Crack Control ACI 2019 Edition

ACI 224R-19 — Control of Cracking in Concrete Structures

Report on the causes of cracking and methods for crack control in concrete structures. Covers flexural, shrinkage, thermal, and restraint cracking with crack width prediction and reinforcement detailing.

Scope

ACI 224R-19 is a state-of-the-art report on the control of cracking in concrete structures. It covers the mechanisms and causes of cracking in reinforced and prestressed concrete, methods for predicting crack widths, and practical detailing guidelines to minimize unsightly or structurally damaging cracks. The report addresses cracking under flexural loads, direct tension, thermal and shrinkage effects, and restraint conditions.

The document serves as a comprehensive reference for structural engineers, providing crack width prediction equations, allowable crack width recommendations for various exposure conditions, and reinforcement detailing requirements that are referenced by ACI 318 Chapter 24 (Control of Cracking). It does not establish mandatory code requirements but presents recommended practice.

Purpose

The primary purpose of ACI 224R is to provide engineers with the tools and knowledge to predict, control, and minimize cracking in concrete structures. While concrete naturally cracks due to its low tensile strength, uncontrolled cracking can compromise structural durability by allowing moisture, chlorides, and other aggressive agents to reach the reinforcement, leading to corrosion and spalling.

By understanding the mechanisms that cause cracking — flexural tension, restrained shrinkage, thermal gradients, and external restraint — the engineer can select appropriate reinforcement detailing, joint spacing, concrete materials, and construction procedures to keep cracks within acceptable limits. The report bridges material science, structural behavior, and practical construction.

[FIGURE — Types of cracks in concrete structures: flexural cracks (bottom of beam), diagonal tension cracks (near supports), shrinkage cracks (slab surface pattern), thermal cracks (mass concrete), and restraint cracks (wall on footing)]

Engineering Applications

Crack control is relevant across virtually all concrete structures:

  • Reinforced concrete beams and one-way slabs — flexural crack control per ACI 318 Table 24.3.2
  • Two-way slab systems — crack control over supports and at midspan
  • Water-retaining structures — tanks, reservoirs, and treatment plants requiring crack widths below 0.15 mm
  • Bridge decks — exposed to deicing salts, require tight crack control for corrosion protection
  • Parking structures — exposed to chlorides from vehicles, requiring crack widths below 0.3 mm
  • Mass concrete elements — thermal crack control through concrete materials selection and placement sequencing

Design Philosophy

Crack control in reinforced concrete follows a fundamental principle: crack width is controlled by limiting the stress in the reinforcement at service loads and by distributing the reinforcement properly within the tension zone. The Gergely-Lutz crack width equation (ACI 224R Equation 3-1) shows that crack width w is proportional to the steel stress fs, the thickness of concrete cover cc, and the area of concrete surrounding each bar A, all raised to fractional powers.

w = 0.011 β fs (dcA)1/3 × 10-3 where: w = maximum crack width (mm) β = ratio of distances from neutral axis to tension face and to centroid of reinforcement fs = steel stress at service load (MPa) dc = concrete cover from extreme tension fiber to center of closest bar (mm) A = effective tension area per bar (mm²)

The crack control philosophy embedded in ACI 318 Chapter 24 simplifies the Gergely-Lutz equation into a maximum bar spacing requirement. Rather than calculating crack widths directly, the code specifies that the maximum bar spacing s = 380(280/fs) - 2.5cc, with the limit that s does not exceed 300(280/fs). This spacing requirement ensures that crack widths remain below approximately 0.4 mm for interior exposure.

Note: The Gergely-Lutz equation predicts crack widths at the tension face of beams and one-way slabs. For two-way slabs, crack control is provided by minimum reinforcement requirements and maximum spacing limits in both directions. The equation is calibrated for crack widths up to 0.6 mm and reinforcement stresses up to 275 MPa.

Important Requirements

Key crack control provisions from ACI 224R and ACI 318 include:

  • Maximum Bar Spacing — Per ACI 318 24.3.2, the center-to-center spacing of flexural reinforcement closest to the tension face shall not exceed s = 380(280/fs) - 2.5cc, nor 300(280/fs). For typical Grade 60 steel and moderate service stresses, this limits spacing to about 300 mm.
  • Minimum Reinforcement for Crack Control — Temperature and shrinkage reinforcement per ACI 318 24.4 must be at least 0.0018 times the gross concrete area in each direction for Grade 60 deformed bars.
  • Skin Reinforcement — For beams with overall depth h > 900 mm, ACI 318 24.3.3 requires longitudinal skin reinforcement distributed along both side faces within h/2 of the tension face, with spacing not exceeding the bar spacing limit.
  • Joint Spacing — For slabs exposed to temperature and moisture changes, contraction joints should be spaced at 4.5 to 6.0 m to control random cracking. For thinner slabs (150 mm), closer spacing is recommended.
  • Cover Considerations — While thicker cover protects against corrosion, it increases crack width at the surface. The engineer must balance durability cover requirements (ACI 318 Table 20.5.1) with crack width implications.

Key Parameters

Parameter Value / Provision
Crack width limit — dry interior 0.40 mm (ACI 224R Table 4.1)
Crack width limit — moisture, exterior 0.30 mm (ACI 224R Table 4.1)
Crack width limit — deicing chemicals, seawater 0.18 mm (ACI 224R Table 4.1)
Crack width limit — water-retaining structures 0.10–0.15 mm (ACI 224R Table 4.1)
Max bar spacing (typical Grade 60, fs = 230 MPa) s = 380(280/230) - 2.5cc; limit 300(280/230) = 365 mm
Steel stress at service fs Typically 0.6fy for Grade 60 (~230–250 MPa)
Crack control coefficient z z = fs(dcA)1/3; limit 30 kN/mm (interior), 25 kN/mm (exterior)
Skin reinforcement depth trigger h > 900 mm; skin reinf on both faces within h/2 of tension face
Shrinkage & temp reinforcement ratio 0.0018 (Grade 60), 0.0020 (Grade 50), 0.0025 (Grade 40)
Exposure Condition Allowable Crack Width (mm)
Dry air or protective membrane0.40
Humidity, moist air, soil0.30
Deicing chemicals, seawater0.18
Water-retaining structures0.10
Prestressed concrete (service)0.10–0.20 (depends on exposure)

Practical Engineering Notes

The Gergely-Lutz equation and the ACI 318 simplified spacing provisions are calibrated for normal-weight concrete with specified compressive strength up to 42 MPa. For high-strength concrete, the relationship between crack width and bar spacing is less well-defined, and direct crack width calculation using the Crack Width Calculator is recommended.

Restrained shrinkage cracking is often more critical than flexural cracking for slabs on grade, walls, and large floor areas. The key parameters for shrinkage crack control are: the magnitude of restrained shrinkage (which depends on aggregate type, w/cm ratio, and curing effectiveness), the degree of restraint (subgrade friction, adjacent elements), and the spacing of contraction joints.

Field Tip: For slabs on grade, the most effective crack control measure is proper jointing. Contraction joints should be cut within 6–12 hours after finishing (or as soon as the concrete can be cut without raveling) at a depth of at least one-quarter of the slab thickness. For 150 mm slabs, cut joints 38–50 mm deep at 4.5 m spacing.

Typical Workflow

  1. Determine exposure condition and corresponding allowable crack width from ACI 224R Table 4.1
  2. Calculate steel stress at service load fs = Ms / (Asjd)
  3. Determine reinforcement spacing using ACI 318 Eq. 24.3.2: s = 380(280/fs) - 2.5cc
  4. Verify spacing does not exceed maximum limit: s ≤ 300(280/fs)
  5. For deep beams (h > 900 mm), add skin reinforcement per 24.3.3
  6. Check crack width directly using Gergely-Lutz equation for critical structures
  7. Specify jointing plan for slabs and walls to control shrinkage cracking
Gergely-Lutz: w = 0.011 β fs (dcA)1/3 × 10-3 ACI 318 spacing: s = 380(280/fs) - 2.5cc Limiting spacing: s ≤ 300(280/fs) Crack coeff z: z = fs(dcA)1/3 ≤ 30 kN/mm (interior)

Common Mistakes

  • Using ultimate strength stress for crack control — Crack width calculations use service load steel stress (fs = Mservice/(Asjd)), not the yield stress. Using fy gives unconservatively wide bar spacing.
  • Ignoring skin reinforcement for deep beams — Beams deeper than 900 mm can develop significant flexural cracks on the side faces. Skin reinforcement controls these cracks and is required by ACI 318 24.3.3.
  • Neglecting shrinkage and thermal cracking — Focus solely on flexural crack control while ignoring restraint and volume change cracks. Proper joint spacing and minimum reinforcement are essential.
  • Excessive reinforcement concentration — Using fewer, larger bars at wider spacing reduces crack control effectiveness. More smaller bars at closer spacing produce tighter, better-distributed cracks.

Best Practices

  • Use smaller diameter bars at closer spacing rather than larger bars at wide spacing — for the same reinforcement area, closer spacing gives up to 40% narrower cracks
  • For exposed structures (parking decks, bridge decks), specify epoxy-coated or galvanized reinforcement and follow the stricter 0.18 mm crack width limit
  • Specify low-shrinkage concrete mixes using well-graded aggregates, low w/cm (≤ 0.45), and adequate curing (7–14 days)
  • Use the Crack Width Calculator to compare crack widths for different bar sizes and spacings during design
  • For water-retaining structures, consider post-tensioning to keep the entire section in compression under service loads, eliminating flexural cracks entirely

Limitations

ACI 224R is a report, not a code — its recommendations are not mandatory unless specifically referenced in the contract documents. The crack width prediction equations are empirical and calibrated primarily for normal-weight concrete with conventional reinforcement. They are less accurate for high-strength concrete, fiber-reinforced concrete, or members with unusual cross-sectional geometries.

The Gergely-Lutz equation predicts surface crack widths at the tension face of beams and one-way slabs. It does not directly apply to two-way slab systems, deep beams, or regions with concentrated reinforcement. For these cases, finite element analysis or specialized crack models may be more appropriate.

Crack Width Calculator

Calculate crack widths per Gergely-Lutz and ACI 318.

RC Beam Design Calculator

Design beams with crack control per ACI 318 Chapter 24.

The Concrete Engineering Formulas and Structural Engineering Formulas sections include the Gergely-Lutz equation, crack spacing provisions, and section analysis formulas for service-level stress calculation.

Refer to the Civil Engineering Handbook for detailed guidance on crack control detailing, reinforcement selection, and serviceability design.

Reinforcement Detailing Guide

Complete guide to rebar detailing for crack control.

Development Length Guide

Understanding bar development and splice requirements.

Common Structural Design Mistakes

Avoid costly errors including crack control oversights.

Deepen your understanding with Concrete Technology and Structural Analysis learning modules.

Review key terms in the Engineering Glossary: Cover, Crack Width, Reinforcement Ratio, Service Load, Neutral Axis, Tensile Strain, Shrinkage, and Thermal Gradient.

References

  • ACI 224R-19. Control of Cracking in Concrete Structures. American Concrete Institute, 2019.
  • ACI 318-19. Building Code Requirements for Structural Concrete. American Concrete Institute, 2019.
  • Gergely, P. and Lutz, L.A. Maximum Crack Width in Reinforced Concrete Flexural Members. ACI SP-20, 1968.
  • Engineering Standards Reference — ACI 318, CivilFlow.
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