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CONCRETE TECHNOLOGY
Why Concrete?
Chemical composition of cement.
Portland cement is made by heating a mixture of limestone and clay, or other materials of similar bulk composition and sufficient reactivity, ultimately to a temperature of about 1450°C. Partial fusion occurs, and nodules of clinker are produced. The clinker is mixed with a few per cent of calcium sulfate and finely ground, to make the cement. The calcium sulfate controls the rate of set and influences the rate of strength development. It is commonly described as gypsum, but this may be partly or wholly replaced by other forms of calcium sulfate. Some specificationsallow the addition of other materials at the grinding stage. The clinker typically has a composition in the region of 67% CaO, 22% SiO2, 5% A12O3, 3% Fe2O3 and 3% other components, and normally contains four major phases, called alite, belite, aluminate and ferrite, which is also called chemical composition of cement
Compound Composition of Cement:
The major compounds of ordinary Portland cement are
i) Tricalcium Silicate (C3S)- 3 CaO SiO2
ii) Di Calcium Silicate (C2S)- 2 CaOSiO2
iii) TriCalcium Aluminate ( C3A)- 3CaOAl2O3
iv) Tetracalcium Aluminoferrite (C3AF)- 4CaO Al2O3 Fe2O3
Functions of ingredients of cement:
The functions of various ingredients of an ordinary Portland cement are given below:
1.Lime(CaO):
It is the major constituent of cement and its proportion is need to be maintained carefully. The lime in excess makes the cement unsound and causes the cement to expand and
disintegrate. If lime is in deficiency, the strength of cement is decreased and cement sets quickly. If lime is in right proportion, it makes the cement sound and strong.
2.Silica(SiO2):
It is an important ingredient of cement and imparts strength to the cement due to the formation of dicalcium and tricalcium silicates.
Silica in excess provides greater strength to the cement but at the same time prolongs its setting time.
3.Alumina(Al2O3):
It imparts quick setting quality to the cement. It acts as a flux and lowers the clinkering temperature. Alumina in excess reduces the strength of cement.
4.Calcium Sulphate(CaSO4):
It is present in the form of gypsum. It helps in increasing the initial setting time of cement.
5.Iron Oxide(Fe2O3):
It also provides colour , hardness , and strength to the cement. It also helps the fusion of raw materials during the manufacture of cement.
6.Magnesium Oxide(MgO):
This ingredient, if present in small amount, imparts hardness and colour to the cement. Magnesium oxide in excess makes the cement unsound.
7.Sulphur trioxide(SO3):
It is present in very small quantity, it makes the cement sound. If present in excess , it causes the cement to become unsound.
8.Alkalies:
They should be present in small quantities. Alkalies in excess will cause efflorescence.
Physical Characteristics of Cement:
The important physical properties to studied are: i) Fineness ii)Setting time iii)Soundness iv)Compressive strength.
Based on the above properties it is possible to compare the quality of cement from different factories.
i) Fineness :
The fineness of cement is a measure of the size of particles of cement. Finer the cement the rate of chemical reaction or rate of hydration is more, since more surface area is available for chemical reaction. This results in greater strength development. If it is fine beyond a certain limit its cementative property reduces due to pre hydration by atmospheric moisture. As per Indian Standards the residue should not exceed 10% when the cement is sieved in IS Sieve no.9. When tested for fineness by Blaine’s air permeability method as described in IS 4031 ( Part 2 ) : 1488. the specific surface of cement shall not be less than 225 m2/kg.
ii) Setting Time: The time at which the cement paste loses its plasticity after the addition of water
is known as initial setting time. The time corresponding to the paste becoming a hard mass is known as final setting time. The setting time of the cements, when tested by the Vicat apparatus method described in IS 4931 ( Part 5 ) : 1988 shall conform to the following requirements:
a) Initial setting time in minutes, not less than 30; and b) Final setting time in minutes, not more than 600.
iii) Soundness:
After the cement has set it should not undergo large change in volume. The cement contains free lime even after final grinding. This lime hydrates very slowly. The free lime is covered by a thin film of cement which also prevents direct contact between lime and water. After the paste has set , moisture penetrates to the free lime and hydration states and the slaked lime occupies larger volume . Hence expansion takes place. Unsoundness due to the presence of MgO is similar to that of lime . Unsoundness may be reduced by :
i) thorough mixing ii)Fine grinding ,iii) Allowing the cement to aerate for several days, and iv ) limiting MgO content to less than 0.5%. When tested by ‘Le Chatelier’ method and autoclave teat described in IS 4031 (Part 3 ) : 1988, un aerated cement shall not have an expansion of more than 10 mm and 0’8 percent, respectively.
iv) Compressive Strength: It is one of the important properties. Rarely pure cement is tested for compressive
strength. The average compressive strength of at least three mortar cubes ( area of face 50 cm2 ) composed of one part of cement, three parts of standard sand ( conforming to IS 650 : 1966 ) by mass and (P/4 +3.0 ) percent ( of combined mass of cement plus sand 1 water and prepared, stored and tested in the manner described in IS 4031 ( Part 6 ): 1988 shall be as follows:
a) 72±1 hour : not less than 16 MPa, b) 168±2 hours : not less than 22 MPa, and c) 672±4 hours : not less than 33 MPa NOTE--P is the percentage of Water required to produce 3a paste of standard
consistency (The quantity of water required to produce a paste of standard consistency, to be used for the determination of water content of mortar for compressive strength tests and for the determination of soundness and setting time, shall be obtained by the method described in IS 4031 ( Part 4 ) : 1988.
Uses of cement:
Following are the uses of cement:
1.It is used in cement mortar for masonry work, plastering, pointing, etc.
2.It is used for making joints for pipes, drains, etc .
3.It is used in concrete for laying floors, roofs and constructing lintels, beams, stairs, pillars, etc.
4.It is employed for manufacturing precast pipes, piulkles, fencing, posts etc.
5.It is used in the construction of important engineering structures such as
bridges, culverts, dams, tunnels, light houses etc.
Grade of Cements
There are at present three grades of cement available in the market as detailed below:
Grades Compressive strength after 28 days
Grade 33 33 N/mm2 (33 M Pa)Grade 43 43 N/mm2 (43 M Pa)Grade 53 53 N/mm2 (53 M Pa)
M Pa = Mega Pascal .. N/mm2 unit for Force/ Stress measurement. All the Cements produced in India conform to Indian Standard Specifications prescribed by Bureau of Indian Standards (BIS)
The Grade of Cement is named after 28 days compressive strength cement is N/mm2.
Details of Manufacture
(i) Collection of raw materials
(ii) Crushing, grinding and mixing of raw materials.
(iii) Burning.
(iv) Grinding of clinker.
I. TESTS ON FRESH CONCRETE
1.Slump Test: Test on workability of concrete:
This method of test specifies the procedure to be adopted, either in the laboratory or during the progress of work in the field, for determining, by the slump test, the consistency of concrete where the nominal maximum size of the aggregate does not exceed 38 mm.
Apparatus :
a) Mould - The mould for the test specimen shall be in the form of the frustum of a cone having the following internal dimensions:
Dimensions
Bottom diameter : 20 cm
Top diameter : 10 cm
Height : 30 cm The apparatus is as shown in the figure.
b)Tamping rod - The tamping rod shall be of steel or other suitable material, 16 mm in diameter, 0.6 m long and rounded at one end.
Procedure:i. The internal surface of the slump cone shall be thoroughly cleaned and should be free from any set concrete before commencing the test. The mould should be placed on smooth horizontal,
rigid and non – absorbent surface such as carefully levelled metal plate. The mould is firmly held in position while filling it.ii. The mould should be filled in 4 layers each approximately one quarter of the height of mould. Each layer shall be tamped with 25 blows. The stroke should be distributed in a uniform manner over the cross section of mould. For the 2nd and subsequent layers tamping rod should penetrate into underlying layer. The bottom layer should be tamped throughout its depth.iii. After the top layer has been rodded the concrete shall be struck off level with trowel or rod. The mortar, which may have leaked out between mould and base plate, shall be cleaned away.iv. The mould shall be removed from concrete immediately by raising it slowly and carefully in vertical direction. This will allow the concrete to subside and the slump shall be measured immediately by determining the difference between height of mould and that of highest point of slumped concrete specimen. The above operation shall be carried out in a place free from shock or vibration. and within a period of two minutes after sampling. Slump: - The slump measured shall be recorded in terms of millimeters of subsidence of the specimen during the test. Any slump specimen which collapses or shears off laterally gives incorrect result and if this occurs the test shall be repeated with another sample. If, in the repeat test also, the specimen should shear, the slump .shall be measured and the fact that the specimen sheared, shall be recorded.
Limitation :
1. It is not suitable for a concrete in which maximum size of the aggregate is more than 40 mm.
2. Slump occurs in the case of plastic mixes only and in case of dry mi8xes slump does not occur.
3. Very wet concretes produce what is called collapse slump.
4.There is no direct relationship between the workability as defined above and the value of slump.
5. Many shapes of slump can occur and it is difficult to decide which is the correct value.
Advantages:
1. The apparatus is cheap, portable and suitable for site use.
2. The difference in water content of the successive batches of concrete of the same identical mix can be easily detected.
2.Flow table
b) Flow Table Apparatus: This method of test specifies the procedure for the use of the flow table to determine the fluidity of concrete, where the nominal size of the aggregate does not exceed 38 mm.
Apparatus:
a) Mould - The mould shall be made of a smooth metal casting, as shown in Fig. in the form of he frustum of a cone with the following internal dimensions. A base 25 cm in diameter, upper surface 17 cm in diameter, and height 12 cm; the base and the top shall be open and at right angles to the axis of the cone. The mould shall be provided with handles.
Procedure - Immediately preceding the test, the table top, and inside of the mould shall be wetted and cleaned of all gritty material and the excess water removed with a rubber squeezer. The mould, centred on the table, shall be firmly held in place and filled in two layers, each approximately one-half the volume of the mould. Each layer shall be rodded with 25 strokes of a straight round metal rod 1.6 cm in diameter and 61 cm long, rounded at the lower tamping end. The strokes shall be distributed in a uniform manner over, the cross-section of the mould and shall penetrate into the underlying layer. The bottom layer shall be rodded throughout its depth. After the top layer has been rodded the surface of the concrete shall be struck off with a trowel so that the mould is exactly filled. The excess concrete which has overflowed the mould shall be removed and the area of the table outside the mould again cleaned. The mould shall be immediately removed from the concrete by a steady upward pull. The table shall then be raised and dropped 12.5 mm, 15 times in about 15 seconds. The diameter of the spread concrete shall be the average of six symmetrically distributed caliper measurements read to the nearest 5 mm.
Flow table mould and Flow table apparatus is as shown in figure.
Recording -The flow of the concrete shall be recorded as the percentage increase in diameter of the spread concrete over the base diameter of the moulded concrete, calculated from the following formula:
Flow , Percent = ((spread Diameter in cm – 25 ) / 25 ) X 100
Advantages:
• The test is simple and can be used in the field.
• The test quickly provides a direct result.
• The test is dynamic, making it especially appropriate for highly thixotropic concrete mixtures.
Disadvantages:
• The test procedure does not represent actual placement conditions—concrete is typically vibrated, not jolted.
• The test results tend to converge as the number of drops is increased. Near the end of the test, the properties of the thin layer of concrete do reflect the bulk properties of the concrete.
• The results are not given in terms of fundamental units. An analytical treatment of the test would be difficult.
3. Compacting Factor Test:
This test specifies a procedure for determining the workability of concrete, where the nominal maximum size of the aggregate does not exceed 38 mm. The test is designed primarily for use in the laboratory, but if circumstances permit, it may also be used in the field. It is more precise and sensitive than the slump test and is particularly useful for concrete mixes of very low workability as are normally used then concrete in to be compacted by vibration., the concrete may fail in slump
Procedure :The sample of concrete to be tested shall be placed gently in the upper hopper, using the hand scoop. The hopper shall be filled level with its brim and the trap-door shall be opened so that the concrete falls into the lower hopper.. Certain mixes have a tendency to stick in one or both of the hoppers. If this occurs, the concrete may be helped through by pushing the rod gently into the concrete from the top. During this process, the cylinder shall be covered by the trowels. Immediately after the concrete has come to rest, the cylinder shall be uncovered, the trap-door of the lower hopper opened, and the concrete allowed to fall into the cylinder. The excess of concrete remaining above the level of the top of the cylinder shall then be cut off by holding a trowel in each hand, with the plane of the blades horizontal, and moving them simultaneously one from each side across the top of the cylinder, at the same time keeping them pressed on the top edge of the cylinder. The outside of the cylinder shall then be wiped clean. The above operation shall be carried out at a place free from vibration or shock. The ‘weight of the concrete
1. Advantages :
1. This test gives a satisfactory performance for both wet and dry mixes.
2. This test gives more sensitive and consistent results than slump test
2. Limitations:
The only limitation seems to be that the equipment is not portable and hence can not be used at the site. in the cylinder shall then be determined to the nearest 10 g. This weight shall be known as c the weight of partially compacted concrete ‘. The cylinder shall be refilled with concrete .from the same sample in layers approximately 5 cm deep, the layers being heavily rammed or preferably vibrated so as to obtain full compaction. The top surface of the fully compacted concrete shall be carefully struck off level with the top of the cylinder. The outside of the cylinder shall then be wiped clean.
Calculation - The compacting factor is defined as the ratio of the weight of partially compacted concrete to the weight of fully compacted concrete. It shall normally be stated to the nearest second decimal place. The equipment is as shown in the figure.
3.
4.Vee bee test:.
Aim : To determine the workability of fresh concrete by using a Vee-Bee consistometer as per IS: 1199 - 1959.Vebe Consistometer
The Vebe consistometer (Bartos 1992; Scanlon 1994; Bartos, Sonebi, and Tamimi 2002) measures the remolding ability of concrete under vibration. The test results reflect the amount of energy required to remold a quantity of concrete under given vibration conditions. The Vebe consistometer is applicable to concrete with slumps less than 2 inches.
PROCEDURE
i) A conventional slump test is performed, placing the slump cone inside the cylindrical part of the consistometer.ii) The glass disc attached to the swivel arm is turned and placed on the top of the concrete in the pot.
iii) The electrical vibrator is switched on and a stop-watch is started, simultaneously.iv) Vibration is continued till the conical shape of the concrete disappears and the concrete assumes a cylindrical shape.v) When the concrete fully assumes a cylindrical shape, the stop-watch is switched off immediately. The time is noted.
REPORTING OF RESULTS
The consistency of the concrete should be expressed in VB-degrees, which is equal to the time in seconds, recorded in Para v), above.
The apparatus, shown in Figure, consists of a metal cylindrical container mounted on a vibrating table, which produces a sinusoidal vibration. In the version of the test standardized in Europe as EN 12350-3, a slump cone is placed in the center of the cylinder and filled in the same manner as in the standard slump test. After the slump cone is removed, a clear plastic disk is set atop the fresh concrete. The Vebe table is started and the time for the concrete to remold from the slump cone shape to the shape of the outer cylindrical container is recorded as a measure of consistency. The sliding clear plastic disk facilitates the determination of the end of the test.
Advantages:
• The Vebe consistometer is a dynamic test and can be used on concretes that are too dry for the slump test.
• The test device is standardized in ASTM and identified by ACI Committee 211 (2002) in its guide for proportioning low slump concrete.
• Test results are obtained directly.
Disadvantages:
• Due to the need to ensure that all vibration is kept within the test device, the size of the test device makes the Vebe consistometer generally unsuitable for field use.
• The test device only works for low slump concretes.
• No analytical treatment of the test method has been developed. Such treatment would be complex because the shear rate declines during the duration of the test as the concrete specimen changes shape.
Defects in concrete
s.no Defect Causes Suggedted solution
1 Segregation &Bleeding
Mix is lean.Over vibration Use richer mix
2 Permeability&
shrinkage
Higher water content in the mix.
Lack of compaction
Improper grading of aggregate
Reduce water cement ratio
Resort sufficient to compaction .
Use proper graded aggregate
3 Blow holes exposed on
faces
Improper mix design
Inadequate cover between reinforcement and mould face which restrains local flow of concrete between mould face and reinforcement
Lack of sufficient vibration.
Use correct water cement ratio.Use agent to improve workability if water cement ratio is to be kept low
Use adequate cover fro reinforcement .Resort to adequate mechanical vibration using vibrator head of larger circumference.(It is also to be noted that vibrator needle has no effect on the concrete below the tip of the needle).
Resort to knifing at the contact face of mould.Resort to vigorous tampling with a wooden mallet on all sides of form work during the concreting process.
4 Plastic cracks (small near horizontal crack at faces)
During compaction higher particles tends to settle down and water rises up and collects below certain points of concrete,rising higher due to arching or interlocking.This causes cracks during drying below such points.
Use cohesive mix.Place and compact concrete in layer avoiding any local point of arching.
5 Crazing(map or starry fine cracks on the
Shrinkage of surface due to surface carbonation(caused due to
Reduce water cement ratio. Use pozzollana cement.
surfaces upto 3cm long)
free lime released during hydration reacting with co2 which reduces resistance of the surface to drying shrinkage)
Resort to membrance curing to keep the surface covered.
Construction chemicals for water proofing:
i) Terrace water proofing
ii) water repellant additives for external plasters
iii) water proofing of toilets and other wet use areas
iv) Water leakage sealing of basements during construction
v) water leakage problems of leaking of basements
vi) water leakage problems of leaking toilets.
vii) Water seepage / leakage problems from external wall facades etc., etc.,
Advantages of concrete: Concrete as a construction material entails the following advantages
1. It has a high compressive strength, and the corrosive and weathering effects are minimized. Its strength equals that of hard material stone when properly prepared.
2. As Compared to other materials, concrete is economical in the long term.
3. It is durable and fire resistant and requires very little maintenance.
4. The green concrete can be easily handled and molded into any shape or size according to specifications.
5. The concrete can be pumped and hence it can be laid in the difficult positions also.
6. Concrete can even be sprayed on and filled in to the cracks for repairs by the grinding process.
7. Being strong in compression, it has limited structured applications in combination with steel reinforcement.
Disadvantages of concrete:
Following are the disadvantages of concrete:
1. Concrete is to be reinforced with steel bars or meshes since it it has low tensile strength and hence cracks easily.
2. Provisions for contraction joints has to be made to avoid the development of cracks due to drying shrinkage and moisture movement in fresh concrete.
3. In order to avoid the formation of cracks due to thermal movement ( concrete expands and contracts with the changes in temperature) , expansion joints have to be provided.
4. Concrete as a material lacks in ductility and this factor proves disadvantages with respect to earthquake resistance design.
5. Concrete is liable to disintegrate by alkali and sulphate attack.
6. Concrete undergoes creep under sustained loading resulting in reduction of pre stress in the pre stressed concrete construction.
Objective:
To learn the principles and procedures of testing Concrete and Highway materials
Concrete Chemicals (Admixtures) and Applications
Admixtures are materials other than cement, aggregate and water that are added to concrete either before or during its mixing to alter its properties, such as workability, curing temperature range, set time or color. Some admixtures have been in use for a very long time in concrete construction, such as calcium chloride to provide a cold-weather setting concrete.
Based on their functions, admixtures can be classified into the following five major categories:
Retarding admixtures Accelerating admixtures Super plasticizers Water reducing admixtures Air-entraining admixtures
Among other important admixtures that do not fit into these categories are admixtures whose functions include bonding, shrinkage reduction, damp proofing and coloring. The following paragraphs provides details on the above-mentioned categories of concrete admixtures.
Retarding Admixtures
Retarding admixtures slow down the hydration of cement, lengthening set time. Retarders are beneficially used in hot weather conditions in order to overcome accelerating effects of higher temperatures and large masses of concrete on concrete setting time. Because most retarders also act as water reducers, they are frequently called water-reducing retarders. As per chemical admixture classification by ASTM-ASTM C 494, type B is simply a retarding admixture, while type D is both retarding and water reducing, resulting in concrete with greater compressive strength because of the lower water-cement ratio.
Retarding admixtures consists of both organic and inorganic agents. Organic retardants include unrefined calcium, sodium, NH4, salts of lignosulfonic acids, hydrocarboxylic acids, and carbohydrates. Inorganic retardants include oxides of lead and zinc, phosphates, magnesium salts, fluorates and borates. As an example of a retardant’s effects on concrete properties, lignosulfate acids and hydroxylated carboxylic acids slow the initial setting time by at least an hour and no more than three hours when used at 65 to 100 degrees Fahrenheit. The concrete contractor, however, need not memorize these chemical-specific results. Given the specific job requirements and goals, the concrete supplier should offer appropriate admixtures and concrete mixes from which to choose.
Accelerating admixtures
Accelerators shorten the set time of concrete, allowing a cold-weather pour, early removal of forms, early surface finishing, and in some cases, early load application. Proper care must be taken while choosing the type and proportion of accelerators, as under most conditions, commonly used accelerators cause an increase in the drying shrinkage of concrete.
Calcium chloride is a common accelerator, used to accelerate the time of set and the rate of strength gain. It should meet the requirements of ASTM D 98. Excessive amounts of calcium chloride in concrete mix may result in rapid stiffening, increase in drying shrinkage and corrosion of reinforcement. In colder climates, calcium chloride should not be used as an anti-freeze. Large amount of calcium chloride is required to lower the freezing point of the concrete, which may ruin the concrete.
Super plasticizers
Super plasticizers, also known as plasticizers, include water-reducing admixtures. Compared to what is commonly referred to as a “water reducer” or “mid-range water reducer”, super plasticizers are “high-range water reducers”. High range water reducers are admixtures that allow large water reduction or greater flowability (as defined by the manufacturers, concrete suppliers and industry standards) without substantially slowing set time or increasing air entrainment.
Each type of super plasticizer has defined ranges for the required quantities of concrete mix ingredients, along with the corresponding effects. They can maintain a specific consistency and workability at a greatly reduced amount of water. Dosages needed vary by the particular concrete mix and type of super plasticizer used. They can also produce a high strength concrete. As with most types of admixtures, super plasticizers can affect other concrete properties as well. The specific effects, however, should be found from the manufacturer or concrete supplier.
Water reducing admixtures
Water reducing admixtures require less water to make a concrete of equal slump, or increase the slump of concrete at the same water content. They can have the side effect of changing initial set time. Water reducers are mostly used for hot weather concrete placing and to aid pumping. A water-reducer plasticizer, however, is a hygroscopic powder, which can entrain air into the concrete mix via its effect on water’s surface tension, thereby also, obtaining some of the benefits of air-entrainment (see below).
Air-entraining admixtures
Air-entraining agents entrain small air bubbles in the concrete. The major benefit of this is enhanced durability in freeze-thaw cycles, especially relevant in cold climates. While some strength loss typically accompanies increased air in concrete, it generally can be overcome by reducing the water-cement ratio via improved workability (due to the air-entraining agent itself) or through the use of other appropriate admixtures. As always, admixtures should only be combined in a concrete mix by a competent professional because some of them can interact in undesirable ways.
Bonding admixtures
Bonding admixtures including addition of compounds and materials such as polyvinyl chlorides and acetates, acrylics and butadiene-styrene co-polymers, can be used to assist in bonding new / fresh concrete with old / set concrete.
Coloring agents have become more commonly used, especially for patios and walkways. Most are surface applied and often have the additional effect of surface hardening. Such surface applied coloring admixtures generally should not be used on air-entrained concrete. Integrally colored concrete is also available.
Waterproofing and damp proofing admixtures
Water proofing and damp proofing admixtures including soaps, butyl stearate, mineral oil and asphalt emulsions, are used to decrease the amount of water penetration into the larger pores of concrete. “Antifreeze” admixtures typically are accelerators used in very high doses, with a corresponding high price, to achieve a very fast set-time, though they do not have properties to protect against freezing on their own. However, in general, these are not used for residential work.
Compressive strength of concrete: Out of many test applied to the concrete, this is the utmost important which gives an idea about all the characteristics of concrete. By this single test one judge that whether Concreting has been done properly or not. For cube test two types of specimens either cubes of 15 cm X 15 cm X 15 cm or 10cm X 10 cm x 10 cm depending upon the size of aggregate are used. For most of the works cubical moulds of size 15 cm x 15cm x 15 cm are commonly used.
This concrete is poured in the mould and tempered properly so as not to have any voids. After 24 hours these moulds are removed and test specimens are put in water for curing. The top surface of these specimen should be made even and smooth. This is done by putting cement paste and spreading smoothly on whole area of specimen.
These specimens are tested by compression testing machine after 7 days curing or 28 days curing. Load should be applied gradually at the rate of 140 kg/cm2 per minute till the Specimens fails. Load at the failure divided by area of specimen gives the compressive strength of concrete.
Following are the procedure for Compressive strength test of Concrete Cubes
APPARATUS
Compression testing machine
PREPARATION OF CUBE SPECIMENS
The proportion and material for making these test specimens are from the same concrete used in the field.
SPECIMEN
6 cubes of 15 cm size Mix. M15 or above
MIXING
Mix the concrete either by hand or in a laboratory batch mixer
HAND MIXING
(i)Mix the cement and fine aggregate on a water tight none-absorbent platform until the mixture is thoroughly blended and is of uniform color
(ii)Add the coarse aggregate and mix with cement and fine aggregate until the coarse aggregate is uniformly distributed throughout the batch
(iii)Add water and mix it until the concrete appears to be homogeneous and of the desired consistency
SAMPLING
(i) Clean the mounds and apply oil
(ii) Fill the concrete in the molds in layers approximately 5cm thick
(iii) Compact each layer with not less than 35strokes per layer using a tamping rod (steel bar 16mm diameter and 60cm long, bullet pointed at lower end)
(iv) Level the top surface and smoothen it with a trowel
CURING
The test specimens are stored in moist air for 24hours and after this period the specimens are marked and removed from the molds and kept submerged in clear fresh water until taken out prior to test.
PRECAUTIONS
The water for curing should be tested every 7days and the temperature of water must be at 27+-2oC.
PROCEDURE
(I) Remove the specimen from water after specified curing time and wipe out excess water from the surface.
(II) Take the dimension of the specimen to the nearest 0.2m
(III) Clean the bearing surface of the testing machine
(IV) Place the specimen in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast.
(V) Align the specimen centrally on the base plate of the machine.
(VI) Rotate the movable portion gently by hand so that it touches the top surface of the specimen.
(VII) Apply the load gradually without shock and continuously at the rate of 140kg/cm2/minute till the specimen fails
(VIII) Record the maximum load and note any unusual features in the type of failure.
NOTE
Minimum three specimens should be tested at each selected age. If strength of any specimen varies by more than 15 per cent of average strength, results of such specimen should be rejected. Average of there specimens gives the crushing strength of concrete. The strength requirements of concrete.
CALCULATIONS
Size of the cube =15cm x15cm x15cm
Area of the specimen (calculated from the mean size of the specimen )=225cm2
Characteristic compressive strength(f ck)at 7 days =
Expected maximum load =fck x area x f.s
Range to be selected is …………………..
Similar calculation should be done for 28 day compressive strength
Maximum load applied =……….tones = ………….N
Compressive strength = (Load in N/ Area in mm2)=……………N/mm2
=……………………….N/mm2
REPORT
a) Identification mark
b) Date of test
c) Age of specimen
d) Curing conditions, including date of manufacture of specimen
f) Appearance of fractured faces of concrete and the type of fracture if they are unusual
RESULT
Average compressive strength of the concrete cube = ………….N/ mm2 (at 7 days)
Average compressive strength of the concrete cube =………. N/mm2 (at 28 days)
Percentage strength of concrete at various ages:
The strength of concrete increases with age. Table shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.
Age Strength per cent1 day 16%3 days 40%7 days 65%14 days 90%28 days 99%
1. Compressive strength
AIM: To determine the compressive strength of concrete specimens- Cube & Cylinder as per IS: 516 - 1959.
APPARATUSi) Compression testing machine conforming to IS: 516 – 1959
AGE AT TESTTests should be done at recognized ages of the test specimens, usually being 7 and 28 days. The ages should be calculated from the time of the addition of water to the drying of ingredients.NUMBER OF SPECIMENSAt least three specimens, preferably from different batches, should be taken for testing at each selected age.PROCEDUREi) The specimens, prepared according to IS: 516 - 1959 and stored in water, should be tested immediately on removal from the water and while still in wet condition. Specimens when received dry should be kept in water for 24hrs. before they are taken for testing. The dimensions of the specimens, to the nearest 0.2mm and their weight should be noted before testing.ii) The bearing surfaces of the compression testing machine should be wiped clean and any loose sand or other material removed from the surfaces of the specimen, which would be in contact with the compression platens.iii) In the case a of cubical specimen, the specimen should be placed in the machine in such a manner that the load could be applied to the opposite sides of the cubes, not to the top and the bottom. The axis of the specimen should be carefully aligned with the centre of thrust of the spherically seated platen. No packing should be used between the faces of the test specimen and the steel platen of the testing machine. As the spherically seated block is brought to rest on the specimen, the movable portion should be rotated gently by hand so that uniform seating is obtained.
iv) The load should be applied without shock and increased continuously at a rate of approximately 140kg/sq.cm/minute until the resistance of the specimen to the increasing load breaks down and no greater load can be sustained. The maximum load applied to the specimen should then be recorded and the appearance of the concrete and any unusual features in the type of failure should be noted.CALCULATIONThe measured compressive strength of the specimen should be calculated by dividing the maximum load applied to the specimen during the test by the cross - sectional area, calculated from the mean dimensions of the section and should be expressed to the nearest kg/sq.cm. An average of three values should be taken as the representative of the batch, provided the individual variation is not more than ±15% of the average. Otherwise repeat tests should be done.A correction factor according to the height/diameter ratio of the specimen after capping should be obtained from the curve given below:-The product of this correction factor and the measured compressive strength is known as the corrected compressive strength, this being the equivalent strength of a cylinder having a height/diameter ratio of two. The equivalent cube strength of the concrete should be determined by multiplying the corrected cylinder strength by 1.25.REPORTING OF RESULTSThe following information should be included in the report on each test specimen:i) identification mark ii) date of testiii) age of specimeniv) curing conditions, including date of manufacture of specimen
v) weight of specimenvi) dimensions of specimenvii) cross-sectional areaviii) maximum loadix) compressive strengthx) appearance of fractured faces of concrete and type of fracture, if unusual.
TYPES OF CEMENT, ITS COMPOSITION AND USES
The following are the types of cement that are in practice:
1. Rapid Hardening Cement
2. Quick setting cement
3. Low Heat Cement
4. Sulphates resisting cement
5. Blast Furnace Slag Cement
6. High Alumina Cement
7. White Cement
8. Coloured cement
9. Pozzolanic Cement
10. Air Entraining Cement
11. Hydrographic cement
Table below shows different types of cement, their composition and uses:
Types of Cement Composition Purpose
Rapid Hardening Cement
Increased Lime content Attains high strength in early days it is used in concrete where form work are removed at an early stage.
Quick setting cement
Small percentage of aluminium sulphate as an accelerator and reducing percentage of Gypsum with fine grinding
Used in works is to be completed in very short period and concreting in static and running water
Low Heat Cement Manufactured by reducing tri-calcium aluminate
It is used in massive concrete construction like gravity dams
Sulphates resisting Cement
It is prepared by maintaining the percentage of tricalcium aluminate below 6% which
It is used in construction exposed to severe sulphate action by water and soil in places like canals linings,
increases power against sulphates
culverts, retaining walls, siphons etc.,
Blast Furnace Slag Cement
It is obtained by grinding the clinkers with about 60% slag and resembles more or less in properties of Portland cement
It can used for works economic considerations is predominant.
High Alumina Cement
It is obtained by melting mixture of bauxite and lime and grinding with the clinker it is rapid hardening cement with initial and final setting time of about 3.5 and 5 hours respectively
It is used in works where concrete is subjected to high temperatures, frost, and acidic action.
White Cement It is prepared from raw materials free from Iron oxide.
It is more costly and is used for architectural purposes such as pre-cast curtain wall and facing panels, terrazzo surface etc.,
Coloured cement It is produced by mixing mineral pigments with ordinary cement.
They are widely used for decorative works in floors
Pozzolanic Cement It is prepared by grindin pozzolanic clinker with Portland cement
It is used in marine structures, sewage works, sewage works and for laying concrete under water such as bridges, piers, dams etc.,
Air Entraining Cement
It is produced by adding indigenous air entraining agents such as resins, glues, sodium salts of Sulphates etc during the grinding of clinker.
This type of cement is specially suited to improve the workability with smaller water cement ratio and to improve frost resistance of concrete.
Hydrographic cement
It is prepared by mixing water repelling chemicals
This cement has high workability and strength
What is mix design?
Concrete mix design is the method of correct proportioning of ingredients of concrete, in order to optimise the above properties of concrete as per site requirements. In other words, we determine the relative proportions of ingredients of concrete to achieve desired strength & workability in a most economical way.
Concrete is an extremely versatile building material because, it can be designed for strength
ranging from M10 (10Mpa) to M100 (100 Mpa) and workability ranging from 0 mm slump to 150 mm slump. In all these cases the basic ingredients of concrete are the same, but it is their relative proportioning that makes the difference
Basic Ingredients of Concrete: -
1. Cement – It is the basic binding material in concrete.
2. Water – It hydrates cement and also makes concrete workable.
3. Coarse Aggregate – It is the basic building component of concrete.
4. Fine Aggregate – Along with cement paste it forms mortar grout and fills the voids in the coarse aggregates.
5. Admixtures – They enhance certain properties of concrete e.g. gain of strength, workability, setting properties, imperviousness etc Concrete needs to be designed for certain properties in the plastic stage as well as in the hardened stage.
Basic steps in mix design are as follows:
a. Find the target mean strength.
b. Determine the curve of cement based on its strength.
c. Determine water/cement ratio.
d. Determine cement content.
e. Determine fine and coarse aggregate proportions
Illustration:
Consider a mix design for M30 grade of concrete, having moderate workability (Slump range 50mm to 75mm).
Material Properties
Cement 53 grade (Although, actual 28 days compressive strength = 63 N/mm2)
Fine aggregate –
FM. = 3.26 (Zone I ) 600 micron passing = 32 %
Specific gravity = 2.75
Coarse aggregate
20mm - Specific gravity - 2.95
Dry Rodded bulk density – 1600 Kg/m3
10mm - Specific gravity 2.86
Dry Rodded bulk density – 1700 Kg/m3
2.1 Find the target mean strength
Concrete is designed for strength higher than characteristic strength as a margin for statistical variation in results and variation in degree of control exercised at site. This higher strength is defined as the target mean strength. It is calculated as follows:
Target mean strength = Characteristic strength + K * s
K= Himsworth Coefficient is taken as 1.65 for 5 % probability of failure.
s = Standard deviation
The values of s are given in IS 10262 for fair, good and very good degree of control. However, IS 456-2000 has given revised values of s to be considered for mix design. Better the degree of control lesser is the value of s and lower is the target mean strength. In other words, the ‘margin’ kept over characteristic strength is more for fair degree of control to that of good degree of control.
Say for M30 grade of concrete, K=1.65 (for 5% failure) and Standard Deviation _ = 5 N / mm2.
Target Mean Strength = 30 + 1.65 * 5 = 38.25 N/mm2
2.2 Determine the curve of cement based on its strength.
The strength of cement is determined either by conventional methods given in IS 4031- 1988 Part 6 or by accelerated curing reference mix method mentioned in IS 10262 – 1982 page15 (Appendix B Clause 3.1.1.) The cement is classified into various curves based on the strength of cement.
Curve Strength of Cement (N/mm2)
A 31.9 to 36.8 N/mm2
B 36.8 to 41.7 N/mm2
C 41.7 to 46.6 N/mm2
D 46.6 to 51.5 N/mm2
E 51.5 to 56.4 N/mm2
F 56.4 to 61.3 N/mm2
After selecting the appropriate curve based on the strength of cement, water/cement ratio is interpolated for a given target mean strength.
2.3 Determine water/cement ratio
The relation between Target Mean Strength and water cement ratio for different cement curves is given in IS 10262
Once the cement curve is fixed, water/cement ratio required for achieving the target mean strength can be interpolated
For Example
For F curve and Target Mean Strength of 37
The water cement ratio is 0.48
For F curve and target mean strength of 38.25
The water cement ratio is 0.46
All the mix design methods follow same procedure up to this stage
2.4 Finding cement content
Most of the mix design methods find cement content with following formula:
Weight of Water per m3
Water /cement ratio = Weight of cement per m3
Weight of Water per m3
Weight of cement = Water/cement ratio
Weight of water required per m3 also called as water demand.
Water demand depends on:
i. Required Workability of concrete: Higher the workability required greater is the water demand.
ii. Aggregate properties: Fineness and silt content of fine aggregate, size, shape and flakiness of coarse aggregate, type of aggregate e.g. crushed, uncrushed.
iii. Use of admixtures: Plasticizers will reduce the water demand Different mix design methods give empirical relations to find the water demand
Say, we want to find water demand for 20 mm maximum size of crushed aggregate and natural sand of zone II (F.M. = 3.0) for a slump range of 60 to 80 mm (compaction factor = 0.9)
A. IS method
Water Demand =186 lit for compaction factor of 0.8
Add 3% water for incremental compaction factor of 0.1
Water demand = 186 + 5.58 = 191.58 Lit
Cement Content =191.58/0.46 = 416.47Kg/m3
C. ACI Method
Water demand for 30 to 50mm Slump =185 Lit
Water demand for 80 to 100 mm Slump = 200 Lit
Water demand for 50 to 80 mm can be interpolated as average of the above =192.5
Cement Content = 192.5 /0.46 = 418.47
Determine fine and coarse aggregate content:
The fine aggregate to coarse aggregate ratio is determined in different methods as follows:
A. IS method
Sand % by volume for zone II sand, compaction factor 0.8 and water cement ratio 0.6 for 20mm down coarse aggregate = 35%
Corrections
Correction for zone 1 sand = +1.5%
Correction for water/cement ratio of 0.46= - 3%
Net Sand content = 35=1.5-3-33.5%
Weights of fine and coarse aggregates are calculated as
V = (W +C/Sc + 1/p(fa/Sfa) ) x 1/1000
V = (W +C/Sc + 1/1-p x (Ca/Sca) ) x 1/1000
V = Absolute volume of fresh concrete i.e. (gross volume –volume of entrapped Air)
= 1-.02=0.98
W = water demand = 190Lit
C = cement content = 416Kg
p = ratio of fine aggregate to total aggregate =0.335
fa = total quantity of fine aggregate in Kg per m3
ca = total quantity of coarse aggregate in Kg per m3
Sc = Specific Gravity of Cement =3.15
Sfa = Specific gravity of fine aggregate =2.75
Sca = Combined Specific Gravity of Coarse aggregate (Assuming 30% of coarse aggregate is 10mm down aggregate =2.90 x 0.7 + 2.86x 0.3=2.89
Fa= 606 Kg/m3
Ca =1278Kg/m3
ACI Method
For F.M = 3.0 Volume of dry rodded coarse aggregate = 0.6
For F.M = 3.26. Volume of dry rodded coarse aggregate per m3 can be extrapolated as = 0.58
Dry rodded density of coarse aggregate is say *1800 kg/m3
Total coarse aggregate content = 1800 x 0.5 = 1044 Kg.
First estimate of weight of fresh concrete is calculated from table2 as =2355Kg/m3. (This plastic density is low for concretes with basaltic aggregates.)
Total sand content per m3 = 2355 - 416 - 192.5 -1044 = 702.5 Kg per m3
The above proportions need to be reworked on actual plastic density of concrete
* should be dry rodded density of combined or graded coarse aggregate.
Problem:
Design a mix with the following stipulations. Characteristic strength at 28 days is 25 N/mm2.
Maximum size of concrete used 20mm. Degree of workability of slump is 80 – 130 mm. Design quality control is good and type of exposure severe.
Technical data on materials used are Cement used Brila SRC
Specific gravity of cement 3.15
Specific gravity of coarse Aggregate 2.88
Specific gravity of fine aggregate 2.62 Water absorption of coarse aggregate 0.70% Water absorption of fine aggregate 0.92% Fineness modulus of sand used is 2.34
CA:FA = 63 : 37 CA 20 mm : CA 12.5 mm = 60 : 40 ( By combined sieve analysis)
Solution:
1. Target mean strength = 25 + 1.65 x 5.3 + 33.745 N/mm2
2. Water cement used = 0.5
3. Water content per cu.m = 200 litres
4. Cement content / cu.m = 400 Kg
5. Expected entrapped air content =2 %
Quantity of CA and FA
FA = 0.980 = [ 200 +( 400/3.15)+(1/0.37) x Fa / 2.64) ] 1/1000 = 638 Kg
Ca = 0.980 = [ 200 + (400/3.15)+(1/0.63) x CA/2.68 )] 1/1000 = 1185 Kg
Density of Concrete = 2423 Kg ( 400 + 638 + 1185 + 200 )
Mix proportion by weight 1: 1.595: 2.963
Tables from ACI Method
Table 1 Approximate mixing water requirements for different slumps and maximum size of aggregate
Notes:
i) The above values are maximum for reasonably well-shaped angular coarse aggregate graded within acceptable limits.
ii) The quantities of mixing water can be used to compute cement factors for trial batches.
iii) For aggregate greater than 40 mm, the slump values are based on slump tests after removal of particles greater than 40 mm by wet screening.
Table 2 Volume of Coarse Aggregate Per Unit Volume of Concrete
Notes :
I) These volumes are selected from empirical relationships to produce concrete with a degree of workability suitable for usual RC construction; for less workable concrete, such as that required for concrete pavements they may be increased by about 10 percent; for more workable concrete, such as that required for pumped concrete, they may be decreased by about 10 percent.
ii) Fineness modulus of sand = sum of cumulative ratios retained on sieves with square openings of about 0.15, 0.3, 0.6, 1.18, 2.36 and 4.75 mm
The IS method suffers from following limitations: -
a. The IS method recommends 35 % sand content by absolute volume for zone II sand with correction of +1.5 % for zone I and –1.5 % for zone III. These zones have wide range and this correction is not adequate to achieve a cohesive mix. Some times a correction may be required even when fine aggregate varies from upper side to lower side of a particular zone.
b. Though sand content is adjusted for lower water cement ratio there is no direct adjustment for cement content. As discussed earlier, the cement particles act, as fines in concrete and richer mixes often require lesser fine aggregate when compared to leaner mixes. A mix in which cement content has been lowered by use of plasticisers may require higher sand content to improve cohesion.
c. The IS method gives different tables for determining sand content for concrete up to M 35 grade and above M 35 grade. There is an abrupt change in sand content from 35% to 25% in the two tables when shifting from M 35 grade concrete to M40 grade concrete. The change may be justified to account for higher cement content but it should be gradual in nature.
d. The IS method considers compaction factor as measure for workability, to calculate the water demand. Compaction factor may not correctly represent workability and the revised IS 456 2000 has excluded compaction factor as a measure of workability. It recommends use of slump as a measure for workability. Relationship between slump and compaction factor is difficult to standardize.
e. The IS method does not take into account the effect of the surface texture and flakiness of aggregate on sand and water content. It does not recommend any corrections when crushed fine aggregate is used against natural fine aggregate as in case of DOE method.
f. The IS method does not easily account for blending of different fine aggregates or coarse aggregates when they individually do not conform to IS requirements. On the other hand in RRL method, coarse sand can be blended with fine sand or stone dust to get the required gradation (Natural sand and stone dust will have different specific gravities). Even coarse aggregates of different sizes, gradation and specific gravities can be blended to achieve the required gradation in RRL method.
g. The IS method gives water demand and fine aggregate content for 10mm 20mm and 40mm down aggregate .In practice the maximum size of coarse aggregate is often between 20mm and 40mm, the estimation of water and sand content is difficult.
h. The quantities of fine aggregate and coarse aggregates are calculated from the yield equation. The yield equation is based on concept, that volume of concrete is summation of absolute volumes of its ingredients. Absolute volume of ingredients are function of specific gravities of
ingredients .The plastic density of concrete if theoretically calculated on the basis of specific gravities, may not match with that actually measured from concrete.
i. The IS method does not have a specific method of combining 10mm aggregates with 20 mm aggregates. The grading limits for combined aggregates in IS383 are too broad and do not help much to arrive at particular ratio of different coarse aggregates. The combined grading curves of RRL method help us to arrive on particular ratio of coarse aggregates.
j. The IS method does not have an adjustment in fine aggregate content for different levels of workability. Higher workability mixes require more fine aggregate content to maintain cohesion of mix.
ACI method
This method is based on determining the coarse aggregate content based on, dry rodded coarse aggregate bulk density and fineness modulus of sand. Thus this method takes into account the actual voids in compacted coarse aggregates that are to be filled by sand cement and water. This method also gives separate tables for air-entrained concrete. This method is most suitable for design of air-entrained concrete. This method gives separate values of water and sand content for maximum size of aggregate up to 150mm. Hence this is most suitable method for designing plum concrete. It also givesseparate values for 12.5 & 25 mm down coarse aggregate.
This method suffers from following limitations: -
a. It gives coarse aggregate contents for sand with FM range of 2.4 to 3.0 .It is found that sand available in many parts of India is extremely coarse with FM more than 3.2.
b. In this method the density of fresh concrete is not given as function of specific gravity of its ingredients. In IS and DOE method the plastic density or yield of concrete is linked to specific gravity of ingredients.
c. The values of density of fresh concrete given in this method range from 2285 kg/m3 for10mm down aggregate to 2505kg/m3 for 150mm down coarse aggregate. It is found that in many parts of our country, the density of fresh concrete (plastic density) of 20 and 10 mm down aggregates vary from 2400 to 2600kg/m3. The weights calculated from the given densities often result in higher cement contents than that assumed.
d. The ACI method also does not take into account the effect of the surface texture and flakiness of aggregate on sand and water content, neither does it distinguish between crushed stone aggregates and natural aggregates.
e. The ACI method does not have a specific method of combining 10mm aggregates with 20 mm aggregates.
f. The fine aggregate content cannot be adjusted for different cement contents. Hence the richer mixes and leaner mixes may have same sand proportion, for a given set of materials.
Process for Concrete production:
The various process involved in concrete production are as given below:
1) Proportioning.
2) Mixing.
3) Transportation.
4) Placement.
5) Compaction
6) Curing.
Important observations regarding curing:
(1)Curing should be started earliest as possible.
(2)For the portion of concrete which is covered with formwork, the curing should be started as soon as the formwork is removed.
(3)On exposed surface, it should be started when concrete doesn’t get distributed by curing.
(4)Ensure uninterrupted curing. If it is discontinued for any reason ,the reaction of hydration will be stopped permanently .The partial hydration makes the capillary pores discontinuous and water can’t enter the concrete even if the curing is started again.
(5)High strength concrete should be cured at an early age.
(6)There is a widespread belief that humid climate is sufficient and curing is not required in rainy season
(7)The person generally entrusted for curing is the most unskilled person. He doesn’t appreciate that importance of curing. In fact he believes that curing is a processs of wastage of water time and money.
(8)It can’t be made a measurable item in the contract.
Factors affecting strength of Concrete:
Surface Hardness Methods
In surface hardness methods the hardness of the surface measured, using the size of the indentation or the amount of rebound, is taken as an indicator of the strength of concrete. The
most popular method in this category is the rebound hammer test. This test is also known as Schmidt rebound hammer, impact hammer, or sclerometer test. The rebound hammer, shown in Fig., consists of a spring-loaded mass and a plunger. When the plunger is pressed against the concrete, it retracts against the force imparted by a spring. When the spring is retracted to a certain position, it releases automatically. Upon release the mass rebounds, taking a rider with it along a guide scale. The distance traveled by the mass, expressed as a percentage of the initial extension of the spring, is called the rebound number. ASTM C805 covers the procedure for conducting the rebound hammer test. The rebound number, which is a measure of the hardness of the concrete surface, can be empirically related to the compressive strength of concrete. However, in certain circumstances the surface hardness may not represent the strength of the concrete inside the structure. For example, the presence of a large aggregate immediately underneath the plunger would result in a large rebound number. A large void underneath the plunger, on the other hand, would provide an unusually low rebound number. Other factors that influence the rebound number include the type of aggregate, smoothness of the surface, moisture condition, size and age of the specimen, degree of carbonation, and position of the hammer (vertical versus inclined or horizontal). The hammer should be used only against a smooth surface. Troweled surfaces should be rubbed smooth using a carborundum stone. If the concrete being tested is not part of a large mass, it has to be supported so that the specimen does not move
during the impact. Concrete in the dry state tends to record a higher rebound number. Because of gravity, the rebound number of floor concrete would be smaller than that of the soffit concrete, ven though both concretes are similar. The rebound number of inclined and vertical surfaces would fall somewhere in between. The rebound hammer is best utilized for checking the uniformity of concrete in a large structure or for comparing the quality of concrete in similar structural components such as precast beams. It can also be used for estimating the strength of concrete that is being cured for the purpose of removing formwork. If the rebound numbers have to be used for estimating strength, correlation should be established between the rebound number and the compressive strength for each type of concrete used on a site. If mix proportions or constituent materials are changed, then a new set of tests should be conducted and a new
correlation equation obtained. Typically there is a large variation in rebound numbers. At least 10 to 12 readings should be taken in each location. ASTM C805 provides guidelines for averaging the rebound numbers. In certain cases some individual readings might have to be omitted from the average. If proper calibration is used, the accuracy of prediction of concrete strength is about ±20% for laboratory specimens and ±25% for in situ concrete.
Limitations :
The results of the Schmidt rebound hammer are affected by:
1. Smoothness of test surface
2. Size, shape, and rigidity of the specimens
3. Age of test specimens
4. Surface and internal moisture conditions of the concrete
5. Type of coarse aggregate
6. Type of cement
7. Type of mold
8. Carbonation of the concrete surface
Ultrasonic Pulse Velocity Method
In this method the longitudinal wave velocity in concrete is used to estimate the compressive strength and check the uniformity of concrete. An exciter is used to initiate a pulse which is picked up at another designated location. The distance between the two locations divided by the
time required for the travel is the pulse velocity. Correlation relationships have been developed between pulse velocity, compressive strength, and modulus of elasticity. The relationships are affected by a number of variables, such as the moisture condition of the specimen, type and volume fraction of the aggregate, water–cement ratio, and age of specimen. In general, pulse velocity alone cannot be used to estimate the strength, unless a correlation exists for the particular type of concrete being tested. But the method is an excellent tool for quality control measures. The test method is covered in ASTM C597. The pulse velocity method can be used for both laboratory and field tests. In field tests, the presence of reinforcement and the vibration of elements being tested (such as piers under a roadway in use) might pose problems. A combination of rebound hammer and pulse velocity methods is sometimes used to evaluate both the surface characteristics and the quality of concrete located well inside the surface.
Principle: Voltage pulses are generated and transformed into wave bursts of mechanical energy by the transmitting transducer (which must be coupled to the specimen surface through a suitable medium). A receiving transducer is coupled to the specimen at a known distance to measure the interval between the transmission and reception of a pulse. There are three practical
arrangements for measuring pulse velocity, namely direct, diagonal and surface techniques. The direct approach provides the greatest sensitivity and is therefore superior to the other arrangements.
Main applications: Determination of the variability and quality of concrete by measuring pulse velocity. Using transmission method, the extent of such defects such as voids, honeycombing, cracks and segregation may be determined. This technique is also useful when examining fire damaged concrete.
User expertise: Low level is required to make measurements. However, expertise is needed to interpret the results.
Advantages Excellent for determining the quality and uniformity of concrete. It can rapidly survey large areas and thick members. Path lengths of 10m to 15m can be inspected with suitable equipment.
Limitations Proper surface preparation is required. The work is very time consuming as it takes only point measurements. Skill is required in the analysis of results as moisture variations and presence of metal reinforcement can affect results. The interpretation of ultrasonic test results based on published graphs and tables can be misleading. It is therefore necessary that correlation with the concrete be inspected is carried out. It works on single homogenous materials.
Steps Involved in conventional construction of Roads:
This class of work is done under the following headings:
(1) Forms and form setting.
(2) Trimming of base and laying sliding membrane.
(3) Setting of expansion and dummy joint assemblies.
(4) Concrete production and transport.
(5) Spreading concrete.
(6) Laying reinforcement - where required.
(7) Compacting and finishing concrete.
(8) Joint forming and sealing.
(9) Texturing of road surfaces.
(10) Curing of road surfaces
