The Importance of Expansion and Control Joints in Infrastructure Preservation
Expansion joint header repairs are one of the more demanding concrete repair applications on bridge and parking structure programs. The header material bonds directly to a Portland cement concrete deck, bears constant vehicular impact and abrasion loads, and must accommodate the structure’s thermal movement without delaminating.
Get the material wrong and you cycle back – cracking, spalling, water intrusion into the substructure, and another mobilization.
What Expansion Joints Do and Why Headers Fail
Expansion joints allow a bridge or structure to move with temperature change. The header, or nosing, is the concrete edge that frames each side of the joint gap. It takes the direct impact of traffic crossing the joint and must stay bonded to the deck while the joint opens and closes with thermal cycling.
Header failure modes include:
- Cracking and spalling from impact and abrasion
- Delamination at the bond line between header and deck
- Water and chloride intrusion through failed joint seals or cracked nosing into the substructure
- Corrosion of the substructure driven by that intrusion
The header material, the prep sequence, and the joint seal installation all affect how long the repair holds.
MPC vs. Elastomeric Concrete for Expansion Joint Headers
Both MPC and elastomeric concrete are specified for expansion joint header repairs. Material cost per cubic foot is comparable. The differences show up in prep requirements, temperature performance, set time, and application constraints.
Surface Preparation: MPC concrete does not require sandblasting in most header applications. Standard block-out preparation — removing unsound concrete, oil, and contaminants — is sufficient. Elastomeric concrete requires grit blasting of the substrate and a primer coat before placement, adding steps and time to the closure window.
Mixing MPC is a two-component mix: liquid activator and dry mix, combined in approximately 45 seconds. Elastomeric concrete is a [3]-component mix requiring 1 to 5 minutes of mix time depending on temperature, plus primer application and cure time.
Set Time and Temperature Performance At 70°F, Phoscrete MPC reaches traffic-ready strength in 30 minutes to [1] hour. At 50°F, set time extends to [1] to [2] hours. In cold temperatures below 50°F, elastomeric concrete typically requires additional heating or blanketing steps to achieve a cure. MPC does not.
At high temperatures above 70°F, Phoscrete slow-set admixture and cooled activator extend working time. Verify applicable admixture use against the TDG for your placement temperature.
Pour Depth MPC can be placed to full depth in a single lift. Elastomeric concrete has a maximum lift of 3 inches, requiring multiple lifts for deeper headers.
Cantilever Applications When expansion joint spacing requires the header to cantilever to close the gap, elastomeric concretes are not recommended. MPC headers in cantilever applications should be reinforced with steel bars.
Sloped Installations: Self-leveling elastomeric concrete can be problematic on sloped installations. MPC does not have this limitation.
Cleanup MPC tools clean with water. Elastomeric concrete tools require acetone or isopropyl alcohol.
Lane Closure Efficiency
Because Phoscrete MPC sets fast, joint seal installation can be completed in the same lane closure as the header pour – eliminating the need for a second mobilization. For a bridge deck with multiple joints, that adds up to significant closure savings per project.
FHWA recommends 2,500 psi compressive strength (ASTM C-109 or C-39) before returning lanes to vehicular traffic. Verify that your material and conditions meet this threshold before reopening.
Control Joints: A Different Application
Control joints manage load stress in large concrete placements and prevent uncontrolled cracking. Proper placement and sealing protect reinforcing steel from corrosion and the concrete from spalling. While the failure modes overlap with expansion joints – water intrusion, chloride penetration, spalling – the repair scope and material selection differ. Consult your project TDG for applicable material requirements.
Request a Spec Review
To review MPC performance data for your expansion joint header repair program or discuss specific project conditions, contact a Phoscrete sales engineer via live chat, call (561) 420-0595, or submit the form below.