Inspection Quality Variation Statistical Control and Acceptance Criteria

Inspection, Quality, Variation, Statistical Control and Acceptance Criteria Dr. J. SUDHAKUMAR Assistant Professor Department of Civil Engg. NIT, CALICUT

Inspection, Quality, Variation, Statistical Control and Acceptance Criteria n INSPECTION AND QUALITY CHECK n Need v. To and scope of Inspection ensure that work is done in accordance with the plans, specifications and good practice, and to prevent mistakes.

§ The other requirements for the same purpose include ØIntelligent design ØAdequate specifications ØReliable construction ØCompetent inspection

§ The following items are covered by the inspection of concrete construction at various stages: Ø Sampling, Identification, Examination, and any field testing of materials Ø Control of concrete proportioning and the measurement of materials Ø Examination of the foundation, forms and other work preparatory to concreting

ØContinuous inspection of the batching, mixing, conveying, placing, compacting, finishing and curing of concrete ØTesting for consistency of concrete, and preparation of any concrete specimens required for laboratory testing

ØGeneral observation of contractor’s plant and equipment, weather, working conditions and other items affecting the concrete ØPreparation reports of records and

n. Choice of an Inspector v. An ideal inspector should have both technical and practical knowhow of the subject. He should essentially understand the following: Ø Bulking of fine aggregate (sand) Ø Well proportioned mix or not Ø Whether the mix has desired slump

Ø Whether the forms are sufficiently tight and braced Ø Whether the concrete is properly compacted or not Ø Whether the forms are removed early or not

n Authority of the Inspector v. An authority should be given to the inspector to : ØProhibit concreting until all preliminary conditions (such as completion of forms) have been fulfilled and inspection report for concreting has been provided

ØStop the use of materials and equipment, which do not comply with the specifications ØStop any work which is not being done in conformity with the plans and specifications

ØRequire the removal or repair of faulty construction or of construction performed without inspection and not accessible to being inspected later

Ø Normally, the inspector is authorised to take direct action in the first three cases above Ø He should report the matter immediately to his superior Ø He should stop the work only as a last resort, when it is clear that unsatisfactory concrete will result from the continuing operations

n Quality check on concrete v. Before concrete is placed, the specification requirements in all aspects must be fulfilled Ø Forms should be of proper size and strength and in their correct location Ø Cement, aggregates, water and any other ingredients should be inspected

1. Batching of materials, time of mixing, possibility of segregation, proper curing etc. , as this will influence the properties of hardened concrete 2. Preparation of test specimens 3. At least 3 specimens for each 200 m 3 of concrete 4. Each sample should be from different points of the structure

v. Sampling should not be from the conveying device v. Taken at irregular times and without prolonged preparations v. Sample should be placed in a water-tight non-absorbent container v. Remixed fast enough to make it uniform, and then moulded into specimens

v. For compression test, 150 mm cube moulds are used v. Moulds are filled in 3 layers v. Each layer is compacted with 25 strokes of a 16 mm diameter and 600 mm long, round bullet pointed steel rod v. After the top surface has been levelled, the specimen is covered to prevent evaporation

v. Flexure specimens are 150 mm x 150 mm in section. Moulds are placed with their long axis horizontal and are filled in 2 layers, each layer being rodded 50 times.

n Quality check on Personnel and Equipment v Should have a well trained team, conversant with different quality control procedure of inspection, testing and data analysis v An adequately equipped field laboratory for carrying out routine control tests

n Table 1: Daily testing programme and composition of team ( refer notes )

n Table 2: Degree of quality control expected under different site conditions ( refer notes )

n Measures of Variability of Concrete Mix Design v. Factors Ø It contributing to Variability is found that strength of concrete varies from batch to batch Ø The following are the sources of variability

v. Variation in the quality of constituent materials used v. Variation in the mix proportions due to batching processes v. Variations in the quality of batching and mixing equipment available

v. Quality of supervision and workmanship v. Variation due to sampling and testing of concrete specimens Ø These variations are inevitable during production of concrete

v. The purpose of quality control using statistical means is to produce concrete of uniform quality Ø If a large number of cube strength test results are plotted on a histogram, the results are found to follow a bell-shaped curve termed as “Normal Distribution Curve”

v. The arithmetic mean of a number of test results gives no indication of the extent of variation of strength Ø However, this can be ascertained by relating the individual strength to the mean strength and determining the variation from the mean, with the help of the characteristics of the N. D. curve

v. Standard Deviation Ø The root mean square deviation of the whole consignment is termed as the ‘standard Deviation”. It can be defined numerically as: S = (x – -x-)2/(n-1)

Ø Where S = S. D. of the test results x = any value in the test results _ x = arithmetic mean of the results n = number of test results

v Coefficient of Variation Ø An alternate method of expressing the variation of results about the mean is by coefficient of variation Ø This is a non-dimensional measure of variation

Ø This is obtained by dividing the standard deviation by the average value, and is expressed as V = 100 S / -X-

where Ø V = coefficient of variation S = S. D. in strength -X- = Mean value in strength v Ø When the C. V. is constant, S. D. increases with the increase in strength of concrete

v STATISTICAL CONCEPTS OF MIX DESIGN Ø Characteristic strength This means that value of the strength of concrete below which, not more than 5 percent of the test results are expected to fall

Ø Target mean strength ft = fck + K. s where ft = target mean strength fck = characteristic strength K = a statistical constant, depending on the definition of fck and is derived from the mathematics of Normal Distribution s = standard deviation

Ø The value of K is equal to 1. 65 where not more than 5% of test results are expected to fall below the characteristic strength ft = fck + 1. 65 s

1. Acceptance Criteria (IS 456 : 2000) Compressive strength Both the following conditions must be met : a) The mean strength of any four consecutive test results compiles with the appropriate limits in Table 3.

Table 3 For M 15 grade Mean of 4 consecutive results in N/mm 2 must be greater than or equal to fck + 0. 825 x SD or fck + 3 N/mm 2 , whichever is greater

Table 3 For M 20 or above grade Mean of the group of 4 consecutive results in N/mm 2 must be greater than or equal to fck + 0. 825 x SD or fck + 4 N/mm 2 , whichever is greater

Acceptance Criteria b) Any individual test result complies with the appropriate limits in Table 3.

Table 3 For M 15 grade Individual test results in N/mm 2 must be greater than or equal to ( fck – 3 ) N/mm 2

Table 3 For M 20 or above grade Individual test results in N/mm 2 must be greater than or equal to ( fck – 4 ) N/mm 2

Acceptance Criteria 1. Flexural strength Both the following conditions must be met :

a) The mean strength of any four consecutive test results exceeds the specified characteristic compressive strength by at least 0. 3 N/mm 2

b) The strength determined from any test result is not less than the specified characteristic strength less 0. 3 N/mm 2

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