Enhancing Pavement Performance Through Stabilization

Stabilizing base pavement materials is crucial for enhancing the durability and strength of flexible pavements. This study by Prof. (Dr.) Dharamveer Singh & Ph.D. Scholar Rohit Kumar Sharma, Department of Civil Engineering, IIT Bombay, evaluates the effectiveness of various cementitious admixtures, their mechanical properties and optimal mix designs for improving pavement performance, and sustainable use of industrial by-products.

the durability and strength of flexible pavementsPicture courtesy: Wirtgen-Group

Introduction

Stabilizing base pavement materials enhances flexible pavement systems' structural and durability performance. Cementitious stabilized aggregates (CSA) endure heavy traffic loads and utilize natural aggregates and industrial by-products efficiently. While cement is commonly used as a stabilizer, researchers also explore additives like commercial chemical admixtures (CCA), flyash, and slag to improve early strength and durability. Early strength in the CSA layer depends on materials, compaction methods, curing duration, and optimum stabilizer content (OSC).

Apart from newly introduced CCA, which are rich in alumina and silica content, to enhance cement, other additives like fly ash and slag can also effectively replace cement as stabilizers. In India, fly ash production reached 226.1 million tonnes annually in 2019-20, partially used in industries like cement, bricks, and as quarry filler material. Steel production hit 100.3 million tonnes in 2021, with steel slag comprising 20-30% of crude steel weight, requiring safe and cost-effective utilization. Slag, known for minimizing heat of hydration and alkali-aggregate reaction, is extensively used as a partial cement replacement in concrete, termed supplementary cementitious materials (SCMs).

Designing CSA layer in composite pavement systems relies on flexural strength (MoR), often derived from unconfined compressive strength (UCS) due to time and site constraints in mechanistic-empirical pavement design. The chosen derivation factor varies (14% to 32%) based on materials and curing conditions, but the influence of admixtures needs to be better addressed. Understanding their impact on CSA layer thickness design and costs is crucial for broader practical applications.

CCA admixturesFigure 1: Stabilizers used in the study

Materials and Methodology

Two CCA admixtures, labelled AC-I and AC-II, were selected and incorporated into the cement at 2% and 4% of its weight, as advised by the supplier. The mixes were denoted as OPC (cement), ACC-I (Cement + 4% AC-I), ACC-II (Cement + 2% AC-II), CFA (70% cement + 30% fly ash), and CSG (50% cement + 50% slag). Moisture density relationship tests were conducted on various mix proportions to determine the Optimum Stabilizer Content (OSC), Optimum Moisture Content (OMC), and Maximum Dry Density (MDD). Mix designs were finalized according to IRC 37, 2018 guidelines and IRC SP 89 parts I & II. Mechanical strength properties such as UCS and MoR were evaluated after 7 and 28 days of curing. Finally, composite pavement designs were developed according to IRC 37, 2018. The stabilizers used are shown in Figure 1.

UCS test results for various mixesFigure 2: UCS test results for the mixes

UCS for different cementitious admixtures

UCS test results for various mixes (OPC, ACC-I, ACC-II, CFA, and CSG) at 7 and 28 days are depicted in Figure 2. A comparison of these results reveals a notable increase in UCS values at 28 days compared to 7 days, particularly for SCMs over CCA and OPC. Since the 28-day strength is standard for CSA layer design, while the seven-day strength is more commonly used for mix design, establishing the 7/28 ratio for different admixtures is crucial. The 7/28 ratios were determined as follows: OPC (65%), ACC-I (84%), ACC-II (69%), CFA (56%), and CSG (67%). Notably, 7/28 ratios ranged from 65% to 70% for OPC, ACC-II, and CSG, with the highest observed for ACC-I and the lowest for CFA. A high 7/28 ratio signifies rapid hydration, as seen with the addition of AC-I admixture. In contrast, a low ratio suggests slower hydration at seven days, potentially indicating pozzolanic reactivity requiring extended curing for strength development.

MoR test results for OPC, ACC-I, ACC-II, CFA, and CSGFigure 3: MoR test results for the mixes

MoR for different cementitious admixtures

Figure 3 presents MoR test results for OPC, ACC-I, ACC-II, CFA, and CSG at 7 and 28 days. The 7/28 ratio was 50% for OPC, 58% for ACC-I, 79% for ACC-II, 46% for CFA, and 71% for CSG. The lowest 7/28 ratio was observed for CFA, while the highest was for ACC-II, suggesting significant variation in MoR values among different admixtures despite being designed through the same process.
 
Correlation between MoR and UCS Figure 4: MoR vs UCS Correlations for the Mixes

Correlations between MoR and UCS

Correlation between MoR and UCS plays a crucial role in designing the CSA layer where laboratory limitations restrict the cast of beam samples. Correlation factors for OPC is 22%, while for other admixtures, it ranges from 21% to 26%. The Indian code of practice suggests a uniform adoption of a 20% correlation factor regardless of the admixture type. This implies that choosing a 20% correlation factor is conservative for design purposes.
 
the durability and strength of flexible pavements

Conclusion

Different admixture combinations were found to affect critical mechanical properties and design considerations for CSA discretely. The UCS, MoR, and correlation factors were higher for SCM than for CCA admixtures.
NBM&CW - JUNE 2024
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