1544Early Age Property Monitoring

Compared to normal concrete, early age property develops very rapidly for HPC. A great amount of hydration products are produced in a relatively short period. After that only a little change can be observed. Thus, monitoring of early age property is very important to understand the setting and hardening processes of HPC, which has a great impact on the final workability, mechanical properties, and durability.

Development of early age properties and strength gain in HPC are intimately related to the hydration of Portland cement. At present, some methods and techniques such as ultrasonic through-transmission measurements [19-22], microwaves monitoring methods [23,24], impact-echo method [25], ultrasonic reflection [26,27], and electrodeless resistivity measurements [28] have been developed for following the property changes of Portland cement at an early stage.

Ultrasonic reflection and electrodatless resistivity measurements are two most promising methods amongst these techniques for in situ monitoring early age properties of HPC. A detailed introduction to the two methods will be given below. Ultrasonic Reflection Method

An ultrasonic reflection technique that continuously monitors the setting and hardening of concrete has been recently been developed by Kolluru V. Subramaniam, J.P. Mohsen, C.K. Shaw, and S.P. Shah from Northwest University, USA. The experimental procedure is based on high-frequency ultrasonic measurements and consists of monitoring the wave reflection factor (WRF) at the interface between a steel surface and concrete. The basic principle, experimental setup, and procedure are described as follows.

The ultrasonic reflection method requires access to only one face or side of the structure. Freshly mixed HPC is placed in a mold. Transducers that excite and receive ultrasonic sound wave pulses are attached to the outside of a steel plate that is placed in the HPC. The top surface of the steel plate is flush with the surface of concrete. The reflection of ultrasonic waves at the steel-concrete interface is recorded during the setting and hardening of HPC. A schematic diagram of the test setup is shown in Figure 15.11.

The experimental procedure consists of collecting the first and second reflected echoes from the steel-air interface prior to placing the concrete. The first and second reflections are then collected at regular intervals of time at the steel-concrete interface as shown in Figure 15.11b. These time domain signals captured during the setting process are collected and digitized. Then, the collected time domain signals are transformed into the frequency domain using the FFT subroutine, as shown in Figure 15.11c.

When concrete is placed in the mold, it is in a plastic state that resembles a fluid. According to wave mechanics, a shear wave traveling through metal that is incident upon a steel-fresh concrete interface is entirely reflected. Thus, at early ages, most of the wave energy is reflected and the amplitude of the received wave is almost of the same magnitude as the input signal. As the concrete stiffens, more of the wave energy is transmitted through the concrete and less is reflected at the interface. The process of wave reflection can be quantified using the ultrasonic reflection measurement that defines the ratio of the amount of incident wave energy that is reflected from an interface between two materials. The ultrasonic reflection method as a function of time, determined under controlled laboratory conditions, is shown in Figure 15.12.

Many laboratory- and field-scale experiments have been conducted by using ultrasonic reflection measurements. The results show that the observed trends in the ultrasonic reflection measurements are very similar to those observed in the strength gain obtained from the same samples. Furthermore, the percentage change in the WRF and strength with time is very similar.

(a) Steel plate embedded in concrete.

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