CONCRETE

Concrete is an artificial material obtained by mixing together cementing material, coarse aggregate, fine aggregate and water.

If cement is used as cementing material in the mix, it is known as plain cement concrete.

If steel rods/bars are embedded in the plain cement concrete, then it is called reinforced cement concrete abbreviated as R.C.C

Instead of cement, lime is used as cementing material in the mix, the resulting mass is termed as lime concrete.

INGREDIENTS OF CONCRETE The ingredients of concrete are as follows:

Cement Fine aggregates (sand) Coarse aggregate (broken stone or gravel) Water And additives are also used to improve the quality of concrete

like plasticizers, retarders, accelerators, air-entraining agents etc.

CONCRETE

CEMENT Cement is a very important binding material. The name cement refers to the material manufactured from limestone

and clay and made available in powder form, which when mixed with water can set to a hard durable mass even under water.

It is always measured by weight, It is available in bags

MANUFACTURE OF PORTLAND CEMENT Cement is manufactured from limestone and clay. The limestone is crushed and the clay is made into a liquid form by

addition of water. They are again mixed together and finely ground. The mixture is called the slurry. It is then conveyed into tanks and then to a long cylinderical rotary

kiln where it is gradually heated to high temperature of 1300 to 1500 deg.

In this process, it is converted to clinker (fused limps), which is then ground in ball mills and tube mills to an extremely fine powder to form cement.

USES OF CONCRETE Concrete is used to make pavements, pipes, architectural

structures, foundations, walls, beams, columns, slabs etc.

INGREDIENTS AND FUNCTIONS OF CEMENTa) Alumina or Clay It is responsible for the setting action of the cement. Larger the amount of alumina present in the cement it will start

setting. Excess quantity of alumina weekens the cement.

b) Silica It is also responsible for imparting strength to the cement.

c) Lime It is also responsible for the strength characteristics of the

cement.

d) Iron Oxide This ingredient mainly imparts colour to the cement It also goes into chemical combination and helps increase

strength and hardness to the cement.

e) Magnesium Oxide (MgO) It also imparts strength and hardness to the cement, but

only present in small amount.

6. Calcium Sulphate(CaSO4) OR Gypsum This ingredient is used to retard or prolong the setting

action of the cement.

CONSTITUENTS OF CEMENT During burning and fusion in rotary kiln, all the ingredients

of cement undergo chemical combination and form what are known as Bogue Compounds in the finely ground cement.

S.No.

Bogue compound Abbrev. as

1. Tricalcium silicate (3CaOSiO2) C3S

2. Di calcium silicate (2CaOSiO2) C2S

3. Tricalcium Aluminate (3CaOAl2O3) C3A

4. •Tetracalcium Alumino Ferrite (4CaOAl2O3Fe2O3)

C4AF

i. After the addition of water in cement Tri-Calcium Aluminate (C3A) is the first compound which starts hydrating and is responsible for early setting cement.

C3A does not contribute any strength to the cement but it is mainly responsible for setting action of the cement.

ii. After C3A, hydration of Tri-Calcium Silicate (C3S) starts.

It is C3S whose hydration os responsible for the initial strength of the cement.

The strength acquired during first 7days is mostly due to hydration of C3S.

iii. Di-calcium silicate (C2S) The strength of concrete after 7 days, mainly due to

hydration of C2S. C2S starts contributing strength after 7days of its

placement and continues for about one year.

iv. Tetra-calcium alumino ferrite is more or less inactive compound and does not play any significant role in setting and hardening properties of the cement.

WHAT IS HYDRATION? Chemical reaction between water and cement

compound is known as hydration.

1. SETTING OF CEMENT Is changing plastic state of cement to stiff solid state

which is not so strong enough and has a very small compressive strength.

2. HARDENING OF THE CEMENT Is the rate of gain of strength. With the passage of time

as the hydration proceeds, the compressive strength of the product goes on increasing.

3. INITIAL SETTING Initial setting is a stage in the process of hardening,

(stage when the cement started gaining strength)

In short it can be said, Initial setting of the cement is controlled by tricalcium

aluminate, C3A. Early strength is controlled by tricalcium silicate, C3S Strength of cement developing after 7-8days of its

placement is dependent on the amount of dicalcium silicate.

LABORATORY TESTS FOR PORTLAND CEMENT1. Fineness Test Is done to verify the standard of grinding of cement. The

rate of hydration of cement depends on its fineness Fineness of cement is determined by sieve test. 100gms of cement is taken and sieved through The residue left over the sieve should not be more than 10%

of the original weight.

2. COMPRESSIVE STRENGTH TEST. Test is carried out to determine the compressive strength of

the cement. Cement and sand are taken in 1:3 ratio and paste is prepared

using water. Paste is filled in 76mm cube mould and vibrated on a

vibrating machine for 2min. Freshly moulded cubes are kept aside for 24 hous. After 24 hrs they are shifted to water bath for curing. The cubes are tested under compression machine (3 cubes

tested) The average of three will give the comp. strength of cube at

1, 7, 14 and 28th day.

3. TENSILE STRENGTH TEST To determine the tensile strength of the cement.

4. INITIAL SETTING TIME TEST AND FINAL SETTING TIME Vicate’s apparatus is used. An apparatus consists of a frame to which a movable rod is attached. An indicator is attached to the movable rod with the help of which

penetration of the needle can be determined.

a. Initial Setting Time Mould is filled with cement paste. The needle is gently brought in contact with the cement paste. In the beginning the needle penetrates completely. After sometime the needle is again brought in contact with the paste

and released. The process is repeated until the needles penetrates upto 5mm

measured from bottom of the mould.

Definition of initial setting time – the interval between the addition of water to cement and the stage when the needle ceases to penetrate 5mm layer of paste measured from the bottom, is known as initial setting time of cement.

In about 30 minutes, it is said to have attained its initial set

b) Final Setting Time Cement paste of standard consistency is prepared. A special needle F is fixed at the bottom of vicate

apparatus. The needle is brought in contact and released. The time upto which the needle makes an impression on

the test block of paste, but it will not penetrate more than 0.5mm.

Definition of final setting time– the time interval between the moment water was added

to cement and the moment, the stage is reached, when the needle will not penetrate more than 0.5mm is taken as the final set.

In about 10 hours, it becomes rock hard and is said to have reached its final set.

TYPES OF CEMENTCement is specified by its grade ie th mortar cube

strength in N/mm2 in 28days.

1. ORDINARY PORTLAND CEMENT The initial setting time of OPC is not less than 30minutes

and final setting time is not more than 10 hours. The rate of gaining strength, heat of hydration, resistance

to cracking are moderate. Used in pavements, RCC works. OPC is available in three gradesI. Grade 33 – designated as C-33II. Grade 43 – Designated as C-43III. Grade 53 – designated as C-53 E.g Grade 33 Cement designated as C-33 where C stands

for cement with standard mortar cube strength of 33N/mm2 in 28days.

2. RAPID HARDENING CEMENT (HIGH EARLY STRENGTH) Manufacturing is same as that of OPC but it differs only in

chemical composition, i.e. clinkering at high temperature and high degree of fineness in grinding is achieved.

It attains high strength in less time. Advantage of this – it enables the removal of form work at earliest. Used for highway slabs, which is to be opened to the traffic at the

earliest. Also used for precast elements.

3. LOW HEAT CEMENT In case of mass concrete structures such as dams, bridges etc. rate

of loss of heat of hydration from the surface is much lower than it is generated.

This causes rise in temperature in the mass of concrete, may develop cracks.

Under such circumstances low heat cements are used.

This cement is proportioned in such a way that C3A and C3S are formed in lesser amount, C2S is formed in increased amount, which result in reduced rate of evolution of heat of hydration.

Low heat cement has not only slower rate of heat generation, but also greater resistance to cracking.

4. AIR ENTRAINING AGENT. Just the ordinary cement in which air entraining agents have been

added, such as vinsol, resins etc. Air entraining agents are added during the process of grinding the

clinkers. Concrete made with such a cement contains minutes, well

distributed air bubbles. Such a cement is resistant to severe frost action.

5. WATER REPELLENT/WATER PROOFING CEMENT This cement is used where water tight conditions are predominant.

Small amount of water proofing compounds (Alor Ca) stearate is mixed with an ordinary cement.

6. WHITE AND COLOURED CEMENT It is an ordinary cement having pure white colour. Iron oxide is limited to less than 1%. Coloured cements are produced by addition of suitable mineral

pigment to ordinary or white cement.

7. SULPHATE RESISTANT CEMENT It is resistant to sulphate attack . In this the amount of C3A is reduced.

8. QUICK SETTING CEMENT When concrete has to be laid under water, this cement is

advantageous. Setting action of this cement starts within 5mins. And it

becomes stone hard in less than one hour. This property is achieved by adding small % of aluminium

sulphate and also by grinding this cement much finer than ordinary cement.