CORROSION EVALUATION OF BRIDGE COLUMN USING IMPACT ECHO METHOD

CORROSION EVALUATION OF BRIDGE COLUMN USING IMPACT ECHO METHOD

Prepared by:R. Hamid, Mohd Azizi Ismail and Syahrul Fithry SeninCivil Engineering Department, UKM

INTRODUCTIONInternal Defect in Concrete: Rebar corrosionRebar corrosion is a problem faced by RC structures when there is access to aggressive corrosion agents such as moisture, oxygen, chloride, acidic gaseous

The quality of the concrete cover will determine how those corrosion agents will be controlled to transport within the pores in cover.

The permeability of concrete cover and its thickness will determine the rate and extent of the corrosion.

2

THE CORROSION INIATIATION IN CONCRETE

Carbonation reduce the pH of the pore water in hardened cement paste from 12.6-13.5 to about 9. When all Ca(OH)2 has become carbonated, the pH reduec further to 8.3

Steel embeded in hydrating cement paste rapidly form a THIN PASSIVE LAYER of oxide which prevent the reaction between O2 and H2O to form rust.

When low pH reached the surface of rebar, the protective passive layer is removed and corrosion take place, provided that O2 and H2O are present.

BS 8110: total chloride content < 0.4 % by mass of cement4CORROSION PROCESS

When there are difference of electrical potential along the steel and concrete (permenant submerged and a part which is wettng and drying, variation of salt concentration in the pore water, non-uniform access of O2), a electrochemical cell is setup (formation of ANODE & CATHODE), connected by an electrolyte in the form of pore water in the hardened cement paste.

The positively charge Fe2+ at the anode is passed in the solution, and the negatively charged electron is thru the steel in to the CATHODE. It will combine with O2 and H2O to form hydroxyl ion (OH-). These travel thru the electrolyte and combine with Fe2+ to form feric hydroxide which converted further to rust.5THE PROBLEM OF CORROSION IN CONCRETENoProblems of Corrosion To Reinforced ConcreteReferences1Loss of Load-Carrying Capacity of Steel Cairns J., Zhao Z., 19992Bond deterioration between steel-concreteFang C. et. Al, 20043Crack promotion due to expansion of rustSeok J.B., Hwan O.B., 2010Cracks parallel to the rebar: product of corrosion occupy a volume several times larger (3 times) than the original steel=> easier for aggressive agents to ingress towards steel

Reduce the cross-sectional areas of the steel => reduce load carrying capacity

Film of rust surrounding rebar

6PROBLEM STATEMENTThe corrosion of rebar in reinforced concrete had been found to reduce the mechanical properties of concrete. As the corrosion process is silently progress inside a concrete, a non-destructive method that capable to detect the process is needed before the concrete structure is severely damage. Therefore, a study on using Impact-Echo Method on possibility to detect rebar corrosion in concrete bridge columns was conducted on-site by studying the wave velocity and frequency response on corroded rebar.IMPACT-ECHO METHOD: PRINCIPLES

ImpactorTransducerIMPACT-ECHO METHOD( CONTINUE)Elastic wave propagates when subjected to impact force

Stress pulses propagates in concrete asP-wave (pressure wave)S-wave (shear/Rayleigh wave)R-wave (surface wave)

Impact echoe is based on elastic wave propagation . The impact force will create stress pulses that propagates in medium. 9

IMPACT-ECHO RESPONSE ON SOLIDP and S-wave will be reflected when encounter boundary with different acoustic impedance

R2P4P6PtAs the R wave passes the receiver, there is no displacement until the arrival of P-waves which is reflected from the bottom of solid.The arrival of P is indicated by 2P, is the first P wave causing downward movement. Multiple reflection of subsequent P wave is labelled as 4P, 6P etc. The reflected wave in time deflection is converted to frequency domain by FFT. As the reflection of P-waves is periodic , there is a coresponding frequency, f, that equal to the inverse of time.11IMPACT-ECHO METHOD (CONTINUE)If wave encounters internal defect:

part of the wave reflected back to surface

Move around the defect, ELONGATING the path

IMPACT-ECHO OPERATING EQUATIONSParameterEquationVelocity of pressure wave, Cp

Cp = 2T/tFrequency of wave, fp

fp =1/tFrequency of rebar, fstFst = Cp/(4 dst)METHODOLOGY OF THE STUDYVisual Inspections on 3 bridge columns (0.3 x 0.3 m) Corrosion areas Submerged water levelMeasurement of the concrete cover at 5 points on the columnCorrosion Rebar Evaluation Using Impact-Echo Method Concrete wave velocity, Cp Frequency and amplitude of rebarComparison Cp with UPV ENDSTARTTHE INVESTIGATED BRIDGE COLUMNS

SELECTED POINTS ON THE INVESTIGATED COLUMN

RESULTS OF THE STUDYMEASURED COLUMN CONCRETE COVERColumn123PointsI IIIIIIVVI IIIIIIVVI IIIIIIVVCover measured (mm)525050494848474545455048474646Average (mm)50FREQUENCY SPECTRUM (COLUMN 1 AT LOCATION i)

ffstFrequency spectrum = the distribution of frequencies (Hz) with various amplitudes (dB) contained in the recorded waveformTo find the dominant frequenciesFFT has been used to convert the recorded time-displacement waveform to frequency domain 19CONCRETE WAVE VELOCITY, CpColumn123PointsI IIIIIIVVI IIIIIIVVI IIIIIIVVFrequency (kHz)5.025.764.984.984.984.96 4.964.905.884.964.984.983.654.924.98Wave Velocity, CP (m/s)301234562988298829882976297629403528297629882988219029522988CONCRETE QUALITY EVALUATIONColumn 123Points I II III IV V I II III IV V I II III IV V Wave Velocity, (m/s) 301234562988298829882976297629403528297629882988219029522988Concrete Quality*DDPPPPPPGPPPPPP*-Classification of normal concrete quality based on UPV value (Whitehurst, 1951) WHITEHURST (1951): CONCRETE QUALITY CLASSIFICATIONConditionUPV values (m/s)Excellent (E)> 4500Good (G)3500-4500Doubtful (D)3000-3500Poor (P)2000-3000Very Poor (VP)