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Mechanical repair of timber beams fractured in flexure using bonded-in reinforcements
Composites: Part B 40 (2009) 95–106
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Mechanical repair of timber beams fractured in flexure using bonded-in reinforcements Parvez Alam a,*, Martin P. Ansell b, Dave Smedley c a
Laboratory of Paper Coating and Converting, Department of Chemical Engineering, Åbo Akademi University, Porthaninkatu 3, 20500-FIN Turku, Finland Materials Research Centre, Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, UK c Rotafix (Northern) Ltd., Rotafix House, Abercraf, Swansea SA9 1UR, UK b
a r t i c l e
i n f o
Article history: Received 1 January 2007 Received in revised form 6 November 2008 Accepted 10 November 2008 Available online 3 December 2008 Keywords: Repair B. Fracture A. Wood (spruce) A. Polymer matrix composites (PMCs) E. Pultrusions
a b s t r a c t The flexural properties of strength class C16 spruce beams have been compared to the flexural properties of the same beams repaired with bonded-in reinforcements in the form of steel or composite pultruded rods. Reinforcing materials included rectangular sections of mild steel, pultruded carbon fibre reinforced plastic (CFRP), glass fibre reinforced plastic (GFRP) and a thermoplastic matrix glass fibre reinforced polyurethane (FULCRUM). Grooves were routed into the faces of the fractured beams following straightening and the reinforcements adhesively bonded into the top, bottom or both faces of the beams. The steel and CFRP reinforcements are most effective in restoring the flexural strength which often exceeds its original value. These reinforcements are also effective in enhancing flexural strength but the CFRP reinforcement endows the greatest transformed flexural strength. The fracture mechanisms in the repaired beams depend on the placement of reinforcement and the quality of the adhesive to reinforcement bond. All properties are optimised by bonding reinforcement into both faces of the fractured beams. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The fracture of timber in buildings may be a function of age, environment or poor design. Moisture-related expansion and shrinkage of timber warps and twists wood, lowering its strength and introduces localised stress concentrations within the structure [1]. Mould growth, rot and insect attack breaks down the internal structure of wood in historic buildings [2] thereby reducing its mechanical functionality. Timber is weaker in compression than in tension, but higher density woods tend to fail in tension whilst lower density woods often exhibit gross compressive buckling before the appearance of any signs of failure in tension. Wood will often fracture in horizontal shear when it is very dry or there are sharp changes in growth ring density [3]. Poorly designed, newer timber buildings may fail prematurely and can benefit from mechanical repair. Remedial techniques for the preservation and restoration of wooden buildings include the application of fungicides and pesticides [4] and, more recently, mechanical methods of repair employing fibre reinforced plastics (FRPs) and thermosetting adhesives [5]. However, the application of empirical research results to timber structures in need of repair are often unrealistic as much of the research is based upon timber reinforcement as opposed to timber repair [6–9]. Nevertheless, strategies have been developed * Corresponding author. Fax: +358 2 215 4858. E-mail address: parvez.alam@abo.fi (P. Alam). 1359-8368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2008.11.010
for reinforcement based upon methods used in repair situations when either aesthetics or impracticalities necessitate intricate positioning of reinforcements. For example, Jones [6] tested beams reinforced with steel rods, which were placed into deep grooves on the compressive face of the timber. The tests proved that slotting five steel rods through the depth of the groove was hardly more beneficial than using two steel rods, each located at the furthermost tensile and compressive ends of the groove. Though the tests were not simulations of repair, they did have economic significance with respect to reinforcement. The use of dowel-type connectors, termed ‘shear spikes’, has also been suggested as a means of repairing timber. Radford et al. [7] inserted shear spikes at regular intervals along the length of two timber beams stacked one upon another. The shear spikes penetrated the full depth of the two beams and the method was geared towards restraining horizontal shear failure. It is questionable as to whether such a doweling system, which simply connects two timber beams together, is any more effective than glued laminated timber systems. Wood pile repair simulation tests have also been reported [9]. Fibre reinforced plastic reinforcements were used to repair damage induced in cylindrical piles, which were subjected to three-point bending loads. Either shear connectors or grout was used to connect FRP reinforcement to the reduced diameter of the beam. It was concluded that the use of grout to connect the FRP was better than the use of shear connectors. A timber rehabilitation system that can be used for repair is described by Duarte et al. [10]. The method exhibits how beam sec-
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tions can be effectively jointed and strengthened using reinforced epoxy plates. The work gives credibility to the use of reinforcements in repair situations. Many papers published on metallic reinforcements for timber [11–19] can be useful in developing ideas for repairing timber. The subject of FRP reinforcement of flexural timber has also generated considerable interest within the research community. Problems associated with vertical shear failure in timber beams have been addressed by Triantafillou [20] who used CFRP vertical laminates as crack resistors. Vertical shear failure is often reported as a failure mode in flexural beams [21–23]. Short vertical laminates used [24] as tensile reinforcements are more effective with wet wood than dry wood. This increased effectiveness can be attributed to the lower flexural modulus of wet wood as compared to dry wood. To achieve the highest strength and stiffness improvement, FRP reinforcements are often situated horizontally as far from the neutral axis as possible. Dagher [25] for example reported a 100% rise in flexural strength is possible by laminating only the underside of the beam. The level of strength enhancement as a function of volume fraction does however have a maximum [21] beyond which any increased volume fraction does not result in a proportional rise in the flexural strength. This is essentially because ordinarily the glue line is the weakest ‘link’ in the composite and will fail at a critical stress level. At higher reinforcement fractions however, the failure mode becomes dominated by shear within the wood and the strength is controlled by this [26]. Reinforcing on the compressive face has also received some attention [27,28]. Compressive reinforcements are prone to buckling due to their long slender geometries [28] and indeed, reinforcing on only the tensile face can have a suitable ductile design feature as composite beams reinforced only in tension often fail gradually in compression before fracturing in tension [29]. Though the flexural modulus of unreinforced timber decreases as the moisture increases, the flexural rigidity (a product of the elastic modulus and the second area moment) is almost entirely unaffected [30]. This is because the second area moment reduces as a function of timber shrinkage. Compressive strength properties in timber are affected significantly more so than in tension as the moisture content increases [30]. Increasing the heterogeneity of the material by layering timber sections (e.g. laminated veneer lumber, glulam) decreases moisture related warping. It could be argued that moisture related movements within the material of timber can also be restrained to some extent through the application of reinforcing material. Bond integrity may nevertheless be compromised when there are fluctuations in the moisture content, especially when there is a large stiffness differential between the timber and reinforcement. This said, a study by Wheeler and Hutchinson [31] demonstrated that typical structural epoxy adhe-
sives (SikadurÒ31 and Rotafix TimbersetÒ) are well tailored for use in bonding timber. In their research, they found that up to moisture contents of 22%, there was little or no compromise in the bonding strength. Similar conclusions were reached by Broughton and Hutchinson [32] who tested the bond integrity by tensile pull out testing of steel or pultruded FRP rods from bonded timber sections. Beyond this critical moisture content of 22%, a decreased resistance to tensile pull out was reported. Bond durability is nevertheless still an area of doubt amongst many timber engineers and long term testing would be a necessity for the purpose of validation. The objective of this paper is to examine repair strategies for timber beams fractured in bending in the laboratory to simulate structural damage. The beams are straightened and repaired by routing out grooves on the beam faces and bonding in reinforcement in the form of steel or composite pultrusions. The repaired beams are then tested in flexure and mechanical properties are compared with the properties of the original beam. The effect of the placement of the reinforcement on the stiffness and strength of repaired beams is examined. 2. Mechanical properties of C16 spruce beams in flexure 2.1. Fabrication of beams Vertically laminated spruce beams were manufactured from the same timber lath, Fig. 1, and tested to destruction in preparation for mechanical repair. The reason for testing the beams to destruction was to measure the flexural properties unique to each beam for direct comparison with the same beam following mechanical repair. Since C16 spruce beams can be highly variable in their properties, this approach was logical. The beams were halved lengthways and then laminated using Rotafix CB10TSS adhesive, which is a slow setting thixotropic epoxy, frequently used for on-site structural repairs. Each beam was 1900 mm in length and the width and depth was on average, 93 mm and 96 mm, respectively. In total, 36 beams were manufactured for evaluation. 2.2. Mechanical testing of beams The beams were subjected to flexure in four-point bending. A linear variable differential transformer (LVDT) displacement transducer was affixed to the crosshead to monitor the beam centre point deflection. The lower roller span was 1800 mm and the span between the upper rollers, located about the centre of the beam, was 600 mm. Fig. 2 shows the test set-up with the beam dimensions. Displacement controlled loading ensued at a crosshead speed of 2 mm min1. The beams were observed vigilantly during testing to ensure that testing would stop at the first drop in load
Fig. 1. Schematic representation of the manufacture of vertically laminated bi-component spruce beams.
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Fig. 2. Schematic of four-point bending test set-up showing positions of rollers and LVDT.
accompanied by an audible crack. On terminating the test, the beam was turned upside down and load was applied on the underside until the beam was flattened to approximately its original shape. 2.3. Fracture modes and flexural strength of beams Various modes of fracture were observed in the beams, Fig. 3, with knots influencing the propagation of cracks in 16 of the 36 beams. Flexural strengths were calculated using Eq. (1).
rf ¼
aF 2W
ð1Þ
where a, distance from a loading roller to the nearest bottom roller; F, highest force reached before the first drop in load occurs; W, sec-
tion modulus of spruce beam = bt2/6; b, breadth of beam t = thickness of beam. Fig. 4 compares the flexural strengths of beams where fracture was influenced by knots with beams where fracture was unrelated to the presence of knots. Power law curves are fitted to the data and coefficients of determination are given. Generally it can be said that the scatter in the strength values of beams, where the presence of knots influences the failure mode, is somewhat higher than beams where the failure mode is independent of knot presence. Furthermore, it can broadly be said that the knots influencing fracture have an effect on the strength in that, generally, beams that contain knots also have lower strength values. This can be substantiated further since the mean and standard deviations are 24.8(±9.38) MPa and 29.6(±7.3) MPa for knot influenced failure and knot independent failure, respectively. 3. Repair of C16 spruce beams 3.1. Reinforcement materials
Fig. 3. Possible modes of failure for beams in flexure.
The fractured beams, which had been re-flattened after testing, were repaired using four rectangular section materials. These included; mild steel, pultruded carbon fibre reinforced plastic (CFRP), pultruded glass fibre reinforced plastic (GFRP) and pultruded glass fibre reinforced thermoplastic polyurethane (FULCRUM). Some mechanical properties of these materials and their dimensions are provided in Tables 1 and 2, respectively. The elastic modulus for CB10TSS is taken to be 1 GPa. The dimensions of the reinforcements differ somewhat due to manufacture and use in structural applications. Therefore, to maintain a working link between research and industrial practice, the reinforcement dimensions were
Fig. 4. Comparison of the flexural strengths of C16 spruce beams in relation to the failure mode.
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Table 1 Mechanical properties of the reinforcing materials.
Tensile modulus/GPa Compressive modulus/GPa Flexural modulus/GPa Tensile strength/MPa Compressive strength/MPa Flexural strength/MPa
bc t3c 12 bc t 2c Wc ¼ 6 Ic ¼
Mild steel
GFRP
CFRP
FULCRUM
210 – – 430–580 – –
40 30 45 1000 450 1000
150 – 150 2800 – –
45 – 45 1000 – 1150
Table 2 Dimensions of reinforcing materials applied as vertically laminating plates. Reinforcement
Length/mm
Depth/mm
Width/mm
Mild steel CFRP GFRP FULCRUM
1900 1900 1900 1900
20 20 20 20
5.0 1.5 4.0 1.2 2
kept the way they are typically used in industry. Repair work was carried out at Rotafix Ltd. on all 36 spruce beams. Three reinforcing configurations were used for repair; with some of the beams repaired on only the tensile face, some on only the compressive face and the remaining beams were repaired in both tension and compression. Surface pre-treatment applied to the reinforcing materials included grit blasting of the mild steel to SA 2.5 using guidance in SIS 055900 (1967), after which it was coated with a primer, removal of peel ply layer from CFRP composites prior to gluing, abrading of the GFRP surface using a sodium carbonate abrasive while the FULCRUM was left untreated. Grooves were routed into the faces of the spruce sections to a width of 12.7 mm and a depth of 20 mm. These dimensions are typically used by Rotafix Ltd. for on-site repairs. The reinforcing plates were inserted vertically into the grooves and bonded in using the thixotropic CB10TSS adhesive. 3.2. Section properties of repaired beams
ð2Þ ð3Þ
Ic denotes the composite second moment of area and Wc is the composite section modulus. The global widths and depths of the composite beams are correspondingly represented by bc and tc. 3.2.2. Transformed section modulus The location of the composite beam centroid must be determined in order to calculate the transformed section properties. This calculation is somewhat idealised, since the beam is already fractured and the true neutral axis will differ according to the magnitude and mode of fracture. Additionally, the beams repaired on only one face are asymmetrical and the neutral axis lies closer to the reinforcement. In order to calculate the transformed strength and make subsequent comparisons with unreinforced equivalents, a theoretical beam centroid needs to be determined. This is achieved by transforming the reinforcement and adhesive sections into theoretically corresponding spruce sections using a modular ratio technique. Figs. 5–7 show the hypothetical transformed sections of the reinforcement and adhesive to equivalent sections of spruce for beams repaired on the compressive, tensile and both faces, respectively. The compressive face sections are transformed according to Eq. (4).
b1 ¼ 2
Er br 2Ea ba þ 4bw þ Ew Ew
ð4Þ
Eq. (5) is used to calculate, from the top down, the geometrical centroid of the transformed beam, Cc, which gives rise to the calculation of the transformed second moment of area, Eq. (6) and the transformed section modulus, Eq. (7).
P
P Qi y i Ai y A1 þ ðt 1 þ y2 ÞA2 i Cc ¼ P ¼ P ¼ 1 Ai Ai A1 þ A2 i
i
ð5Þ
i
3 b1 t31 b2 t 2 þ ðb1 t1 ÞðC c y1 Þ2 þ þ ðb2 t 2 Þðt w C c y2 Þ2 It;c ¼ 12 12
ð6Þ
The mechanical properties calculated for a material in bending are implicitly related to the section properties, which can be very different depending on how the section properties are calculated. Two methods were used for calculating the section properties. The first was a classical method often used in fibre and particle composite mechanics, whilst the second was the transformed section method, frequently employed by structural engineers.
where t1 = tr = ta and t2 = tw t1.
3.2.1. Composite section properties The second moment of area and the section modulus of beams repaired on the compressive face, the tensile face and on both faces can be calculated using Eqs. 2 and 3, respectively.
Eq. (9) is used to calculate, from the bottom up, the geometrical centroid of the transformed beam, Ct, which gives rise to the calculation of the transformed second moment of area, Eq. (10) and the transformed section modulus, Eq. (11).
W t;c ¼
2It;c tw
ð7Þ
The tensile sections are transformed according to Eq. (8).
b1 ¼ 2
Er br 2Ea ba þ þ 4bw Ew Ew
Fig. 5. Transforming of spruce beams repaired on the compressive face.
ð8Þ
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Fig. 6. Transforming of spruce beams repaired on the compressive face.
Fig. 7. Transforming of spruce beams repaired on the both faces.
P
P Qi yi Ai y A1 þ ðt1 þ y2 ÞA2 i i Ct ¼ P ¼ P ¼ 1 Ai Ai A1 þ A 2 i
ð9Þ
i
3 3 b1 t 1 b2 t 2 It;t ¼ þ ðb1 t 1 ÞðC t y1 Þ2 þ þ ðb2 t 2 Þðt w C t y2 Þ2 12 12
ð10Þ
where t1 = tr = ta and t2 = tw t1.
W t;t
2It;t ¼ tw
4. Mechanical properties of repaired C16 spruce beams
ð11Þ
Sections on both faces are transformed according to Eq. (12).
Er br 2Ea ba b1 ¼ b3 ¼ 2 þ þ 4bw Ew Ew
ð12Þ
The geometrical centroid of the transformed beam is assumed to be an intersection point between lines of symmetry and the transformed second moment of area, together with the transformed section modulus are determined by Eqs. 13 and 14, respectively.
It;b
( 2 ) 3 b1 t31 t2 t1 b2 t2 þ ¼ þ ðb1 t 1 Þ þ 12 2 2 12 ( ) 2 b3 t33 t2 t3 þ þ ðb3 t 3 Þ þ 12 2 2
ð13Þ
where t1 = t3 = tr = ta and t2 = tw (t1 + t3).
W t;b ¼
2It;b tw
both faces are denoted by the added subscripts c, t and b, respectively. In Figs. 7–9 and Eqs. (4)–(14); the first moment of area Qi = yiAi, the surface areaAi = tibi, the vertical distance from the edge to the local centroid yi = ti/2, the local centroid is Ci and ti is the depth of the vertical beam section, i.
ð14Þ
The theoretical centroid for each beam is calculated using its own dimensional properties. The transformed second area moments, It, and transformed section moduli, Wt, are calculated utilising the parallel axis theorem. Beams repaired in compression, tension and on
4.1. Test method Repaired spruce beams were subjected to four-point bending at a crosshead rate of 2 mm min1. The beams were tested under service class 2 conditions (between 12% and 20% timber moisture content) according to Euro Code 5. An LVDT displacement transducer was attached to the moving crosshead and was used to monitor the centre point deflection (refer to Fig. 2). 4.2. Flexural modulus of repaired beams The difference in the flexural modulus of beams repaired with mild steel, CFRP, GFRP and FULCRUM reinforcement are shown in Figs. 8–10. These differences are relative to their original unreinforced properties. Values for beams repaired in compression are given in Fig. 8, for beams repaired in tension the values are shown in Fig. 9 and the values for beams repaired in both compression and tension are provided in Fig. 10. Each histogram gives the original flexural modulus for the undamaged C16 spruce beam adjacent to its repaired equivalent. The increase or reduction of the repaired beam relative to its unreinforced equivalent is indicated as a plus or minus percentage value. The x-axis scale is constant in each of the histograms to make it easier to compare the reinforcing materials as well as their location within the beams.
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Fig. 8. Flexural moduli of all beams repaired on the compressive face.
Fig. 9. Flexural moduli of all beams repaired on the tensile face.
Figs. 8–10 demonstrate that mild steel reinforcements generally yield the highest values of flexural modulus for the repaired spruce beams, as well as the most clear-cut trends between repairs made in compression, tension or on both faces. In the beams repaired by steel, repair on both faces shows generally higher stiffnesses than repairs on the tensile face, which in turn are superior to repairs to the compressive face of the beam. Although the reinforcement
volume fraction is equal for beams repaired in tension and compression only, the fracture paths tend to occur on the tensile face and consequently, repairing in tension is more effective as a result of the superior constraint of fracture. Constraining the fracture path in tension increases the effective geometrical depth of the composite beam, which would otherwise be reduced to a depth relative to the fracture depth.
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Fig. 10. Flexural moduli of all beams repaired on both faces.
The increased flexural modulus values of all steel repairs as compared to the pultruded composites can be attributed to the high stiffness characteristics of the steel. The extra load required to bend the composite beam results from the increased resistance to loading from the steel insert. Logically, the beams reinforced on both faces yield the highest stiffness enhancement at the very least due to the higher volume fraction of stiff reinforcing steel. The flexural modulus values for beams reinforced by the pultruded composite reinforcements, do not follow the trends observed for the steel reinforced beams. The CFRP, GFRP and FULCRUM all have significantly lower E-modulus values compared to the steel and lower loads are consequently required to flex the beams to the same extent as the steel repaired beams. As a result, the steel manages to enhance the flexural modulus of the repaired beam much more effectively than the pultruded composite reinforcements. Direct conclusions regarding repaired beam stiffness enhancements are difficult to make based upon the results in Figs. 8–10. However, there are some general observations that can be made. For example, higher stiffness reinforcements are, more effective as stiffness enhancing materials for repaired spruce beams than are lower stiffness reinforcements. Tensile face repairs are more effective in enhancing the flexural modulus than compressive face repairs, provided that equal volumes of reinforcement are used. Repairs on both faces are superior to repairing solely on the tensile and compressive faces because the volume fraction of reinforcement is higher and randomly located micro-cracks can be restrained more effectively. The effectiveness of repairing on only the compressive face depends critically upon the extent of the crack path. Larger fracture paths generally render compressive face reinforcements less effective. The tensile fracture paths have less effect on the flexural modulus when the beams are repaired in tension and on both faces. In this case the tensile repair restricts the effects of the presence of the crack and the effective depth of the composite beam extends to the outermost edge of the reinforcement.
4.3. Flexural strength of repaired beams Figs. 11–13 show the flexural strengths of beams and the difference in strength after repair. The figures give the results for all the reinforcing materials used in the three different repair configurations. The strength values of the unreinforced beams were measured at the first drop in load and the first drop in load is also used for the repaired beams, regardless of whether or not the peak load is higher. The flexural strengths, rf, are calculated using Eqs. (15)–(17) where a is the distance from the loading point to the bottom rollers, Ffirstdrop is the load at the first drop in load and Wc is the composite section modulus. The subscripts c, t and b make reference to beams repaired in compression, tension and on both faces, respectively.
aF firstdrop 2W c;c aF firstdrop rf ;t ¼ 2W c;t aF firstdrop rf ;b ¼ 2W c;b
rf ;c ¼
ð15Þ ð16Þ ð17Þ
Trends for the strength of spruce beams repaired with mild steel show essentially the same trends observed for the repaired modulus values as a function of reinforcement location and volume. The strength values and the percentage enhancement generally increase in the order of beams repaired with steel in compression to those repaired in tension to those repaired on both faces. Whereas the flexural moduli of the beams repaired with pultruded composite showed some level of randomness between the reinforcing series, their flexural strengths seem to increase in the order of beams repaired in compression, to those repaired in tension to those repaired on both sides, as for the beams repaired with steel. Generally, fracture paths have been observed to initiate on the tensile face of the beam. Repairing the compression face increases
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Fig. 11. Flexural strength for all beams repaired on the compressive face.
Fig. 12. Flexural strength for all beams repaired on the tensile face.
the strength more because of the added compressive strength and stiffness of the reinforcing material than as a result of constraining crack propagation. During flexure of a beam reinforced on the compression face but containing tensile fractures the load is taken by the compressive face reinforcement only. Once the elastic limits of the reinforcement are exceeded, a significant load has already been borne and yielding develops in the beam components. As
the tensile fractures increases in size and distribution within the spruce, the strength of the composite is reduced and the effectiveness of the repair is reduced. Repairing the tensile face with the same volume fraction of reinforcement does the job of both restraining (to some extent) the development of the fracture path as well as increasing the strength as a function of the added tensile strength of the repairing material. Repairing on both faces doubles
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Fig. 13. Flexural strength for all beams repaired on both faces.
the volume fraction of the reinforcement and constrains crack propagation on both faces. 4.4. Transformed strength of repaired beams Figs. 14–16 compare the repaired transformed strength values with the original strengths of the unreinforced beams for all configurations and reinforcing materials. The strength values of the unreinforced beams were measured at the first drop in load and once again the first drop in load is also used for the repaired beams regardless of whether or not the peak load is higher. The transformed flexural strengths are calculated using Eqs. (18)–(20). The subscripts c, t and b denote repair in compression, tension and on both faces, respectively.
aF firstdrop 2W t;c aF firstdrop rt;t ¼ 2W t;t aF firstdrop rt;b ¼ 2W t;b
rt;c ¼
ð18Þ ð19Þ ð20Þ
An advantage of using the transformed section method is that it permits the analysis of the flexural strength in relation to the materials and geometrical properties of the reinforcement used. Regardless of the assumptions involved in a transformed section analysis, such as the estimation of the geometrical properties from material properties, it is still a widely used method that allows the comparison of different volume fractions of reinforcing materials possessing different mechanical properties. Observation of the trends between the three different reinforcement configurations shows that beams repaired on the compression face are less effective than beams repaired in tension, which in turn are less effective than beams repaired on both faces. This complements the conclusions made from observations of the flexural strengths (refer to Figs. 11–13). In contrast to the results for the flexural strengths, the percentage enhancement in transformed flexural strength compared to
beams in their undamaged states is highest for the beams repaired with CFRP and not steel. In terms of transformed flexural strength, the CFRP pultrusions are therefore more effective for repairing fractured beams than GFRP and FULCRUM pultrusions and mild steel. In general the repair of beams has a more significant effect on improving the flexural strength and the transformed flexural strength of beams than it does on flexural stiffness (compare Figs. 10, 13 and 16 for beams repaired on both faces). It is suggested that the reinforcing material is less effective at increasing the flexural modulus of the original beam because of its relatively low volume fraction. Other factors such as bond integrity, position of the reinforcement and whether reinforcement is located on one or both faces clearly influence the flexural stiffness and higher modulus reinforcements will have a greater stiffening effect than their lower stiffness counterparts. In contrast, flexural strength, is not only a function of the reinforcement properties but is also dependent upon the extent to which the reinforcement manages to halt crack growth. 4.5. Post-repair failure modes of beams Various modes of failure observed in unreinforced spruce beams are illustrated in Fig. 3. Clear differences in failure mode were observed as a function of the position of the reinforcement. In most of the beams repaired in compression, fracture develops from the tension face to a level close to the reinforcing material after which the fracture path takes a horizontal path and proceeds closely parallel to the reinforcing material, but without touching it. Furthermore, for beams repaired on the compression face, neither buckling of this face in the spruce nor de-bonding between reinforcement and adhesive was observed. In contrast, spruce beams repaired on the tensile face often buckled appreciably on the compression face. In beams repaired on both faces, fracture paths propagate by horizontal shear around the axis of neutrality in these beams. More reinforcement to adhesive de-bonding was observed in the beams reinforced on both faces than was observed for the other configurations. This is believed to be because higher stresses
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Fig. 14. Transformed flexural strength for all beams repaired on the compressive face.
Fig. 15. Transformed flexural strength for all beams repaired on the tensile face.
were experienced by beams repaired on both faces relative to the beams repaired on only one face. Although beams repaired in tension using CFRP experienced, in some cases, stresses as high as the steel plate repaired beams, the stress required to activate de-bonding between the steel plate and the adhesive is perhaps lower than that between the CFRP and the adhesive. The consequence is steel-adhesive de-bonding, but no CFRP-adhesive de-bonding. GFRP-repaired beams fail at lower
stresses than the CFRP or steel-repaired beams and hence de-bonding was not observed. Fractures will always take the path that is mechanically weakest. Hence, when fracture initiates in the wood, it tends to propagate through the wood and works its way around the adhesive and reinforcement. In that sense, the reinforcement configuration, rather than material, plays a significant role in resisting cross fracture initiating from the tensile face. Tensile face fractures that may ordinarily run across the width of the beam
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Fig. 16. Transformed flexural strength for all beams repaired on both faces.
would need to take a different path and most likely would resist a higher level of loading.
ing and Physical Sciences Research Council (EPSRC) for financing his studentship at the University of Bath.
5. Conclusions
References
Class C16 spruce beams were fractured in four-point bending and the flexural modulus and strength were recorded. The beams were straightened to allow repair using adhesively bonded-in reinforcement in the form of rectangular section steel, CFRP and two grades of GFRP. The reinforcement was bonded into routed slots on the fractured beam faces with a thixotropic epoxy adhesive. The repaired beams were tested to failure and the effect of the position of the reinforcement on the flexural modulus, flexural strength and transformed flexural strength were assessed. The reinforcements bonded into the top and bottom faces of the beams were, not surprisingly, more effective than reinforcements bonded into the top or bottom face. Steel reinforcement has the greatest effect on beam stiffness which was often considerably greater than the stiffness of the original grade C16 beam. Steel and CFRP have the greatness ability to enhance flexural strength but CFRP has the greatest effect on the transformed flexural strength. The position of reinforcement and the integrity of the reinforcement to adhesive bond influence the fracture mode of the repaired beams. Bonded-in reinforcement is an effective method for repairing damaged timber structures.
Acknowledgments The authors thank Stan Bowen, Rotafix Ltd. for constructing and repairing the test beams. The first author thanks the UK Engineer-
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Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Mechanical repair of timber beams fractured in flexure using bonded-in reinforcements Parvez Alam a,*, Martin P. Ansell b, Dave Smedley c a
Laboratory of Paper Coating and Converting, Department of Chemical Engineering, Åbo Akademi University, Porthaninkatu 3, 20500-FIN Turku, Finland Materials Research Centre, Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, UK c Rotafix (Northern) Ltd., Rotafix House, Abercraf, Swansea SA9 1UR, UK b
a r t i c l e
i n f o
Article history: Received 1 January 2007 Received in revised form 6 November 2008 Accepted 10 November 2008 Available online 3 December 2008 Keywords: Repair B. Fracture A. Wood (spruce) A. Polymer matrix composites (PMCs) E. Pultrusions
a b s t r a c t The flexural properties of strength class C16 spruce beams have been compared to the flexural properties of the same beams repaired with bonded-in reinforcements in the form of steel or composite pultruded rods. Reinforcing materials included rectangular sections of mild steel, pultruded carbon fibre reinforced plastic (CFRP), glass fibre reinforced plastic (GFRP) and a thermoplastic matrix glass fibre reinforced polyurethane (FULCRUM). Grooves were routed into the faces of the fractured beams following straightening and the reinforcements adhesively bonded into the top, bottom or both faces of the beams. The steel and CFRP reinforcements are most effective in restoring the flexural strength which often exceeds its original value. These reinforcements are also effective in enhancing flexural strength but the CFRP reinforcement endows the greatest transformed flexural strength. The fracture mechanisms in the repaired beams depend on the placement of reinforcement and the quality of the adhesive to reinforcement bond. All properties are optimised by bonding reinforcement into both faces of the fractured beams. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The fracture of timber in buildings may be a function of age, environment or poor design. Moisture-related expansion and shrinkage of timber warps and twists wood, lowering its strength and introduces localised stress concentrations within the structure [1]. Mould growth, rot and insect attack breaks down the internal structure of wood in historic buildings [2] thereby reducing its mechanical functionality. Timber is weaker in compression than in tension, but higher density woods tend to fail in tension whilst lower density woods often exhibit gross compressive buckling before the appearance of any signs of failure in tension. Wood will often fracture in horizontal shear when it is very dry or there are sharp changes in growth ring density [3]. Poorly designed, newer timber buildings may fail prematurely and can benefit from mechanical repair. Remedial techniques for the preservation and restoration of wooden buildings include the application of fungicides and pesticides [4] and, more recently, mechanical methods of repair employing fibre reinforced plastics (FRPs) and thermosetting adhesives [5]. However, the application of empirical research results to timber structures in need of repair are often unrealistic as much of the research is based upon timber reinforcement as opposed to timber repair [6–9]. Nevertheless, strategies have been developed * Corresponding author. Fax: +358 2 215 4858. E-mail address: parvez.alam@abo.fi (P. Alam). 1359-8368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2008.11.010
for reinforcement based upon methods used in repair situations when either aesthetics or impracticalities necessitate intricate positioning of reinforcements. For example, Jones [6] tested beams reinforced with steel rods, which were placed into deep grooves on the compressive face of the timber. The tests proved that slotting five steel rods through the depth of the groove was hardly more beneficial than using two steel rods, each located at the furthermost tensile and compressive ends of the groove. Though the tests were not simulations of repair, they did have economic significance with respect to reinforcement. The use of dowel-type connectors, termed ‘shear spikes’, has also been suggested as a means of repairing timber. Radford et al. [7] inserted shear spikes at regular intervals along the length of two timber beams stacked one upon another. The shear spikes penetrated the full depth of the two beams and the method was geared towards restraining horizontal shear failure. It is questionable as to whether such a doweling system, which simply connects two timber beams together, is any more effective than glued laminated timber systems. Wood pile repair simulation tests have also been reported [9]. Fibre reinforced plastic reinforcements were used to repair damage induced in cylindrical piles, which were subjected to three-point bending loads. Either shear connectors or grout was used to connect FRP reinforcement to the reduced diameter of the beam. It was concluded that the use of grout to connect the FRP was better than the use of shear connectors. A timber rehabilitation system that can be used for repair is described by Duarte et al. [10]. The method exhibits how beam sec-
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tions can be effectively jointed and strengthened using reinforced epoxy plates. The work gives credibility to the use of reinforcements in repair situations. Many papers published on metallic reinforcements for timber [11–19] can be useful in developing ideas for repairing timber. The subject of FRP reinforcement of flexural timber has also generated considerable interest within the research community. Problems associated with vertical shear failure in timber beams have been addressed by Triantafillou [20] who used CFRP vertical laminates as crack resistors. Vertical shear failure is often reported as a failure mode in flexural beams [21–23]. Short vertical laminates used [24] as tensile reinforcements are more effective with wet wood than dry wood. This increased effectiveness can be attributed to the lower flexural modulus of wet wood as compared to dry wood. To achieve the highest strength and stiffness improvement, FRP reinforcements are often situated horizontally as far from the neutral axis as possible. Dagher [25] for example reported a 100% rise in flexural strength is possible by laminating only the underside of the beam. The level of strength enhancement as a function of volume fraction does however have a maximum [21] beyond which any increased volume fraction does not result in a proportional rise in the flexural strength. This is essentially because ordinarily the glue line is the weakest ‘link’ in the composite and will fail at a critical stress level. At higher reinforcement fractions however, the failure mode becomes dominated by shear within the wood and the strength is controlled by this [26]. Reinforcing on the compressive face has also received some attention [27,28]. Compressive reinforcements are prone to buckling due to their long slender geometries [28] and indeed, reinforcing on only the tensile face can have a suitable ductile design feature as composite beams reinforced only in tension often fail gradually in compression before fracturing in tension [29]. Though the flexural modulus of unreinforced timber decreases as the moisture increases, the flexural rigidity (a product of the elastic modulus and the second area moment) is almost entirely unaffected [30]. This is because the second area moment reduces as a function of timber shrinkage. Compressive strength properties in timber are affected significantly more so than in tension as the moisture content increases [30]. Increasing the heterogeneity of the material by layering timber sections (e.g. laminated veneer lumber, glulam) decreases moisture related warping. It could be argued that moisture related movements within the material of timber can also be restrained to some extent through the application of reinforcing material. Bond integrity may nevertheless be compromised when there are fluctuations in the moisture content, especially when there is a large stiffness differential between the timber and reinforcement. This said, a study by Wheeler and Hutchinson [31] demonstrated that typical structural epoxy adhe-
sives (SikadurÒ31 and Rotafix TimbersetÒ) are well tailored for use in bonding timber. In their research, they found that up to moisture contents of 22%, there was little or no compromise in the bonding strength. Similar conclusions were reached by Broughton and Hutchinson [32] who tested the bond integrity by tensile pull out testing of steel or pultruded FRP rods from bonded timber sections. Beyond this critical moisture content of 22%, a decreased resistance to tensile pull out was reported. Bond durability is nevertheless still an area of doubt amongst many timber engineers and long term testing would be a necessity for the purpose of validation. The objective of this paper is to examine repair strategies for timber beams fractured in bending in the laboratory to simulate structural damage. The beams are straightened and repaired by routing out grooves on the beam faces and bonding in reinforcement in the form of steel or composite pultrusions. The repaired beams are then tested in flexure and mechanical properties are compared with the properties of the original beam. The effect of the placement of the reinforcement on the stiffness and strength of repaired beams is examined. 2. Mechanical properties of C16 spruce beams in flexure 2.1. Fabrication of beams Vertically laminated spruce beams were manufactured from the same timber lath, Fig. 1, and tested to destruction in preparation for mechanical repair. The reason for testing the beams to destruction was to measure the flexural properties unique to each beam for direct comparison with the same beam following mechanical repair. Since C16 spruce beams can be highly variable in their properties, this approach was logical. The beams were halved lengthways and then laminated using Rotafix CB10TSS adhesive, which is a slow setting thixotropic epoxy, frequently used for on-site structural repairs. Each beam was 1900 mm in length and the width and depth was on average, 93 mm and 96 mm, respectively. In total, 36 beams were manufactured for evaluation. 2.2. Mechanical testing of beams The beams were subjected to flexure in four-point bending. A linear variable differential transformer (LVDT) displacement transducer was affixed to the crosshead to monitor the beam centre point deflection. The lower roller span was 1800 mm and the span between the upper rollers, located about the centre of the beam, was 600 mm. Fig. 2 shows the test set-up with the beam dimensions. Displacement controlled loading ensued at a crosshead speed of 2 mm min1. The beams were observed vigilantly during testing to ensure that testing would stop at the first drop in load
Fig. 1. Schematic representation of the manufacture of vertically laminated bi-component spruce beams.
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Fig. 2. Schematic of four-point bending test set-up showing positions of rollers and LVDT.
accompanied by an audible crack. On terminating the test, the beam was turned upside down and load was applied on the underside until the beam was flattened to approximately its original shape. 2.3. Fracture modes and flexural strength of beams Various modes of fracture were observed in the beams, Fig. 3, with knots influencing the propagation of cracks in 16 of the 36 beams. Flexural strengths were calculated using Eq. (1).
rf ¼
aF 2W
ð1Þ
where a, distance from a loading roller to the nearest bottom roller; F, highest force reached before the first drop in load occurs; W, sec-
tion modulus of spruce beam = bt2/6; b, breadth of beam t = thickness of beam. Fig. 4 compares the flexural strengths of beams where fracture was influenced by knots with beams where fracture was unrelated to the presence of knots. Power law curves are fitted to the data and coefficients of determination are given. Generally it can be said that the scatter in the strength values of beams, where the presence of knots influences the failure mode, is somewhat higher than beams where the failure mode is independent of knot presence. Furthermore, it can broadly be said that the knots influencing fracture have an effect on the strength in that, generally, beams that contain knots also have lower strength values. This can be substantiated further since the mean and standard deviations are 24.8(±9.38) MPa and 29.6(±7.3) MPa for knot influenced failure and knot independent failure, respectively. 3. Repair of C16 spruce beams 3.1. Reinforcement materials
Fig. 3. Possible modes of failure for beams in flexure.
The fractured beams, which had been re-flattened after testing, were repaired using four rectangular section materials. These included; mild steel, pultruded carbon fibre reinforced plastic (CFRP), pultruded glass fibre reinforced plastic (GFRP) and pultruded glass fibre reinforced thermoplastic polyurethane (FULCRUM). Some mechanical properties of these materials and their dimensions are provided in Tables 1 and 2, respectively. The elastic modulus for CB10TSS is taken to be 1 GPa. The dimensions of the reinforcements differ somewhat due to manufacture and use in structural applications. Therefore, to maintain a working link between research and industrial practice, the reinforcement dimensions were
Fig. 4. Comparison of the flexural strengths of C16 spruce beams in relation to the failure mode.
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Table 1 Mechanical properties of the reinforcing materials.
Tensile modulus/GPa Compressive modulus/GPa Flexural modulus/GPa Tensile strength/MPa Compressive strength/MPa Flexural strength/MPa
bc t3c 12 bc t 2c Wc ¼ 6 Ic ¼
Mild steel
GFRP
CFRP
FULCRUM
210 – – 430–580 – –
40 30 45 1000 450 1000
150 – 150 2800 – –
45 – 45 1000 – 1150
Table 2 Dimensions of reinforcing materials applied as vertically laminating plates. Reinforcement
Length/mm
Depth/mm
Width/mm
Mild steel CFRP GFRP FULCRUM
1900 1900 1900 1900
20 20 20 20
5.0 1.5 4.0 1.2 2
kept the way they are typically used in industry. Repair work was carried out at Rotafix Ltd. on all 36 spruce beams. Three reinforcing configurations were used for repair; with some of the beams repaired on only the tensile face, some on only the compressive face and the remaining beams were repaired in both tension and compression. Surface pre-treatment applied to the reinforcing materials included grit blasting of the mild steel to SA 2.5 using guidance in SIS 055900 (1967), after which it was coated with a primer, removal of peel ply layer from CFRP composites prior to gluing, abrading of the GFRP surface using a sodium carbonate abrasive while the FULCRUM was left untreated. Grooves were routed into the faces of the spruce sections to a width of 12.7 mm and a depth of 20 mm. These dimensions are typically used by Rotafix Ltd. for on-site repairs. The reinforcing plates were inserted vertically into the grooves and bonded in using the thixotropic CB10TSS adhesive. 3.2. Section properties of repaired beams
ð2Þ ð3Þ
Ic denotes the composite second moment of area and Wc is the composite section modulus. The global widths and depths of the composite beams are correspondingly represented by bc and tc. 3.2.2. Transformed section modulus The location of the composite beam centroid must be determined in order to calculate the transformed section properties. This calculation is somewhat idealised, since the beam is already fractured and the true neutral axis will differ according to the magnitude and mode of fracture. Additionally, the beams repaired on only one face are asymmetrical and the neutral axis lies closer to the reinforcement. In order to calculate the transformed strength and make subsequent comparisons with unreinforced equivalents, a theoretical beam centroid needs to be determined. This is achieved by transforming the reinforcement and adhesive sections into theoretically corresponding spruce sections using a modular ratio technique. Figs. 5–7 show the hypothetical transformed sections of the reinforcement and adhesive to equivalent sections of spruce for beams repaired on the compressive, tensile and both faces, respectively. The compressive face sections are transformed according to Eq. (4).
b1 ¼ 2
Er br 2Ea ba þ 4bw þ Ew Ew
ð4Þ
Eq. (5) is used to calculate, from the top down, the geometrical centroid of the transformed beam, Cc, which gives rise to the calculation of the transformed second moment of area, Eq. (6) and the transformed section modulus, Eq. (7).
P
P Qi y i Ai y A1 þ ðt 1 þ y2 ÞA2 i Cc ¼ P ¼ P ¼ 1 Ai Ai A1 þ A2 i
i
ð5Þ
i
3 b1 t31 b2 t 2 þ ðb1 t1 ÞðC c y1 Þ2 þ þ ðb2 t 2 Þðt w C c y2 Þ2 It;c ¼ 12 12
ð6Þ
The mechanical properties calculated for a material in bending are implicitly related to the section properties, which can be very different depending on how the section properties are calculated. Two methods were used for calculating the section properties. The first was a classical method often used in fibre and particle composite mechanics, whilst the second was the transformed section method, frequently employed by structural engineers.
where t1 = tr = ta and t2 = tw t1.
3.2.1. Composite section properties The second moment of area and the section modulus of beams repaired on the compressive face, the tensile face and on both faces can be calculated using Eqs. 2 and 3, respectively.
Eq. (9) is used to calculate, from the bottom up, the geometrical centroid of the transformed beam, Ct, which gives rise to the calculation of the transformed second moment of area, Eq. (10) and the transformed section modulus, Eq. (11).
W t;c ¼
2It;c tw
ð7Þ
The tensile sections are transformed according to Eq. (8).
b1 ¼ 2
Er br 2Ea ba þ þ 4bw Ew Ew
Fig. 5. Transforming of spruce beams repaired on the compressive face.
ð8Þ
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99
Fig. 6. Transforming of spruce beams repaired on the compressive face.
Fig. 7. Transforming of spruce beams repaired on the both faces.
P
P Qi yi Ai y A1 þ ðt1 þ y2 ÞA2 i i Ct ¼ P ¼ P ¼ 1 Ai Ai A1 þ A 2 i
ð9Þ
i
3 3 b1 t 1 b2 t 2 It;t ¼ þ ðb1 t 1 ÞðC t y1 Þ2 þ þ ðb2 t 2 Þðt w C t y2 Þ2 12 12
ð10Þ
where t1 = tr = ta and t2 = tw t1.
W t;t
2It;t ¼ tw
4. Mechanical properties of repaired C16 spruce beams
ð11Þ
Sections on both faces are transformed according to Eq. (12).
Er br 2Ea ba b1 ¼ b3 ¼ 2 þ þ 4bw Ew Ew
ð12Þ
The geometrical centroid of the transformed beam is assumed to be an intersection point between lines of symmetry and the transformed second moment of area, together with the transformed section modulus are determined by Eqs. 13 and 14, respectively.
It;b
( 2 ) 3 b1 t31 t2 t1 b2 t2 þ ¼ þ ðb1 t 1 Þ þ 12 2 2 12 ( ) 2 b3 t33 t2 t3 þ þ ðb3 t 3 Þ þ 12 2 2
ð13Þ
where t1 = t3 = tr = ta and t2 = tw (t1 + t3).
W t;b ¼
2It;b tw
both faces are denoted by the added subscripts c, t and b, respectively. In Figs. 7–9 and Eqs. (4)–(14); the first moment of area Qi = yiAi, the surface areaAi = tibi, the vertical distance from the edge to the local centroid yi = ti/2, the local centroid is Ci and ti is the depth of the vertical beam section, i.
ð14Þ
The theoretical centroid for each beam is calculated using its own dimensional properties. The transformed second area moments, It, and transformed section moduli, Wt, are calculated utilising the parallel axis theorem. Beams repaired in compression, tension and on
4.1. Test method Repaired spruce beams were subjected to four-point bending at a crosshead rate of 2 mm min1. The beams were tested under service class 2 conditions (between 12% and 20% timber moisture content) according to Euro Code 5. An LVDT displacement transducer was attached to the moving crosshead and was used to monitor the centre point deflection (refer to Fig. 2). 4.2. Flexural modulus of repaired beams The difference in the flexural modulus of beams repaired with mild steel, CFRP, GFRP and FULCRUM reinforcement are shown in Figs. 8–10. These differences are relative to their original unreinforced properties. Values for beams repaired in compression are given in Fig. 8, for beams repaired in tension the values are shown in Fig. 9 and the values for beams repaired in both compression and tension are provided in Fig. 10. Each histogram gives the original flexural modulus for the undamaged C16 spruce beam adjacent to its repaired equivalent. The increase or reduction of the repaired beam relative to its unreinforced equivalent is indicated as a plus or minus percentage value. The x-axis scale is constant in each of the histograms to make it easier to compare the reinforcing materials as well as their location within the beams.
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Fig. 8. Flexural moduli of all beams repaired on the compressive face.
Fig. 9. Flexural moduli of all beams repaired on the tensile face.
Figs. 8–10 demonstrate that mild steel reinforcements generally yield the highest values of flexural modulus for the repaired spruce beams, as well as the most clear-cut trends between repairs made in compression, tension or on both faces. In the beams repaired by steel, repair on both faces shows generally higher stiffnesses than repairs on the tensile face, which in turn are superior to repairs to the compressive face of the beam. Although the reinforcement
volume fraction is equal for beams repaired in tension and compression only, the fracture paths tend to occur on the tensile face and consequently, repairing in tension is more effective as a result of the superior constraint of fracture. Constraining the fracture path in tension increases the effective geometrical depth of the composite beam, which would otherwise be reduced to a depth relative to the fracture depth.
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101
Fig. 10. Flexural moduli of all beams repaired on both faces.
The increased flexural modulus values of all steel repairs as compared to the pultruded composites can be attributed to the high stiffness characteristics of the steel. The extra load required to bend the composite beam results from the increased resistance to loading from the steel insert. Logically, the beams reinforced on both faces yield the highest stiffness enhancement at the very least due to the higher volume fraction of stiff reinforcing steel. The flexural modulus values for beams reinforced by the pultruded composite reinforcements, do not follow the trends observed for the steel reinforced beams. The CFRP, GFRP and FULCRUM all have significantly lower E-modulus values compared to the steel and lower loads are consequently required to flex the beams to the same extent as the steel repaired beams. As a result, the steel manages to enhance the flexural modulus of the repaired beam much more effectively than the pultruded composite reinforcements. Direct conclusions regarding repaired beam stiffness enhancements are difficult to make based upon the results in Figs. 8–10. However, there are some general observations that can be made. For example, higher stiffness reinforcements are, more effective as stiffness enhancing materials for repaired spruce beams than are lower stiffness reinforcements. Tensile face repairs are more effective in enhancing the flexural modulus than compressive face repairs, provided that equal volumes of reinforcement are used. Repairs on both faces are superior to repairing solely on the tensile and compressive faces because the volume fraction of reinforcement is higher and randomly located micro-cracks can be restrained more effectively. The effectiveness of repairing on only the compressive face depends critically upon the extent of the crack path. Larger fracture paths generally render compressive face reinforcements less effective. The tensile fracture paths have less effect on the flexural modulus when the beams are repaired in tension and on both faces. In this case the tensile repair restricts the effects of the presence of the crack and the effective depth of the composite beam extends to the outermost edge of the reinforcement.
4.3. Flexural strength of repaired beams Figs. 11–13 show the flexural strengths of beams and the difference in strength after repair. The figures give the results for all the reinforcing materials used in the three different repair configurations. The strength values of the unreinforced beams were measured at the first drop in load and the first drop in load is also used for the repaired beams, regardless of whether or not the peak load is higher. The flexural strengths, rf, are calculated using Eqs. (15)–(17) where a is the distance from the loading point to the bottom rollers, Ffirstdrop is the load at the first drop in load and Wc is the composite section modulus. The subscripts c, t and b make reference to beams repaired in compression, tension and on both faces, respectively.
aF firstdrop 2W c;c aF firstdrop rf ;t ¼ 2W c;t aF firstdrop rf ;b ¼ 2W c;b
rf ;c ¼
ð15Þ ð16Þ ð17Þ
Trends for the strength of spruce beams repaired with mild steel show essentially the same trends observed for the repaired modulus values as a function of reinforcement location and volume. The strength values and the percentage enhancement generally increase in the order of beams repaired with steel in compression to those repaired in tension to those repaired on both faces. Whereas the flexural moduli of the beams repaired with pultruded composite showed some level of randomness between the reinforcing series, their flexural strengths seem to increase in the order of beams repaired in compression, to those repaired in tension to those repaired on both sides, as for the beams repaired with steel. Generally, fracture paths have been observed to initiate on the tensile face of the beam. Repairing the compression face increases
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Fig. 11. Flexural strength for all beams repaired on the compressive face.
Fig. 12. Flexural strength for all beams repaired on the tensile face.
the strength more because of the added compressive strength and stiffness of the reinforcing material than as a result of constraining crack propagation. During flexure of a beam reinforced on the compression face but containing tensile fractures the load is taken by the compressive face reinforcement only. Once the elastic limits of the reinforcement are exceeded, a significant load has already been borne and yielding develops in the beam components. As
the tensile fractures increases in size and distribution within the spruce, the strength of the composite is reduced and the effectiveness of the repair is reduced. Repairing the tensile face with the same volume fraction of reinforcement does the job of both restraining (to some extent) the development of the fracture path as well as increasing the strength as a function of the added tensile strength of the repairing material. Repairing on both faces doubles
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Fig. 13. Flexural strength for all beams repaired on both faces.
the volume fraction of the reinforcement and constrains crack propagation on both faces. 4.4. Transformed strength of repaired beams Figs. 14–16 compare the repaired transformed strength values with the original strengths of the unreinforced beams for all configurations and reinforcing materials. The strength values of the unreinforced beams were measured at the first drop in load and once again the first drop in load is also used for the repaired beams regardless of whether or not the peak load is higher. The transformed flexural strengths are calculated using Eqs. (18)–(20). The subscripts c, t and b denote repair in compression, tension and on both faces, respectively.
aF firstdrop 2W t;c aF firstdrop rt;t ¼ 2W t;t aF firstdrop rt;b ¼ 2W t;b
rt;c ¼
ð18Þ ð19Þ ð20Þ
An advantage of using the transformed section method is that it permits the analysis of the flexural strength in relation to the materials and geometrical properties of the reinforcement used. Regardless of the assumptions involved in a transformed section analysis, such as the estimation of the geometrical properties from material properties, it is still a widely used method that allows the comparison of different volume fractions of reinforcing materials possessing different mechanical properties. Observation of the trends between the three different reinforcement configurations shows that beams repaired on the compression face are less effective than beams repaired in tension, which in turn are less effective than beams repaired on both faces. This complements the conclusions made from observations of the flexural strengths (refer to Figs. 11–13). In contrast to the results for the flexural strengths, the percentage enhancement in transformed flexural strength compared to
beams in their undamaged states is highest for the beams repaired with CFRP and not steel. In terms of transformed flexural strength, the CFRP pultrusions are therefore more effective for repairing fractured beams than GFRP and FULCRUM pultrusions and mild steel. In general the repair of beams has a more significant effect on improving the flexural strength and the transformed flexural strength of beams than it does on flexural stiffness (compare Figs. 10, 13 and 16 for beams repaired on both faces). It is suggested that the reinforcing material is less effective at increasing the flexural modulus of the original beam because of its relatively low volume fraction. Other factors such as bond integrity, position of the reinforcement and whether reinforcement is located on one or both faces clearly influence the flexural stiffness and higher modulus reinforcements will have a greater stiffening effect than their lower stiffness counterparts. In contrast, flexural strength, is not only a function of the reinforcement properties but is also dependent upon the extent to which the reinforcement manages to halt crack growth. 4.5. Post-repair failure modes of beams Various modes of failure observed in unreinforced spruce beams are illustrated in Fig. 3. Clear differences in failure mode were observed as a function of the position of the reinforcement. In most of the beams repaired in compression, fracture develops from the tension face to a level close to the reinforcing material after which the fracture path takes a horizontal path and proceeds closely parallel to the reinforcing material, but without touching it. Furthermore, for beams repaired on the compression face, neither buckling of this face in the spruce nor de-bonding between reinforcement and adhesive was observed. In contrast, spruce beams repaired on the tensile face often buckled appreciably on the compression face. In beams repaired on both faces, fracture paths propagate by horizontal shear around the axis of neutrality in these beams. More reinforcement to adhesive de-bonding was observed in the beams reinforced on both faces than was observed for the other configurations. This is believed to be because higher stresses
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Fig. 14. Transformed flexural strength for all beams repaired on the compressive face.
Fig. 15. Transformed flexural strength for all beams repaired on the tensile face.
were experienced by beams repaired on both faces relative to the beams repaired on only one face. Although beams repaired in tension using CFRP experienced, in some cases, stresses as high as the steel plate repaired beams, the stress required to activate de-bonding between the steel plate and the adhesive is perhaps lower than that between the CFRP and the adhesive. The consequence is steel-adhesive de-bonding, but no CFRP-adhesive de-bonding. GFRP-repaired beams fail at lower
stresses than the CFRP or steel-repaired beams and hence de-bonding was not observed. Fractures will always take the path that is mechanically weakest. Hence, when fracture initiates in the wood, it tends to propagate through the wood and works its way around the adhesive and reinforcement. In that sense, the reinforcement configuration, rather than material, plays a significant role in resisting cross fracture initiating from the tensile face. Tensile face fractures that may ordinarily run across the width of the beam
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Fig. 16. Transformed flexural strength for all beams repaired on both faces.
would need to take a different path and most likely would resist a higher level of loading.
ing and Physical Sciences Research Council (EPSRC) for financing his studentship at the University of Bath.
5. Conclusions
References
Class C16 spruce beams were fractured in four-point bending and the flexural modulus and strength were recorded. The beams were straightened to allow repair using adhesively bonded-in reinforcement in the form of rectangular section steel, CFRP and two grades of GFRP. The reinforcement was bonded into routed slots on the fractured beam faces with a thixotropic epoxy adhesive. The repaired beams were tested to failure and the effect of the position of the reinforcement on the flexural modulus, flexural strength and transformed flexural strength were assessed. The reinforcements bonded into the top and bottom faces of the beams were, not surprisingly, more effective than reinforcements bonded into the top or bottom face. Steel reinforcement has the greatest effect on beam stiffness which was often considerably greater than the stiffness of the original grade C16 beam. Steel and CFRP have the greatness ability to enhance flexural strength but CFRP has the greatest effect on the transformed flexural strength. The position of reinforcement and the integrity of the reinforcement to adhesive bond influence the fracture mode of the repaired beams. Bonded-in reinforcement is an effective method for repairing damaged timber structures.
Acknowledgments The authors thank Stan Bowen, Rotafix Ltd. for constructing and repairing the test beams. The first author thanks the UK Engineer-
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