Deductive verification of concurrent programs under weak memory has thus far been limited to simple programs over a monolithic state space. For scalabiility, we also require modular techniques with verifiable library abstractions. We address this challenge in the context of RC11 RAR, a subset of the C11 memory model that admits relaxed and release-acquire accesses, but disallows, so-called, load-buffering cycles. We develop a simple framework for specifying abstract objects that precisely characterises the observability guarantees of abstract method calls. Our framework is integrated with an operational semantics that enables verification of client programs that execute abstract method calls from a library it uses. We implement such abstractions in RC11 RAR by developing a (contextual) refinement framework for abstract objects. Our framework has been mechanised in Isabelle/HOL.