Bond Strength Development Research Articles

A discussion on the various aspects of the minimum pressing time for polyurethane adhesives

ABSTRACT For many purposes related to the bonding of wood substrates, one-component polyurethane adhesives (1C-PUR) are of great significance. 1C-PUR adhesives are widely used, especially in the cross laminated timber (CLT) production. One major benefit is their ability to be pressed at room temperature. However, not only pressing temperature but also the pressing time is an important economic aspect for manufacturers. Knowing the minimum possible press time offers an economic advantage, however, there have been only few publications on this topic so far. The aim of this work was to investigate and evaluate various aspects of a 1C-PUR adhesive with regard to the minimum pressing time comparing newly developed knowledge-based approaches. The analysis included the strength development of the advanced curing of the adhesive, the development of the final internal bond strength and the minimum required press time for bonded joints to withstand imposed stress. The influence of the wood moisture content and closed assembly time were also investigated. The present study found that closed assembly time had a favourable influence on the required press time period. With immediate testing of the glued joint it also revealed a quicker strength development with lower strength values at a higher wood moisture content.

Relationship between thermal curing behavior and bond strength development in formaldehyde-based resins with different molecular weights

Urea–formaldehyde (UF) and phenol–formaldehyde (PF) resins are the most widely used adhesives in the production of wood composites, and are cured through hot-pressing to provide adhesion strength with the composites. This study investigates the relationship between thermal curing behavior monitored by differential scanning calorimetry (DSC) and adhesive bond strength development measured by an automated bonding evaluation system (ABES) for UF and PF resins with different molecular weights (M w). DSC results demonstrated that UF resins cured faster than PF resins, as indicated by a lower apparent activation energy (E a). In addition, UF and PF resins with lower M w cured faster in the early stages of curing and slower in the latter stages of curing as compared to higher-M w resins. Furthermore, according to the ABES results, when increasing the molecular weight, both UF and PF resins exhibited greater adhesive bond strength, irrespective of pressing parameters (temperature and time). The isothermal DSC and ABES results demonstrated that the adhesive bond strength increased with an increase in the degree of cure of the resins, indicating a correlation between them. It also showed that UF resins attained optimal adhesive bond strength at low temperature (110–130 °C) in roughly 180 s while PF resins attained it at high temperature (120–130 °C) in about 300 s. However, the degree of cure under isothermal scans was inconsistent with temperatures. These results indicate that the degree of cure is closely related to the adhesion strength development of UF and PF resins which are curing at low and high temperatures to obtain the optimum adhesion strength.

Investigation on bonding between aggregates and asphalt binder containing warm mix additives

Increased moisture susceptibility has been identified as one of the potential failures for warm-mix asphalt (WMA). In order to critically understand the moisture damage mechanism, the evaluation of bond strength between aggregate and asphalt binder holds significant importance. The bonding mechanism can be easily explained by Pneumatic Adhesion Test (PAT). This study evaluated the bond between various aggregates and warm mix (WM) modified asphalt binders using PAT. Five aggregates, two base binders, and four WM additives with varying dosages have been selected for the present experimental study. The objective is to identify the relative contribution of these factors towards the development of bond strength and their consequence against moisture damage. Bond strength was evaluated in terms of the pull-off tensile strength (POTS), whereas moisture susceptibility was determined using the bond strength ratio (BSR) parameter. Results showed that the bonding between aggregate and asphalt binder relies on the aggregate type, base binder, WM additives and their dosages. Since PAT was carried out on both wet and dry-conditioned samples, it was found that the conditioning state (wet and dry) also affects the interfacial bonding of the asphalt-aggregate matrix. Wet-conditioned samples tend to exhibit lower POTS values than dry-conditioned samples. The BSR values were found to be different for various aggregate-asphalt binder combinations. The minimum BSR was found to be 0.82 for a moisture-resistant asphalt mixture.

The influence of calcium sulphoaluminate on the properties of low-cost magnesium phosphate cement mortars

Magnesium phosphate cement (MPC) exhibited many excellent performances, however, the high cost of raw materials of MPC limited its application in construction areas. Calcined dolomite powders containing CaO could not be applied in MPC production. The dolomite-bauxite-gypsum powders after calcination were used to replace dead burned MgO and the free-CaO in dolomite was transferred into calcium sulphoaluminate (ye'elemite, C4A3S‾). This study intends to investigate the performance of the low-cost MPC mortars prepared with calcined dolomite-bauxite-gypsum powders containing C4A3S‾. The setting times, fluidities, hydration temperature developments, compressive strength developments and volume changes of MPC mortars with C4A3S‾ were investigated. Furthermore, the differences in compressive strength, flexural strength and bond strength development of MPC mortars with C4A3S‾ in air and water curing were compared. Phase developments, microstructures and pore diameter distribution of MPC mortars with ye'elemite were also investigated. MPC mortars prepared with the calcined dolomite powders containing 51% C4A3S‾ and 26% MgO showed a setting time of 27 min, free-fluidity of 150 mm, 3-h compressive strength of 41 MPa, 7-d compressive strength of 58 MPa and 180-d compressive strength of 74 MPa. C4A3S‾ in MPC mortars could form some amorphous hydration products and its hydration may be promoted in water curing. The new amorphous phases may have filling and cementitious effects, leading to denser structures and higher strengths.

A Novel CNN-LSTM Hybrid Model for Prediction of Electro-Mechanical Impedance Signal Based Bond Strength Monitoring.

The recent application of deep learning for structural health monitoring systems for damage detection has potential for improvised structure performance and maintenance for long term durability, and reliable strength. Advancements in electro-mechanical impedance (EMI) techniques have sparked attention among researchers to develop novel monitoring techniques for structural monitoring and evaluation. This study aims to determine the performance of EMI techniques using a piezo sensor to monitor the development of bond strength in reinforced concrete through a pull-out test. The concrete cylindrical samples with embedded steel bars were prepared, cured for 28 days, and a pull-out test was performed to measure the interfacial bond between them. The piezo coupled signatures were obtained for the PZT patch bonded to the steel bar. The damage qualification is performed through the statistical indices, i.e., root-mean-square deviation (RMSD) and correlation coefficient deviation metric (CCDM), were obtained for different displacements recorded for axial pull. Furthermore, this study utilizes a novel Convolutional Neural Network-Long Short-Term Memory (CNN-LSTM)-based hybrid model, an effective regression model to predict the EMI signatures. These results emphasize the efficiency and potential application of the deep learning-based hybrid model in predicting EMI-based structural signatures. The findings of this study have several implications for structural health diagnosis using a deep learning-based model for monitoring and conservation of building heritage.

Performance assessment of bamboo bond strength in cement–fly ash mortar

The novelty of this study is the combined experimental and statistical approach, which is showcased to predict and optimise bamboo–mortar bond strength, incorporating the effects of various material factors. The design of experiments method was used to quantify the impact of multiple factors affecting the bond strength. Firstly, different experimental pull-out tests were conducted in the laboratory to determine the effect of various factors (water/cement ratio, curing duration, cement/sand ratio, fly ash content and bamboo treatment) on bond strength development. Three failure patterns were observed – bamboo failure, bond failure and mortar failure. Secondly, using a two-level full factorial design method, different statistical models were formulated. These models were developed based on the surface treatment of the bamboo (model M1 (untreated) and model M2 (treated and untreated)). The effects of single factors and their interactions on bond strength development were quantified. The developed models were validated for the prediction of bond strength based on different factors. The developed statistical models were found to predict and simulate the experimental behaviour of bond strength with sufficient accuracy.

Intimate contact development for automated fiber placement of thermoplastic composites

The consolidation step in the automated fiber Placement (AFP) of thermoplastic composites is crucial for development of interlaminar bonding strength. Two mechanisms contribute to the interlaminar bond strength development in the interface between the incoming tow and the substrate: (a) the development of intimate contact between the two surfaces and (b) the formation of a fusion bond across those contacting surfaces known as healing. So far, a number of different theoretical intimate contact models have been proposed in the literature to study intimate contact development for different manufacturing processes. However, very limited experimental methods are available to measure degree of intimate contact. To fill this gap, a new experimental approach based on topology of the tape surface is introduced to quantify the degree of intimate contact for the AFP process. The new approach is based on the Bearing Area Curve (BAC) of the surface profile of the tape. To implement the new procedure, an unidirectional strips of carbon fiber/PEEK were laid down on the polished steel tool with different placement rates, compaction forces, hot gas torch (HGT) temperatures, and tool temperatures. The degree of intimate contact is calculated based on the proposed experimental approach and compared with prediction models from the literature. Furthermore, the effect of AFP process parameters on the degree of intimate contact is studied using BAC method and the effective intimate contact model. The results showed that the development of intimate contact is significantly governed by the torch and tool temperature.

The bond strength development of coconut wood in relation to its density variations

ABSTRACT This study examines the influence of density variations in coconut palm wood (Cocos nucifera L.) on bond strength development using three types of adhesives: urea formaldehyde (UF), melamine urea formaldehyde (MUF), and polyvinyl-acetate (PVAc). A special testing device called Automated Bonding Evaluation System (ABES) was used to investigate the bond strength development between coconut veneers as a function of adhesive type, wood density (moisture content of 10%), pressing time, and bond-temperature development. Coconut wood density was shown to greatly affect the bond line. The bond strength of medium-density material (600–700 kg/m3) was lower than that of high-density material (900–1,000 kg/m3). Interestingly, the medium-density coconut veneers achieved higher bond strength with shorter pressing times, while the high-density coconut veneers reached better results with longer pressing times. Moreover, the bond strength improved by increasing the hot-pressing time resulting in higher bond-line temperatures. A comparison was made between coconut wood and beech wood for similar bonding parameters.

Experimental evaluation on mechanical and thermal insulation properties of shotcrete under constant‐variable temperature

AbstractOne of the commonly used stabilization systems for rock tunnel is shotcrete. However, the hydration of shotcrete is usually accompanied by the development of internal defects and micro damage under high temperature. Additionally, in the actual high‐temperature engineering, the temperature of the rock tunnel exhibits irregular changes. Therefore, in this study, the curing conditions of constant temperature (23°C, 40°C, 60°C, 80°C) and variable temperature (23–40–60–80°C, 80–60–40–23°C) are set, respectively. The mechanical properties and structural properties of polyurethane shotcrete and ordinary Portland shotcrete are introduced. It is determined that the limit temperature for inhibiting the development of sprayed concrete's compressive and splitting strength is between 60°C and 80°C, and the limit temperature for inhibiting the development of bond strength is about 40°C. Meanwhile, the influence of high temperature on the thermal insulation performance of shotcrete is considered. The results that the self‐foaming performance of polymer(polyurethane) reduces the thermal conductivity of shotcrete. Based on this phenomenon, the heat transfer model of shotcrete at room and high temperature is established to clarify the heat transfer process of shotcrete. Results will provide theoretical basis for further research of durability and heat insulation performance of protective spraying material for tunnel under high temperature.

In-mould assembly of functionally integrated structures: A surrogate model for fast quality assessment

For the in-mould assembly of functionally integrated structures, a proper bond strength development between the different components has to be ensured during the actual manufacturing step. Physically detailed models and simulations are able to compute a reliable bond strength distribution. However, these computations are very time consuming. In this contribution, a simulation based surrogate model for the in-mould assembly of thermoplastic composite structures is developed. It predicts the bond strength within seconds using data from process simulations combined with machine learning, achieving a remaining error of less than 5% compared to high-fidelity computations.

Property comparison of steel slag cement mortar with hydroxyethyl methyl cellulose having different degrees of substitution and PAAm modification

The aim of this study is to explore the influence of HEMC with different total substitution degrees and polyacrylamide (PAAm) modification on the properties of steel slag cement mortar. The results show that HEMC increases the water retention rate, consistency, tensile bond strength, and flexibility of all types of steel slag cement mortars. The addition of HEMC, however, leads to a decrease in compressive strength and flexural strength. The total substitution degree or PAAm modification of HEMC has a minor effect on the water retention rate, compressive strength, flexibility, and pore size distribution of steel slag cement mortar. HEMC with a higher total substitution degree is more beneficial to the development of tensile bond strength except for SL1 cement mortar at an early age (3 days and 7 days), whereas PAAm weakens the positive effect of HEMC on the tensile bond strength. Mercury intrusion porosimeter (MIP) tests show that the addition of HEMC increases the porosity of steel slag cement mortar.

Improving oil well cement strengths through the coupling of metakaolin and nanosilica

The research study aimed to investigate the effect of coupled metakaolin and nano-silica (nano-SiO2) on improving the compressive and shear bond strengths of class G oil well cement. Samples curing was done at 80 °C for 3, 7, 14, and 28 days. XRD and TG-DSC techniques were used for cement phase characterization. The results indicated that the cement system with 2% nano-SiO2 and 20% metakaolin increased the compressive strength by 20.0 and 21.4%, and shear bond strength by 205.5 and 90.8% for 3 and 28 days, respectively. This means the synergistic effect is more significant for cement-formation bond strength development. The results of this study, therefore, points to a potential ternary cement system for durable oil wells.

Heat damage and bond-slip performance of steel arch frame-concrete in high-geothermal tunnels

According to the real construction environment in high-geothermal tunnel, push-out test on fifty-four I-shaped steel and concrete samples subjected to different relative humidity and variable temperature curing conditions were carried out to evaluate the bond slip performance at the interface. The study results indicated that there are three models of crack evolution in failure process under different curing conditions. Compared with the samples cured under standard condition, higher curing temperature contributes to the early development of bond strength, but the long-term bond strength decreased significantly. Relative humidity is beneficial for increasing the intensity and shearing stiffness, enhancing bonding properties and reducing the loss of long-term bond strength of samples cured under VTC condition. More initial defects lead to the weak slip resistance of the interface under hot and dry environment. According to the regression analysis of experiment data, the formula of characteristic strength and segmental constitutive equation of the bond-slip under coupling effect of temperature and humidity are obtained. The parametric finite element model (FEM) of the new constitutive relation proposed in this paper was established using parametric design language. The final results may contribute to provide a reference for related support design and construction in high geothermal tunnel.

Polyethyleneimine as a surface activator for low surface energy substrates bonded with cyanoacrylate adhesives

The modification of surface properties of polypropylene (PP) substrates has attracted much attention in fundamental science and industrial applications. Primer treatment is one of the most simple and facile surface modification methods. This study used polyethyleneimine (PEI) as a surface activator for bonding polypropylene substrates with ethyl cyanoacrylate (ECA) adhesive. We have understood the effect of various factors such as the open time, solvent composition, optimum concentration and molecular weight of the primer. The bonding strength was maximum when the substrates were bonded immediately on priming. It was also observed that the solvent composition plays a minor role in the development of bond strength. The bond strength of substrates coated with a primer having a PEI concentration as low as 0.05% showed better results and was, in fact, three times better than the unprimed ones. It was also concluded that a low molecular weight PEI performs better than one having a higher molecular weight. DSC results confirmed that an improvement in strength is due to an increase in the molecular weight of the ECA polymer, which occurs in the presence of PEI molecules.

Monitoring bond strength of adhesively planted rebars in concrete using electro-mechanical impedance (EMI)

Electro-mechanical impedance (EMI) is among the many types of gauges that can be used for structural health monitoring (SHM) of concrete structures. The bond strength between concrete and planted rebar plays an important role in the integrity of concrete structures strengthened by post-installed anchors. This paper presents a method to monitor the development of epoxy bond strength between concrete and a planted rebar by the EMI technique. Three concrete cubes each with a planted rebar were fabricated and used for a set of monitoring experiments. To conduct experiments, a lead zirconate titanate (PZT) patch was attached to the polished side surface of the planted rebar. During the monitoring test, conductance spectra signals from three PZT patches were acquired at the 0th, 20th, 40th, 60th, 80th, 100th, and 120th min of the experiment. Furthermore, a pull-out test was conducted on two concrete beams with three planted rebars each. The root mean square deviation (RMSD) and mean absolute percentage deviation (MAPD) were applied to compare instantaneous conductance signatures with the initial, baseline conductance signature across different frequency ranges to ascertain the growth of bond strength during the curing process of epoxy. The results of the monitoring experiments and the pull-out test demonstrate the effectiveness of EMI technique in monitoring bond strength development between concrete and planted rebars. The results also show potential application prospects of the EMI technique in the SHM of reinforced concrete structures.

Development of polyvinyl acetate adhesive bond strength when gluing veneer

This article presents the development of the shear strength of two polyvinyl acetate (PVAc) adhesives: Mekol D3 and Mekol SPECIAL when bonding veneer depending on pressing time. Besides testing the strength of the adhesive bond after hot pressing, we also investigated the effect of cooling on the final strength of PVAc adhesive bonds. The shear strength development of the PVAc adhesive bond during hot pressing was determined using an Automated Bonding Evaluation System (ABES). The pressing temperature was constant and was 80 °C, while the pressing time was extended from 30 to 300 seconds in 30-second intervals. The effect of cooling on the final strength of the PVAc adhesive bond was determined after cooling times of 5 and 30 seconds. Based on the investigations, we found that the shear strength of PVAc adhesive bonds increased with the pressing time and approached a constant value after a certain time. We found significant differences in the strength of adhesive bonds glued with Mekol D3 and Mekol SPECIAL and a statistically significant effect of cooling on the strength of the adhesive bonds.

Influence of magnetic field treated water on the compressive strength and bond strength of concrete containing silica fume

The overall objective of this study is to investigate the effect of magnetic field treated water and silica fume content on the compressive strength and bond strength of concrete. A total of ninety specimens were cast and tested under compression and pullout tests. In order to fully understand the concrete strength development, both tests were carried out at 7 and 28 days of concrete age. Normal water and magnetic water were used in the mix using different silica fume content. The results showed significant increase in the compression strength when magnetic water was used regardless of the silica fume content. Specimens prepared with magnetized water showed an increase in the bond strength compared to those prepared with normal water at all silica fume contents. The results also showed the use of high content of silica fume reduces the bond strength due to the reduction in adhesive properties between steel and concrete. The ultimate pullout load for 16 mm diameter bar was larger than that for use of larger than that for the 12 mm bar size but the bond strength of the larger diameter bar size is smaller since it has larger surface embedded area. The effect of silica fume on bar size illustrated that for a smaller bar size a lower development of bond strength is recorded because the distribution of silica fume particles around the small surface area reduces the adhesion forces between concrete and steel bars. Finally, as expected, the results showed that age of concrete has significant influence on the bond strength. The main contribution of this work is to prove that magnetic field treated water can enhance not only the compressive strength of concrete but also the bond strength between steel bar and concrete.

The Influence of Rib Configuration on Bond Strength Development between Steel and Concrete

The bond strength between rebar and concrete is important for the quality performance of reinforced concrete structures. At the interface, bond strength development mainly depends on surface configuration. Different rib configuration improves the strength significantly in high yield rebars as compared to mild steel. This study examines the bond strength behavior of ordinary MS (Mild Steel) rebars, HYSD (High Yield Strength Deformed) parallel rib, and HYSD diamond rib rebars. Experimental analysis to obtain pull-out behavior of rebar in concrete was based on IS 2770 Part I – 1967: Reaffirmed 2007; Indian Standard Methods of Testing Bond in Reinforced Concrete. Importantly, the concrete of M30 grade was used and a total of nine specimens were tested. The cubes of size 150mm x 150mm x 150mm were cast with centrally embedded rebar provided up to 20 mm from their bottom faces. Additionally, the pull-out test was conducted in 1000 kN capacity Universal Testing Machine. The usable bond strength values were calculated from the load at 0.025 mm free and 0.25 mm loaded end slips. The results showed that the usable bond strength value of HYSD diamond rib rebars is very large compared to MS and appreciably greater than HYSD parallel rib. Moreover, the usable bond strength of HYSD diamond rib rebars is 60.06% and 35.60 % greater than that of the MS rebars and HYSD parallel rib pattern rebars, respectively. The high frictional resistance developed in the bond strength test of HYSD diamond rib rebars because of the better mechanical interlocking. This was primarily due to the presence of a more frictional surface area of lugs.

Standard Test Method ASTM D 7998-19 for the Cohesive Strength Development of Wood Adhesives.

The properties of cured wood adhesives are difficult to study because of the loss of water and other components to the wood, the influence of wood on the adhesive cure, and the effect of adhesive penetration on the wood interphase; thus, normal testing of a neat adhesive film is generally not useful. Most tests of wood adhesive bond strength are slow, laborious, can be strongly influenced by the wood and do not provide information on the kinetics of cure. Test method ASTM D 7998-19, however, can be used for fast evaluation of the strength of wood bonds. The use of a smooth, uniform, and strong wood surface, like maple face-veneer, and sufficient bonding pressure reduces the adhesion and wood strength effects on bond strength. This method has three main applications. The first is to provide consistent data on bond strength development. The second is to measure the dry and wet strengths of bonded lap shear samples. The third is to better understand the adhesive heat resistance by quickly evaluating thermal sensitivity and distinguishing between thermal softening and thermal degradation.

Flexural behaviour of bamboo-reinforced wall panels with varying fly ash content

This study investigates the effect of fly ash in bamboo-reinforced mortar wall panels. The effect of fly ash with bamboo has been quantified by evaluating the material properties of bamboo, the compressive strength of mortar mixes, composite panel properties through non-destructive testing techniques and by evaluating the flexural strength of panels through two-point loading tests. Nine panels were fabricated with mortar mixes categorised into three groups based on mix proportion. Test results showed that bending cracks promote debonding of mortar layers under flexure with the addition of fly ash in wall panels due to lower bond strength development. Because of the reduced surface hardness and resilience, the first crack load and ultimate load were lowered compared to panels developed without fly ash. Debonding failure under flexure loads was observed, which leads to a reduction in flexural strength of the panels with the addition of fly ash, although significant compressive strength and cost-effectiveness was achieved.