Exploration of Mechanisms of Joint Deterioration in Concrete Pavements regarding Interfacial Transition Zone

Ph.D. Student, Civil, Construction and Environmental Engineering, Town Engineering Building, Iowa State University, Ames, Iowa 50014, USA Research Associate, College of Transportation Engineering, Tongji University, Shanghai, China Associate Research Scientist, Qinghai University-Tsinghua University Sanjiangyuan University Sanjiangyuan Research Institute, Qinghai University, Xinning, China Associate Professor, School of Highway Chang'an University, Xi’an, China Director, National Concrete Pavement Technology Center, Ames, Iowa 50014, USA Professor, Civil, Construction and Environmental Engineering, Town Engineering Building, Iowa State University, Ames, Iowa 50014, USA Associate Professor, School of Hydraulic and Electirc Engineering, Qinghai University, Xinning, China State Key Laboratory of Plateau Ecology and Agriculture, Xinning, China


Introduction
Sawn joints in concrete pavements are used to control both transverse and longitudinal cracking, which are induced by shrinkage or contraction of the pavement.According to the American Concrete Pavement Association [1], an efficient joint system should accommodate slab movements, transfer load between pavement segments, and divide the concrete slab into practical construction increments.Contraction joints are normally made by saw cutting, with a minimum depth of ¼ the thickness of slab but not less than 25.4 mm and a minimum width of 3.18 mm.Saw cutting is conducted when the concrete is mature enough to be cut without spalling, but not too late to allow random cracks in the slab.A sealant is often placed to prevent water from getting into the joint.However, sealants may deteriorate and so allow water to penetrate the saw cut over time, thus increasing the risk of damage within the kerf.
Performance and service life of concrete pavements are heavily dependent on the performance of the joints.Hence, premature joint deterioration is a great concern, particularly in cold weather regions.Figure 1 shows multiple forms of pavement joint deterioration observed in different states [2].To control or eliminate such pavement deterioration, it is necessary to understand the mechanisms of joint damage.e objective of this research was to review the current state of knowledge of possible causes of joint deterioration and further investigate the mechanism behind this form of deterioration.

Mechanisms of Joint Deterioration
Damage is typically observed in two different forms.e first is the formation of small flakes in the paste near the joint (Figure 2(a)).e other form is where cracks form about 25.4 mm from the exposed face, and exposed aggregate particles are observed (Figure 2(b)).In the latter case, the concrete between the crack and the face is often in good condition.Typically, the crack will form around the coarse aggregate leaving it unusually free of mortar.e cause of this distress has been less well understood.
e current knowledge of joint deterioration indicates that there are four major mechanisms by which concrete joints may deteriorate: (1) Freezing-thawing (F-T) damage (2) Salt crystallization (3) Oxychloride expansion (4) Interfacial transition zone damage 2.1.Freezing-awing Damage.It is widely accepted that the freeze-thaw mechanism is typically associated with concrete that contains marginal air-void systems that are exposed to abundant moisture.Moisture in a joint may be higher than that at the surface because water is trapped due to insufficient drainage of the base layer or the cracks not opening up.Li et al. [3] have observed that for samples with a degree of saturation higher than a critical value of 86-88%, concrete may exhibit damage in a few freeze-thaw cycles.ey also found that saturated concrete is unable to resist cyclic F-T, regardless of the air content of concrete, although a properly entrained-air system is effective at delaying critical saturation.

Salt Crystallization.
Deicing salts are applied to roadways in areas that experience snow and ice in order to ensure the safety of traffic on roadways.Salts in pore solutions may crystallize out and form crystals due to changes in external humidity or temperature.A state of supersaturation is required for crystallization to occur.Under supersaturation, the cumulative crystals generate stress on the confining pore walls and cause damage in the solid matrix of a concrete mixture [4].Using an equation proposed by Correns in 1949 [5] which describes the growth and dissolution of crystals under pressure (Equation (1)), aulow and Sahu [4] concluded that potential pressure created by crystallization (159 atm) could be 5 to 10 times the tensile strength of concrete in their example: where P is the pressure exerted by growing crystals, R is the gas constant, 0.082 L-atm/mol, T is the absolute 2 Advances in Civil Engineering temperature, K, V s is the molar volume of solid salt in L/mole, and C/C s is the degree of supersaturation, where C is the existing solute concentration and C s is the saturation concentration.

Oxychloride Formation.
Certain deicers contain chemicals that may deteriorate concrete pavement.Chloride deicers will increase the concentration of calcium and magnesium in pore water of concrete.At the same time, the chloride is able to react with cement paste and forming a new expansive phase that may be expansive, calcium chloride [6].e expansive nature of calcium oxychloride can block the concrete pore connection, decrease the fluid transfer property in concrete, and influence the entire concrete matrix [6][7][8].
Formation of calcium oxychloride occurs at temperature just above freezing [6,8].Both calcium chloride and magnesium chloride can form calcium oxychloride by reacting with calcium hydroxide from the cement paste.
ough various forms of calcium oxychloride can be produced based on environmental conditions such as temperature and relative humidity, a common reaction is illustrated in Equation ( 2). e reaction between magnesium chloride and calcium hydroxide is complex, producing not just calcium oxychloride but also magnesium chloride in secondary reactions [9]: When MgCl 2 is used as a deicer on concrete pavements, a reaction between Ca(OH) 2 from cement hydration and magnesium chloride will produce brucite (Mg(OH) 2 (Equation ( 3)).At the same time, the calcium in calcium silicate hydrate (C-S-H) will be replaced by magnesium to produce noncementitious magnesium silicate hydrate (M-S-H) (Equation ( 4)) [8]: As the secondary reaction, brucite produced from Equation (3) will react with remaining MgCl 2 to produce magnesium oxychloride (Equation ( 5)).Two phases of magnesium oxychloride are reported to exist, with either 3 or 5 Mg(OH) 2 molecules (( CaCl 2 produced from Equation (3) may also contribute to formation of calcium oxychloride [10], and the process of calcium oxychloride formation is in a similar stepwise reaction as MgCl 2 solutions (Equation ( 2)).Both calcium oxychloride and magnesium oxychloride have been reported to be expansive and destructive in the concrete system [8,11]: Formation of magnesium oxychloride is dependent on the concentration of magnesium chloride [12,13].M-S-H and Mg(OH) 2 are thermodynamically more stable than C-S-H and Ca(OH) 2 , so it is preferable for C-S-H and Ca(OH) 2 to change into M-S-H and Mg(OH) 2 , both of which are expansive.Monosi and Collepardi also reported that the oxychloride formation proceeds faster at temperatures between 4 and 10 °C, while Peterson et al. [13] proposed that a phase change of calcium oxychloride may occur between 0 °C and 50 °C.

Interfacial Transition Zone Damage.
e interfacial transition zone (ITZ) in concrete is a 10-50 μm zone between cement paste and coarse aggregates formed during hydration of cement particles against aggregate surfaces [14,15].ITZ has a higher porosity than bulk-hydrated cement paste.Due to the high porosity and their high connectivity, property of transportation of fluids is increased in the ITZ, Advances in Civil Engineering facilitating movement of chloride ions [16] thus increasing the amount of paste that may react with the chlorides as discussed above.
In a previously reported project, Taylor et al. [17] observed mass loss in samples cycled in 3% sodium chloride solution using ASTM C666 (Figure 3).In comparing with samples with no silica fume, samples with an improved ITZ by silica fume were more durable.It was also reported that chloride and calcium ions appeared concentrated in the ITZ near the surface in these samples, indicating that the interfacial transition zone could be preferentially penetrated and attacked by deicers (Figure 4).ey concluded that the ITZ is playing a signi cant part in the formation of incremental cracks about 25.4 mm from, and parallel to, the joint surface.
It is well known that the solubility of calcium solutions increases with decreasing temperature.It was hypothesized that, as temperatures are reduced in a freezing system, then an ITZ containing high amounts of Ca(OH) 2 might tend to dissolve more readily than the bulk C-S-H system nearby.Such dissolution would encourage separation of the coarse aggregate particles from the paste.In addition, chloride ions can penetrate into hydrated cement pastes [18], and leach out Ca(OH) 2 , consequently increasing porosity and susceptibility to freeze-thaw deterioration [19].

Summary of Mechanisms.
e F-T damage and salt crystallization stresses discussed above can be quanti ed.Based on an equation proposed by Correns [5], Zhang [21] calculated the crystallization pressure to be about 2600 kPa in the ITZ.e osmotic pressure is about 1100 kPa according to Valenza and Scherer [20].e total pressure due to these mechanisms is thus about 3700 kPa.
Figure 5 shows a deteriorated sample exposed to 3% NaCl solution, and part of aggregate on the corner is smooth and clean with no mortar after 10 cycles of F-T. is is a concrete sample with 0.45 water-to-cementitious material ratio and 10% silica fume.e exural strength of the sample was likely above 4500 kPa [22] which is greater than the imposed stress.e cumulative pressure caused by osmotic pressure and salt crystallization is therefore not enough to cause the deterioration in Figure 5. is suggests that some other mechanism is in play, such as calcium oxychloride expansion, resulting in the paste being separated from the aggregate in the ITZ.

Materials and Methods
erefore, the purpose of the work discussed below was to study whether the formation and expansion of oxychloride was a part of mechanism causing the deterioration observed in Figures 2 and 5  4 Advances in Civil Engineering a cementitious material content of 335 kg/m3.Mixture parameters are shown in Table 1.
Concrete mixing and curing were conducted in accordance with ASTM C 192.Air content, slump, and unit weight were measured after mixing.Two 7.6 × 10.2 × 40.6 cm beams were cast from each mix.Sample preparation comprised 3 days wet curing followed by exposure to air until 28 days of age.Beams were cut into 7.6 × 10.2 × 6.35 cm prisms using a diamond saw at 28 days (Figure 6).e section thickness was selected to increase the likelihood that a single aggregate, and its associated ITZ, would percolate from face to face thus accelerating distress effects associated with the ITZ.

Advances in Civil Engineering
All samples were vacuum saturated in accordance with ASTM C1202 for 24 hours using the following solutions: 3% sodium chloride (NaCl), 3% magnesium chloride 2 ), and water.

Test Procedures.
Six slices were placed in a pan, partially submerged in assigned solutions (Figure 7), and stored at 4 °C for 56 days.e low, constant temperature was selected to remove the effects of freezing, while enhancing potential

Results and Discussion
It is observed that the samples soaked in water did not exhibit any distress, as was expected, since no F-Tcycles were applied.Likewise, neither osmotic pressure nor critical saturation should occur.
For samples soaked in NaCl, there was some microcracking in the paste matrix, and the zone between paste and aggregate was undamaged.A possible explanation for this damage is the reactions between aluminate phases of the matrix and deicer solutions [8].
SEM images of samples soaked in MgCl 2 showed cracking in the boundary between aggregate and paste (Figure 8).In addition, Figure 9 shows that the paste was pushed above the aggregate, indicating expansion in the paste.It is likely that the damage is due to the paste expanding away from the aggregate particles.is type of deterioration was more prevalent closer to the face of the sample exposed to solution, which is consistent with distress being associated with solutions penetrating the system from an exposed face.
Zhang et al. [23] used the same mix proportions and materials to observe the influence of the ITZ on F-T resistance of concrete mixtures.Samples with 0, 5, and 10% silica fume were tested in a variety of salt solutions (NaCl, MgCl 2 , CaCl 2 , and water) in F-T cycles.ey found that samples tested in water for 35 cycles remained in good condition.is indicated that the paste system was able to resist F-T cycling without becoming critically saturated as discussed in F-T damage.
For samples soaked in 3% MgCl 2 , whole aggregate particles were observed to be removed in several samples (Figure 10). is is again an indicative of aggregate particles being separated from the paste, likely due to expansion of the paste tied with damage in the ITZ.
To explain the phenomenon in Figure 2(b), the following mechanisms may be occurring: (i) Salt solution is trapped in the kerf of a sawn joint, either because it has not cracked out or the base material below the crack is impermeable.(ii) Salt solution is preferentially transported around coarse aggregate particles through the ITZ.Advances in Civil Engineering (iii) Oxychloride compounds are formed in the paste around and near the aggregate particles.(iv) e paste expands, setting up tensile stresses and so cracking in the ITZ.Any dissolution of the ITZ will accelerate this damage.(v) Cracks are propagated under traffic through the paste to the top surface, typically parallel to and about 1.9 to 2.5 cm from the sawn face.Aggregates are either removed and left, paste free, loose in the joint, or remained in the concrete, projecting into the joint.

Conclusion
Joint deterioration in sawn pavements has been found in many cold regions of the United States.e major mechanisms by which concrete joints may deteriorate include (1) freezing-thawing (F-T) damage, (2) salt crystallization, and (3) oxychloride expansion.e interfacial transition zone (ITZ) between cement paste and aggregate permits more salt solution to penetrate around aggregate particles and potentially accelerates the joint deterioration.By reviewing the previous work and testing samples subjected to soaking and F-T cycles, a mechanism has been described that explains the observed formation of so-called incremental cracks at joints.Solutions containing magnesium and calcium chloride appear to be penetrating the exposed ITZ around coarse aggregate particles in nondraining joints (Figure 11).e solution is reacting with hydrated cement paste to form oxychloride compounds near the aggregate particles.e paste expands, separating from the aggregates and causing cracks to propagate to the surface under traffic.

Figure 4 :
Figure 4: SEM map analysis on distressed samples under cyclic freeze thaw in CaCl 2 (calcium and sodium).

Figure 5 :
Figure 5: Sample exposed to 3% NaCl solution showing washed aggregate on the corner.

Figure 8 :
Figure 8: SEM images of sample soaked in water, NaCl, and MgCl 2 for 56 days.

Figure 9 :
Figure 9: Photo of sample after 56 days in MgCl 2 : paste was pushed above aggregate.