Repair of fractured spruce beams with bonded-in reinforcements

Parvez Alam Postdoctoral researcher

Department of Chemical Engineering, Åbo Akademi University Turku, Finland.

Martin P. Ansell Senior Lecturer

Department of Mechanical Engineering, University of Bath Bath, United Kingdom.

David Smedley

Technical Manager Rotafix Ltd.

Abercraf, Swansea, United Kingdom. Summary Constructional timber is susceptible to fracture damage. This paper researches methods and

materials used for repairing pre-fractured class C16 spruce beams loaded in flexure. Strength and

stiffness values for repaired beams are referenced against the original beam strength and stiffness

values. Repair configurations comprise mechanical reinforcement on both tensile and compressive

faces of the beam. Reinforcement materials include mild steel, CFRP and GFRP. Stiffer

reinforcements have a greater effect on improving the stiffness qualities of repaired beams. Repair

strengths rely more on the crack restraining capabilities of the reinforcement and less on their

unique mechanical properties. Steel reinforcements are superior to CFRP and GFRP with respect to

improving the composite stiffness and strength. When compared using the transformed section

method however, the CFRP composites show the greatest enhancement in repaired beam strength

compared with the initial strength.

1. Introduction The restoration of timber structures holds historical, economical, conservational and ecological

significance. Mechanical methods for repairing timber are under-researched and unrealistic

comparisons are often made between empirical research and the subsequent application of the

research to timber structures in need of repair. Jones (1997), Radford et al (2002), Ogawa (2000)

and Lopez-Anido et al (2003) have all conducted experimental investigations into what is

effectively, reinforcement of undamaged timber beams and have assumed the validity of the results

in repair scenarios. Jones (1997) for example, conducted tests on beams reinforced with steel rods

located in deep grooves cut in from the compressive face of timber beams. This method is often

used in repair when aesthetics or, indeed, impracticalities necessitate non-standard methods of

applying reinforcements. Though Jones’s tests have significance with respect to reinforcement, they

were not simulations of repair. Radford et al (2002) suggested the use of ‘shear spikes’ for repair.

The spikes were inserted vertically at intervals along the length of two solid timber beams stacked

on top of each other, each spike penetrating the full depth of each beam. However, the repair

method was geared to the horizontal shear failure mode, where the shear spikes were simply

connecting two horizontally laminated wood sections and might have been less effective than repair

systems using adhesives (such as in adhesively bonded glued laminated timbers). Lopez-Anido et al

(2003) conducted wood pile repair simulation tests. In their tests, damage was simulated by

geometrically reducing the cross sectional diameter of the wood piles. The reinforcement was then

applied around the area of the reduced cross section and the improvement measured against an

unreinforced wood pile. Duarte et al (2004) developed a timber replacement system comprised of

three steel rods placed in a groove cut into the centre of the tensile face of a wood beam. Duarte and

co-workers tested solid pine beams in flexure and replaced fractured central portions with the

replacement system using the steel rods and adhesive to join the sections. The method is used

practically and is an efficient way of repairing damaged timber provided support can be applied to

the original timber beam-ends during replacement. The method does not however, address the issue

of repairing fracture paths, but rather, effectively joins reinforced and unreinforced flexural beam

sections. The objective of this paper is to investigate fracture repairs in timber beams that have been

subjected to flexural failure.

2. Procedure C16 spruce beams were manufactured by halving a wider beam into two equal segments and gluing

them together, Figure 1, using Rotafix CB10TSS, a slow setting thixotropic epoxy adhesive. Beams

assembled in this way were tested in flexure, Figure 2, to the point of the first audible fracture

accompanied by a drop in load. A linear variable differential transformer (LVDT) displacement

transducer was used to monitor the centre point deflection of the beam as a function of time.

Cut centrally along length

Original timber beam

Beam segments are vertically

laminated and are subsequently

bonded together to form a single

composite beam with dimensions b, t,

and l.

b

t

l

Fig 1 Manufacture of the C16 spruce beams

6h

6h

6h

F/2 F/2

F/2 F/2 18h

h = 100mm

h = depth

LVDT

1800mm

600mm 600mm

600mm

Fig 2 Four-point bending test arrangement

The beams were then repaired on both the compressive and tensile faces, Figure 3. Prior to repair,

the beams were flattened out by applying load to the underside of the flexed beam. The materials

used for repairing the fractured beams comprised mild steel, carbon fibre reinforced plastic (CFRP)

and glass fibre reinforced plastic (GFRP). Further repair tests using FULCRUM (glass fibre

reinforced polyurethane composites) as well as alternative repair configurations (compressive only

and tensile only) are described by Alam (2004). The dimensions of the reinforcements used in

repair are provided in Table 1.

Surface pre-treatments of the mild steel comprised grit blasting to SA 2.5 using guidance in

SIS055900 (1967) after which it was coated with a primer. A peel-ply layer was removed from the

CFRP prior to adhesion. Wet and dry grade 320 emery papers were used to roughen the surface of

the GFRP.

Table 1 Dimensions of reinforcement materials used to repair fractured C16 spruce beams

Reinforcement Length/mm Depth/mm Width/mm MILD STEEL 1900 20 5.0

CFRP 1900 20 1.5

GFRP 1900 20 4.0

3. Section properties of composite beams The composite second area moment, I, and section modulus, W, for each beam is a function of beam

width, b, and beam depth, t, (Eqns. 1 & 2).

12

3bt

I = [1] 6

2bt

W = [2]

The transformed section method was used to analyse beam properties, Figure 3. Each beam was

transformed using specific individual beam dimensions. In these figures, b refers to a width

dimension, t is a thickness dimension, y is the distance from a local centroid to the local edge and E

is an elastic modulus. The subscripts r, a and w are designated to reinforcement material, adhesive

material and wood respectively. The subscripts 1, 2 and 3 refer to transformed elements.

w

w

aa

w

rr bE

bE

E

bEbb 4

2231 +

+== ar tttt === 31 ( )312 tttt w +−=

Fig 3 Transforming section for beams repaired in both tension and compression

The transformed second moments of area are calculated using Eqn. 3. The transformed section

modulus for all repair configurations can then be calculated from Eqn. 4.

ba

br

bw

tr, ta

tw

b3

b2

t3

t2

b1

t1

( ) ( )

+++

+

++=

2

3233

3

33

3

22

2

1211

3

11,

2212122212

tttb

tbtbtttb

tbI bt [3]

w

bt

btt

IW

,

,

2= [4]

The flexural strengths and transformed flexural strengths, σ, can then be calculated according to

Equation 5 using the composite section modulus (Equation 2) for the flexural strength calculations

and the transformed section moduli (Equation 4) for the transformed flexural strength calculations.

In Eqn. 5, F is the load value at first fracture and a is the distance from the loading point to the

nearest lower roller.

W

aF

2=σ [5]

4. Flexural properties of C16 spruce beams Flexural strength properties of 36 as-received C16 beams are plotted in Figure 4 and related to the

influence of knots in the failure mode. Power law curves are fitted to the data sets and coefficients

of determination are given. Beam fractures influenced by knot presence have a higher scatter in

properties than beams that fracture independent of knot presence. Furthermore, beam strengths are

generically lower where knots have influenced the fracture mode. Examples of fracture are given in

Figure 5. Where knots are believed to influence fracture; indications are given by black dots. The

arithmetic mean flexural modulus of the spruce beams was measured as 9.6GPa with a standard

deviation of ±2.3GPa. The mean flexural strength for all beams was measured as 27.3MPa with a

standard deviation of ±8.1MPa about the mean. prEN 338 (2000) considers the mean modulus and

strength values of C16 timber to be 8GPa and 16MPa respectively.

Fig 4 Comparison of the flexural strengths of C16

spruce beams in relation to their mode of failure

1

2

3

4

5

6

Fig 5 Examples of beam fracture for the first

6 beams tested.

5. Flexural properties of repaired C16 spruce beams Figures 6-8 show the flexural modulus values of spruce beams and the post-repair flexural moduli

for beams reinforced by mild steel (beams 14, 20 and 34), CFRP (beams 1, 13 and 18) and GFRP

(beams 17 and 22) respectively. Beams were reinforced on both faces and they were selected

randomly from the 36 beams evaluated. The percentage increase or loss in flexural modulus

properties post-repair is provided for each beam.

Beams repaired with higher modulus reinforcements show less loss in stiffness, or indeed in the

case of steel, a significant improvement over the original flexural modulus values. Steel is weight-

for-weight considerably cheaper than CFRP and GFRP and would be the material of choice with

respect to stiffness enhancement. This said, the pultruded composites do effectively bridge fracture

paths and hence restore the original beam depth. For this reason, the contribution of the pultruded

composites to stiffness should not be ignored and it can be said that at least, CFRP and GFRP bring

the beams closer to their original stiffness than otherwise would be the case without repair.

Figures 9-11 show the flexural strength values of spruce beams and the post-repair flexural

strengths for beams reinforced by mild steel, CFRP and GFRP respectively. The percentage

increase or loss in flexural modulus properties post-repair is provided for each beam. In every case,

repaired beam strengths exceed original beam strengths, with the superior repair material being mild

steel followed by CFRP and finally GFRP. It is quite likely that the most effectual reinforcement is

that which is placed in tension since it closes tensile fractures and restrains their growth. Indeed in

many cases, failure fractures actually initiated from a different area of the repaired beam

independent of the original pre-repair fracture path. Mild steel reinforcement is ductile, can

maintain high stresses over large deformations and strain hardens as a function of increasing strain

(past the limit of elastic proportionality). This, coupled with the higher volume fraction of steel

used, is perhaps one aspect that makes them better than the pultruded composite reinforcements at

enhancing the flexural strength of the fractured beams relative to their original strength values.

Figures 12-14 show the transformed section flexural strength values of spruce beams and the post-

repair flexural strengths for beams reinforced by mild steel, CFRP and GFRP respectively. The

percentage increase or loss in flexural modulus properties post-repair is provided for each beam.

Since the method of transforming section takes into account the dimensions and modulus of the

reinforcement material relative to the wood, it is in some ways a better indicator of how effective

the reinforcement is compared with the equivalent dimensions of wood. Using the transformed

section method, steel is found to enhance beam strength similarly to GFRP composites but

noticeably less than CFRP composites. Therefore, using this method of analysis, CFRP composites

are more effective as repairing materials.

Observing the stiffness and strength histograms, Figures 6-11, it can be seen that strength properties

are considerably easier to enhance in a repair situation than stiffness properties. It is postulated that

the reinforcing material is less effective at increasing the flexural modulus because of the relatively

small volume of reinforcement. The significance of the reinforcement is that it increases the

stiffness as a function of the reinforcement stiffness properties, the dimensions of the unfractured

spruce and its location within the composite beam. Steel, therefore, has the greatest effect on

repaired beam stiffness. Contrarily, strength is a non-recoverable attribute of deformation. The

function of the reinforcement is not only to offer a contribution to beam strength as a function of its

own strength; but also to effectively constrain the growth of pre-existing cracks in the timber

material.

Fig 6 Flexural modulus values for C16 beams

before and after steel repair

Fig 7 Flexural modulus values for C16 beams

before and after CFRP repair

Fig 8 Flexural modulus values for C16 beams

before and after GFRP repair

Fig 9 Flexural strength values for C16 beams

before and after steel repair

Fig 10 Flexural strength values for C16 beams

before and after CFRP repair

Fig 11 Flexural strength values for C16 beams

before and after GFRP repair

Fig 12 Transformed strength values for C16

beams before and after steel repair

Fig 13 Transformed strength values for C16

beams before and after CFRP repair

Fig 14 Transformed strength values for C16

beams before and after GFRP repair

6. Failure observations Examples of failure in repaired spruce beams are shown using cut-through cross sections for mild

steel reinforcements, Figure 15, and GFRP reinforcements, Figure 16. It is interesting in both of

these cases to notice that de-bonding between the reinforcement and the adhesive leads on to the

final fracture path. This suggests that the reinforcement is not necessarily being fully utilised as de-

bonding may be taking place before reinforcement failure. Since the CFRP repaired beams did not

de-bond from the adhesive, the shear stress required for de-bonding is higher at the CFRP-adhesive

interfaces than it is for steel-adhesive or GFRP-adhesive interfaces. It is clear therefore, that the

CFRP reinforcement is utilised more effectively in the composite beam as failure occurs away from

this critical interface.

Fig 15 Example of failure in a steel repaired

beam under flexure Fig 16 Example of failure in a GFRP repaired

beam under flexure

7. Conclusions The method of repair reported on herein has been used in on-site repairs carried out on a c.a. 400-

year-old timber building in St. Albans, UK. The job is described in detail in an accompanying

paper, Smedley et al (2006), in this conference proceeding. The research presented in this paper has

shown that mechanical repairs made on the tensile and compressive faces of fractured C16 timber

beams can not only restore the strength and stiffness that existed in the timber prior to fracture, but

indeed, in many cases, a stronger and stiffer beam can be generated. Considering that the beams are

almost completely structurally unsound after major fracture, these findings are not only exciting,

but also hold conservational and economical significance. Steel repairs for example, were shown to

improve the stiffness and strength of a broken spruce beam by up to 114% and 255% respectively

of its original stiffness and strength values. Pultruded composites did not fare as well for stiffness

but did at least bring the stiffness back close to the original beam stiffness values in both cases.

With respect to strength enhancement, all mechanical reinforcements enhanced broken timber

beams to between 11% and 255% of the original beam strengths (prior to breakage). Steel and

CFRP reinforcements performed similarly well with respect to flexural strength improvement with

GFRP materials showing the smallest improvement.

On transformation of sections however, CFRP composites were seen to be superior to both GFRP

and steel reinforcements. The repaired beam strength is a function of the bond strength coupled with

the restraining capabilities of the reinforcing material. It is the ability of the material to restrain the

growth of the original fracture path that determines its applicability as a mechanical repair material.

Both steel and GFRP reinforcements experienced de-bonding at the adhesive interface, which led to

premature fracture in the timber beam. However, CFRP composites were utilised to their full

potential as no premature interfacial failure was observed for these materials.

8. References Alam, P. (2004); The reinforcement of timber for structural applications and repair; Ph.D. Thesis,

Department of Mechanical Engineering, University of Bath, United Kingdom.

Duarte, A., Negrão, J. and Cruz, H. (2004); Rehabilitation of timber beams with reinforced epoxy

plates; Proceeedings of the 8th

World Conference on Timber Engineering (WCTE2004); Lahti,

Finland, Vol. 1 pp. 371-376.

Jones, R. (1997); Upgrading of timber members in historic buildings; Journal of the Institute of

Wood Science; Vol. 14 No. 4 pp. 192-203.

Lopez-Anido, R., Michael, A. P. and Sandford, T. C. (2003); Experimental characterization of FRP

composite-wood pile structural response by bending tests; Marine Structures; Vol. 16 pp. 257-274.

Ogawa, H. (2000); Architectural application of carbon fibres. Development of a new carbon fibre

reinforced glulam; Carbon; Vol. 38 pp. 211-226.

prEN (2000); Structural timber – strength classes; British Standards Institution, London.

Radford, D. W., Van Goethem, D., Gutkowski, R. M. and Peterson, M. L. (2002); Composite repair

of timber structures; Construction and Building Materials Vol. 16 pp. 417-425.

SIS 055900 (1967); Pictorial surface preparation standards for painting steel surfaces; Swedish

Standard.

Smedley, D., Alam, P. and Ansell, M. P. (2006); George Street, St. Albans, UK – a case study in

the repair of historic timber structures using bonded-in pultruded plates, Proceedings of the 9th

World Conference on Timber Engineering (WCTE2006); Portland, Oregon, United States of

America.