EN 1992-1-1 §6.2.5: Shear at the Interface Between Concrete Cast at Different Times

About this Interface Shear Between Concrete Cast at Different Times Calculator

This calculator checks the longitudinal shear at the interface between concrete cast at different times, such as an in situ topping on a precast unit, to EN 1992-1-1 Clause 6.2.5. It works the shear stress at the interface, the resistance with no reinforcement crossing it, the reinforcement ratio needed, the crushing limit and the limits on the normal stress and the angle of the reinforcement.

• Precast and composite floor designer confirm a topping is tied to its planks or beams, and size any ties needed across the joint.
• Bridge engineer check the joint between a precast girder and a cast in place deck, including a reduced cohesion under fatigue or dynamic loads.
• Contractor's engineer assess a construction joint in a wall or beam and decide what surface preparation is needed for it to work without extra steel.

The surface factors sit in an editable table, so they can be matched to a national annex. It is an engineering-grade calculation on CalcTree that keeps every step of Expression (6.25) visible.

More info on Interface Shear Between Concrete Pours

Interface surface factors

Clause 6.2.5(2) gives a cohesion factor c and a friction factor μ for very smooth, smooth, rough and indented surfaces. The page reads them for the selected surface; c is taken as zero where the normal stress across the interface is tensile, and it is halved under fatigue or dynamic loads (6.2.5(5)).

Shear stress and resistance

The design shear stress v_Edi from Expression (6.24) is the share of the longitudinal force carried by the new concrete, β, times the transverse shear, over the lever arm and the interface width. The resistance without reinforcement is the cohesion and friction terms of Expression (6.25), using the design tensile strength of the weaker concrete.

Reinforcement across the interface

Setting v_Edi equal to v_Rdi in Expression (6.25) gives the reinforcement ratio ρ = A_s / A_i, which the page also reports as an area per square metre of interface. The page checks the crushing limit of 0.5 ν f_cd, that the normal stress is below 0.6 f_cd, and that the reinforcement angle is in the range Expression (6.25) allows.

Common Calculation Errors to Avoid

• Claiming a rough surface that is not specified the surface category must match what is actually prepared on site; a rough or indented interface needs to be called up in the drawings.
• Keeping cohesion under tension where the normal stress across the joint is tensile, the cohesion term c f_ctd drops out.
• Using the stronger concrete the tensile and compressive strengths should come from the weaker of the two concretes either side of the interface.
• Forgetting fatigue under fatigue or dynamic loading the cohesion factor is halved, which can turn a joint that needed no ties into one that does.
• Taking β as one by habit β is the ratio of the longitudinal force in the new concrete to the total, so it depends on where the interface sits in the section.

FAQs

What is β in the interface shear check?

β is the ratio of the longitudinal force in the new concrete to the total longitudinal force at the section considered. It scales the transverse shear into the stress that the interface must carry.

When is no reinforcement needed across the joint?

When the design shear stress at the interface is within the cohesion and friction resistance alone, the page reports a pass for the interface without reinforcement and the required ratio is zero.

How is the reinforcement area per square metre found?

The required ratio ρ is the reinforcement area divided by the interface area, so multiplying by one square metre of interface gives the steel that must cross each square metre. You then choose bars and spacing to suit.

Can I change the c and μ values?

Yes. The factors are held in an editable table. For a very smooth surface the code allows a range of c, and the table starts at the lower bound.

What does the page not cover?

Stepped transverse shear diagrams, joints between segments and the anchorage of the reinforcement on both sides of the interface are outside its scope.