30 Haziran 2014 Pazartesi

Deterioration by Frost Action

The frost action is one of the major problems in concrete structures, such as pavements, retaining walls, bridge decks and railings, requiring heavy expenditures for repair and replacement. The deterioration of hardened concrete by frost action is attributed to the complex microstructure of the material as well as to the specific environmental conditions. Thus, a concrete that is frost resistance under a given freeze-thaw condition may be damaged under a different condition. The frost action in concrete can take several forms. The most common is cracking and spalling of concrete causing by progressive expansion of cement paste matrix from repeated freeze-thaw cycles. Concrete slabs exposed to freezing and thawing in the presence of moisture and deicing chemicals are susceptible to scaling. (i.e. finished  surface or ‘skin’ flake or peels off). The following theories, each may contribute partially to an explanation of the ultimate failure of concrete surfaces, are proposed to explain scaling:

-          Pressure developed by expulsion of water from saturated aggregate particles.
-          Hydraulic pressure developed in capillaries just below the concrete surface
-          Acceleration of moisture to ice crystals in capillaries below the surface
-          Osmotic pressure caused by concentration of salt in capillaries immediately beneath the concrete surface
-          Finishing operation which may create a surface condition where the finished surface is dissimilar to the underlying concrete. Improper finishing may also density the surface and destroy the effectiveness of air entrainment.
-          Additional freezing of subsurface ice crystals caused by melting snow and ice with deicing salts,
-          Replenishment of surface moisture during freezing by melting snow and ice with deicing salts.

Certain coarse aggregates, which contain a relatively high pore volume confined to a narrow pore size range (0.1 to 1 mm), in slabs are known to cause  cracking, usually parallel to joints and edges, which eventually acquires a pattern resembling a large capital letter D (cracks curving is described as D-line (deterioration line) cracking or simply D-cracking.
Fig. 4.1. D-cracks run approximately parallel to joints or edges of concrete surface. As deterioration progresses these parallel cracks occur further away from the joint. The deterioration is further advanced to cause disintegration and spalling of concrete near the joint.

4.1.Frost Action on Hardened Cement Paste

According to Powers a saturated cement paste containing no entrained air expands on freezing due to generation of hydraulic pressure. With increasing air entrainment, the tendency to expand decreases because the entrained air voids provide escape boundaries for the hydraulic pressure. A diagrammatic representation of Power’s hypothesis is shown below.
Fig.4.2. Response of saturated cement paste to freezing and thawing both without entrained air.
Powers also proposed that, in addition to hydraulic pressure caused by water freezing in large cavities, the osmotic pressure resulting from partial freezing of solutions in capillaries may be another source of destructive expansion in hcp. Water in capillaries is not pure and solutions freeze at lower temperatures than pure water. (The higher the concentration of a salt solution, the lower the freezing point). The existence of local salt concentration gradients between capillaries is envisaged as the source of osmotic pressure.
The hydraulic pressure due to an increase in the specific volume of water on freezing in large cavities and osmotic pressure due to salt concentration differences in the pore fluid are not the only causes of expansion of hcp exposed to frost action. This became clear by observing the expansion of cement even when benzene was used as pore fluid instead of water (Benzene contracts on freezing).
A capillary effect involving large-scale migration of water from small pores to large cavities is believed to be the main cause of expansion in porous hcp body. Accordingly, the rigidly held water by C-S-H (both interlayer and absorbed in gel pores) can not rearrange itself to from ice at normal freezing point of water because the mobility of water existing in an ordered state is rather limited.
It is estimated that water in gel pores does not freeze above -78 °C. Therefore, in a saturated cement paste subjected to freezing the water in large capillaries turns into ice, while the gel water continuous to exist as liquid water in a super-cooled state. This creates a thermodynamic disequilibrium between the frozen water in capillaries, which acquire a low-energy state, and the super cooled gel water which is in a high-energy state. This causes the gel water to migrate to lower-sites (large cavities where it can freeze. This fresh supply of water from the gel pores to the capillary pores increases the volume of ice in the capillary pores steadily until there is no room to accommodate more ice. Any subsequent tendency for the super-cooled water to flow toward the ice bearing regions would obviously cause internal pressure and expansion of the system.
It may be noted that during frost action on cement paste, the tendency for certain regions to expand is balanced by other regions that undergo contraction (e.g., loss of absorbed water from C-S-H). The net effect on the specimen is obviously the result of two opposite tendencies. This satisfactorily explains why cement paste containing air entrainment shows contraction during freezing.


4.2.Frost Action on Aggregate

Depending on the response of aggregate on frost action, an air entrained concrete may still be damaged. The mechanism involved in the development of internal pressure on freezing a saturated cement paste is also applicable to porous aggregates. However, not all porous aggregates are susceptible to frost damage; the behavior of an aggregate particle to freeze-thaw cycles primarily depends on its pore size distribution (size, number and continuity of pores) and its permeability.
From the stand point of lack of concrete durability to frost action attributed to the aggregate, three classes of aggregate are proposed.

1-    The low permeability and high strength aggregate: so that upon freezing of water elastic strain is accommodated without causing fracture,

2-    The aggregates of intermediate permeability: i.e., those having a significant proportion of total porosity represented by small pores of the order of 500 nm and smaller. Capillary forces in those small pores cause the aggregate to get easily saturated and to hold water. On freezing the magnitude of pressure developed depends on the rate of temperature drop and the distance that water under pressure must travel to find an escape boundary to relieve the pressure. Pressure relief may be available either as an empty pore inside the aggregate or at the aggregate surface. The critical distance for pressure relief in hcp is of the order of 0.2 mm.
      These considerations have given rise to the concept of critical aggregate size with respect to frost action damage. For a given pore size distribution, permeability, degree of saturation and freezing rate, the larger the aggregate, the larger is the frost action.
      There is no single critical size for an aggregate type because this depends on freezing rate, degree of saturation and permeability of aggregate. Generally, when aggregates larger than the critical size are present in concrete, freezing is accompanied by pop-outs, that is failure of the aggregate in which a part of the aggregate piece remains in the concrete and other part comes out with mortar flake.

3-    Aggregates of high permeability; which generally contains a large number of relatively big pores. Although they permit easy entry and expelling of water, they may cause durability problems. This is because the transition zone may be damaged by expelled water from an aggregate particle. In such cases, the aggregate particles themselves are not damaged as a result of frost action. This shows why the results of freeze-thaw and soundness tests on aggregate alone are not always reliable in predicting its behavior in concrete.

4.3.Factors controlling Frost Resistance of Concrete

The ability of concrete to resist damage due to frost action depends on the characteristics of both the cement paste and the aggregate. In each case, however, the outcome is controlled actually by the interaction of several factors, such as;
-          The location of escape boundaries (the distance by which water has to travel for pressure relief),
-          The pore structure of the system (size, number and continuity of pores),
-          The degree of saturation (amount of freezable water present),
-          The rate of cooling and the tensile strength of the material that must be exceed to cause rupture.
The provisions of escape boundaries in the cement paste matrix and modification of its pore structure are the two parameters that are relatively easy to control; the former can be controlled by means of air entrainment in concrete and the latter by the use of proper mix proportions and curing.
 Air entrainment:
It is not the total air, but void spacing of the order of 0.1 to 0.2 mm within every point in the hardened cement, which is necessary for protection of concrete against frost damage. By adding small amounts of certain air-entraining agents to the cement paste (e.g. 0.05 % by weight of the cement) it is possible to incorporate 0.05 to 0.1 mm bubbles. Thus, for a given volume of air depending on the size of air bubbles, the number of voids, void spacing and degree of protection against frost action can vary a great deal.
Although the volume of entrained air is not a sufficient measure of protection of concrete against frost action, assuming that mostly small air bubbles are present, it is the easiest criterion for the purpose of quality control of concrete mixtures. Since the cement paste content is generally related to the maximum aggregate size, lean concretes with large aggregates have less cement paste than rich concretes with small aggregates, therefore, rich mixes would need more air entrainment for an equivalent degree of frost resistance. The recommended air contents for frost resistance, by ACI are given below:
Nominal Dmax
(mm)
Air Content (%)
Severe Exposure
Moderate Exposure
9.00
7.5
6.0
12.5
7.0
5.5
19.0
6.0
5.0
25.0
6.0
4.5
37.5
5.5
4.5
50.0
5.0
4.0
76.0
4.5
3.5

The aggregate grading affects the volume of entrained air, which is decreased by an excess of very fine sand particles. Addition of mineral admixtures or the use of cements of higher fineness has a similar effect. Generally, a more cohesive mix is able to hold more air than either a very wet or very stiff concrete. Besides, insufficient mixing or over mixing, excessive time of handling or transportation of fresh concrete, and over vibration tend to reduce the air content. For these reasons, it is recommended that air content should be determined on concrete as placed.
W/C ratio and Curing:
 It was earlier explained that the pore structure of hcp is determined by the W/C ratio and degree of hydration. In general, the higher the W/C ratio for a given degree of hydration or the lower the degree of hydration for a given W/C ratio, the higher will be the volume of large pores in the hcp. Since the readily freezable water resides in large pores, it can therefore be accepted that at given temperature of freezing the amount of freezable water will be more with higher W/C ratios and at earlier ages of curing. The relationships between freezable water, concrete temperature and W/C ratio are shown in Fig. 4.3 (a). The figure shows that the amount of water that can be frozen in concrete with a given W/C ratio increases with decreasing temperature. It also shows that the amount of water that will be freeze at a given temperature increases with W/C ratio (Higher W/C ratio results in larger and greater number of capillaries in which more freezable water can be present). Fig. 4.3 (b) shows that a combination of low W/C ratio and entrained air ensures a high durability factor to frost action.


Fig 4.3. Effect of W/C ratio and air content on durability of concrete to frost action
The importance of W/C ratio on the frost resistance of concrete is recognized by ACI 318 building code, which requires that normal weight concrete subjected to frost action in a moist condition should have a maximum W/C ratio of 0.45 for thin sections such as curb, and 0.5 for other elements. These W/C ratio limits are based on assumption of adequate cement hydration; therefore, at least 7 days of moist curing at normal temperature is recommended prior to frost exposure.
Degree of saturation:
Dry or partially dry materials do not suffer frost damage. There is a critical degree of saturation above which concrete is likely to damage when exposed to very low temperatures. In fact, the difference between the critical and actual degree of saturation determines the frost resistance of concrete. A concrete may fall below the critical degree of saturation after adequate curing, but depending on the permeability it may again reach or exceed the critical degree of saturation when exposed to a moist environment. Thus, the permeability is vital in frost resistance of concrete because it controls not only the hydraulic pressure associated with internal water movement on freezing but also the critical degree of saturation prior to freezing. From the standpoint of frost damage the effect of increase in permeability, as a result of cracking due to any physical or chemical causes, should be apparent.
Strength:
The direct relationship between strength and durability does not hold in the case of frost damage. Relatively low strength air entrained concrete may show higher frost resistance than a non-air entrained concrete having higher strength, because of protection against development of high hydraulic pressures. Generally, in medium and high strength concretes, every 1% increase in air content reduces the strength by ~5%.  Without any change in W/C ratio, 5% air entrainment would, therefore, lower the concrete strength by 25%. Due to improved workability as a result of entrained air, it is possible to make up a part of strength loss by reducing the W/C ratio a little while maintaining the desired level of workability. Nevertheless, air entrained concrete is generally lower in strength than the corresponding non air entrained one.

4.4.Concrete Scaling

The resistance of concrete against the combined influence of freezing and deicing salts (chlorides of ammonia, calcium or sodium) which are commonly used to melt ice and snow from pavements is generally lower than its resistance to frost alone. The maximum damage to the concrete surface by scaling occurs at salt concentrations of ~4 to 5%.
It seems that the use of deicing salts has both negative and positive effects on frost damage, and the most dangerous salt concentration is a consequence of both effects. The super cooling effect of salt on water (i.e., the lowering the temperature of ice formation) may be viewed as a positive effect. On the other hand, the negative effects are:
1-    An increase in degree of saturation of concrete due to the hygroscopic character of the salts,
2-    An increase in the disruptive effect when the super-cooled water in pores eventually freezes,
3-    The development of differential stresses caused by layer-by-layer freezing of concrete due to salt concentration gradients,
4-    Temperature shock as a result of dry application of deicing salts on concrete covered with snow and ice,
5-    Crystal growth in supersaturated solutions in pores.

Overall, the negative effects of deicing salt application far outweigh the positive effect; therefore, the frost resistance of concrete under combined influence of freezing and deicing salts is significantly lowered.

4.5.Factors Affecting the Amount of Entrained Air

As it was mentioned earlier, the amount and spacing of entrained air bubbles with in the paste are major factors controlling frost resistance of concrete. Entrained air content may not be distinguished from entrapped air by routine tests. Thus, it is the total air content can be measured in the fresh concrete.
In addition to the dosage of the air entraining agent, there are some other factors influencing the air content and the characteristics of the air voids (even when the dosage of air entraining agent is kept constant):

Cement Content : Increasing cement content reduces the amount of air entrained in concrete (at a given dose of air entraining agent, lean mixes entrain more air than the rich mixes).

Cement Composition: Alkali content of the cement is the major factor affecting entrained air. For a given dosage of admixture an increase in alkali content, increases air content.

Cement Fineness: Fine cements entrain less air than coarse ones.

Dmax: An increase in the Dmax reduces the mortar content, thus increasing air content for a given dosage of the air entraining agent.

Fine Aggregate Quantity & Grading: Fine aggregate contributes to air entrainment by retention of air bubbles within the interstitial voids between its particles. For a given dosage of air entraining admixture, air content increases as the amount of fine sand decreases and as the amount of middle sized sand particles (0.3 to 0.6 mm) increases.

Mineral Admixture: The finely divided mineral admixtures reduce air content. The effect is more pronounced with fly ash having higher carbon content.

W/C Ratio:Increase in W/C ratio results in an increase in air content.

Initial Consistency of the Concrete: Within practical slump range (2.5 – 15 cm), increase in initial slump is accompanied by an increase in air content. Each 2.5 cm increase in slump (in the range of 2.5 to 12.5 cm) is accompanied by an increase of about 0.5% point in air content. Further increase in slump cause a rapid decrease in air content. Above a slump of 15 to 18 cm, many mixtures become too fluid to retain entrained air.

Type & Condition of Mixer: The type of mixer (stationary or transit) and its condition affect air content considerably. For a given air content decreases by worn blades & impaired mixing action (due to accumulation of hardened mortar in the drum and on the blades). Under loading the mixer increases the air content, overloading reduces it.

Duration of Mixing Time: Air content increases with increased time up to ~2 minutes in stationary and up to ~15 minutes in most transit mixers after which air content may remain approximately constant for a considerable period before it begins to drop off.

Chemical Admixtures: Generally, the presence of chemical admixture increases the air content to a certain extent, from a slight to a significant amount. In the case of using chemical admixtures, trial mixes are definitely necessary to determine the required dosage of the air entraining agent.

Temperature: For a given dosage of air entraining agent, air content varies inversely with the temperature. (Increasing temperature from 10 to 38 °C halves the air content).

Vibration: A proper application of vibration, just enough to compact the mix (~15 seconds) eliminates only the large accidentally formed entrapped air voids. Prolonged vibration and pumping decrease the air content. Excessive finishing reduces the air content at the surface zone.

4.6.  Recommended Air Contents for Frost Resistance

Concrete may withstand frost action when the cement paste contains proper amount of entrained air. There is an optimum air bubble size distribution and spacing of bubbles above which further increase does not increase the durability of concrete.
Actually the entrained air in the concrete is the air content occurring within the paste fraction. When the entrained air is expressed as a function of content, the optimum air content is generally in the range of about 9% of the mortar volume. However, it is not practical to express air content as a part of the mortar volume. It is easier to express it as a % of the total volume of concrete.
The air content recommended for a satisfactory frost resistance varies in the range of 3.5 to 8% of the concrete volume. The actual amount of air necessary for frost resistance depends on the mix proportions, Dmax, and exposure conditions to which the concrete will be subjected. ACI 201.1-81, recommends the following levels of air content for different exposure conditions and aggregate sizes. For concretes of compressive strength greater than 40 MPa the air contents given below may be reduced by ~1%.

ACI Air Content Recommendations for Frost Resistance
Dmax
(mm)
Air Content *(%)
Mild Exposure
Moderate Exposure*
Severe Exposure **
9.5
4.5
6.0
7.5
12.5
4.0
5.5
7.0
19.0
3.5
5.0
6.0
38.0
2.5
4.5
5.5
50.0
2.0
4.0
5.0
75.0
1.5
3.5
4.5

* Air content in the field may be tolerated by + 1.5%.
* Outdoor exposure in the cold climate, where concrete is occasionally exposed to moisture prior to freezing, e.g. exterior walls, slabs not in direct contact with soil.
** Outdoor exposures in the cold climate, where concrete is in almost continuous contact with moisture prior to freezing or where deicing salts are used, e.g., pavements, bridge decks, water tanks, side wall..





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