Generally at site concrete is produced in batches with the locally available materials of variable characteristics. Thus concrete of one batch is likely to be different from the other.
The magnitude of this variation depends upon many factors as follows:
(a) Variation in the quality of the constituent materials.
(b) Variation in the mix proportions due to batching.
(c) The quality of overall workmanship and supervision at site.
Further concrete has to go under a number of operations such as:
(iv) Curing etc.
During all these operations considerable variation may develop partly due to the quality and efficiency of machinery and plants available and partly due to the difference in the efficiency of techniques used. Thus there is no single feature to define the quality of concrete. Under such conditions concrete generally is referred as good, fair or poor quality concrete. This interpretation is subjective.
Thus it is necessary to define the quality of concrete in terms of desired performance characteristics, economics, safety and other such factors. Due to the large number of variables influencing the performance of concrete, quality control is essential. As the concrete serves the needs of safety and serviceability including durability, which vary from one situation and type of construction to another. Therefore uniform standards for general application to all the works are not practical.
Thus the aim of quality control is to reduce the above noted variations and produce uniform material of desired characteristics for the envisaged job. Therefore the quality control is a dynamic corporate programme to assure that all aspects of equipment, materials and workmanship are looked after well.
Factors Causing Variations in the Quality of Concrete:
The main factors which cause variations in the quality of concrete are as follows:
Personnel are the basic key for the successful execution of any plan or job. Therefore the basic requirement for the success of any quality control plan is the availability of trained and experienced personnel at all levels of concrete production and its execution. The designer must be well versed with the construction operations and the site engineer must be able to understand the statements of specifications fully and clearly. In quality control everything cannot be specified and much depends on the skill and experience of the people involved in it.
For producing concrete of uniform quality, its ingredients play a major role. Thus the ingredients used in concrete must be from the same source as far as possible, specially the cement. When ingredients from different sources are used, the strength and other qualities of the materials are likely to change.
Thus the ingredients from different sources must be tested before use, the same type of cement from different sources and at different times from the same source exhibits variation in its properties, especially the compressive strength.
Thus cement received from each source must be tested initially, once from each source of supply and after wards every two months as the variation in compressive strength is related to the composition of raw materials and the variation in the process of manufacturing. Cement should be protected from moisture. Set cement with hard lumps should be rejected.
The maximum size, moisture content, grading and shape of coarse aggregate are major source of variability in the concrete. Hence aggregates should be stock piled separately in single sizes. The graded aggregate should not be allowed to segregate.
Rule for Grading of Aggregate:
i. For fine aggregate long and continuous grading’s are preferable. The material passing through 300 micron (0.3 mm) sieve and 150 micron (0.15 mm) sieve should be minimum.
ii. The grading’s that are at the coarser end of the range are more suitable for rich mixes and those at finer end of the range are suitable for lean mixes.
The equipment used for batching, mixing and compaction should be of the right type. The weight batchers should be checked frequently for their accuracy. Generally weight batching should be preferred than volume batching. If volume batching has to be adopted, the volume measures should be checked frequently for the weight volume ratio.
Mixer’s performance also should be checked to the requirements of the relevant standards. Concrete should be mixed for the stipulated time as under mixing and over mixing both affect the compressive strength of concrete and should be avoided. The vibrators should be of required frequency and amplitude.
The green concrete should be handled, transported and placed in such a manner that it does not segregate. The time interval between mixing and placing should be as minimum as possible. The targeted strength, impermeability and durability can be achieved only by thorough compaction. 1% voids left due to incomplete compaction will reduce the compressive strength by 5%.
Quality Control of Concrete:
It consists of inspection and testing. They play a vital role in the overall quality control plan.
Inspection may be divided into the following two groups:
1. Quality control inspection.
2. Acceptance inspection.
1. Quality Control Inspection:
For repeated type activities or operations an early inspection is important. After the plant is stabilized, occasional checks may be sufficient to ensure the continued satisfactory results. On the other hand for the activities or operations which are not of repetitive type more constant checks are necessary.
Apart from the tests of concrete material, concrete should be tested at the fresh or green and hardened stages. Tests in the green stage of concrete offer an opportunity for necessary corrective steps to be taken before it is too late. These tests include unit weight, air content and workability test. The accelerated strength tests from which a reliable idea of the 28 day strength can be obtained with in few hours are effective quality control tools.
2. Acceptance Test:
The tests whose results are at universal acceptance level are known as accepted tests. The 28 day concrete strength test is the only acceptance test on the basis of which the acceptance or rejection of concrete mix depends.
Advantages of Quality Control in Concrete:
Following are the advantages of quality control:
i. Quality control is the rational use of available resources after testing their characteristics resulting in the reduction of material cost.
ii. Quality control reduces the maintenance cost.
iii. In the absence of quality control there is no guarantee that the weakness of a certain area can be compensated by over spending in other areas. For example the loss of strength due to incomplete compaction or in adequate curing of concrete cannot be compensated by adding more cement in concrete at some other place.
iv. In the absence of quality control at the site, the designer may be tempted to overdesign to minimise the risk. This will result in more cost.
v. Checks at every stage of production of concrete and rectifications of defects at the proper time will result in early completion of work reducing the delay and cost.
Statistical Quality Control:
Cement concrete is a mixture of cement, aggregate, and water and thus have certain amount of variability in materials as well as in construction methods. This results in variation of concrete strength from batch to batch as well as in the same batch. To assess the strength of the final product is very difficult. To evaluate the strength of end product requires a very large number of destructive tests, which is very costly and time consuming.
Thus we have to resort to sample tests. To adopt very rigid criteria to reject a structure on the basis of a single or few standard samples will prove very costly. Thus to have a reasonable control on concrete work, the acceptance test basis of samples may be adopted, by ensuring that the probability of test result falling below the design strength is not more than a specified tolerance level.
The aim of quality control is to limit the variability of concrete as much as possible. Statistical quality control method provides a scientific approach to concrete designer to understand the realistic variability of materials. With the knowledge of variability of materials, he may lay down the design specifications with proper tolerance to provide for the unavoidable variations. The acceptance criteria are based on statistical evaluation of the test results of samples taken at random during the execution period of the work. For this purpose various statistical techniques are available.
The quality control of concrete will be of great importance on large contracts, where the specifications insist on certain minimum requirement. The efforts put in will be more than the benefits from the resulting savings in the overall concreting operations.
The control over the quality of concrete generally is carried out by testing 150 mm cubes or 150 x 300 mm cylinders prepared at site from the concrete used in the structure. These tests measure the potential strength of the concrete but do not indicate any variation in the compaction or curing conditions of the concrete in the structure. The compressive strength of tests cubes from random sampling of a mix show variations. These variations are inherent in the various operations involved in the preparation and testing of concrete. The variation is shown in Fig. 21.1. This Fig. has been drawn on the basis of table 21.1.
In the Fig. 21.1, the strength interval is taken constant along the x-axis and no of specimens in each interval (called frequency, along y-axis. The figure obtained is called histogram. The area under the curve represents the total number of specimens to an appropriate scale.
If the number of specimen is very large and at the same time the size of the interval is decreased to a limiting value of zero. The histogram would become a continuous curve known as the distribution curve. Such a distribution curve is called a Normal or Gaussian distribution curve. The assumptions of this distribution are very close to reality and are an extremely useful tool in the computation.
The doted smooth curve of Fig. 21.1 is normal distribution curve. The curve is described in terms of mean strength fm and standard deviation σ or S. The standard deviation is the measure of the scatter or dispersion of strength about the mean.
The theoretical normal distribution is represented in Fig. 21.2. It can be seen from the Fig. 21.2 that the curve is symmetrical about the mean value and extends to plus and minus infinity. Actually in practice, very low and very high values of strength do not occur, but these extremes can be ignored as most of the area under the curve (99.6%) lies with in ± 3.0 S or σ. In practice this area can be taken to represent all the strength values of concrete.
In other words, it can be said that the probability of a value of strength failing with in ± 3 5 from the mean value is 99.6%. Similarly the probability of a value falling between any given limits about the mean value (fm + K.S.) can be stated. The values of probability for various values of probability factor K together with the probability of falling strength below (fm – KS).
In the mix design the mean strength can be calculated by the following relation:
fm = fmin + K.S. …(21.1)
where fmin. = minimum strength which is also known as specified characteristics strength or specified design strength.
The probability factor K usually is chosen as 1.64 or 2.33, i.e. there is a probability that 1 in 20 or 1 in 100 respectively of the strength values will fall below the minimum strength. The term K.S. is known as margin. In the calculation of the margin, the standard deviation ‘S’ or σ should be based on the results obtained using the same plant, materials and supervision.
In the absence of such data, the value of S may be taken depending upon the number of available results V and the characteristic strength fmin as follows:
The above values of standard deviation should be used till the sufficient data for the calculation of standard deviation is not available.
In the British method of mix design for air entrained concrete, it is assumed that a loss of 5.5% in compressive strength results for each 1% air entrained by volume in the mix.
This reduction is compensated by aiming for a higher strength as:
where a is the percentage of air entrained.
Himsworth has suggested the value of coefficient of probability K as shown in Table 21.3.
In the table 21.2 the value of K is shown as 1.64 if 5% results are allowed to fall below the minimum value, but generally it is taken as 1.65.
Find the mean strength of concrete for an overhead tank concrete from the following data:
1. The characteristics strength of concrete = 18 MPa
2. The number of test results available = 75
3. Probability factor K = 2.33.
fm = fmin + K.S.
As the value of n is greater than 20, and fmin is less than 20 MPa, the standard deviation 5 is taken as-
S = 0.2 x fmin = 0.2 x 18 = 3.6
Then fm = fmin + K.S.
= 18 + 2.33 x 3.6 = 18 + 8.388 = 26.388 say
= 26.4 MPa Ans.
From the following data, find the mean strength if air entrainment is 4%:
1. The characteristics strength of concrete = 18 MPa
2. The number of test results available = 75
3. Probability factor K = 2.33.
fm = [(fmin + K.S)/(1 – 0.055 a)]
the value of fmin + K.S. from example (1) is 26.4 MPa
the fm = [26.4/(1 – 0.220)] = 26.4/0.78 = 33.85 MPa (app.) Ans.
Sampling of Concrete:
1. Sampling Procedure:
To ensure that each concrete batch should have a reasonable chance of being tested, random sampling procedure should be adopted. Thus the sampling should be spread over the total period of concreting to cover all mixing units and all the batches.
2. Frequency of Sampling:
The minimum frequency of sampling of concrete of all grades should be as follows as per IS 456-1978 shown in table 21.4.
At least sample should be taken from each shift.
Test Specimens for Concrete:
Three specimens should be prepared from each sample to be tested at 28 days. Additional cube specimens may be prepared for determining the strength at 7 days or at the time of stripping the shuttering or to determine the duration of curing etc. The specimens should be tested as per IS 516-1959 or latest edition.
The test strength of sample should be the average of the strength of three specimens. The individual variation in strength should not be more than 15% of the average strength.
As stated above, statistical methods may be used to analyse the variation in strength. The standard deviation and coefficient of variation are the most accepted methods. The procedure of the calculation of standard deviation and coefficient of variation is illustrated with the help of test results of a site-shown in Table 21.1.
Acceptance Criteria for Concrete:
Concrete is considered to comply the requirement of strength if it fulfills the following conditions as per IS 456-1978:
1. The test strength of every sample is not less than the characteristic value.
2. The strength of one or more samples may be less than the characteristic value but in each case it is not less than the greater value given by the following relations:
(a) Characteristic strength minus 1.35 times the standard deviation.
(b) 0.8 times the characteristic strength.
(c) The average strength of the sample is not less than the characteristic strength plus.
Thus the compressive strength of any sample of concrete should not be less than the greater value given by relation- (a) and (c) above.
To obtain the 28 day specified strength with in the permissible range of limits is called the compliance of specification. According to BS 5328 1990, the compliance of the characteristic strength is based on groups of consecutive test results, as well as on single results. Each result is the average of two cubes, made in the specified manner from concrete which is sampled at a prescribed rate and normally tested at 28 days.
The compliance is assumed to be met, if both of the following requirements are satisfied:
i. The average strength determined from the first two, three or four consecutive test results or from any group of four consecutive results, complies with the results of the following table 21.7
ii. No individual test result should fall short of the specified characteristic strength more than the value given in table 21.7.
2. Flexural Strength:
The compliance with respect to flexural strength of concrete is met when the following both conditions are satisfied:
i. The mean strength determined from any group of four consecutive test results exceeds the specified characteristic strength by at least 0.3 MPa (0.3 N/mm2).
ii. The strength determined from any test result is not less than the specified characteristic strength less than 0.3 MPa (0.3 N/mm2).
Establishing the Compressive Strength of Concrete:
Concrete is a very useful and faithful material. If due care is taken of all the ingredients and workmanship of the concrete structure, it generally does not fail to give the specified results.
1. Core Test:
The selection of point from where the core is to be taken depends upon the discretion of the inspecting authority or it can be decided from the cube testing register. The number of test specimens (cores) should not be less than three and they should represent the whole section of the doubtful concrete.
The core strength should be converted to equivalent cube strength. If the strength of equivalent cube obtained is at least 85% of the characteristic strength of the concrete grade, no strength of an individual core is obtained less than 75%, then strength of the concrete may be considered satisfactory.
In case such core tests are not possible or core test results are not found satisfactory then load test may be conducted.
2. Load Tests for Flexural Members:
To verify the performance of the structure i.e., the characteristic strength of the concrete, load tests may be carried out after the expiry of 28 days from the time of placing the concrete.
i. The structure should be subjected to a load equal to full dead load of the structure plus 1.25 times the live load for a period of 24 hours. After 24 hours the live load should be removed.
ii. The deflection due to live (imposed) load should be recorded. If within 24 hours of removal of live load, the structure does not recover at least 75% of the deflection under super impose load, the test should be repeated after 72 hours of the first test. If the recovery is less than 80% the structure should be rejected.
iii. If the max. deflection in mm shown in 24 hours under load is less than 40 l2/D , where I is the effective span in metres and D the overall depth of the section in mm, then it is not necessary to measure the recovery and the recovery criteria is not applicable.
3. Non-Destructive Tests:
Nondestructive testing provides alternative method to core testing for the estimation of the strength of the concrete in the structure. It can also be used to supplement the data obtained from limited number of cores.
Use of Standard Deviation or Coefficient of Variation:
The use of standard deviation or coefficient of variation is based on the following argument. If the control over all materials and operations involved in the production of concrete including sampling and testing was perfect, then every result would be the same and would correspond to the mean value. In practice to have each operation fully perfect is impossible.
The more uniform the operations; closer will be the result to the mean value. Hence lower will be the value of standard deviation. Thus the quality of concrete can be changed by standard deviation. At site it is observed that it is more difficult to achieve consistent results with high strength concrete and the value of standard deviation is higher for high strength concrete than medium or low strength concrete.
Coefficient of variation = standard deviation/mean strength = a constant
It is seen that with a constant coefficient of variation, the standard deviation increases with the strength and is larger for higher strength.
On the basis of their research work Murdock and Erntroy have shown that the coefficient of variation more nearly represent a particular standard of control at relatively low strengths, while standard deviation more nearly represent the standard of control at high strength. Thus Indian standard method has adopted the standard deviation method.
American Concrete Institute (ACI) has recommended following values of standard deviation for different.
Control standards as shown in Table 21.8:
IS 456-1978 has suggested the following values of standard deviation for different grades of concrete: