Testing and E-valuation

There are several techniques for evaluating civil structures [i. e., 13] with pulse echo techniques including radar (reflection and scattering of electromagnetic pulses) [14­16], impact echo (propagation, reflection, and scattering of elastic waves after mechanical impact) [17, 18] and ultrasonic pulse echo (propagation and dispersion of sound waves produced with ultrasonic transducers) [19]. The more widely known techniques are perhaps impulse response (IR) and impact-echo testing. The IR method uses a low-strain impact to send stress waves through a specimen via a sledgehammer with a built-in load cell. The maximum compressive stress at the impact point is related to the elastic properties of the hammer tip. Response to the input stress is measured with a velocity transducer [20]. The IR method is suggested for evaluation of reinforced concrete structures such as floor slabs, pavements, bridge decks and piers, fluid retaining structures, chimneystacks, and silos.

More recently, the impact-echo technique is used to determine the position of inter­mediate and large defects in concrete structures. It is used to assess the bonding condi­tion between facing stones, mortar, and inner rubble core in stone masonry [21] and the structural integrity of high-performance/high-strength concrete of existing build­ings using SAWS (spectral analysis of surface waves) [22]. A mechanical impact on
the surface of the structure is used to generate an elastic stress wave that travels through the structure, reflects off external boundaries and internal flaws, and returns to the surface. A receiver located near the point of impact is used to measure the nor­mal surface displacements. Fast Fourier transform (FFT) is used to determine the cor­responding frequency spectrum and analyze the location of flaws or (internal or external) surfaces by using [23].

2 Подпись: (1)f

where Cp is the P-wave (primary/pressure wave) speed within the material (~2,000- 4,000 m s-1 for concrete depending on age and other characteristics [24, 25]) and f is the characteristic frequency. Limitations and associated problems of the method include spectral effects of the impactor, the receiving device, and the interpretation of other reflected waves [26]. Improvements are reported using noncontacting devices for both impact generation (shock waves) and response monitoring (laser vibrometer to measure surface velocity) [27].

Flexural strength is determined according to ASTM C 947 [28] and density is determined according to ASTM C 948 [29]. GFRC made of cement, AR-glass fibers, sand, and water is a noncombustible material and should meet the criteria of ASTM E 136 [30]. Single skin GFRC panels can be designed to provide resistance to the passage of flame, but fire endurances of greater than 15 min, as defined in ASTM E 119 [31], are primarily dependent upon the insulation and fire endurance characteris­tics of the drywall or back-up core.