Setting and early-age hydration

The non-destructive monitoring of concrete behaviour during setting and part of hardening stage was conducted using ultrasonic equipment called CELplus®, a product of

Geotron-Elektronik. As schematically shown in Figure 3.8, the apparatus consists of main unit and the supplementary components, among which the most important are waveform generator board, pair of piezoelectric broadband ultrasonic transducers and mould meeting high damping specification.

The instrument operates according to so-called ultrasonic pulse velocity (UPV) method, a special type of stress wave propagation method also called direct transmission method. In accordance with type of transducers used and their arrangement (Figure 3.8), it allows primary P-waves, a fastest type of mechanical/stress wave called interchangeably longitudinal or compressional waves, to be transmitted most directly which is through sample. This guarantees that high sensitivity of measurement could be obtained at simultaneous provision of possible maximum energy of emitted signal transmitted. Both transmitter and receiver can operate in the middle resonant frequency range between 10 and 200 kHz, which was important for few reasons as following: i) it tuned out to be sufficient for reliable measurement of the P-wave velocity, ii) it fell in typical range used in field of testing cement- based materials, iii) in the study at hand it provided wavelength larger than aggregate size. Other relevant technical information was impulse strength and rate of change, reported by the producer to be approximately 2 kV and 0.3 s/kV, respectively.

Figure 3.8: Block diagram of the apparatus for pulse velocity measurement (to the left) and the main part of CELplus® equipment as arranged for test (to the right).

CELplus PC Transmitter Receiver Test piece Mounting frame

Prior to each experiment, transducers were fixed with the frame’s screws. Once the rubbery and highly damping mould had been situated in place as well, silicon jelly/grease chosen as couplant was distributed over the exposed area of sensors. Such step was necessary to enhance contact between the transducers and the material as well as to secure that the sensors could be easily removed after testing without damaging. The next preparation step was then executed by pouring the concrete into the mould and sealing it, taking place not later than 10 minutes from mixing finalization. From this moment, transducers were in direct and full face contact with the specimen under study and the measurement started. At command, the instrument in general and transmitter in particular transmitted pulses of longitudinal wave. After travelling though material, the signal was automatically picked up by receiver. Knowing the distance between transducers (being fixed at constant 2.5 cm) and the time delay between start of transmission and the start of receiving the signal (based on threshold onset picking algorithm), P-wave velocity was calculated as the ratio between the two. Particular and other signal parameters captured, this including e.g. root mean square voltage (i.e. measure of relative energy and complex waveforms resultant from pulses transmitted), were provided in real time of experiment by the attached software. Preliminary tests showed that the monitoring intervals could be limited to range of 1 to 5 minutes, which was held throughout the tests. If not mentioned otherwise, these were continued until approximately 24th hour of age and were processed under quasi-constant temperature 20 ± 1 °C and RH of ambient of 65 %. Regrettably, no measurement on coarse-grained UHPC could be taken owing to early stage of ultrasonic apparatus development when producing the mixes.

The destructive testing of a setting concrete followed a well-known penetration resistance

test according to the DIN EN 480-2 standard, in literature being sometimes referred to as Vicat test for mortars and concrete. In this test, a steel right cylinder (i.e. the needle) having diameter of 1.13 ± 0.05 mm and the length of 50 ± 1 mm is lowered into sample-filled frustum 40 mm in height and the penetration depth is measured. Final setting time as counted from the addition of water, being the only setting phenomena aimed at, is when needle penetration is less than 2.5 mm. In the standard, maintaining 90 % of relative humidity is required. However, since the result was nearly identical for sample kept sealed, and, furthermore, such condition corresponded to one met is concrete shrinking autogenously, the latter was chosen and was commonly used. Eventually, more testing samples were produced at once for coarse-grained UHPC to provide sufficiency of measuring points. This step was necessary to ensure that no aggregate grain is hit and needle movement isn’t stopped by the

interlocked steel fibres, signs of which would be remarkable change of needle immersion depth at different locations. The ambient temperature was 20 ± 1 °C.

Changes of concrete’s in-situ temperature were recorded by means of thermocouple PT100

embedded in the sample of interest. The sensor had a protective hybrid cover to facilitate its extraction after test finalization and further reuse. This cover showed however to have no influence on results as proved/validated in preliminary studies. Experiments themselves involved usage of specimens from parallel measurement of other variables (shrinkage deformation under free or restrained conditions, ultrasonic measurement) or casting of individual specimen of identical boundary conditions from selfsame mix batch. In doing so, selfsame material characteristics and curing conditions have been always maintained. Frequency of data collection by the data acquisition system and, for some tests, its duration was similar if not identical to measurement being validated, substantiated or compared to. The ambient temperature was 20 ± 1 °C.