Concrete Substrate Research Articles

Experimental Comparison of Efficiency of Water-Soluble and Solvent Hydrophobic Agents for Concrete

The paper presents the results and assessment of the properties and capabilities of new types of surface hydrophobic agents for the treatment of fresh and matured surfaces of concrete and other cement-bound layers. Hydrophobisation prevents the premature evaporation of water and thus plastic shrinkage, which inevitably leads to the formation of cracks in cement-based structures. The influence of the new type of hydrophobic agents, epoxy water-based (EWH) and acrylate (AH) containing solvent, on the physical and mechanical properties of the treated concrete samples was assessed, including the adhesion of hydrophobisation on the concrete surface layers. It was confirmed that surface hydrophobisation successfully prevents premature evaporation of water, and thus, plastic shrinkage (concrete treated with EWH_0.3 showed more than two times lower shrinkage than reference concrete). The concrete samples treated by hydrophobisation agents showed higher strength after 120 days (37.5 MPa) in comparison with untreated concrete (32.8 MPa). Different properties were recorded with different types of hydrophobisation agents, including compressive strength after 90 days (31.2 MPa with EWH_0.15, and 35.9 MPa with AH_0.15). Water absorption after 120 days was lowest with EWH_0.3 samples (3.77%), two times lower than AH_0.15 (6.98%). The layer of hydrophobisation agent EWH_0.3 was thicker than AH_0.15, leading to lower water absorption of treated concrete and higher resistance to defrosting chemicals—waste 8.5. g/m2 with EWH_0.3 in comparison to 35.7 g/m2 with AH_0.15. Furthermore, a difference in the hydrophobisation behaviour was shown with blocking of infiltration from the chemically aggressive environment into the concrete substrate. Deterioration of the surface concrete exposed to 10% HCl solution was worse with AH_0.15; but the concrete surface treated with AH_0.15 showed better resistance than the EWH treated surface, when exposed to 5% CH3COOH. Degradation of the hydrophobisation integrity and the loss of cohesion between the concrete and the surface treatment after exposure to acetic acid and hydrochloric acid was observed using scanning electron microscopy (SEM).

A Lagrangian approach to ex-vessel corium spreading over ceramic and concrete substrates using moving particle hydrodynamics

Understanding the spreading of molten core materials (corium) is vital for achieving post-accident heat removal and maintaining the integrity of the reactor containment during a severe nuclear accident. The spreading of highly viscous corium over sacrificial concrete and ablation-resistant ceramic substrates, consistent with recent VE-U9 experiments at the VULCANO facility, is investigated using a Lagrangian moving particle hydrodynamics (MPH) method. The spreading dynamics are coupled with a heat transfer model to account for the increase in melt viscosity due to solidification. Three alternative boundary conditions are considered at the melt-substrate interface to mimic the influence of low viscosity concrete ablation products on the spreading dynamics. Simulation of spreading under no-slip conditions closely resembled the spreading behavior observed over the inert ceramic substrate, advancing with a crawling motion with a steep profile in the proximity of the melt front. Spreading terminated due to a combination of thermal radiation from the free surface and the heat transfer to the substrate, leading to the formation of a continuous crust. Introduction of slip boundary models, to mimic lubrication of the interface by a film of low-viscosity concrete decomposition products, enabled the prediction of the gliding flow observed over the sacrificial concrete substrate, characterized by a shallow melt topology near the spreading front. The simulation results demonstrate excellent potential for Lagrangian models such as MPH to predict complex spreading dynamics in the presence of both melt solidification and lubrication at the substrate.

Experimental study on factors influencing ERC-NSC interface bonding performance

The short-term behavior and long-term durability of materials for structural strengthening are significantly affected by their bond properties and compatibility with the existing substrate. In this study, the bonding performance between a normal strength concrete (NSC) substrate and an epoxy resin concrete (ERC) layer was investigated by applying direct tensile, splitting tensile, bi-shear, and slant shear tests, and the interfacial bond strength and corresponding failure modes were explored. Several factors that may affect the interfacial bond strength, including the ERC age, substrate moisture degree, and interface inclination angle, were examined to explore their influences. The results reveal that the increase in ERC age resulted in various degrees of increment for the bond strength among the four tests. The saturated surface dry (SSD) preparation method slightly increased the bond strength by 3 % in the splitting tensile tests. However, moisture at the substrate interface reduced the bond values by 25.5 %, 29.67 %, and 16.9 % in the direct tensile, bi-shear, and slant shear tests, respectively. As the inclination angle increased, the bonding strength of the slant shear test increased, while a larger inclination angle reduced the likelihood of interfacial failure. Furthermore, the minimum cohesion and shear friction coefficients were determined to be 2.26 MPa and 1.68 for air surface dry preparation and 1.62 MPa and 1.48 for SSD preparation.

Importance of Concrete Mesoscale Structure on Bond-Slip Behavior of Externally Bonded CFRP Strips

Currently, the effect of the concrete substrate on the load-bearing capacity of externally bonded carbon fiber-reinforced polymer (CFRP) strips is determined in the guidelines solely on the basis of the concrete strength. In addition to strength, surface preparations can improve the bond resistance by exposing the aggregates of the substrate. However, the interlocking also depends on the parameters that derive from the inner concrete structure such as the aggregate grain dimension and shape, as well as the air void distribution. Therefore, the influence of the concrete inner structure on the bond is investigated with double shear tests by comparing specimens with equivalent concrete strength and varying grain shape and dimension. Bond parameters are calculated using a novel procedure based on optical measurements. Furthermore, crack paths are analyzed by means of computed tomography (CT) scans, which prove to be an essential inspecting technique for characterizing the inner concrete structure. The fracture energy is approximately 30% higher for concrete with a maximum grain size of 8 mm compared to an equivalent mortar with 4 mm maximum grain size.

Waste granite powders as fillers in epoxy coatings: A case study in a car repair workshop

Epoxy coatings of floors are a very good solution for car repair workshops. Epoxy coatings provide the floor with resistance to oil, fuel, and grease staining. The floor surface is very easy to clean. To support sustainable development, a filler in the form of waste granite powder was added to the epoxy coating. In a car repair workshop, an epoxy coating of the floor was made according to the recipe previously developed in laboratory tests. The pull-off strength of the concrete substrate was tested before and after the mechanical treatment of concrete surface. The pull-off strength of the epoxy coating was also tested. The obtained results were compared with the laboratory results. It has been proved that the waste granite filler does not deteriorate the pull-off strength of the epoxy coating and thus its durability. The waste granite powder can be successfully applied as a filler in real epoxy coatings of floors.

Experimental and numerical analysis of CFRP-concrete joint bond behavior after exposure to wet-dry cycles

Fiber-reinforced polymer (FRP) composites have been largely used for strengthening and retrofitting reinforced concrete members in the last few decades. However, limited information is available regarding their performance after prolonged exposure to different environmental conditions. In this paper, the effect of wet-dry cycles on the bond behavior of carbon FRP-concrete joints tested using a single-lap direct shear test set-up is investigated. Specimens were exposed to 50 wet-dry cycles, each comprised of 6 h of immersion in water at 25 °C followed by 18 h of drying at 50 °C. The digital image correlation technique was used to obtain the displacement field of the composite and determine the cohesive material law and fracture energy of the FRP-concrete interface. A finite element model was then used to reproduce the load response and reinforcement strain profile observed experimentally. The results show that wet-dry cycles affect the FRP-concrete bond properties and change the failure mode from a cohesive debonding within the concrete substrate to an adhesive debonding within the interface.

Interfacial behavior of externally bonded BFRP-to-concrete joints using different epoxy adhesives

The purpose of this study is to investigate the influence of adhesive types on the interfacial behavior of externally bonded basalt fiber reinforced polymer (BFRP) sheet-concrete substrate. Twelve manufactured double-lap shear samples were tested using four bonding adhesives. The strains on the surface of BFRP‐concrete joint were determined using electrical resistance strain gauge together with two‐dimensional digital image correlation technique (2D‐DIC). Experimental results show that properties of different bonding adhesives i.e., stiff (linear elastic, higher strength) and soft (nonlinear elastic, lower modulus, higher ultimate strain) has a significant impact on the failure mode, stiffness, ultimate load, strain and shear stress distribution nonetheless not so much on bond-slip relationship. The failure modes of specimens changed from interfacial debonding or cohesive failure in concrete substrate for stiff adhesive specimens to debonding in adhesive layer when soft adhesives are used. Stiff adhesive interfaces experienced higher ultimate loads from 1.1 to 1.3 times higher than soft adhesive interface. Meanwhile soft adhesives are shown to possess a lower maximum shear stress, higher interfacial fracture energy and longer stress transfer length.

Molecular-level investigation on the effect of surface moisture on the bonding behavior of cement-epoxy interface

Epoxy resins are widely used as construction and repair materials. However, their bond with the concrete substrate under wet conditions is of concern as the loss of compatibility will lead to the failure of the bonded system. The reported study investigated the effect of surface moisture at the epoxy/cement interface using molecular dynamics (MD) simulation. The model of the interface was built using calcium silicate hydrate (C–S–H), as a hydrated cement substrate and diglycidyl ether of bisphenol-F (DGEBF) as an epoxy resin. Subsequently, different layers of water molecules were inserted into the model to represent varying moisture concentrations at the interface. The interfacial properties of the dry and wet models were evaluated in terms of adhesion, debonding, and energy ratios. In addition, the radial distribution function (RDF) was employed to deeply investigate the atomic interaction between the cement surface and epoxy. The results of the model indicated that the adhesion properties were adversely affected by the presence of moisture at the epoxy/cement interface. The degradation in the adhesion at moisture contents of 3% and 6% was 37.49% and 72.02%, respectively. Additionally, it was found that the detachment increased, as evident from the values of debonding, at higher moisture contents due to an increase in the adhesion between the C–S–H and water molecules where the latter form hydrogen bonds with the substrate. This work provides valuable molecular-level insights into the epoxy/cement bond that can be used to design new epoxy resins suited for cement and concrete applications.

Beam tests for the determination of the interfacial properties of FRCM composites

Fiber Reinforced Cementitious Matrix (FRCM) materials are widely used for strengthening reinforced concrete (RC) structures. Experimental studies show that the collapse of RC elements reinforced with FRCM composites occurs due to the loss of bond of the reinforcing material. For this reason the issue of the shear stress transfer between reinforcement and concrete substrate has been extensively studied in the last years. The single-lap shear test is the most frequently adopted test configuration. In this paper the results of bond tests between a FRCM composite and concrete performed in the beam test configuration are presented. The results are used to determine the cohesive material law (CML) of the tested composite and the obtained CML is compared with that obtained on the basis of the results of single-lap shear tests. The comparison shows that the two test configurations provide similar CML.

Assessment of bonding, durability, and low-temperature performance of cement-based rapid patching materials for pavement repair

ABSTRACT A high-performance concrete rapid patching repair material needs to develop high early strength to satisfy traffic opening requirements, yet provides sufficient durability to sustain different environmental conditions and primary deterioration mechanisms for full-depth concrete repair. In this study, a set of experiments was conducted to evaluate the performance of developed pavement repair mixtures subjected to different environmental conditions, including low ambient temperature, highly reactive siliceous aggregate, wet/dry cycling, and moisture loss. Full-scale slab patching was also carried out to assess the rapid patching mixtures’ constructability, compatibility, and interface bonding with the concrete substrate. Patching materials with a reduced cement content and Type IP cement exhibited a lower drying shrinkage rate and less deterioration from wet/dry cycles compared with commonly used patching mixtures that typically use high cement content and Type I cement. The tendency of alkali-silica reaction deterioration was reduced significantly by replacing 50% Type III cement with Type IP cement. The rapid patching mixtures generally displayed a high degree of early-age shrinkage but with satisfactory compatibility and good bonding with the concrete substrate. Test results also indicate that Type III cement-based rapid patching mixtures can be a promising option to reduce traffic closure duration at lower ambient temperatures.

The reasons for the loss of polyurea coatings adhesion to the concrete substrate in chemically aggressive water tanks

The article discusses the use of one of coatings protecting and strengthening concrete surfaces. It is polyurea coatings to strengthen waterproofing in tanks for water and other related liquids. Details of polyurea application technologies for specific tank structures and preparatory works that must be performed before the application of spray polyurea coatings have been discussed, as well as essential points related to minor and major cracks, cover reprofiling, type of coating used in chemically aggressive environments, causes and examples of structures where destructive corrosion processes of concrete surfaces and, consequently, the polyurea coatings covering them have been activated. The presented examples of failures in the application of polyurea coatings arise due to insufficient knowledge about the coatings themselves and the substrate on which the coatings are applied, but above all in failing to follow the strict procedure for their application.

RETRACTED: Efficiency and durability of carbon dots grafted TiO2-based coatings applied on concrete under peeling and washing trials

In engineering practice, the durability of photocatalytic coatings is a major concern. Here two types of unique visible light-responsive coatings are proposed and placed on the concrete surface, consisting of vinyl chloride/vinyl ester/ethylene copolymer (as a binder) and carbon-dots-grafted TiO 2 (TiO 2 -CDs) but different in fabrication. The first type is mono-layer coating (MLC), which involves directly spraying the TiO 2 -CDs suspension containing the binder onto the concrete. The second type was called double-layer coating (DLC), which involves applying a binder layer to the concrete surface before spraying the TiO 2 -CDs layer. These show that adding binder improves the coatings' anchoring on both MLC and DLC concrete substrates, along with desirable resistance to peeling and washing, when compared to the TiO 2 -CDs coated concrete without the binder. The well distributed binder in the TiO 2 -CDs-based coating inevitably decreases the photocatalytic efficiency of the MLC concrete due to the binder's masking effect on the coated surface. However, the DLC concrete fully exploits the binder adhesion by inserting a polymer matrix layer and therefore maintains a strong resistance to peeling and washing without compromising its photocatalytic efficiency. The proposed DLC approach enables the fabrication of highly cost-effective and durable photocatalytic coatings applied to cementitious materials. • The surface of TiO 2 nanoparticles was modified by APTM and integrated with CDs. • Two coating methods were used to bond the TiO 2 -CDs on the surface of substrate. • Effect of coating method on the stability of TiO 2 -CDs against peeling and washing. • Effect of environmental condition on removal HCHO of TiO 2 -CDs was studied. • DLCM of TiO 2 -CDs substrate had better stability compared with MLCM.

Effect of overlay thickness, fiber volume, and shrinkage mitigation on flexural behavior of thin bonded ultra-high-performance concrete overlay slab

This study investigated the flexural performance of 10 reinforced concrete slabs rehabilitated using thin ultra-high-performance concrete (UHPC) bonded overlay. The performance was compared to two slabs repaired using latex-modified concrete (LMC) that is widely used for bridge deck rehabilitation. Test parameters included the overlay thickness (25, 38, and 50 mm), fiber volume (2% and 3.25%), and shrinkage mitigation methods for the UHPC. The overlay slab specimens were stored outdoor for 30 months before conducting the four-point bending test. Experimental results indicated that the overlay slabs exhibited flexural failure pattern with yield of reinforcing steel, concrete crushing of the reinforced concrete substrate, and multiple cracks in the overlay. The use of UHPC thin bonded overlay enhanced cracking load by 295%–395% and flexural capacity by 35%–75% compared to the LMC overlay slabs. The maximum crack width of UHPC overlay slabs at the stage of the yield of reinforcement was less than 0.1 mm compared to 0.25–0.3 mm for the LMC overlay slabs. No slip occurred at the interface between the UHPC overlay and substrate, whereas the slip initiated following LMC overlay cracking. The increase of overlay thickness from 25 to 50 mm and fiber volume from 2% to 3.25% enhanced the flexural load capacity of the UHPC overlay slabs by 30%–40% and 10%–25%, respectively. These two strategies also reduced crack opening and propagation before the yield of reinforcing steel. The use of pre-saturated lightweight sand and CaO-based expansive agent was effective in mitigating autogenous and drying shrinkage without significantly influencing the flexural capacity of UHPC overlay slab.

Single-Side Shear Bond Strength and OTZ Microstructure of UHPC Repair Materials with Concrete Substrate

Single-Side Shear Bond Strength and OTZ Microstructure of UHPC Repair Materials with Concrete Substrate

The effect of the amount and particle size of the waste quartz powder on the adhesive properties of epoxy resin coatings

One of the most important adhesive properties of coatings is their pull-off strength. The research investigated the effect of quartz powders on the adhesive properties of epoxy resin coatings. As a filler to epoxy resin, three fractions of quartz powders were used. The main aim of the research was to determine the effect of the amount and particle size of the waste quartz powder on the adhesive properties of epoxy resin. Three series of epoxy resin coatings were carried out with the content of each of the three selected waste quartz powders in a weight amount from 0% to 29%. The coatings were made on a concrete substrate with two surface treatment methods (raw and ground). The adhesive properties of the coatings were tested using the pull-off method, determining the pull-off strength value according to ASTM D4541 . The average pull-off strength of the modified coatings ranged from 2.48 to 3.36 MPa, and the reference coatings (without the waste quartz powder) from 2.89 to 2.91 MPa. Studies have shown that all selected types of waste quartz powder can be used as potential fillers in epoxy coatings in an amount of up to 29% by weight of the coating mass, without significantly deteriorating the adhesion of this coating.

Evaluation of interface rapid bond strength between normal concrete and ternary system fast setting and rapid hardening self-compacting concrete

• The rapid bonding performance between NC substrate and TSSCC was investigated. • The bond strength meets the requirements of the executive standard within 2 hours. • Surface-milled revealed higher bond strength of NC/TSSCC composite. • The microstructure and bonding model of the NC/TSSCC interface were studied. • TSSCC is suitable for rapid repair of the reinforced concrete structures. The ternary system (Ordinary Portland cement-sulfoaluminate cement-anhydrite) fast-setting and rapid-hardening self-compacting concrete (TSSCC) is a new type of green and low-carbon material. In this study, the interfacial rapid bond strength and bonding mechanism between ternary system self-compacting concrete (TSSCC) as rapid repair material and normal concrete (NC) substrate was investigated at different ages. Two surface treatment methods, including milling and grooving, were employed to improve the bond properties. Meanwhile, the hydration products of TSSCC and the microstructure of the bonding interface were investigated using TGA-DSC and SEM. The results show that the bond strength of the surface-milled NC/TSSCC composite is about 1.2 times that of the surface-grooved composite. Due to the fast setting and rapid hardening of the ternary system, the 2 h bond strength of NC/TSSCC attained about 50 % of the 28d bond strength. Similarly, the 2 h slant shear bond strength of the NC/TSSCC composite satisfied the 7 d slant shear strength value recommended in the ACI 546R-04 Concrete Repair Guide. The composite NC matrix/TSSCC consists of three zones: the old concrete zone, the transition zone, and the new concrete zone. The NC/TSSCC rapid bond strength is derived from the main hydration product AFt of TSSCC to the pinning of pore cracks in the old concrete, the meshing between AFt, the penetration filling of the hydration product, and the van der Waals force between the old and new concrete hydration products. This study indicates that TSSCC has the potential application for the rapid repair of reinforced concrete structures.

Experimental and analytical behavior of one-way fiber foamed concrete slabs reinforced with/without glass fiber grid under 4-point flexural test

PurposeThe primary objective of this study is to produce one-way slabs made of LWFC with low density and sufficient compressive strength suitable for structural purpose then investigate their flexural behavior under various types of reinforcement and thickness of the slab and the influence of addition of PP fibers reinforcement on the mechanical behavior of reinforced concrete slabs. The specimens were tested using four-point loading. The results concerning load capacity, deflection and failure mode and crack pattern for each specimen were obtained. Also, an analytical investigation of PP fiber and GFG contribution on the flexural behavior of foamed concrete slabs is studied to investigate the significant role of PP fiber on the stress distribution in reinforced foam concrete and predict the flexural moment capacity.Design/methodology/approachThe materials used in this study are cement, fine aggregate (sand), water, PP fibers, foaming agent, chemical additives if required, steel reinforcing rebars and glass fiber grid. The combination of these constituent materials will be used to produce foamed concrete in this research Then this study will present the experimental program of one-way foamed concrete slabs including slabs reinforced with GFR grids and another with steel reinforcements. The slabs will be tested in the laboratory under static loading conditions to investigate their ultimate capacities. The flexural behavior is to the interest of the slabs reinforced with GFR grids reinforcements in comparison with that of one with steel reinforcing rebars. Three groups are considered. (1) Group I: two slabs of PP fiber foamed concrete with minimum required reinforcements. (2) Group II: two slabs of PP fiber foamed concrete with glass fiber grids. (3) Group III: two slabs of PP fiber foamed concrete with the minimum required reinforcements and glass fiber grids.FindingsThe experimental results proved the effectiveness and efficiency of this the new system in producing a low density of concrete below 1900 kg/m3 had a corresponding strength of about 17 MPa at least. Besides, the presence of PP fibers had a noticeable improvement on the flexural strength values for all the examined slabs. It was found that the specimens reinforced with steel reinforcement mesh carried higher flexural capacity compared to these reinforced with GFG only. The specimens reinforced with GFG exhibited the lowest flexural capacity due to GFG separation from the concrete substrate. Also, an analytical investigation to predict the flexural strength of all tested specimens was carried out. The analytical results were agreed with the experimental results. Therefore, LWFC can be used as a substitute lightweight concrete material for the production of structural concrete applications in the construction industries today.Research limitations/implicationsFoamed concrete is a wide field to discuss. To achieve the objectives of the project, the study is focused on the foamed concrete with the following limitations: (1) because the aim of this research is to produce foamed concrete suitable for structural purposes, it is decided to produce mixes within the density range 1300–1900 kg/m3. (2) Simply-supported slabs are of considered. (3) This study also looks out by using GFR and without it.Originality/valueThe main objectives of this study were producing structural foamed concrete slabs and investigate their flexural response for residential uses.

Recycling of waste limestone powders for the cleaner production of epoxy coatings: Fundamental understanding of the mechanical and microstructural properties

Epoxy resins are very well suited for floor coatings, but their ingredients are harmful to the environment. Therefore, their content in coatings should be reduced, e.g. by adding a filler. In this paper, the fundamental understanding of the behavior of epoxy coatings modified with the addition of waste limestone powder was systematically assessed in terms of viscosity, mechanical properties (hardness, tensile strength, flexural strength, pull-off strength) and microstructure (powder distribution in the coating, porosity of the concrete substrate). The replacement ratio with limestone powder ranged from 0% to 29% of the mass of the epoxy resin. The following research methods were used: a flow cup, a Shore D durometer, a tensile and flexural testing machine, a pull-off tester, and a scanning electron microscope (SEM). It was observed that an increasing amount of waste limestone powder in the epoxy resin resulted in an increased viscosity of the liquid coating by 56%, an increased hardness of the cured epoxy coating by 10%, a decreased tensile and flexural strength by 62% and 59%, respectively, and a decreased pull-off strength of the coating by a maximum of 21%. Although the limestone powder caused a large increase in viscosity, the surface of the coating remained even, and the pull-off strength was still well above the standard minimum values. Microstructural analysis showed that most of the waste limestone was sedimented at the bottom of the sample, which resulted in a local increase of hardness. The novelty of the study is the investigation of the effects of replacing part of the epoxy resin and curing agent in epoxy coatings with limestone powder sourced from the production of coarse aggregate in aggregate quarries. Adding limestone powder to the epoxy coating will reduce the content of harmful substances in it, lower its price, and mean that mineral waste is recycled. The paper presents an analysis of the properties of a modified epoxy resin with regards to its use in floor coatings.

Flexural performance of RC beams strengthened with inorganic adhesive bonded NSM CFRP bar

Organic adhesives (e.g. epoxy) have been widely utilized in near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strengthening technique for reinforced concrete (RC) structural members. Their inevitable shortcomings, such as poor fire resistance, emission of toxic fumes, and incompatibility with concrete substrate, necessitate the advancement of eco-friendly inorganic adhesives. In this study, two types of inorganic adhesives, including ordinary Portland cement mortar (OPCM) and alkali-activated slag mortar (AASM), were adopted in the NSM CFRP bar strengthening method for RC beams. The inorganic adhesive type, bonding length, and additional anchorage device on CFRP bar were considered as parameters in the strengthening scheme. One control beam and four strengthened beams were fabricated and tested, followed by comparison of their failure modes, loading behaviour, and reinforcement strains. The results show that all the strengthened beams exhibit higher ultimate loads than the control beam. Comparing to the OPCM, the AASM is more effective in mitigating debonding of CFRP bar and enhancing the ultimate load of beams. Furthermore, it is essential to provide a sufficient bonding length for AASM bonded NSM CFRP bars to guarantee their strengthening effectiveness. The addition of anchorage device for CFRP bars can further enhance the synergistic action between CFRP bar and adhesive, leading to a further enhancement in flexural strength and CFRP bar strain. In addition, the flexural strengths of OPCM or AASM bonded NSM CFRP bar-strengthened beams can be accurately predicted by the equations provisioned in ACI 440.2R-2017.

Experimental, analytical, and numerical investigation on flexural behavior of hybrid beams consisting of ultra-high performance and normal-strength concrete

In this study, the flexural behavior of hybrid beams consisting of an ultra-high-performance concrete (UHPC) substrate and normal-strength concrete (NSC) overlay is investigated. From the experimental perspective, five UHPC-NSC hybrid beams were loaded to failure, and their failure patterns, load–deflection curves, ductility, strain distributions, and crack evolutions were compared. The UHPC substrate was found to significantly enhance the load-carrying capacity and suppress massive cracking of the hybrid beams. Interestingly, the increasing thickness of the UHPC substrate was observed to impair the beams’ overall ductility, leading to the apparent descending post-peak branch of the load–deflection curve. In other words, the agreed ductility of UHPC at the material level did not always benefit the deformation capacity of the beams at the structural level. Furthermore, an analytical derivation is proposed to capture the load–deflection curve of the hybrid beams. Specifically, sectional analysis in OpenSees was adopted to obtain the moment–curvature relation. This numerical strategy significantly alleviates the complicated derivation of the moment–curvature relation from conventional analytical formulas, owing to the significant tensile contribution of the UHPC material. Finally, finite element (FE) modeling of the hybrid beam specimens was performed. A nonlocal phase-field approach was implemented for UHPC and NSC materials. The numerical simulations are in good agreement with the experimental observations in terms of stiffness, load–deflection relation, and crack evolution.