Basic Concepts Related to Frost Resistance
Chapter 4 Laboratory Tests
4.1 CONCRETE TESTS
4.1.4 Other Test Methods Paving Block Testing
In the last few years, paving blocks have been used increasingly for the construction of concrete pavements, both for residential and commercial applications. Paving blocks are made by compacting dry concrete mixtures, and their internal pore structure is thus very different from that of standard concretes. In addition to the usual spherical air voids (which are very difficult to entrain in dry concretes because of the small quantity of water available, Chapter 8), paving blocks also contain a lot of irregularly shaped air voids resulting from compaction operations. These voids cover a range of sizes similar to the range of sizes of spherical air voids (approximately 10 µm to 1 mm, see Chapter 8 on the durability of dry concretes) and they can have either a beneficial or a detrimental effect on the frost durability of concrete, depending on whether they reach a high degree of saturation or whether they remain empty under field conditions. Due to the particular nature of these concretes, and considering the fact that paving blocks in a pavement are expected to freeze while being completely surrounded by water, the Canadian Standard CSA A231.2-M85 (1985) describes a test method specifically designed to assess the frost resistance of paving blocks (this test method could also be extended to other types of dry concretes such as roller-compacted concrete).
Table 4.3 Visual rating of scaled surfaces according to Canadian Standard CAN3-A231.2- M85 (paving block test).
Rating Condition of surface
0 No scaling
1 Very light scaling (maximum depth 3 mm, no coarse aggregate visible) 2 Slight to moderate scaling
3 Moderate scaling (some coarse aggregate visible over 50% of the surface)
4 Moderate to severe scaling (some coarse aggregate visible over 75% of the surface) 5 Severe scaling (coarse aggregate visible over 100% of the surface)
Prior to testing, the specimens are oven dried for a minimum of 24 h at 110±5 °C, allowed to stand in the laboratory until the temperature returns to the ambient temperature (23±3 °C), and placed in sealed containers filled with a 3% (by weight) sodium chloride solution, for
24 h. The specimens are then exposed to 50 daily freezing and thawing cycles (16±1 h of freezing at −15±2 °C and 8±1 h of thawing at 23±3 °C). After 10, 25 and 50 cycles respectively, the specimens are carefully washed with a 3% NaCl solution and the scaled- off particles are recovered, filtered, dried and weighed. The extent of scaling is also visually assessed according to the rating given in Table 4.3. According to this Standard, the paving blocks are rejected if the mass of scaled-off particles exceeds 1.0% of the initial dry mass of the specimens. This percentage corresponds approximately to a loss of mass of 0.4–0.5 kg/m2, depending on the geometry of the block.
This test procedure exposes the concrete specimens to very severe conditions, the oven drying being expected to be particularly harmful, especially for concretes with a high porosity. It is quite possible that the severity of these conditions tends to reduce the differences observed between different concretes. This would of course make it more difficult to classify different paving blocks as a function of their relative durabilities, particularly for those having marginal durabilities. The presumably higher severity of this test method, however, is not yet supported by strong experimental evidence and research is still needed to assess more precisely the validity of this test procedure.
Critical Degree of Saturation
The determination of the critical degree of saturation is another method that was developed to assess the frost durability of concrete. This method (recommended by the RILEM 4-CDC Committee (1977)) is based on the experimental evidence that concrete can safely resist freezing and thawing cycles if, and only if, its degree of saturation is lower than a threshold value called the critical degree of saturation SCR. The critical degree of saturation is an intrinsic property of concrete (Chapter 3), and different values of SCR are associated with different concrete mixtures. A given concrete is considered as frost resistant if its degree of saturation under field exposure conditions SACT never exceeds SCR (Fagerlund, 1971).
Experimentally, the critical degree of saturation can be determined in two different ways. First, the modulus of elasticity of concrete specimens kept at different levels of relative humidity and subjected to six freezing and thawing cycles can be measured. The relationship between the modulus of elasticity and the degree of saturation shows a clear break point which corresponds to the critical degree of saturation. A typical example of such a relationship is given in Figure 4.16. Another method consists of measuring the residual length change of concrete specimens after one or two cycles of freezing and thawing. The relationship between the residual length change and the degree of saturation also indicates a critical SCR value, as illustrated in Figure 4.17. Both methods give similar values of SCR.
The frost immunity period can be determined by studying the kinetics of capillary water absorption. A thin concrete specimen is immersed in water and weighed at different time intervals in order to obtain the relationship between the degree of saturation reached by water absorption SCAP and the time of immersion. Figure 4.18 shows a typical example of such a relationship. This figure indicates that, in the first few hours, SCAP increases very rapidly until it reaches a break point but, afterwards, the water content increases very slowly.
CO <D O ft"^ CD o CO a> m « CD £• o C) CO =3 CO CO CD o *o E CO o 3 E 3 CO ■D c O >* E■D o co E c~ CO c >s Q 0 0.2 0.4 0.6 0.8 1.0 Degree of saturation 1.0 0.8 0.6 0.4 0.2 Scn-oj
'AJ
Figure 4.16 Relationship between the relative dynamic modulus of elasticity and the degree of saturation of concrete after six freezing and thawing cycles (after Fagerlund, 1977).
-0.2 0 |—I 0.2 <D C 0.4 <0 ■5 JC CD C J9> 0.6 "c53 ■D CO CO 0.8 1.0 1.2. o
°t
o 1 cycle • 2 cycles o°t
o 1 cycle • 2 cycles w "0 II w "0 II D SCF I'■°N.
J 0.5 0.6 0.7 0.8 0.9 Degree of saturation 1.0Figure 4.17 Relationship between the residual length change and the degree of saturation of concrete after one and two cycles of freezing and thawing (after Fagerlund, 1977).
4-<>
72 120 144168
y/T\me (h) 288 336
Figure 4.18 Relationship between the degree of saturation reached by water absorption (SCAP) and the period of immersion (after RILEM 4-CDC Committee, 1977).
The degree of saturation corresponding to the break point (86.5% for the example given in Figure 4.18) roughly represents the highest humidity level that can be reached in concrete walls which are never kept in contact with water for very long periods of time (between rainfalls, the surface of the concrete is allowed to dry since the relative humidity of the surrounding air is lower than 100%). However, slabs and other building components can reach a higher degree of saturation if they are in contact with water for longer periods of time.
The frost durability of concrete is related to the difference between the critical degree of saturation SCR and the degree of saturation under field exposure conditions SACT:
F=SCR−SACT (4.5)
Concrete will be frost resistant if it never becomes critically saturated, i.e. if F>0. The frost immunity period can thus be determined by studying the difference between SCR and SCAP, i.e.
F=SCR−SCAP (4.6)
The value of F decreases with time (since SCR is a constant and SCAP increases with time), and the frost immunity period corresponds to the period of water absorption required for the value of F to reach zero, as shown, for example, in Figure 4.19. Concrete will thus be considered frost resistant if, under field exposure conditions, it is not in contact with water for a period of time longer than this frost immunity period.
0.10 0.08 <0.06 CO oc 0.04 o CO ." 0.02 0 -0.02
Idl
OI
urablee;oncret<i II
3I
j44<xo
kyjNon^urable cbncn*»}/<■/<■/<■■' '••rVrirrtl 1 10 10*Period of water absorption (h) 10°
Figure 4.19 Relationship between the parameter F and the period of water absorption in the critical degree of saturation test (after RILEM 4-SDC Committee, 1977).
The most interesting aspect of the critical degree of saturation method is that the assessment of the frost durability of concrete is based on field exposure conditions. The concept of the frost immunity period emphasizes the fact that frost resistance is not only a function of the properties of concrete, but also a function of the field exposure conditions. However, the method requires a large number of long and tedious experiments which must be carried out with great care (the degree of saturation of the concrete specimens is especially important, and is not always easy to control during the testing procedure). This represents a serious drawback, and makes it difficult to use the technique as a routine test procedure.
It is important to point out that, in this test method, concrete is considered fully saturated (i.e. S=100%) when all voids (including both the capillary pores and the air voids) are full of water. Since the air voids cannot be filled by capillary suction, the role of air entrainment is thus simply to reduce below the critical SCR value the maximum degree of saturation
that concrete can reach (SACT or SCAP). In properly air-entrained concretes, the degree of
saturation obtained by capillary absorption should be lower than SCR, which simply means
that such concretes will almost never become critically saturated, and thus that their frost immunity period tends to infinity. The fact that the air voids cannot be easily filled by capillary suction also means that the very high degrees of saturation (near 100%) required to determine the critical degree of saturation (Figures 4.16 and 4.17) can only be obtained with vacuum saturation procedures.
The concept of the critical degree of saturation was initially developed to study the frost durability of porous materials such as stones or clay bricks for which the use of air entrainment is not possible. For such materials, the concept of the frost immunity period is particularly useful, since the real question is to determine if they can safely sustain the moisture levels reached under field exposure conditions. For concrete, however, the real question is to determine the volume of entrained air (or, preferably, the spacing factor of the air voids) required for good frost durability. From a practical point of view, it is simple and safe to assume that only the capillary pores will be full of water at the time of freezing. Consequently, it is less important to determine the maximum degree of saturation that
concrete will reach under natural exposure conditions, and other laboratory procedures such as described in this chapter will generally yield satisfactory results.