1. Introduction
Mass concrete is a fundamental material in modern civil engineering, extensively employed in large-scale infrastructure such as high-rise buildings, dams, bridges, and nuclear power plants due to its structural integrity and durability [
1,
2,
3]. However, a significant challenge arises from its inherent propensity for early-age thermal cracking, which is primarily driven by the exothermic nature of cement hydration, the low thermal conductivity of concrete, and the restrained boundary conditions prevalent in massive sections [
4,
5]. This temperature rise generates internal thermal gradients and stress, and when coupled with autogenous and drying shrinkage, can lead to cracking that severely compromises structural integrity, long-term durability, and service life [
6,
7,
8].
The thermal and shrinkage behavior of mass concrete is a complex phenomenon influenced by a multitude of interrelated factors. The composition of the cementitious matrix is paramount. Ordinary Portland Cement (OPC), as the primary binder, releases substantial heat, predominantly from the hydration of tricalcium aluminate (C
3A) and tricalcium silicate (C
3S) [
9]. Cement fineness (specific surface area) further accelerates early hydration kinetics, increasing both the rate and magnitude of temperature rise, albeit with potential benefits to early strength [
10,
11]. Consequently, research has extensively focused on modifying the binder system to mitigate thermal risks.
The partial replacement of cement with supplementary cementitious materials (SCMs) is a well-established strategy. Granulated Blast Furnace Slag (GBFS) and fly ash (FA) are particularly effective. GBFS exhibits latent hydraulic properties, reducing the peak hydration heat and delaying the time to reach maximum temperature, thus lowering the early-age thermal cracking potential [
12,
13]. Fly ash contributes through its pozzolanic reaction and micro-aggregate filling effect, leading to a denser microstructure, reduced permeability, and lower cumulative heat evolution over time [
14,
15]. Studies have shown that high-volume fly ash can progressively mitigate temperature rise and cracking potential [
16,
17]. The combined use of GBFS and fly ash has been demonstrated to synergistically enhance durability against sulfate attack and chloride migration [
18].
Beyond SCMs, the use of specialized low-heat cements (LHC), characterized by a higher content of dicalcium silicate (C
2S), offers a direct approach to reducing hydration heat. LHC has been successfully applied in major dam projects to control thermal stress and cracking risks [
19,
20,
21]. However, its slower strength development necessitates careful design and sometimes combination with other SCMs [
22].
The cracking resistance of concrete is not solely dependent on thermal properties but also on its mechanical response under restraint. The development of thermal stress is a direct indicator of cracking risk and is influenced by the concrete’s tensile strength, elastic modulus, creep, and coefficient of thermal expansion, all of which are time-dependent at early ages [
23,
24,
25]. The Temperature Stress Testing Machine (TSTM) has become an invaluable tool for experimentally evaluating these parameters under controlled restraint and temperature conditions, providing insights into early-age cracking behavior [
26,
27,
28].
Recent advancements have also introduced functional materials to actively control shrinkage and enhance crack resistance. High-performance expansive agents can generate chemical expansion to compensate for thermal and autogenous shrinkage, effectively inducing beneficial pre-compressive stress [
29,
30]. The incorporation of fibers, such as polypropylene fibers, provides a physical bridging mechanism that controls plastic shrinkage and mitigates crack propagation [
31,
32]. Furthermore, the development of self-healing agents, including cementitious capillary crystalline materials, offers a pathway to autonomously repair microcracks, thereby improving long-term durability [
33,
34].
The prediction and control of the temperature field remain critical. This involves considering the multi-physics coupling of hydration, temperature, humidity, and constraint [
35]. Factors such as ambient temperature, casting temperature, cooling pipe layout, water temperature, and flow rate significantly influence the internal temperature distribution and the resulting inside-outside temperature difference [
36,
37,
38]. Finite element analysis and numerical modeling have become essential for simulating these complex interactions and assessing cracking risks under realistic conditions [
39,
40,
41]. In addition, recent advances in computer vision offer promising solutions for automated crack detection and characterization during concrete curing. Semantic segmentation models such as DeepLab and EfficientNet [
42,
43] have been successfully applied to concrete crack detection, enabling pixel-level identification of crack regions with high accuracy.
Although the use of fly ash, expansive agents, and polypropylene fibers has been widely reported for mass concrete, there is a persistent need for a holistic mix design approach that systematically integrates material selection, proportioning, and functional additives to achieve the dual objectives of “low temperature rise” and “high crack resistance” in mass concrete. Furthermore, robust validation through field monitoring is crucial to bridge the gap between laboratory findings and actual structural performance [
44].
Therefore, aiming to achieve synergistic thermal control and crack resistance, this study conducts a systematic investigation encompassing optimized raw material selection, experimental mix proportioning, and field application validation. The influence of key parameters, including fly ash content, high-performance expansive agent, and polypropylene fibers, on workability, mechanical properties, hydration heat, and volumetric stability is thoroughly evaluated. By linking material design with construction-stage performance, this research provides critical insights into the early-age behavior of mass concrete and offers a practical basis for safety control during construction. The findings aim to provide a theoretical foundation and practical guidance for the design of high-performance, durable mass concrete structures.
2. Raw Materials: Selection and Control Parameters
Aiming at thermal control and crack prevention in mass concrete, this chapter discusses the rationale behind the selection of key constituents and their respective performance requirements. By analyzing the interaction mechanisms of cement, mineral admixtures, and functional materials, this section provides the necessary theoretical support for the mix design and field validation presented in later sections.
2.1. Cement
As the primary binder in concrete, cement plays a decisive role in governing the heat of hydration, which is a critical factor in managing thermal stress and cracking risks in mass concrete. The hydration behavior is fundamentally dictated by its mineral composition. Among the major clinker phases, tricalcium aluminate (C3A) exhibits the most rapid hydration rate and the highest heat output per unit mass. While tricalcium silicate (C3S) hydrates slightly slower than C3A, it contributes most significantly to the total heat release due to its high concentration. In contrast, dicalcium silicate (C2S) hydrates slowly with low heat evolution, primarily contributing to long-term strength development.
Beyond mineralogy, the alkali content is another pivotal parameter influencing crack resistance. Excessive alkali levels (typically exceeding 0.6%) accelerate early-age hydration, leading to a concentrated release of hydration heat. Furthermore, high alkalinity can alter the morphology of calcium silicate hydrate (C-S-H) gels, inducing a transition from an idealized acicular (needle-like) structure to a coarser rod-like morphology, thereby reducing the overall ductility of the concrete. Consequently, P·II 52.5 Portland cement with an alkali content restricted to below 0.6% was selected for this study. All technical properties of the selected cement comply with the national standard Common Portland Cement (GB175-2023) [
45] (
Table 1).
2.2. Mineral Admixtures
Fly ash was selected to serve a dual purpose: decreasing the cumulative heat of hydration and improving the long-term microstructural integrity of the concrete.
The integration of fly ash is a well-established practice for enhancing concrete performance. It contributes to a more compact microstructure through two main mechanisms: the physical micro-aggregate filling effect and the chemical pozzolanic activity, which consumes calcium hydroxide to form additional C-S-H gel. These mechanisms collectively improve impermeability and crack resistance. The technical indicators of the fly ash used in this research are presented in
Table 2. Strict limits were imposed on ammonium and chloride ion concentrations, fulfilling the requirements of national standard Fly Ash Used for Cement and Concrete (GB/T 1596-2017) [
46] and Limit and Test Method of Ammonium Ion Content Fly Ash (GB/T 39701-2020) [
47] to prevent potential adverse effects on concrete properties.
2.3. Functional Materials
To specifically enhance the crack resistance, thermal control, and self-healing capacity of the concrete, several functional materials were incorporated into the mixture.
2.3.1. Polypropylene Fibers
Polypropylene (PP) fibers (
Table 3) are utilized to effectively mitigate plastic shrinkage cracking through the interfacial bonding between the fibers and the concrete matrix. This is manifested by a significant reduction in total crack area, maximum crack width, and the overall frequency of cracks. The technical properties of the fibers must comply with Synthetic Fibers for Cement Concrete and Mortar (GB/T 21120-2018) [
48]. For this project, a variety with a fiber length of 8–20 mm was selected to ensure optimal distribution and bridging capacity.
2.3.2. High-Performance Expansive Agent
A high-performance hydration heat-inhibiting composite expansive agent was employed. This specialized additive is a synergistically blended product comprising a hydration heat inhibitor and a calcium-magnesium composite expansive agent. It is designed to effectively retard the hydration rate of cement and lower the adiabatic temperature rise in the concrete. Furthermore, it provides stage-wise compensation for concrete shrinkage throughout the hydration process, thereby substantially reducing the risk of thermal and autogenous cracking. This study utilizes the HME-V model (
Table 4), which meets the requirements specified in Calcium and Magnesium Oxides Based Expansive Agent for Concrete (T/CECS 10082-2020) [
49] and Concrete Temperature Rise Inhibitor (JC/T 2608-2021) [
50].
2.3.3. Inorganic Self-Healing Agent
The inorganic self-healing agent employed in this study is a cementitious capillary crystalline waterproofing (CCCW) material. The reaction between the CCCW agent and the cementitious matrix generates crystalline precipitates that fill the internal pores, thereby refining the microstructure and increasing the overall density of the material.
When cracks form in the matrix modified with cementitious capillary crystalline waterproofing (CCCW) materials and water infiltrates, the chemically active components in the CCCW agent are transported by water as a medium, penetrating deeper into the matrix via osmotic action. These active constituents promote the further hydration of unreacted cement particles or interact with existing hydration products to generate insoluble crystalline compounds. The resulting crystalline deposits effectively fill and seal the cracks, thereby restoring the structural integrity of the material. The performance of this self-healing agent (
Table 5) meets the requirements specified in the national standard Cementitious Capillary Crystalline Waterproofing Materials (GB 18445-2012) [
51].
2.4. Aggregates
The quality of aggregates exerts a decisive influence on the workability, mechanical strength, volumetric stability, and long-term durability of concrete. The quality control protocols primarily focus on clay (fines) content, particle size distribution, morphology, and alkali-aggregate reactivity.
2.4.1. Fine Aggregate
Naturally graded Zone II river sand with a fineness modulus ranging from 2.3 to 3.0 was selected as the fine aggregate. To mitigate adverse effects on the interfacial bonding strength and volumetric stability of the cement paste, the clay content and clay lump content were strictly limited to ≤2.0% and ≤1.0%, respectively. Furthermore, the chloride ion content was restricted to ≤0.01%, and the sand was required to be non-reactive regarding potential alkali-silica reactivity (ASR).
2.4.2. Coarse Aggregate
The coarse aggregate consisted of crushed limestone with a continuous grading of 5–31.5 mm. Optimized particle grading is essential for minimizing the required binder content and enhancing the packing density of the concrete. The content of elongated and flaky particles was restricted to ≤8%, and the crushing value was maintained below 10% to ensure high compressive strength and crack resistance. Additionally, the clay content and clay lump content were controlled within ≤1.0% and ≤0.1%, respectively, with a mandatory requirement for zero alkali-aggregate reactivity.
3. Experimental Study
To systematically investigate the influence of key mix design parameters on the performance of low-temperature-rise and crack-resistant mass concrete, a comprehensive series of experiments was conducted. This chapter evaluates the effects of various factors—including total binder content, water-to-binder (w/b) ratio, sand ratio, and the dosages of fly ash, high-performance expansive agent, fibers, and self-healing agents—on workability, mechanical properties, hydration heat characteristics, and volumetric stability. Based on the experimental findings, the optimal parameter combinations for different structural components were determined.
3.1. Specimen Design and Experimental Grouping
In accordance with the specific requirements of the engineering project, the ranges for critical parameters such as binder content and functional material dosages were preliminarily established, as detailed in
Table 6. The experimental materials were supplied by Hefei Tiancheng Concrete Co., Ltd., Hefei, Anhui Province, China. All test mixtures were prepared using P·II 52.5 Portland cement as the primary binder to ensure consistency across the experimental groups.
3.2. Concrete Preparation and Performance Characterization
The preparation and experimental evaluation of the concrete specimens were conducted in strict accordance with the relevant national and industrial standards to ensure the reliability and reproducibility of the results. Initially, the workability of the fresh concrete was characterized immediately after mixing, with key parameters including slump, slump flow, air content, and apparent density being recorded. Subsequently, the mechanical performance was determined primarily through compressive strength testing at specified curing ages.
To systematically investigate the thermal behavior of the mass concrete, the hydration kinetics—specifically the heat evolution rate and cumulative heat of hydration—were measured, alongside the adiabatic temperature rise, which serves as a critical indicator for thermal stress analysis. Furthermore, the volumetric stability was evaluated by monitoring the autogenous volumetric deformation (autogenous shrinkage) to assess the material’s crack resistance potential during the early-age hydration phase. A comprehensive overview of the representative experimental procedures and testing setups is illustrated in
Figure 1.
3.3. Experimental Results and Discussion
3.3.1. Influence of Binder Content, Sand Ratio, and Water-to-Binder Ratio
The influence of total binder content on the workability of fresh concrete was first investigated under the conditions of a fixed water consumption of 160 kg/m
3, a fly ash content of 30%, and a sand ratio of 39%. Four mixtures (Groups 1#–4# in
Table 6) with binder contents ranging from 380 kg/m
3, 400 kg/m
3, 420 kg/m
3, 440 kg/m
3 were evaluated, and the results are summarized in
Table 7.
The results indicate that at a binder content of 380 kg/m3, the concrete exhibited slight aggregate exposure and poor coating properties (cohesiveness). In contrast, mixtures with a binder content of 400 kg/m3 or higher demonstrated excellent workability, characterized by a slump of 200 ± 10 mm and a slump flow exceeding 460 mm. Considering both workability and cost-effectiveness, 400 kg/m3 was preliminarily selected as the baseline binder content.
Subsequently, the effect of the sand ratio (38% vs. 40%) was examined based on the 400 kg/m3 binder dosage (Groups 5# and 6#). It was observed that increasing the sand ratio to 40% resulted in excessive viscosity, whereas reducing it to 38% led to insufficient coating of the aggregates. Consequently, an optimal sand ratio of 39% was determined to achieve the best balance between fluidity and cohesiveness.
Based on a binder content of 400 kg/m
3, a sand ratio of 39%, a fly ash replacement level of 30%, and a water content of 160 kg/m
3 (corresponding to Mix 2# in
Table 6), the water-to-binder ratio was further adjusted to 0.40, 0.39, and 0.37 (corresponding to Mixes 2#, 7#, and 8# in
Table 6). The results indicate that reducing the water-to-binder ratio significantly enhances compressive strength. As shown in
Table 8, a 14-day compressive strength of 59.1 MPa was achieved at a ratio of 0.37. However, this reduction also leads to decreased workability of fresh concrete and is unfavorable for temperature control. Consequently, a water-to-binder ratio of 0.40 with a water content of 160 kg/m
3 was selected for subsequent experiments.
3.3.2. Influence of Fly Ash Content
With the total binder content fixed at 400 kg/m3, the influence of fly ash replacement levels ranging from 25% to 45% was systematically investigated (Mixes 2#, 9#, 10#, 11#, and 12#).
As shown in
Table 9, increasing the fly ash content led to a pronounced improvement in the workability of fresh concrete, as evidenced by higher slump and flow spread values. This enhancement is mainly attributed to the “ball-bearing effect” of fly ash particles. However, a gradual reduction in early-age compressive strength was observed with increasing fly ash content. At a replacement level of 35%, the 7-day and 14-day compressive strengths reached 41.1 MPa and 49.8 MPa, respectively, satisfying the early-strength requirements of C45 concrete. In contrast, when the fly ash content increased to 45%, a marked decrease in early strength occurred, with the 7-day compressive strength dropping to 33.9 MPa.
The results of autogenous deformation tests, summarized in
Table 10, indicate that the autogenous shrinkage of concrete was significantly reduced as the fly ash content increased. Considering both the early-age strength requirements and the effectiveness in shrinkage mitigation, an optimal fly ash replacement level of 35% was therefore selected.
3.3.3. Influence of High-Performance Expansive Agent
Based on a total binder content of 400 kg/m
3, a sand ratio of 39%, a water content of 160 kg/m
3, and a fly ash replacement level of 35%, the effects of different dosages of a high-performance expansive agent (0%, 8%, and 10%, corresponding to Mixes 10#, 13#, and 14# in
Table 6) on the fresh and mechanical properties of concrete were investigated.
As summarized in
Table 11 and
Table 12, the incorporation of the expansive agent exhibited no significant adverse effects on the fresh properties of concrete. When the expansive agent was used as an equivalent replacement for cement, the compressive strength at 7–14 days decreased by approximately 10–15%; however, this strength difference gradually diminished with increasing curing age.
In terms of volumetric stability (
Figure 2), the addition of the expansive agent transformed the autogenous deformation behavior of concrete from shrinkage to expansion. At dosage levels of 8% and 10%, the 7-day expansion reached 225.2 με and 270.1 με, respectively, effectively compensating for shrinkage. The incorporation of fly ash significantly reduced the hydration heat release rate and early cumulative heat evolution of concrete. Furthermore, the addition of the high-performance expansive agent (high-efficiency anti-cracking agent) further suppressed the early hydration heat release rate and total heat evolution. In combination with appropriate heat dissipation conditions, this led to a reduction in concrete temperature rise, thereby enhancing its resistance to cracking.
Considering both temperature control requirements and the need for shrinkage compensation, an expansive agent dosage of 8% was selected for the foundation slab, while a higher dosage of 10% was adopted for the sidewall concrete.
3.3.4. Influence of Polypropylene Fibers
The effects of polypropylene fiber contents of 1.0 kg/m
3, 1.2 kg/m
3, and 1.5 kg/m
3 (corresponding to Mixes 15#~17# in
Table 6) on the fresh and mechanical properties of concrete were investigated, as summarized in
Table 13.
The results indicate that increasing the fiber content leads to reduced flowability and increased viscosity of fresh concrete. Meanwhile, the spatial dispersion and interfacial bridging effect of the fibers can limit the development of plastic shrinkage cracks and enhance the crack resistance of concrete. When the fiber dosage exceeded 1.2 kg/m3, workability deteriorated markedly. The incorporation of polypropylene fibers resulted in a slight reduction in early-age compressive strength (less than 5%); however, this effect was not significant. To achieve a balance between crack resistance enhancement and satisfactory workability for construction, an optimal fiber content of 1.2 kg/m3 was selected.
3.3.5. Influence of Inorganic Self-Healing Agent
The influence of incorporating a self-healing agent at a dosage of 2.0 kg/m
3 (Mix 18#) was evaluated in comparison with the reference mixture (Mix 13#). The results are summarized in
Table 14.
In terms of fresh properties, the addition of the self-healing agent had no noticeable adverse effect on the workability of fresh concrete. The inorganic self-healing agent is a cementitious capillary crystalline waterproofing agent. Its active chemicals react with the cement matrix to form insoluble crystals, which fill pores and microcracks. When water enters cracks, these reactions are reactivated, further sealing the cracks and improving compactness and durability. Regarding mechanical performance, the self-healing agent led to an approximately 8% reduction in early-age compressive strength at 7 days, while its influence on the 14-day strength was relatively minor.
With respect to hydration heat characteristics, as illustrated in
Figure 3, the self-healing agent significantly prolonged the hydration induction period and simultaneously increased both the peak hydration heat release rate and the cumulative heat evolution. Specifically, the induction period of cement hydration was extended by more than twofold, resulting in a setting time exceeding 24 h, and the cumulative heat release at 5 days increased by approximately 15% compared with the reference mixture.