Concrete Technology Complete notes of unit 4

Desired Properties of Concrete, Strength, Durability & Im-permeability, Characteristic Strength, Compressive, Tensile and Flexure of Concrete, Bond Strength, Tests on Concrete, Modulus of Elasticity, Effect of W/C Ratio and admixtures on Strength, Types of concrete, High Strength and High Performance Concrete Creep and Shrinkage of Concrete, Significance, Types of Shrinkage and Their Control,

1. Unit-4 Strength of Concrete By Dr. Prathik Kulkarni Assistant Professor Bajaj Institute of Technology, Wardha 01-11-2023 U-IV Strength of Concrete by Dr.PK 1

2. General The compressive strength of concrete is one of the most important and useful properties of concrete . In most structural applications concrete is employed primarily to resist compressive stresses. In those cases where strength in tension or in shear is of primary importance, the compressive strength is frequently used as a measure of these properties. Therefore , the concrete making properties of various ingredients of mix are usually measured in terms of the compressive strength. Compressive strength is also used as a qualitative measure for other properties of hardened concrete. No exact quantitative relationship between compressive strength and flexural strength, tensile strength, modulus of elasticity, wear resistance, fire resistance, or permeability have been established nor are they likely to be. However, approximate or statistical relationships , in some cases, have been established and these give much useful information to engineers. 01-11-2023 U-IV Strength of Concrete by Dr.PK 2

3. For a given cement and acceptable aggregates, the strength that may be developed by workable , properly placed mixture of cement, aggregate and water (under the same mixing, curing and testing conditions) is influenced by: (a ) Ratio of cement to mixing water; (b) Ratio of cement to aggregate; (c ) Grading, surface texture, shape, strength and stiffness of aggregate particles; (d) Maximum size of aggregate. In the above it can be further inferred that water/cement ratio primarily affects the strength , whereas other factors indirectly affect the strength of concrete by affecting the water/ cement ratio. 01-11-2023 U-IV Strength of Concrete by Dr.PK 3

4. Water/Cement Ratio Strength of concrete primarily depends upon the strength of cement paste. It has been shown in Chapter I that the strength of cement paste depends upon the dilution of paste or in other words, the strength of paste increases with cement content and decreases with air and water content . In 1918 Abrams presented his classic law in the form: S = A/ B x where x =water/cement ratio by volume and for 28 days results the constants A and B are 14,000 lbs /sq. in. and 7 respectively. Abrams water/cement ratio law states that the strength of concrete is only dependent upon water/cement ratio provided the mix is workable. 01-11-2023 U-IV Strength of Concrete by Dr.PK 4

5. Strictly speaking, it was Feret who formulated in as early as 1897, a general rule defining the strength of the concrete paste and concrete in terms of volume fractions of the constituents by the equation: where S = Strength of concrete c, e and a = volume of cement, water and air respectively K = a constant. 01-11-2023 U-IV Strength of Concrete by Dr.PK 5

6. 01-11-2023 U-IV Strength of Concrete by Dr.PK 6 Gel/Space Ratio Many research workers commented on the validity of water/cement ratio law as propounded by Duff Abrams . They have forwarded a few of the limitations of the water/ cement ratio law and argued that Abrams water/cement ratio law can only be called a rule and not a law because Abrams’ statement does not include many qualifications necessary for its validity to call it a law. Some of the limitations are that the strength at any water/cement ratio depends on the degree of hydration of cement and its chemical and physical properties, the temperature at which the hydration takes place, the air content in case of air entrained concrete , the change in the effective water/cement ratio and the formation of fissures and cracks due to bleeding or shrinkage.

7. 01-11-2023 U-IV Strength of Concrete by Dr.PK 7 Instead of relating the strength to water/cement ratio, the strength can be more correctly related to the solid products of hydration of cement to the space available for formation of this product . Powers and Brownyard have established the relationship between the strength and gel/space ratio. This ratio is defined as the ratio of the volume of the hydrated cement paste to the sum of volumes of the hydrated cement and of the capillary pores. Power’s experiment showed that the strength of concrete bears a specific relationship with the gel/space ratio . He found the relationship to be 240 x 3 , where x is the gel/space ratio and 240 represents the intrinsic strength of the gel in MPa for the type of cement and specimen used . The strength calculated by Power’s expression holds good for an ideal case . Gel/space ratio can be calculated at any age and for any fraction of hydration of cement. The following examples show how to calculate the gel/space ratio.

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9. 01-11-2023 U-IV Strength of Concrete by Dr.PK 9 Example: Calculate the gel/space ratio and the theoretical strength of a sample of concrete made with 500 gm. of cement with 0.5 water/cement ratio, on full hydration and at 60 per cent hydration . Sol.

10. 01-11-2023 U-IV Strength of Concrete by Dr.PK 10 Grades of Concrete as per IS - 456 of 2000

11. 01-11-2023 U-IV Strength of Concrete by Dr.PK 11 Permissible stresses in Concrete All values in N/mm2 (IS 456 of 2000)

12. 01-11-2023 U-IV Strength of Concrete by Dr.PK 12 Permissible shear stress in concrete as per IS 456 of 2000

13. 01-11-2023 U-IV Strength of Concrete by Dr.PK 13 Accelerated Curing test ( IS:9013-1978 ) Prepare the specimen and store it in moist air of at least 90% relative humidity and at a temperature of 27+2 o C for 23 hrs + 15 minutes. Lower the specimen, into a curing tank with water at 100  C and keep it totally immersed for 3 ½ hours + 5 minutes The temperature of water shall not drop more than 3oC after the specimens are placed and should return to boiling within 15 minutes. After curing for 3 ½ hours + 5 minutes in the curing tank, the specimen shall be removed from the moulds and cooled by immersing in cooling water 27+2 o C for a period of at least one hour.   The corresponding strength at 28 days can be found out from the following correlation. (It is however suggested that a new specific correlation should be developed for the specific concrete used at site.) R 28  (Strength at 28 days) = 8.09 + 1.64 Ra Where, Ra = Accelerated Curing Strength in MPa .    

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15. 01-11-2023 U-IV Strength of Concrete by Dr.PK 15 Maturity Concept of Concrete While dealing with curing and strength development, we have so far considered only the time aspect . It has been pointed out earlier that it is not only the time but also the temperature during the early period of hydration that influence the rate of gain of strength of concrete . Since the strength development of concrete depends on both time and temperature it can be said that strength is a function of summation of product of time and temperature . This summation is called maturity of concrete. Maturity = Σ (time x temperature) The temperature is reckoned from an origin lying between –12 and –10°C. It was experimentally found that the hydration of concrete continues to take place upto about – 11°C. Therefore , –11°C is taken as a datum line for computing maturity.

16. 01-11-2023 U-IV Strength of Concrete by Dr.PK 16 Maturity is measured in degree centigrade hours (°C hrs ) or degree centigrade days (° C days). A sample of concrete cured at 18°C for 28 days is taken as fully matured concrete. Its maturity would be equal to 28 x 24 x [18 – (–11)] = 19488°C h. However, in standard calculations the maturity of fully cured concrete is taken as 19,800°Ch. Maturity concept is useful for estimating the strength of concrete at any other maturity as a percentage of strength of concrete of known maturity. In other words, if we know the strength of concrete at full maturity (19,800°Ch), we can calculate the percentage strength of identical concrete at any other maturity by using the following equation given by Plowman . Strength at any maturity as a percentage of strength at maturity of

17. 01-11-2023 U-IV Strength of Concrete by Dr.PK 17 The values of coefficients, A and B depend on the strength level of concrete. The values are given Below. The values of A and B are plotted against the cube strength at the maturity of 19,800 ° Ch. A straight line relationship will be obtained indicating that they are directly proportional to the strength. Plowman divided the strength into 4 zones as shown in Table and has assigned the values of A and B for each zone. It is to be noted that the maturity equation holds good for the initial temperature of concrete less than about 38°C.

18. 01-11-2023 U-IV Strength of Concrete by Dr.PK 18 Example 1: The strength of a sample of fully matured concrete is found to be 40.00 Mpa find the strength of identical concrete at the age of 7 days when cured at an average temperature during day time at 20°C, and night time at 10°C . Sol. Maturity of concrete at the age of 7 days = Σ ( time x Temperature) = 7 x 12 x [20 – (–11)] + 7 x 12 x [10 – (– 11)] = 7 x 12 x 31 + 7 x 12 x 21 = 4368°Ch. The strength range of this concrete falls in Zone III for which the constant A is 32 and B is 54. ∴ the percentage strength of concrete at maturity of 4368°Ch. = A + B log 10(4368)/1000 = 32 + 54 x log10 (4.368) = 32 + 54 x 0.6403 = 66.5 ∴ The strength at 7 days = 40.0 x 66.5/100. = 26.5 MPa

19. 01-11-2023 U-IV Strength of Concrete by Dr.PK 19 Example 2: Laboratory experiments conducted at (Pune) on a particular mix showed a strength of 32.5 MPa for fully matured concrete. Find whether formwork can be removed for an identical concrete placed at Srinagar at the age 15 days when the average temperature is 5°C if the concrete is likely to be subjected to a stripping stress of 25.0 MPa . Sol.

20. 01-11-2023 U-IV Strength of Concrete by Dr.PK 20 If the strength at a given maturity is known, then the number of days required to reach the same strength at any other temperature can also be calculated from, where, M = Maturity for the given strength, and t the alternative temperature in centigrade. In the above example for reaching the same strength, number of days required, This is to say that the concrete cured at 5°C would take about 52 days to achieve full maturity .

21. 01-11-2023 U-IV Strength of Concrete by Dr.PK 21 Effect of Maximum size of Aggregate on Strength At one time it was thought that the use of larger size aggregate leads to higher strength. This was due to the fact that the larger the aggregate the lower is the total surface area and, therefore , the lower is the requirement of water for the given workability. For this reason, a lower water/cement ratio can be used which will result in higher strength . However , later it was found that the use of larger size aggregate did not contribute to higher strength as expected from the theoretical considerations due to the following reasons . The larger maximum size aggregate gives lower surface area for developments of gel bonds which is responsible for the lower strength of the concrete. Secondly bigger aggregate size causes a more heterogeneity in the concrete which will prevent the uniform distribution of load when stressed . When large size aggregate is used, due to internal bleeding, the transition zone will become much weaker due to the development of microcracks which result in lower compressive strength.

22. 01-11-2023 U-IV Strength of Concrete by Dr.PK 22 Relation Between Compressive and Tensile Strength In reinforced concrete construction the strength of the concrete in compression is only taken into consideration . The tensile strength of concrete is generally not taken into consideration . But the design of concrete pavement slabs is often based on the flexural strength of concrete. Therefore, it is necessary to assess the flexural strength of concrete either from the compressive strength or independently . The type of coarse aggregate influences this relationship . Crushed aggregate gives relatively higher flexural strength than compressive strength. This is attributed to the improved bond strength between cement paste and aggregate particles. The tensile strength of concrete , as compared to its compressive strength, is more sensitive to improper curing. This may be due to the inferior quality of gel formation as a result of improper curing and also due to the fact that improperly cured concrete may suffer from more shrinkage cracks. The use of pozzolanic material increases the tensile strength of concrete.

23. 01-11-2023 U-IV Strength of Concrete by Dr.PK 23 From the extensive study, carried out at Central Road Research Laboratory (CRRI) the following statistical relationship between tensile strength and compressive strength were established . (i ) y = 15.3x – 9.00 for 20 mm maximum size aggregate. (ii ) y = 14.1x – 10.4 for 20 mm maximum size natural gravel. (iii) y = 9.9x – 0.55 for 40 mm maximum size crushed aggregate. (iv ) y = 9.8x – 2.52 for 40 mm maximum size natural gravel. Where y is the compressive strength of concrete MPa and x is the flexural strength of concrete MPa . Subjecting all the data to statistical treatment the following general relationship has been established at CRRI between flexural and compressive strength of concrete: y = 11x – 3.4

24. 01-11-2023 U-IV Strength of Concrete by Dr.PK 24 The flexural strength of concrete was found to be 8 to 11 per cent of the compressive stength of the concrete for higher ranges of concrete strength (greater than 25 MPa ) and 9 to 12.8 per cent for lower ranges of concrete strength (less than 25 MPa ) as shown in Table below.

25. 01-11-2023 U-IV Strength of Concrete by Dr.PK 25 Compression Test Compression test is the most common test conducted on hardened concrete, partly because it is an easy test to perform, and partly because most of the desirable characteristic properties of concrete are qualitatively related to its compressive strength.

26. 01-11-2023 U-IV Strength of Concrete by Dr.PK 26 The compression test is carried out on specimens cubical or cylindrical in shape. Prism is also sometimes used, but it is not common in our country. Sometimes, the compression strength of concrete is determined using parts of a beam tested in flexure. The end parts of beam are left intact after failure in flexure and, because the beam is usually of square cross section , this part of the beam could be used to find out the compressive strength . The cube specimen is of the size 15 x 15 x 15 cm. If the largest nominal size of the aggregate does not exceed 20 mm , 10 cm size cubes may also be used as an alternative. Cylindrical test specimens have a length equal to twice the diameter. They are 15 cm in diameter and 30 cm long. Smaller test specimens may be used but a ratio of the diameter of the specimen to maximum size of aggregate, not less than 3 to 1 is maintained . Strength = Load/Area

27. 01-11-2023 U-IV Strength of Concrete by Dr.PK 27 Strength of Cubes and Cylinders

28. 01-11-2023 U-IV Strength of Concrete by Dr.PK 28 The Flexural Strength of Concrete Concrete as we know is relatively strong in compression and weak in tension. In reinforced concrete members, little dependence is placed on the tensile strength of concrete since steel reinforcing bars are provided to resist all tensile forces. However, tensile stresses are likely to develop in concrete due to drying shrinkage, rusting of steel reinforcement, temperature gradients and many other reasons . Therefore, the knowledge of tensile strength of concrete is of importance .

29. 01-11-2023 U-IV Strength of Concrete by Dr.PK 29 Flexural strength of concrete using I.S . 516-1959 The standard size of the specimens are 15 x 15 x 70 cm. Alternatively, if the largest nominal size of the aggregate does not exceed 20 mm, specimens 10 x 10 x 50 cm may be used .

30. 01-11-2023 U-IV Strength of Concrete by Dr.PK 30 The mould should be of metal, preferably steel or cast iron and the metal should be of sufficient thickness to prevent spreading or warping. The mould should be constructed with the longer dimension horizontal and in such a manner as to facilitate the removal of the moulded specimens without damage. The tamping bar should be a steel bar weighing 2 kg, 40 cm long and should have a ramming face 25 mm square. The testing machine may be of any reliable type of sufficient capacity for the tests and capable of applying the load at the rate specified. The permissible errors should not be greater that ± 0.5 per cent of the applied load where a high degree of accuracy is required and not greater than ± 1.5 per cent of the applied load for commercial type of use.

31. 01-11-2023 U-IV Strength of Concrete by Dr.PK 31 The bed of the testing machine should be provided with two steel rollers, 38 mm in diameter, on which the specimen is to be supported, and these rollers should be so mounted that the distance from centre to centre is 60 mm for 15 cm specimen or 40 cm for 10.0 cm specimens. The load is applied through two similar rollers mounted at the third points of the supporting span, that is , spaced at 20 or 13.3 cm centre to centre. The load is divided equally between the two loading rollers, and all rollers are mounted in such a manner that the load is applied axially and without subjecting specimen to any torsional stresses or restrains. The loading set up is shown in Fig below..

32. 01-11-2023 U-IV Strength of Concrete by Dr.PK 32 Indirect Tension Test Methods Cylinder Splitting Tension Test : This is also sometimes referred as, “Brazilian Test ”. This test was developed in Brazil in 1943. At about the same time this was also independently developed in Japan. The test is carried out by placing a cylindrical specimen horizontally between the loading surfaces of a compression testing machine and the load is applied until failure of the cylinder, along the vertical diameter. Below figure shows the test specimen and the stress pattern in the cylinder respectively. H orizontal stress = 2P/∏DL where, P is the compressive load on the cylinder L is the length of cylinder D is its diameter

33. 01-11-2023 U-IV Strength of Concrete by Dr.PK 33 BOND STRENGTH The bond strength is the measure of the effectiveness of the grip between concrete and steel.  In pull out tests on plain bars, the maximum load generally represents the bond strength that can be developed between concrete and steel. With plain bars the maximum load is not very different from the load at the first visible slip, but in the case of deformed bar, the maximum load may corresponds to a large slip which may not be obtained in practice before other types of failure occur. It is preferable therefore when comparing plain and deformed bars to determine not only the maximum load but also the load at the arbitrary amount of slip and also plot the complete load slip curves for the plain and deformed bars under comparison. One such basis of comparison is the load at a relative movement (slip) between steel and concrete of 0.125 mm at the free end of the bar in a pull out test.

34. 01-11-2023 U-IV Strength of Concrete by Dr.PK 34 Modulus of elasticity of concrete is defined as the ratio of stress applied on the concrete to the respective strain caused . The accurate value of modulus of elasticity of concrete can be determined by conducting a laboratory test called compression test on a cylindrical concrete specimen. Modulus of Elasticity Compressometer A compressometer is a device used in the compression test of the concrete cylinder to determine its strain and deformation characteristics. The set up involves the following procedures. 1) The compressometer consists of two frames(top and bottom), as shown in figure below. The frames are initially assembled by the help of spacers. The spacers are held in position during the assembling. 2) The pivot rod is kept on the screws which are then locked in position. The tightening screws of the top and bottom frames are kept in loose condition.

35. 01-11-2023 U-IV Strength of Concrete by Dr.PK 35 3) Once the compressometer is arranged, it is placed on the concrete specimen kept on a level surface. The compressometer is centrally placed on the specimen. 4) Once the position is set, the screws are tightened and the compressometer is held on the specimen. 5) Once the set up is done, the spacers can be unscrewed and removed.

36. 01-11-2023 U-IV Strength of Concrete by Dr.PK 36 Slope of Initial Tangent gives: Initial tangent modulus = stress/strain Result Initial tangent modulus of given concrete = ..................N / mm² Tangent modulus at working stress =...................N / mm² Secant modulus (Modulus of elasticity of given concrete) = .............. N / mm²

37. 01-11-2023 U-IV Strength of Concrete by Dr.PK 37 Types of concrete 1. Normal Strength Concrete 2. Plain or Ordinary Concrete 3. Reinforced Concrete 4. Prestressed Concrete 5. Precast Concrete 6. Lightweight Concrete 7. High-Density Concrete 8. Air-Entrained Concrete 9. Ready-Mix Concrete 10. Volumetric Concrete 11. Decorative Concrete 12. Rapid-Set Concrete 13. Smart Concrete 14. Pervious Concrete 15. Pumped Concrete 16. Limecrete 17. Roll Compacted Concrete 18. Glass Concrete 19. Asphalt Concrete 20. Shotcrete Concrete

38. 01-11-2023 U-IV Strength of Concrete by Dr.PK 38 High strength concrete High-strength concrete is a term used to describe concrete with special properties not attributed to normal concrete . High-strength means that the concrete has one or more of the following properties: low shrinkage, low permeability, a high modulus of elasticity, or high strength. According to Henry Russell, ACI defines high strength concrete as "concrete that meets special performance and uniformity requirements that cannot always be achieved routinely by using only conventional materials and normal mixing, placing, and curing practices.

39. 01-11-2023 U-IV Strength of Concrete by Dr.PK 39 The requirements may involve enhancements of placement and compaction without segregation, long-term mechanical properties, early-age strength, toughness, volume stability, or service life in severe environments" (Concrete International, p. 63). High-strength concrete is typically recognized as concrete with a 28-day cylinder compressive strength greater than 6000 psi or 42 Mpa . More generally, concrete with a uniaxial compressive strength greater than that typically obtained in a given geographical region is considered high-strength, although the preceding values are widely recognized. Strengths of up to 20,000 psi (140 Mpa ) have been used in different  applications . Laboratories have produced strengths approaching 60,000 psi (480 Mpa ).

40. 01-11-2023 U-IV Strength of Concrete by Dr.PK 40 Applications of High-Strength Concrete High-strength concrete is typically used in the erection of high-rise structure s. It has been used in components such as columns (especially on lower floors where the loads will be greatest), shear walls, and foundations. High strengths are also occasionally used in bridge applications as well. High strength concrete has found its invaluable position in the construction of High rise buildings. Bridges with long spans. High load carrying buildings built on weak soil ( low bearing capacity)

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42. 01-11-2023 U-IV Strength of Concrete by Dr.PK 42 Advantages of High Strength Concrete Reduced maintenance and repair. The size of structural members like beams and columns are reduced since smaller sections are enough to carry high loads. Since the size of members are reduced the cost of formworks are less. In bridge construction the use of high strength concrete becomes inevitable because longer spans with minimum supports can be constructed. Construction of slabs and floors with thinner sections is made possible

43. 01-11-2023 U-IV Strength of Concrete by Dr.PK 43 Mix design procedure for High Strength Concrete The mix design of high strength concrete is similar to conventional concrete with minor variations because the volume of mineral admixtures is calculated too. It can be done by two methods IS code method and ACI method. The design procedure explained below is based on ACI method Determination of mean design strength based on the required concrete strength. Determination of optimum coarse aggregate content based on nominal size of coarse aggregate required. Estimation of the water content required. Selection of water cement ratio from the table based on the field strength and maximum size of coarse aggregate. Calculation of cement content based on the water cement ratio. Providing the required adjustments in the mix based on the admixtures used.

44. 01-11-2023 U-IV Strength of Concrete by Dr.PK 44 High-performance concrete (HPC) is  concrete that has been designed to be more durable and, if necessary, stronger than conventional concrete . High Performance Concrete In simpler words, HPC is a concrete that has atleast one outstanding property viz.  Compressive Strength, High Workability, Enhanced Resistances to Chemical or Mechanical Stresses, Lower Permeability, Durability  etc. as compared to normal concrete. Ultra-high-performance-concrete (UHPC) is generally defined as concrete with a compressive strength greater than 150 MPa . UHPC typically is made with  high-strength steel fibers , fine sand, cement, fly ash, a large volume of SF, and a low amount of water (a w/cm ratio less than 0.20) .

45. 01-11-2023 U-IV Strength of Concrete by Dr.PK 45 Material Consideration for High Performance Concrete The materials used in high performance concrete are nothing different from the conventional concrete, only that the mineral admixtures like silica fume, granulated blast furnace slag and fly ash are used in higher concentrations . Care is to be taken during the mix design to maintain appropriate quantities of these admixtures and other materials within the permissible limits to achieve the properties of HPC. Cement with lower percentage of tricalcium aluminate C 3 A is required to make HPC When aggregates are concerned coarse sand can be used as fine aggregate to reduce the water content since more fine particles like silica fume are to be used which will increase the water demand .

46. 01-11-2023 U-IV Strength of Concrete by Dr.PK 46 And coarse aggregate nature plays an important role in the properties of HPC. Stronger the coarse aggregate, stronger will be the aggregate cement interface (transition zone) thus resulting in high performance. Mostly well graded cubic aggregates are preferred. Chemical admixtures like super plasticizers, retarders and air entraining agents are used in HPC to have better workability (mobility) at lower water contents.

47. 01-11-2023 U-IV Strength of Concrete by Dr.PK 47 Advantages and Application of HPC Reduced maintenance cost — though the initial cost of HPC is high when compared to conventional concrete, since less maintenance is required it results to be more economical. The service life of a structure is more since the concrete is more durable and resist abrasion and chemical attack. As a result of high strength the size of structural members is reduced. On the account of its superior mechanical properties, HPC finds its application in various types of structures like High rise buildings Tunnels Bridges with long spans. Nuclear structures Constructions at sever exposure conditions

48. 01-11-2023 U-IV Strength of Concrete by Dr.PK 48 Mix design Procedure for High Performance Concrete The steps involved in the mix design of high performance concrete is given below Based on the rheological properties (e.g. Workability) and compressive strength desired, the yield stress and plastic velocity is determined The dosage of high range water reducers (HRWR) used to reduce the water content is determined, usually it is 1.5% of the weight of cement Sand content required is determined from code as per zones of construction Coarse aggregate content required is calculated per m 3  of concrete Cement content based on the water cement ratio is calculated. Finally correction factors are applied based on IS CODE 10262.

49. 01-11-2023 U-IV Strength of Concrete by Dr.PK 49 High strength concrete is defined as concrete that has compressive of 55 MPa or greater.  Chemical resistant concrete, early drying concrete, ultra-water resistant concrete, heat resistant concrete, and impact and abrasion resistant concrete . Types of HPC

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51. 01-11-2023 U-IV Strength of Concrete by Dr.PK 51 Desired Properties of Concrete, Strength , Durability & Im -permeability, Characteristic Strength, Compressive , Tensile and Flexure of Concrete, Bond Strength, Tests on Concrete, Modulus of Elasticity, Effect of W/C Ratio and admixtures on Strength , Types of concrete, High Strength and High Performance Concrete Till now we have discussed

52. 01-11-2023 U-IV Strength of Concrete by Dr.PK 52 Thank you