One of the main requirements of brick wall elements is their durability. Salt crystallization and cycles of water freezing–thawing are considered to be among the main factors of brick degradation [1
]. Porous building materials such as brick wall elements always contain a certain amount of moisture in their structure, which directly influences the material properties such as strength, shrinking/expansion properties, vapor permeability and resistance to external conditions. According to the European regulations, the resistance of bricks to freeze–thaw cycles is assessed through CEN/TS 772-22 standard [4
]. However, in addition to the aforementioned direct method of testing the resistance of bricks to freeze–thaw cycles, the literature mentions some indirect procedures/methods and limits critical values per procedure/method to grade the resistance of bricks in freeze–thaw cycles. A highly acknowledged indirect procedure for predicting the resistance of bricks to freeze–thaw cycles is the well-known Maage coefficient [5
]. The Maage coefficient is based on experimental results and presents a statistical model with two main variables: the total volume of pores (PV), and the share of pores of a certain diameter, i.e., pores larger than 3 µm (P3). According to Maage, pores larger than 3 μm in diameter have a beneficial effect on frost resistance of bricks. Other authors claim that it is precisely the large pores that are responsible for the good resistance of clay bricks to freeze–thaw cycles [1
]. Furthermore, small pores are not considered to significantly affect the resistance of brick to freeze–thaw cycles, while those of medium size are considered to be critical pores [1
]. According to Elert et al. [11
], small pores are defined as ones whose diameter is <0.2 μm, medium-sized pores are of diameter 0.2–2 μm while large pores are of diameter >2 μm. Pores with diameters <0.25 μm are considered to be small pores and those with diameter >1.4 μm as large pores according to Ravaglioli [15
], while Koroth [1
] and Kung [16
] consider pores in the range 0.1–1.0 μm to be critical (medium-sized pore). Culturone et al. [12
] divided pores in two groups; pores with radius <1 μm (small pores or micropores) and pores with radius >1 μm (large pores or macropores) considering the small pores harmful. The presence of small pores negatively affects the quality of bricks, since their capacity to absorb and retain water increases [12
]. Stryszewska and Kanka [14
] divided pores into as many as five groups (<0.1 μm, 0.1–1 μm, 1–3 μm, 3–10 μm and >10 μm). They set up a correlation between the particular form of frost damage and the prevailing pore group. The final conclusion in their research is that bricks without any signs of damage are clearly characterized by the prevalence of pores with diameters in the range of 3–10 μm and that the pores with diameter <0.1 μm do not affect the resistance of brick to freeze–thaw cycles.
Pore-size distribution in a brick unit is influenced by the characteristics of raw materials mixture, shaping procedure (hand-made or machine-made bricks) and thermal treatment. Elert et al. [11
] have studied the frost resistance of hand-made bricks shaped from two different raw materials (calcareous and non-calcareous clay) and fired at 700, 800, 900, 1000 and 1100 °C each. The conclusion of their research was that the durability of non-calcareous bricks fired at 1000 °C or above is generally superior. These bricks display lower total porosity and a higher degree of vitrification than the corresponding calcareous specimens. For non-calcareous clays, a firing temperature of 1000 °C is high enough to produce durable bricks, while a temperature of 1100 °C is necessary in the case of calcareous clays. Elert et al. [11
] observed that brick specimens with high porosity and a high percentage of pores with a diameter of <2 μm underwent significant damage during freeze–thaw cycles. The authors in [12
] also concluded that the clay with a lower CaCO3
content would ensure more exceptional durability as the final products. For raw materials with slightly higher carbonate content, it is necessary to increase the firing temperature in order to reduce the amount of medium-sized (harmful) pores, which are formed by CO2
-separation from the raw material [5
The authors in [19
] claim that hand-made bricks contain a higher content of large pores than machine-made bricks, which renders hand-made bricks more resistant to freeze–thaw cycles. On the other hand, the research presented in [22
] clearly shows that machine-made brick contains significant content of large pores. It should be noted here that the hand-made and machine-made bricks explored in [22
] were not made from the same raw material. Machine and hand-made bricks produced from the same raw material were studied in [23
] and it is observed that hand-made bricks achieved a higher content of large pores than machine-made bricks fired at the same temperature. The basic difference between machine and hand-made production of bricks, based on the same raw material, lies in their textural properties. Elert et al. [11
] and Netinger et al. [23
] studied the influence of firing temperature on the pore structure of clay bricks and concluded that the total porosity of brick specimens decreases and pore-size distribution changes towards larger pore sizes as the firing temperatures rise. The authors in [24
] came to the same conclusion regarding the pore structures of clay roofing tiles. The retention at the maximum firing temperature is also an important parameter considering the structural characteristics of the final product. A more extended retention period has evidently a positive influence on the final properties of the product [25
In order to assess clay product’s resistance to freeze–thaw cycles, researchers have been studying the changes of the following properties during freeze–thaw cycles: surface appearance of the specimens [14
], flexural strength and toughness [24
], compressive strength and dynamic modulus of elasticity [27
], propagation speed of ultrasonic waves through specimens [12
], weight of specimens [11
] and the structure of pores [24
]. During freeze–thaw cycles, the surface of brick specimens becomes damaged, compressive and the flexural strength (as well as dynamic modulus of elasticity, toughness and weight) is decreased, and so is the propagation speed of ultrasonic waves through the specimens. With each freeze–thaw cycle, new micropores and cracks appear.
As the literature shows, the influence of pore-size distribution on brick resistance to freeze–thaw cycles has been an interesting topic for many years. However, the literature fails to cover all the factors that simultaneously influence the frost resistance of a brick, a process that is not simple and easy to investigate. The authors of this paper study the simultaneous influence of the following factors: the characteristics of raw material, shaping method and influence of firing regime (with particular emphasis on the retention time) on frost resistance of the bricks exposed to freeze–thaw cycles, by monitoring the changes in surface appearance and the changes of compressive strength as well as the Maage factor. A correlation was set up between the Maage factor and the ratio of compressive strength before and after freezing as a quantitative indicator of brick frost-resistance.
The distribution of pores of the unfired bricks for each specified size, determined by Hg porosimetry, is shown in Figure 5
. Based on the reference literature, pores with a diameter of less than 0.1 µm are considered to be small, while the ones with the diameter over 1.0 µm are considered to be large [1
]. Maage [5
] in his equation considers pores to be large only if their diameter is over 3.0 µm. Based on this fact, the authors decided to use the Maage methodology and his equation will be considered for further analysis. This approach was used for both the unfired and fired bricks. In line with this, medium-sized pores are considered to be those with the diameter in the range of 0.1–3.0 µm. The used specimen labels are given in Table 5
The pore-size distribution of the unfired and fired brick specimens is given in Figure 5
and Figure 6
. Furthermore, Figure 7
shows pores of each size, according to Maage methodology categorized into groups of large–medium–small, in the unfired and fired bricks. This Figure was designed based on the previous Figure 6
, that describes the pores’ dimensions into the interval 100–0.002 μm (Hg-porosimetry).
The appearance of the bricks after the exposure to freeze–thaw cycles are shown in Figure 8
Values of compressive strengths (average of ten measurements) of the bricks before and after the exposure to freeze–thaw cycles are presented in Figure 9
while these values in comparison with the ratio of compressive strength before and after the cycles are show in Table 6
shows that the bricks made from S2 raw material achieved higher compressive strength than the ones made from S1 raw material fired at the maximum reached temperature (1060/1030 °C). As the microstructure of S1 bricks was obtained based on the vitrification process [34
], evidently, the content of glass phase decreased the mechanical value of the bricks. Considering the retention time of bricks at the highest temperatures, the obtained results showed that this parameter has a positive influence on the value of compressive strengths value. This fact is the consequence of the formed microstructure with a higher content of larger pores and a lower content of dangerous medium and small pores, Figure 7
Based on the appearance of the bricks after their exposure to freeze–thaw cycles (Figure 8
), it was concluded that none of the brick groups had any damage caused by freezing/thawing procedure and all were assessed as resistant. However, the ratio of compressive strength before and after freeze–thaw test (Table 6
) shows that some bricks have undergone more pronounced inside changes. According to the literature cited in the Introduction [11
], a clay material with a lower CaCO3
content should ensure better durability of the final products than the one with a higher content of CaCO3
. Based on the results, only the machine-made bricks confirmed this conclusion. As shown in Figure 7
b), the machine-made bricks produced from S2 raw material developed a similar share of large pores (except the group of bricks S2M1060-0.5h) as those made from S1 raw material, but a larger share of medium-sized pores (harmful pores), which proves a better freeze–thaw resistance of the bricks made of S1 clay.
The opposite was true for the hand-made bricks: the ratio of medium pores was lower for bricks made of clay with a higher content of CaCO3 (S2) (e.g., S1H1030-1.5h vs. S2H1060-1.5h; S1H1030-0.5h vs. S2H1060-0.5h), but the share of large pores was higher. Considering that the trends in the pore ratios were different for hand and machine-made bricks, it was not possible to conclude whether the raw material unambiguously affected the durability values or if it was due to the shaping procedure.
Analyzing the results from Figure 7
a, it is evident that the raw materials have a more significant effect on the total share of pores than on their distribution. In fact, in the case of the fired bricks made from S1 raw material, the total porosity decreased in comparison with the unfired bricks, while in the case of the fired bricks made from S2 raw material, the total porosity increased in comparison with the unfired ones. This is the reason why machine shaping bricks made from S2 raw material, even though they achieved better compressive strengths in comparison with the ones made from S1, did not reach a satisfactory frost resistance.
Comparing the ratio of compressive strengths (Table 6
) of the bricks made from the same raw material and with the same retention time at the maximum reached temperature, but shaped differently (hand-made/machine-made) (S1H1030-1.5h vs. S1M1030-1.5h; S2H1060-1.5h vs. S2M1060-1.5h), it could not be concluded that shaping procedure directly affect their resistance to freeze–thaw cycles, which is contrary to the conclusions given in [19
]. Evidently, the shaping procedure influenced the structure of pores in the case of the unfired bricks (Figure 7
a), but the microstructure changes, after the firing process, defined their final resistance.
Reviewing the ratio of compressive strengths before and after the freeze–thaw cycles, it could be firmly concluded that the bricks with a longer retention time at the maximum temperature (1.5 h), generate a better resistance to freeze–thaw cycles than bricks with a shorter retention time (0.5 h): the ratio of compressive strengths was higher in the case of S1H1030-1.5h than in S1H1030-0.5h, again higher in the case of S1M1030-1.5h than in S1M1030-0.5h. The explanation could be found in the change of microstructure but also of the pore structure, Figure 7
b. These results point out that the bricks retained longer at the maximum temperature (1.5 h), regularly report a greater share of large pores and a smaller share of medium pores than the bricks made from the same raw material, using the same type of manufacture (hand-made or machine-made), but fired at the same temperature for a shorter amount of time (e.g., S1H1030-1.5 h vs. S1H1030-0.5 h; S1M1030-1.5 h vs. S1M1030-0.5 h). This result is compatible with the reference literature [5
Taking into account the share of large pores (P3), Figure 7
b and the total volume of pores (PV), the Maage factor, FC
= 3.2 × PV + 2.4 × P3, for the analyzed group of bricks was calculated, Table 6
In addition to the Maage factor (FC
), Table 6
, shows again the assessment of brick resistance to freeze–thaw cycles according to HRN B.D8.011 [36
]. This presentation summarizes the obtained results of the used methods in order to assess the brick resistance to freeze–thaw cycles.
It is evident (Table 6
) that the bricks, which are assessed to be resistant to freeze–thaw cycles according to HRN B.D8.011 [36
], have the ratio of compressive strengths over 0.72 and the Maage factor in range 27 to 100. Based on the reference literature [5
], only the bricks with the Maage factor higher than 70 are considered resistant to freeze–thaw cycles. However, in the research, even the bricks with a smaller Maage factor (68:27) proved to be resistant. While trying to make a relation between the ratio of compressive strengths and the Maage factor (Figure 10
), a low coefficient of correlation of R2
= 0.26 for the equation y = 209.64x − 95.19 (y is the Maage factor and x is the ratio of compressive strengths) was obtained in the case that all bricks were analyzed together regardless of the method of brick forming. However, if the results are processed separately for hand-made and machine-made bricks (Figure 10
), it is evident that machine-made bricks have a significantly higher correlation coefficient value than hand-made bricks. Namely, the coefficient of correlation of R2
= 0.28 for the equation y = 135.88x − 30.60 was achieved for hand-made bricks, while machine-made bricks achieved the coefficient of correlation of R2
= 0.60 for the equation y = 710.00x − 491.80. A higher correlation coefficient value for machine-made bricks could be due to greater uniformity characteristics of those bricks compared to hand-made bricks. If such a correlation between the Maage factor and the ratio of compressive strengths before and after the freeze–thaw cycles for machine-made bricks is confirmed on a larger number of testing results, this parameter could be used as an additional method for assessing brick resistance to freezing and thawing cycles. This interesting observation will be further explored by the authors.