Concrete | Definition, Ingredients and Curing You Should Know

Plain concrete, commonly known as mass concrete, is a mixture of binding material, fine aggregate, coarse aggregate, and water.

This can be easily moulded to the desired shape and size before it loses plasticity and hardens. Plain concrete is strong in compression but very weak in tension.

Tensile properties are introduced into concrete by incorporating various materials, leading to RCC, RBC, PSC, FRC, cellular concrete, and Ferro cement.

In this post, proportioning, mixing, curing, properties, tests, and uses of plain concrete are dealt with in detail.

The other improved concrete formulations are explained, and their special properties and uses are highlighted.

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Plain Concrete

The major ingredients of concrete are:

1. Binding material (like cement, lime, polymer)
2. Fine aggregate (sand)
3. Coarse aggregates (crushed stone, jelly)
4. Water.

A small number of admixtures, like air-entraining agents, waterproofing agents, workability agents, etc., may also be added to impart special properties to the plain concrete mixture.

Depending upon the proportion of ingredients, the strength of concrete varies. It is possible to determine the proportions of ingredients for a particular strength using the mix design procedure.

In the absence of a mix design, the ingredients are proportioned as 1:1:2, 1:1.5:3, 1:2:4, 1:3:6, and 1:4:8, which are the weight ratios of cement to sand to coarse aggregate.

In proportioning concrete, it is kept in mind that voids in coarse aggregates are filled with sand, and the voids in the sand are filled with cement paste. The proportion of ingredients usually adopted for various works is as follows:

1. Proportion 1:2:3 is used for machine foundation, footings for steel columns, and concreting underwater
2. A Proportion of 1:1.5:3 is used For Water tanks, shells, and folded plates, for other water-retaining structures.
3. Proportion 1:2:4 Commonly used for reinforced concrete works like beams, slabs, tunnel lining, bridges
4. Proportion 1:3:6 is commonly used for Piers, abutments, concrete walls, the sills of windows, and floors.
5. The Proportion 1:4:8 Mass concretes are likely used for dams, foundation courses for walls, and concrete blocks.

Also Read: Reinforced Cement Concrete

Functions of Various Ingredients

Cement

 Is the binding material. After adding water, it hydrates, binds aggregates, and adheres to surrounding surfaces, such as stone and brick.

Generally, a richer mix (with more cement) gives more strength. The setting time starts after 30 minutes and ends after 6 hours.

Hence, concrete should be placed in its mould within 30 minutes of mixing water and should not be subjected to any external forces until final setting occurs.

Coarse aggregate 

Consists of crushed stone. It should be well-graded, and the stones should be of igneous origin. They should be clean, sharp, angular, and hard.

They add mass to the concrete and prevent cement shrinkage. Fine aggregate consists of river sand.

It prevents the shrinkage of cement. When surrounded by cement, it gains mobility, enters the voids in the coarse aggregates, and the ingredients bind. It adds density to concrete by filling voids. The denser the concrete, the higher its strength.

Water 

When used for making concrete, it should be clean. It activates the hydration of cement and forms a plastic mass. As it sets, it becomes a hard mass.

Water improves the workability of concrete, making it easier to mix and place in the final position. The more water, the better the workability.

However, excess water reduces concrete’s strength. To achieve the required workability and good strength, a water-cement ratio of 0.4 to 0.45 is used for machine mixing, and 0.5 to 0.6 for hand mixing.

Preparing and Placing Concrete

The following steps are involved in the concreting:

1. Batching
2. Mixing
3. Transporting placing and
4. Compacting.

Batching

The measurement of materials used to make concrete is known as batching. The following two methods of batching are practised:

1. Volume Batching: In this method, cement, sand, and concrete are batched by volume. A gauge box is made from wooden plates and has a volume equal to that of one bag of cement. One bag of cement has a volume of 35 litres. The required amounts of sand and coarse aggregate are added by measuring them into the gauge box. The quantity of water required to make concrete is determined after selecting the water-cement ratio. For example, if the water-cement ratio is 0.5, for one bag of cement (50 kg), the water required is 0.5 × 50 = 25 kg, which is equal to 25 litres. A suitable measure is used to select the required quantity of water. Volume batching is not the ideal method. Wet sand has a higher volume for the same weight as dry sand. It is called sand bulking. Hence, it upsets the calculated volume required.
2. Weigh Batching: This is the recommended method. A weighing platform is used in the field to accurately measure the proportions of sand and coarse aggregates. Large weigh-batching plants have automatic weighing equipment.

Mixing

To produce uniform, high-quality concrete, mix cement, sand, and coarse aggregate first in a dry condition, then add water and mix again in a wet condition.

The following methods are practised:

1. Hand Mixing: The required amount of coarse aggregate for a batch is weighed and spread on an impervious platform. Then the sand required for the batch is spread over the coarse aggregate. They are mixed in dry conditions by overturning the mix with shovels. Then the cement required for the batch is spread over the dry mix and mixed with shovels. After the uniform texture is observed, water is added gradually, and mixing is continued. The full amount of water is added, and mixing is complete when a uniform colour and consistency are observed. The mixing process is complete in 6–8 minutes after adding water. This mixing method is not very effective, but it is commonly adopted for small works.
2. Machine Mixing: In large and important works, machine mixing is preferred. The required quantities of sand and coarse aggregate are placed in the mixer drum. 4 to 5 rotations are performed for dry mixing, then the required quantity of cement is added, and the mixture is mixed again with another 4 to 5 rotations. Water is gradually added, and the drum is rotated for 2 to 3 minutes, during which period it makes about 50 rotations. At this stage, uniform and homogeneous mixes are obtained.

Transporting and Placing of Concrete

After mixing, the concrete should be transported to the final position. In small works, it is transported in iron pans from hand to hand by a set of workers.

Wheelbarrows and hand carts also may be employed. In large-scale concreting, chutes, belt conveyors, or pipes with pumps are employed.

During transportation, care should be taken to prevent aggregate segregation from the cement matrix.

Concrete is placed on formwork. The form works should be cleaned and properly oiled. If concrete is to be placed for the foundation, the soil bed should be well compacted and free of loose soil.

Concrete should be dropped in its final position as closely as possible. If it is dropped from a height, the coarse aggregates fall first, followed by the mortar matrix. This segregation results in weaker concrete.

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Compaction of Concrete

During concrete placement, air is entrapped. The entrapped air reduces concrete strength by up to 30%. Hence, it is necessary to remove this entrapped air. This is achieved by compacting the concrete after it is placed in its final position. Compaction can be carried out by hand or with vibrators.

1. Hand Compaction: In this method, concrete is compacted using tools such as rammers, tampers, spades, or slicing tools. In intricate portions, a pointed steel rod of 16 mm in diameter and about a meter long is used for poking the concrete
2. Compaction by Vibrators: Concrete can be compacted by using high-frequency vibrators. Vibration reduces friction between particles and sets their motion. As a result, entrapped air is removed, and the concrete is compacted. The use of vibrators reduces the compaction time. When vibrators are used for compaction, the water-cement ratio can be reduced, thereby improving concrete strength. Vibration should be stopped as soon as cement paste appears on the concrete surface. Over-vibration is not good for concrete.

The following types of vibrators are commonly used in concrete:

1. Needle or immersion vibrators
2. Surface vibrators
3. Form or shutter vibrators
4. Vibrating tables.

Needle vibrators are used in concrete beams and columns. Surface vibrators and form vibrators are useful in concrete slabs. Vibrating tables are useful in preparing precast concrete elements.

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Curing of Concrete

The Curing may be defined as maintaining satisfactory moisture and temperature conditions for freshly placed concrete for a specified period to ensure proper hardening.

Curing in the early ages of concrete is more important. Curing for 14 days is very important. Better to continue it for 7 to 14 days more.

During curing, hydration occurs, allowing calcium-silicate hydrate (C-S-H) to form. Over 90% of a mix’s final strength is typically reached within four weeks, with the remaining 10% achieved over years or even decades.

If curing is not done properly, the concrete’s strength decreases. Cracks develop due to shrinkage. The durability of concrete structures is reduced.

Hydration and hardening of concrete during the first three days are critical. Abnormally fast drying and shrinkage due to wind-driven evaporation during placement may lead to increased tensile stresses when it has not yet gained sufficient strength, resulting in greater shrinkage cracking.

The early strength of concrete can be increased by keeping it damp during curing. Minimising stress before curing minimises cracking.

High-early-strength concrete is designed to hydrate faster, often by using more cement, which increases shrinkage and cracking.

The following curing methods are employed:

1. Spraying of water: Walls, columns, and plastered surfaces are cured by sprinkling water.
2. Covering the surface with wet gunny bags, straw, etc.: Columns and other vertical surfaces may be cured by covering the surfaces with wet gunny bags or straw
3. Ponding: Horizontal surfaces, such as slabs and floors, are cured by stagnating water to a height of 25 to 50 mm by providing temporary small hands with mortar.
4. Steam curing: In the manufacture of prefabricated concrete units, steam is passed over the units kept in closed chambers. It accelerates the curing process, reducing the curing period.
5. Application of curing compounds: Calcium chloride may be applied to the curing surface. The compound shows affinity to moisture and retains it on the surface. It keeps the concrete surface wet for a long time.

Properties of Concrete

Concrete exhibits completely different properties in the plastic and hardened states. Concrete in the plastic stage is also known as green concrete. The properties of green concrete include:

1. Workability
2. Segregation
3. Bleeding
4. Harshness

The properties of hardened concrete

Hardened concrete is a type of concrete that is strong and can withstand both structural and service loads. Hardened concrete is one of the strongest and most durable construction materials.

The concrete is fully set and ready to take loads. The following are the properties of hardened concrete;

1. Strength
2. Resistance to wear
3. Dimensional changes
4. Durability
5. Impermeability.
6. Fire resistance.
7. Thermal and acoustic insulation properties.
8. Impact resistance.

Properties of Green Concrete

Concrete made from eco-friendly concrete waste is called “Green concrete”.  Green Concrete is concrete that has undergone additional steps in the mix design and placement to ensure a sustainable structure with a long life cycle and a low-maintenance surface. e.g., energy savings, CO2 emissions, wastewater.

The following are the properties of Green Concrete;

1. Workability: This is defined as the ease with which concrete can be compacted fully without segregating and bleeding. It can also be defined as the amount of internal work required to fully compact the concrete to optimum density. Workability depends on the quantity of water, grading, shape, and the percentage of aggregates in the concrete.

Workability is measured by

1. The slump is observed when the frustum of the standard cone filled with concrete is lifted and removed
2. The compaction factor was determined after allowing the concrete to fall through the compaction testing machine.
3. The time taken in seconds for the shape of the concrete to change from cone to cylinder when tested in the Vee-Bee sensitometer.

The suggested values of workability for different works are shown in the table below:

Factors Affecting the Workability of Concrete

Proportions and characteristics of materials and properties of admixtures all have an impact on the workability and other qualities of every concrete mix design. Factors affecting workability include:

Water/Cement Ratio

A higher proportion of cement or cementitious materials usually means greater strength, and with the proper amount of water, more paste coats the surface of the aggregates, facilitating easier consolidation and a better finish.

Not enough water for proper hydration means poor strength development and an uncooperative mix that resists easy placement and finishing.

Adding excessive water could be said to increase workability because it makes it easier to place and consolidate.

However, the negative impact on segregation, finishing operations, and final strength can be so severe that it warrants caution.

A water-to-cementitious material (w/cm) ratio of 0.45 to 0.6 is the sweet spot for producing workable concrete.

Aggregate Size and Shape

As aggregate surface area increases, more cement paste is needed to cover the entire surface of aggregates. So mixes with smaller aggregates are less workable than those with larger aggregates.

Elongated, angular, and flaky aggregates are difficult to mix and place and have a greater surface area to cover, decreasing workability.

Rounded aggregates have a lower surface area, but lack the angularity to develop sufficient bond strengths with the cement paste.

Crushed aggregate with the proper proportions provides a better bond with the cement matrix and adequate workability.

Admixtures

Many types of admixtures alter the workability of fresh concrete, either by design or as a side effect.

Surfactants such as superplasticisers reduce the attraction between cement and aggregate particles, allowing mixes to be quite flowable without the negative strength and segregation effects of too much water.

Air-entraining admixtures for freeze/thaw durability produce air bubbles of a controlled size that make finishing easier, though using too much produces a sticky mix with the opposite effect.

Segregation

The separation of coarse particles from the green concrete is called segregation. This may occur due to insufficient fine particles in the concrete or to throwing the concrete from greater heights during placement.

Because of segregation, the concrete’s cohesiveness is lost, leading to honeycombing. Ultimately, it results in the loss of strength of hardened concrete. Hence, utmost care is to be taken to avoid segregation.

Bleeding

This refers to the appearance of water and cement particles on the surface of freshly laid concrete. This happens when there is an excessive amount of water in the mix or when compaction is excessive.

Bleeding causes the formation of pores and weakens the concrete. Bleeding can be avoided by controlling the concrete’s water content and using a finer aggregate grading.

Harshness

Harshness is the resistance concrete offers to its surface finish. Harshness is due to the presence of a lesser quantity of fine aggregates, lesser cement mortar, and the use of poorly graded aggregates.

It may also result from insufficient water. With harsh concrete, it is difficult to get a smooth surface finish, and the concrete becomes porous.

Properties of Hardened Concrete

Strength

The characteristic strength of concrete is defined as the compressive strength of 150 mm size cubes after 28 days of curing, below which not more than 5 per cent of the test results are expected to fail.

The unit of stress used is N/mm2. IS 456 grades the concrete based on its characteristic strength as shown in the table below.

The strength of concrete depends on the cement content, aggregate quality and grading, water-cement ratio, compaction, and curing.

The strength of concrete is gained in the initial stages. In 7 days, the strength gained is about 60-65 per cent of the 28-day strength.

It is customary to assume the 28-day strength as the concrete’s full strength. However, concrete also gains strength after 28 days. The characteristic strength may be increased by the factor given in the table below.

Effect of the age factor on the strength of concrete

Factors that Affect the Strength of Concrete

Water/Cement Ratio

The ratio of water weight to cement weight is called the Water/Cement ratio. It is the most important factor in determining concrete strength.

A lower w/c ratio leads to higher concrete strength. Generally, the water/cement ratio of 0.45 to 0.60 is used.

Too much water leads to segregation and voids in concrete. The water/Cement ratio is inversely proportional to the strength of concrete.

As shown in the chart below, when the w/c ratio has increased, the strength of concrete decreases, and when the w/c ratio decreases, the strength of concrete increases.

Compaction of Concrete

Compaction of concrete increases its density by removing air voids from freshly placed concrete, thereby making it more compact.

The presence of air voids in concrete greatly reduces its strength. Approximately 5% of air voids can reduce strength by 30-40%.

As shown in the chart above, even at the same water/cement ratio, strength varies with different compaction accuracies.

In fully compacted concrete, the strength is higher than in insufficiently compacted concrete.

Ingredients of Concrete

The main ingredients of concrete are cement, sand, aggregate, and Water. The quality of each material affects the concrete’s strength. All materials, therefore, should fulfil the standard criteria for use in concrete-like,

(a) Type and Quantity of Cement

The quantity of cement greatly affects concrete strength. A higher cement content increases the tendency for shrinkage cracks during curing and hardening. Types of cement also have a great impact on the properties of hardened concrete.

According to IS 456 2000, the minimum cement content specified ranges from 300 to 360 kg per cubic meter of concrete, depending on exposure condition and concrete grade.

The maximum cement content in concrete is also limited to 450 kg per cubic meter. The grade of cement, i.e. 33 grade, 43 grade, 53 grade, will also affect the strength of concrete. The higher the grade, the higher the strength, particularly high early strength.

(b) Types and Quantity of Aggregate

The strength of concrete depends upon the strength of aggregates. The low quality of aggregate reduces the strength of concrete.

The quantity of aggregate also affects the hardened concrete’s properties. At constant cement content, a higher aggregate content reduces concrete strength. The shape and grading of aggregate play a major role in determining concrete strength.

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(c) Quality of Water

Water quality plays a significant role in the setting and hardening of concrete. Acidic, oily, silty, and seawater should not be used in the concrete mix.

Impurities of water have an adverse effect on the strength of concrete. Therefore, potable water is always used in the concrete mix.

In particular, impure water may lead to corrosion, carbonation, or acid attack, thereby reducing the concrete’s lifespan.

# Curing of Concrete

The curing of concrete is essential to prevent plastic shrinkage, control temperature, develop strength, and ensure durability.

Curing provides the desired moisture and temperature at depth and near the surface after concrete is placed and finished, enabling strength development.

In other words, curing provides sufficient water to concrete for completing the hydration process without interruption, which is important for strength development. Commonly, 7-day curing corresponds to 70% of the compressive strength. 

The curing period depends on the types of cement and the nature of the work. Generally, it’s about 7 to 14 days for Ordinary Portland Cement.

There are many methods of curing, such as ponding and immersion, Spraying and fogging saturated wet coverings, etc.

Hence, please remember to use as little water as possible during concrete mixing and use as much as possible after concreting.

# The Shape of Aggregate

There are many aggregate shapes, including angular, cubical, elongated, flaky, irregular, and rounded.

Angular aggregates are rough-textured, and rounded aggregates are smooth-textured. Thus, the rounded aggregates create a lack of bonding between the cement paste and the aggregate.

Angular aggregates exhibit a better interlocking effect in concrete, but the angular aggregate contains a larger amount of voids.

For this, you needed a well-graded aggregate. The shape of aggregates becomes more important in high-strength, high-performance concrete, where a very low w/c ratio is used.

In such cases, cubical shape aggregates with uniform grading are required for better workability.

# Maximum Size of Aggregates

Larger aggregates have lower strength because they have a lower surface area for the formation of gel bonds, which confer strength.

A larger aggregate size makes concrete heterogeneous. It will not distribute the load uniformly when stressed.

Due to internal bleeding, microcracks develop in concrete when larger aggregates are used.

# Grading of Aggregate

The grading of aggregates determines their particle-size distribution. It’s the most important factor for a concrete mix. 

There are three types of graded aggregate: Gap-Graded Aggregate, Poorly-Graded Aggregate, and Well-Graded Aggregate.

Well-graded aggregate contains all sizes of particles of aggregate. So, they have fewer voids. The use of well-graded aggregates increases concrete strength.

# Weather Condition

Weather conditions also affect concrete strength for several reasons. In cold climates, exterior concrete is subjected to repeated freezing and thawing due to sudden weather changes.

It produces deterioration in concrete. With the change in moisture content, materials expand and contract. It produced cracks in the concrete.

# Temperature

With a certain degree of temperature increase, the rate of the hydration process increases; it gains strength rapidly. Sudden temperature changes create a thermal gradient that causes concrete to crack and spall. So, the final strength of concrete is lower at a very high temperature.

# The Rate of Loading

The strength of concrete increases with increasing loading rate because at higher rates, there is less time for creep.

Creep is the permanent deformation of a material under constant loading. So, the failure occurs at limiting strain rather than at limiting stress. In rapid loading, the load resistance is better than in slow loading.

# Age of Concrete

With increasing concrete age, the degree of hydration would increase. The hydration process is the chemical reaction of water and cement.

Hydration produces the gel, which plays a significant role in the bonding of particles of the concrete ingredients.

Therefore, the strength of concrete increases with its age. Normally, concrete strength doubles after 11 years, provided there are no adverse factors.

The knowledge about factors that affect concrete strength is helpful in many ways, particularly during designing the structure, choosing material for concrete, observing precautions for different weather conditions, choosing different methods for concreting, aiming for a better life of building structures, for low maintenance of buildings after construction, longer durability and better serviceability, etc.

Dimensional Change

Concrete shrinks with age. The total shrinkage depends on the concrete constituents, the member size, and the environmental conditions. Total shrinkage is approximately 0.0003 of the original dimension.

Durability

Environmental factors such as weathering, chemical attack, heat, freezing, and thawing can degrade concrete.

The period during which concrete remains unaffected by these forces is known as its durability. Generally, dense, strong concrete is more durable.

The cube-crushing strength alone is not a reliable guide to durability. Concrete should have an adequate cement content and should have a low water-cement ratio.

Impermeability

This is the resistance of concrete to the flow of water through its pores. Excess water during concreting leaves a large number of continuous pores, resulting in increased permeability.

Since permeability reduces concrete durability, it should be kept very low by using a low water-cement ratio, dense, well-graded aggregates, good compaction, and continuous curing under low-temperature conditions.

The cement content should be sufficient to provide adequate workability, given the low water-cement ratio and the available compaction method.

Tests on Concrete

The following are some of the important tests conducted on concrete:

1. Slump test.
2. Compaction factor test.
3. Crushing strength test.

Desirable Properties of Concrete

Appropriate quality and quantity of cement, fine aggregate, coarse aggregate, and water should be used so that the green concrete has the following properties:

1. Desired workability.
2. No segregation in transporting and placing
3. No bleeding and
4. No harshness.

Hardened concrete should have

1. the required characteristic strength.
2. minimum dimensional changes.
3. good durability
4. impermeable,
5. good resistance to wear and tear

Uses of Concrete

1. As bed concrete below column footings, wall footings, and on the wall at supports to beams
2. As still concrete.
3. Over the parapet walls as coping concrete
4. For flagging the area around buildings
5. For pavements
6. For making building blocks.

However, concrete is primarily used as a major ingredient in reinforced and prestressed concrete. Many structural elements, such as footings, columns, beams, chejjas, lintels, and roofs, are made of R.C.C. Cement concrete is used to make storage structures such as water tanks, bins, silos, and bunkers. Bridges, dams, and retaining walls are R.C.C. structures in which concrete is the major ingredient.

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