The problem domain
\(\varOmega =\varOmega _ c \cup \varGamma _\text {int}\) comprises the concrete (
\(\varOmega _\text {c}\)) and reinforcement (
\(\varGamma _\text {int}\)) parts. It is assumed, that the reinforcement does not cross the external boundary
\(\varGamma _\text {ext}:= \varGamma _\text {u} \cup \varGamma _\text {t}\). For the 2D problem, displacement fields pertinent to concrete and steel are denoted
\(\varvec{u}_ c \) and
\(\varvec{u}_\text {s}\), respectively. Along
\(\varGamma _\text {int}\) it is possible to split
\(\varvec{u}_ s \) and
\(\varvec{u}_ c \) into components that are parallel and perpendicular to
\(\varGamma _\text {int}\), i.e.,
$$\begin{aligned} \varvec{u}_ s&= u_\text {s,l} \varvec{e}_{\text {l}} + u_{\text {s,}\perp } \varvec{e}_\perp , \end{aligned}$$
(1)
$$\begin{aligned} \varvec{u}_ c&= u_\text {c,l} \varvec{e}_{\text {l}} + u_{\text {c,}\perp } \varvec{e}_\perp . \end{aligned}$$
(2)
It is noteworthy that the directions
\(\varvec{e}_{l}\) and
\(\varvec{e}_{\perp }\) need not be constant throughout the whole structure, but may be a function of the position. Similarly, as the unit vectors
\(\varvec{e}_{l}\) and
\(\varvec{e}_{\perp }\) are associated with each bar (and can vary along it), the orthogonality of the reinforcement bars is not required. Taking
\(t_ c \) to be the thickness of the structure,
\(\varvec{b}\) is the body force and
\(\varvec{\sigma }_ c \) is the stress in the concrete phase, the equilibrium can be stated in strong form as follows
$$\begin{aligned} \begin{aligned} - \left( t_ c \varvec{\sigma }_ c \right) \cdot \varvec{\nabla }&= t_ c \varvec{b} \quad \text {in } \varOmega _ c , \\ \varvec{u} = \varvec{u}_\text {p} \quad \text {on } \varGamma _\text {u}, \varvec{t}&:= \varvec{\sigma }_ c \cdot \varvec{n} = \varvec{\hat{t}} \quad \text {on } \varGamma _\text {t}, \end{aligned} \end{aligned}$$
(3)
For steel, it is assumed that the reinforcement can sustain both normal force and bending moments, i.e., it can be subjected to both longitudinal and transverse loads. In the former case, we have the normal force
\(N_ s \) linked to the bond stress
\(t_\varGamma \) (distributed around the circumference of the bar
\(S_ s \)). The latter mechanism couples the bending moment
\(M_ s \) and the transverse distributed load
\(\lambda \). Therefore, for each reinforcing bar equilibrium can be expressed as
$$\begin{aligned} \begin{aligned} -\dfrac{\partial N_ s }{\partial l} + S_\text {s} t_{\varGamma }&= 0 \quad \text {in } \varGamma _\text {int}, \\ -\dfrac{\partial ^2 M_\text {s}}{\partial l^2} + \lambda&= 0 \quad \text {in } \varGamma _\text {int}, \\ N_ s = 0, \ T_ s = 0 , \ M_ s&= 0 \quad \text {on } \partial \varGamma _\text {int} . \end{aligned} \end{aligned}$$
(4)
Next, we consider the steel/concrete interface as presented in Fig.
2, where the reinforcement bar was fictitiously protruded out of the concrete. Summing all the forces acting on the concrete along
\(\varGamma _\text {int}\), the equilibrium condition for the interface can be found to be
$$\begin{aligned} \left[ t_ c ^+ \varvec{\sigma }_ c ^+ \right] \cdot \varvec{e}_\perp -\left[ t_ c ^- \varvec{\sigma }_ c ^- \right] \cdot \varvec{e}_\perp + \lambda \varvec{e}_\perp + S_\text {s} t_{\varGamma } \varvec{e}_{\text {l}} = \mathbf {0}. \end{aligned}$$
(5)
Lastly, it is assumed that there is no relative motion between the steel and the concrete in the transverse direction (no normal displacement jump), i.e., we introduce the following interface constraint:
$$\begin{aligned} u_{\text {s,}\perp } - u_{\text {c,}\perp } = 0. \end{aligned}$$
(6)
By adopting this formulation we assume the contact deformations in the transverse direction to be negligible. To maintain generality, only implicit (algorithmic) definitions of the constitutive relations are considered. Hence, evolving internal variables are considered, but omitted in the abstract notation. Thus, for both the concrete and the steel we consider the implicit relations
$$\begin{aligned} \varvec{\sigma }_ c&= \varvec{\sigma }_ c \left( \varvec{\varepsilon }\left[ \varvec{u}_ c \right] \right) , \end{aligned}$$
(7)
$$\begin{aligned} N_ s&= N_ s \left( \dfrac{\partial u_\text {s,l}}{\partial l} , \dfrac{\partial ^2 u_{\text {s,}\perp }}{\partial l^2} \right) , \end{aligned}$$
(8)
$$\begin{aligned} M_ s&= M_ s \left( \dfrac{\partial u_\text {s,l}}{\partial l} , \dfrac{\partial ^2 u_{\text {s,}\perp }}{\partial l^2} \right) , \end{aligned}$$
(9)
$$\begin{aligned} t_{\varGamma }&= t_{\varGamma } \left( u_\text {s,l} - u_\text {c,l} \right) = t_{\varGamma } \left( s \right) , \end{aligned}$$
(10)
where
\(\varvec{\varepsilon }=\left[ \varvec{u}_ c \mathbin {\otimes }\varvec{\nabla }\right] ^{sym}\) denotes the strain and
\(s = \left[ u_\text {s,l} - u_\text {c,l} \right] \) is the reinforcement slip.
In order to state the variational format, (
3), (
4), and (
6) must be recast into weak forms. Multiplying each equation by a suitable test function, integrating over the domain (and employing (
5) in (
3) together with neglecting body forces), we arrive at the definition of the fully-resolved problem: Find
\(\varvec{u}_ c , u_\text {s,l}, u_{\text {s,}\perp }, \lambda \in \mathbbm {U}_ c \times \mathbbm {U}_\text {s,l} \times \mathbbm {U}_{\text {s,}\perp } \times \mathbbm {L}\) such that
$$\begin{aligned}&a_ c \left( \varvec{u}_ c ; \delta \varvec{u}_ c \right) - b \left( u_{\text {s,l}} - \varvec{e}_{\text {l}} \cdot \varvec{u}_ c ; \varvec{e}_{\text {l}} \cdot \delta \varvec{u}_ c \right) \nonumber \\&\quad -c \left( \lambda ; \varvec{e}_\perp \cdot \delta \varvec{u}_ c \right) = l_ c \left( \delta \varvec{u}_ c \right) \qquad \forall \ \delta \varvec{u}_ c \in \mathbbm {U}_ c ^0, \end{aligned}$$
(11)
$$\begin{aligned}&a_\text {l} \left( u_\text {s,l}, u_{\text {s,}\perp } ; \delta u_\text {s,l} \right) + b \left( \right. u_{\text {s,l}} - \varvec{e}_{\text {l}} \cdot \varvec{u}_ c ; \delta u_\text {s,l} \left. \right) = 0 \nonumber \\&\quad \forall \ \delta u_\text {s,l} \in \mathbbm {U}_\text {s,l}, \end{aligned}$$
(12)
$$\begin{aligned}&a_\text {b} \left( u_\text {s,l}, u_{\text {s,}\perp } ; \delta u_{\text {s,}\perp } \right) + c \left( \lambda ; \delta u_{\text {s,}\perp } \right) = 0 \nonumber \\&\quad \forall \ \delta u_{\text {s,}\perp } \in \mathbbm {U}_{\text {s,}\perp }, \end{aligned}$$
(13)
$$\begin{aligned}&c \left( \delta \lambda ; u_{\text {s,}\perp } - \varvec{e}_\perp \cdot \varvec{u}_ c \right) = 0 \qquad \qquad \ \forall \ \delta \lambda \in \mathbbm {L}, \end{aligned}$$
(14)
for suitable trial sets
\(\mathbbm {U}_ c , \mathbbm {U_\text {s,l}}, \mathbbm {U_{\text {s,}\perp }}, \mathbbm {L}\) defined as
$$\begin{aligned} \mathbbm {U}_ c= & {} \left\{ \phantom {\dfrac{1}{2}} \right. \varvec{u}(\varvec{x}): \varOmega \mapsto \mathbbm {R}^{2}, \left. \right. \nonumber \\&\int _{\varOmega _\text {c}} \varvec{u}^2 + \left[ \varvec{u} \mathbin {\otimes }\varvec{\nabla }\right] ^2 \mathrm {d}\varOmega < \infty , \ \varvec{u}=\varvec{u}_\text {p} \ \text {on } \varGamma _\text {p} \left. \phantom {\dfrac{1}{2}} \right\} ,\nonumber \\ \end{aligned}$$
(15)
$$\begin{aligned} \mathbbm {U}_\text {s,l}= & {} \left\{ \phantom {\dfrac{1}{2}} \right. v(l): \varGamma _\text {int} \mapsto \mathbbm {R}, \int _{\varGamma _\text {int}} v^2 + \left( \dfrac{\partial v}{\partial l} \right) ^2 \mathrm {d}\varGamma < \infty \left. \phantom {\dfrac{1}{2}} \right\} ,\nonumber \\ \end{aligned}$$
(16)
$$\begin{aligned} \mathbbm {U}_{\text {s,}\perp }= & {} \left\{ \phantom {\dfrac{1}{2}} \right. v(l): \varGamma _\text {int} \mapsto \mathbbm {R}, \left. \right. \nonumber \\&\int _{\varGamma _\text {int}} v^2 + \left( \dfrac{\partial v}{\partial l} \right) ^2 + \left( \dfrac{\partial ^2 v}{\partial l^2} \right) ^2 \mathrm {d}\varGamma < \infty \left. \phantom {\dfrac{1}{2}} \right\} , \end{aligned}$$
(17)
$$\begin{aligned} \mathbbm {L}= & {} \left\{ \lambda (l): \varGamma _\text {int} \mapsto \mathbbm {R}, \int _{\varGamma _\text {int}} \lambda ^2\ \mathrm {d}\varGamma < \infty \right\} , \end{aligned}$$
(18)
and the test space
\(\mathbbm {U}_ c ^0\), defined as
$$\begin{aligned} \begin{aligned} \mathbbm {U}_ c ^0 = \left\{ \phantom {\dfrac{1}{2}} \right.&\varvec{u}(\varvec{x}): \varOmega \mapsto \mathbbm {R}^2, \left. \right. \\&\int _{\varOmega _\text {c}} \varvec{u}^2 + \left[ \varvec{u} \mathbin {\otimes }\varvec{\nabla }\right] ^2 \mathrm {d}\varOmega < \infty , \varvec{u}=\varvec{0} \ \text {on } \varGamma _\text {p} \left. \phantom {\dfrac{1}{2}} \right\} . \end{aligned} \end{aligned}$$
(19)
The coupling terms indicated in the system (
11)–(
14) are defined as:
$$\begin{aligned} b \left( v ; w \right)&:= \int _{\varGamma _\text {int}} S_\text {s} t_\varGamma (v) w \ \mathrm {d} \varGamma , \end{aligned}$$
(20)
$$\begin{aligned} c \left( \lambda ; v \right)&:= \int _{\varGamma _\text {int}} \lambda v \ \mathrm {d}\varGamma . \end{aligned}$$
(21)
The following forms are introduced pertinent to
(i)
Concrete:
$$\begin{aligned} a_ c \left( \varvec{u}_ c ; \delta \varvec{u}_ c \right)&:= \int _{\varOmega _\text {c}} t_ c \varvec{\sigma }_ c \left( \varvec{\varepsilon } \left[ \varvec{u}_ c \right] \right) \mathbin {:}\left[ \delta \varvec{u}_ c \mathbin {\otimes }\varvec{\nabla }\right] \ \mathrm {d}\varOmega , \end{aligned}$$
(22)
$$\begin{aligned} l_ c \left( \delta \varvec{u}_ c \right)&:= \int _{\varGamma _\text {ext}} t_ c \hat{\varvec{t}} \cdot \delta \varvec{u}_ c \ \mathrm {d}\varGamma . \end{aligned}$$
(23)
(ii)
Bar action of the rebars:
$$\begin{aligned} a_\text {l} \left( u_\text {s,l}, u_{\text {s,}\perp } ; \delta u_\text {s,l} \right)&:= \int _{\varGamma _\text {int}} N_ s \dfrac{\partial \delta u_\text {s,l}}{\partial l} \ \mathrm {d}\varGamma . \end{aligned}$$
(24)
(iii)
Beam action of the rebars:
$$\begin{aligned} a_\text {b} \left( u_\text {s,l}, u_{\text {s,}\perp } ; \delta u_{\text {s,}\perp } \right)&:= - \int _{\varGamma _\text {int}} M_ s \dfrac{\partial ^2 \delta u_{\text {s,}\perp }}{\partial l^2} \ \mathrm {d}\varGamma . \end{aligned}$$
(25)