Mass Concrete Hydration Heat
and Temperature Crack Control

The "mass" in mass concrete is not about absolute geometric dimensions but about whether heat can dissipate efficiently. When hydration heat accumulates within the structure forming significant temperature gradients, compounded by external restraints and concrete shrinkage, thermal stress races against the tensile strength at the corresponding age — cracks are fundamentally the outcome of this race. This article unpacks the "why it cracks, how to calculate it, and how to control it" through four dimensions: heat source mechanisms, stress evolution, quantitative prediction, and control systems.

I. Heat Source Fundamentals: How Hydration Heat "Ignites" Mass Concrete

Cement hydration is a strongly exothermic reaction, releasing 300–400 kJ of heat per kilogram of cement. For mass concrete, three core characteristics stand out: large structural dimensions resulting in relatively insufficient heat dissipation surfaces, high binder content concentrating the heat source, and low thermal conductivity of concrete (approximately 1.7 W/(m·K)) causing heat accumulation.

The mineral composition of cement determines the heat release profile: C₃S (tricalcium silicate) and C₃A (tricalcium aluminate) hydrate rapidly with high heat release, driving early temperature rise; C₂S (dicalcium silicate) hydrates slowly with low heat output. Low-heat cement achieves peak delay and reduction precisely by increasing the C₂S ratio. This is why selecting moderate-heat or low-heat cement can fundamentally alter the shape of the temperature rise curve at the source.

II. Two-Stage Crack Evolution Mechanism

Cracks do not appear "suddenly" — they follow a clear temporal evolution pattern, with fundamentally different mechanical mechanisms during the heating and cooling phases.

Heating Phase (0–3 d after casting): Predominantly Surface Cracks

Concentrated cement hydration heat release causes rapid internal temperature rise. Internal expansion is restrained by the cooler surface, generating tensile stress in the surface layer. At this stage, the concrete has low elastic modulus and high creep, so internal compressive stress is not yet significant. However, the surface layer, cooling faster, has already developed a temperature gradient — when surface tensile stress exceeds the tensile strength at that age, surface cracks appear.

Cooling Phase (3–14 d and beyond): The Main Battlefield for Through-Cracks

After peak temperature, hydration heat weakens and internal temperature gradually declines, causing overall concrete shrinkage. The elastic modulus is now significantly higher and creep capacity has diminished. Under external boundary restraints (foundation, adjacent members, formwork), substantial tensile stress develops. This is the primary cause of deep cracks and through-cracks, and the critical window for thermal crack prevention.

Two Fundamental Types of Thermal Stress

Understanding the distinction between these two types is the basis for deciding whether to prioritize "insulation" or "restraint reduction" strategies.

Restraint Degree: The True "Switch" of Thermal Stress

Professor Xu Youlin's classical derivation: under full restraint, a 10 ℃ temperature difference in C30 concrete can generate 3.0 MPa of tensile stress — far exceeding its characteristic tensile strength of 2.01 MPa — theoretically making cracking "inevitable."

In reality, cracks are not ubiquitous. The fundamental reason is that the restraint degree β (0 ≤ β ≤ 1) is far less than 1. Foundation frictional restraint, pile horizontal stiffness, and wall-to-slab restraint all have limited rigidity. Actual thermal stress is calculated as:

This means that reducing restraint (sliding layers, post-cast strips, alternate bay construction, induced joints) is equally important as reducing temperature differential — the physical foundation of the "combined resistance and release" philosophy.

Time Effects: Creep as Nature's "Pressure Relief Valve"

Concrete is not an ideal elastic material. Under prolonged thermal loading, stress relaxation occurs — the initial elastic stress decays over time, with relaxation coefficients typically between 0.3–0.5. This explains why concrete with relatively low early strength may not necessarily have poor crack resistance: high early-age creep capacity can absorb a portion of restraint stress. However, this protective mechanism weakens significantly during the later cooling phase, making temperature differential control more critical in later stages.

III. Quantitative Prediction: From Post-Pour Firefighting to Pre-Pour Simulation

Modern thermal control has shifted from "post-pour monitoring and alarming" to "pre-pour simulation and prediction." Three levels of predictive capability form a technology pyramid:

1. Adiabatic Temperature Rise Estimation — Determining the "Energy Ceiling"

Adiabatic temperature rise is the theoretical upper limit assuming no heat exchange with the environment, determining the energy scale of the entire temperature field. Its calculation depends on the coupling of total binder content, equivalent hydration heat, specific heat capacity, and density.

For blended cement systems, equivalent hydration heat must be corrected using mineral composition methods, conversion formulas, or numerical fitting. Research shows that fly ash and slag exhibit "overlap effects" when combined — they cannot be simply linearly superimposed. This is why a binary blend of 30% fly ash + 20% slag typically achieves equivalent hydration heat at 75–85% of the pure cement system.

2. 1D/3D Finite Difference — Solving the True Temperature Field

In actual structures, heat is simultaneously generated and diffused outward, requiring discretization along the thickness direction to solve the heat conduction equation. The one-dimensional finite difference method recommended in GB 50496-2018 Appendix B is the industry-standard tool. The stability condition for the explicit scheme is Fourier number r ≤ 0.5 — violating this condition will cause the calculation to diverge.

3. Thermal Stress Simulation — Predicting Crack Risk

A more advanced approach couples the temperature field with the stress field, using finite element software (such as Midas, Abaqus, PLAXIS, etc.) to simulate stress evolution and determine whether the crack resistance safety factor meets requirements. For complex structures or critical projects, such simulation has become essential.

IV. Control System: Synergy of Materials, Construction, Monitoring, and Curing

Temperature crack control can never be solved by any single measure — it requires multi-dimensional synergy of "reducing heat source — controlling temperature rise — releasing restraint — protecting the surface."

4.1 Material Source Control: Suppressing Hydration Heat

• Cement selection: Prioritize moderate-heat Portland cement (hydration heat ≤ 310 kJ/kg) or low-heat slag Portland cement (≤ 270 kJ/kg) instead of ordinary Portland cement (≥ 350 kJ/kg) to significantly reduce temperature rise
• Ternary binder system: Low-heat cement + fly ash + ground granulated blast-furnace slag blends, utilizing pozzolanic and micro-aggregate effects to delay hydration
• Aggregate optimization: 5–31.5 mm continuously graded crushed stone with clay content ≤ 1%; fine aggregate using medium sand with fineness modulus 2.3–3.0 to increase packing density and reduce binder demand
• Mix design red lines: Total binder 350–400 kg/m³, water-to-binder ratio ≤ 0.45, water reducer efficiency ≥ 25%
• Expansive agent compensation: 8–12% UEA expansive agent to offset shrinkage stress through micro-expansion during hardening

4.2 Placement Temperature: Lowering the Starting Point

Placement temperature directly determines the baseline for temperature rise — summer construction is critical:

• Aggregate pre-cooling: Sunshades, spray water cooling (water temperature ≤ 15 ℃), air cooling, or vacuum cooling of aggregates
• Mixing water cooling: Chilled water or partial ice flake substitution
• Pouring timing: Schedule for nighttime, early morning, or low-temperature seasons
• Transport protection: Insulated mixer trucks, minimize transfers, temperature rise from discharge to compaction completion ≤ 5 ℃
• Pour area cooling: Mist spraying and sunshading during hot seasons

4.3 Pouring Techniques: Releasing Restraint

• Layered pouring: 300–500 mm per layer, sloped layer advancement, symmetric pouring from far to near, avoiding single-point discharge causing local heat concentration
• Alternate bay construction: Divide mass concrete into bays ≤ 40 m, pour at intervals ≥ 7 d, utilizing later-age strength to release early stress
• Sliding layers: Double-layer polyethylene film or flexible cushion on rock foundations to reduce base frictional restraint
• Induced joints and post-cast strips: Guide cracks to occur in controlled locations, avoiding random cracking

4.4 Cooling Pipes: A Necessity for Thick Structures

When structural thickness exceeds 1.5 m, surface insulation alone cannot maintain the internal-to-surface temperature difference within 25 ℃ — cooling pipes become mandatory.

Pipe Layout Principles:

• Pipe material: Φ25 mm HDPE (mainstream) or Φ40–50 mm galvanized steel pipe, horizontal spacing 1.0–1.5 m, distance from surface ≥ 0.5 m
• Water circulation schedule: Begin 6–24 h after initial set, continue 7–14 d; water-to-concrete internal temperature difference ≤ 20 ℃ to avoid thermal shock
• Flow control: Estimate required flow using Q ≈ 0.6 × V × ΔT
• Staged circulation: Strong restraint zone (0–0.2 L) control base temperature difference at 15–17 ℃; weak restraint zone (0.2–0.4 L) at 17–20 ℃

4.5 Surface Curing: Protecting the Temperature Differential

The core objective of curing is "early covering, adequate moisture, extended duration, dynamic adjustment":

4.6 Intelligent Monitoring: From "Seeing" to "Predicting"

Modern monitoring has evolved from single-point thermocouples to an integrated system of distributed optical fiber + wireless sensors + cloud platform early warning:

• Sensor placement principle: Three layers along the thickness direction at 50 mm below surface, center, and 50 mm above bottom; sensor spacing 1.5–2.0 m in a staggered arrangement
• Monitoring frequency: Every 2–4 h during heating phase, every 4–12 h during cooling phase
• Alarm thresholds: Core-to-surface difference ≥ 20 ℃ → immediately increase water flow; core temperature > 70 ℃ → immediately cover and insulate while reducing water flow
• Simulation integration: BIM and FEM-based intelligent early warning platforms enabling full closed-loop: "monitor — simulate — warn — adjust"

V. Engineering Significance of Key Control Indicators

The four indicators in GB 50496-2018 Section 3.0.4 form the "constitution" of thermal control plans:

Of particular note: hydraulic engineering standards (e.g., JTS/T 202-1-2022, water conservancy thermal control guidelines) impose stricter base temperature difference controls in restrained zones, with strong restraint zones requiring ≤ 15 ℃, and segmented controls at 0–0.2 L (strong restraint) and 0.2–0.4 L (weak restraint). This means hydraulic structures such as bridge pier caps and dam foundations cannot simply adopt the four civil building indicators.

VI. Conclusion: From "Resistance" to "Release" — An Engineering Philosophy

The essence of temperature crack control is managing a fundamental contradiction — allowing concrete free deformation to release stress while maintaining structural integrity and waterproofing. Pure "resistance" (relying on higher strength) is neither economical nor reliable; pure "release" (no protection at all) cannot meet functional requirements.

The evolution of modern thermal control technology is precisely the shift from singular "strength confrontation" to "combined resistance and release, integrated measurement and adjustment":

• Material level: Reduce heat source, compensate shrinkage
• Structural level: Release restraint through alternate bay construction, sliding layers, induced joints
• Construction level: Regulate temperature field through pre-cooling, layering, and cooling pipes
• Monitoring level: Achieve dynamic control through distributed sensing and cloud platforms

This methodology applies not only to traditional dams, raft foundations, and pier caps, but also provides a reliable technical foundation for mega-projects such as sea-crossing bridges, the Shenzhen-Zhongshan Link, and plateau railways — where thermal crack prevention has evolved from a "construction procedure" to a "core process" that directly determines structural lifespan and operational safety.