3 Experimental 1 Introduction
3.3 Test methods and procedures 1 SAP material characterization
3.3.4 Free and restrained shrinkage
Autogenous shrinkage free of external restraint was measured
using two similar set-ups based on corrugated tube method developed by Jensen and co-workers [Jen 95][Qia 08] and, for smaller one, being standardized in ASTM C1698–09. The main differences between the apparatus to be followed in Figure 3.9 regarded the size of the tubes (diameter x length: approximately 30 x 400 vs. 80 x 380 mm x mm), solution for sealing the moulds from their ends (plastic end plugs vs. steel ones), presence of anchorage and fixing (none vs. mounted screws and springs) and finally the number of specimens measured and evaluated at once (three vs. one tube).
Figure 3.9: Apparatus used in autogenous shrinkage measurement
In gist in common to both, the special design of the measuring device (dilatometer) and the use of corrugated, tube-shaped polyethylene moulds enable continuous monitoring of the concrete deformations beginning immediately after the filling and encapsulating of the tubes. Having executed small modification to regulation of original ASTM proposal, the autogenous strain of sample at time t was calculated according to Eq. 3.4:
) ( ) ( 2 ) ( 10 ) ( ) ( ) ( 0 0 6 0 0 t L t L L t R L t L t L t L ref plug autogenous − ⋅ − + = ⋅ − = ε (3.4)
where L(t) is the length of sample at time t (in mm), L(t0) is the length of sample at time-zero
(in mm), Lref is the set-up associated length of reference bar (in mm), R(t) is the reading of
gauge with sample in dilatometer (in mm) and Lplug is the set-up associated length (thickness)
of one end plug (in mm).
Time-zero needed in Eq. 3.4 was acquired by applying the method described Section 4.3.1.
To better understand relation between changes in courses of autogenous shrinkage and hydration process, temperature evolution was monitored for some mixtures. A small hole was drilled in the middle of both types of corrugated tubes and, subsequently, was sealed. After specimen of interest was prepared and placed in the measurement position, the tiny gap was freed from the cover and PT100 temperature sensor was inserted. Having connection with data acquisition system, actual temperatures were recorded for the first 24 hours at minimum. The data were collected when the temperature changed more than 0.5 °C or, but not rarer than, every 5 minutes.
Autogenous shrinkage combined with the effect of drying to ambient, often described as total
shrinkage, was measured using improved protocol of DIN 52450. The main modification was
automatization of testing procedure and extension to more hardened concrete samples measured at once, see Figure 3.10.
Figure 3.10: Apparatus used in total shrinkage measurement: custom-built device for automatic data recording
(to the left) and Graf-Kaufmann set-up for manual records (to the right).
The examination was performed on small beams having cross-section of 40x40 mm² and length of 160 mm, hereafter referred to as prism. Prior to casting, two measuring pins were fixed in every mould compartment, and thus becoming part of test specimen. The sealed configuration followed after the concrete had been poured. At age of demoulding of 1 day, the specimens were either transported to the measuring device or wrapped in thick plastic foil to avoid moisture loss until examination day (additional age of 7 days presented in work at hand; for other demoulding ages, please see e.g. [Dud 10a]). During test, automatic data collection was processed under temperature of 20 °C and 65 % of relative humidity. To limit amount of data acquired, the measuring intervals of 5 to 30 minutes were adapted based on current deformation rate recorded. Depending on measuring place availability, three and exceptionally two nominally identical specimens were produced and the average value is presented. Each specimen removed from automatic device underwent immediate manual measurement on original Graf-Kaufmann set-up and the manual measurement continued with few days intervals.
Additionally, some extra prisms though deposed of measuring pins were produced from the selfsame batch of concrete as the shrinkage prisms and the mass loss was recorded. Whenever total shrinkage-mass loss relationship knowledge was aimed at, automatic data acquisition was executed. This involved using balance with accuracy of 0.01 g connected to data acquisition system and record intervals similar to ones set in shrinkage measurements. One specimen could be tested at once; however, as manual control of mass changes using selfsame balance and more samples showed (results will not be presented), the difference between
Measuring frame Reference prism Specimen holder Apparatus main frame Gauge
replicates is negligible, i.e. one specimen was sufficient for this kind of measurement. Mass loss measurements on samples with other geometries than prisms were executed as well, see discussion of Section 4.6.3 for details. Given manual control and number of samples (minimum three per mix and each testing age), less frequent data collection was possible in this case.
The measurement of deformations due to shrinkage under external restraint was carried
out using the instrumented ring test in two various set-ups: One as prepared acc. to ASTM C1581 – 04 and another one built by a project partner, Dr. Eppers (formerly German Cement Works Association) [Epp 09][Epp 10]. The smaller geometry of the latter set-up showed in Figure 3.11 allowed favourable hydration heat dissipation and thus minimising of temperature effects.
Figure 3.11: Apparatus used in instrumented ring test measurements: C1581-04 based set-up (to the left) and Dr.
Eppers’ adapted apparatus (to the right).
Addressing idea of test, a annulus made of fresh cementitious material is cast around a steel ring. As the cement hydration progresses and material transforms from fluid to solid, the annulus starts to shrink. In turn, the concrete ring is pressurized by the steel one, resulting in development of tensile stresses in the former (in circumferential direction). The main quantitative outcome of the test is the steel ring strain change measured on its inner surface which, using appropriate analytical solution, can be translated into the corresponding tensile stresses in concrete.
Until age of 24 hours, a sealed configuration was always applied. This typically involved covering the rings with saturated burlap and/or few layers of plastic foil. Some minimal distance between the cover and concrete surface was however maintained. This was found
necessary to ensure that the cracking, if occurred, was strictly attributed to restraining of shrinkage and not stress concentration around uneven surface area, for instance. To maintain sealed conditions during measurement in following days, the assistance of the outer steel ring was preferred; meanwhile, the top surfaces of the concrete annuli were protected by a dual layer of self-gluing aluminium tape. Sometimes, when new concrete rings were to be cast, the outer rings had to be however removed. In such case, exposed surfaces were immediately sealed in selfsame manner as the top. The outer mould elements were eliminated intentionally only when measurement of combined effect of restraint and drying from top and bottom
or, alternatively, from circumference was planned (age of 1 day and, for some mixtures,
also 7, 14 and 28 days; for most important results see e.g. [Dud 14]). The conditions during examinations were identical: 20 °C and 65 % RH.
At maximum, three concrete rings could be produced from selfsame concrete batch, including two for identical and one for contrary curing condition. To increase reliability of the result, however, some mix repetitions were done as well. The data were collected from the four strain gauges glued to inner surface of the inner steel ring and the average for each ring is reported. To limit amount of data acquired, the measuring intervals of 5 to 30 minutes were adapted based on current deformation rate recorded. The measurement continued until through-cracking appeared (signalized by abrupt drop of strain to approx. zero), the strain approached asymptotic value or both outer and inner steel rings had to be reused in next experiments.
To assess tensile stresses developed due to restrained deformations and hence to be able to estimate quantitatively the tendency of these concretes to crack, computation based on strains recorded in ring measurements (case of big IRT) was performed acc. to Eq. 3.5 [Hos 04][Yoo 14]:
( )
2 2 2 2 2 2 2 2 OS IS OS OS OC OC OS S Steel Max Actual R R R R R R R E t ⋅ − ⋅ − + ⋅ ⋅ − = − ε σ (3.5)where σActual-Max is the maximum residual tensile stress (i.e. theoretical elastic stress minus
relaxed stress) in the circumferential direction, εSteel(t) is the measured strain at time t, ROS is
mm), ROC is the outer radius of concrete ring (= 203 mm) and ES is the Young’s modulus of
steel ring. For most important results, please see Chapter 4 and Appendix I.
Cracking potential, i.e. indication of how close the uncracked concrete is to cracking, was
determined according to Eq. 3.6:
) (t ft Max Actual CR − = Θ σ (3.6)
where ΘCR is cracking potential estimated at time of interest t, σActual-Max is the maximum
residual tensile stress in the circumferential direction at time of interest t and ft(t) is the
splitting tensile strength at time of interest t, which in own study replaced by corresponding flexural strength.
For sake of correct interpretation of results, it should be pointed out that the tensile strength of unloaded concrete (short-term tensile strength) is typically higher than that of subjected to sustained loading [ref. 8, 10, 11 Ibid. Sch 02]. In the instrumented ring tests, this load will be present and will result from tensile stresses generated due to restraining autogenous shrinkage deformations. Simultaneously, measurement of flexural strength should give higher values of tensile strength than one measured in tests of splitting tensile strength, as confirmed, e.g. average of 10.3 MPa (Appendix I) vs. approx. 7 MPa [Epp 10] recorded for concretes of very similar compositions at the age of 1 day. That is to say, by using Eq. 3.6 and input data as declared, cracking potential would be underestimated. Nonetheless, the results were used for different purpose, in particular for assessment of relative changes enabling evaluation of IC effect also under the influence of external restraint. For most important results and ages at which concretes cracked, please see Appendix I.
Pressure transducers RVAP015GV from SensorTechnics allowing measurement in the vacuum range were used to monitor the capillary pressure evolution in concrete. The equipment was arranged in such way so that sensors were protected from destructive action of pore solution while entrance of fine concrete components (air, tiny aggregates) was prohibited. For this reason, beside the transducer, one full test set-up consisted of a tube (3 mm inner diameter), sponge-like membrane, sealant, sample mould (analogous to that used in ultrasonic measurement, although made of a stiffer material) and connection to data logging system. In principle of measurement, the vacuum created by the self-desiccation of the matrix is replaced by the de-aired water passing from the tube though the membrane to
concrete. This causes a depression in tube which is measured by the pressure sensor connected to the data logging system.
For sake of accurate measurements, special preparation protocol was followed. First, the transducer and attached sealant, i.e. flexible 2 cm-long rubber hose, was filled with de-aired distilled water. A medical syringe equipped with needle smaller than transducer opening was used for this purpose, allowing injection of liquid from the sensor’s bottom. This action continued until sensor contained nothing but water and only water filled the volume of the transparent sealant. As next step, the same procedure was repeated for the translucent stiff tube, however, using rubber hose instead of needle for the syringe ending. When no air bubbles remained in the tube and a drop of water covered its tip, both water-filled elements were connected with each other. Finally, after disconnecting of syringe and removal of air from the tube, if any, the other free end was closed with wet sponge underwater using needle as help.
Such prepared measuring element was inserted into a hole in the mould drilled beforehand and its horizontal position was fixed at height identical to that used in ultrasonic or temperature measurements. In subsequence, concrete was poured, followed by very gentle vibration of sample by own hands. When this finished, the top surface of sample was covered with few layers of plastic foil to ensure that sealed conditions are maintained and the measurement began. It was deduced that the apparatus operated correctly when the pressure record was slightly positive, approx. 2 kPa. Four samples were examined for each mixture, with automatic records being taken with interval of 5 min. For collection of data, software written for Labview® was utilized. The ambient temperature was 20 ± 1 °C.