
M15 10 15
M20 13.5 20
M25 17 25
M30 20 30
M35 23 35
M40 27 40
M45 30 45
The strength of PCC with The Passage of Time?
Days Strength %
1 16 %
3 40 %
7 65 %
14 90 %
28 99 %
Factors Affecting the Strength Of PCC
There are six main factors that may affect the strength of PCC.
1: Quality of raw ingredients (Materials)
- Cement should be according to standards and it can be stored in dry places.
- Aggregate size, texture, shape affects the strength of PCC, the use of the angular size of aggregate to gain maximum strength.
- Silt, clay and other organic impurities reduce the strength of PCC.
- Water used in PCC should be free from clay, impurities and also fit for drinking.
2: Water to cement ratio:
- W/c ratio is the main factor that affects the strength.
- Higher the water-cement ratio reduces the strength and age of PCC (More workability).
- Lower will be the w/c ratio higher strength will be (Less workability).
3: Coarse/fine aggregate ratio:
- If the coarse/fine aggregate ratio is higher, than the overall aggregate surface area will increase (which may affect the strength.
- When the area of the aggregate surface is increases, then water demand will also increase.
4: Compaction of PCC:
- If any voids or entrapped air present in PCC it will result in the reduction of strength.
- 5% voids reduce the 15% strength of PCC.
- PCC cured regularly for 7 days (As per standards).
- Cracks will be developed if curing is inappropriate.
- The temperature depends on the reaction of hydration. If the temperature increases than the hydration process will also increase(With curing we have to maintain the temperature).
- At higher temperature, the PCC will gain strength more quickly but decreases the final strength.
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