Use of ultrasonic-test data for quality and strength evaluation of concrete

During mobilization for construction of the designed 79-story ODTs "Okhta" tower in Saint Petersburg, experimental barrette foundations were prepared for determination of bearing capacity with respect to the soil [1]. Moreover, the Arabtec Construction Co. and the JCS Soletanshstroi performed non-destructive ultrasonic tests by order of the ODTs "Okhta" to evaluate the uniformity and quality of the concrete placed in the body of the barrette foundations. The N. M. Gersevanov Scientific-Research Institute of Foundations and Underground Structures (NIIOSP) had previously stated the problem − additional investigations, interpretation and processing of ultrasonic test data in conformity with recommendations of Russian regulatory documents, and evaluation of the quality and strength class of concrete in the barrette foundation. Five experimental barrette foundations with a length of � 78 m and a rectangular section of 2,800� 1,500 mm along the external perimeter, and 2,500� 1,300 mm inside the casing were prepared. The basic lower portion of the barrette foundation with a length of � 65 m is fabricated from heavy Class V40 concrete, while the upper section, which is eliminated during excavation of the pit, is Class 12.5. Construction of the basic part of the barrette foundation includes installation of "Osterberg cells" 0.4 m high, which are designed to measure axial forces transferred through its body during loading [2]. The "upper" cells (barrette piles Nos. 1-3) are situated at a height of approximately 20 m, and the "lower" cells approximately 4 m above the lower elevations of the barrette foundation. To monitor the quality of the concrete placed in the barrette foundations, a series of cube specimens (100� 100� 100 mm) were prepared from batches of a mix, and then tested for strength under uniaxial compression in a laboratory operated by the JSC Soletanshstroi. The average strength values R of the concrete in the specimens tested at ages T = 3.7 and 28 days, were, respectively, 37.2, 38.6, and 54.7 MPa. The last value corresponds to Class V40 concrete. Since specimens prepared on the surface cannot reflect the actual properties of the concrete placed in the barrette foundations the quality and uniformity of the concrete should be monitored directly within the body of the barrette foundations. A procedure and results of studies conducted by the N. M. Gersevanov ScientificResearch Institute of Foundations and Underground Structures to evaluate the uniformity and quality of concrete in experimental barrette foundations on the ODTs project "Okhta" are described. The curves obtained for the dependence of the strength of concrete on depth made it possible to assess the uniformity of the concrete in the barrette foundations, and obtain verified assessments of the strength class of the concrete.