2.1.1 Latex

SB latexes were synthesized via seeded emulsion polymerization using styrene and butadiene monomers (60:40 wt.%) sourced from the Ulsan Synthetic Rubber Plant, Korea. Five latex samples were prepared with 1-3 wt.% of comonomers (AA, MAA, IA, MMA, and N-MAM) selected to enhance dispersion stability, workability, and film formation. Sodium lauryl sulfate (2.2 wt.%) and potassium persulfate (1.2 wt.%) served as emulsifier and initiator, respectively. Polymerization was conducted in a 5 L autoclave reactor, with a pre-emulsion stirred at 200 rpm for 5 min before adding butadiene and heating to 80°C. Post-polymerization, latex pH was adjusted to 8-12 using NaOH, filtered, and mixed with defoamer (1.1 wt.%). Detailed formulations are shown in Table 1.

The butadiene-to-styrene ratio was a key formulation parameter, initially explored across a range from 30/70 to 45/55 to optimize film flexibility and mechanical performance. A lower Tg associated with increased butadiene content improved softness, adhesion, and flexible strength. However, excessive butadiene (e.g., >45 %) led to overly soft films with reduced compressive strength, undermining structural integrity in LMC applications. Conversely, higher styrene content improved stiffness but compromised flexibility. The final BD/ST ratio of 40/60 was therefore selected as an optimal balance, offering sufficient flexibility and compressive strength required for durable bridge deck pavements.

URHC combined with LMC enables rapid, durable repairs, ideal for minimizing downtime in bridge deck maintenance. URHC provides fast setting and early strength, critical for reopening traffic quickly. To be effective, the latex must ensure polymerization stability and help concrete achieve over 21 MPa compressive strength within 4 hours. Simultaneously, good workability is essential for field application. Achieving this balance depends on comonomer selection, which affects mechanical properties and latex stability. Commonly used comonomers (AA, MAA, IA) and film-enhancing monomers (MMA, N-MAM) contribute to crack resistance, chloride barrier performance, and environmental durability, making LMC highly suitable for demanding bridge deck conditions.

2.1.2 Preservatives

To improve latex storage stability, three preservatives were evaluated: chloromethylisothiazolinone (CMIT), methyl-isothiazolinone (MIT), and BIT. CMIT offers strong antimicrobial activity for rapid microbial control but is limited by potential skin sensitivity, making it suitable for non-skin-contact applications. MIT is less potent but safer, making it preferable for broader use, especially where long-term protection with minimal irritation is needed. BIT provides excellent stability and low reactivity, making it ideal for high-pH latex systems. Its durability reduces degradation and extends shelf life. Preservative selection depends on latex requirements such as longevity, exposure conditions, and safety ^{(Luo, 2024)}.

2.1.3 Aggregates

The construction mix utilizes 19 mm crushed gravel as coarse aggregates to enhance the concrete’s structural integrity and durability. Sharp sand is used as fine aggregates, chosen for its angular particles which improve cohesion and workability.

2.1.4 Comparing Ordinary Portland Cement (OPC) and URHC

One of the key distinctions between the two cements lies in the quantity and function of specific chemical compounds, particularly Tricalcium Aluminate (C3A), Tricalcium Silicate (C3S), and Calcium Sulfoaluminate (CSA). These compounds play a crucial role in determining the workability and early strength development of the cement. The chemical compositions of OPC and URHC are summarized in Table 2.

2.1.5 URHC and mobile mixer vehicle

The URH cement and retardant employed in the study were supplied by UNION Co., Seoul, Korea. The cement is generally used in installation on the bridge, repair of concrete surface. The composition of the cement is listed in Table 3.

A mobile mixer vehicle was used for more reliable on-site concrete application. This vehicle differs from traditional batching plants by allowing continuous on-site concrete pouring. It transports individual materials separately to the site and mixes them on-site, enabling immediate production and discharge. This method avoids the transportation of pre-mixed, partially set concrete, which can change properties over time. The on-site process of mixing, blending, and pouring at a single location ensures high-quality pavement concrete.

2.2.1 Analyses of latex

To characterize the prepared latex formulations, key properties were evaluated, including particle size distribution, solid content, viscosity, surface tension, polymerization stability, mechanical stability, gel content, and freeze-thaw resistance. Particle size and solid content were measured via dynamic light scattering (DLS, Nano-Zs, Malvern), respectively. Viscosity and surface tension were assessed using a Brookfield viscometer and a surface tension meter. Polymerization stability was monitored by measuring coagulum content. Mechanical stability followed JIS K-6392-1995 using a Maron tester, while gel content, reflecting crosslinking density, was determined gravimetrically. Latex properties are summarized in Table 4. Stability during polymerization was assessed by coagulum content after filtration (200-mesh), where higher coagulum indicated reduced stability. Concrete workability was evaluated via slump tests (target: 180±30 mm) and slump loss after 20 min, with lower slump loss indicating better workability retention, which is crucial for field applicability and pavement durability.

2.2.2 URHC in LMC mix design

The LMC application method evaluation criteria followed the KS standard method KS F 2402. The mix design for LMC involves blending latex with concrete to enhance strength, durability, and workability. This process adjusts the optimal ratio of latex and other materials to achieve the desired concrete characteristics. Standard mixing conditions are shown in Table 5 ^{(Said, 2016)}.

2.2.3 Compressive strength

The compressive strength evaluation of URHC-LMC was performed according to the KS F 2405 standards. The results showed that the mix achieved the target 4-hour compressive strength of ≥21 MPa. Furthermore, after a curing period of 28 days, all samples satisfied the strength requirement of ≥35 MPa, demonstrating the effectiveness of the URHC in achieving both early and long-term strength goals. Tests were performed using controlled load until the specimen failed. To minimize eccentricity effects, both ends of the concrete specimen were tested while maintaining levelness using a grinder.

2.2.4 Flexural strength

The flexural strength test was performed using 100×100× 460 mm beam specimens to evaluate the material's resistance to bending, in accordance with KS F 2408 standards, using the three-point loading method. Three specimens were tested for each variable, and the average value was used as the final result. In this test, the beam is supported at two points, and a load is applied at the center until the specimen fractures. The maximum recorded load represents the flexural strength, providing a reliable assessment of materials subjected to bending stresses, such as beams and slabs.

2.2.5 Permeability and surface scaling tests

This test, governed by standards like ASTM C 1202, AASHTO T 259, and KS F 2711, measures how easily chloride ions can penetrate concrete. Chloride ions, often from de-icing salts or seawater, can cause significant damage to the steel reinforcement in concrete by initiating corrosion. For concrete pavements in environments where de-icing salts are used, such as highways and bridges, preventing chloride penetration is crucial to maintaining structural integrity.

2.2.6 URHC-LMC for aging bridge repair

The URHC-LMC method provides an efficient solution for repairing aging bridges initially constructed with LMC. Over time, traffic loads and environmental exposure cause surface deterioration and cracking. Fig. 2 illustrates the URHC-LMC pavement repair process.

The image illustrates the sequential construction procedure for applying URHC-LMC in bridge deck repair projects. The process begins with the identification of the deteriorated area (a), followed by precise cutting of the damaged pavement (b) to ensure proper bonding of the repair material. Next, high-pressure water jetting is conducted to remove surface contaminants and ensure a clean substrate (c), which is further refined through vacuum cleaning to eliminate residual moisture and debris (d). After surface preparation, URHC-LMC is placed using specialized equipment (e), and finally, surface finishing is performed (f) to ensure uniform texture and proper integration with adjacent pavement. This method enables rapid construction and early opening to traffic while maintaining high durability and adhesion performance.

Fig. 2. Step-by-step Process of URHC-LMC Application for Bridge Deck Repair. (a) Target Area Selection, (b) Road Cutting, (c) Water Jetting for Surface Cleaning, (d) Vacuum Cleaning, (e) URHC-LMC Pavement Placement, (f) Surface Finishing

3.1.1 Comonomers hydration and performance in URHC-LMC

LMC incorporating URHC is widely used in infrastructure repairs, especially bridge decks requiring rapid setting and early strength. Achieving a compressive strength of ≥21 MPa within four hours is critical to minimize traffic disruptions. URHC reduces curing time while ensuring structural integrity, and latex further enhances mechanical performance by improving flexibility, adhesion, and environmental resistance. The latex forms a continuous polymer film in the cement matrix, strengthening the bond between particles and improving resistance to dynamic loads and harsh environments (Soroushin and Spinkel, 1999).

Carboxylic acid-based comonomers AA, MAA, and IA, accelerate hydration, promoting early strength gain. Their high heat of hydration (AA: 385 J/g, MAA: 360 J/g, IA: 345 J/g) results from strong ionic interactions with calcium hydroxide. Fig. 3 compares heat generation, showing carboxylic acids have higher exothermic profiles than non-carboxylic counterparts. However, excessive heat release can cause thermal cracking, requiring careful control of hydration kinetics.

Non-carboxylic comonomers like (MMA, 310 J/g) and (N-MAM, 290 J/g) generate less heat, reducing thermal cracking risk but may delay strength development. Therefore, URHC-LMC performance relies on balancing hydration rate and thermal effects through targeted comonomer selection and additive optimization to ensure durability and safety in large-scale repairs.

The heat of hydration was quantitatively evaluated using an isothermal conduction calorimeter (TAM Air, TA Instruments, USA), operated at a constant temperature of 25±0.1°C in accordance with ASTM C1702-17, wherein cement pastes were formulated with a water-to-cement ratio of 0.35, incorporating 90 wt.% cement and 10 wt.% styrene-butadiene latex modified with 2 wt.% of a specific comonomer (AA, MAA, IA, MMA, or N-MAM), thoroughly mixed under ambient laboratory conditions (22-25°C, RH 50-60 %) for a total of 2.5 minutes to ensure homogeneity, after which 5 grams of the freshly mixed paste was carefully transferred into sealed glass ampoules and immediately placed into the calorimeter chamber, where continuous heat flow data was recorded over a 72-hour period to capture both initial and long-term exothermic profiles, and the total heat release (in J/g) was subsequently calculated by integrating the resulting heat.

Fig. 3. Comparison of the Heat of Hydration for Different Comonomers Used in LMC

3.1.2 Effect of comonomers

An experiment was conducted to evaluate the polymerization stability and workability of latex in LMC incorporating URHC, a system widely used in infrastructure repairs such as bridge decks that require fast setting and early strength development. A key performance target is achieving a compressive strength of at least 21 MPa within four hours to minimize traffic disruption and enable rapid reopening. To investigate the influence of formulation, 2 wt.% of various comonomers were added to the latex, and their effects on polymerization stability, workability, and strength were assessed ^{(Buch et al., 2008)}.

To assess latex stability after polymerization, each sample was filtered through a 200-mesh stainless steel screen directly within the reactor to evaluate the degree of coagulation and particle aggregation. The amount of residue retained on the mesh was collected and dried at 150°C for 30 minutes, then weighed to determine the percentage of aggregated solids. A lower residue value indicated higher colloidal stability. All stability tests were conducted at a controlled pH of 11.5.

When URHC is incorporated into LMC, the early hydration behavior is strongly influenced by the interaction among key cementitious compounds such as C3A, C3S, and CSA.

In this study, three carboxylic acid-based comonomers (AA, MAA, and IA) and two non-carboxylic monomers (MMA and N-MAM) were evaluated. The carboxyl groups in AA, IA, and MAA react strongly with CSA, forming calcium- carboxylate complexes that accelerate hydration and contribute to rapid strength gain. However, this also leads to latex coagulation, reducing fluidity and negatively affecting workability.

On the other hand, MMA and N-MAM exhibit lower reactivity with cement, allowing for better latex dispersion and improved workability. These formulations remain fluid longer, making them easier to handle on-site. However, their slower hydration kinetics delay the formation of calcium silicate hydrate (C-S-H), resulting in insufficient strength development within the required time frame. Fig. 4 illustrates these findings. These results highlight the trade-off between early strength and workability, underscoring the importance of optimizing comonomer selection based on project-specific performance needs.

Fig. 4. Comparison of Latex Stability, Workability, and Compressive Strength Based on Different Comonomers. (a) Latex Stability and Workability, (b) Compressive Strength

3.1.3 Enhancing latex stability and strength in URHC-LMC system

To enhance polymer stability and early-age performance in URHC-LMC, this study investigated the effect of increasing total comonomer content to 3 wt.% relative to the primary monomers. Formulations combined 1 wt.% of carboxylic acid-based comonomers (AA, MAA, IA) with 2 wt.% of N-MAM to evaluate their synergistic effects on latex dispersion, workability, and early compressive strength. Carboxylic acids introduced negatively charged carboxyl groups into the polymer matrix, enhancing colloidal stability through electrostatic repulsion. Among them, AA showed the highest ionization, promoting rapid ionic interaction with calcium ions and superior early strength. IA and MAA also improved stability but were less effective due to their bulkier structures and lower ionization.

N-MAM further improved polymer flexibility and film formation; however, its hydrophilic amide groups increased the risk of latex coagulation at higher concentrations by promoting water absorption and chain entanglement. Fig. 5(a) illustrates the impact of each comonomer combination on stability and workability. The AA-N-MAM blend offered the strongest initial stabilization, while IA and MAA combinations better preserved workability over time. Excess N-MAM (>2 wt.%) negatively affected stability, highlighting the importance of dosage control.

Workability and early strength were also assessed based on comonomer reactivity. As shown in Fig. 5(b), AA accelerated hydration and achieved the highest early strength but reduced workability due to rapid stiffening. IA offered the best balance, providing extended working time and satisfactory strength. MAA allowed moderate hydration control, while MMA and N-MAM improved workability through reduced ionic interaction but delayed strength gain. Notably, N-MAM contributed to long-term dispersion and durability, though excessive use impaired storage stability and field performance. Overall, increasing comonomer content to 3 wt.%, particularly through optimized combinations of AA or IA with N-MAM, significantly improved latex stability, early strength, and workability in URHC-LMC systems, provided that N-MAM dosage is carefully controlled.

Fig. 5. Effects of Combining Carboxylic Acid (1 wt.%) and N-MAM (2 wt.%) Comonomers on (a) Latex Stability and Workability, (b) Compressive Strength

3.1.4 Enhancing URHC-LMC flexibility and durability in bridges

To enhance the performance of URHC-LMC, this study addressed key vulnerabilities such as aggregate microcracking and environmental stressors including chloride ingress and freeze-thaw cycles. Specific polymer modifications were made namely, increasing BD content, reducing ST content, optimizing the Tg to 0-10°C, and decreasing latex particle size. These adjustments aimed to improve flexural strength, permeability resistance, and surface scaling durability ^{(Su et al., 2024)}.

Fig. 6(a) shows the relationship between Tg, particle size, and flexural strength. Crushed stone aggregates often develop internal microcracks during processing, which act as stress concentrators and weaken the concrete’s flexural capacity under dynamic loads. Increasing BD content improves the flexibility and elasticity of the latex film, enabling it to bridge microcracks and fill microvoids, thereby enhancing stress distribution and crack resistance throughout the matrix.

Fig. 6(b) illustrates the correlation between Tg, particle size, and permeability. Bridge decks exposed to marine environments or de-icing salts suffer from chloride-induced corrosion and freeze-thaw damage. Chloride ingress leads to steel corrosion, while freeze-thaw cycles cause internal expansion, resulting in cracking and surface degradation. By maintaining Tg within 0-10°C, the latex film remains flexible under thermal stress, accommodating volume changes during freeze-thaw events. Reducing particle size enhances film density, effectively sealing microvoids and limiting water and ion penetration. This improves barrier performance, protects reinforcement, and reduces corrosion rates.

Fig. 6. Effects of Glass Transition Temperature (Tg) and Particle Size on Flexural Strength and Permeability of LMC

Furthermore, reducing latex particle size (from 190 nm to 160 nm) significantly enhances the latex’s film-forming ability and impermeability after drying. Smaller particles create a thinner but more uniformly distributed polymer film, increasing the specific surface area of coverage and allowing the latex to more effectively fill and bridge capillary pores and microvoids within the cementitious matrix. This results in denser film formation and reduced porosity, which in turn improves chloride penetration resistance by physically blocking ion transport pathways. In addition, finer particles contribute to a more cohesive and continuous hydrophobic surface layer, which further limits moisture ingress and protects embedded steel reinforcement from corrosion. These mechanisms are consistent with barrier theory, where smaller particle size enhances tortuosity and reduces permeability through denser packing and more efficient void filling.

These improvements significantly reduce surface scaling, chloride-induced corrosion, and microcrack propagation, thereby strengthening the URHC-LMC system against both mechanical and environmental deterioration. The optimized latex formulation is thus highly suitable for demanding bridge deck applications exposed to aggressive conditions.

3.1.5 Effect of preservatives

Maintaining latex stability at high pH (>11) is critical, as degradation under alkaline conditions significantly reduces the workability of LMC. This study evaluated the effectiveness of three preservatives (CMIT, MIT, and BIT) in preserving latex under conditions typical of cementitious environments ^{(Shang et al., 2022)}. Latex is also vulnerable to microbial degradation during storage, especially in hot and humid climates. Previous studies identified Pannonibacter phragmitetus and Halomonas hamiltonii as key degraders in alkaline settings, producing acidic by-products that lower pH and disrupt polymer structure, leading to impaired latex performance.

Initial compatibility tests were conducted at pH 11 using each preservative at 400 ppm. After 10 minutes of vigorous stirring and filtration through a 200-mesh screen, coagulum content was measured to assess stability. As summarized in Table 6, CMIT caused significant coagulation (>4,500 ppm), reflecting poor stability at high pH. In contrast, MIT and BIT showed excellent compatibility with minimal coagulum formation (<1 ppm).

Further analysis evaluated preservative durability over seven days at pH 11 and 65°C, as shown in Fig. 7. MIT degraded rapidly, with over 70 % loss of concentration, while BIT retained over 88 % of its original level, indicating superior chemical resilience. This stability is attributed to BIT’s aromatic benzene ring and robust sulfur-nitrogen (S-N) bonds, which resist hydrolysis and oxidative degradation. In contrast, MIT’s structure is prone to breakdown in alkaline media.

BIT also interacts with latex particles via hydrogen bonding and van der Waals forces, forming a stable film that enhances microbial resistance and colloidal stability, even in environments rich in multivalent cations (e.g., Ca2⁺, Al3⁺). When combined with N-MAM, BIT’s stabilizing effect is amplified, as N-MAM forms hydrated steric barriers that minimize ionic interactions and coagulation (Tamira et al., 2020).

BIT’s antimicrobial efficacy is multifaceted, involving membrane disruption, metabolic inhibition, and ROS generation. It provides long-term protection at low concentrations (400 ppm) without compromising safety. BIT complies with global safety standards (REACH, OSHA), and its safe handling is ensured through PPE and exposure guidelines. By preserving latex integrity during storage and use, BIT supports consistent flowability, mechanical strength, and cement compatibility, making it a highly effective and practical preservative for URHC-LMC systems.

Fig. 7. Degradation Behavior of (a) MIT and (b) BIT Preservatives Over Time at pH 11 and 65°C

3.1.6 Effect of gel content and TDDM optimization on mechanical stability and field performance of latex in LMC

This study investigated the influence of latex gel content, adjusted via tert-dodecyl mercaptan (TDDM), on mechanical stability and workability of LMC incorporating URHC. Initial lab-scale evaluations, as shown in Fig. 8(a), tested slump and slump loss across formulations with gel content ranging from 30.7 % to 88.9 %. An optimal gel content of 50.3 % exhibited the lowest slump loss (24.2 %), effectively balancing flowability and structural cohesion under static conditions.

However, when scaled to a field test, this same formulation exhibited severe coagulation during pumping, particularly at mechanical components such as rotating shafts, as illustrated in Fig. 8(b). These issues were attributed to insufficient structural integrity under dynamic shear forces and elevated temperatures, conditions not fully replicated in laboratory settings.

To address this, additional field trials were conducted by increasing the gel content above 85 % through precise TDDM adjustment. A formulation containing 0.24 wt.% TDDM achieved a gel content of 85.1 %, significantly reducing coagulation and maintaining an acceptable slump loss of 29.5 %, as shown in Fig. 8(b). Conversely, higher TDDM levels (0.58 wt.%) led to reduced gel content due to excessive chain transfer, resulting in instability.

Mechanical components remained clean post-application, as shown in Fig. 8(c), and the modified LMC was successfully placed without flow interruptions. These findings demonstrate that, while lower gel contents may enhance workability under controlled conditions, a higher gel content (>85 %) is essential to ensure mechanical stability in real-world field applications. Therefore, optimizing TDDM dosage is critical for maintaining both pumpability and performance of latex-modified LMC in bridge deck repairs.

Fig. 8. (a) Effects of Latex Gel Content on Slump and Slump Loss, (b) Coagulation Observed on a Pump Shaft during Field Test, (c) Clean Pump Shaft after Optimizing Gel Content

3.1.7 Freeze-thaw stability

Latex, which contains over 50 % water, is highly vulnerable to coalescence and particle agglomeration during freeze-thaw cycles, leading to performance degradation. Fig. 9 illustrates the significant improvement in freeze-thaw stability achieved by incorporating hydrophilic N-MAM as a comonomer. This enhancement is driven by several interconnected physicochemical mechanisms.

N-MAM, a water-soluble monomer, increases steric stabilization by introducing hydrophilic groups on latex particle surfaces. These groups form a hydrated barrier that enhances repulsive forces and prevents particle aggregation under freeze-thaw stress. Through hydrogen bonding, N-MAM also interacts with surrounding water molecules, limiting ice crystal formation and reducing mechanical damage.

In addition, the modified latex exhibits a more porous particle morphology, which acts as a buffer against internal pressure during freezing. This structure helps absorb expansion stress, reducing coalescence risk. Upon thawing, the hydrated layer facilitates particle mobility and re-dispersion, preventing permanent agglomeration.

Furthermore, N-MAM promotes a more uniform distribution of hydrophilic domains within the latex matrix, minimizing weak zones susceptible to destabilization. Together, these effects contribute to significantly enhanced freeze-thaw resilience of the latex system.

Fig. 9. Influence of N-MAM Concentration on the Freeze-thaw Stability of Latex