Noninvasive Assessment of Existing Concrete.

摘要

The nondestructive evaluation (NDE) of concrete has been a long-standing challenge. In the last three decades, many NDE methods have been proposed, and some of them resulted in commercial products. The most common method is probably the one based on the measurement of the velocity of bulk ultrasonic waves propagating through concrete. The measurement helps to assess the strength of concrete or to monitor the curing of fresh concrete. Another popular commercial system is the Schmidt hammer that consists of a springdriven steel hammer that hits the specimen with a defined energy. Part of the impact energy is absorbed by the plastic deformation of the specimen, and the remaining impact energy is rebounded. The rebound distance depends on the hardness of the specimen and the conditions of the surface. As these methods are not universally accepted, much research is still ongoing on the NDE of concrete. In this report, we preset an NDE method based on the propagation of highly nonlinear solitary waves (HNSWs) along a 1-D chain of spherical particles in contact with the concrete to be tested. With respect to ultrasonic-based NDE, the proposed approach: (1) exploits the propagation of waves confined within the grains; (2) employs a cost-effective transducer; (3) measures different waves’ parameters (time of flight, speed, and amplitude of one or two pulses); (4) does not require any knowledge of the sample thickness; (5) does not require an access to the sample’s back-wall. Moreover, the method differs from the Schmidt hammer because it can be applied also onto fresh concrete, multiple HNSWs features can be exploited, and it does not induce plastic deformation. In this project, the propagation of HNSWs is used to measure the strength of cured concrete under concrete’s control mix design and under excessive water/cement (w/c) ratio. The objective was the assessment of the modulus of hardened concrete to predict the compressive strength of bridge concrete decks, or other concrete structures. In the work presented here many HNSW-based transducer were used to test concrete cylinders cast with well controlled w/c ratios and short beams cast with a certain w/c ratio but corrupted with excessive water. The latter mimicked rainfall prior, during, and after construction. We monitor the characteristics of the waves reflected from the transducer/concrete interface in terms of their amplitude and time-of-flight (TOF). The latter denotes the transit time at a given sensor bead in the granular crystal between the incident and the reflected waves. When a single HNSW interacts with a 'soft' neighboring medium, secondary reflected solitary waves (SSW) form in the granular crystal, in addition to the primary reflected solitary waves (PSW). We observed that the waves propagating within the transducer are affected by the amount of water present in the mix design. Future studies shall expand the research by including more samples and by conducting field tests in existing bridges.