Experimental, Analytical, and Numerical Assessments for the Controversial Elastic Stiffness Enhancement of CFRP-Strengthened Timber Beams

Abstract

The strengthening of timber beams with carbon-fibre-reinforced polymers (CFRP) has been widely used in the last decades to enhance the behaviour of historical or new timber structures, usually for bending. While considerable improvement in capacity and ductility is typically achieved, the increase in stiffness was, in many cases, well short of analytical expectations, which tend to overestimate stiffness. This study addresses the problem by investigating the underlying mechanical behaviour using experimental, analytical, and numerical tools on a sample of Norway spruce (Picea abies) beams reinforced with carbon-fibre fabric. In the experimental program, each beam is tested for bending with and without CFRP reinforcement in order to determine specimen-specific stiffness increase on an individual basis. The reinforcement yielded an increase of 27% in capacity, 53% in ultimate displacement, and 133% in compliance, verifying its efficiency. Axial compression tests on an independent sample are also performed to verify modulus of elasticity in compression. Numerical computations based on a beam model and a three-dimensional finite element model are performed with the introduction of separate moduli of elasticity for tension and compression in timber. Inverse computation using the experimental load–deflection curves yielded the moduli and the compression yield stress of timber to provide the best match between tests and simulations. The mean difference of only 6% in stiffness between FEM and the tests is obtained. The dominance of normal stresses in the longitudinal direction is found, in correspondence with the experimentally observed tensile failure of timber (apart from a few defected specimens). Compression yield stresses are within 7% (beam model) and 2% (FEM) error compared with the control axial tests. The differences between FE simulations and tests in ultimate load and compliance are within 1%. This study concludes that the application of CFRP in the composite beams enables the determination of timber material properties opposed to pure timber beams without reinforcement, and the adoption of separate moduli of elasticity for tension and compression leads to adequate modelling of reinforced timber beams.

Comparing the efficiency of strengthening timber beams reinforced with carbon composite rods and plates

“…In [8], the authors confirm an increase in stiffness by 13 % to 30 % for timber beams (45×45×1000 mm) reinforced with fabric-type carbon fibers on the lower face of the beam with a percentage of reinforcement of 0.268 % and 0.307 %, respectively. The increase in bearing capacity was recorded at the level of 27 %.…”

“…Research results showed an increase in the bearing capacity of reinforced beams from 28 % to 41 %. Works [8,9] focused on the research of fabric-type CFRP, which is rarely used in beams with a significant cross-section.…”

“…Studies [6][7][8][9][10][11][12][13][14][15][16][17][18] show that the reinforcement with CFRP materials, in general, increases the strength and stiffness of timber beams. However, in the above studies, the strengthening of medium-sized timber elements made of solid wood reinforced with plates and rods is not fully disclosed, so conducting such a study is expedient.…”

Improving the Flexural Response of Timber Beams Using Externally Bonded Carbon Fiber-Reinforced Polymer (CFRP) Sheets

“…When subjected to bending loads, timber beams are relatively weak on the bottom side where high tensile stresses occur. Given this, FRP sheets are often adhesively bonded to the soffit of the timber beams to increase flexural stiffness, strength, ductility, and energy absorption as well as prevent premature failure of timber beams [11][12][13][14][15]. In this case, the FRP-timber bonded structures can exhibit various failure modes and damage mechanisms, which inherently depend upon several parameters, such as the timber strength, FRP sheets strength, bulk adhesive strength, adhesive-timber interface strength, and adhesive-FRP interface strength.…”