ppt-of-concrete-cube-testing-ndt. pdf
- 1. 26-03-2021 1 Concrete Cube Testing & NDT RAVI RANADE ( NDT, Repair / Rehab. Consultant, Concrete Technologist ) Construction Diagnostic Centre Pvt. Ltd. CDC CDC Standard Cube Strength The measured compressive strength of a cube made, cured and tested in accordance with IS – 516 Core Strength The compressive strength of a core, cut, prepared and tested in accordance with the requirements of IS– 516 - Part 4 - 2018, for a stated length/diameter ratio Estimated in-situ Cube Strength ( Equivalent Cube Strength ) The strength of concrete at a location in a structural member estimated from indirect means and expressed in terms of specimens of cubic shape Understanding Concrete Strength CDC Understanding Concrete Strength Potential Cube Strength ( BS 6089 – 2010 ) Determining the average potential strength is a method for estimating the 28-day compressive strength of the concrete supplied to the structure. It takes the corrected core strength and modifies it to take account of excess voidage, differences caused by different curing conditions and differences between the maturity of the core at testing and 28 days at 20°C.
- 2. 26-03-2021 2 CDC Understanding Concrete Strength Most direct method of assessing in-situ strength of concrete in a structural element is by Core tests. CDC Understanding Concrete Strength 22.25 25.58 CDC • When Cube test is carried out according to standard procedures, the results of the Cubes compression test represent the potential strength of the concrete as delivered to a site. (Ref : ACI 228.1R-19) • The test is used mainly as a basis for quality control of the concrete to ensure that contract requirements are met. • It is not intended for determining the in-place strength of the concrete because, it makes no allowance for the effects of placing, compaction, or curing. It is unusual for the concrete in a structure to have the same properties as a standard-cured cube at the same test age. Cube Strength v/s In-Situ Strength CDC • Sometimes, the strength of the concrete in the structure can be obtained by using field cured cylinders (Cube) prepared and cured in accordance with ASTM C 31/C 31M. • These Cubes are cured on or in the structure under, as nearly as possible, the same conditions as the concrete in the structure ( Normally kept over slab). • Still, measured strengths of field cured cubes may be significantly different from in-place strengths because it is difficult, and often impossible, to have identical bleeding, consolidation, and curing conditions for concrete in Cube and concrete in structures Cube Strength v/s In-Situ Strength
- 3. 26-03-2021 3 CDC • Sample of concrete for test specimen shall be taken at the end discharge point • For final acceptance of concrete for Structural strength purpose, the samples should only be taken at end discharge point. • If pumping / placing is not in the scope of the RMC supplier, then RMC supplier may collect additional sample for his contractual requirements. Sampling of Concrete CDC We believe ONLY on Cube strength But !!! The millennium dollar question is does a cube strength truly represent the in-situ concrete strength??. The parameters on which the CUBE strength of concrete depends Does cube really fail in pure axial compression ? Size of cube- Higher the size -> lower the strength Shape – Cube or Cylinder V/s Shape of RCC members – Cube Strength = 1.25 x Cylinder Strength Mould material & Texture of moulds - Compared to Cast Iron moulds – strength of concrete casted in M.S., Timber, Plywood and PVC moulds is found to be on lower side. Compaction – Layers, no. of blows, dia. of compaction rod. Curing temperature – 27 0 C ( +- 2 0 C) – Higher Temp -> Higher strength Moisture conditions at the time of testing – SSD – Air dried cubes yield 20 -25 % more strength Type of compression testing machine – (Rocker & roller plate) – The tolerance for cube dimensions as per IS : 10086 – 1981 is ONLY +- 0.2 mm Direction of casting & Direction of Testing - Normally the strength is more in the casting direction Rate of loading – 140 Kg/Sqcm/ Min @ 315 KN/Min. At field the cubes are crushed with a speed of more than 500 to 1500 Kg/Sqcm/Min. With such an increase in rate of loading a compressive strength can get increased up to 55 %. Curing period – 28 days for cubes , Site - 10 days for OPC, 15 days for Blended CDC Age – Blended cement concrete- slower strength development CDC As per IS – 516 the standard rate of loading should be – 140 Kg/Sqcm/ Min i.e. @ 315 KN/Min for 150x150x150 cube Rate of Loading Kg / Sqcm. / Min Rate of Loading
- 4. 26-03-2021 4 CDC A truly representative concrete is not used for filling the cubes. A special batch with lower w/c ratio and higher cement content is made to cast the cubes. The samples are very rarely collected at the end discharge point. The sample size is always less than the IS – 456 requirements. The cube samples do not have the correct size and shape. It is very important to measure the dimensions of the specimens to the nearest 0.2 mm as per IS – 516. In most of cases dimensions not are not measured, but are simply assumed be perfect 150 x 150 x 150 mm, which is NEVER the case. It is very important to know the type of fracture, which can indicate that, whether testing is properly carried out or not or whether the size and shape of the cube is responsible for erroneous results. Cube Testing - Observations CDC At almost ALL sites, temperature control facility for curing tanks is not available. Most of the time, cubes are taken out from the tank much before the testing , and they do not have the saturated surface condition (SSD). The resulted strength is on higher side. Very few labs in India have Rate of loading controlled testing machines. As reported above, rate of loading with which cubes are tested at site or in labs is much higher than the required. Thus the resulted strength is on very much higher side. The curing of concrete at site is simply an eye wash The entire procedure from Cube casting, Cube testing and interpretation is taken very very casually. Cube Testing - Observations a) Lower Cube Strength b) Doubt concerning workmanship involved in batching, mixing, placing, compacting or curing of concrete. c) Deterioration of concrete due to: overloading; fatigue; chemical action; fire; explosion; weathering. d) To ascertain whether the in-situ strength of Concrete is acceptable for: the designed / actual / Projected loading system; CDC Why NDT ? Accuracy of @ ± 75 to 85 % Remember – We are dealing with a complex and heterogeneous material - Concrete CDC What is the reliability of NDT ??
- 5. 26-03-2021 5 The Accuracy & Reliability depends upon - Proper selection of ND Tests Proper surface preparation for every test Combination of various ND Tests Sampling & Extent of Testing Co-relation of ND Tests with standard site samples Cognizance of various factors and site conditions affecting the test Site Conditions accuracy and reliability of available correlations (e.g. between pulse velocity and strength). CDC NDT Results Always - Combine two or more NDT methods like – Rebound Hammer + UPV Rebound Hammer + UPV + Core Half-Cell Potential + Resistivity Half-Cell Potential + Resistivity + Carbonation + Sulphate & Chloride Test Pile Integrity + UPV + Core Pile Integrity + Pile Load test ( Static / Dynamic ) RADAR + Thermography Maturity test + Pullout Test CDC NDT Results CDC EN 13791:2019 REBOUND HAMMERS / CALIBRATION ANVIL CDC
- 6. 26-03-2021 6 REBOUND HAMMER TEST (IS- 516 - Part 5 Sec 4- 2020) CDC REBOUND HAMMER OPERATION CDC Hardness measurement of surface layer of 25 to 50 mm Influenced by – Smoothness of test surface, Size, shape, and rigidity of the specimens, Age of test specimens, Surface and internal moisture conditions of the concrete, Type of coarse aggregate, Type of cement, Type of mould, Carbonation of the concrete surface ( Calcium Bi Carbonate a Harder substance ) Prediction of strength 25 % CDC REBOUND HAMMER TEST Silver Schmidt V/s Classical Rebound Hammer Parameter Silver Schmidt Classical Rebound Hammer Rebound Value Q R Measurement Rebound Velocity Rebound Distance Friction & Gravity Effect Negligible ( Measures velocity immediately before and after impact) Present Weight of hammer & plunger Weight of hammer reduced & that of plunger adjusted accordingly Standard Correction for impact direction Not required Required Rebound Value as against “R” value > “R” value CDC
- 7. 26-03-2021 7 Rebound Hammer – ONLY a surface Test CDC 25 mm ULTRASONIC PULSE VELOCITY TEST (IS- 516 - Part 5 Sec 1- 2018) CDC ULTRASONIC PULSE VELOCITY TEST V2 = Ed (1-v) p (1+v) (1-2v) Thus V Ed Ed Es and Es = 5000 fck Thus V fck CDC ULTRASONIC PULSE VELOCITY TEST - Methods Direct Semi - Direct In Direct CDC
- 8. 26-03-2021 8 CDC Gradation of Quality of concrete - as per IS 516 (part 5/Sec1) : 2018 AMENDMENT NO. 1 NOVEMBER 2019 - Direct velocity Km/Sec Quality of Concrete Grade <= M - 25 > M - 25 Excellent Above 4.500 More than 4.500 Good 3.500 to 4.500 3.750 to 4.500 Doubtful Below 3.500 Less than 3.750 ULTRASONIC PULSE VELOCITY TEST CDC Gradation of Quality of concrete ( as per CDC ) - Direct & Semi Direct velocity Km/Sec Quality of Concrete Grade < M - 15 M - 20 to M - 25 M - 30 to M - 35 > M - 40 Excellent Above 4.250 More than 4.400 More than 4.600 More than 4.900 Good 3.500 to 4.250 3.750 to 4.400 3.900 to 4.600 4.150 to 4.900 Medium 3.000 to 3.500 3.400 to 3.750 3.600 to 3.900 3.800 to 4.150 Doubtful Below 3.000 Less than 3.400 Less than 3.600 Less than 3.800 ULTRASONIC PULSE VELOCITY TEST Influenced by – • Type of cement • Type of Aggregate • Surface condition & moisture content, • Type of Mix & water / cement ratio, • Reinforcement, • Stress level CDC ULTRASONIC PULSE VELOCITY TEST Applications:- • The ultrasonic pulse velocity method is used to assess • The homogeneity / Quality of the concrete • The presence of cracks, voids and other imperfections, depth of crack • Changes in the structure of the concrete which may occur with time • The quality / compressive strength of concrete of one element in relation to another element / standard requirement. • The values of dynamic elastic modulus of concrete CDC ULTRASONIC PULSE VELOCITY TEST
- 9. 26-03-2021 9 CDC ULTRASONIC PULSE VELOCITY TEST CDC ULTRASONIC PULSE VELOCITY TEST Core Test ( IS -516 - Part 4 – 2018, IS - 456 ) CDC CDC “Core Tests Easy to Perform , Not Easy to Interpret” - Adam Neville
- 10. 26-03-2021 10 Age – Upto M-25 more than 14 days, for higher grades can be less Location – • Not near construction joints, • Preferably from the middle part of the member • For Slab or from foundation top, trim top 15 to 20 % depth as top part of the core may not contain uniform distribution of aggregates (maximum up to 60 mm). • In case of cores which are not across full depth of member, trim 10 to 15 % portion of the bottom side, as the portion near to the broken end may contain some micro cracks/fractures. • From Compression zone CDC Core Test - IS -516-Part 4 – 2018 - Test requirements - Number of Cores – Minimum 3 Nos Diameter of Cores – • Minimum Dia. Of Core shall be 3 times nominal Aggregate Size • The core diameter shall generally be 100 mm to 150 mm (± 10 mm), with the preferred diameter being 100 mm for nominal maximum aggregate size up to 20 mm. Length of Cores – • Core length to Dia. Ratio shall be between 1.0 to 2.0 • For l/d ratio lower than 2.0 correct the compressive strength Reinforcement – No reinforcement in the core sample to be tested CDC Core Test - IS -516-Part 4 – 2018 - Test requirements - Preparation of Core sample – • Grinding (Most precise method) - (For any value of strength) – Grinding after curing , Why not before curing ?? • Capping – o Up to 50 Mpa - Capping with calcium aluminate cement mortar), Capping with sulphur mixture o Up to 100 MPa Capping with high strength sulphur mixture o Or Other capping materials may also be used provided that, at the time of testing, it has a strength at least equal to the anticipated strength of concrete. Cores may be tested generally in saturated condition except if specifically required to be tested in air dry condition. • Specimen shall be soaked in water for at least 40 hours to maximum of 48 hours before testing at 27 ± 3°C CDC Core Test - IS -516-Part 4 – 2018 - Test requirements - CDC Core Test - IS -516-Part 4 – 2018 - Test requirements - Perpendicularity Tolerance for 0.5 0 for 100 mm dia core - 0.87 mm 75 mm dia core - 0.65 mm 60 mm dia core - 0.52 mm All for 100 mm dia core
- 11. 26-03-2021 11 Diameter correction factor for less than 100 mm dia. • What about diameters from 80 to 100 mm ?? Correction factor according to the l/d ratio – The equivalent cube strength = 1.25 x corrected of the concrete cylinder strength CDC Core Test - IS -516-Part 4 – 2018 - Test requirements - Core Test - IS -516-Part 4 – 2018 - Test requirements - CDC As per IS 456- 2000 Cl – 17.4.3 • Min. Avg. equivalent cube strength of core shall be o > 85 % of design grade of concrete o But no individual core shall have strength < 75 % As per IS -516-Part 4 – 2018 Cl - B-2.5.2 For assessing strength of a particular member by taking THREE cores • Min. Avg. equivalent cube strength of core shall be o > 85 % of design grade of concrete o But no individual core shall have strength < 75 % For overall assessment requirement or where large number of cube sets (each set consisting of 4 consecutive samples) have failed , by taking TEN cores • Min. Avg. equivalent cube strength of core shall be o f‘(avg) > 0.85 (fck + 3) o F’ (i) > 0.75 (fck) CDC Core Test – Acceptance Criteria CDC Factors affecting Core Test Variation of In-place concrete strength in structures (ACI - 214.4R-4) • Core test results may not represent the quality of concrete as delivered to the site if mixing water was added at the site, or poor placing, consolidation, or curing practices were followed Consolidation • Strength is reduced by approximately 7% for each percent by volume of entrapped air remaining when concrete is insufficiently consolidated Curing • Low initial curing temperatures reduce initial strength development rate but can result in higher long-term strength. Conversely, high initial-curing temperatures increase initial strength development but reduce long-term strength
- 12. 26-03-2021 12 CDC Variation of In-place concrete strength in structures (ACI - 214.4R-4) Micro-Cracking • Micro-cracks can be present in regions of the structure that were subjected to stress from applied loads or restraint of imposed deformations. Rough handling of the core specimen can also cause micro-cracking. Curing • Low initial curing temperatures reduce initial strength development rate but can result in higher long-term strength. Conversely, high initial-curing temperatures increase initial strength development but reduce long-term strength Factors affecting Core Test CDC Where to take the cores • At a number of locations spread over the whole structure • Load carrying capacity of the structure is not impaired by removing some concrete Length/diameter preferred ratios • a) 2,0 if the strength result is to be compared to cylinder strength; • b) 1,0 if the strength result is to be compared to cube strength. • a) To be classified as a 2:1 core the capped or ground length to diameter ratio shall be within the range between 1.95 to 1 and 2.05 to 1.0 • b) To be classified as a 1:1 core the capped or ground length to diameter ratio shall be within the range between 0.90 to 1 and 1.10 to 1.0 Factors affecting Core Test CDC Correction factor for l/d ratio • stronger concretes are less affected by the value of L/D than concretes of lower strength; this is shown in Fig. 1. Factors affecting Core Test CDC Effect of Reinforcement • In general, reinforcement reduces the strength of a core, • The exceptions being 1:1 cores with not more than 2.0 % volume fraction of reinforcement and 2:1 cores where the reinforcement is completely within 30 mm of the ends of the core and the volume fraction of reinforcement is not more than 2.0 %. In these cases, the presence of reinforcement may be regarded as having no impact on the core strength Which Moisture Condition ? • Dried core samples give about 5 to 10 percent more strength than saturated samples. Therefore, for comparison with the cube test results, saturated sample testing is recommended in the test procedure of IS -516-Part 4 – 2018 Factors affecting Core Test
- 13. 26-03-2021 13 CDC Direction of Core Drilling • In one of the experiment, upto 7 – 8 % less strength is noticed for horizontally drilled cores • Bartlett and MacGregor found that the direction of the core axis relative to the direction of casting has no effect on the strength of cores taken from high-strength Concrete • IS -516-Part 4-2018 & EN 13791:2019, does not differentiate between either direction of coring • In case of Slip formed Concrete there would be no lowering of the apparent strength of concrete in cores drilled horizontally. Factors affecting Core Test Core Test – Some Observations Corrections for excess voidage • Estimating excess voidage, BS EN 12504-1 National Annex NA. An indication of the adequacy of placing and compaction. • From BS EN 13791 National Annex NA an estimate of in situ strength assuming fully compacted concrete may be calculated from: = kv x fc,is CDC CDC Core Test – Some Observations Consensus and Unanimity • Before any Test program is commenced, it is desirable that there is complete agreement between the interested parties on the validity of the proposed testing procedure, the criteria for acceptance, and the appointment of a person and/or laboratory Variability of Strength – • For 100 mm diameter cores with ends prepared by grinding, there is a 95 % probability that the true mean value is within ±14% / n of the calculated value ( Ref: EN 13791:2019 ) e.g. - for 03 cores - 8.1 % for 10 cores - 4.4 % for 30 cores - 2.6 % EN – 13791 – 2019 - Estimation of compressive strength CDC
- 14. 26-03-2021 14 Introduces the two applications of in situ strength assessment • a) to estimate in situ characteristic compressive strength fck,is of a test region and/or in situ strength at specific locations (Clause 8) • b) assessment of compressive strength class of concrete supplied to a structure under construction where there is doubt about the compressive strength based on results of standard tests or doubt about the quality of execution (Clause 9) EN – 13791 – 2019 - Estimation of compressive strength CDC EN – 13791 – 2019 - Test regions, test location and number of tests CDC Test region The test region shall be defined: • a single concrete • a series of similar elements, or a large element • may include concrete from different production units using the same materials Clause 8 Small test region • Shall not include significantly different concrete • ≤10 m3, or ≤30 m3 where there are no supply issues and where indirect testing is used to identify locations of lower compressive strength Clause 9 • a volume is not more than about 30 m3, supplied in a single day and no indication that any load is different to the others • The region may comprise up to six volumes, so ≤180 m3 EN – 13791 – 2019 - Estimation of compressive strength for structural assessment of an existing structure CDC CDC
- 15. 26-03-2021 15 CDC EN – 13791 – 2019 - Core testing and the determination of the in situ compressive strength CDC core length factor (CLF'). CDC EN – 13791 – 2019 - Estimation of compressive strength for structural assessment of an existing structure - Based only on Core test data For test region comprising not more than 30 m3 – Take the mean value of three or more cores (provided the spread of test results is not more than 15 % of the mean value) as the in situ compressive strength (fck,is) CDC EN – 13791 – 2019 - Estimation of compressive strength for structural assessment of an existing structure - Based only on Core test data Small Test Region Less than around 10 m3 • At least three cores, and at least one core from each element. The lowest value of the three or more cores may be assumed to be fck,is for structural assessment purposes. Less than around 30 m3 • Indirect testing is used to determine variability and locations of lower strength. The mean value of the three or more cores may be assumed to be fck,is for structural assessment purposes. Provided: • Each core represents concrete that is to remain in the structure and the spread of results is less than 15% of the mean value result. A wider spread of results indicate further investigation is required.
- 16. 26-03-2021 16 The regression equations of Indirect Tests values shall be used to estimate the characteristic in situ compressive strength Estimation of the characteristic in situ compressive strength for a test region m is the number of estimated strength values. Estimation of the in situ compressive strength at a specific location CDC EN – 13791 – 2019 - Estimation of compressive strength for structural assessment of an existing structure - Based on a combination of Indirect test data and Core test data CDC EN – 13791 – 2019 - Assessment of compressive strength class of concrete in case of doubt Doubt about the in situ quality may arise from doubts about the quality of the concrete supplied to the site, problems during the execution of the works or after some exceptional event on site. Doubt often includes, but is not limited to, the following: • insufficient compressive strength of samples taken for identity testing e.g. poorly made or cured cubes; • workability is excess of that specified, e.g. addition of water at behest of site personnel • problems during execution of the works, e.g. movement of forms, too few internal vibrators The assessment criteria are based on concrete that is under production control certification. CDC Where doubt remains there are three practical methods to assess the compressive strength class from in situ concrete: • Comparative indirect test testing - The recommendation is to take not less than 20 indirect test measurements, rebound hammer or UPV, in the region under investigation and compare with a 20 indirect test measurements for a reference region for which the compressive strength class is confirmed. • Indirect testing combined with a minimum or cores, 9 –20 indirect tests with 3 cores for up to 180 m3 (2 cores for a single ≤ 30 m3 volume) • Cores only, 3 to 12 cores for 30 to 180 m3 insufficient compressive strength of samples taken for identity testing e.g. poorly made or cured cubes; EN – 13791 – 2019 - Assessment of compressive strength class of concrete in case of doubt EN – 13791 – 2019 - Assessment of compressive strength class of concrete in case of doubt - Based only on Core test data Minimum of three core results for a single ≤30 m3 volume placed in a single day, where there is no indication that any load is different to the others delivered to that volume. Up to 12 core results required to assess up to 180 m3. CDC
- 17. 26-03-2021 17 EN – 13791 – 2019 - Assessment of compressive strength class of concrete in case of doubt - Based on Indirect testing and minimum cores Minimum 9 –20 indirect tests with 3 core results for up to 180 m3 2 cores for a single ≤ 30 m3 volume placed in a single day, where there is no indication that any load is different to the others delivered to that volume. Note - 10m3 a day for three days is to be treated as three different volumes. CDC CDC • Do not confuse the Clause 8 Estimation of compressive strength for structural assessment of an existing structure procedures with Clause 9 Assessment of compressive strength class of concrete in case of doubt procedures, • They have different approaches that may lead to significantly different outcomes. EN – 13791 – 2019 - Estimation of compressive strength CDC The potential strength is of interest when we want to know that the mixture used in construction conforms to what is specified. The actual strength is of interest when we want to know how good the concrete in the structure is And the Reality is - The intrinsic or “true” strength of concrete cannot be measured. Concrete Complexities CDC Most of the R & D was done on OPC 33 cement and M-15 grade of concrete There is an immediate need to reinvent the Concrete Extensive National level R & D is required to find out – Effect of Size of Cube , Cylinder on strength Effect of length to diameter ratio Correlation of Cylinder strength v/c Cube strength Effect of Cube Mould material and texture on compressive strength and on Rebound number Effect of Compaction Effect of curing period, temperature and curing regime R & D is required
- 18. 26-03-2021 18 CDC Extensive National level R & D is required to find out – Moisture condition of core sample Effect of direction of core drilling – Direction of Casting and Direction of Testing Effect of Rate of Loading on concrete testing Effect of Location of coring in the member Effect of reinforcement in the core sample Effect of Grinding and different capping materials Effect of flatness, Perpendicularity , Parallality on Cube and Core strength And our Indian Standards shall be based on the outcome of this R & D R & D is required Concrete Seems to be Easy to Prepare & Cast but Very Difficult to Understand CDC CDC RAVI RANADE Contacts CDC Construction Diagnostic Centre Pvt. Ltd. 020 – 2543 2643 94220 07479 cdc@ndtconcrete.com info@concretepune.com W www.ndtconcrete.com www.ndtconcrete.net