Title:
High Strain Rate Properties of CFRP Sheets Surface Bonded to Concrete
Author(s):
Jonathan Harman, Emmanuella O. Atunbi, and Alan Lloyd
Publication:
Symposium Paper
Volume:
347
Issue:
Appears on pages(s):
21-38
Keywords:
FRP sheet, CFRP, surface bonded; high strain rate
DOI:
10.14359/51732656
Date:
3/1/2021
Abstract:
Many common building materials, such as concrete and steel, are understood to experience a change in
apparent material properties under high strain rates. This effect is often incorporated into impact and blast design by
using dynamic increase factors (DIFs) that modify properties of the material such as strength and stiffness when
subjected to high strain rates. There is currently limited guidance on dynamic properties of fiber reinforced polymer
(FRP) sheets bonded to concrete. Since FRP is a common retrofit material for blast and impact load vulnerable
structures, it is important to have a full understanding on the behaviour of the FRP material and of the composite
action between the FRP sheet and the substrate it is bonded to. Important parameters for blast and impact resistant
design of reinforced concrete structures retrofitted with surface bonded FRP include dynamic measures of debonding
strain, development length, and bond stress. This paper presents the results of an experimental program measuring the
dynamic properties of carbon fiber reinforced polymer (CFRP) sheets bonded to concrete under impact induced high
strain rates.
A series of rectangular concrete prisms were cast and fitted with surface bonded CFRP sheets to facilitate pull-out
shear tests that directly measure the FRP to concrete bond. The bonded length of the CFRP sheet was variable with
three different lengths explored. A series of static tests have been conducted to measure the strain fields on the FRP
sheets under load up to failure. These strain fields, which were measured with digital image correlation techniques,
were used to determine development length, bond stress, and ultimate strain of the FRP sheet prior to debonding. A
companion set of prisms have also been cast and will be tested under impact loading to explore the same properties at
high strain rates of around 1 s-1. Initial test results indicate a potential increase in both ultimate strain and bond stress,
and a decrease in development length under high strain rates. The results of the larger study will be compiled and,
when compared with the static companion set, be used to propose DIFs for FRP sheets bonded to concrete for use in
design in high strain rate applications.
However, the main constitutive phases of SHCC, i.e. matrix, fibers and interphase between them, are highly rate
sensitive. Depending on the SHCC composition, the increase in loading rates can negatively alter the balanced
micromechanical interactions, leading to a pronounced reduction in strain capacity. Thus, there is need for a detailed
investigation of the strain rate sensitivity of SHCC at different levels of observation for enabling a targeted material
design with respect to high loading rates.
The crack opening behavior is an essential material parameter for SHCC, since it defines to a large extent the tensile
properties of the composite. In the paper at hand, the rate effects on the crack opening and fracture behavior of SHCC
are analyzed based on quasi-static and impact tensile tests on notched specimens made of three different types of
SHCC. Two SHCC consisted of a normal-strength cementitious matrix and were reinforced with polyvinyl-alcohol
(PVA) and ultra-high molecular weight polyethylene (UHMWPE) fibers, respectively. The third type consisted of a
high-strength cementitious matrix and UHMWPE fibers. The dynamic tests were performed in a split Hopkinson
tension bar and enabled an accurate description of the crack opening behavior in terms of force-displacement
relationships at displacement rates of up to 6 m/s (19.7 ft/s).