Concreting works

What is concrete?

Concrete is one of the modern-day construction materials which is of pivotal importance in construction projects. It is composed of cement, fine aggregates and coarse aggregates along with water and air.

In a construction project, concrete is used for the casting of various building elements like pile, raft, footings, columns, beams, slabs etc.

Concrete is made up of following components

Cement is one of the components of concrete which is a binding material, it is used extensively in construction works for concrete and mortar preparation. There are various types of cement which can be used for various works as per application. Few of them are ordinary Portland cement (OPC), rapid hardening cement, low heat Portland cement, sulphate resisting Portland cement, high alumina cement, blast furnace slag cement, coloured cement, pozzolona cement, air entraining cement, white cement.

Fine aggregates are essentially natural sand or crushed stone particles which pass through 4.75 mm sieve and retain on 0.075 mm sieve.

Coarse aggregates are the stone particles or metal which retain on 4.75 mm sieve.

After mixing of above materials along with water in required proportions gives desired grade of concrete with respective strength.

Along with above material, admixtures are also added to modify the properties of ordinary concrete to obtain desired results.

Concrete mixing processes

 There are two methods of concrete mixes which are used for proportioning different ingredients.

1. Nominal mix concrete, here all the ingredients and their respective proportions are as per standard specifications. These proportions are specified for certain strength achievement. These proportions are formulated based on past empirical studies without scientific base. Thus, it is preferred for small scale concrete works only. As per IS code nominal mix concrete may be used for concrete of M20 grade or lower grade.
2. Design mix concrete, here all the ingredients and their respective proportions are determined with their relative ratio to achieve the desired strength. In design mix concrete, properties of fresh concrete like workability and other certain specifications are taken in detail consideration.

Once desired mix proportions are established, either of two methods can be used for mixing i.e. hand mixing or machine mixing. Based on quantity and quality required, the suitable method of mixing is selected.

Fresh Concrete

Fresh concrete is a freshly mixed material which can be moulded into any shape, wherein relative quantities of cement, aggregates and water mixed together, control the properties of concrete in the wet state as well as hardened state. Quantity of water plays a pivotal role in concrete preparation. Quantity of water used can be altered on site due to the presence of free surface moisture in the aggregates or due to the absorption characteristics of dry and porous aggregates. Hence the water cement ratio is required to be adjusted based on various site conditions. Another aspect for deciding the water cement ratio is workability of concrete.

Workability is the relative ease with which concrete can be placed, compacted and finished with minimum loss of homogeneity. The term workability signifies a deeper meaning than consistency which is often loosely used in place of workability. Consistency merely indicated degree of mobility or fluidity. A mix designer uses the parameter workability to specify in the mix design process along with other involved parameters. With right workability assumption and backed by experience will make concreting activity feasible and durable.

The compressive strength of concrete is one of the elemental and useful properties of concrete. Concrete is essentially employed to resist compressive stress. Hence, various ingredients of concrete mix are usually measured in terms of the compressive strength. Moreover, compressive strength is also used as a quantitative measure of other properties of hardened concrete. It can be inferred that water cement ratio primarily affects the strength of the concrete; however other factors indirectly affect the strength of concrete by affecting the water cement ratio.

In reinforced concrete theory, it is assumed that concrete is elastic, homogenous, isotropic and that it conforms to Hook’s law. However, these assumptions are practically not true, the concrete is not a perfect elastic material.

Concrete is largely considered to be a very durable material requiring very little or no maintenance at all. All of it is true except when the concrete is subject to highly aggressive environments. However, in today’s time usage of concrete has spread more to harsher and hostile conditions, which has threatened the impression of concrete as a very durable material. It has to noted that strength of concrete has direct relationship with durability of concrete. However, it does not hold good in all situations.

Grade of Concrete

Concrete is classified based on various grades with respect to its compressive strength. The compressive strength is measured at 28 days in standard conditions.

Grades of concrete indicated by the letter M means mix which is followed by compressive strength at 28 days in N/mm2. For example M-25 indicates that crushing strength of the cube of size 150 mm x 150 mm x 150 mm at 28 days is 20 N/mm2.

oncrete Grade	Mix Ratio	Compressive Strength
MPa (N/mm2)	psi
Normal Grade of Concrete
M5	1 : 5 : 10	5 MPa	725 psi
M7.5	1 : 4 : 8	7.5 MPa	1087 psi
M10	1 : 3 : 6	10 MPa	1450 psi
M15	1 : 2 : 4	15 MPa	2175 psi
M20	1 : 1.5 : 3	20 MPa	2900 psi

Standard Grade of Concrete
M25	1 : 1 : 2	25 MPa	3625 psi
M30	Design Mix	30 MPa	4350 psi
M35	Design Mix	35 MPa	5075 psi
M40	Design Mix	40 MPa	5800 psi
M45	Design Mix	45 MPa	6525 psi

High Strength Concrete Grades
M50	Design Mix	50 MPa	7250 psi
M55	Design Mix	55 MPa	7975 psi
M60	Design Mix	60 MPa	8700 psi
M65	Design Mix	65 MPa	9425 psi
M70	Design Mix	70 MPa	10150 psi

Types of concrete

There are generally two types of concrete in construction: –

1. Plain cement concrete construction (PCC)
2. Reinforced cement concrete construction (RCC)

PCC, Plain cement concrete as the name suggests concrete in this case is poured and casted without use of any reinforcement. Plain cement concrete (PCC) is also referred to as cement concrete or binding concrete. It is basically a mix of cement, fine aggregates and coarse aggregates. Plain cement concrete (PCC) is essentially used as concrete bed under foundation which helps ease out in levelling and placing reinforcement. Plain cement concrete (PCC) is also used as coping on walls and parapet walls. Moreover, plain cement concrete (PCC) is also used under plinth beams. Its application is basically at places where structural members are subject to compressive forces only.

Characteristics of plain cement concrete (PCC) :

• Compressive strength of plain cement concrete (PCC) is usually between 200 kilogram per sq.cm. to 500 kilograms per sq.cm.
• Tensile strength of plain cement concrete (PCC) is usually between 50 kilogram per sq.cm. to 100 kilograms per sq.cm.
• Density of plain cement concrete (PCC) is usually between 2200 kilogram per cum to 100 kilogram per cum.
• Stability of plain cement concrete (PCC) is outstanding.

An ideal plain cement concrete (PCC) should have high compressive strength. Moreover, tensile strength of plain cement concrete (PCC) is also benchmarked between 8 to 12 percent of the compressive strength and shear strength of plain cement concrete (PCC) is also benchmarked between 8 to 10 percent of the compressive strength. The compressive strength for PCC broadly depends on the factors like cement content, water cement ratio, concrete age, material quality which is used for concrete preparation, mixing method, placing of concrete on the location, concrete compaction, and curing after the casting of concrete. Plain cement concrete (PCC) should be durable enough to resist the exposed climatic conditions, along with that it should also show durability against chemical exposure. Plain cement concrete (PCC) should be workable enough for mixing, and mainly transporting and placing. Plain cement concrete (PCC) should not display segregation and bleeding. Plain cement concrete (PCC) should be fire resistant.

The various Do’s for plain cement concrete (PCC) are as follows. For the plain cement concrete (PCC) in the substructure works loose material around it mostly from side of pits, trenches should be removed, the surface should be properly dressed. In case the water table around the excavated site is high, proper dewatering should be done before PCC activity. Water cement ratio to be maintained as per the site situation and climatic condition. Shuttering work used around the sides to be done neatly and precisely.

A plain cement concrete (PCC) requires following processes for its preparation. First step is of selection and preparation of various ingredients, then second step is the mixing of the ingredients, then third step is transportation and placing, then fourth step is the compaction of the concrete, the fifth step is to do finishing of surface and finally curing of concrete. These steps are discussed in detail in the next section. A general work procedure for placing of plain cement concrete (PCC). The PCC could be placed in the desired location directly by the RMC truck, or it can also be placed using the concrete pump machine for more precision and even placement on site. One must also note that as the distance from the RMC plant increases the plain cement concrete (PCC) in transportation becomes less homogeneous. Based on the designed mix suggested for plain cement concrete (PCC), respective ingredients are prepared for mixing. Based on the quantum of plain cement concrete (PCC) required on site hand mix or machine mix or RMC is ordered. Plain cement concrete (PCC) should maintain the required water cement ratio and its workability. During the pour of plain cement concrete (PCC), claws and spade is used for spreading. Vibrator is used for compaction and placing of plain cement concrete (PCC).

The various Don’ts for plain cement concrete (PCC) includes, avoiding mixing of concrete on bare land. Without proper formwork in place PCC work should not be carried out. If height of concrete pour exceeds 1.5 mt. adequate support slopes should be made during concrete pour.

Curing of completed PCC work should be carried out for a minimum of 14 days, in best case scenarios it is advisable to do curing up to 28 days for PCC.

Note : Usually Portland pozzolona cement (PPC) is used in case of plain cement concrete (PCC). Compressive strength for plain cement concrete (PCC) varies in the range of 200 to 500 kilogram per square centimetre. Depending on the location of use size of aggregate in plain cement concrete (PCC) varies from 10mm to 40mm. For foundation work allowable plain cement concrete (PCC) slump is 75 mm to 100 mm. Based on the site requirement or as per specification proportion for plain cement concrete (PCC) is done, generally mix of ratio 1:3:6 or 1:2:4 for plain cement concrete (PCC) is used. The required quantity of water which is used against a 50 kg cement bag in plain cement concrete (PCC) mix to achieve required consistency shall be as follows. For 1:3:6 plain cement concrete (PCC) mix not more than 34 lit of water to be used, for 1:2:4 plain cement concrete (PCC) mix not more than 30 lit of water to be used, for 1:1.5:3 plain cement concrete (PCC) mix not more than 27 lit of water to be used, for 1:1:2 plain cement concrete (PCC) mix not more than 25 lit of water to be used. The plain cement concrete (PCC) is laid in layers of up to 150 mm thick. After placing of plain cement concrete (PCC) post 24 hrs start the curing activity which be done for at least seven days. The strength of plain cement concrete (PCC) is defined as the achieved compressive strength after 28 days of placing PCC.     

RCC, Reinforced cement concrete as the name suggests concrete in this case is poured and casted with the use of reinforcement. In this case the concrete is casted around the reinforcement bars to withstand the tensile forces along with bending and compressive forces in the construction elements. When the concrete is set and hardened in this case is called as reinforced concrete construction. Concrete and reinforcement both have their unique abilities, concrete resists compression whereas reinforcement resists bending and elongation. This improves ductility and in turn reduces long term deflections. Reinforced cement concrete (RCC) is used in various structures like residential buildings, commercial buildings, roads, dams, tunnels, bridges, and highways.

As we all know by now that concrete has a very high compressive strength, however we also know that concrete is low in tensile strength. In case tensile forces are involved like beams and slabs, high risk of failure is involved if only plain cement concrete (PCC) is used. However, steel displays a very high tensile strength and also has good compressive strength. Hence, a combined mix of these two materials i.e. steel and plain concrete is made called reinforced cement concrete (RCC). This combination in turn is capable of withstanding three forces which are most likely to occur on a construction structure. The three forces are compressive load, tensile stress, and shear forces. The RCC is considered to be reliable and extremely useful in construction industry.

Reinforced cement concrete (RCC) properties

• A good reinforced cement concrete (RCC) is capable of withstanding bending, tensile, compressive and shear forces.
• A good reinforced cement concrete (RCC) gives complete insulation against heat and sound.
• A good reinforced cement concrete (RCC) gives a durable structure.
• A good reinforced cement concrete (RCC) prevents corrosion of steel.
• A good reinforced cement concrete (RCC) can be moulded into desired shape.

RCC has proven to be very advantageous. RCC Structure made from good reinforced cement concrete (RCC) are stable which possess high tensile as well as compressive strength. Moreover, its maintenance is very less and highly economical also. A good reinforced cement concrete (RCC) reduces the chances of rusting and buckling. Further it possesses resistance to climatic changes and fire also.

Compatibility between steel and concrete has led to the creation of the stable composite RCC. Due to the alkaline nature of concrete whose principal component is calcium hydroxide, it helps prevent rusting of the steel reinforcement used within RCC. The bond and grip between the concrete and steel is also established easily. Another critical condition which makes the composite stable is that the coefficient of thermal expansion of that of steel and concrete is almost identical which helps prevent cracking which otherwise would have happened due to different rate of expansion.

There are various types of reinforcement used up in RCC. Various grades of steel can be used as reinforcement along with concrete for RCC. Reinforcement can be made in required shape and volume. A few common types of reinforcement are Mild steel bars, Hot rolled bars & cold rolled bars and steel fabric. Mild steel bars come in various diameters. They have their respective characteristic strength in tension as specified in IS code. These steel bars which is used as reinforcement when usually bent does not crack and show high ductility. In case of hot rolled and cold worked bars, these are specially prepared reinforcement. Hot rolled bars characteristic strength in tension is almost double that of mild steel bars. Thickness of the section in this case is also high. These bars are highly resilient which can bend in temperature up to 100 degrees Celsius without any defects, the same is not possible in ordinary mild steel bars. The cold worked steel bars usually come in stretched or twisted forms; it has elongated ribs along their length. These bars also have high characteristic strength of about 425 N/mm2 against 250 N/mm2 of mild steel bars. Such bars cannot be subject for bending and re-bending under high temperature. In case of steel fabric, it is made up of variety of bars and wires. Such type of reinforcement my include deformed steel bars, plain round wires, indented and deformed wires of cold-worked type. In this case mesh is prepared by welding together straight lengths strictly and carefully as per the specified norms, otherwise the intended mechanical properties of the reinforcement may get altered.

Placement of reinforcement before concreting as per careful design considerations is one of the most crucial activities for RCC. On site an appointed structural design consultant provides all the reinforcement details for every construction element. The location, shape, size and spacing of the reinforcement shall be different for respective construction elements like a slab or beam or column. Say for example in beam in the lower part of the cross section, it is mostly subject to higher tensile stress hence as a result more steel bars are provided in the zone. The specific details for the same is obtained by doing detail structural analysis. Likewise, reinforcements which are placed horizontal are usually tied up by stirrups at suitable intervals. These stirrups give additional support against the shear stress. For respective elements concrete covers are provided to reinforcement bars, it is essential in order to protect the reinforcement from deterioration under attack from fire, water and climatic conditions. Also, the concrete cover depends on various environmental condition, usually varies from 25mm to 80mm. Before placement of reinforcement, one should take care that it must be clear of rust, grease and dust, this ensures a better bonding between concrete and reinforcement.

RCC is actively and widely used on almost every construction site. Reinforced cement concrete (RCC) is used in various structural elements like columns, slab, beam, footing, lintel, stairs and roofs. Reinforced cement concrete (RCC) is also used for paving in high traffic areas like roads, flyover, airports etc. Various structures like multi-storey buildings, towers, underwater structures, docks, harbours, bridges, bunkers, silos, bins, water tanks and dams. Moreover, it is also used in construction of atomic plants which prevents radiation emission from the plant.

Above two types of concrete i.e., PCC and RCC are the two terms which are actively used on day-to-day basis for any construction project, be it a real estate project or infra project. Out of the various grades of concrete used, usually M15 grade is used in case of plain cement concrete (PCC) works and a minimum of M20 grade and above is used in case of reinforced cement concrete (RCC) works.

Why plain cement concrete (PCC) is provided under reinforced cement concrete (RCC) ?

The most important reason for providing plain cement concrete (PCC) is to provide a impervious rigid bed to reinforced cement concrete (RCC) before starting any work activity on site, the same is in case of masonry work wherein plain cement concrete (PCC) avoids contact with direct excavated soil. The whole idea is to create a levelled surface using plain cement concrete (PCC) and create a barrier between naked ground and reinforcement place for RCC works and also to prevent contact of water coming out from ground. Plain cement concrete (PCC) prevents weakening of RCC.

How to work out quantity of various materials for respective design mix ?

Say we consider an example of M 10, whose ratio is 1:3:6, 1 proportion of cement, 3 proportion of fine aggregate or sand and 6 proportion of coarse aggregate. Note that dry volume is equal to 1.54 times of wet volume as per standard practice. Quantity of respective material i.e. cement or sand or aggregate is obtained by using formula (= (Dry volume x cement or sand or aggregate ratio)/(sum of ratio)). Cement density is 1440 kg/m3 and a bag of cement is of 50 kg.

Say we work out cement quantity, following steps to be followed:

1. Dry volume = 1×1.54 = 1.54 m3
2. Cement = (1.54×1)/(1+3+6) = 1.54/10 = 0.154 m3
3. Weight of cement = 0.154 x 1440 = 221.76 kg
4. No. of bag = 221.76 / 50 = 4.4352 bags

Application of plain cement concrete (PCC) :- PCC pavement

As we know by now that plain cement concrete (PCC) can sustain heavy compression load. Rigid pavement made of plain cement concrete (PCC) are so named due to their capacity to sustain structural deflection under loading. This is due to high modulus of elasticity of their surface course of plain cement concrete (PCC) pavement. This rigid structure of pavement is usually composed of plain cement concrete (PCC) surface course which is again built on either subgrade or an underlying base course. Due to the relative rigidity of plain cement concrete (PCC) pavement, it distributes loads over a wide area with just one or utmost two layers. The surface course which is the top layer is the plain cement concrete (PCC) slab. The layer below the plain cement concrete (PCC) layer is called as the base course which is of aggregate or a stabilized subgrade. The layer below the base course is called as the sub base course. This layer may always not be needed. Out of all the layers the surface course made of plain cement concrete (PCC) is the stiffest w.r.to its resilient modulus and gives majority of strength. The underlying layers of the plain cement concrete (PCC) surface course have less magnitude of resilient modulus but they also play important part in strength contribution. Moreover, the also serve the purpose of drainage and frost protection.

Surface course made of plain cement concrete (PCC) which is the top most layer is in contact with traffic load. The plain cement concrete (PCC) pavement provides characteristics such as friction, noise control, smoothness and drainage. The thickness of the plain cement concrete (PCC) pavement varies depending upon the traffic load over itself. It may vary from 150 mm to 300mm for light traffic load and heavy traffic load respectively.

Immediately below the plain cement concrete (PCC) pavement is the base course. The function of the base course is to provide additional load distribution, acts as uniform support to plain cement concrete (PCC) pavement, drainage and frost resistance. It also helps prevent soil movement in subgrade due to slab pumping. These base courses are further constructed using various materials like aggregate base, stabilised aggregate or soil, lean concrete, dense graded HMA, permeable HMA.

Below the base course is the portion of pavement structure called sub base course. The primary function of the sub base course is to minimize contact of fines by preventing its penetration to the plain cement concrete (PCC) pavement structure. This layer also helps improve drainage, minimize frost action in cold weather areas, and also gives a good platform for conducting construction activity. This layer constitutes of material which are of lower quality than that of base course but better than sub grade soil. This layer is however not needed or used.

Concrete pouring procedure on site

Concrete pouring can be broadly divided into following parts: –

1. Site preparation
2. Form work erection
3. Mixing
4. Concrete placing
5. Finishing
6. Curing

Site Preparation Before concreting activity can be done on site first step is that, site has to be prepared by cleaning and clearing. Mostly earth-moving equipment’s are employed to remove trees, grass, previous structures, or anything else. Further base is prepared for pouring activity.

Formwork erection Once site is cleared formwork erection activity is started. Depending on various construction elements like footing, stub column, staircase, beam, slab etc. respective formwork are set on site. These formworks can be made up of different materials like plywood, steel, aluminium etc. Based on site requirement and feasibility suitable formwork can be used on site. Formwork should be in good condition, this in turn provides good finish concrete surface after curing. These formworks are designed to secure easy removal once the concrete is set and cured.

Mixing  In concrete mixing operation, various ingredients of concrete are mixed such as cement, fine aggregate, coarse aggregate, admixture and water together. Mixing can be done using two methods i.e. hand mixing of concrete and machine mixing of concrete. Hand mixing of concrete is usually practised for small scale works only. There are fair chances of inferior quality of concrete produced by hand mixing method. In case of mass concreting, machine mix concrete is preferred. Concrete produced from concrete mixer proves to be economical and efficient where quantity of concrete required is high.

Concrete Placing Fresh concrete is poured in the formwork for casting. Concrete level is monitored to ensure pouring is done up to the required level. Concrete vibrators are employed to achieve consistent placing of concrete and to avoid issues like honeycombing.

Finishing After the concrete is placed, the top of concrete is levelled and edged are made uniform. The surface is first screed, then the surface is evened out using trowel or float. Based on concrete finish obtained, additional trowel finish is applied to achieve smooth surface.

Curing Once the placed concrete is set and rested curing process is started. The curing process is carried out for 28 days of which first week is most important. Concrete surface can be used with light traffic in 3 to 4 days.  

Concrete quality

Usually for fresh concrete workability tests, air content measurement and setting time are the parameters which are tested to ascertain its quality. For hardened concrete common quality tests are compressive strength, tensile strength, modulus of elasticity, permeability test and In situ tests.

Various types of special concrete

1. Light weight concrete: The density of light weight concrete is anywhere between 300 to 1850 kg/m3 as compared to the conventional concrete density 2400 kg/m3. Light weight concrete helps reduce dead load of structure, lowers handling costs and improves speed of construction. The basic concept behind preparation of light weight concrete is by including air in concrete. Light weight concrete has become more popular these days due to its numerous advantages.
   1. Aerated concrete: This concrete is also made by introducing air or gas Portland cement or lime or finely crushed silicious filler slurry concrete, when the mix sets a cellular structure is formed. It is also referred as cellular concrete, gas concrete or foam concrete. This concrete possesses the properties like low density and thermal insulation properties. These are usually used in pre-fabricated structural members also.
   2. No-fines concrete: This concrete is produced by omitting fines from the concrete ingredients. No fines concrete is made up of only water, cement and coarse aggregate.
2. High density concrete: This type of concrete possesses density of the range of 3360 kg/m3 to 3840 kg/m3. In order to achieve higher densities up to 5280 kg/m3 iron is used as both fine aggregate and coarse aggregate. This type of concrete is usually used in construction of structures for radiation shield. These are used in structures like nuclear reactors, particle accelerator, industrial radiography, X-ray and Gamma ray centres.  
3. Sulphur Infiltrated concrete: Sulphur is one of the newest composites used in construction. This type of concrete possesses greater strength ability, greater water impermeability and greater resistance to corrosion. This concrete is also used in precast industry.
4. Fibre reinforced concrete: The concrete reinforced with discrete, discontinuous and uniformly dispersed suitable fibres is called as fibre reinforced concrete. The advantages of fibre reinforced concrete its increased tensile strength, better fatigue strength and energy absorbing characteristics. Fibre reinforced concrete is used on road pavements, air fields, bridge decks, industrial flooring, canal lining, refractory linings, explosive resistant structures etc. This concrete can also be used for fabrication of precast members.
5. Cold weather concrete: The conventional methods adopted for concreting cannot be applied to the concreting works in cold weather. However, such regions in India is relatively small. The cold weather causes delay in setting and hardening, freezing and thawing. There are various precautions to be taken for concreting in sub zero temperatures. Utilisation of hydration of heat of cement and practical measures for insulation by using membrane, saw dust, burlap or hessian cloth. Suitable cement should be used like high alumina cement. Heating of material of concrete using economical measures. Admixtures to be adopted for use in the concrete mix. Electrical heating of concrete mass and use of air entraining agents. These measures will ensure concreting in cold weather.
6. Hot weather concrete: Various problems are encountered when concreting in hot weather like rapid rate of hydration of cement, quick setting, rapid evaporation of water used in mixing of the mix, less time for finishing, difficulty in continuous and uninterrupted curing. To improve the quality of concrete various measures should be adopted to keep the temperature of concrete as low as possible.
7. Prepacked concrete: Conventionally concrete is mixed in concrete mixer, but in this case a concrete member can be constructed by first placing the coarse aggregate in the formwork mould and then a specially prepared mortar is used for grouting in the member. This type of method is used when the reinforcement is complicated or any other members are also incorporated for casting.
8. Vacuum concrete: This process helps achieve high strength in a high workable concrete.
9. Self-compacting concrete: This concrete minimizes or eliminates the use of vibrator for compaction. This concrete provides advantages like increased speed in construction, reduction in manpower used for the activity, good surface finish, improved durability, reduced noise level and safer working environment.