What is a Load Transfer Device in Concrete Pavements?

Load Transfer Devices in Concrete Pavements: Complete Technical Reference

Function of Load Transfer Devices

A load transfer device is a structural component installed at or formed within a joint or crack in a Portland cement concrete (PCC) pavement that transfers a portion of an applied wheel load from one slab to the adjacent slab through shear action. When a wheel load approaches a transverse joint, the loaded slab deflects downward. Without a load transfer mechanism, the unloaded slab does not share this deflection, creating a differential vertical displacement (a step) at the joint, a condition known as faulting. The fundamental purpose of any load transfer device is to minimize this differential movement by transmitting vertical shear forces across the joint interface.

The physics of the mechanism operates as follows. As the wheel approaches the joint, the loaded slab bends under the applied load. The load transfer device, whether a dowel bar, aggregate interlock, or keyway, transmits a portion of the vertical shear force to the adjacent unloaded slab. The unloaded slab deflects partially in response, sharing the load and reducing the differential deflection. This load sharing reduces tensile stresses in the loaded slab, preventing corner breaks, and reduces pumping of subbase fines. The FHWA Guide for Load Transfer Restoration identifies three key effects of good load transfer: reduced tensile stresses in concrete (reducing potential for cracking and corner breaks), reduced corner deflections (reducing pumping and faulting development), and reduced differential deflections (reducing reflection crack deterioration in overlays).

The consequences of poor load transfer are severe and progressive. Faulting, a vertical step between adjacent slabs, develops within 5–10 years in undoweled pavements under heavy traffic. Every vehicle crossing the joint produces an impact loading that accelerates slab edge fatigue. Pumping of fines from beneath the slab edge creates voids, leading to loss of support. Corner breaks and mid-panel cracking develop as the structural capacity of the edge-loaded slab is exceeded. In airport pavements, faulting of even 2.5 mm (0.1 in) produces objectionable ride quality for aircraft and can accelerate foreign object debris (FOD) generation from spalled joint edges.

The shear transfer mechanism at a joint depends on the type of device. For dowel bars, shear force is transmitted through the dowel acting in double shear across the joint plane, with bearing stresses developed between the dowel and the surrounding concrete. For aggregate interlock, shear is transmitted by the interlocking of irregular aggregate particles on the fractured crack faces. For keyed joints, the interlocking slab geometry provides direct mechanical shear transfer. For all mechanisms, the stiffness of the load transfer system, characterized by the joint shear stiffness parameter, determines the proportion of load shared between slabs.

Dowel Bars

Dowel bars are the most common and effective load transfer device for transverse joints in concrete pavement. They are smooth, plain round steel bars installed horizontally across transverse contraction joints and construction joints. One half of each bar is embedded in one slab, and the other half extends into the adjacent slab. The smooth surface, combined with a bond breaker applied to one half, allows horizontal joint movement (opening and closing with thermal expansion and contraction cycles) while preventing relative vertical displacement. This differential movement capability, horizontal freedom combined with vertical restraint, is the defining characteristic that distinguishes dowel bars from all other load transfer devices.

Material Standards

Dowel bars must conform to rigorous material standards that govern steel grade, coating requirements, and dimensional tolerances. The primary standards applicable to dowel bar specification include ASTM A1078/A1078M (epoxy-coated steel dowels for concrete pavement), ASTM A615 Grade 60 (plain carbon steel base material, smooth hot-rolled bars), AASHTO M254 (corrosion-resistant coated dowel bars for US DOT projects), and ACPA M254-23 (comprehensive specification covering Type A, B, C, and D dowel categories). For corrosion-resistant applications, ASTM A775 and ASTM A934 govern fusion-bonded epoxy coating, ASTM A1094 covers hot-dip galvanized dowels, ASTM A955 (304, 316, and 316LN grades) covers stainless steel dowels, and ASTM D7957 covers fiber-reinforced polymer (FRP/GFRP) dowels. European applications follow EN 13877-3 for load transfer at concrete pavement joints.

Dimensions by Slab Thickness

Dowel bar diameter, length, and spacing are functions of slab thickness. The general rule established by the AASHO Road Test and validated by decades of field performance is that dowel diameter (D) equals slab thickness (T) divided by 8. FAA AC 150/5320-6G, Table 3-6 provides the authoritative dimensions for airfield pavement dowels.

The edge distance requirement specifies that the outermost dowel must be minimum 150 mm (6 in) and maximum 450 mm (18 in) from the free edge of the pavement. For highway and industrial applications, common industry practice per ACPA and FHWA uses 25 mm diameter dowels at 300 mm spacing for 150 mm slabs, increasing to 38 mm diameter at 300 mm spacing for 250–400 mm slabs.

Alignment Requirements

Dowel bar alignment is critical to joint performance. Misaligned dowels can lock the joint, preventing thermal movement and causing pavement distress. Per the ACPA Dowel Bar Alignment and Location Guide Specification (November 2018), five types of misalignment are defined: horizontal skew (rotation in the horizontal plane), vertical tilt (rotation in the vertical plane), side shift (longitudinal translation along the joint direction), vertical translation (depth deviation from design position), and lateral translation (horizontal offset across the joint width).

The Composite Misalignment (CM) quantifies total angular misalignment as the root-sum-square of horizontal skew and vertical tilt: CM = √(Horizontal Skew² + Vertical Tilt²). The Joint Score (JS) quantifies the cumulative impact of all misaligned dowels in a single joint on the joint’s ability to open and close freely. Each dowel is assigned a weighting factor based on its CM value: CM ≤ 0.6 in (15 mm) receives weight 0 (no penalty), CM between 0.6–0.8 in receives weight 2, CM between 0.8–1.0 in receives weight 4, CM between 1.0–1.5 in receives weight 5, and CM exceeding 1.5 in receives weight 10. The Critical Joint Score (JS_critical) is calculated as JS_critical = C × 15 × (Joint Width in ft / 12), where C is an adjustment factor ranging from 0.6 to 1.5 depending on climate, panel length, and base type.

Shear Transfer Mechanism and Performance

When a wheel load deflects the loaded slab, the dowel bar embedded in both slabs is forced into bending. The bending bar transmits a vertical shear force from the loaded slab to the adjacent slab. Simultaneously, the dowel generates bearing stresses against the concrete on both sides of the joint. The adjacent slab deflects partially in response, sharing the load. Properly designed dowels achieve Load Transfer Efficiency of 85–95% at transverse joints, compared to less than 50% for undoweled joints relying only on aggregate interlock. Pavement service life is extended by a factor of 2–3× under equivalent traffic loading, and joint faulting is virtually eliminated for the design life.

FAA Requirements

Per FAA AC 150/5320-6G, dowels are required in the last three transverse contraction joints from a free edge of pavement. Dowels are required in ALL construction joints (both transverse and longitudinal) unless a thickened or reinforced edge is provided. On stabilized bases, FAA notes there is “little benefit to providing more than minimum of dowels in last three joints from a free edge.” These requirements apply to all airfield concrete pavements serving aircraft operations.

Tie Bars

Tie bars are deformed steel bars used across longitudinal joints in concrete pavement to hold slab faces in close contact. Unlike dowel bars, tie bars prevent joint opening, they do NOT allow horizontal movement. The deformed surface of tie bars bonds with the surrounding concrete, creating an axial restraint that prevents adjacent slabs from separating.

Key Distinction from Dowel Bars

The fundamental differences between dowel bars and tie bars relate to surface finish, primary function, joint movement capability, orientation, and load transfer mechanism. Dowel bars have a smooth surface, are oriented across transverse joints, allow horizontal joint movement through a bond breaker on one half, and transfer load through shear and bending. Tie bars have a deformed surface, are oriented across longitudinal joints, prevent horizontal movement through full bonding with concrete, and do NOT act as load transfer devices themselves, they keep the joint tight so that aggregate interlock can provide load transfer.

FAA Specifications

Per FAA AC 150/5320-6G, Paragraph 3.16.10.1, tie bars must be deformed bars conforming to Item P-501 specifications. For panels 6 inches or less in thickness, No. 4 bars (1/2 inch diameter) are used, 20 inches (510 mm) long, spaced at 36 inches (900 mm) on center. For panels exceeding 6 inches in thickness, No. 5 bars (5/8 inch diameter) are used, 30 inches (762 mm) long, spaced at 30 inches (762 mm) on center. FAA explicitly warns against using tie bars to create continuous tied joints greater than 75 feet (23 m) in length. Tie bars are required at longitudinal contraction joints within 20 feet (6 m) of a free edge for panels 9 inches or less in thickness.

FHWA Guidance

The FHWA Technical Advisory T 5040.30 specifies that tie bars are sized based on slab cross-sectional area, concrete density, coefficient of friction at the slab/subbase interface, and allowable steel stress (typically 27,000–36,000 psi for Grade 40–60 steel). The design equation ensures that the steel cross-section can resist the frictional force generated as the concrete contracts during temperature drops and drying shrinkage. Tie bars do not act as load transfer devices themselves, their function is exclusively to prevent joint opening so that aggregate interlock can provide effective load transfer across the longitudinal joint.

Aggregate Interlock

Aggregate interlock is the natural load transfer mechanism provided by the interlocking of rough, irregular aggregate particles exposed on the fractured face of a saw-cut and cracked joint. When a concrete pavement joint is constructed by saw-cutting a weakened plane, the controlled cracking that occurs below the saw cut produces irregular fracture surfaces. The aggregate particles on one crack face bear against those on the opposite face when a load is applied, transmitting shear forces through the joint.

The mechanism depends on the roughness of the fracture surface. In concrete with larger maximum aggregate size and stronger aggregate particles, the fracture path propagates around the aggregate (debonding at the aggregate-paste interface), creating a rough surface with aggregate particles protruding from the paste matrix. In concrete with weaker aggregate or very high strength paste, the fracture may propagate through the aggregate itself, creating a smoother surface with less interlock potential. The key parameter controlling aggregate interlock effectiveness is the joint opening width, experimental studies documented by FHWA HIPERPAV II show that a joint opening of 0.6 mm (0.024 in) or greater can translate to total loss of load transfer by aggregate interlock.

Degradation Factors

Aggregate interlock degrades over time through multiple interacting mechanisms. Traffic loading produces repeated load cycles that cause micro-crushing of aggregate at contact points, progressively wearing down the interlock surfaces. Thermal cycles, daily and seasonal temperature changes, cause the joint to open and close, gradually widening the joint gap and reducing the contact area between aggregate particles. Moisture infiltration softens the subbase material, increases slab deflections under load, and accelerates the wear of interlock surfaces. Freeze-thaw cycles during spring thaw conditions create saturated subbase conditions that accelerate pumping and erosion. Erodible subbase materials, particularly fine-grained soils, erode under repeated loading, creating voids that increase slab deflections and joint openings.

Applications and Limitations

Aggregate interlock is used primarily in low-volume roads and light-duty pavements where traffic loads are moderate and joint spacing is short enough to keep joint openings below the critical 0.6 mm threshold. For airfield pavements, FAA allows Type D (Dummy) contraction joints (without dowels) and Type B (Hinged) contraction joints for panels less than 9 inches (230 mm) thick. In these applications, aggregate interlock is only effective when joints remain tight, requiring short joint spacing (12.5–15 ft) to minimize thermal movement at each joint.

Keyed Joints

A keyed joint (also called a keyway joint) is a structural joint where one slab has a protruding “key” that fits into a matching groove in the adjacent slab, providing mechanical load transfer through interlocking geometry. The key is typically formed using a prefabricated U-shaped galvanized steel or metal form placed before concrete pouring. Common cross-sectional shapes include trapezoidal, rectangular, and dovetail configurations.

Keyed joints are used in applications with thicker slabs (≥ 250–300 mm / 10–12 in) where construction challenges of thinner keyways are less severe. They are most commonly applied at longitudinal construction joints between paving lanes and for load transfer between adjacent concrete sections where dowel bars are less practical. Keyed joints can be used in conjunction with dowels for additional support.

However, FAA AC 150/5320-6G does not include keyed joints as a standard joint type in Table 3-5. FAA standard joints are Types A through F (thickened edge, hinged, doweled, dummy, doweled construction, and butt construction). Keyed joints are generally not recommended for airfield pavements serving aircraft exceeding 30,000 lbs. Documented problems include cracking and damage due to inability to accommodate shrinkage and temperature changes, joint faulting creating height differences between slabs, potential misalignment during construction, and limited durability compared to properly doweled joints.

Load Transfer Efficiency (LTE) Definition and Measurement

Load Transfer Efficiency (LTE) is the quantitative measure of a joint’s ability to transfer load from a loaded slab to an adjacent unloaded slab. LTE is expressed as a percentage, where 0% indicates no load transfer (free edge condition) and 100% indicates perfect transfer (equal deflections on both sides of the joint). LTE is the single most important parameter for evaluating the condition of existing concrete pavement joints and determining the need for load transfer restoration.

Formulas

The deflection-based LTE formula (most widely used, per FHWA-RD-02-088) is LTE = (du / dl) × 100%, where dl is the maximum deflection at the joint of the loaded slab and du is the corresponding deflection of the unloaded slab. An alternate deflection formula computes LTE* = [2du / (dl + du)] × 100%. The two deflection formulas are mathematically related: LTE = 2 / [1 + (1/LTE*)] and LTE* = 2 / [1 + (1/LTE)].

A stress-based LTE formulation also exists: LTE_σ = (σu / σl) × 100%, where σl and σu are the stresses on the loaded and unloaded sides respectively. Critically, there is no one-to-one relationship between deflection-based and stress-based LTE. Research documented in the FHWA Guide for Load Transfer Restoration demonstrates that a deflection LTE of 60% may correspond to a stress LTE of only approximately 22%. This means the stress reduction benefit from load transfer is much smaller than the deflection reduction would suggest.

Acceptable Values

LTE by Falling Weight Deflectometer (FWD)

The Falling Weight Deflectometer (FWD) is the standard instrument for measuring load transfer efficiency in concrete pavements. Per ASTM D4694 (deflection measuring equipment) and ASTM D4695 (test method), the FWD system consists of a weight dropped from a specified height onto a load plate (producing an impulse force of typically 40–120 kN / 9,000–27,000 lbf), a load cell measuring the applied force, and 7–8 geophone sensors measuring pavement surface deflections at precise distances from the load plate center.

Sensor Configuration

Per FAA AC 150/5320-6G, Appendix C (Table C-2), the recommended sensor configuration for concrete pavement testing places Sensor 1 at the load plate center or edge, Sensor 2 at 12 inches (305 mm), Sensor 3 at 24 inches (610 mm), Sensor 4 at 36 inches (914 mm), Sensor 5 at 48 inches (1,219 mm), Sensor 6 at 60 inches (1,524 mm), and Sensor 7 at 72 inches (1,829 mm) from the load plate center. For joint load transfer testing, the load plate is positioned at the joint edge with sensors placed on both sides of the joint, the load is applied on the approach side of the joint, and the sensor on the loaded side measures dl while the sensor on the unloaded side measures du.

For rigid pavement testing, FAA specifies testing at three critical locations per panel: the panel center (to determine concrete stiffness/modulus), the joint (to determine LTE), and the panel corner (to determine support conditions and void detection).

Temperature Effects

LTE varies significantly throughout the day as joints close in heat (producing higher LTE) and open in cold (producing lower LTE). Best testing conditions occur when ambient temperature is ≤ 21°C (70°F) and when slabs are flat (not curled). Nighttime testing should be avoided in summer and fall when the slab top is cooler than the bottom, causing upward curl. Optimal testing windows are early morning and late afternoon when thermal gradients through the slab are minimized.

Interpretation of Results

Some agencies apply a bending correction factor to account for the fact that the ratio of load-plate sensor deflection to the nearest adjacent sensor is not 1.0 even at the slab interior: Bending Factor = D_load_center / D_adjacent_sensor (from interior test) and Corrected LTE = LTE(measured) × Bending Factor.

FHWA faulting and condition thresholds from the LTR Guide specify: deflection LTE requiring retrofit is ≤ 60%, faulting requiring attention is > 2.5 mm (0.1 in), differential deflection requiring attention is ≥ 250 μm (10 mils), and project-level total faulting trigger is ≥ 500 μm.

LTE Deterioration

Load transfer efficiency deteriorates over time through multiple interacting mechanisms that progressively reduce the joint’s ability to share loads between adjacent slabs.

Pumping

Pumping is the ejection of water and suspended fine particles from beneath the concrete slab through joints and cracks under traffic loading. The mechanism begins when free water accumulates in the pavement structure from precipitation or poor drainage. When a heavy load passes over a joint, the loaded slab deflects downward, forcing the trapped water out at high velocity through the joint opening. This water carries suspended fine particles from the subbase and subgrade. The consequences are progressive: loss of subbase support beneath the slab, void formation at the slab corner, accelerated faulting as fines deposit under the approach slab raising it relative to the departure slab, corner breaks from loss of support, and surface staining with visible mud pumping at joints.

Subbase materials that resist shear stress of 25–50 Pa do not erode for the pavement’s design life. Erodible subbase materials, particularly fine-grained soils with high silt content, are most susceptible to pumping-induced failure. The presence of free water combined with heavy traffic loading and poor LTE creates a positive feedback loop: poor LTE increases slab deflections, which increases pumping, which further reduces LTE.

Erosion and Faulting Progression

Erosion of subbase material under repeated loading creates voids under the slab, increasing slab deflections, corner stresses, and accelerating fatigue cracking. Faulting progression follows a characteristic pattern documented in FHWA HIPERPAV II: initially growing linearly with load applications, then growing nonlinearly as deterioration accelerates. Objectionable faulting for ride quality is defined as exceeding 3.3 mm (0.13 in) .

Dowel Bar Corrosion

When the protective coating on dowel bars is compromised, through coating defects, improper handling during installation, or inadequate concrete cover, chlorides from deicing salts or marine environments penetrate to the steel surface, initiating corrosion. Corrosion products (rust) occupy a larger volume than the original steel, creating tensile stresses in the surrounding concrete that can cause cracking and spalling along the joint. Loss of effective dowel cross-section reduces shear capacity, increased bearing stress on concrete creates dowel looseness, and in severe cases joint movement becomes locked by corrosion buildup.

Other Deterioration Mechanisms

Dowel looseness develops as repeated bearing stresses enlarge the hole around the dowel, reducing shear transfer efficiency. Loss of slab support from erosion voids increases deflections at the joint. Freeze-thaw damage to the concrete around the joint reduces dowel bearing capacity. Aggregate degradation from crushing and wearing of aggregate at interlock surfaces progressively reduces shear transfer capacity. Joint sealant failure allows water and incompressibles into the joint, accelerating all deterioration mechanisms.

Inspection for Load Transfer Condition

Inspection of load transfer condition combines visual inspection, non-destructive testing (NDT), and direct sampling to evaluate joint performance and determine the need for rehabilitation. Per FAA AC 150/5320-6G and ICAO Annex 14, a comprehensive inspection program addresses all mechanisms of load transfer deterioration.

Visual Inspection

Visual inspection identifies visible indicators of load transfer problems. Faulting, the vertical step between adjacent slabs, is measured with a straightedge or faultmeter. Spalling, cracking and chipping at joint edges, indicates stress concentrations from poor load transfer or dowel corrosion. Corner breaks, diagonal cracks emanating from the slab corner, indicate loss of support from pumping and erosion. Pumping stains at joints provide direct evidence of active subbase erosion. Joint sealant condition assessment identifies adhesive failure, cohesive failure, or intrusion of incompressibles that can lock the joint.

ACPA and PCI concrete pavement condition surveys categorize severity by faulting magnitude: Low severity (barely noticeable, less than 3 mm faulting), Medium severity (3–10 mm faulting with visible spalling), and High severity (more than 10 mm faulting with extensive spalling, exposed dowels, and structural distress).

FWD / HWD Testing

Per FAA AC 150/5320-6G Appendix C, FWD testing computes LTE from loaded and unloaded side deflections at each joint, computes Impulse Stiffness Modulus (ISM) = load / peak deflection, and calculates ISM_ratio (center/corner or center/joint) for concrete durability assessment: ISM_ratio < 1.5 indicates good condition, ISM_ratio 1.5–3.0 indicates questionable durability, and ISM_ratio > 3.0 indicates poor durability at the corner or joint.

Ground Penetrating Radar (GPR)

Per FAA AC 150/5320-6G Appendix E and ASTM D6432, GPR uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) to measure layer thickness, locate subsurface moisture or voids, detect delamination, and locate dowel bar positions. GPR is limited in effectiveness when used in fine-grained soils (silt and clay) due to signal attenuation.

Core Sampling and Dowel Imaging

Core sampling provides direct measurement of slab thickness, reveals concrete condition at depth (durability issues, ASR, freeze-thaw damage), and verifies dowel bar position and coating condition. A core that drops out during extraction indicates a void beneath the slab and loss of support. Magnetic Imaging Tomography (MIT) per ASTM E3013/E3013M uses magnetic pulse induction (MPI) equipment for non-destructive rapid testing of dowel bar alignment, measuring horizontal skew, vertical tilt, side shift, depth, and lateral position, and computing Joint Score (JS) for comparison with JS_critical values.

Acceptance Criteria Per FAA and ICAO

FAA AC 150/5320-6G specifies that new construction dowel alignment must conform to approved plans. For existing pavements, LTE ≥ 75% is generally acceptable, LTE ≤ 60% indicates rehabilitation (load transfer restoration) is recommended, and faulting > 2.5 mm (0.1 in) means load transfer restoration should be considered. ICAO Doc 9157, Part 3 requires reporting of pavement strength via the ACR-PCR method, with technical evaluation including NDT (FWD/HWD), core sampling, and visual surveys, and mandates re-evaluation of pavement condition if traffic changes significantly.

Load Transfer and Pavement Life

The relationship between load transfer efficiency and pavement structural capacity is direct and quantifiable. Poor LTE increases the effective edge stress in the loaded slab, accelerating fatigue damage and reducing remaining life. Free-edge stresses can be 25–40% higher than interior stresses. The relationship between LTE and edge stress follows a predictable pattern: with LTE at 100% (perfect transfer), stress approximates interior condition, with LTE at 75%, stress is approximately 15–20% above interior, with LTE at 50%, stress is approximately 30% above interior, and with LTE at 0% (free edge condition), stress exceeds interior by 40% or more.

Per the FAA FAARFIELD design methodology, edge stress is reduced by 25% to account for assumed joint load transfer in design. When evaluating existing pavements, edge stresses for evaluation are back-calculated by dividing the computed edge stress by 0.75.

Life Extension with Dowel Bar Retrofit

The FHWA Guide for Load Transfer Restoration documents that properly installed retrofitted dowels restore LTE to 70–90% from ≤ 60%, produce large improvement in LTE with decreases in maximum vertical deflections (confirmed by Heavy Vehicle Simulator testing), extend service life of structurally sound pavements by 10–15+ years, and are cost-effective when the pavement has significant remaining structural life but poor load transfer. Dowel bar retrofit is NOT recommended for pavements with D-cracking, alkali-silica reaction (ASR), or extensive slab cracking.

Joint Spacing and Pavement Life

FAA AC 150/5320-6G specifies maximum joint spacing based on slab thickness and base type. For slabs without stabilized base: ≤ 6 in (152 mm) thickness allows 12.5 ft (3.8 m) spacing, 6.5–9 in (165–229 mm) allows 15 ft (4.6 m), > 9 in (> 229 mm) allows 20 ft (6.1 m). For slabs with stabilized base: 8–10 in (203–254 mm) allows 12.5 ft (3.8 m), 10.5–13 in (267–330 mm) allows 15 ft (4.6 m), ≥ 16 in (406 mm) allows 20 ft (6.1 m). The panel aspect ratio must not exceed width-to-length ratio of 1:1.25 per FAA. FHWA specifies maximum panel length of 15 ft (4.6 m) for highways, which can be exceeded with technical analysis. Shorter joint spacing provides better performance in freeze-thaw environments and reduces joint openings, improving aggregate interlock longevity and reducing the demand on load transfer devices.