Verification of the resistance of concrete paving blocks to freeze-thaw with de-icing salt in accordance with DIN EN 1338 [DIN38 has been required from manufacturers since 2005. The test method for determining the so-called “resistance to frost and freeze-thaw cycle with de-icing salt” in FTV’saccordance with Annex D will in the following be called “slab test” for short. Slab-type test specimens at least 28 days old are subjected to 28 frost-thaw cycles with de-icing salt, with a 3 % NaCl test solution.
Despite specifying the test method to be used, public tenders frequently specify testing based on the CDF method to DIN CEN/TS 12390-9. For some construction projects, paving blocks are specified that show a surface scaling far below the limit value of DIN EN 1338 (1.000 g/m²) as given in the slab test. References to the Austrian Standard are also requested. One reason for the non-uniform tendering practice, among others, is a certain degree of insecurity regarding the question of whether the resistance to frost and freeze-thaw cycles with de-icing salt can be sufficiently reliably assessed by the slab test. The clearly higher mass, as a rule, of scaled material obtained by the CDF test alone, apparently provides a good feel for being on the safe side. Whether the two test methods in their current form sufficiently represent the resistance of installed concrete paving blocks to freeze-thaw cycles with de-icing salt under environmental conditions was investigated by the described research project by VDZ gGmbH and the University of Kassel.
Realistic utilization simulation
Freeze-thaw cycles (FTC), rainfall and spreading of de-icing salt on concrete block pavements were simulated in frost chambers and subjected to acceleration. Concrete paving blocks from large-scale production (see table) and concrete paving blocks from a testing facility (additionally manufactured for the purpose of investigation, owing to insufficient differences in the quality of concrete blocks from large-scale production) were laid in a rectangular bond on an underground of crushed basalt 2/5 mm. Basalt chipping 1/3 mm was brushed into the joints between the concrete blocks (see Fig. 1).
Twice a week, rainfall was simulated with 1 liter per m² paved area and salt-spreading procedures, with 130 g/m² of de-icing salt. Minimum temperatures and precipitation quantities were determined following assessment of measurement series by the German meteorological service for Düsseldorf and Kassel recorded over 30 to 50 years. In 80 % of the FTC, a minimal air temperature of -6 °C (5 % fractile value of the basic population, of the values measured by the German weather service) was regulated. A total of 20 % of the FTC were performed down to -15 °C (1 % fractile value). The condition of the surface of the concrete block pavements was visually monitored, and the mass of the pallets with the experimental pavements was continuously gravimetrically determined. The temperatures in the air and in the concrete block pavers were recorded.
Concrete paving blocks from large-scale production
Three manufacturers of concrete paving blocks delivered blocks from their regular production for the investigations: i.e., complete production sections from the manufacturers’ pallets. Various types of concrete blocks were manufactured on blockmakers with various adjustments (types W1, W2 and W3 in Table 2), and block types made with different formulations (types W4 to W6) were investigated. The density of the blocks (footbath method) reacted inversely proportionally to randomly determine capillary water absorption. The block density was used as criterion for choosing a range as large as possible of the blocks to be investigated.
Results: The facings of the concrete paving blocks from large-scale production (types W1 to W6 in Table 3) exhibited scaling far below the respective criterion established both in the slab-test and in the CDF procedure (1,000 g/m² for the slab test and 1,500 g/m² for the CDF procedure.)
Block types W1 to W4 passed both the slab test and the CDF procedure and exhibited no damage to the facing concrete, including during storage in the frost chamber. The ultrasonic run time was uneventful as well.
The concrete pavers of type W5, after passing the slab test and the CDF procedure, after 958 FTC during storage in the frost chamber, experienced initial cracking to the facing concrete. After 1,334 FTC, clearly recognizable loosening of the structure had developed on the corners and edges of the facing concrete. There, the facing concrete crumbled in fine grains (see Fig. 2.) Following removal, partial horizontal cracking became visible in the facing concrete (see Fig. 3).
The facing concrete in the type W6 concrete paver had partially detached itself in large sheets from the damaged backing concrete (see Figs. 4 and 5.)
Following the testing in the frost chamber, a total of two thirds of the blocks of type W5 and W6 placed in the frost chambers exhibited visible damage to the facing concrete. Damage to type W5 facing concrete was preceded by a marked change in ultrasonic run time. Testing in accordance with the standard (the facing concrete is the test side) of the pavers from the testing facility with the slab test (EN 1338 Annex D) showed no negative results, the same as with the CDF procedure.
Concrete block pavers from the testing facility
The mix formulations of the concrete block pavers manufactured on the HeidelbergCement AG testing facility for the purpose of the investigations were based on formulations provided by two manufacturers. Concrete block paver types B8 and B9 were manufactured with various vibration times. Types K10 und K11B differed in the composition of the concrete.
The densities of the concrete block pavers manufactured on the testing facility were by up to 0.14 kg/dm³ lower than the pavers manufactured by large-scale production. All of the 4 paver types from the testing facility met the requirements of DIN EN 1338 DIN EN 1338 for the maximum water absorption of 6 M. %.
Results: after approx. 400 FTC in the frost chamber, the surfaces of types B8, B9 and K11b showed no visually recognizable damage. The ultrasonic run times up to this point of time were not significantly changed. For type K10, in contrast, marked initial cracking occurred below a lifted corner of the facing 256 FTC during investigation in the frost chamber. After 326 FTC (which corresponds to approx. 7.5 years of exposure to frost conditions 1 [RST12]), several corners of the facing of type K10 exhibited vaulting. At random locations, starting on the sides of the lifted corners, cracks running parallel to the surface were visible. The average ultrasonic run time (USL) after investigation in the frost chamber, at 53.9 ms was significantly higher compared to the reference value (35.0 ms). Fig. 6 shows an overall picture on the USL of type K10. At 109.4 ms, the maximum USL occurred in paver 10/11/3 (i.e., the number of the type / the pallet of the manufacturer / the paver). After 480 FTC, the facing concrete completely detached itself from the backing concrete (Figs. 7 and 8.)
The concrete block pavers on the testing facility were tested for comparison in four laboratories using two standard testing procedures (slab test und CDF procedure). Testing took place both on the facing and on the backing concrete (turned block pavers). In addition, pavers from the testing facility were stored in the frost chamber. The results are shown in Table 4.
The behavior of type K10 in the frost chamber in installed condition agreed with the result of the slab test, when the slab test was performed on the backing concrete (turned paver.) However, testing of the facing concrete by the slab test provided no indication of the behavior of type K10 in installed condition.
In all of the tests performed by the four laboratories, the facings of types K10 and K11b tested with the CDF procedure showed scaling in excess of 1,500 g/m² (criterion in accordance with SET94). For all four-block types, one laboratory determined scaling in excess of the limit value (1,500 g/m².) This is assumed to be due to the influence of not entirely complete sealing on the sides of the blocks.
While the blocks of type K11b showed sufficient resistance to freeze-thaw on the facing and the backing concrete, the test CDF procedure resulted in failure in all the laboratories. The backing concrete of the uneventful types K9 and K11b experienced very high scaling in the CDF procedure and would therefore not be assessed for this test regarding their behavior in installed condition.
Bond strength between facing and backing concrete
A possible damage pattern of installed pavers is the spalled facing concrete. Investigations of the bond strength adhesive bond strength in accordance with DIN EN 1542) between facing and backing concrete showed that failure, as a general rule, occurs in the backing concrete, when it contains less than 300 kg/m³ of cement. If the backing concrete, in addition, is not resistant to freeze-thaw with de-icing salt, it will thereby be damaged and the bond to the facing concrete is lost (see Fig. 9).
Currently, in testing the resistance to freeze-thaw with de-icing salt, only the degree of scaled material from the facing concrete is assessed, in practice with the slab test and CDF procedure laboratory testing methods, also for two-layer concrete paving blocks. An assessment of the core concrete is not included in either tests.
Comparison of the results on concrete block pavements under realistic conditions with the results of the slab tests and the investigations with the CDF procedure permits the following conclusions to be drawn:
Transferability of the results from the laboratory method for determining the freeze-thaw resistance with de-icing salts of two-layer concrete paving blocks to behavior in installed conditions is enhanced when, in addition to the facing concrete, also the backing concrete is investigated by a slab test (in accordance with DIN EN 1338).