Performance of Soy Methyl Ester-Polystyrene as a Concrete Protectant: A State-of-the-Art Review

Abstract

Freeze–thaw cycles, application of deicing salts, and rebar corrosion are becoming main sources of concrete deterioration in bridge decks and pavements. During the past few decades, concrete surface treatments have begun to receive wide acceptance because of their effectiveness in sealing the concrete. Surface treatments achieve this by limiting fluid ingress, thereby reducing damage associated with freeze–thaw cycles, deicing salt application, and rebar corrosion. Soy methyl ester-polystyrene blends (SME-PS) have been shown to be an innovative, promising method of topical sealing. SME-PS, which is a derivative of soybean oil and expanded polystyrene, has been continuously studied since 2008. In this context, a comprehensive literature review compared the performance of SME-PS with that of traditional concrete sealers, including organic, inorganic, hybrid, and biotic sealers. These sealers were reviewed for performance in reducing water absorption and chloride penetration and improving freeze–thaw durability of concrete. The reviewed papers indicate that SME-PS possesses superior performance as a concrete protectant for reducing water absorption, chloride penetration, and freeze–thaw damage. To further enhance the feasibility of SME-PS, future studies may include investigating the physical and chemical protecting mechanisms of SME-PS, understanding the factors that affect the penetration behavior of SME-PS in concrete, and evaluating the short and long-term effectiveness of SME-PS in concrete.

Keywords

infrastructure materials durability of concrete durability polymer concretes adhesives and sealers waterproofing

Concrete infrastructure deterioration during winter is often caused by freeze–thaw (F/T) cycles and application of chloride-based deicing salts ( 1 – 5 ). F/T cycles mainly cause the microcracking in concrete by the expansive formation of ice ( 6 , 7 ) while chloride-based deicing salts can physically attack concrete surface in the form of scaling ( 8 – 11 ), chemically react with concrete matrix ( 2 , 3 , 12 –17), and exacerbate the F/T damage ( 18 , 19 ). Moreover, the diffused chloride ions from deicing salts can depassivate the reinforcing steel bars in concrete and initiate the corrosion ( 20 – 23 ). Therefore, it is important to protect concrete infrastructure (i.e., concrete bridge decks and pavements) from weather and deicing, which will consequently extend the service life and reduce maintenance cost.

The common solutions for improving concrete durability include modifying the concrete mixture, coating the steel rebar, applying concrete sealers, adding corrosion inhibitors in concrete, and concrete re-alkalization ( 24 – 27 ). Compared with other solutions, applying concrete sealers is often considered as the optimal solution. For existing concrete structures, modifying the concrete mixture, coating the steel rebar and adding corrosion inhibitors are not applicable and the cost of applying a concrete sealer is much lower than that of concrete re-alkalization ( 24 , 26 ). For new concrete structures, modifying the concrete mixture can be insufficient for some extreme conditions and may lead to overdesign of the whole structure ( 24 , 25 ). The rebar coating can lose its effectiveness if there is any localized damage on it ( 27 ). Moreover, the inhibiting mechanism of corrosion inhibitors has not been clearly established so far and some corrosion inhibitors are reported to have negative impacts on the surrounding environment ( 27 , 28 ).

According to European standard EN 1504-2 ( 29 ), concrete sealers are divided into three function categories, including (i) hydrophobic impregnation, (ii) impregnation, and (iii) coating ( 25 , 30 ). Inspired by this standard and previous studies, Pan et al. revised the names of sealer categories and added another group, which were hydrophobic impregnation, pore blocking surface treatment, surface coating, and multifunctional surface treatment ( 24 ). There is a more widely accepted classification of concrete sealers into organic and inorganic sealers, based on the chemical composition ( 24 , 30 , 31 ). However, this classification has become incomplete since an increasing number of hybrid concrete sealers ( 32 – 41 ) and biotic sealers ( 42 – 48 ) have been used to meet multiple practical requirements in the last decade. Therefore, there are four categories of concrete sealers in this review (i.e., organic concrete sealer, inorganic concrete sealer, hybrid concrete sealer, and biotic concrete sealer) for performance comparison.

In 2008, Coates et al. ( 49 , 50 ) first developed an innovative concrete protectant named soy methyl ester-polystyrene blend (SME-PS) by dissolving expanded polystyrene (PS) (plain Styrofoam cups) in SME. They evaluated its potential of being used as a topical application on concrete, which also led to the patenting of the technology in US8129459B2. It is hypothesized that SME-PS blend can penetrate into concrete, making the concrete surface hydrophobic, while the PS molecules will precipitate on the walls of pores in concrete surface and prevent possible reactions between deicing salts and concrete matrix ( 49 , 50 ). Afterward, a series of studies continuously investigated the performance of SME-PS blends as a new type of concrete protectant ( 51 – 60 ). The latest two studies revealed that SME-PS can effectively mitigate the formation of calcium oxychloride, an expansive and destructive phase, in concrete exposed to CaCl₂ ( 60 ), and SME-PS can enhance the resistance of concrete to salt-scaling damage and postpone chloride-induced rebar corrosion by modifying the hydrophobicity and porosity of concrete surface ( 59 ).

In this work, the authors review the feasibility of SME-PS as a concrete protectant for mitigating concrete deterioration and compare its performance with that of typical concrete sealers from those four categories. The comparison is based on the results of water absorption, relative dynamic modulus (RDM) of elasticity and mass loss, and chloride penetration. In addition, the field performance of SME-PS in reducing chloride penetration in concrete pavement joints and surfaces is reviewed and future research work about SME-PS for promoting its application in practice is proposed as well.

Concrete Sealers

Deterioration versus Protection

In aggressive environments, concrete may experience damage and deterioration as a result of a combination of multiple interrelated causes (i.e., F/T cycles, application of deicing salts, and rebar corrosion); these are mainly associated with the transport of aggressive fluid and ionic species in concrete ( 30 , 61 –64). The main factors that influence concrete deterioration caused by F/T cycles are porosity ( 65 – 67 ) and degree of saturation ( 7 , 68 , 69 ). Deicing salts can cause both physical and chemical damage to concrete infrastructure ( 4 , 70 ), which are typically manifested as salt scaling and chemical reactions between deicing salts and cement hydration products, respectively. In addition, joint deterioration is becoming an increasingly popular topic because concrete pavement joints frequently exhibit premature deterioration and the associated repair cost is very high ( 57 , 61 , 71 , 72 ). Rebar corrosion is one of the most common problems in reinforced concrete structures and the critical chloride content (Cl⁻_crit) at the depth of reinforcement is a key parameter ( 20 , 73 ), which is dependent on the chloride binding capacity, chloride diffusivity, and pH value of concrete ( 73 ).

The protecting mechanism of concrete sealers is mainly the reducing of water absorption along with ion diffusion by forming a physical barrier on concrete or modifying the hydrophobicity of the concrete ( 24 ). Other types of concrete deterioration, including acid attack, sulfate attack, and carbonation caused by the ingress of aggressive chemicals, can also be addressed by using concrete sealers ( 31 ).

Types of Concrete Sealers

Table 1 reports the molecular structure/formula and curing mechanism of representative sealers from the proposed four categories. For organic concrete sealers, polyurethane, epoxy, acrylic, methyl methacrylate, chlorinated rubber, and mineral oils are commonly used to improve concrete durability ( 24 , 74 –77). Generally, organic sealers cure and form a film bonded to the concrete surface, which acts like a physical barrier that prevents the ingress of water and any deleterious substances ( 24 ). Polyurethane (PU) and polymethyl methacrylate (PMMA) film is formed by the reaction between monomer and initiator ( 75 , 78 ) while the epoxy resins react with curing agent (amines) and form the cross-linking polymer film ( 24 ). Acrylic and chlorinated rubber are solvent-based sealers that cure with the evaporation of the solvent ( 27 , 79 , 80 ) while linseed oil and soybean oil can penetrate and stay in the pores of concrete surface ( 76 , 77 ). SME-PS can also be regarded as a type of oil-based organic concrete sealer that penetrates into the pores of a concrete surface and acts like a concrete protectant (49 –51, 56 , 58 ).

Table 1.

Molecular Structure/Formula and Curing Mechanism of Common Concrete Sealers

Unlike organic sealers, inorganic concrete sealers usually penetrate into concrete and react with the hydration products of concrete. For instance, silicate-based sealers can react with calcium hydroxide and water to produce gel-like products that fill the capillary pores and block the ingress path of deleterious substances ( 82 , 83 ). Silane, siloxane, and ethyl silicate (TEOS), which have similar molecular structures, penetrate into concrete, react with the hydroxyl groups of concrete, and form hydrogen bonding with concrete substrate ( 25 , 30 , 84 –87), as shown in Table 1. The alkali in the concrete acts as the catalyst for this reaction and the alkyl groups in the sealers determine the hydrophobicity of the concrete surface ( 24 ).

Hybrid concrete sealers are typically organic sealers modified with inorganic nanomaterials (e.g., nano-SiO₂ or nano-TiO₂) or inorganic sealers modified with polymers (e.g., acrylate or polyurethane). The incorporation of inorganic nanomaterials significantly decreases the permeability and flammability ( 32 , 39 , 40 ) and improves the durability and weathering resistance of organic sealers ( 34 , 38 , 41 ), while added polymers in inorganic sealers act as a skeleton to improve toughness ( 24 ) and decrease the pores larger than 100 nm in the inorganic coating ( 37 ). The curing mechanism of hybrid sealers is determined by each component. For instance, the relative humidity significantly affects the compressive strength, flexural strength, and sorptivity of polymer-modified cementitious coating ( 89 ).

Bacteria like Bacillus sphaericus and Bacillus cohnii can produce a dense protective layer of calcite through microbiologically induced calcite precipitation (MICP) ( 42 – 48 ). The morphology of the calcite layer is very crucial and can be influenced by the type of bacteria and environmental conditions ( 90 ). Some biotic sealers have exhibited comparable effectiveness to that of conventional concrete sealers and good compatibility with concrete ( 47 , 48 ). But the cost of using biotic sealers is relatively higher because of the nature of the bacteria, difficulty of application, and a narrow effective temperature range ( 90 ). Moreover, the ammonia produced by ureolytic bacteria and excessive nutrient solution need to be carefully handled after application ( 90 ).

Properties of Concrete Sealers

Table 2 summarizes the physical properties, sealing effectiveness, and durability characteristics of representative sealers. The properties of bonding strength, tensile strength, and elasticity determine the failure mode of organic sealers ( 24 ), while hydrophobicity and permeability determine whether film or impregnated concrete sealers are able to prevent the ingress of water and other deleterious substances. In respect of the durability of sealers, the attack from acids, alkalis, sulfate, temperature change, ultraviolet radiation, and exposure to fire are major issues in practice ( 31 ). Table 2 exhibits the advantages and disadvantages of all the concrete sealers by grading the tabulated properties, in which “○” stands for good, “×” stands for poor, and “NA” stands for not applicable since such information is not available.

Table 2.

Physical, Sealing, and Durability Effectiveness of Concrete Sealers

Sealer	Physical effectiveness of sealers			Sealing effectiveness of sealers		Durability characteristics of sealers				References
Sealer	Bond strength	Tensile strength	Elasticity	Hydrophobicity	Permeability	Chemical resistance	Thermal resistance	Ultraviolet resistance	Fire resistance	References
Organic sealer
Polyurethane (PU)	NA	○	×	NA	○	○	×	NA	×	( 24 , 31 , 91 )
Epoxy (EP)	○	○	×	×	○	○	×	×	×	( 24 , 81 )
Acrylic	×	○	×	NA	○	○	NA	○	×	( 24 , 31 , 79 )
Chlorinated rubber (CR)	○	○	○	×	○	NA	○	NA	×	( 92 )
Inorganic sealer
Silicate-based	NA	NA	NA	NA	○/×^a	NA	NA	NA	NA	( 24 , 31 )
Silane				○	○		×	×
Siloxane				○	○		×	×
Ethyl silicate				○	○		NA	NA
Hybrid sealer
Polymer-modified cementitious coating	○	○	○	NA	○	○	NA	○	NA	( 24 , 37 )
Nanomaterials-modified polymer coating	NA	○	NA	NA	○	NA	○	○	○	( 24 , 34 , 41 )
Biotic sealer
Biotic sealer	○	NA	NA	○	○	○	○	NA	NA	( 90 , 93 )

Note: NA = not available; ○ = good; × = poor.

Pan et al. ( 24 , 31 ) reported contradictory results of the water permeability of silicate-based sealers (i.e., sodium silicate).

As can be seen in Table 2, all the representative organic sealers have low permeability and low fire resistance. This indicates that these sealers could effectively prevent the ingress of water and other deleterious substances. However, they may easily be destroyed if exposed to fire. It should be noted that good tensile strength of the organic sealers somehow makes them capable of crack-bridging ( 24 ). In respect of chemical, thermal, and ultraviolet resistance, organic sealers do not show consistent performance. Since some information is not available, the selection of correct organic concrete sealers under different conditions becomes a challenge.

Currently, hydrophobicity and permeability are the most important parameters of impregnated inorganic concrete sealers whereas many other parameters of inorganic concrete sealers are still unavailable. Jones et al. reported that the chloride ion permeability of silane would significantly decrease when the temperature was raised up to 45°C ( 94 ). Moreover, Levi et al. found that the protection of silane and silicone on concrete water absorption would decrease by 50% and 90% after 300 h ultraviolet aging, respectively ( 95 ). The combination of organic and inorganic components may significantly improve the tensile strength, permeability, and ultraviolet resistance as shown in the hybrid sealers ( 24 ). For biotic sealers, since the protecting layer is formed by bacteria via MICP, they have good compatibility with concrete substrate and are capable of self-healing. It should be noted that although the film of calcite has good thermal and chemical resistance, the activity of bacteria is dependent on ambient temperature and water access. Therefore, biotic sealers should not be applied on concrete structures in regions with extreme weather.

Preparation and Properties of SME-PS Blend

As a derivative of soybean oil, soy methyl ester (SME) has been the focus of the biodiesel industry since 1994 ( 96 ). It is biodegradable, nontoxic, renewable ( 96 ) and was reported to be a green alternative solvent for organic substances ( 97 ). SME-PS blends are composed of SME as a solvent and expanded PS as a solute. Coates found that concrete samples could be completely saturated with SME ( 49 ). This is because SME molecules have 16 to 22 hydrocarbons and generally have a molecular size of about 2 nm, which is close to the C-S-H gel pores (0.5 ∼ 10 nm) ( 98 ). While SME, by itself, can be used as a concrete sealer to make the concrete surface hydrophobic, the dissolved PS molecules also act as a “blocking material” ( 58 ). Moreover, the addition of PS makes the viscosity of SME-PS blend become tunable, which makes the penetration rate of SME-PS blend tunable as well ( 49 ).

Introduction of SME and PS

Based on a patented technology (US8378132B2), SME is produced by mixing soybean oil with an alcohol (i.e., methanol) and an alkaline catalyst (i.e., NaOH) ( 99 ). As shown in Figure 1, each molecule of soybean oil (triglyceride) reacts with three molecules of methanol, producing three molecules of methyl esters and one molecule of glycerin. This process describes the transesterification of SME, which is a kind of fatty acid methyl ester (FAME). First, the alkaline catalyst helps the reaction by removing a hydrogen atom from the methanol molecule, which makes the latter become methoxide ion and increases its reactive potential. Then the three methoxide ions replace the triglycerides that are attached to one fatty acid chain and form a methyl ester molecule. Finally, the detached triglyceride molecules attach to the free hydrogen atoms from methanol molecules and form the byproduct of this process, glycerin.

Figure 1.

Transesterification process of soy methyl ester (SME).

As a derivative of soybean oil, SME is biodegradable and nontoxic (having a lower level of volatile organic compound than conventional organic solvents have) and has some properties that soybean oil does not have, including high solvent capacity and penetration depth into concrete ( 49 ). Therefore, SME could be used as a carrier for other substances, especially polymers like PS and polyvinyl chloride (PVC). According to ASTM D1133 ( 100 ), the kauri-butanol value (KBV) is a parameter evaluating the solvent power of hydrocarbon solvents, which is determined by the maximum amount of kauri-resin/gum that can be dissolved. A higher KBV indicates a greater solvent power of the solvent in dissolving certain materials ( 101 ). Figure 2 demonstrates the KBV of different FAMEs, soybean oil, and hexane. SME has a comparable KBV with methyl acetate, methyl oleate, and methyl linoleate, which is much higher than that of soybean oil, methyl formate, and hexane. Hu et al. reported that the KBV of SME is similar to those of fatty acid methyl esters derived from sunflower oil, corn oil, and canola oil ( 102 ).

Figure 2.

Comparison of kauri-butanol values (KBV) of different fatty acid methyl esters.

Another reason for choosing SME as the solvent is its high flash point and great penetration depth. As shown in Figure 3, SME exhibits the highest flash point among the methyl esters derived from vegetable oils. When being used as biodiesel, the relatively high flash point of SME is not desirable since it would lead to slow evaporation and residual film on the applied surface ( 101 ). However, if the SME is used to seal the concrete surface, it becomes a great advantage because the SME will penetrate into the concrete instead of easily evaporating into the atmosphere. The greater penetration depth of concrete sealers could lower the penetration of deicing salts into concrete more effectively ( 103 ). As shown in Figure 4, the penetration depth of SME is much greater than that of linseed oil and soybean oil, which have been previously used as concrete sealers ( 76 , 77 ). It should be noted that the penetration depth of SME decreases with the increase of relative humidity (RH) in concrete because of SME’s hydrophobicity nature ( 51 ).

Figure 3.

Flash point of different FAMEs originated from different oils.

Figure 4.

Penetration depth of linseed oil, soybean oil and SME in concrete (linseed oil: w/c = 0.58, cured in RH of 100% for 14 days, air content = 5.5 ± 0.5%; soybean oil: w/c = 0.4, cured in moisture for 14 days and in air for 14 days, air-entraining agent = 0.03%; SME: w/c = 0.4, conditioned in RH of 50%, 65% and 80% for 18 months, air-entraining agent = 20 mL/100 kg cement) under different relative humidity.

Since PS is not degradable and has low density, the disposable cups and food plates, packaging materials, and insulation materials produced by PS occupy large volumes of landfill and cause serious environmental issues. But PS was discovered to be capable of being used as an additive in biodiesel to improve the heating value, cold flow properties, and oxidation stability, and to decrease the high NO_x emission ( 101 ). Yamane and Kawasaki investigated the solubility of polyethylene (PE), polypropylene (PP) and PS in fatty acid, FAME and methanol ( 105 ). They defined a simple composite affinity parameter, that is relative energy distance (RED), which determines the solubility potential of polymers in different organic solvents. A good solvent shows a RED value smaller than 1.0 while a poor solvent has a RED value larger than 1.0 ( 105 ). As expressed in Equation 1,

RED = R_{a} / R_{0},

(1)

the RED value could be obtained via the Hansen solubility parameter distance (R_a) divided by the interaction radius (R₀) between dissolved polymer and solvent ( 105 ). The value of R_a is somehow dependent on the molecular structure of dissolved polymer. The results in Table 3 indicate that FAME is a selective solvent for PS. Vera Morales et al. reported that the dissolved PS molecules in FAME deriving from wasted vegetable oil did not affect the hygroscopic properties, density, or acid value (106). Furthermore, neither did they interact with FAMEs, such as by decomposing it or forming bonds with fatty acid ( 106 ). Moreover, Zhang et al. ( 107 ) found that PS was completely soluble in methyl esters over a wide range of temperatures and molecular weights.

Table 3.

Relative Energy Distance (RED) Values of PS, PE and PP in FAME

Polymer	RED
Polystyrene (PS)	0.93
Polyethylene (PE)	1.08
Polypropylene (PP)	1.05

Note: FAME = fatty acid methyl ester.

Source: Data extracted from Yamane and Kawasaki ( 105 ).

Properties of SME-PS Blend

SME easily dissolves expanded PS (plain Styrofoam cups). Since PS is a long-chain molecule that could entangle and hinder the fluid flow of SME, the density, viscosity, and surface tension of SME-PS are different from those of SME. Coates et al. found that the viscosity of SME-PS blends substantially increased with an increase of PS content (by mass of SME), while the surface tension was primarily determined by SME ( 49 , 50 ). Figure 5 demonstrates the change in viscosity of SME-PS with respect to PS content (i.e., 0%, 1%, 5%, 10%, 20% and 40% by mass of SME). As indicated in Figure 5, there is a dramatic increase in viscosity between the PS contents of 5% and 10%. Coates proposed a concept of critical PS content at which PS molecules begin to percolate in the SME-PS blend ( 49 ). By using a Padé-type approximation and defining the aspect ratio (length to diameter: L/D) of PS molecules in the SME-PS blend, the approximate range of the critical PS content was theoretically obtained to be near 3.9% to 12.8%. This is shown by the red dashed line in Figure 5. The theoretical approximation correlated well with experimentally measured viscosity, which confirms that the entanglement of PS molecules as a result of percolation is the main reason for the dramatic increase in viscosity of SME-PS blends. It also indicates that for practical application, the optimal PS content of SME-PS shall not exceed 3.9% by mass of SME. Otherwise, the penetration rate of the SME-PS blend would be significantly reduced because of the dramatic increase in the viscosity of the SME-PS blend.

Figure 5.

Viscosity of SME-PS blend as a function of PS content.

It should be also noted that the viscosity of SME-PS is sensitive to temperature ( 49 , 50 ). This affects the penetration rate of SME-PS under low temperatures. There are two critical temperature points for SME-PS blends. The first critical temperature is known as the cloud point at which SME loses its solubility and forms waxy crystal conglomerates ( 52 ). The cloud point of pure SME is roughly 0°C while the cloud point of SME-PS with a PS content of 5% and 10% is near 5°C ( 52 ). The second critical temperature is named the pour point (−4°C) where SME becomes waxy crystal conglomerates and ultimately turns into a gel-like substance ( 52 ). For SME-PS blend that has been applied on concrete and finished penetration, such solidification could possibly be an advantage because it helps to fill the pores of concrete and reduces the transport of fluid in winter. However, the application temperature (i.e., ambient temperature) is recommended to be above the first critical point during the field application of SME-PS protectant ( 52 ).

The penetration depth in cementitious materials is a very important parameter for SME-PS protectant, which has been investigated by different methods ( 49 , 50 , 56 ). Coates et al. ( 49 , 50 ) investigated the penetration depth of SME-5%PS (PS content of 5% by mass of SME) in a dry cement paste sample with a water-to-cement (w/c) ratio of 0.3 in the lab. By using an x-ray camera, they tracked the ingress of SME-PS protectant in the saw cuts of cement paste samples with a penetration depth of 4 mm after 5 h ( 49 , 50 ). Thomas ( 56 ) investigated the penetration depth of SME-2%PS in concrete slabs in the field by observing the particle agglomeration after mixing deionized water with the powder samples at different depths of concrete slabs. All the concrete slabs had a curing age of at least 28 days before the application of SME-PS. The SME-PS protectant was applied twice with a dosage of 12.3 mL/ft² for each time. Figure 6 shows the penetration depth of the SME-2%PS in concrete slabs of different w/c ratios (i.e., 0.42, 0.49, and 0.56) and with/without air entrainment (i.e., AE: air entrained and NAE: non-air entrained). As can be seen, the penetration depth of SME-PS increased with the increase of w/c ratio. Also, air entrainment could increase the penetration depth of SME-PS. Thomas reported the porosities of concrete samples of 0.42AE, 0.49AE, 0.56AE and 0.49NAE to be 18.5%, 21.5%, 22.3% and 13.8%, by volume, respectively ( 56 ).

Figure 6.

Penetration depth of SME-PS sealer in concrete of different w/c ratios and air entrainments.

In addition, the penetration depth of SME-PS was compared with those of inorganic and organic concrete sealers, respectively. Since all the concrete sealers were applied on the concrete samples without air entrainment, the penetration depth of SME-2%PS in concrete slabs with a mixture design of 0.49NAE was selected. As shown in Figure 7, the penetration depth of SME-2%PS is greater than that of most organic sealers, silicate-based sealers, and acrylic sodium silicate. It should be noted that concrete properties, sealer types, and sample conditioning all exhibit effects on the penetration depth ( 108 ). Attanayake et al. ( 108 ) developed a function of penetration depth, which was associated with the concrete properties (i.e., porosity and mean pore radius) and sealer properties (i.e., viscosity, contact angle, and surface tension). They also found that the moisture content of the first 6 mm depth of concrete surface controls the penetration depth. The penetration depth of solventless silicone concentrate and SME-2%PS both increase with the increase of w/c ratio of concrete, which significantly changes the porosity of concrete ( 56 , 109 ). But such a finding cannot confirm that the protecting performance of SME-PS blends as a concrete protectant is comparable to or even better than that of other concrete sealers since some information about sample conditioning is missing and the relationship between the penetration depth and the performance of concrete sealer is not established yet.

Figure 7.

Penetration depth of: (a) SME-PS and organic sealers and (b) SME-PS and inorganic sealers.

After application, SME penetrates into concrete and polymerizes over time, which reduces the risk of leaching and wash-out of the SME-PS ( 49 ). Polymerization can occur with the aid of alkalis available in the concrete pore solution ( 49 ). This also facilitates the deposit of PS molecules on the wall of concrete pores, which may possibly prevent the reaction between deicing salts and the cement matrix ( 49 ) by providing a physical protective barrier on the pore wall. Such properties indicate that SME-PS blends have good compatibility with concrete.

As a derivative of the largest oil crop in the U.S. ( 96 ), SME is renewable, biodegradable, nontoxic, cost-effective, and environmentally friendly. Moreover, waste expanded PS can be dissolved in SME for preparation of SME-PS blend instead of being disposed to waste landfills. The overall properties of the SME-PS blend make it a promising concrete sealer. However, there are still some drawbacks with SME-PS blends. With the presence of unsaturated fatty acids, SME may slowly and irreversibly become oxidized during the storage process, which will cause dissolved PS molecules to precipitate. Therefore, it is necessary to add antioxidants into SME-PS blend. Second, storage containers need to be carefully selected ( 54 ). Low density polyethylene (LDPE) and polytetrafluoroethylene (PTFE) are the best choices while acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polymethylpentene (PMP), polystyrene (PS) and copper should be avoided ( 54 ). A shelf-life of 2 to 3 years has been recommended for the storage duration of SME-PS blend. Additionally, extremely low ambient temperatures during the application of SME-PS could delay the absorption of SME-PS in concrete. It is recommended that SME-PS should be applied at a temperature above the first critical temperature (i.e., 5°C), which makes the application temperature of SME-PS wider than the ideal application temperature (i.e., 10∼32°C) of commercial concrete sealers. The advantages and disadvantages of SME-PS blends as a concrete protectant are summarized in Table 4.

Table 4.

Advantages and Disadvantages of SME-PS Blend

Advantages	Disadvantages
•Renewable •Biodegradable •Cost-effective •Nontoxic •Environmentally friendly •High flash point •Non-corrosive •Good compatibility with concrete •Wider application temperature	•Oxidization over time after application •Need storage containers made of proper materials

Note: SME-PS = soy methyl ester-polystyrene.

Comparative Evaluation of SME-PS Performance to Mitigate Concrete Deterioration

Generally, the performance of concrete sealers can be evaluated by using standard testing methods in the laboratory or by customized testing methods in the field. So far, many studies have investigated different aspects of the protecting performance of concrete sealers in the laboratory, including water absorption (ASTM C1585), F/T durability (ASTM C666), chloride diffusion/penetration (ASTM C1556), carbonation, sulfate attack, and rebar corrosion ( 31 ). The results of water absorption, F/T durability, and chloride diffusion/penetration are selected for comparison with conventional concrete sealers in this review.

ASTM C1585 is the standard testing method of determining the absorption and absorption rate of water in unsaturated hydraulic cement concrete ( 114 ). As one of three main mechanisms governing transport in concrete (i.e., permeability, diffusion, and absorption), water absorption is often employed as an important parameter to quantify the durability of concrete ( 68 , 115 ). A modified version suggested by Farnam et al. considers the intrinsic water absorption for various types of fluids ( 15 ). ASTM C666 is a standard testing method employed to investigate the F/T durability of concrete in the laboratory ( 116 ). RDM of elasticity is the parameter showing the remaining service life of concrete; and concrete samples are regarded as failed when the RDM drops below 60%. ASTM C1556 is one of the commonly used standard testing methods for quantitively assessing the resistance of concrete to the diffusion of external chloride ions ( 117 ). A chloride concentration profile with respect to depth and the apparent chloride diffusion coefficient can be obtained for predicting the service life ( 118 ).

Compared with the laboratory environment, field conditions are more complicated and unpredictable. Therefore, the protecting performance of concrete sealers on the pavement joints and pavement surfaces are evaluated separately. For traditional sealants (i.e., hot-poured sealants, cold-poured sealants, and preformed compression sealers) in pavement joints, the mechanical properties and weathering resistance of sealants have been frequently investigated because the main failure types of traditional sealants are adhesive, cohesive, intrusion, and extrusion ( 119 – 123 ). However, regardless of whether a sealer is applied in the pavement joints or on the pavement surface, the horizontal chloride profile and vertical chloride profile seem to be the most important parameters of penetrating sealers that represent the durability enhancement.

Performance of Concrete Sealers in Laboratory Environment (Water Absorption)

Using ASTM C1585, Thomas ( 56 ) investigated the water absorption of concrete samples treated with SME-2%PS and compared it with control concrete samples with no treatments. All the samples with a curing age of 28 days were conditioned under RH of 50% and temperature of 23°C for over 6 months before the test. As shown in Figure 8, the water absorption of both control and surface-treated concrete samples increased with w/c ratio. Compared with control samples, the reduction of water absorption of corresponding samples treated with SME-2%PS were all between 61.0% and 69.8%. Such results indicate that the influence of w/c ratio and air entrainment on the performance of reducing water absorption is limited when SME-PS is used to seal the surface of concrete.

Figure 8.

Water absorption of concrete samples treated with SME-2%PS compared with control samples with no surface treatment.

In addition, Xiao et al. ( 124 ) compared the performance of reducing water absorption of SME-PS with that of other commonly used concrete sealers (i.e., water-based silane with a concentration of 40%, silane/siloxane mixture with a concentration of 7% or 10%, lithium silicate/potassium methyl siliconate mixture, and poly-alpha-methylstyrene) on two types of concrete samples. One type was made of pure ordinary Portland cement and the other type was made of ordinary Portland cement with 30 wt% (percentage by weight) replacement of class C fly ash. All the concrete samples had a constant w/c ratio of 0.42 and were demolded and cut 24 h after casting. Two application times were selected, which were 30 min after cutting and 7 days after cutting. Before the application of sealers, all the samples were placed in an environmental chamber with a temperature of 23°C and RH of 50% and they were placed back into the same chamber for another 7 days after the application of sealers. Then the water absorption test was conducted in accordance with ASTM C1585 ( 114 ) for 90 days. As shown in Figure 9a, only the cement concrete sample treated with silane 40% 30 mins after cutting had a smaller water absorption than the sample treated with SME-PS, and the reduction of 90-day water absorption of SME-PS was about 40%. Interestingly, Figure 9b shows that the cement/fly-ash concrete sample treated with silane 40% 7 days after cutting had a smaller water absorption than the sample treated with SME-PS did, and the reduction of 90-day water absorption of SME-PS was only 23%. This could be attributed to the pozzolanic reaction of the fly-ash replacement, which needs further investigation to confirm.

Figure 9.

Water absorption of (a) cement and (b) fly-ash concrete samples treated with SME-PS compared with samples treated with penetrating sealers.

Figures 10a, 11a, 12a, and 13a exhibit the comparison of water absorption (WA) between concrete samples treated with SME-2%PS and those treated with common organic sealers, inorganic sealers, biotic sealers, and hybrid sealers, respectively. Since the water absorption of concrete could be significantly influenced by w/c ratio and conditioning environment ( 115 ), Table 5 summarizes the w/c ratio, RH of conditioning, and curing age of all the samples reviewed in this section. Castro et al. claimed an increasing linear relation between w/c ratio and the 8-day water absorption, initial sorptivity and secondary sorptivity of concrete samples conditioned under different RH, and the slope of the linear relation decreased with the increase of conditioning RH ( 115 ). Moreover, the 105°C oven drying can significantly increase the water absorption of concrete samples ( 115 ). As suggested by Pan et al. ( 31 ), the comparison of the relative water absorption, given by

W A_{re} = \frac{WA of samples with sealer}{WA of corresponding control samples} \times 100 %,

of concrete samples treated with different sealers would be more precise. Figures 10b, 11b, 12b, and 13b show the comparison of relative water absorption between concrete samples treated with SME-2%PS and organic sealers, inorganic sealers, biotic sealers, and hybrid sealers, respectively.

Figure 10.

Comparison of water absorption of concrete samples treated with SME-2%PS and organic sealers: (a) water absorption and (b) relative water absorption.

Figure 11.

Comparison of water absorption of concrete samples treated with SME-2%PS and inorganic sealers: (a) water absorption and (b) relative water absorption.

Figure 12.

Comparison of water absorption of concrete samples treated with SME-2%PS and biotic sealers: (a) water absorption and (b) relative water absorption.

Figure 13.

Comparison of water absorption of concrete samples treated with SME-2%PS and hybrid sealers: (a) water absorption and (b) relative water absorption.

Table 5.

Summary of Concrete Sample Preparation and Conditioning before Treatment with Different Sealers

Sealer	W/C ratio	Conditioning RH	Curing age	Reference
Organic sealers
Polymer emulsion coating	0.45	Oven dried under 70°C for 24 h	14 days (moist cured)	Almusallam et al. ( 27 )
Polyurethane
Chlorinated rubber
Acrylic coating
Epoxy
Acrylate coating	0.6	60 ± 10%	28 days (water cured)	De Muynck et al. ( 43 )
Inorganic sealers
Fluorinated silane	0.33	Oven dried under 100 ± 5°C until constant weight	2 months (na)	Levi et al. ( 95 )
Silane
Siloxane
Nanosilica (NS)	0.65	50 ± 5%	28 days (first water cured and then air cured)	Franzoni et al. ( 30 )
Sodium silicate (SS)
NS+SS
Ethyl silicate
Silane/siloxane water	0.52	50 ± 4%	91 days (moist cured)	Medeiros and Helene ( 125 )
Silane/siloxane Solvent	0.52	50 ± 4%	91 days (moist cured)	Medeiros and Helene ( 125 )
Biotic sealers
Bacillus sphaericus	0.6	60 ± 10%	28 days (water cured)	De Muynck et al. ( 43 )
Bacillus sphaericus with CaAc
Bacillus sphaericus with CaCl₂
Bacillus cohnii	0.5	Under 30°C for 14 days	28 days (moist cured)	Xu et al. ( 42 )
Bacillus cohnii with Ca glutamate
Bacillus cohnii with Ca lactate
Hybrid sealers
CP+C30B (4%)	0.53	60 ± 5%	28 days (moist cured)	Scarfato et al. ( 39 )
AS+C30B (4%)	0.53	60 ± 5%	28 days (moist cured)	Scarfato et al. ( 39 )

Note: W/C ratio = water/cement ratio; RH = relative humidity; na = not applicable.

As can be seen in Figure 10a, the concrete samples treated with SME-2%PS had a slightly smaller value of water absorption than most of the samples treated with organic sealers. But Figure 10b indicates the relative water absorption of samples treated with SME-2%PS is greater than that of all the samples treated with organic sealers except polymer emulsion coating. Compared with concrete samples treated with SME-2%PS, all the concrete samples treated with organic sealers (except those treated with acrylate coating and polymer emulsion coating) have a similar w/c ratio and are oven dried under 70°C while exhibiting a similar water absorption and a smaller relative water absorption. The water absorption curve of the concrete sample treated with polymer emulsion coating shows the sealer cannot effectively reduce water absorption of concrete. The concrete sample treated with acrylate coating has a higher w/c ratio and is conditioned under higher RH while showing a similar water absorption. Such results indicate that the performance of reducing water absorption of organic sealers (except polymer emulsion coating and acrylate coating) is slightly better than that of SME-2%PS. This could be attributed to the difference between the protecting mechanism of organic sealers and SME-PS. Organic sealers inhibit the water ingress by forming a dense film while the penetrated SME-PS in the concrete pores allows water ingress and prevents contact between water and cement matrix.

In Figure 11, silane exhibited the best performance of reducing water absorption while nanosilica (NS), sodium silicate (SS) and nanosilica plus sodium silicate (NS+SS) seem to have limited effect on water absorption of concrete with a w/c ratio of 0.65. As can be seen, the reduction in water absorption of concrete samples treated with fluorinated silane, silane/siloxane water, and silane siloxane solvent are slightly greater than that of siloxane. The curve of the sample treated with siloxane was close to those of samples treated with SME-2%PS. Considering w/c ratio, curing age, and conditioning, the effectiveness of SME-2%PS in reducing water absorption can be considered close to siloxane, better than NS, SS, NS+SS, and ethyl silicate, but slightly less competitive than silane, fluorinated silane, and water- or solvent-based silane/siloxane. This is consistent with the results discussed above. Though SME-PS, silane, fluorinated silane, and water- or solvent-based silane/siloxane could all be categorized as penetrating concrete sealers, the difference in reducing water absorption between SME-PS and the others could be attributed to their different reactivity with concrete. It would be interesting and valuable to investigate the possible chemical reaction between SME-PS and cement matrix.

As shown in Figure 12, only the performance of Bacillus sphaericus with CaCl₂ (nutrient) in reducing water absorption is comparable to that of SME-2%PS when considering w/c ratio and sample conditioning. The hybrid sealers in Figure 13 are an acrylic and vinylidenefluoride based polymer (CP) and a solvent-based siloxane resin (AS) modified by layered sodium montmorillonite (C30B) with a dosage of 4 wt%. As can be seen, the performance of SME-2%PS in reducing water absorption was better than montmorillonite-modified siloxane sealer while not as good as montmorillonite-modified polymer sealer.

Figure 14 presents the comparison of 48 h water absorption between samples treated with different sealers and corresponding control samples. The reduction of 48 h water absorption of SME-PS is 69%∼75%, which is higher than that of most biotic sealers (8%∼63%) and silicate-based sealers (5%∼50%) while lower than that of most organic sealers (74%∼85%) and silane-based sealers (77%∼92%). Such results confirmed the previous findings based on the water absorption curves and considerations of w/c ratio, curing age, and sample conditioning. To sum up, in respect of reducing water absorption, SME-2%PS shows much better performance than most of the biotic sealers and silicate-based sealers, comparable performance to siloxane, while giving a slightly less competitive performance than most of the selected organic sealers and penetrating sealers containing silane.

Figure 14.

Comparison of 48 h water absorption of concrete samples treated with different sealers: (a) SME-2%PS and inorganic sealers and (b) organic sealers, biotic sealers, and hybrid sealers.

Performance of Concrete Sealers in Laboratory Environment (F/T Durability)

Although concrete sealers cannot be used as an alternative solution to air-entraining agents, they can provide additional protection to concrete that is subjected to F/T cycles ( 31 ). Unlike air-entraining agent, concrete sealers mainly protect concrete by delaying the ingress of water into concrete so that the time needed for reaching the critical degree of saturation increases ( 81 ). Since concrete sealers can only prevent the ingress of water after application, the degree of saturation of concrete before the application becomes an important factor that influences the performance of concrete sealers ( 31 ). In addition, the protecting mechanisms of organic, inorganic, hybrid, and biotic sealers are different. For organic, hybrid, and biotic sealers, a physical film is formed after application, which prevents the ingress of water. If the film cracks or becomes porous, the sealer is no longer effective. For penetrating inorganic sealers, the concrete surface becomes hydrophobic after application and water does not stay on the concrete surface if sealers can keep their hydrophobicity. When the concrete cracks after the application of sealer because of external loads, or when the water pressure formed in the F/T process exceeds the repulsive force provided by sealers ( 31 ), the sealers will lose their function.

In accordance with ASTM C666, Golias investigated the evolution of RDM and mass loss of concrete samples treated by SME-5%PS (PS content of 5% by mass of SME) with two application dosages and by an even coating of solvent-based silane (>50% alkyalkoxysilane) ( 51 , 58 ). The SME-PS protectant was applied by immerging the samples in the SME-PS blend for 6 h (dosage 1) and 24 h (dosage 2) ( 51 ). Moreover, the performance of concrete sealers—including polyurea, epoxy, long-chain/fluorinated silane, ACM&HG (spray coating for polyurea while brush coating for all the other sealers with a dosage of 4 m²/kg) ( 33 ), linseed oil, silane, siloxane (one-time brush coating with a dosage of 3.07 m²/L) ( 76 ), acrylate-copolymer-/acrylic-latex-modified cementitious coating (a controlled thickness of 3–5 mm) ( 126 ), and 40% silane-based water repellent (two-time brush coating with an interval of 6 h and a dosage of 6.9 m²/L) ( 127 )—in improving F/T durability of concrete samples is compared with that of SME-5%PS. Although bacteria have been used in repairing systems to improve the F/T resistance of concrete pavement ( 128 ) and used as surface deposition to improve the F/T resistance of limestone ( 129 ), there are few studies investigating the influence of biotic sealers on concrete. To make the comparison more precise, the w/c ratio, usage of air-entraining agent, and conditioning that influence the sealer performance should also be considered. All the concrete samples were cured over 60 days and conditioned by being immersed in water over 7 days after the application of selected sealers, then the tests were conducted. The air-entraining agent was used in the samples treated with linseed oil, silane, siloxane, and 40% silane-based water repellent only.

Figure 15, a to c , presents the RDM evolution of concrete samples treated with SME-5%PS and organic, inorganic, and hybrid concrete sealers as a function of F/T cycles, respectively. In Figure 15a, the RDM of concrete samples treated with SME-5%PS remains over 85% while that of the control sample reached 53.3% after being subjected to 300 F/T cycles. Compared with concrete samples treated with SME-5%PS, the samples treated with organic sealers exhibited a faster decreasing rate of RDM, which indicates that the samples treated with linseed oil, polyurea, and epoxy would probably fail after being subjected to 300, 120, and 150 F/T cycles, respectively. As shown in Figure 15b, the concrete samples treated with inorganic sealers can survive more F/T cycles than those treated with organic sealers but not as many as the samples treated with SME-5%PS. The curves in Figure 15c exhibit that the acrylate-copolymer- or acrylic-latex-modified cementitious coating could not protect the concrete samples as effectively as other concrete sealers.

Figure 15.

Evolution of RDM of concrete samples treated with different sealers: (a) comparison between SME-5%PS and organic sealers, (b) comparison between SME-5%PS and inorganic sealers, and (c) comparison between SME-5%PS and hybrid sealers.

Figure 16, a to c , presents the mass loss evolution of concrete samples treated with SME-5%PS and organic, inorganic, and hybrid concrete sealers as a function of F/T cycles, respectively. The curves in Figure 16a show that SME-5%PS could completely prevent concrete samples from mass loss after being subjected to 300 F/T cycles while most of the samples treated with organic sealers had lost or would lose at least 1% of the initial weight after being subjected to only 100 F/T cycles. In Figure 16b, the mass loss of all the concrete samples treated with inorganic sealers except >50% alkyalkoxysilane and silane is less than 2% of the initial weight after being subjected to 100 F/T cycles. Figure 16c shows that acrylic-latex-modified cementitious coating could effectively prevent the mass loss of concrete samples while acrylate-modified cement coating could not.

Figure 16.

Evolution of mass loss of concrete samples treated with different sealers: (a) comparison between SME-5%PS and organic sealers, (b) comparison between SME-5%PS and inorganic sealers, and (c) comparison between SME-5%PS and hybrid sealers.

Compared with the concrete samples treated with organic, inorganic, and hybrid sealers, the samples treated with SME-5%PS demonstrate superior F/T durability even without air entrainment. Since the significance of air entrainment is greater than that of concrete sealer to the F/T durability of concrete ( 130 ), such results confirm the advantage of using SME-PS to protect concrete from F/T deterioration. However, the influence of application dosage of SME-PS protectant on its protecting performance on concrete F/T durability is not fully understood and relevant future research is needed.

Performance of Concrete Sealers in Laboratory Environment (Chloride Penetration)

Although a considerable number of tests have been conducted on chloride penetration in concrete treated with traditional sealers, a comparative study is somewhat complicated because different testing methods are employed ( 31 ). To have a more precise comparison between SME-PS and other sealers, four studies of concrete samples treated with organic, inorganic, hybrid, and biotic sealers are selected ( 27 , 43 , 88 , 131 ). All the concrete samples are cured for over 28 days and subjected to unidirectional chloride diffusion of NaCl solution; the only differences in the testing method are the concentration of NaCl solution and exposure time ( 27 , 43 , 56 , 88 , 131 ). Since only ordinary Portland cement was used in all studies, the effect of chloride binding on the performance of different sealers is assumed to be the same. It has been shown that the chloride binding is mainly influenced by the cement hydration products and the degree of hydration; and it is relatively independent of w/c ratio and aggregate content ( 132 ). Costa and Appleton ( 133 ) reported that the chloride penetration is strongly dependent on w/c ratio, exposure condition, and time. Thus, the main influencing factors of these three studies would be the w/c ratio, concentration of NaCl solution, and exposure time.

Figure 17 demonstrates the chloride profile of concrete samples treated with different sealers. As can be seen in Figure 17a, the chloride concentration of all the concrete samples decreases with the increase of depth. SME-2%PS has been shown to effectively reduce the chloride penetration in concrete samples with different w/c ratios. Compared with control samples, the chloride concentration in the first 10 mm depth of concrete samples treated with SME-2%PS is reduced nearly 45% to 80%. Results shown in Figure 17b indicate that all the organic sealers except polymer emulsion coating are comparable to SME-2%PS in mitigating chloride penetration. However, Figure 17, c and d , indicates that SME-2%PS shows superior performance to inorganic sealers and polymer-modified cementitious sealers in reducing chloride penetration. Such a finding is consistent with other studies reported elsewhere ( 31 ).

Figure 17.

Chloride concentration profile of concrete samples treated with various types of sealers: (a) comparison between SME-2%PS and control samples, (b) comparison between SME-2%PS and inorganic sealers, (c) comparison between SME-2%PS and organic sealers, and (d) comparison between SME-2%PS and hybrid sealers.

According to Fick’s second law, the chloride diffusion coefficients of concrete samples treated with different sealants can be obtained. To eliminate the effects of w/c ratio, exposure condition, and time, the relative chloride diffusion coefficients, that is, the chloride diffusion coefficients of samples treated with sealers divided by those of corresponding control samples multiplied by 100%, are exhibited in Figure 18. All the relative chloride diffusion coefficients of samples treated with SME-2%PS are smaller than 20% (i.e., more than 80% reduction); additionally, w/c ratio seems to have limited effect on the performance of SEM-2%PS in reducing the chloride penetration in concrete. On the other hand, only the samples treated with acrylic coating and polyurethane have relative chloride diffusion coefficients smaller than 20%. The relative chloride diffusion coefficients of samples treated with inorganic sealers are between 36% and 92%. Moreover, the relative chloride diffusion coefficients of concrete samples treated with acrylic modified cementitious sealer and Bacillus sphaericus sealer are about 80% and between 60% and 95%, respectively. The nutrient plays an important role in the chloride-penetration-reduction performance of the biotic sealers. Considering the exposure time and the concentration of sodium chloride, SME-2%PS exhibits a better performance in mitigating chloride penetration in concrete than most of the investigated four types of concrete sealers.

Figure 18.

Relative chloride diffusion coefficient of concrete samples treated with various types of sealers.

Performance of Concrete Sealers in Field Environment (Chloride Diffusion at Pavement Joint)

There are few studies that have evaluated the performance of SME-PS blends to seal pavement joints and protect pavement surface in the field ( 54 , 56 , 124 ). Wiese applied SME-2%PS on 12-year-old and newly constructed pavement joints and investigated the horizontal chloride profile of cored samples collected from joints after 3 years’ field service ( 54 ). Xiao et al. applied SME-PS and three other types of concrete sealers (i.e., saline 40%, silane/siloxane, and lithium silicate) in the joints of newly constructed (21-day old) concrete pavements and did a visual inspection on the joints from the shoulder after 113 days, 202 days, and 283 days of field service. They also investigated the water-contact angle of core samples taken from the joints and found that, compared with control core samples, the core samples treated with silane and silane/siloxane mixture showed a greater value of water-contact angle while samples treated with SME-PS showed a slightly smaller value after 1 year of field service ( 124 ). Such results indicate that it would be valuable to investigate and enhance the long-term effectiveness of SME-PS in the future. A study investigating the 2-year field service of silane-based sealers applied on 19-year-old pavement joints ( 134 ) was selected for comparison with the field performance of SME-PS protectant.

The SME-2%PS blend was applied on 13 longitudinal joints on US231 in Lafayette, Indiana on August 6, 2011. The old traditional sealing systems (i.e., backer rod and silicon) of all the joints were first cleaned off and then six of them were treated with SME-2%PS, six of them were left with no treatment, and the last one was left with the original backer rod and silicon sealant to leave a slab between the SME-PS section and the control. The field site was revisited on November 26, 2014, and three cores were taken from the joints (including those treated with SME-2%PS, traditional sealing system, and no sealing). A titration test was conducted to obtain the chloride profile perpendicular to the saw cut so that the chloride content from the exposed saw-cut face inwards could be presented.

The silane/siloxane-based sealers were applied on five transverse joints of I94 in Minneapolis, Minnesota on August 18, 2013. Similarly, the old sealing system was cleaned off first and then four of the five joints were treated with four types of penetrating silane-based sealers and one of them was left untreated. The five joints were revisited on July 13, 2015, and five cores were taken for observation of scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) to obtain the horizontal chloride profile.

The chloride profiles of concrete samples from joints on US231 are shown in Figure 19a. The application of SME-2%PS reduced the chloride concentration substantially (more than 50% reduction) in comparison with joints with no sealing; however, it allowed slightly higher concentrations of chlorides into the concrete compared with joints with traditional sealing systems. This is mainly because the traditional sealing systems act as a physical barrier and keep water out; however, in the case of SME-PS, the large capillary pores and air voids might not be completely filled with SME-PS, providing room for the ingress of chlorides ( 54 ). Less chloride was seen near the surface of the pavement joint, which could be attributed to the rainwater during summer that washed the deicing salts from the surface of the concrete ( 54 ).

Figure 19.

Horizontal chloride concentration profile of field samples: (a) treated with SME-2%PS compared with traditional joint-sealing system and (b) treated with silane-based sealers compared with control concrete with no sealing.

Figure 19b reports the chloride profiles of concrete pavement joint treated with silane/siloxane-based sealers after 2 years of service. The application of silane/siloxane-based sealer effectively reduced the chloride concentration in the first 15 mm depth. However, the difference of chloride concentration between the joints with/without treatment seems to be negligible when the depth is larger than 15 mm. It should be noted that the chloride concentration of the pavement joint treated with silane/siloxane-based sealer is much greater than that of the pavement joint treated with SME-PS protectant, which could probably be attributed to the difference between the quality of these two concrete pavements. Unfortunately, the mixture designs of these two concrete pavements are not available. To have a more precise comparison between these two sealers, the relative chloride concentration of pavement joints treated with these two sealants should be investigated.

The relative chloride concentration of pavement joints treated with SME-PS and silane/siloxane-based sealers could be obtained via the chloride concentration of the pavement joint treated with sealers divided by that of the untreated pavement joint. Figure 20a shows the reduction of chloride concentration with the application of SME-PS and a traditional joint-sealing system. The joint treated with a traditional joint-sealing system and SME-PS showed a similar performance in reducing chloride penetration in concrete (near 40%–50% reduction) in the first 7 mm depth. As illustrated in Figure 20b, silane-based sealers were relatively less effective in reducing chloride penetration (near 20%–30% reduction).

Figure 20.

Relative horizontal chloride concentration profile of field samples treated with different types of sealers: (a) SME-2%PS and traditional joint-sealing system and (b) silane-based sealers.

Performance of Concrete Sealers in Field Environment (Chloride Diffusion from Concrete Surface)

Thomas applied SME-2%PS on concrete slabs in the field, which were exposed to solutions of different deicing salts twice per week, and investigated the vertical chloride profile of cored samples collected from concrete slabs after 9-month exposure ( 56 ). In this study, 36 concrete slabs of four different mixtures were prepared and cured for at least 28 days to evaluate the effect of SME-PS protectant on the diffusion of chloride from the concrete surface. The surfaces of 12 slabs were treated just once with SME-2%PS with an application rate of 12.3 mL/ft², 12 slabs were treated twice with SME-2%PS at the same application rate, and the remaining 12 slabs were left untreated as control slabs. Solutions of commercial-grade deicing salts (i.e., 10% NaCl, 10% MgCl₂ and 10% CaCl₂) were applied on the treated and control concrete slabs twice a week with a rate of 23.1 g sol./ft² for 9 months. The initial exposure of deicing salts was in December of 2014 and the concrete slabs were subjected to F/T cycles in Indiana. After 9 months’ exposure to salt solutions and F/T cycles in the winter, samples were cored from the concrete slab and titration tests were conducted to obtain the chloride content profile in the vertical direction.

Another field study conducted at Dodge and Pierce county of Wisconsin is also reviewed in this section to compare the performance of inorganic sealer with SME-PS protectant on reducing chloride diffusion ( 135 ). Two bridge decks were treated with a low-viscosity oligomeric organosiloxane sealer (tri-siloxane sealer) right after construction (SO) while another two bridge decks received treatment with the same sealer on a periodic basis starting from 4 years after construction (SP). After about 12 years of field exposure (application of 50/50 sand/salt in the winter), the chloride concentration profiles of four bridge decks were obtained to investigate the long-term performance of the sealer. In addition, six concrete slabs (122 × 91 × 20 cm³) treated with low-modulus silicone and polyurethane sealer were placed in a simulated field environment ( 136 ). All the concrete slabs were cured for 7 days in the wood molds and then were treated with sealers and exposed to wet/dry cycles of sodium chloride solution (3 days of ponding and 4 days of drying) ( 136 ). After being subjected to 40 wet/dry cycles, concrete powder samples were collected from different depths of the concrete slabs for chloride concentration profile assessment. Another two studies evaluating 12 years’ field service of a low-viscosity oligomeric organosiloxane sealant applied on bridge decks ( 135 ) and investigating the chloride penetration of concrete slabs treated with polyurethane and low-modulus silicone sealers after exposure to 40 wet/dry cycles of sodium solution ( 136 ) were selected for comparison. To the best of the authors’ knowledge, the field conditions of the selected three studies are the closest ones to that of the field evaluation of SME-PS sealer.

Figure 21, a to c , shows the chloride concentration profile of concrete slabs with two applications of SME-2%PS after 9 months’ exposure to NaCl, MgCl₂, and CaCl₂ solutions, respectively. Compared with the control concrete slabs of 0.49AE, the control concrete slabs of 0.49NAE had higher chloride concentration at shallow depth but lower chloride concentration at greater depth. The explanation given was that no air entrainment led to a greater volume of cement paste, absorbing more fluid at shallow depth ( 115 ) and increasing tortuosity at deeper depth ( 56 ). Compared with the control concrete slabs, all the slabs treated with SME-2%PS exhibited decreased amounts of chloride penetration and chloride concentrations. Varying w/c ratio seemed to have limited effect on the performance of SME-PS in reducing chloride concentration. Thomas conducted a non-linear regression analysis on the chloride concentration profiles of slabs treated with SME-PS to obtain the apparent diffusion coefficient (D_app) and surface chloride concentration (C_s) using Fick’s second law as shown in Figure 22 ( 56 ). The correlation factors (R²) of all the cases were found to be larger than 0.9, meaning that Fick’s second law is applicable for approximation of the chloride-penetration profile of concrete slabs treated with SME-PS in the field ( 56 ). Compared with control slabs, SME-PS protectant was able to reduce 30% to 55% of C_s regardless of the types of deicing salts. Interestingly, the value of D_app of MgCl₂ and CaCl₂ increased with the increase of w/c ratio while that of NaCl decreased. Further studies can investigate such differences. Compared with control concrete slabs, the chloride-penetration depth of concrete slabs treated with SME-2%PS twice was reduced by 57%, 74%, and 75% while that of concrete slabs treated with SME-2%PS once was reduced by 38%, 43%, and 52% for NaCl, MgCl₂, and CaCl₂, respectively. Such a result indicates that the second application of SME-PS is necessary to increase the long-term durability of concrete slabs.

Figure 21.

Comparison of chloride concentration profile of concrete slabs with/without treatment of SME-2%PS after being exposed to different deicing salts for 9 months: (a) NaCl, (b) MgCl₂, and (c) CaCl₂.

Figure 22.

Comparison of chloride diffusion coefficient (a) and surface chloride concentration (b) of concrete slabs with/without SME-2%PS after being exposed to different deicing salts for 9 months.

Figure 23a demonstrates the chloride concentration profile of concrete bridge decks treated with tri-siloxane sealers after about 12 years of exposure to field conditions. As can be seen, one-time application of tri-siloxane sealer after construction did not prevent the chloride penetration while periodic application of tri-siloxane sealer showed about 30% reduction in chloride penetration. A previous study reported that the reduction of chloride penetration in concrete would increase with the penetration depth of inorganic concrete sealers ( 110 ). This explains the effective performance of periodic application of tri-siloxane sealer in reducing chloride penetration in concrete. In Figure 23b, there is little difference between the chloride concentration profiles of concrete slabs with/without the treatment of low-modulus silicone or polyurethane sealer. Based on such findings, the field performance of reducing chloride penetration of SME-2%PS protectant (two-time application) would be better than that of low-modulus silicone sealer and polyurethane sealer. But since the investigation of long-term effectiveness of SME-PS protectant has not been done yet, it is not applicable to conclude whether SME-PS or tri-siloxane is better and whether SME-PS needs periodic application.

Figure 23.

Chloride concentration profile of concrete bridge decks treated with inorganic sealers after serving in the field for about 12 years: (a) and treated with organic sealers after serving in the field for 9.5 months (b).

Cost Comparison between SME-PS and Commercial Concrete Sealers

Wiese compared the cost of joint protection via the application of SME-PS versus traditional sealing systems ( 54 ). Since the traditional sealing system includes backer rod and filler, two saw cuts are necessary for placing the backer rod while the application of SME-PS only needs one saw cut. Table 6 summarizes the cost associated with the application of SME-PS and a traditional sealing system. In addition to the cost of joint application, the authors also investigated the cost of surface application of common concrete sealers available in the market. With the available unit prices and recommended application rate, the costs of different concrete sealers are calculated and tabulated in Table 7. As can be seen, the cost of SME-PS is substantially lower than those of organic or inorganic sealers. It should be noted that all the prices are completely dependent on the market conditions at the time being considered and thus the costs of concrete surface treatment can vary over time.

Table 6.

Cost Comparison of Joint Protection by Applying SME-PS and Traditional Sealing System

Item cost	SME-PS	Traditional
Saw cut ($/ft)	1.78	2.72
SME-PS sealant ($/ft)	0.06	NA
Backer rod ($/ft)	NA	0.03
Filler ($/ft)	NA	0.25
Total cost ($/ft)	1.84	3.00
Total cost per mile ($/ft)	With saw cut: 9,715	15,840
	Without saw cut: 317

Note: SME-PS = soy methyl ester-polystyrene; NA = not available.

Table 7.

Cost Comparison of Surface Protection by Applying SME-PS and Other Concrete Sealers

Sealers	Cost ($/ft²)
SME-PS	0.046~0.076
Sodium silicate	0.06~0.22
Lithium silicate	0.09~0.14
Silane	0.16~0.3
Siloxane	0.13~0.28
Fluorinated silane	0.09~0.3
Acrylic	0.075~0.25
Polyurethane	0.24~0.26
Epoxy	0.48~0.58
Latex	0.22~0.37

Note: SME-PS = soy methyl ester-polystyrene.

Conclusions and Future Work

Based on the results and findings provided in this literature review, SME-PS blends have shown great potential for being used as an efficient concrete protectant to protect concrete infrastructure from deterioration during winter and the corrosion of reinforcement. Important conclusions of this study are:

(1) The application of SME-PS can modify the hydrophobicity of concrete surface, which leads to a significant reduction in water absorption and salt-scaling damage. Accordingly, major improvement in the F/T durability of concrete has been reported. In addition, surface treatment with SME-PS can mitigate chloride diffusion in concrete and thus postpone the potential chloride-induced rebar corrosion.

(2) Compared with other commercial concrete sealers, SME-PS has exhibited a comparable and promising behavior in reducing water absorption and chloride diffusion, and a greater enhancement in F/T durability of concrete.

(3) In the field tests, the application of SME-PS has shown effectiveness in reducing chloride penetration in concrete slabs as well as pavement joints. But it is necessary to further evaluate the long-term performance of SME-PS protectant in the field.

The protecting nature of SME-PS protectant requires further investigation in respect of whether the SME-PS penetrated in concrete surface can chemically interact with hydration products of concrete. Additionally, the penetration behavior of SME-PS in concrete, short-term and long-term stability of SME-PS in concrete, and long-term performance of SME-PS in lowering the corrosion rate of reinforcing steel bar in concrete require further study.

Future work on using SME-PS as a concrete protectant may include the following:

(1) Understanding the protecting mechanism of SME-PS protectant by confirming the existence and stability of precipitated PS molecules on the wall of pores in concrete surface and exploring the possible chemical reactions between the SME-PS and cementitious matrix;

(2) Understanding the influence of pore structure of concrete surface on the penetration behavior of SME-PS in concrete;

(3) Understanding the effect of temperature, diffused oxygen, ultraviolet light, and alkaline environment on the short-term and long-term stability and effectiveness of SME-PS in concrete.

Footnotes

Acknowledgements

The authors gratefully acknowledge Indiana Soybean Alliance for providing the financial support and Drexel Advanced and Sustainable Infrastructure Materials (ASIM) Lab for providing the equipment used in this study.

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: Jialuo He, Yaghoob Farnam; data collection : Jialuo He; analysis and interpration of results: Jialuo He, Mohammad Balapour, Yaghoob Farnam; draft manuscript preparation: Jialuo He; All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Indiana Soybean Alliance provided financial support for this work.

ORCID iDs

Jialuo He

Mohammad Balapour

Any opinions and or discussions provided in this paper are those of the authors.

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