Durability of concrete and cement composites

Testing of hardened concrete specimens

Reference to tests on hardened concrete specimens may become commonplace in future specifications. These tests are described in documents from standards organisations and currently, or in the recent past, comprise ASTM C666, ASTM C671, ASTM C672, ASTM C682 and CEN/TS 12390-9. The CEN technical specification (12390-9) is based on Swedish Standard SS 13 72 44, DAfStb Heft 422 (Bunke, 1991) and RILEM recommendation TC 117-FDC. The essential features of the methodologies employed in current tests are outlined.

Microcrack development, ASTM C666 The American standard method ASTM C666 involves checking internal damage associated with microcrack development by determining the dynamic elastic modulus during severe freeze-thaw cycles. The test specimens are exposed to a maximum of 300 freeze-thaw cycles in a temperature range of ‡4.4ëC to ÿ17.8ëC. The cycles may be conducted in containers of water or the specimens may be frozen in air and thawed in water. The cycles are short with rapid freezing and cooling. The dynamic elastic modulus is monitored after various numbers of cycles to yield a `durability factor’ based on the initial dynamic modulus, the dynamic modulus after 300 cycles and the number of cycles required to reduce the relative dynamic modulus to 60%.

Surface scaling, ASTM C672 Surface scaling may be encouraged by ponding water or deicing solution on concrete slabs that are subjected to freeze-thaw cycles. American standard method ASTM C672 involves checking the visual appearance of the surface at intervals over a programme that extends to 50 cycles. Rectangular specimens with a minimum surface area of 72 square inches (approximately 46 500mm2) and minimum thickness of three inches (approximately 75 mm) are cast, cured and surmounted by a freeze-resistant dyke to form a pond. The test regime is illustrated in Fig. 8.7.

Scaling, CEN/TS 12390-9 The convergence of practice towards a common European test has focussed on tests for scaling resistance. A pre-normative standard was submitted to CEN members for enquiry in 2002 as prEN 12390-9, Part 9 of the test method series on hardened concrete. It was recommended that it be published as a European Technical Specification and not as a Euronorm. The specification provides three methods for testing of slabs and the reference method is based on a Swedish standard. The alternative methods are based on a German cube test and a RILEM test that uses specimens cast in standard cube moulds (two specimens per mould). The reference method involves monitoring the amount of material scaled from the surface of sawn concrete specimens exposed to 56 cycles of freezing and thawing, while covered with water or salt solution. Four specimens are prepared per test as slabs, 50mm thick, with 150mm x 150mm sawn surfaces derived from 21-day-old cubes. Rubber sheets are glued to the faces other than the sawn test face. The 25-day-old specimens are sealed into an insulated mould and covered, at an age of 28 days, by a freezing medium to a depth of three millimetres, creating the test set-up illustrated in Fig. 8.8. The freezing medium may be either demineralised water or a three per cent sodium chloride solution. The medium is covered by a polyethylene sheet to prevent evaporation. A daily freeze-thaw cycle is achieved by exposing the specimen to a temperature range of ‡20ëC (4-+4C) to -20C (+-2C)

in accordance with a specified time-temperature curve, illustrated in Fig. 8.9. The amount of scaled material is assessed after 7, 14, 28, 42 and 56 cycles. The cumulative value after 56 cycles is used for evaluating the scaling resistance.

 

that cast against the PTFE ± is untreated but all other sides are sealed by aluminium foil and butyl rubber or by epoxy resin. The 28-day-old specimens are partially immersed to a depth of approximately five millimetres in the freezing medium. The nature of the prepared specimen is such that capillary suction is encouraged through the test surface only. The specimens are subjected to two freeze-thaw cycles per day by cycling from ‡20ëC to ÿ20ëC in each twelve-hour period. The test set-up and the time-temperature curve are illustrated in Figs 8.12 and 8.13, respectively. The test specimens are placed in containers which are put into a liquid cooling bath. The amount of scaled material is determined after 4, 6, 14 and 28 freeze-thaw cycles in the case of sodium chloride immersion, and after 14, 28, 48 and 56 freeze-thaw cycles in the case of demineralised water immersion. The scaled material is detached from the specimens by placing the containers in an ultrasonic bath. The mass of scaled material in the solution is then analysed. Scaling resistance is evaluated by determination of the mean and individual values after 28 cycles (CDF) and 56 cycles (CF).

 

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