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Nondestructive testing (NDT) includes several highly efficient techniques for the estimation of the physical and mechanical properties of structural timber. However, NDT results are affected by several factors [moisture content (MC), temperature, specimen dimensions, sensor positioning and grain angle, timber-sensor coupling, etc.], of which wood MC is the most important. Scientific research on MC influence in NDT measurements started in the 19th century, and 2018 was the first year that a MC adjustment for NDT measurements was published in a European standard (EN14081-2). Although MC influence on NDT has been studied for more than 170 years, MC adjustment factor research is important nowadays because it is widely used in the industry. Currently, NDT device manufacturers develop their own MC adjustment factors adapted to their equipment and species. This paper presents an exhaustive review of MC adjustments for different types of NDT equipment, as proposed by several authors from the 19th century to date, and a discussion of factors affecting MC adjustments. [ABSTRACT FROM AUTHOR]

Nondestructive testing (NDT) includes several highly efficient techniques for the estimation of the physical and mechanical properties of structural timber. Apart from visual grading, scientific research using Nondestructive testing on timber has been used in Spain since the 1990s. Nondestructive testing can be used for two different purposes: timber grading and the assessment of existing timber structures. The most common devices used in Spain are portable ones based on ultrasound, stress waves, vibration and probing techniques. Many statistical linear models for estimating the mechanical properties of new sawn timber and timber from existing structures have been proposed. Furthermore, several factors that affect Nondestructive testing measurements have been studied (moisture content, temperature, specimen dimensions, sensors position-grain angle, among others) and adjustment factors have also been proposed. Species have been characterized for visual grading standards from the 1980s to date. The large number of research works using different species, devices and procedures shows the need of homogenization and standardization of Nondestructive testing use. This paper presents a review of research works using Nondestructive testing on timber in Spain, in order to add to knowledge, elucidate the concepts to unify Nondestructive testing used and promote research group collaboration in the near future. [ABSTRACT FROM AUTHOR]

The predictability of modulus of elasticity (MOE), modulus of rupture (MOR) and density of 120 samples of Scots pine (Pinus sylvestris L.) were investigated using various non-destructive variables (such as time of flight of stress wave, natural frequency of longitudinal vibration, penetration depth, pullout resistance, visual grading and concentrated knot diameter ratio), and based on multivariate algorithms, applying WEKA as machine learning software. The algorithms used were: multivariate linear regression (MLR), Gaussian, Lazy, artificial neural network (ANN), Rules and decision Tree. The models were quantified based on the root-mean-square error (RMSE) and the coefficient of determination (R2). To avoid model overfitting, the modeling was built and the results validated via the so-called 10-fold cross-validation. MLR with the "greedy method" for variable selection based on the Akaike information metric (MLRak) significantly reduced the RMSE of MOR and MOE compared to univariate linear regressions (ULR). However, this reduction was not significant for density prediction. The predictability of MLRak was not improved by any other of the tested algorithms. Specifically, non-linear models, such as multilayer perceptron, did not contribute any significant improvements over linear models. Finally, MLRak models were simplified by discarding the variables that produce the lowest RMSE increment. The resulted models could be even further simplified without significant RMSE increment. [ABSTRACT FROM AUTHOR]

Needle penetration resistance (NPR), screw withdrawal resistance (SWR), core drilling (CD) and drilling chips extraction (DCE) are nondestructive and semi-destructive techniques used to estimate density in timber structures. In most of the previous studies, these techniques were tested in clear sawn timber and clear specimens. The goal of the present paper is to study the relationship between density and these techniques by means of five different devices in whole pieces of timber from built-in engineering structures, which are from 12 4.5-m long structural timber joists of Norway spruce from a 19th century building in Barcelona (Catalonia, Spain). Although determination coefficients (R2) for density estimation models were lower than those from clear timber, the results obtained confirmed that these four techniques are suitable for in-situ density estimation of woods in buildings. The best results were obtained by CD (the bigger the bit, the higher the correlation), followed by DCE, and SWR. The worst correlation was found for NPR devices, but the results could be probably improved with more measurements. [ABSTRACT FROM AUTHOR]

• Recovered timber from demolition is a reliable structural material. • Visual strength grading standards do not properly grade recovered timber. • Longitudinal vibration accurately estimates mechanical properties. • Non-destructive probing methods allow accurate prediction of recovered timber density. An increasing amount of timber is recovered from demolition and can be value-added through reuse or recycling for structural applications. This study investigates the possibilities of using visual and non-destructive parameters to estimate the mechanical properties of recovered timber so that it can be graded for new applications. A batch of nineteen recovered Norway spruce (Picea abies (L.) Karst.) rafters was studied. The application of two visual grading standards produced a rejection of 95% of the specimens. Estimation of modulus of elasticity was performed using ultrasound and vibration NDT techniques, with determination coefficients (R2) of 0.66 and 0.75 respectively. In the case of bending strength estimation, R2 were 0.60 and 0.77 respectively. Visual knottiness parameter was only a statistically significant variable in the estimation model of local modulus of elasticity. Density was estimated using drilling chips extraction and drilling penetration resistance with R2 of 0.66 and 0.58 respectively. [ABSTRACT FROM AUTHOR]

Non-destructive techniques (NDTs) are well suited for rapid estimation of timber properties, but NDT results are affected by several factors, the most important of which is the moisture content (MC) of wood. Much of the research in this context was limited to ultrasound measurement of a few wood species, mainly to Norway spruce. The present paper investigates the MC influence on the NDT results obtained by instruments based on ultrasound (two devices), impact stress waves (one device) and longitudinal vibrations (two devices). A hundred large cross-section specimens of four timber species were tested, namely: radiata pine, Scots pine, Salzmann pine and maritime pine. The influence of MC on velocity was found to be stronger below the fiber saturation point (FSP) than above FSP. MC adjustment factors below FSP are proposed for these wood species. [ABSTRACT FROM AUTHOR]

The main objective of this paper is to define the reference conditions and to propose modification factors to standardize the variables recorded by different nondestructive techniques. This practice will make it possible to generate consistent practical data while developing a procedure for the standardization of timber structure assessment. The nondestructive variables analyzed are: time of flight, natural frequency, pullout resistance, penetration depth and drill resistance. The factors which affect these variables are: moisture content, temperature, size/length and positioning of sensors and angle to the grain. Reference conditions are defined and equations and modifications factors are proposed to help in the evaluation of existing timber structures and decision making. [ABSTRACT FROM AUTHOR]

Both non-destructive and semi-destructive tests can potentially be very efficient methods for the assessment of structural timber density. This paper describes an investigation into the suitability of three techniques: core drilling, probing, and screw withdrawal. It presents the results after testing 150 pieces of large cross-section (80 mm x 120 mm) structural timber of radiata pine (Pinus radiata D. Don.) from a Spanish source. A strong correlation was found between specimen density and core drilling. Meanwhile, there was also a meaningful correlation with the screw withdrawal, and an acceptable correlation with probing. Even though differences were observed in their predictive capacity, none of these procedures should be rejected as a way of estimating density, as each has its own respective advantages and limitations. [ABSTRACT FROM AUTHOR]

The non-destructive testing (NDT) of timber using the longitudinal vibration method is based on the natural frequency of wood which is in relation to its quality. In the present paper, the suitability of this tool is investigated and the results of grading 395 pieces are presented. Structural timber of Radiata pine ( Pinus radiata D. Don.), Scots pine ( Pinus sylvestris L.), and Laricio pine [ Pinus nigra ssp. salzmannii (Dunal) Franco] from Spanish sources were investigated. The specimens were tested for bending according to the European standard EN 408 (2003) and the values of strength and stiffness were compared with the results estimated by means of NDT. The vibration equipment applied permits the measurement of the longitudinal natural frequency and mass of the specimen, and then the density and the dynamic modulus of elasticity can be calculated. There is a strong relationship between the static modulus of elasticity obtained from the bending test and the dynamic modulus of elasticity obtained by the NDT technique. There is an acceptable relationship between modulus of rupture and dynamic modulus of elasticity if the visual defects (knot sizes) are taken into account. Acoustic measurements have become widely acceptable, and they have great potential for stress grading of coniferous timber. [ABSTRACT FROM AUTHOR]

The use of large cross-section timber for structural purposes has increased in Spain, and knowledge of its properties is strategically necessary. The Spanish visual strength-grading standard UNE 56544 (2011) efficiency applied to large cross-section structural timber was analyzed using a sample of 363 specimens of radiata pine (Pinus radiata D. Don.) from the Basque Country and Catalonia, Spain. Different sizes were tested (80 × 120 × 2400 mm³, 150 × 250 × 5600 mm³, 150 × 250 × 4300 mm³, and 200 × 250 × 5000 mm³). Bending strength, modulus of elasticity, and density were obtained, and characteristic values were determined in order to assign strength class according to European standard EN 338 (2010). Knots and twists were the most relevant singularities for visual strength grading. It was concluded that large crosssection Spanish radiata pine timber was suitable for structures, and it was assigned to the C20 strength class. [ABSTRACT FROM AUTHOR]

Three samples of sawn Radiata pine (Pinus radiata D. Don) timber, consisting of 150 pieces with dimensions of 80 mm x 120 mm x 2400 mm, 80 pieces with dimensions of 150 mm x 200 mm x 4500 mm, and 75 pieces with dimensions of 150 mm x 250 mm x 5600 mm, were visually graded according to the Spanish standard UNE 56544 (2011) to analyze the influence of visual grading requirements on both the grading yield and mechanical properties. The resulting percentages of the rejected pieces stood at 19 and 28% for 150 x 200 mm and 150 x 250 mm cross-section pieces, respectively. That percentage increased to 73% for 80 x 120 mm cross-section pieces. The grading yield and mechanical properties were then analyzed to determine the influence of warp defects. Three different criteria for limiting warp defects were considered, reducing the standard specifications. The modulus of elasticity, bending strength, and density were obtained. The results concluded that loosening specified requisites improves the visual grading output in the smallest cross-section, with no significant reduction in the mechanical properties. A proposal for a less rigorous specification of twist defects in the Spanish visual grading standard has been introduced. [ABSTRACT FROM AUTHOR]

This presents an exploratory analysis of a method for the determination of elastic constants using strain gauges in compression tests. The elastic constants of Scots pine ( Pinus sylvestris L.) wood from the “Valsaín” sawmill (Segovia, Spain) were obtained. Compression tests were performed on small clear specimens, varying grain direction with respect to the direction of the load applied. Deformations were measured by strain gauges bonded on specimen surfaces. The modulus of elasticity parallel to the grain was obtained for prismatic specimens measuring 20 × 20 × 60 mm. The modulus of elasticity perpendicular to the grain in both radial and tangential directions was obtained for 16 × 16 × 48 mm prismatic specimens and for 48 × 48 × 48 mm cubic specimens. The shear modulus in the LR , LT and TR (longitudinal ( L ), radial ( R ) and tangential ( T )) planes was obtained for 16 × 16 × 48 mm prismatic specimens with the grain at 45°. Larger stiffness values than typical average values for softwood were obtained. It can be concluded that this method is suitable for determining the modulus of elasticity (longitudinal and transversal), and that the values obtained are greater than the average values for softwoods. On the other hand, the Poisson ratios obtained parallel to the grain were similar to values in the literature, though values for ν RT and ν TR were greater. [ABSTRACT FROM AUTHOR]

• 21 150 × 200 × 11,000 mm3 Salzmann pine rafters from 18th century building were analysed. • Criteria were established to assign the most adequate nominal cross-section dimension. • NDT measurements were used to optimize the nominal size dimensions assignment. • Central area cross-section showed the best MOE estimation with ToF methods. • Global mean cross-section showed the best MOE estimation with vibration analysis. The frequent highly irregular geometry of the elements in existing timber structures complicates the structural verification of members and influences the use of non-destructive testing (NDT), affecting the inspection time, cost and results obtained. This process therefore needs to be improved. The main aim of this paper is to analyse how to obtain a representative nominal cross-section (NCS) of members, and its influence on determination of the static modulus of elasticity (MOE sta). 21 150 × 200 × 11,000 mm3 Salzmann pine (Pinus nigra subsp. salzmannii (Dunal) Franco) timber rafters from the 18th century Royal Coliseum of Charles III theatre (in Aranjuez, Madrid, Spain) were acoustically tested (by ultrasound, stress wave and vibration). The MOE sta was determined by mechanical testing, and different criteria for determining the NCS were analysed. Good correlation was obtained between the MOE sta and NDT parameters, and the best criterion to assign NCS was proposed to improve the accuracy of the models obtained by reducing the number of NCS measurements. [ABSTRACT FROM AUTHOR]

The mechanical properties of timber can be estimated from wave propagation velocity by measuring wave time-of-flight (ToF). However, a time-lag complicates the measurements, which produces an apparent velocity dependency on length and this species and instrument dependent property is also influenced by knottiness. This research is dealing with time-lag determination by different sensor positioning in situ ToF measurements. ToF longitudinal measurements were conducted on 120 90 mm × 140 mm specimens of the coniferous species radiata pine, Scots pine, laricio pine and maritime pine. The following commercially available acoustic devices were used: Sylvatest Duo, USLab, and Microsecond Timer. The sensors were arranged for the measurement types "end-to-end", "on the same surface" and "on opposite surfaces". ToF data were obtained from the full-length (4 m) specimens and then from the same specimens shortened to 3 m, 2 m and 1 m in length. The in situ procedures of ToF are applicable for a reliable length determination independently from the time-lag (tL) and velocity. The differences observed by end-to-end measurements, with respect to velocity, are below 4.4%. A velocity correction factor can be deduced for each instrument, which is independent of species. [ABSTRACT FROM AUTHOR]

• Cross laminated timber from reclaimed timber has good structural properties. • Visual strength grading standards do not properly grade reclaimed timber. • Bending strength in CLT from reclaimed timber is lower than in CLT from new timber. • Transversal vibration accurately estimates the modulus of elasticity of CLT panels. The demolition sector generates a large amount of timber waste that could be directly reused or recycled in other products for structural purposes. Timber should be graded before it is used for structural purposes, and visual strength grading standards designed for new timber do not properly grade recovered timber. Cross Laminated Timber (CLT) is now one of the most common wood products used in construction. CLT would therefore be a good option for recycling timber due to the high quantity of material used in CLT manufacturing. This paper investigates the possibilities of using recovered timber from demolition to manufacture CLT. Twelve CLT panels from recovered and new timber were manufactured and tested. The static modulus of elasticity was found to be the same between recovered and new timber, while the bending strength of CLT from recovered timber was lower than it was for CLT from new timber. Non-destructive testing for the estimation of mechanical properties of boards and CLT panels was successfully developed. [ABSTRACT FROM AUTHOR]

• Strength capacity of an 18th century timber structure using NDT verified with load tests. • Structural behavior of the floor slab considering the transverse stiffness of the system. • Strength estimation (bending, compression and shear) through their relationship with stiffness. • Verification of structural safety in accordance with Eurocode 5 Timber structures. The timber structural members of a seven-story building from the end of the 18th century were evaluated following a procedure based on previous research works. The structure consisted of timber-framed brick walls and heavy timber floors with mortar filling. Non-destructive measurements were taken in 13 sampling areas of the floors and framed walls. Detailed measurements of at least 3 pieces in each sample were taken (stress wave Time-of-Flight and probing tests). Mean density and modulus of elasticity values and characteristic values of bending and compression strength were estimated. On-site load tests were carried out on three floor slabs to deduce their actual bending stiffness and compare it with estimation by NDT. They behaved more rigidly than expected, which is explained by floor screed stiffness, the transverse stiffness effect and possible semi-rigid connection behaviour at the supports. Structural verification determined the required strength of the critical members to compare this with the estimated strengths, showing a correct safety level. [ABSTRACT FROM AUTHOR]