간벌재로 생산된 낙엽송 제재목은 원주목이나 칩 등의 저부가가치산업에 활용된다. 국내산 낙엽송 각재의 고부가가치화를 위하여 철근 및 FRP(Fiber Reinforced Polymer)로 보강하여 적층한 복합적층재를 제작하여 건축부재로써의 활용가능성을 검토하였다.
국내산 낙엽송 각재의 휨 강도 성능을 예측을 위해 비파괴적인 지표 검토가 필요하다. 낙엽송 각재를 초음파법과 종진동법 및 소하중법의 비파괴 방법으로 예측탄성계수를 측정하였다. 종진동법으로 측정된 예측탄성계수가 실측 탄성계수와 가장 높은 유의성을 보였다. 휨 파괴계수와 실측 탄성계수의 유의성은 매우 밀접하게 측정되어 종진동법에 의한 비파괴 탄성계수로 각재의 휨 파괴계수를 예측하였다.
각재를 대단면 건축부재로 활용가능한 복합각재적층목은 각재와 각재간 다중접착성능과, 보강재와 각재간 접착성능 평가가 필요하다. 제작된 시험편들은 침지박리시험과 삶음박리시험 및 블록 전단 시험으로 접착성능을 평가하였다. 실험결과, 수 위치에 따른 접착면의 접착강도는 수가 각재의 중앙과 접착면 사이에 존재(접선단면과 방사단면이 혼재된 단면)한 두 단면을 접착한 시험편이 가장 양호하였다. 수가 각재의 접착면에 있는 단면(방사단면)을 접착한 시험편들은 높은 박리율을 보였다. 낙엽송 각재와 GFRP간 접착강도는 PVAc 접착제가 양호하였으며, CFRP는 Epoxy접착제, 강판의 경우 Poly-urethane접착제가 양호한 접착강도를 보였다.
각재를 이용한 복합각재적층목을 기둥-보 목조건축 구조재로 이용하기 위해선 장대재 제작이 가능하여야 한다. 따라서 낙엽송 각재의 종접합 성능을 평가하였다. 시험편들은 종 접합 방법에 따라 6가지로 제작하였으며 인장 및 휨 강도시험으로 종 접합성능을 검토하였다. Rod+Lap 시험편의 평균 휨 파괴계수는 30.5 MPa로 종 접합한 시험편들 중 가장 양호한 성능을 발휘하였다. Rod+Lap 시험편의 평균 휨 강도는 종접합하지 않은 대조군 시험편 평균 휨 파괴계수의 66%에 해당되었다. 낙엽송 각재의 종접합 방법은 봉형 GFRP를 삽입 접착한 방법이 가장 유효한 강도를 보였다.
철근 및 FRP로 보강한 복합각재의 단기내력성능은 보부재와 기둥부재 및 접합부에 대하여 실시하여야한다. 복합각재 보부재의 휨 성능평가를 위한 시험편들은 종류에 따라 7가지 유형으로 제작하였으며 유형별 5개씩 총 35개의 보강시험편과 보강하지 않은 5개의 대조군 시험편을 제작하였다. 휨 강도 시험 결과, 보강각재들의 휨 강도는 대조군 시험편보다 평균 69% ~ 130% 향상되었다. 각재의 인장부 전체를 CFRP로 보강한 CFRP60 시험편의 휨 성능은 대조군 대비 2.3배 향상되어 강도 증가 폭이 가장 컸다. 대부분의 시험편들의 파괴형상은 보부재의 인장부에서 보강재와 목재가 같이 파괴되었다. 보강재는 파괴되지 않고 인장부위의 목재만 파괴된 시험편들은 각재가 파괴된 후에도 최대하중의 약 80%를 유지하는 경향을 보였다. 최대 휨 모멘트 예측결과 실측 최대 휨 모멘트와 -2 ~ 21%의 편차를 보였다. 목질 기둥재의 내진설계 성능수준을 만족하기 위하여 철근 및 FRP로 보강된 복합적층 기둥재를 제작하여 압축강도 성능을 평가하였다. 복합적층재들은 압축강도 향상을 위한 유형(CFRP bar, GFRP rod, Steel bar)과 섬유경사각에 의한 파단 억제 유형(CFRP plate, GFRP plate) 두 가지로 제작하였다. 실험결과, CFRP bar로 보강한 시험편의 평균 압축강도는 대조군보다 약 7% 증가하였으며, 봉형 GFRP rod와 Steel bar로 보강한 시험편들의 평균 압축강도는 대조군보다 각각 38%, 37% 증가하였다. Steel bar로 보강한 시험편은 옹이 유무에 따른 강도편차가 미비하였다. 보강각재의 실측 압축강도와 예측압축강도를 비교해본 결과, GFRP plate로 보강한 시험편을 제외한 모든 시험편의 강도 차이가 2~10.4%로 양호하였다. 접합내력은 인장형 전단시험편을 제작하여 검토하였다. 강판삽입형으로 제작된 시험편은 비보강 Control시험편과 CFRP를 강판 슬릿부위와 목질부위에 보강한 보강시험편(체적비 6.5%)으로 제작하였다. 실험결과, 최대 접합내력과 항복전단내력은 CFRP보강위치에 따른 차이를 보이지 않았으나, CFRP보강 위치가 강판이 삽입된 부위에 가까운 유형일수록 접합변형은 적었다. 보강된 시험편들이 대조군 시험편들보다 평균접합내력은 65%~95%, 평균항복전단내력은 24%~29%향상되었다. 5D에 CFRP로 보강한 모든 시험편들이 보강하지 않은 7D 시험편보다 평균 70% 향상된 평균접합내력과 평균 26% 향상된 항복전단내력을 보였다. 낙엽송 각재와 CFRP로 보강된 각재의 지압강도를 측정하여 항복전단내력을 예측하였다. 예측항복전단내력 추정식이 본 연구의 실측항복전단내력과 유사하였다.
복합각재적층재의 장기내력성능은 FRP를 보강한 낙엽송 각재 적층목의 온·습도 변동에 따른 크리프 실험을 하였다. 시험편은 낙엽송 각재 2개를 적층한 대조군과 동일 단면적의 집성재 그리고 FRP(CFRP, GFRP)로 인장부위가 보강된 시험편 2종류를 제작하였다. 시험편들은 각각2본씩 총 8본의 시험편을 실내에 설치하여 약 14000시간동안 오전 9시 저녁 6시에 크리프거동을 측정하였고, 온도와 습도는 1시간마다 측정하였다. 연구결과, 평형함수율과 습도는 비례관계이고, 시험편의 함수율과 습도 및 평형함수율은 상관관계를 보이지 않았다. 상대크리프의 평균 값은 Control이 약0.67로 측정되었고 Glulam은 0.4, CFRP로 보강한 시험편은 0.43, GFRP로 보강한 시험편도 0.43으로 측정되었다. 보강재 유형에 따라 상대크리프 값의 차이는 보이지 않았다.
따라서 국내산 낙엽송 각재로 제작된 복합각재적층목은 구조용 건축부재로써 접착성능과 부재의 단개내력성능 및 장기내력성능이 검토되어 대단면 건축부재로 활용 가능성을 보았다.

Larch timber fabricated from thinning wood is used in low-added value industry such as solid wood and chips. For high-added valuing of domestic larch timber, the availability as a construction member was reviewed by reinforcing rebars and FRP (Fiber Reinforced Polymer) and fabricating laminated composites.
It is necessary to review non-destructive indexes in order to estimate the bending strength performance of a domestic larch lumber. In this study, MOE''s (modulus of elasticity) of the larch lumber were measured through non-destructive methods such as an ultra-sonic method, a longitudinal vibration method, and a non-destructive bending method. The estimated MOE measured through a longitudinal method showed the highest correlation with the actual MOE. As significance between the beding MOR (modulus of rupture) and the actual MOE was very closely measured, the non-destructive MOE measured through a longitudinal vibration method was used to estimate the bending MOR of the lumber.
Composite laminated timber of which a timber can be used as a large structural member requires an evaluation of multi-bonding performance between timbers and of bonding performance between a reinforcement and a timber. The bonding performance of the fabricated specimens was experimented through a water soaking delamination test, a water boiling delamination test, and a block shear strength test. The experiment results showed that the bonding strength of the bonding surface according to the pith poisition was the highest in the specimen for which the two sections with the pith at the center of the cross-section on a timber and between the bonding surfaces (the tangential and radial sections were mixed) were bonded. Furthermore, the specimens for which the section (radial section) with the pith on the bonding surface of the timber was bonded showed a high delamination percentage. The PVAc adhesive showed the highest bonding strength between a larnch timber and GFRP, while Epoxy adhesive showed the highest bonding strength for CFRP, and Poly-urethane adhesive for a steel plate.
In order to use composite laminated timber as a structural member for posts and beam wooden construction, manufacturing long-splan lumber must be possible. Accordingly, in this study, the researchers conducted a performance evaluation for longitudinal bonding of larch lumber. The specimens were prepared in six different forms in accordance with the longitudinal bonding method. The longitudinal bonding strength of those specimens were experimented through tensile strength tests and bending strength tests. The average MOR strength of the Rod + Lap specimen was 30.5MPa, which was the highest among the longitudinally bonded specimens. The bending strength of the Rod + Lap specimen showed an effective strength that was 66% of that of the Control group, which was not longitudinally bonded. The longitudinal bonding method of the larch timber showed the highest strength when rod GFRP insertion method was applied.
The short-term strength performance of composite timbers reinforced with a steel bar and FRP must be evaluated for beams, columns, and joints. Specimens for bending performance evaluation of the composite timber beams were fabricated in seven types according to the reinforcement type. A total 35 of reinforced specimens, five specimens for each type and five unreinforced Control specimens, were fabricated. It was determined in the bending test that the bending strength of the composite timbers improved by 69~130% over the Control specimens on average. The bending performance of the CFRP60 specimen, which was reinforced with carbon-fiber-reinforced plastic (CFRP) for the entire tensile zone of timber, was 2.3 times higher than that of the Control, showing the greatest increase in strength. Most of the specimens showed a fracture shape accompanied by the fracture of the reinforcement and timber in the tension part of the beam member. The specimens of which reinforcements were not fractured and in which only the timber on the tension part was fractured showed a tendency to maintain approximately 80% of the maximum load even after the timber was fractured. The predicted result of the maximum bending moment showed -2~21% deviations from the measured maximum bending moment. Laminated timbers composed of small-diameter timber columns reinforced with a steel bar and FRP were fabricated to satisfy the seismic design performance level of wooden columns, and their compression strength was evaluated. Laminated composite members were fabricated in two types: a type (a CFRP bar, a GFRP rod, a steel bar) to improve compression strength and a type (a CFRP plate, a GFRP plate) to suppress fractures by inclination angle of grain. The experiment results showed that the average compression strength of the specimen reinforced with a CFRP bar increased by approximately 7% compared to that of the Control. The average compression strength of the specimens reinforced with a GFRP bar and a steel bar increased by 38% and 37% respectively compared to that of the Control. The strength difference of a specimen reinforced with a steel bar depending on the existence of a gnarl was inadequate. The experiment result of comparing the actual compression strength and the estimated compression strength of the reinforced timber showed that all specimens, except the one reinforced with a GFRP plate, showed satisfactory strength differences of 2~10.4%. The bonding strength was evaluated by fabricating a tention-shear specimen. The specimen frabricated by inserting a steel plate was fabricated with an unreinforced Control specimen and a reinforced specimen (volume ratio 6.5%) with CFRP reinforced on a steel plate slit part and lignum part. The experiment result showed that the maximum bonding strength and the yield shear strength did not show any difference in accordance with the reinforcement location of CFRP. However, the bonding deformation was smaller when the CFRP-reinforced locatin was closer to the part where the steel plate was inserted. The average bonding strength of the reinforced specimens improved by 65~95%, while the average yield shear strength improved by 24~29% compared to those of the Control specimens. The bonding strength and the yield shear strength of 5D specimens reinforced with CFRP improved by 70% and 26% more, respectively, on average than those of the unreinforced 7D specimens did. The bearing strength of a larch timber and a CFRP-reinforced larch was measured to estimate the yield shear strength. The estimation equation for the estimated yield shear strength was similar to the actual yield shear strength figured in this study.
Duration of load of a composite laminated timber member went through a creep experiement according to changes in the temperature and the humidity of a larch timber laminated wood reinforced with FRP. The specimen was fabricated in two types: a Control laminated with two larch timbers and a specimen with the tensile part reinforced with the lamination wood of the same cross sectional area and FRP (CFRP, GFRP). A total 8 of the specimens, two of each, were installed indoors and their creep behaviour was measured from 9am to 6pm, for approximately 14,000 hours. The temperature and the the humidity were measured every hour. The experiment results showed that the equilibrium moisture content and the humidity were in a proportional relationship and that the mositure content, the humidity, and the equilibrium moisture content of the specimens did not have any correlation. The average value of relative creep for each specimen was measured: approximately 0.67 for the Control, 0.4 for Glulam, 0.43 for the CFRP-reinforced specimen and the GFRP-reinforced specimen. The result also showed that the relative creep value did not depend on the type of reinforcement.
In conclusion, a composite timber laminated wood made of domestic larch timber showed a possibility of use as a large construction member, as it was reviewed to have the bonding performance, the short-term strength performance, and the duration of load performance as a structural construction member.