What is NDT? Powerful technique for evaluating exiting concrete structures with regard to their strength and durability apart from assessment and control of quality of hardened concrete without or partial damage to the concrete. Concrete is not loaded to the failure, therefore, the strength inferred or estimated can not be expected to yield absolute value of strength. NDT measures some other properties of concrete from which an estimate of its strength , durability and elastic properties are obtained. 1
WHY NDT INVESTIGATION IS REQUIRED IN POST ENGINEERING? It is required to assess the quality control exercised by the agency at the time of concreting and precaution can be taken if found unsatisfactory. Environmental, manmade or natural impact may deteriorate the concrete very fast in sustained loading condition. Ongoing corrosion activity can be trapped and stopped further. To evaluate structural strength at present and use it to increase the load carrying capacity or reduce load to prevent collapse. Gives master print for repair and retrofitting work to be carried out. To predict residual life of the structure for future planning. To get idea about extent of the damage occurred to the structure due to a particular cause. 2
HOW NDT INVESTIGATION HELPS IN ASSESSMENT? Rebound Hammer Test: To estimate strength and hardness of the concrete in the cover region (IS: 13311 Part II) UPV Test: To measure homogeneity, uniformity, compactibility and presence of crack or void, thus representing durability of concrete (IS: 13311 Part I). Core Test: To evaluate in situ compressive strength of concrete (Represents real and actual picture of concrete) IS 516 & ASTM C42/41) Corrosion Assessment: Half Cell Potential (ASTM C876- 9), Resistivity and RCPT (ASTM C1202), to know about extent and rate of ongoing corrosion activity. Bar Locater and Cover Meter: To know the structural strength and protection to the reinforcement by cover concrete. Carbonation: To evaluate extent of carbonation for strength 3 and corrosion estimation.
REBOUND HAMMER• OBJECTIVES• To assess quality of the concrete in relation to the standardrequirements• To estimate compressive strength of concrete with the help ofsuitable correlations between rebound index & compressivestrength• To delineate regions of poor quality or deteriorated concretein a structure• To assess quality of one element of the concrete in relation toanother
REBOUND HAMMER FACTORS AFFECTING THE TEST RESULTS• Type of Cement• Type of Aggregate• Surface Condition and Moisture Content of Concrete• Curing and Age of Concrete• Carbonation of Concrete Surface
ULTRASONIC PULSE VELOCITY OBJECTIVES To assess uniformity and homogeneity of concrete To assess quality of concrete in relation to standard requirements Detection of the presence & approximate extent of cracks, voids &other imperfection in concrete Measurement of changes occurring with time in the properties of the concrete Correlation of pulse velocity & strength as a measure of concretequality Determination of the dynamic modulus of elasticity & Poissonsratio of the concrete
ULTRASONIC PULSE VELOCITY FACTORS AFFECTING TEST RESULTS Surface Conditions and Moisture Content of Concrete UPV α Moisture content Path Length, Shape and Size of the Concrete Member Min. Path Length–100 mm for Conc. with 20 mm down 150 mm-for Conc. With 20-40 mm Grid of 30 Cm X 30 Cm for Results Path Length Prob Minimum KHz Transverse Dimension of Members Upto 500 mm 150 25 500-700 ≥60 70 700-1500 ≥40 150 Above 1500 ≥20 300
ULTRASONIC PULSE VELOCITY FACTORS AFFECTING TEST RESULTS Temperature of Concrete 50 – 300 Ideal 300 – 600 (Reduction in UPV upto 5%) Below 50 (Increase upto 7.5%) Stress level in Concrete At higher stress UPV reduces due to development of microcracks Significant effect is observed when stress is higher than 60% of its ultimate. Reinforcing Bars UPV of R/f is 1.2 to 1.9 times than normal concrete
ULTRASONIC PULSE VELOCITYEVALUATION OF DYNAMIC MODULUS OF ELASTICITY Ed = ρf(μ)v2 Where ρ is density of concrete in Kg/m3, v is pulse velocity in m/sec, f(μ) = (1+ μ)(1-2μ)/(1- μ)where μ is dynamic poisson’s ratio of concrete Es = 1.25Ed-19 Es is static modulus of elasticity
Methods of Conduct of UPV Test
Detection of Flaws and Cracks by UPV TestUPV value in Indirect method lowers by 5-20%compared to direct method (velocity is increased by1km/sec.)
ULTRASONIC PULSE VELOCITYINTERPRETATION OF THE TEST RESULTS Velocity criteria for concrete quality grading Sr. No. Pulse Velocity ( km / sec ) Concrete Quality Grading 1 Above 4.5 Excellent 2 3.5 to 4.5 Good 3 3.0 to 3.4 Medium 4 Below 3.0 Doubtful
OBJECTIVES OF HALF CELLPOTENTIAL TEST To evaluate potential of concrete in promoting corrosion activity of reinforcement by Half Cell Potential test. To evaluate reinforcing steel in concrete that has carbonated to the level of the embedded steel To evaluate indoor concrete that has not been subjected to frequent wetting unless it has been protected from drying after casting To compare corrosion activity in outdoor reinforced concretes of highly variable moisture or oxygen content, or both, at the embedded steel
Mechanism of Chloride Attack Ingress of Cl- and O2 , Formation of OH- Preferential migration of Cl- to neutralize Fe++ formed within the pit and formation of FeCl2. Hydrolysis of FeCl2 and acid formation ( HCl ) which accelerates further attack on metal. Intensive localized corrosion within the pit leading to failure.
Corrosion Mechanism Corrosion of Steel in Concrete is an Electrochemical process. In Steel, one part becomes anode and other part becomes cathode connected by electrolyte in the form of pore water in the hardened cement paste. Positively charged Fe++ at the anode passes into solution. Negatively charged free electrons e- passes through the steel into cathode where they are absorbed by the constituents of the electrolyte and combine with water & oxygen to from hydroxyl ions (OH)-. (OH)- travel through the electrolyte and combine with the ferrous ions to form ferric hydroxide and by further oxidation converted to rust.
HALF CELL POTENTIAL• FACTORS AFFECTING TEST RESULTSo WETTNESS OF SURFACEo CARBONATIONo ELECTRIC CURRENT
HALF CELL POTENTIAL• INTERPRETATION OF TEST RESULTSIf potentials over an area are more positive than -0.20 VCSE, there is a greater than 90% probability that noreinforcing steel corrosion is occurring in that area at thetime of measurement. If potentials over an area are in the range of -0.20 to-0.35 V CSE, corrosion activity of the reinforcing steel inthat area is uncertain. If potentials over an area are more negative than -0.35 VCSE, there is a greater than 90 % probability thatreinforcing steel corrosion is occurring in that area at thetime of measurement.
CONCRETE CORE• OBJECTIVES To assess the compressive strength of concrete onsamples by cutting core with help of suitable correlationbetween L/D ratio. To assess the quality of concrete in relation tostandard requirements.
PROFOMETER REBAR DETECTION The measuring method is based on the eddy current principle with pulse induction. The basic unit can perform the functions such as: (i) locating rebars, (ii) measuring concrete cover, (iii) storing individual cover values and statistical evaluation, and (iv) determining the bar diameter. It measures more accurately cover in the range of 60 mm – 150 mm, and bar diameter in the range of 10 – 26 mm with bar spacing in the range of 70 mm – 200 mm. The instrument has Neighboring bar correction and 2-Layer correction facility. Neighboring bar correction mainly used for determination of the bar diameter in close spaced parallel bar arrangements. 2-Layer Correction mode helps in congested standard orthogonal bar arrangements. The congested bars induce a too strong measuring signal which can be corrected.
OBJECTIVES OF PROFOMETER REBAR DETECTION Formeasurement of concrete cover. To find the diameter of bars in the concrete. To find the spacing between the bars.