13 FROST EFFECTS ON THE MICROSTRUCTURE OF HIGH STRENGTH CONCRETE

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1 13 FROST EFFECTS ON THE MICROSTRUCTURE OF HIGH STRENGTH CONCRETE H. KUKKO Technical Research Centre (VTT), Building Technology, Finland 1. Introduction The purpose of this study was to clarify the internal changes in high strength concrete during destruction caused by freezing and thawing. High strength concretes were studied experimentally with five different compositions. Medium strength concrete was included for comparison. The concretes were subjected to freezing and thawing without deicing salts. In different states of degradation the strength of concretes and changes in ultrasonic pulse velocity and capillary water uptake were measured. The microstructure was analysed using mercury porosimetry and microscopical methods in order to determine the structural changes in the concrete. 2. Test concretes The mixes used in the tests are shown in Table 1. Cement grade P40191 refers to lowheat, sulphate resistant Portland cement with prism compressive strength 40 MPa at age 91 days. Cement P4013 is a very rapid hardening Portland cement with prism strength 40 MPa at age 3 days and cement P4017 is a rapid hardening cement with prism strength 40 MPa at age 7 days. 100

2 P - - P Table 1. Mix proportions. 1) WaterPortland cement ratio in parentheses. The first three mixes were planned for in-situ concreting and the rest of the high strength concretes (mixes 4-5) for precast element production. Mix number 6 was an ordinary medium strength concrete. The workability of the mixes was good and slump varied between 9.5 and 18 cm. In the remainder.of the text the test concretes are referred to as shown in Table 2. Table 2. Concrete mixes according to binder composition. Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6 Slow hardening cement with silica Rapid hardening cement Rapid hardening cement with fly ash Rapid hardening cement with silica Rapid hardening cement with slag Medium strength concrete 3. Compressive strength results The compressive strengths (all converted to 100 mm cube strengths without capping) of different mixes at and days are presented in Table 3.

3 Table 3. Compressive strength results of concretes. - Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6 Concrete Slow hardening cement + silica Rapid hardening cement Rapid hardening cement + fly ash Rapid hardening cement + silica Rapid hardening cement + slag Medium strength Age days 1) Time in wet curing. Additional stiring in 70% RH. Compressive strength (MPa) wet 5 or 3 d no curing curing 1) curing Freeze-thaw tests Test specimens used in freeze-thaw tests were ( mm31 beams. All concretes were cured for 3 or 5 days at >95% RH (wet curing) and freeze-thaw testing started at age. days. Additional freeze-thaw tests for high strength in situ concretes, with strength measurements alone, were done at the same time with uncured specimens in order to study the effect of eventual poor curing at building sites. The freeze-thaw test was performed according to Finnish standard SFS 5447 and comprised 1000 cycles. In each cycle the specimens were frozen in air at -20 'C and thawed in water at +20 'C. The flexural and compressive strengths of the concretes were measured. The flexural strength results are shown in Tables 4 and 5.

4 Table 4. Flexural strength results of test concretes in freeze-thaw tests. 3 or 5 days wet curing. 1) (150 cycles) Table 5. Flexural strength results of test concretes in freeze-thaw tests. No curing. 5. Mercury porosimetry tests The pore structure of concretes was studied using mercury porosimetry prior to and intermittently during freeze-thaw treatment. The purpose of the measurements was to measure changes in porosity of concretes due to freeze-thaw action. Some of the results of mercury porosimetry tests are presented in Figs. 1 and 2.

5 comparison cycles " 500 cycles * 1000 cycles Fig. l. Pore size distribution of slow hardening cement and silica (mix 1) with no curing, after 0, 300, 500 and 1000freeze-thaw cycles. Fig. 2. Pore size distribution of concrete with rapid hardening cement (mix 2) and no curing, after 0, 300, 500 and 1000ffeeze-thaw cycles. Clear changes have occurred in the mercury porosimetry results of degrading concretes. It can be stated from Figs. 1 and 2 that in the capillary pore range of diameter below about 50 nm, the pore volume first gradually increases with the seriousness of freezing

6 defects. The increase of capillary pore volume was most significant in concrete with slow hardening cement and silica (mix 1). Once the maximum capillary pore volume was reached and frost defects became serious, the mean capillary pore size seemed to decrease. The principle of these changes is shown in Fig. 3. pore radius 7 Fig. 3. Principles of porosity changes during freeze-thaw tests. 6. Image analysis The development and use of the new method for image analysis of thin sections was an essential part of this project. The method was used in this study to examine changes in pore and crack skcture in specific test concretes. For analysis of individual features (pore or crack) e.g. the following measurements were made: Area of feature, its longest dimension, feature breadth, volume of equivalent sphere and elongation ratio. In these analysis, features were expressed in terms of pores and cracks. Some of the results are graphically presented in Fig. 4. There is usually no clear limit value between pores and cracks in high strength concretes. Unlike air pores in normal strength concrete, which are typically round, the feature shape in high strength concretes changes gradually in practice. Because no clear border value was found, the border of definition between pores and cracks was chosen as an elongation ratio of 0.33, i.e. a feature is a crack if its maximum dimension is more than three times its breadth.

7 compor~son at 1 m 1 5 F 5 0- > >z 0 <$ Y U 0 n L O L 0.5 5' 0.5 U -0- U 300 cycles 5 g C 0 22 E 0 Q 0 owc I am2 I s I l.0 m 2 0 O.m oaa 051 o m oaa ereadth (mm1 BREADTH (mm) m cracks ez2 pores m cracks pores J -- 1 ' 1000 cycles BREAOTH (mm1 m cracks a pores m cracks ed pores BREADTH (mm1 Fig. 4. Pores and cracks in concrete with slow hardening cement with silica (mix 1). As seen from the figures above, the sorting of features into pores and cracks is not accurate. When compared with visual investigation, the share of pores is overestimated at the coarse end of the measurement range. The main reason is the error resulting from analysis of large cracks compared with the length of the edge of each measurement area in thin section. This appears in the results as wide cracks cut into pores. 7. Electron rnicroscopy In the mercury porosimetric studies, changes were observed in the capillary pore size area during freeze-thaw tests in the degrading concretes (item 5). The purpose of this part of the study was to clarify the reasons for these unexpected changes using scanning electron microscopy (SEM). Samples of test concretes were studied prior to and after frost tests. Fig. 5 shows an example of the observed microstructure.

8 Fig. 5. Concrete with rapid hardening cement and slag after 500 freeze-thaw cycles. Aggregate bond crack with ettringite crystals. When frost defects occur in high strength concretes, needle-like crystals form more frequently in the cracked or porous areas of test concretes than in the comparison concretes, stored in water curing. Both the size and amount of these needle-like crystals are clearly greater than in comparison ccncretes. At the start of the frost tests and in comparison concretes, roselike formations of plate crystals or a less orientated structure was the predominant form of material precipitated on free surfaces. Elongated crystals were less common and large (length several tens of micrometers). Calcium hydroxide-type precipitation was not commonly seen in the SEM studies. Elsewhere in the inner structure of high strength concretes, morphological details are usually not clearly detectable by SEM due to the dense structure and small size of crystals. The structure seems most often amorphous. In the electron microscope study it was discovered that the evident reason the changes observed in high strength concrete porosities with mercury porosimetry is the precipitation of solid material in newly formed cracks. The needlelike crystals first either form a relatively loose network on the walls of cracks, or fill narrow cracks loosely. In the mercury porosimetry results this is seen as an increase in the amount of capillary pores, as mercury cannot easily penetrate these cracks through the crystal network. The seeming later reduction of the mean paste pore size may be due to the felt-like precipitations becoming denser, and perhaps partly also because the growth of crack breadth exceeds the rate of crystallisation. The latter seems to cause loosening of the

9 felt-like formations of elongated crystals from the crack edges, and presumably these collapsed in the mercury porosimetry test. 8. Reference Kukko, Heikki. Frost effects on the microstructure of high strength concrete, and methods for their analysis. Espoo 1992, Technical research Centre of Finland, VTT Publications p. + app. 44 p.