Influence of temperature and relative humidity on the drying of concrete

Transcription

1 Concrete Science and Engineering, Vol. 2, March 2000, pp Influence of temperature and relative humidity on the drying of concrete L. Wirtanen and V. Penttala Helsinki University of Technology, Building Materials Technology, P.O. Box 2100, FIN HUT, Finland RESEARCH PAPERS ABSTRACT The influence of ambient conditions on the moisture distribution of concrete has been studied in order to make it easier to specify the starting time for floor covering works. Four different types of concrete were prepared to quantify the influence of the external conditions on the humidity level of the concrete pore system. The water/cement (w/c) ratios used were 0.89, 0.55, 0.39 and The concrete with w/c ratio 0.39 had an air content of 9.5 %. Another group of four concretes of similar w/c ratios was used to determine the pore structure and the amount of chemically bound and evaporated water in order to give information about the factors influencing the drying process. The test results indicate that the RH-value of the concrete pore system decreases more rapidly when the w/c ratio decreases and its response to changes in the ambient conditions is also more pronounced. A decrease in the w/c ratio decreases the amount of evaporated water and the continuity of the pore structure irrespective of curing conditions. It also reduces both the capillary and total porosities. A high air content makes the concrete pore structure more accessible and allows a more even humidity distribution and greater diffusion. 1. LITERATURE REVIEW 1.1 Moisture flow and diffusion The main characteristic influencing the moisture properties of concrete is the water/cement (w/c) ratio. Water appears in concrete in two forms; chemically and physically bound water. In the following presentation the terms evaporable and non-evaporable water will be used since the distinction between chemically and physically bound water may be controversial. The evaporable water is defined as the water evaporated when drying at 105 C and it determines the moisture content of concrete. Evaporable water is present both as interlayer water bound between layers of hydrated material, adsorbed water molecular layers fixed to the pore surfaces, and as capillary condensed water in the pores. Some of the nonevaporable water is also present as interlayer water while the main part of it has reacted with cement during the hydration process and is bound in the hydration products. The principal factor generating moisture transport in a porous material is differences in the water vapor pressures in the pores (pore pressures). These differences are caused by a humidity gradient at constant temperature or a temperature gradient. Moisture in the bulk sample is transported from a zone with higher RH towards a zone with lower RH. It can also be transported to or from the sample. Moisture is transported either in a vapor phase, a liquid phase or in a combination of the two. Vapor transport is driven by a vapor content gradient in the air-containing pores and starts at relatively low levels of relative humidity. It is, thus controlled by the partial gas pressures and is a rather slow process. Higher levels of relative humidity develop water meniscus in the narrow pores. Water is transported in such pores by capillary forces resulting from the surface tension at curved water menisci [1]. The rate of moisture flow q m (kg/m 2 s) at pure diffusion can be described by Fick s law with the water vapor content c (kg/m 3 ) as a gradient: qm = Dw c (1) D w is the water vapor transport coefficient (coefficient of diffusion, m 2 /s) and x is width (m). The rate of capillary suction (liquid water transport) can be described by Darcy s law where the pore water pressure, p w (Pa) is the driving force: qm = k pw (2) k is the capillary suction coefficient (s) and x is width (m). Water content is often used as the potential for liquid water transport since it is difficult to measure the ISSN /00 RILEM Publications S.A.R.L. 39

2 Concrete Science and Engineering, Vol. 2, March 2000 pore water pressure in hardened concrete. A possible flow equation for the total flow would be: qm = Dw c we k (3) w e is the amount of evaporable water (kg/m 3 ) and the other coefficients are as mentioned in formulas (1) and (2). It is, however, difficult to separate diffusion and capillary suction when measuring the rate of moisture flow. The two phases can be considered as one total flow described as a potential of some quantity times a transport coefficient. The transport coefficient is greatly dependent on the moisture content of the material or the relative humidity in case of isothermal conditions. A common description is: qm = D m we (4) D m is moisture diffusivity. If, however, a temperature gradient does exist, both phases must be considered separately since vapor transport and capillary suction can function in opposite directions [1]. The coefficient of diffusion, D w depends on a number of factors. Bazant and Najjar [2] showed that it is greatly affected by the water content and it decreases to almost 1/20 when the relative water content decreases from 100% to 60%. This implies that a linear diffusion equation is not appropriate for analysis, and that a non-linear diffusion equation must be adopted. Another factor that affects the coefficient of diffusion and favors the use of a non-linear diffusion model is changes in the concrete properties with time i.e. the transport parameters are dependent on the degree of hydration [2, 3]. Šelih et al. [4] indicated that there are two distinct periods during the drying of concrete. In the initial stage of drying, a high and constant rate of moisture content change was observed for all types of concrete tested. The moisture content profile kept its initial shape as long as the drying rate remained unchanged. The existence of a constant drying rate period indicates the presence of free liquid water whose outward flow is driven by capillary forces. This type of drying occurs typically during the first 3 to 7 days of curing. The connection between the capillary pores will break down as the water filled pores in the capillary pore system become smaller during drying [5]. If and when this happens is a function of water/cement ratio and the degree of hydration. A gradual transition from a capillary flow region to a diffusion flow region is then observed. Drying rates are greatly reduced when diffusion becomes the prevailing mechanism of moisture flow. The drying of concrete becomes diffusion controlled, according to experimental results, when the average moisture content decreases below 70 to 80% of the initial saturation. [4]. The composition of a concrete mix is typically non-uniform in the direction of diffusion due to the inhomogeneity inherent in the concrete construction practice and the effect of thermal gradients. This fact affects the coefficient of diffusion and further favors the use of a non-linear diffusion model [3]. Sadouki & Mier [6] indicated additionally that moisture diffusivity is independent of crack growth. This hypothesis seemed justified by experiments demonstrating that the drying rate increased very slightly only when hygral cracks, i.e. cracks caused by hindered drying shrinkage, developed. 1.2 Drying of concrete The humidity level of concrete is affected by composition, age and curing conditions. Moisture variations in concrete and other cement-based composites results from internal drying induced by the hydration reactions of cement and water at an early age, and from climatological conditions during the life-span of the structure. The temperature of the ambient air affects the water vapor pressure in the pores. The equilibrium moisture content at the structure s surface is affected by the relative humidity. A change in the curing conditions causes a redistribution of moisture. The magnitude of change in the concrete is dependent on the magnitude of change in the external conditions (temperature/humidity interval). To observe the influence of external conditions on the humidity distribution of concrete is of importance when defining the starting time for floor covering works. 2. EXPERIMENTAL PROCEDURE 2.1 Materials, mixture characteristics and initial curing Four different mixtures were cast for the relative humidity and temperature measurements. Two different normal strength (NSC), one high-strength (HSC) and one air-entrained normal strength test concrete was studied (AEC). The two latter concretes were chosen because they have significantly shorter drying times than ordinary normal strength concretes. Another group of four different mixtures was cast for the determination of the chemically bound and evaporated water amounts and the changes in the pore structure during curing. These consisted of one NSC, one HSC and two AEC test concretes. The AEC test concretes had 40

3 Wirtanen, Penttala w/c ratios of 0.54 and 0.39, respectively. These were chosen to study further the influence of air entraining, because the w/c ratio limit for self-desiccation is approximately The composition of all mixtures prepared for the measurements, test results for the fresh concrete and the compressive strengths are summarized in Table 1. The cement used was a rapid hardening blended cement CEM II A 42,5 R. The chemical analysis of the cement is given in Table 2. A naphthalenebased superplasticizer was added to HSC and the AEC test concrete having w/c ratio 0.39 in order to obtain a consistency of 1-2 in the VB -consistometer test. The air-entraining agent used is based on a fatty acidic soap. Silica fume was added only to the high-strength concrete. The moulds used for humidity measurements were plastic cylinders (height and diameter 250 mm). The bottom of the cylinders were cut out and fastened again to the cylinders with adhesive tape before the casting was done in order to allow the test specimens to dry from two surfaces when the humidity measurements began. Their compaction was carried out with an immersion vibrator. The 100 mm test cubes were compacted using a table vibrator. The cylindrical specimens were first cured for 5 days under a tight plastic sheet. The cubes were demoulded after 1 day and wrapped into plastic for another 4 days. This was followed by the removal of the plastic covers and the bottoms of the cylinders. The first humidity and temperature probes were next installed into the cylindrical specimens and all test specimens were moved to respective curing conditions: +5 C (RH45% or 70%), +12 C (RH45% or 70%) and +20 C (RH45% or 70%). These curing conditions represent common outdoor climates in Finland not requiring any special concreting techniques. Table 1 Composition, fresh concrete properties and compressive strengths of the test concretes Composition: NSC25 NSC40 AEC25 AEC30 HSC70 Cement (kg/m 3 ) Water (kg/m 3 ) w/c ratio Aggregate (kg/m 3 ) Silica fume (kg/m 3 ) Superplasticizer (kg/m 3 ) Air-entraining agent (kg/m 3 ) Properties: Consistency svb Air content (%) Density (kg/m 3 ) Compressive strength (28d) Table 2 Chemical composition of the cement (CEM II A 42,5 R) Chemical analysis (%) Shipment I Shipment II Calcium oxide (CaO) Silicon dioxide (SiO 2 ) Aluminium Oxide (Al 2 O 3 ) Ferric oxide (Fe 2 O 3 ) Magnesium oxide (MgO) Potassium oxide (K 2 O) Sodium oxide (Na 2 O) Sulphur trioxide (SO 3 ) Physical properties: Ignition loss 950 C Insoluble SO Average compressive strength 1 d (MPa) d (MPa) d (MPa) Relative humidity measurements and subsequent curing The probes used were HMP44 relative humidity and temperature measuring probes manufactured by Vaisala Oy in Finland. The accuracy of the probes is ± 2 % at RH 0 90 % and ± 3 % at RH % according to the manufacturer. However, the accuracy of the RH-change in a probe is much better, the resolution is 0.05 %. The probes were installed into drilled holes, 16 mm, and were left there for the whole testing period. The measuring depth was 20% of the total height of the specimen, and the distance between the probes at the same surface was 10 cm. The relative humidity and temperature mea- Fig. 1 Relative humidity and temperature measuring probe installed in the concrete. 41

4 Concrete Science and Engineering, Vol. 2, March 2000 suring method is shown in Fig. 1. The probes were calibrated in the HMK13B Humidity Calibrator manufactured by Vaisala Oy before and after each testing period. The calibration was done in saturated salt solutions having a relative humidity of 11.3% (LiCl), 75.5% (NaCl) and 97.6% (K 2 SO 4 ) according to the calibration unit recommendations. The environments of +5 C and +12 C were created in freezers, Fig. 2. The freezers were provided with a thermostat, a fan for air-circulation and a saturated salt solution (sodium nitrite, NaNO 2 ) for RH 70% and silica gel for RH 45%. The environment in the freezers and the climate room (+20 C) was monitored with measuring devices and plotters that recorded the temperature and relative humidity continuously. The curing conditions for one specimen of each composition at each low temperature curing condition were changed in order to simulate temperature elevation to room temperature before the starting of the concrete floor covering works. Two different curing cycles were used. The first one was 5 days at a low temperature and 2 days in +20. The short time at +20 C was used to see how fast the temperature stabilizes at the measuring depth and to check that the probes function satisfactorily. The second cycle was 14 days at a low temperature and 7 days in +20 C. The longer time spans were chosen to ensure that the conditions in the drilled holes had stabilized before changing the curing conditions. Both cycles were repeated twice. New probes were installed in new, freshly drilled holes before each change of curing condition in the first cycle. Probes were thus installed at 5, 10, 12, 17 and 19 days of curing. The rest of the specimens were cured at constant conditions throughout the whole testing period, and probes were installed at 5, 21 and 80 days of curing. 2.3 Evaporated and non-evaporated water amounts and pore size distributions The curing environments were +5 C, +12 C and +20 C with RH 45% and +20 C with RH 70%. The curing conditions were kept unchanged over the whole testing period, tests being made at 5, 28 and 91 days. The evaporated and non-evaporated water was measured as a weight loss. The test cubes for determining the non-evaporated water were crushed and ground in order to get a maximum particle size of 1 mm. All visible aggregate particles were removed during grinding. The samples Fig. 2 Photograph of one freezer. The container to the left contains silica gel. were put into cups and dried for 5 days at 105 C and ignited at 550 C for 16 hours. The rather low ignition temperature was used to minimize the influence of carbon dioxide evolution on the test results. The results are thus relevant only for this test series, and should not be compared with other published data. The evaporated water was recorded from 5 to 180 days from intact specimens drying from all surfaces. The specimens for the MIP tests were drilled in the casting direction from the middle of the test cube. The cylinders had a diameter of 25 mm and a variable height. The test specimens were cut from the middle of the cylinders, two from each cylinder. They were dried in air prior to drying for at least 6 weeks in vacuum. The MIP apparatus (Carlo Erba 2000 WS) consists of two units, one low-pressure and one high-pressure unit. The pore volumes of the capillary and macropores were calculated from the test results according to [7]. The capillary suction test employed a procedure described in [8]. The term capillary suction is somewhat misleading, since the water suction stage fills all empty gel and capillary pores. The test specimens, two from each cube, were approximately 25 mm thick and cut in the casting direction from the middle of the cube. The surfaces of the specimens were dried in air prior to drying in an incubator. The drying temperature chosen affects the pore structure and influences thereby the suction rate [9]. A drying temperature of 105 C was chosen in this test sequence even though a lower drying temperature would probably give higher water resistance values, m- values. The reason for the use of a high drying temperature was that it is less time consuming and is almost independent of both initial moisture content and moisture distribution [9]. 42

5 Wirtanen, Penttala Fig. 3 Measured relative humidity of test concrete NSC40 stored at three different temperatures. Relative humidity of the ambient air was 45% (left) and 70% (right). 3. TEST RESULTS AND DISCUSSION 3.1 Curing at constant conditions Test results from the relative humidity measurements of NSC40 cured at constant conditions is shown in Fig. 3. The observed decrease of internal relative humidity is due to the combined effects of external drying and cement hydration. The drying trend for the other concrete types were similar, i.e. curing at a lower temperature and in a higher relative humidity of the ambient air slows down the decrease of the relative humidity measured in the concrete. The concrete composition influences strongly its humidity level, Figs 4 and 5. The humidity level of normalstrength concrete with a w/c ratio of is about 15 to 20 %-units higher compared with a concrete with a w/c ratio of 0.4 or less. The RH-level decrease of a concrete with a high w/c ratio is further almost non-existent in severe conditions (+5 C, RH70%). It is also worth noting that a concrete with a high air-content (8-10%) is influenced more pronounced by the humidity level of the surrounding air than a concrete with equivalent w/c ratio but produced without air-entrainment. Fig. 4 Measured relative humidity of four different test concretes. Curing conditions were +5 C, RH45% (full lines) and RH70% (dashed lines). Fig. 5 Measured relative humidity of four different test concretes. Curing conditions were +20 C, RH45% (full lines) and RH70% (dashed lines). 3.2 Humidity level changes caused by alterations in curing conditions A change in the curing conditions causes a redistribution of humidity in concrete, Figs. 6 and 7. In the first curing cycle, Fig. 6, the short time at +20 C was used to see how fast the temperature stabilizes at the measuring depth. It was noticed that the temperature in concrete reached the temperature of the surrounding air in approximately 18 hours. In the latter cycle, Fig. 7, the higher temperature prevailed for a week and the influence of a change in the curing conditions on the humidity level of Fig. 6 Changes in the measured relative humidity as a consequence of alternating the curing conditions between +5 C, RH45% (5 days) and +20 C, RH45% (2 days). 43

6 Concrete Science and Engineering, Vol. 2, March 2000 Fig. 7 Changes in the measured relative humidity as a consequence of alternating the curing conditions between +5 C, RH45% (14 days) and +20 C, RH45% (7 days). Fig. 8 Weight changes of test concretes NSC25 (full lines) and HSC70 (dashed lines) in four different curing conditions. concrete can be seen. Transposition from +12 C to +20 C showed similar trends as presented below. The change in the humidity level in the concrete pore system varied depending on the composition of concrete. It changed in the same manner as the relative humidity of air at constant volume changes when the measured relative humidity value of concrete was high (NSC25), i.e. a temperature rise caused a lowering of the relative humidity value and vice versa. Transposition to a higher temperature caused a rise in the measured relative humidity value when the initial humidity level of the concrete was lower. This was probably caused by a humidity transition by diffusion from the surrounding concrete into the drilled hole that can be considered a large pore. The way in which the humidity is redistributed in concrete as a consequence of a change in the curing conditions is thus dependent on the relative humidity level in the concrete pore system, the concrete pore structure and the magnitude of change in the surrounding environment. 3.3 Non-evaporable and evaporated water The evaporation of pore water slowed down after approximately 70 days of curing irrespective of the concrete considered, Figs. 8 and 9. The amount of evaporated water changed with curing conditions and composition. It diminished with decreasing temperature and increasing ambient relative humidity. It diminished also with a decreasing w/c ratio. Test concrete HSC70 had an evaporated water amount that was about 1/3-1/5 of NSC25 s for samples cured at identical conditions. A high air content influences the pore structure and increased the amount of evaporated water. The evaporated water amount, w e, can be calculated from equation (5): we = wi wn (5) w i is the initial amount of water and w n is the amount of Fig. 9 Weight changes of test concretes AEC25 (full lines) and AEC30 (dashed lines) in four different curing conditions. Table 3 Amount of non-evaporable water per gram of cement (g/g cem ) 5d; wrapped in plastics d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% non-evaporable water, see also Table 3. The results in Table 3 are mean values of three measurements for each concrete and curing condition. The calculated standard deviations were for NSC25 and AEC25 and for AEC30 and HSC70. It is rather difficult to estimate the influence of curing conditions on the amount of nonevaporable water on the basis of the results obtained. The amount increased with curing age and decreased with a decreasing w/c ratio in all other test concretes except for test concrete AEC25. The difference in the amounts of non-evaporable water of test concretes AEC30 and 44

7 Wirtanen, Penttala Table 4 Volume of the capillary pores, V c (pore radius between 5 ~ 80 nm) measured by MIP (mm 3 /cm 3 ) Fig. 10 Division of the porosity into capillary and macropores and the pore-size distribution of the test concretes after curing for 5 days wrapped in a plastic sheet and additionally 23 days in +5 C, RH45%. 5d; wrapped in plastics d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% HSC70 was rather large. This is probably a consequence of the more accessible pore system of AEC30 (see also next section). 3.4 Pore structure measured by MIP The total porosity obtained in the MIP test was divided into capillary and macropores according to the principle shown in Fig. 10. The macropores include the air entrained volume and interfacial bond cracks. The boundary between capillary and macropores was chosen at the point where the line representing the pore size distribution reaches its minimum to the right of the capillary porosity peak. The equivalent pore radius was approximately 80 nm. The capillary pore volumes, V c, are shown in Table 4. The macropore volume, V m, was significantly higher in the air-entrained concretes than in concretes without airentraining, Table 5. A larger pore volume together with an increased accessibility to the pore system allows a higher diffusion compared with concretes without air-entrainment. This phenomenon is most probably valid for all occasions. A statistical analysis (paired t-test, 95% confidence interval) verified that both the capillary and macroporosity of the different test concretes was affected by the different curing conditions and curing age. 3.5 Pore structure measured by capillary suction test The results obtained in the capillary suction test give information about the w/c ratio, the air content and the continuity of the concrete pore structure. Two parameters describing the pore volume of the test specimens were calculated. These are the suction porosity, ε suc (%) and Table 5 Volume of the macropores, V m (pore radius between ~ 80-30,000 nm) measured by MIP (mm 3 /cm 3 ) 5d; wrapped in plastics d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% the total porosity, ε total (%). wsat wdry ε suc = (6) V wpair wdry ε total = (7) V w sat is the weight of a saturated specimen after capillary suction and water immersion (g), w dry is the weight of a specimen dried in an incubator at 105 C (g), w pair is the weight of a specimen weighed in air after being stored in 15 MPa water pressure for 24 hours (g) [10] and V is the volume of the specimen (g/cm 3 ). The volume was determined according to the buoyancy principle by weighing the specimens in air and in water. The calculated suction and total porosity values are shown in Tables 6 and 7. The air porosity, ε air (%), of the different concretes is the difference between total and suction porosity i.e. pores not filled in the capillary suction test [9]. The air porosity had decreased % units between 28 and 91 days of curing, Table 8. The decrease was greatest in the airentrained concretes. It should be noticed that both test concretes AEC25 and AEC30 had exceptionally high air porosity values in specimens cured at +5 C compared with the other curing temperatures. The corresponding value for test concrete HSC70 was approximately 1 % 45

8 Concrete Science and Engineering, Vol. 2, March 2000 Table 6 Suction porosity values, ε suc, calculated from the capillary suction test (%) 5d; wrapped in plastics d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% Table 8 Air porosity values, ε air, calculated from the capillary suction test (%) 5d; wrapped in plastics d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% Table 7 Total porosity values, ε total, calculated from the capillary suction test (%) 5d; wrapped in plastics d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% Table 9 The resistance numbers, m-values, calculated from the capillary suction tests (s/m 2 ) 5d; wrapped in plastics d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% d; +20 C, RH45% d; +20 C, RH70% d; +12 C, RH45% d; +5 C, RH45% Fig. 11 Capillary suction diagrams of test concretes NSC25 (left) and HSC70 (right) after curing of 5 days wrapped in a plastic sheet and additionally 23 days in +5 C, RH45%. unit less at +5 C than in the other curing temperatures. The capillary suction test gives also information about the continuity of the pore structure. This is best denoted by the resistance number, m-value (s/m 2 ), which is determined as: m = t cap 2 (8) h t cap (s) corresponds to the time needed to fill all the gel and capillary pores and h (m) is the thickness of the test specimen. The m-values are shown in Table 9 and two capillary suction diagrams are shown in Fig. 11. The m- value increased with decreasing w/c ratio with some exceptions in the air-entrained concretes. This indicates that the air-entrained and especially the high-strength test concrete had a less continuous pore structure with more unconnected pores than the normal strength test concrete. A statistical analysis (paired t-test, 95% confidence interval) showed that the suction, total, and air porosity as well as the m-value were all influenced by the chosen curing conditions and curing age. 46

9 Wirtanen, Penttala 4. SUMMARY AND CONCLUSIONS The influence of temperature and relative humidity of the ambient air on the humidity distribution in the concrete pore system has been studied. The issue has been dealt with from a theoretical and an experimental aspect. The theoretical study considered moisture flow and drying mechanisms. The experiments aimed at clarifying the influence of external conditions on the moisture distribution in concrete in order to specify the starting time for floor covering works. The experimental part was divided into two sections. One dealt with humidity changes in the concrete pore system and the other with evaporation and changes in the pore structure. Five different concretes were studied; two ordinary normal strength concretes, two 9-9.5% air-entrained normal strength concretes, and one high-strength concrete. The test specimens were cured at six different environments. Following conclusions were obtained on the basis of the experimental study: 1. The relative humidity level in the concrete pore system decreases more slowly if the ambient temperature decreases or the relative humidity increases. The effect of this known phenomenon diminishes if the w/c ratio of concrete is decreased. 2. A change in the curing conditions causes a redistribution of the humidity in the concrete pore system. The magnitude of change increases with a decreasing w/c ratio, and an increasing humidity or temperature change. 3. The influence of air-entrainment can be noticed in the macropore volume when measured by MIP. Airentrainment enables the casting of concrete with a low w/c ratio that has similar drying properties with that of high-strength concrete. A large macropore volume allows a greater diffusion than is the case in high-strength concrete, due to the more accessible pore structure. 4. The decrease in the air porosity due to hydration is higher in the concretes that possess a high initial air content compared with ordinary concretes. 5. The resistance number, m, of the concrete that is obtained by the capillary suction test describes the continuity of the pore structure. It increases with decreasing w/c ratio and is affected by air-entrainment. This, together with the other test results, implies that the differences in the evaporated water amounts and the way in which different concretes react to changes in the curing conditions depend on w/c ratio and air content. REFERENCES [1] Norling Mjörnell, K., Self-desiccation in Concrete, (Chalmers University of Technology, Gothenburg, Sweden, 1994). [2] Bažant, Z. P. and Najjar L. J., Nonlinear water diffusion in nonsaturated concrete, Mater. Struct. 5 (25) (1972) [3] Xin, D., Zollinger, D. G. and Allen, G. D., An approach to determine diffusivity in hardening concrete based on measured humidity profiles, Advanced Cement Based Materials 2 (4) (1995) [4] Šelih, J., Sousa, A. C. M. and Bremner, Th. W., Moisture transport in initially fully saturated concrete during drying, Transport in Porous Media 24 (1) (1996) [5] Nilsson, L-O, Hygroscopic Moisture in Concrete-Drying, Measurements and Related Material Properties, (Lund Institute of Technology, Report TVBM-1003, Lund, Sweden, 1980). [6] Sadouki, H. and van Mier, J. G. M, Simulation of hygral crack growth in concrete repair systems, Mater. Struct. 30 (203) (1997) [7] Persson, B., Effects of microporosity on the compression strength and freezing durability of high-strength concretes, Magazine of Concrete Research 41 (148) (1989) [8] Determination of the capillary suction rate and porosity, (only available in Norwegian), in Kvalitetssikring. Konstruksjoner og betong. Betong, (SINTEF, Trondheim, Norway, 1993).. [9] Punkki, J. and Sellevold, E. J., Capillary suction in concrete: Effects of drying procedure, Nordic Concrete Research (15) (1994) [10] Concrete. Frost resistance. Protective pore ratio, (only available in Finnish), in Betoninormit, SFS 4475, (Suomen Standardisoimisliitto SFS, Finland, 1988). 47