Evaluation of Early-age Concrete Strength using Surface Wave Velocity for Slipform Operation

Abstract

The setting and hardening of concrete cast inside the form constitute important processes during the construction work and influence the form removing time. As the latest construction method, slipforming enables continuous placing of concrete using moving forms. If exposed prematurely before developing early compressive strength of between 0.2 and 0.3 MPa, concrete may stream down and deform or may fail to develop sufficient strength. Besides, delayed exposure of concrete may generate construction joint or lead the slipform to deform due to the excessive lifting force required to subdue the adhesion between the form and concrete. Accordingly, the efficiency of slipform operation depends on the appropriate form removal timing. Despite of the state-of-the art technology, domestic slipform construction has relied on the experience of foreign engineers to assess the initial setting properties of in-place concrete. Therefore, there is urgent need to develop a self-relying method for the evaluation of compressive strength development of early-age concrete inside the form. This dissertation proposes an experimental formula evaluating the early-age concrete compressive strength using surface wave velocity. To investigate the relationship between compressive strength and surface wave velocity for early-age concrete, a series of experiments including surface wave velocity measurement, cylindrical compressive strength test, and penetration resistance test on in-place concrete during the first 24 hours after placing were conducted. Numerical simulation is presented to demonstrate the applicability of surface wave velocity measured by a pair of ultrasonic transducers attached to the same side of concrete wall with form panel. The proposed formula deals with diverse concrete mixtures and curing temperatures, and the 95 % confidence bounds of the formulation are suggested to improve field application. Various slipform concrete constructions are presented to validate the proposed formula. To that goal, a pair of ultrasonic modules is installed at the bottom of the panel to acquire the surface wave signal, and the proposed formula is applied to evaluate compressive strength and slipform operation for form moving. The results show that the proposed experimental formula is applicable for maintaining optimal slipforming conditions. Extended application of the proposed formula is expected to other construction technology for determining the appropriate form removing time.