Concrete |Definition, Ingredients and Curing You Should Know

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Plain concrete, commonly known as concrete, is a mixture of binding material, fine aggregate, coarse aggregate, and water. This can be easily molded to desired shape and size before it loses plasticity and hardens. Plain concrete is strong in compression but very weak in tension. The tensile property is introduced in concrete by inducting different materials and this attempt has given rise to RCC, RBC, PSC, FRC, cellular concrete, and Ferro cement. In this chapter proportioning, mixing, curing, properties, tests, and uses of plain concrete are dealt with in detail. The other improved versions of concrete are explained and their special properties and uses are pointed out.

Plain Concrete

The major ingredients of concrete are:

Binding material (like cement, lime, polymer)
Fine aggregate (sand)
Coarse aggregates (crushed stone, jelly)
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 proportion of the ingredients for a particular strength by 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 is the ratio of weights 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:

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

Also Read: Reinforced Cement Concrete

Functions of Various Ingredients

Cement is the binding material. After the addition of water, it hydrates and binds aggregates and the surrounding surfaces like stone and bricks. Generally richer mix (with more cement) gives more strength. Setting time starts after 30 minutes and ends after 6 hours. Hence concrete should be laid in its mold before 30 minutes of mixing water and should not be subjected to any external forces till the final setting takes place.

Coarse aggregate consists of crushed stones. It should be well-graded and the stones should be of igneous origin. They should be clean, sharp, angular, and hard. They give mass to the concrete and prevent shrinkage of cement. Fine aggregate consists of river sand. It prevents the shrinkage of cement. When surrounded by cement it gains mobility and enters the voids in coarse aggregates and the binding of ingredients takes place. It adds density to concrete since it fills the voids. The denser the concrete higher is its strength.

Aggregates for concrete mixing — Aggregates

Water used for making concrete should be clean. It activates the hydration of cement and forms a plastic mass. As it sets completely concrete becomes a hard mass. Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in the final position. The more water better is the workability. However excess water reduces the strength of concrete. To achieve the required workability and at the same time good strength a water-cement ratio of 0.4 to 0.45 is used, in the case of machine mixing, and a water-cement ratio of 0.5 to 0.6 is used for hand mixing.

Preparing and Placing Concrete

The following steps are involved in the concreting:

Batching
Mixing
Transporting and placing and
Compacting.

Batching: The measurement of materials for making concrete is known as batching. The following two methods of batching are practiced:

Volume Batching: In this method cement, sand, and concrete are batched by volume. A gauge box is made with wooden plates, its volume being equal to that of one bag of cement. One bag of cement has a volume of 35 liters. The required amount of sand and coarse aggregate is added by measuring onto the gauge box. The quantity of water required for making concrete is found after deciding 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 liters. A suitable measure is used to select the required quantity of water. Volume batching is not the ideal method of batching. Wet sand has a higher volume for the same weight as dry sand. It is called bulking of sand. Hence it upsets the calculated volume required.
Weigh Batching: This is the recommended method of batching. A weighing platform is used in the field to pick up the correct proportion of sand and coarse aggregates. Large weigh batching plants have automatic weighing equipment.

Mixing: To produce uniform and good concrete, it is necessary to mix cement, sand, and coarse aggregate, first in dry condition and then in wet condition after adding water.

The following methods are practiced:

Hand Mixing: 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 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. A full amount of water is added and mixing is completed when uniform color and consistency are observed. The process of mixing is completed in 6–8 minutes of adding water. This method of mixing is not very good but for small works, it is commonly adopted.
Machine Mixing: In large and important works machine mixing is preferred. Required quantities of sand and coarse aggregates are placed in the drum of the mixer. 4 to 5 rotations are made for dry mixing and then the required quantity of cement is added and dry mixing is made 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 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 and belt conveyors or pipes with pumps are employed. In transporting care should be taken to see that the segregation of aggregate from the matrix of cement does not take place.

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

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

Compaction of Concrete: In the process of placing concrete, the air is entrapped. The entrapped air reduces the strength of concrete by up to 30%. Hence it is necessary to remove this entrapped air. This is achieved by compacting the concrete after placing it in its final position. Compaction can be carried out either by hand or with the help of vibrators.

Hand Compaction: In this method concrete is compacted by ramming, tamping, spading, or slicing with tools. In intricate portions, a pointed steel rod of 16 mm in diameter and about a meter long is used for poking the concrete
Compaction by Vibrators: Concrete can be compacted by using high-frequency vibrators. Vibration reduces the friction between the particles and set the motion of particles. 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 less, which also helps in improving the strength of concrete. Vibration should be stopped as soon as cement paste is seen on the surface of the concrete. Over-vibration is not good for concrete.

The following types of vibrators are commonly used in concrete:

Needle or immersion vibrators
Surface vibrators
Form or shutter vibrators
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

Curing may be defined as maintaining satisfactory moisture and temperature conditions for freshly placed concrete for some specified time for proper hardening of concrete. 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 strength of the concrete reduces. Cracks develop due to shrinkage. The durability of concrete structures reduces. Hydration and hardening of concrete during the first three days are critical. Abnormally fast drying and shrinkage due to factors such as evaporation from wind during placement may lead to increased tensile stresses at a time when it has not yet gained sufficient strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased if it is kept damp during the curing process. Minimizing stress prior to curing minimizes cracking. High-early-strength concrete is designed to hydrate faster, often by increased use of cement that increases shrinkage and cracking.

The following curing methods are employed:

Spraying of water: Walls, columns, and plastered surfaces are cured by sprinkling water.
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
Ponding: The horizontal surfaces like slabs and floors are cured by stagnating the water to a height of 25 to 50 mm by providing temporary small hands with mortar.
Steam curing: In the manufacture of prefabricated concrete units steam is passed over the units kept in closed chambers. It accelerates the curing process, resulting in the reduction of the curing period.
Application of curing compounds: Compounds like calcium chloride may be applied on 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 has completely different properties when it is in the plastic stage and when hardened. Concrete in the plastic stage is also known as green concrete. The properties of green concrete include:

Workability
Segregation
Bleeding
Harshness

The properties of hardened concrete are:

Hardened concrete is a type of concrete that is strong and have the capacity to bear the structural as well as service loads that are applied to it. Hardened concrete is one of the strongest and most durable construction materials. The concrete is completely set and able to take the loads. The following are the properties of hardened concrete;

Strength
Resistance to wear
Dimensional changes
Durability
Impermeability.
Fire resistance.
Thermal and acoustic insulation properties.
Impact resistance.

Properties of Green Concrete

Concrete that is made from concrete wastes that are eco-friendly is called “Green concrete”. Green Concrete is a term given to concrete that has had extra steps taken in the mix design and placement to ensure a sustainable structure and a long life cycle with a low maintenance surface. e.g. Energy saving, 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. The workability depends upon the quantity of water, grading, shape, and percentage of the aggregates present in the concrete.

Workability is measured by

The slump is observed when the frustum of the standard cone filled with concrete is lifted and removed
The compaction factor was determined after allowing the concrete to fall through the compaction testing machine.
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 as shown in the table below:

Application	Slump	Compa. factor	Time in Vee-Bee
Concreting of shallow sect. with vibrations	–	0.75-0.80	10-20
Concreting of lightly reinforced sections with vibrators	–	0.80-0.85	5-10
Concreting of lightly reinforced sections without vibrations and heavily reinforced sections with vibrations	25-75 mm	0.85-0.92	2-5
Concreting of heavily reinforced sections without vibration	75-125 mm	more than 0.92	–

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 is coating the surface of aggregates for 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 detrimental that it should be approached very cautiously. Water to a cementitious material ratio (w/cm) of 0.45 to 0.6 is the sweet spot for the production of 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 compared to larger-size 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.

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# Admixtures:

Many types of admixtures alter the workability of fresh concrete, either by design or as a side effect. Surfactants such as superplasticizers reduce attraction between cement and aggregate particles, allowing mixes that can be quite flowable without the negative strength and segregation effects of too much water. Air entraining admixtures for freeze/thaw durability produce air bubbles within a controlled size that can make for easier finishing, although using too much produces a sticky mix with the opposite effect.

2. Segregation: The separation of coarse particles from the green concrete is called segregation. This may happen due to a lack of a sufficient quantity of finer particles in concrete or due to the throwing of the concrete from greater heights at the time of placing the concrete. Because of the segregation, the cohesiveness of the concrete is lost, and honeycombing results. Ultimately it results in the loss of strength of hardened concrete. Hence utmost care is to be taken to avoid segregation.

3. Bleeding: This refers to the appearance of the water along with cement particles on the surface of the freshly laid concrete. This happens when there is an excessive quantity of water in the mix or due to excessive compaction. Bleeding causes the formation of pores and renders the concrete weak. Bleeding can be avoided by suitably controlling the quantity of water in the concrete and by using finer grading of aggregates.

4. Harshness: Harshness is the resistance offered by concrete to its surface finish. Harshness is due to the presence of a lesser quantity of fine aggregates, lesser cement mortar, and due use of poorly graded aggregates. It may result due to insufficient quantity of water also. With harsh concrete, it is difficult to get a smooth surface finish and concrete becomes porous.

Properties of Hardened Concrete

1. 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 percent 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.

Grade	M10	M15	M20	M25	M30	M35	M40
Charac. strength in MN/mm2	10	15	20	25	30	35	40

The strength of concrete depends upon the amount of cement content, quality and grading of aggregates, water-cement ratio, compaction, and curing. The strength of concrete is gained in the initial stages. In 7 days the strength gained is as much as 60 to 65 percent of 28 days’ strength. It is customary to assume the 28 days strength as the full strength of concrete. However concrete gains strength after 28 days also. The characteristic strength may be increased by the factor given in the table below.

Effect of the age factor on strength of concrete

Min age of member when design load is expected.	1 month	3 months	6 months	12 months
Age factor	1.0	1.10	1.15	1.20

Factors that Affect the Strength of Concrete

# Water/Cement Ratio

The ratio of the weight of water to the weight of cement is called the Water/Cement ratio. It is the most important factor for gaining the strength of concrete. The lower w/c ratio leads to the higher strength of concrete. 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 gets decreases and when the w/c ratio is decreased then the strength of concrete increases.

# Compaction of Concrete

Compaction of concrete increases the density of the concrete because it is the process in which air voids are removed from freshly placed concrete which makes the concrete compact and dense. The presence of air voids in concrete greatly reduces its strength. Approximately 5 % of air voids can reduce the strength by 30 to 40 %. As we can see in the above chart, even at the same water/cement ratio strength is different with different compaction accuracies. In fully compacted concrete, strength is higher than insufficiently compacted concrete.

# Ingredients of Concrete

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

(a) Type and Quantity of Cement

The quantity of cement greatly affects concrete strength. The higher cement content increases the tendency of shrinkage cracks when the concrete is getting cured and hardened. 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 for various exposure conditions and for various grades of concrete. Maximum cement content in concrete is also limited to 450 kg per cubic meter of concrete. 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 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 properties of hardened concrete. At constant cement content, the higher amount of aggregate reduces the concrete strength. The shape and grading of aggregate play a major role as far as the strength of concrete is concerned.

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(c) Quality of Water

The quality of water plays a significant role in the setting and hardening process of concrete. Acidic, oily, silty, and seawater should not be used in the concrete mix. Impurities of water give an adverse effect on the strength of concrete. Therefore, potable water is always used in the concrete mix. Particularly the impure water may lead to corrosion, carbonation, or acid attack, therefore, reducing the life of the concrete.

# Curing of Concrete

The curing of concrete is the most essential to prevent plastic shrinkage, temperature control, strength development, and durability. Curing provides the desired moisture and temperature at the depth and near the surface after placing and finishing concrete for the development of strength. 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 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 like Ponding and immersion, Spraying and fogging saturated wet coverings, etc.

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

# The Shape of Aggregate

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

Angular aggregates are rough-textured, and rounded aggregates are smooth-textured. Thus, the rounded aggregates, create the problem of a lack of bonding between cement paste and 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 the case of high-strength and 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 size aggregates give a lower strength because they have a lower surface area for the development of gel bonds which is responsible for strength. Larger size aggregate makes concrete heterogeneous. It will not distribute loading uniformly when stressed. Due to internal bleeding, the problem of development of the microcracks in concrete happens when larger size aggregates are used in concrete.

# Grading of Aggregate

The grading of aggregates determines the particle size distribution of aggregates. It’s the most important factor for 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 that, they have fewer amount of voids. The use of well-graded aggregates gives higher strength to the concrete.

# Weather Condition

Weather condition also affects the strength of concrete due to different reasons. In cold climates, exterior concrete is subjected to repeated freezing and thawing action due to the sudden change in weather. It produces deterioration in concrete. With the change in moisture content, materials expand and contract. It produced cracks in concrete.

# Temperature

With a certain degree of temperature increase, the rate of the hydration process increases in it, it gains strength rapidly. Sudden temperature changes create a thermal gradient, which causes the cracking and spalling of concrete. So, the final strength of concrete is lower at a very high temperature.

# The Rate of Loading

The strength of concrete increases with the increase in the rate of loading because at high rates of loading, there is less time for creep. Creep produces permanent deformation in the structure at constant loading. So, the failure occurs at limiting values of strain rather than stress. In rapid loading, the load resistance is better than the slow loading.

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# Age of Concrete

With the increase in the age of concrete, the degree of hydration would be more. 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 gets doubled 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 better life of building structures, for low maintenance of building after construction, longer durability and better serviceability, etc.

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

3. Durability: Environmental forces such as weathering, chemical attack, heat, freezing, and thawing try to destroy concrete. The period of existence of concrete without getting adversely affected by these forces is known as durability. Generally dense and strong concretes have better durability. 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.

4. 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 leading to permeability. Since the permeability reduces the durability of concrete, it should be kept very low by using a low water-cement ratio, dense and well-graded aggregates, good compaction, and continuous curing at low-temperature conditions. The cement content used should be sufficient to provide adequate workability with a low water-cement ratio and the available compaction method.

Tests on Concrete

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

Slump test.
Compaction factor test.
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:

Desired workability.
No segregation in transporting and placing
No bleeding and
No harshness.

Hardened concrete should have

the required characteristic strength.
minimum dimensional changes.
good durability
impermeable
good resistance to wear and tear

Uses of Concrete

As bed concrete below column footings, wall footings, on the wall at supports to beams
As sill concrete.
Over the parapet walls as coping concrete
For flagging the area around buildings
For pavements
For making building blocks.

However major use of concrete is a major ingredient of reinforced and prestressed concrete. Many structural elements like footings, columns, beams, chejjas, lintels, and roofs are made with R.C.C. Cement concrete is used for making storage structures like water tanks, bins, silos, bunkers, etc. Bridges, dams, and retaining walls are R.C.C. structures in which concrete is the major ingredient.

FAQ;

1. What is Concrete?

Concrete is a mixture of binding material, fine aggregate, coarse aggregate, and water.

2. What are the ingredients of Concrete?