Recovery of Performance-Deficient Asphalt Mixtures: A Comparative Assessment of Additive Modification and Balanced Mix Redesign

Abstract

Traditional volumetric asphalt mix design methods do not directly evaluate cracking and rutting resistance, which may result in mixtures with inadequate field performance despite satisfying conventional volumetric requirements. Balanced Mix Design (BMD) has been introduced to address this limitation by incorporating performance testing. However, it remains unclear whether additive modification alone can recover mixtures that do not satisfy BMD criteria, or whether volumetric redesign (binder content optimization) is necessary. Therefore, this study investigated the effectiveness of additive modification before and after BMD redesign using IDEAL-CT and IDEAL-RT performance indices. Three additive systems were evaluated: SBS polymer, nano-silica, and hybrid modification. Performance threshold values for CT and RT Indices were established using a percentile-based statistical approach derived from an experimental database of 18 asphalt mixtures prepared with different aggregate gradations, filler types, and asphalt binder contents. The 25th percentile (P25) was adopted as the minimum performance criterion. The experimental program consisted of three evaluation cases. The first case investigated the ability of additives to recover initially performance-deficient mixtures before and after BMD redesign. Results showed that additive incorporation alone was insufficient to recover deficient mixtures and, in some cases, reduced cracking resistance due to excessive stiffness. However, after BMD redesign, the modified mixtures achieved balanced cracking and rutting performance above the adopted threshold values. The second case, based on a single initially compliant mixture, suggested that additives may be more effective when applied to mixtures that already satisfy performance requirements; however, this observation is preliminary and requires validation with a broader range of passing mixtures. The hybrid modification system provided the best overall balance. The third case isolated the effect of SBS polymer after BMD redesign using paired t-tests and Cohen’s d analysis. Statistical results confirmed a significant and very large positive effect of polymer modification on rutting resistance, whereas the effect on cracking resistance depended strongly on filler type. Overall, the findings indicate that additive modification cannot reliably compensate for deficient mixture design, while the combination of BMD redesign and additive modification can successfully achieve balanced performance. In addition, filler characteristics were found to play a critical role in controlling modifier effectiveness. These findings are based on a limited number of mixtures and additive combinations, and the proposed thresholds (CT ≥ 52, RT ≥ 44) should be validated using independent datasets before general application.

1. Introduction

Traditional volumetric asphalt mix design methods have long been used to design asphalt mixtures; however, these approaches do not directly evaluate cracking and rutting resistance, which are among the primary causes of pavement deterioration [1]. Consequently, mixtures that satisfy conventional volumetric requirements may still exhibit inadequate long-term field performance.

To address these limitations, the Balanced Mix Design (BMD) concept has emerged as a performance-based framework that integrates cracking and rutting evaluation into the asphalt mix design process [2]. Previous investigations have demonstrated that BMD can significantly improve pavement durability by establishing a balance between cracking resistance and rutting resistance [3,4,5]. In addition, BMD provides an improved capability to account for material variability, aging effects, and performance-related characteristics that are not adequately captured by traditional volumetric parameters [5,6].

Among the available performance evaluation methods within the BMD framework, the IDEAL-CT and IDEAL-RT tests have received increasing attention because of their simplicity, repeatability, and suitability for performance-based asphalt mixture evaluation [7,8,9]. The CT Index and RT Index are commonly used as performance indicators for evaluating cracking and rutting behavior of asphalt mixtures [10,11]. These indices provide a practical framework for assessing the influence of additive modification on mixture behavior.

In parallel, additive modification has become one of the most widely adopted approaches for improving asphalt mixture performance. Styrene–butadiene–styrene (SBS) polymer is commonly incorporated to enhance elastic recovery, stiffness, and rutting resistance of asphalt binders and mixtures [12]. However, the influence of polymer modification on cracking resistance remains inconsistent across studies. Some researchers reported improvements in fatigue and cracking resistance after polymer incorporation [13], whereas others observed reductions in cracking performance in intermediate-temperature fracture tests [14,15,16,17,18]. These contradictory findings suggest that modifier effectiveness is not governed solely by binder properties but is also strongly influenced by mixture composition and internal aggregate structure.

In addition to polymer modification, nanomaterials such as nano-silica have received increasing attention because of their ability to enhance binder–aggregate interaction and improve asphalt mixture stability. Previous studies demonstrated that nano-silica can improve rutting resistance and mechanical performance of asphalt materials [19,20]. Furthermore, hybrid modification systems combining SBS polymer and nano-silica have shown promising performance improvements due to the synergistic interaction between polymer elasticity and nano-scale reinforcement [19]. Nevertheless, excessive stiffening associated with nano-modification may adversely affect cracking resistance under intermediate-temperature conditions [20].

Another important factor affecting asphalt mixture performance is mineral filler type. Mineral fillers play a significant role in controlling asphalt mastic behavior, internal mixture structure, and stress distribution within asphalt mixtures [21]. Despite the recognized importance of filler properties, limited studies have investigated the interaction between filler type and modifier effectiveness within a BMD framework.

While previous studies have examined the individual effects of SBS, nano-silica, and hybrid modification on asphalt mixture performance, most have focused on mixtures that already satisfied volumetric or performance requirements. The specific question of whether these modifiers can recover mixtures that initially do not satisfy performance thresholds, and whether BMD redesign is a prerequisite for their effectiveness, has not been systematically addressed. Furthermore, the role of filler type in governing modifier effectiveness has received limited attention. This study addresses these gaps by comparing additive modification alone with combined BMD redesign and additive modification, while also evaluating the influence of mineral filler characteristics.

From an implementation perspective, failure to satisfy cracking or rutting performance requirements may result in costly redesign procedures, additional laboratory testing, production delays, and modifications to mixture composition. In such cases, highway agencies and asphalt producers must determine whether performance deficiencies can be corrected through additive modification of the existing mixture or whether a complete volumetric redesign is necessary. Despite the practical significance of this decision, limited studies have systematically evaluated whether additives can function as a stand-alone corrective measure or whether volumetric redesign is required. Furthermore, the interaction between BMD redesign, additive modification, and filler characteristics—specifically how these factors collectively influence the recovery of performance-deficient mixtures—remains insufficiently understood.

Therefore, this study investigates the recovery of performance-deficient asphalt mixtures through additive modification and Balanced Mix Design redesign. SBS polymer, nano-silica, and hybrid modification systems are evaluated before and after BMD redesign using different mineral filler types. Particular emphasis is placed on determining whether additive modification alone can recover deficient mixtures or whether volumetric redesign is necessary to achieve balanced cracking and rutting performance. The study further evaluates the influence of filler type on modifier effectiveness and employs statistical analysis to quantify the contribution of SBS polymer modification following BMD redesign. The findings are expected to provide a practical framework for recovering deficient asphalt mixtures and selecting appropriate performance-enhancement strategies.

2. Research Objectives

The specific objectives of this study are as follows:

• To evaluate the effectiveness of SBS polymer, nano-silica, and hybrid modification systems in improving the cracking and rutting performance of asphalt mixtures as measured by the CT Index and RT Index.
• To determine whether additive modification alone can recover asphalt mixtures that do not satisfy the adopted BMD performance thresholds, or whether mixture redesign is required prior to modification.
• To evaluate the influence of mineral filler type on modifier effectiveness and the resulting balanced performance of asphalt mixtures.
• To quantify the contribution of SBS polymer modification after BMD redesign through paired statistical analysis and effect size evaluation.
• To compare the effectiveness of additive modification and BMD redesign as alternative strategies for recovering performance-deficient mixtures.

3. Materials

3.1. Asphalt Cement

A penetration-grade asphalt cement (AC 40–50) obtained from the Doura Refinery, Iraq, was used throughout this study. The binder was selected because it is commonly utilized in pavement construction projects in Iraq and complies with the requirements of the Iraqi Standard Specification for paving asphalt binders (SCRB, 2003) [22]. Conventional physical characterization, including penetration, softening point, ductility, specific gravity, and loss on heating after the Thin Film Oven Test (TFOT), was conducted in accordance with the relevant ASTM standards. The measured properties of the virgin binder and its corresponding specification limits are presented in

Table 1. Physical Properties of AC (40–50).

Property	Result	SCRB Limit
Penetration (25 °C, 100 g, 5 s)	44	40–50
Ductility (25 °C)	149 cm	≥100
Softening point	50 °C	—
Flash point	294 °C	≥232
Specific gravity (25 °C)	1.037	—
Penetration after TFOT	57%	≥55
Ductility after TFOT	98 cm	>25

.

3.2. Aggregate

Crushed quartz aggregates obtained from the Al-Niba’i and Al-Sudoor quarries were used in the preparation of the asphalt mixtures. These aggregate sources were selected because they are commonly used in pavement construction in Iraq and satisfy the requirements of the Iraqi Standard Specification for asphalt paving mixtures (SCRB, 2003) [22]. The physical properties of the coarse and fine aggregate fractions were determined in accordance with the relevant ASTM standards. Since aggregates were obtained from two different sources, the reported values represent the range of measured properties for the tested materials. The physical characteristics of the coarse and fine aggregates are summarized in

Table 2. Physical Properties of Coarse Aggregates.

Property	Result	SCRB Specification Requirement
Bulk specific gravity	2.584–2.598	—
Apparent specific gravity	2.608–2.615	—
Water absorption (%)	0.53–0.58	—
Los Angeles abrasion (%)	13.94–15.65	≤30

and

Table 3. Physical Properties of Fine Aggregates.

Property	Result
Bulk specific gravity	2.581–2.602
Apparent specific gravity	2.634–2.662
Water absorption (%)	0.953–1.418

, respectively.

3.3. Mineral Filler

Two mineral filler types, namely limestone dust (LSD) and Portland cement (PC), were investigated in this study. These fillers were selected because of their widespread use in asphalt mixture production and their distinct physical and chemical characteristics, which may influence asphalt mastic properties and overall mixture performance. Both fillers satisfied the requirements of SCRB (2003) [22] and passed the No. 200 sieve (0.075 mm), ensuring their suitability for use as mineral fillers in asphalt mixtures. The physical properties of the fillers are summarized in

Table 4. Physical Properties of Filler.

Filler Type	Specific Gravity	% Passing No. 200
Limestone dust	2.71	93
Cement	3.15	95

.

3.4. Polymer

Styrene–butadiene–styrene (SBS) polymer used in this study was supplied by Kraton, Berre, France. The polymer was received in solid form and used as the primary elastomeric modifier for asphalt mixture modification. SBS was selected because it is one of the most widely used polymer modifiers in pavement applications and has demonstrated compatibility with conventional paving-grade asphalt binders. The physical properties of the SBS polymer, obtained from the manufacturer’s technical data sheet, are summarized in

Table 5. Physical Properties of SBS.

Property	Value
Physical state	Solid
Appearance	White
Density (kg/m3)	1247
Melting point (°C)	197

.

3.5. Nano-Silica

Nano-silica (NS) used in this study was supplied by SkySpring Nanomaterials, Houston, TX, USA. The material was received in powder form and consisted of untreated amorphous silica nanoparticles with a nominal particle size of 20 nm. According to the manufacturer’s specifications, the nano-silica possessed a high purity (99.5% SiO₂) and a large specific surface area, making it suitable for incorporation into asphalt modification systems. The physical properties of the nano-silica, as provided by the manufacturer, are summarized in

Table 6. Physical Properties of Nano-Silica.

Property	Value
Particle size	20 nm
SiO2 content	99.5%
Appearance	White powder
Specific surface area	160 m2/g
Bulk density	0.08 g/cm3
Surface Treatment	None

.

3.6. Selection of Aggregate and Filler Gradation

Three dense-graded aggregate gradations were selected in accordance with the requirements of SCRB (2003) [22] for Type IIIA surface mixtures with a nominal maximum aggregate size of 12.5 mm. The selected gradations consisted of a design gradation located near the midpoint of the specification envelope, together with relatively coarse and fine gradations positioned within the allowable specification limits. This approach was adopted to generate asphalt mixtures with different aggregate skeleton structures and volumetric characteristics, thereby producing mixtures with varying levels of cracking and rutting resistance. The use of multiple gradations was intended to simulate practical variations that may occur during asphalt mixture production and to establish mixtures exhibiting different performance levels for subsequent BMD evaluation [23]. The selected gradations also provided a basis for assessing the effectiveness of additive modification and BMD redesign in recovering mixtures that did not satisfy the adopted performance criteria. The aggregate gradations used in this study are presented in

Table 7. Selected Aggregate Gradation.

Sieve Size (mm)	Design Gradation	Production Tolerance	Coarse Gradation	Fine Gradation	Specification Range
19	100	-	100	100	100
12.5	95	±6	90	100	90–100
9.5	83	±6	77	89	76–90
4.75	59	±6	53	65	44–74
2.36	43	±4	39	47	28–58
0.3	13	±4	9	17	5–21
0.075	7	±2	5	9	4–10

, while Figure 1 illustrates the selected gradations relative to the SCRB specification limits for dense-graded surface mixtures.

4. Methodology

Figure 2 presents the methodological framework adopted in this study to evaluate the recovery of asphalt mixtures through additive modification and BMD redesign. The experimental program included mixture preparation using three aggregate gradations (Coarse, Design, Fine), two filler types (limestone dust and Portland cement), and three asphalt contents (OAC −0.3%, OAC, OAC +0.3%). Performance evaluation was conducted using the IDEAL-CT and IDEAL-RT tests. The IDEAL-CT (ASTM D8225) [24] was performed at 25 °C with a loading rate of 50 mm/min on cylindrical specimens (150 mm diameter, 62 mm height) compacted to 7 ± 0.5% air voids. The IDEAL-RT (ASTM D8360-22) [25] was performed at 60 °C with the same loading rate and specimen geometry. Three replicate specimens were tested for each mixture condition. Threshold values for the CT Index and RT Index were established from an experimental database of eighteen unmodified mixtures using the 25th percentile (P25) approach. Based on these thresholds, mixtures were classified as either passing or not meeting the criteria.

Three evaluation cases were then investigated: (i) Case 1: recovery of initially performance-deficient mixtures through additive modification and/or BMD redesign, (ii) Case 2: additive modification of initially passing mixtures, and (iii) Case 3: statistical evaluation of SBS polymer modification after BMD redesign. Cracking and rutting performance were re-evaluated after modification, and paired t-tests together with Cohen’s d effect size analysis were used to quantify the influence of SBS modification. The adopted framework provides a comparative assessment of additive modification and BMD redesign as alternative strategies for achieving balanced asphalt mixture performance.

4.1. Mixture Design and Specimen Preparation

Three aggregate gradations were used in this study to prepare asphalt mixtures with different volumetric and performance characteristics. Two mineral filler types, namely limestone dust and cement filler, were incorporated into the investigated mixtures. Asphalt mixtures were prepared at optimum asphalt content (OAC), OAC +0.3%, and OAC −0.3% to generate mixtures with varying cracking and rutting behavior. A total of 18 asphalt mixtures were prepared and evaluated to establish the laboratory performance database used in the BMD analysis and threshold determination. The investigated mixtures included combinations of different gradations, filler types, and asphalt binder contents.

The OAC was initially determined using Marshall mix design procedures in accordance with ASTM D6927.

Table 8. Marshall Properties of Design Mixtures.

Filler	Gradation	OAC (%)	Va (%)	VMA (%)	VFA (%)	Gmb (g/cm3)	Gmm (g/cm3)	Stability (kN)	Flow (mm)
Limestone	Coarse	4.5	4.2	14.8	71.6	2.250	2.349	10.1	3.2
Limestone	Design	4.8	4.0	15.2	73.7	2.307	2.403	11	3.5
Limestone	Fine	5.1	4.1	15.6	73.7	2.299	2.397	13.3	3.8
Cement	Coarse	4.85	4.3	15.1	71.5	2.238	2.339	8.7	3.1
Cement	Design	5.1	4.0	15.5	74.2	2.295	2.391	9.5	3.4
Cement	Fine	5.5	4.2	15.9	73.6	2.287	2.387	11.5	3.7

presents the Marshall properties for each of the six design mixtures. All performance test specimens were compacted using a Superpave Gyratory Compactor (SGC) following ASTM D6925 to produce cylindrical specimens with dimensions of 150 mm diameter and 62 ± 1 mm height at a target air void content of 7 ± 0.5%. The asphalt mixtures were prepared under laboratory conditions following conventional hot mix asphalt production procedures. Aggregate materials and asphalt binder were heated separately prior to mixing, after which the mixtures were compacted using the SGC to produce specimens suitable for performance evaluation.

The BMD redesign process involved evaluating each aggregate gradation (Coarse, Design, and Fine) at three binder contents (OAC −0.3%, OAC, and OAC +0.3%) and selecting the binder content that satisfied both CT ≥ 52 and RT ≥ 44. For each gradation, the binder content that achieved the best balance between cracking and rutting performance was selected without changing the aggregate gradation itself. It is important to clarify that the BMD redesign process in this study involved adjusting the asphalt binder content only, while the aggregate gradation and source remained unchanged. Thus, references to ‘redesign’ throughout this paper refer to volumetric optimization of the binder content, not modification of the aggregate skeleton.

The use of Marshall OAC with SGC-compacted specimens reflects common practice in Iraq and ensures volumetric consistency while allowing performance evaluation under realistic compaction conditions. The Marshall procedure was used to determine OAC because it is the local standard (SCRB, 2003) [22]; however, performance specimens were compacted using SGC to better reflect field compaction conditions and to ensure consistent air voids (7 ± 0.5%) for all performance tests.

To evaluate the influence of additive modification on mixture recovery and post-BMD performance, selected mixtures were modified using SBS polymer, nano-silica, and hybrid modification systems consisting of combined polymer and nano-silica additives. SBS polymer was incorporated at 3% by weight of asphalt binder, while nano-silica was added at 0.5% by weight of binder. The hybrid modification system consisted of the combined incorporation of 3% SBS and 0.5% nano-silica. Similar dosage ranges have been adopted in previous investigations on polymer-, nano-, and hybrid-modified asphalt systems [26,27,28,29,30,31,32,33]. Accordingly, the selected dosages were considered suitable for comparative performance evaluation rather than dosage optimization.

The modifiers were incorporated into the asphalt binder using the wet mixing process prior to mixture preparation. The main mixing conditions adopted for each modification system are summarized in

Table 9. Mixing Conditions and Procedures for Modified Asphalt Binders.

Modification Type	Temperature (°C)	Mixing Speed (rpm)	Mixing Time (min)	Mixing Procedure
SBS polymer	175 ± 5	4000	60	High-shear mixing to ensure polymer swelling and homogeneous dispersion
Nano-Silica	160 ± 5	3000	45	Mechanical mixing to achieve adequate nanoparticle dispersion
Hybrid	175 ± 5 (SBS stage)	4000	60	SBS initially blended with base binder
	160 ± 5 (SiO2 stage)	3000	30	Nano-Silica gradually added after SBS swelling

.

4.2. Balanced Mix Design (BMD) Evaluation

The performance of the investigated mixtures was evaluated within the framework of BMD, which aims to achieve a balance between cracking resistance and rutting resistance using performance-based indicators.

Cracking resistance was evaluated using the IDEAL Cracking Test (IDEAL-CT) in accordance with ASTM D8225 [24] at a temperature of 25 °C and a loading rate of 50 mm/min. The resulting Cracking Tolerance Index (CT Index) was used as the primary indicator of fracture resistance which is calculated as per Equation (1).

$$CT Index={\displaystyle \frac{t}{62}}\times {\displaystyle \frac{l75}{D}}\times {\displaystyle \frac{Gf}{\left|m75\right|}}\times {10}^6$$

(1)

where

• Gf = failure energy (Joules/m²), |m75| = absolute value of the post-peak slope m75 (N/m), l75 = displacement at 75 percent the peak load after the peak (mm), D = specimen diameter (mm), and t = specimen thickness (mm).

Rutting resistance was evaluated using the IDEAL Rutting Test (IDEAL-RT) following ASTM D8360-22 [25] at a testing temperature of 60 °C and a loading rate of 50 mm/min. The corresponding Rutting Tolerance Index (RT Index) was adopted to characterize mixture resistance to permanent deformation under shear loading. The RT Index is calculated as per Equation (2).

$$RT Index=6.618\times {10}^{-5}\times {\displaystyle \frac{\tau f }{1 \mathrm{P}\mathrm{a}}}$$

(2)

where:

• τf = $0.356\times {\displaystyle \frac{Pmax}{t\times w}}$ = shear strength (Pa), Pmax = maximum load (N), t = specimen thickness (m), and w = width of upper loading strip (=0.0191 m).

Three specimens with dimensions of 150 mm diameter and 62 ± 1 mm height were tested for each mixture condition at a target air void level of 7 ± 0.5%. The obtained CT Index and RT Index values were used to classify asphalt mixtures in terms of cracking and rutting performance.

5. Results and Discussion

5.1. Establishment of Threshold Values

Figure 3 and Figure 4 present the CT Index and RT Index results obtained for the eighteen asphalt mixtures used to establish the performance database for threshold determination. The investigated mixtures incorporated different aggregate gradations, filler types, and asphalt contents, thereby providing a wide range of cracking and rutting performance levels. As shown in Figure 3, the CT Index values ranged from 39 to 99. The coefficient of variation (COV) for CT Index values ranged from 9.1% to 16.7%, with an overall average of approximately 14.1%. This wide range reflects realistic experimental variability in asphalt mixture testing, with all values remaining well below the acceptable limit of 20%. The variation observed across different mixture types and filler combinations confirms the reliability and repeatability of the test results while capturing genuine differences in material behavior. In general, increasing asphalt content from OAC−0.3% to OAC+0.3% resulted in a substantial increase in CT Index, indicating improved cracking resistance. Fine gradations consistently exhibited higher CT Index values than design and coarse gradations, while mixtures incorporating limestone dust generally achieved higher cracking resistance than those prepared with cement filler.

The RT Index results presented in Figure 4 ranged from 39 to 66. The COV for RT Index values ranged from 4.7% to 10.1%, with an overall average of approximately 8.4%. All COV values remained near or below the acceptable limit of 10%, confirming the high repeatability of the IDEAL-RT test and supporting the reliability of the rutting performance comparisons across different mixture types and filler combinations. Unlike the CT Index, the RT Index exhibited a less pronounced response to asphalt content. Nevertheless, mixtures incorporating fine gradations generally produced higher RT Index values than coarse and design gradations. In addition, cement-filled mixtures tended to provide slightly higher rutting resistance than limestone dust mixtures, particularly at the higher asphalt content levels.

Several statistical approaches were evaluated to establish representative threshold values for the CT Index and RT Index based on the experimental database developed in this study, as summarized in

Table 10. Proposed Threshold Approaches Based on IDEAL-CT and IDEAL-RT Results.

Threshold Approach	CT Index	RT Index
Minimum value	39	39
Average–SD	46	41
25th percentile (P25)	52	44
Average value	62	50

. The lower-bound approaches based on the minimum value and the average minus one standard deviation produced relatively low threshold values that could permit mixtures with inadequate cracking or rutting resistance to be classified as acceptable. Conversely, the average-value criterion represented a comparatively restrictive limit that would classify a considerable proportion of the investigated mixtures as failing despite demonstrating acceptable overall performance. Based on the distribution of the experimental results and the need to establish balanced and practically representative performance criteria, the 25th percentile (P25) approach was considered the most appropriate threshold method. This approach excludes the lowest-performing mixtures while maintaining realistic acceptance limits for practical implementation. Therefore, the P25 values were adopted as provisional performance thresholds for this study. Accordingly, CT Index = 52 and RT Index = 44 are proposed as internal criteria for classifying mixtures in this study, but they should be validated using independent datasets before being considered for general specification purposes.

5.2. Case 1: Performance-Deficient Mixtures Before and After BMD

Case 1 investigated three mixtures at OAC that did not satisfy one or both of the adopted CT Index and RT Index thresholds, as summarized in

Table 11. Performance-Deficient Mixes Before the Addition of Additives.

Aggregate Source	Aggregate Gradation	Average CT-Index	Average RT-Index	Type of Additives
A Filler (Limestone Dust)	Coarse	51	39	Hybrid
	Design	61	40	Nano
B Filler (Cement)	Coarse	45	46	Polymer

. These mixtures were selected to evaluate whether additive modification alone could recover deficient performance or whether BMD redesign was required prior to modification.

Each deficient mixture was treated with the additive system theoretically most suitable for its primary deficiency (e.g., hybrid for low CT and low RT, polymer for low RT only). While this limits direct comparison across additives, it reflects a practical scenario where a specific deficiency is targeted.

As shown in Figure 5a, additive incorporation without BMD redesign resulted in substantial reductions in cracking resistance. The CT Index decreased from 51 to 20 for the coarse limestone dust mixture and from 61 to 21 for the design limestone dust mixture, corresponding to reductions of approximately 61% and 66%, respectively. The cement-filled mixture exhibited a smaller reduction, with the CT Index decreasing from 45 to 42. These results indicate that additive modification alone was unable to recover the cracking performance of the deficient mixtures and, in some cases, further aggravated the cracking susceptibility. It should be noted that the BMD redesign in this study involved adjusting the binder content only, while the aggregate gradation remained unchanged.

Following BMD redesign, significant improvements in cracking resistance were observed before additive incorporation (Figure 5b). The CT Index increased from 51 to 72, from 61 to 85, and from 45 to 63 for the coarse limestone dust, design limestone dust, and cement-filled mixtures, respectively. After additive modification, the redesigned limestone dust mixtures experienced moderate reductions in CT Index (72 to 59 and 85 to 67), but both mixtures remained above the adopted threshold value of 52. In contrast, the cement-filled mixture exhibited a substantial increase in CT Index from 63 to 112 after polymer modification, indicating excellent compatibility between the BMD-redesigned mixture, cement filler, and modifier system. The RT Index results shown in Figure 6 reveal a different trend. Even without BMD redesign, additive incorporation improved rutting resistance, with RT Index values increasing from 39 to 54, from 40 to 69, and from 46 to 65 for the three investigated mixtures. Nevertheless, the simultaneous reduction in CT Index prevented the mixtures from achieving balanced performance. After BMD redesign, further improvements in rutting resistance were observed following additive incorporation. The RT Index increased from 44 to 68 for the coarse limestone dust mixture, from 44 to 80 for the design limestone dust mixture, and from 54 to 74 for the cement-filled mixture. These results confirm the effectiveness of polymer and nano-modification in enhancing resistance to permanent deformation when applied to mixtures possessing an adequate aggregate structure and volumetric balance.

Overall, the results demonstrate that additive modification alone cannot reliably recover mixtures exhibiting deficient cracking and rutting performance. Although additives improved rutting resistance, their incorporation into poorly balanced mixtures resulted in significant reductions in cracking resistance. Conversely, the combination of BMD redesign and additive modification successfully achieved balanced performance by improving both CT Index and RT Index beyond the adopted threshold values. The results also highlight the influence of filler type, as cement-filled mixtures exhibited superior compatibility with additive modification and achieved the highest overall performance after redesign.

While these results suggest that additive modification alone cannot reliably recover deficient mixtures, this conclusion is based on a limited number of mixtures (three) and additive combinations. The observation is therefore preliminary and warrants further investigation with a broader range of mixtures and additive systems.

5.3. Case 2: Performance of Initially Passing Mixtures

Case 2 evaluated a mixture at OAC that satisfied the adopted CT Index and RT Index thresholds prior to modification, as summarized in

Table 12. Passing Mix before the Addition of Additives.

Aggregate Source	Aggregate Gradation	Average CT-Index	Average RT-Index	Type of Additives
B Filler (Cement)	Design	54	47	1—Nano 2—Polymer 3—Hybrid

. The mixture was selected to investigate the effectiveness of additive incorporation when applied to a mixture that already possessed balanced cracking and rutting performance. As shown in Figure 7, all modification systems enhanced both cracking and rutting performance relative to the unmodified mixture. The CT Index increased from 54 for the control mixture to 68, 71, and 95 after nano-silica, polymer, and hybrid modification, respectively (Figure 7a). The corresponding improvements were approximately 26%, 31%, and 76%, indicating that the hybrid modification system produced the most significant enhancement in cracking resistance.

Similarly, the RT Index increased from 47 for the control mixture to 66, 58, and 61 following nano-silica, polymer, and hybrid modification, respectively (Figure 7b). Nano-silica exhibited the greatest improvement in rutting resistance, increasing the RT Index by approximately 40%, whereas polymer and hybrid modification increased the RT Index by about 23% and 30%, respectively. The results indicate that the hybrid modification system provided the best overall balance between cracking and rutting performance. The combination of SBS polymer and nano-silica appears to have generated a synergistic effect, resulting in the highest CT Index while maintaining substantial improvement in rutting resistance. In contrast, nano-silica alone was particularly effective in enhancing rutting resistance, whereas polymer modification produced a more balanced improvement in both performance indicators.

Unlike the deficient mixtures discussed in Case 1, the investigated mixture already satisfied the adopted BMD performance thresholds prior to modification. Consequently, the additives functioned primarily as performance enhancers rather than corrective measures. An important observation is that improvements in rutting resistance were accompanied by simultaneous increases in cracking resistance, particularly for the hybrid system. This behavior suggests that mixtures possessing an adequate aggregate structure and volumetric balance can benefit from modifier incorporation without experiencing the cracking deterioration observed in deficient mixtures.

For the single initially compliant mixture evaluated in this study, additive modification enhanced both cracking and rutting performance. However, this observation is based on a limited dataset (only one mixture) and should therefore be interpreted with caution; further validation across a broader range of passing mixtures is needed before generalized conclusions can be drawn.

5.4. Case 3: Statistical Evaluation of Polymer Effect After BMD

Case 3 was conducted to isolate the influence of SBS polymer modification after all mixtures had been redesigned using the BMD approach. Six redesigned mixtures representing different aggregate gradations and filler types were evaluated before and after polymer modification, as illustrated in Figure 8.

The RT Index results shown in Figure 9 demonstrate that SBS polymer consistently improved rutting resistance for all investigated mixtures. The RT Index increased from 44 to 71, 44 to 76, and 54 to 85 for the limestone dust mixtures with coarse, design, and fine gradations, respectively. Similarly, the cement-filled mixtures exhibited increases from 54 to 74, 47 to 58, and 61 to 77. These results confirm the effectiveness of SBS polymer in enhancing resistance to permanent deformation through increased binder elasticity and recovery characteristics.

In contrast, the influence of polymer modification on cracking resistance was strongly dependent on filler type (Figure 10). For limestone dust mixtures, the CT Index decreased from 72 to 63, from 85 to 74, and from 61 to 55 for the coarse, design, and fine gradations, respectively, indicating that polymer incorporation reduced cracking resistance despite the improvement in rutting performance. Conversely, all cement-filled mixtures exhibited improvements in CT Index after polymer modification, increasing from 63 to 112, from 54 to 71, and from 54 to 65 for the coarse, design, and fine gradations, respectively. The most notable improvement was observed for the coarse cement-filled mixture, where the CT Index increased by approximately 78%.

The following statistical analysis is presented as exploratory, given the limited number of paired observations (three per mixture group). The results should be interpreted with caution and considered preliminary.

The statistical analysis presented in

Table 13. Statistical Evaluation of SBS Polymer Effect on CT Index and RT Index.

	CT Index			RT Index
Mixture Group	Paired t-Test (p-Value)	Cohen’s d	Classification	Paired t-Test (p-Value)	Cohen’s d	Classification
All 6 Mixes	0.404	0.37	Small to Moderate Positive Effect	0.0012	2.68	Extremely Large Positive Effect
A—Limestone Dust	0.027	−3.44	Extremely Large Negative Effect	0.0026	11.34	Extremely Large Positive Effect
B—Cement	0.162	1.26	Very Large Positive Effect	0.0265	3.47	Extremely Large Positive Effect

supports these observations. Considering all six mixtures together, SBS modification produced a statistically significant improvement in RT Index (p = 0.0012) with an extremely large positive effect size (Cohen’s d = 2.68). In contrast, the overall CT Index response was not statistically significant (p = 0.404), reflecting the opposite behavior observed between limestone dust and cement-filled mixtures. When the mixtures were grouped according to filler type, the influence of filler characteristics became more evident. Limestone dust mixtures exhibited a statistically significant reduction in CT Index (p = 0.027) accompanied by an extremely large negative effect size (d = −3.44), indicating a substantial deterioration in cracking resistance after polymer modification. However, these mixtures simultaneously showed a highly significant improvement in RT Index (p = 0.0026) with an extremely large positive effect size (d = 11.34). In contrast, cement-filled mixtures exhibited positive improvements in both performance indicators. Although the improvement in CT Index was not statistically significant at the 95% confidence level (p = 0.162), the corresponding effect size (d = 1.26) indicated a very large practical improvement. The RT Index also improved significantly (p = 0.0265) with an extremely large positive effect size (d = 3.47).

A possible explanation for the filler-dependent behavior is that limestone dust, being chemically inert, may not interact effectively with the SBS-modified binder, potentially leading to localized stiffening and reduced stress relaxation. In contrast, cement filler, due to its hydraulic activity and alkaline surface chemistry, may promote better adhesion and more uniform stress distribution within the mastic, thereby enhancing cracking resistance despite its higher specific surface area. However, these mechanisms are inferred from the literature and indirect measurements; direct binder or mastic characterization (e.g., rheology, microscopy) would be required to confirm these hypotheses.

6. Summary and Conclusions

This study investigated the recovery of performance-deficient asphalt mixtures through additive modification and BMD redesign using CT Index and RT Index. Based on the experimental findings, the following conclusions are drawn:

• The P25 approach provided practical, albeit internally derived, thresholds (CT = 52, RT = 44) for classifying mixture performance in this study. These thresholds should be validated using independent datasets before general application.
• Additive modification alone did not reliably recover performance-deficient mixtures; it often improved rutting but reduced cracking resistance, particularly for limestone dust mixtures. This conclusion is based on a limited number of mixtures and additive combinations and should be considered preliminary.
• BMD redesign (through binder content optimization) was more effective in recovering deficient mixtures, enabling them to meet performance criteria even before additive incorporation.
• The effectiveness of additives was enhanced after BMD redesign, suggesting that volumetric balance is a prerequisite for beneficial modification.
• For the single initially passing mixture evaluated, all additives improved both CT and RT Index, with the hybrid system providing the best balance.
• Statistical analysis (Cohen’s d) confirmed that SBS polymer has a very large positive effect on rutting resistance (d = 2.68), but its effect on cracking is filler-dependent: negative for limestone (d = −3.44), positive for cement (d = +1.26).
• The inferred modification mechanisms (e.g., mastic desiccation, improved adhesion) require direct microstructural or rheological confirmation.

Future research should validate the proposed thresholds using plant-produced mixtures, incorporate direct microstructural characterization (e.g., SEM, rheometry) to confirm the inferred modification mechanisms, and extend the evaluation to a broader range of passing and performance-deficient mixtures to strengthen the generalizability of the findings. Pilot-scale implementation studies are also recommended to assess the practical feasibility of the proposed recovery strategies.