Engineered Cementitious Composites Matrix Research Articles

Experimental investigation of corrosion-damaged RC beams strengthened in flexure with FRP grid-reinforced ECC matrix composites

In this study, composites fabricated using a fiber-reinforced polymer (FRP) grid-reinforced engineered cementitious composite (ECC) matrix were used for the flexural strengthening of corrosion-damaged reinforced concrete (RC) beams in combination with a patch repair technique. After the longitudinal reinforcements were corroded to the expected ratio by the electrochemically accelerated method, the corrosion-damaged concrete cover was removed, and then repaired with FRP grid-reinforced ECC matrix (FGREM) composites. The investigated test variables were mainly FRP grid types (carbon and basalt), strengthening amounts, corrosion level, and longitudinal reinforcement ratio. The experimental results indicated that the bearing capacities of the corroded beams were sufficiently improved using this strengthening technique. The intermediate debonding at the concrete substrate-to-ECC interface caused by flexural cracks could be completely suppressed. The use of a carbon fiber reinforced polymer (CFRP) grid with higher stiffness could produce a weaker ductile behavior and prematurely trigger critical diagonal crack-induced debonding. Furthermore, the flexural bearing capacities of the basalt-FGREM strengthened beams could be reasonably predicted according to the ACI-549.4R-13 provisions. However, the predicted results were relatively dangerous for carbon-FGREM strengthened beams, which failed under the control of the critical diagonal crack-induced debonding mode.

Residual bond strengths of epoxy and cement-bonded CFRP reinforcements to concrete interfaces after elevated temperature exposure

This paper presents the results from a comparative study on the residual bond performance of epoxy and cement-bonded carbon fiber-reinforced polymer (CFRP) reinforcement to concrete interfaces that experienced different levels of elevated temperature exposure. In total twelve groups of concrete beams strengthened with CFRP sheet and grids of the same tensile stiffness, through epoxy and cement-based adhesive bonding, respectively, were carefully prepared and exposed to different test temperature levels from 20 °C to 600 °C. Both polymer cement mortar (PCM) and ductile engineered cementitious composite (ECC) were adopted as the cement binder. The strengthened beams were tested under three-point monotonic bending after different levels of elevated temperature exposure. It was found that with increasing the exposure temperature, the failure modes of the strengthened beams shifted from debonding in a thin layer of concrete substrate to debonding near the fiber-adhesive interface and tearing off of CFRP grids from the ECC matrix. The peak loads of concrete beams strengthened with cement-bonded CFRP grids are at least 50% higher than their counterparts strengthened with epoxy-bonded CFRP sheets given the same temperature exposure.

Shear strengthening of RC beams with corrosion-damaged stirrups using FRP grid-reinforced ECC matrix composites

This paper presents an experimental investigation of the shear behavior of reinforced concrete (RC) beams with corrosion-damaged stirrups strengthened using a fiber-reinforced polymer (FRP) grid-reinforced engineered cementitious composite (ECC) matrix. A total of 18 beam specimens, including four referential beams and 14 strengthened beams, were prepared and tested. After corroding the stirrups to the expected ratio using the electrochemically accelerated method, the lateral corrosion-damaged concrete cover was removed, and then repaired using the FRP grid-reinforced ECC matrix (FGREM). The main test parameters included the stirrup corrosion level, shear-span ratios, and types and strengthening amounts of the FRP grid. Based on the experimental results, the shear bearing capacity of the beams with corrosion-damaged stirrups improved significantly after being restored using this technique. The embedded FRP grid could effectively share the shear force borne by the stirrups and the ECC improves the deformation characteristics and energy absorption of the specimen owing to its excellent strain-hardening behavior. Furthermore, a theoretical calculation formula was proposed to predict the shear bearing capacity of the strengthened beams based on the truss-arch model, which was confirmed to be safe and accurate after being assessed using the modified version of the demerit points classification (DPC) method.

Assessment of bond-slip behavior of hybrid fiber reinforced engineered cementitious composites (ECC) and deformed rebar via AE monitoring

This paper investigates the efficiency of acoustic emission (AE) technique for monitoring and assessing the bond-slip behavior and damage characteristics of deformed rebar embedded in hybrid fiber reinforced engineered cementitious composites (ECC). A series of pull-out tests were conducted on 48 prism samples with variable rebar diameters (14 mm, 20 mm, and 25 mm), embedded length (3d, 4d and 5d), and cover thickness (2.5d, 4d, 5d and 6d). Two AE sensors, attached on ECC surface and rebar surface respectively, were utilized to critically monitor the AE activity emitted from the ECC matrix and the interface between ECC and rebar throughout the tests. The bond-slip behavior of ECC-rebar presented a more ductile performance than conventional concrete (CC)-rebar because of fiber bridging effect. The damage classification and damage characteristics were clarified by measuring AE activity. The debonding process of ECC-rebar could be divided into four main stages based on the slope changes of cumulative AE hits, and further refined into several substages based on the sudden rises of AE energy. Besides AE cumulative parameters, the Ib-values calculated from AE signals of ECC matrix and ECC-rebar interface were analyzed and compared, which behaved more effectively in identifying the development of cracks and distinguishing the maximum bond stress. Also, the AE intensity analysis was implemented to identify the micro-mechanism transformation during different bonding stages. Moreover, the AE intensity results of CC-rebar obtained from previous research and ECC-rebar in this study were compared.

Chemical attack on concrete containing a high volume of crumb rubber as a partial replacement for fine aggregate in engineered cementitious composite (ECC)

Today, scrap tire is disposed of by open burning which harms the environment. To reduce the wastage of the resource, a sustainable approach is needed to dispose of the waste tire as a partial replacement of the fine aggregate in concrete mixes. This research has been conducted with a focus on the chemical attack of an engineered cementitious composite (ECC) containing a high volume of crumb rubber in terms of durability, behaviour, and comparison with conventional concrete. Two variables have been considered in developing rubberized ECC mixtures, that is, the amount of crumb rubber as a replacement for fine aggregate by volume of 0%–30% and PVA fibres by volume of 0%–2% to cementitious materials. The resistance properties of ECC incorporating crumb rubber were investigated for 13 different variable combinations developed using response surface methodology (RSM). The experimental results revealed that the presence of crumb rubber in the ECC matrix enhanced the resistance of the ECC in both acidic and sulphate environments. It was also revealed that by incorporating 15% crumb rubber, the loss of compressive strength significantly reduced from 38% to 15%.

Experimental Study on Eccentric Compressive Performance of Concrete Column Strengthened with CFRP Grid Reinforced ECC Matrix

Carbon‐fiber‐reinforced polymer (CFRP) grid and engineered cementitious composite (ECC) were combined in this study to strengthen concrete columns. The influences of the number of layers, the overlap length of CFRP grids, and the eccentricity on the bearing capacity and rigidity of reinforced concrete columns were determined. The results show that the principal failure of the reinforced column was debonding of external ECC from FRP grids at the compressive area, edges, or sides. Significant enhancement in the ultimate bearing capacity and rigidity of eccentrically loaded columns was observed after they were externally reinforced by CFRP grids and ECC; such enhancement increased with the number of reinforced layers. Eccentricity made little difference to the enhancement rate of bearing capacity when the number of reinforced layers was the same. At different eccentricities, the composite layers at the tensile area and the compressive area had different contributions to the bearing capacity. An effective bond and efficient stress transfer could be ensured as long as the overlap length between CFRP grids reached 120 mm.

Eco-friendly high strength, high ductility engineered cementitious composites (ECC) with substitution of fly ash by rice husk ash

In this study, an agricultural waste-rice husk ash (RHA) is proposed to substitute fly ash in high strength, high ductility engineered cementitious composites (ECC). The experimental results showed that the substitution of fly ash by RHA accelerated hydration process, promoted pozzolanic reaction, and refined pore distribution in ECC matrix, thereby increased compressive strength of ECC mixtures significantly from 82 MPa to 108 MPa. On the other hand, tensile properties of ECC mixtures were improved with addition of RHA, except ECC mixture with substitute ratio of 50% showed a slight reduction in strain capacity yet exhibited highest strength. At micro-scale, incorporating RHA into ECC reduced the theoretical complementary energy (Jb′) as a result of enhanced fiber/matrix interface, meanwhile, lowered crack tip toughness (Jtip) in ECC matrix which mainly due to the evidently increased modulus of ECC; subsequently lead to the increment of pseudo strain-hardening PSH (=Jb′/Jtip) index, and thus ductility.

Effect of sub-elevated temperature on mechanical properties of ECC with different fly ash contents

In this paper, the mechanical properties of Engineered Cementitious Composites (ECC) with two different fly ash contents after sub-elevated temperature exposure were investigated systematically at multiple scales. At composite level, uniaxial compressive and tensile tests were performed on specimens underwent 20 °C, 50 °C, 100 °C, and 200 °C exposure. The results indicate that, as temperature rise from 20 °C to 200 °C, the compressive strength of both ECC mixtures increased due to the refinement of the matrix pore structure. In term of tensile properties, after exposed to 100 °C and 200 °C, ECC M1 with relatively lower fly ash content shows a substantial reduction in strain capacity, but still retaining strain-hardening behavior, whereas ECC M2 with higher fly ash content exhibits a comparable or even enhanced tensile strength and strain capacity. At micro-scale, ECC matrix toughness was increased and PVA fiber strength was degraded under 200 °C exposure, which deteriorates the tensile properties of ECCs. However, the above negative effects are diminished due to the reduced fiber/matrix interfacial bond. Furthermore, the rather low matrix toughness and fiber/matrix interfacial bond associated with high fly ash content are the main reasons for the highly remained ductility in ECC M2 after sub-elevated temperature exposure.

Chloride diffusion behavior of engineered cementitious composite under dry-wet cycles

This study investigated the chloride diffusion behavior and mechanism of engineered cementitious composite (ECC) under dry-wet cycles and NaCl immersion. A standard chloride diffusion test was conducted to evaluate the effects of water-to-binder ratio (w/b), NaCl solution concentration and exposure time on the chloride diffusion depth, content and coefficient in ECC prisms. The microstructures of chloride-eroded ECC were also characterized using X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP). The results show that the chloride diffusion depth, content, and coefficient were decreased with w/b increasing. The increment of dry-wet cycle number facilitated chloride erosion but reduced late diffusion coefficient and rate. Moreover, the increased chloride concentration made the chloride diffusion depth and content ascend, while its effect on apparent chloride diffusion coefficient was marginal. After 15 dry-wet cycles, Ca(OH)2 in ECC matrix was gradually consumed and Friedel salt and CaCO3 were formed on the chloride diffusion surface. In addition, the porosity of ECC was reduced by decreasing macropores/voids and increasing mesopores under dry-wet cycle. ECC exhibited the worst resistance to chloride diffusion under dry-wet cycles.

Flexural behavior of RC beams strengthened with BFRP bars-reinforced ECC matrix

This paper has proposed a new strengthening system made of basalt fiber-reinforced polymer (BFRP) bars-reinforced engineered cementitious composite (ECC) matrix (hereinafter referred to as “BFREM” for brevity) for repairing of RC beams. Six specimens including a control beam and five beams strengthened with the BFREM layers were prepared and tested. The test variables included the reinforcement ratio of BFRP bars (0.94% or 1.41%) and the installation method (prefabricated or cast-in-place) used for the BFREM layer as well as the presence of the end anchorage system (with steel bolts). The test results showed that two failure modes of the strengthened beams were the tensile rupture of BFRP bars and the concrete cover separation with the strengthening layer attached. The cast-in-place method incorporating the end anchorage system was capable of inhibiting the debonding failure of the BFREM layer, thereby making better utilization of the strengthening materials. Moreover, the proposed strengthening system was efficient in enhancing the ultimate loads of the strengthened beams, which were 34%–48% higher than that of the control beam. A theoretical analysis was then conducted to predict the flexural strengths of the tested beams, and its reliability was verified by comparing the test results and the theoretical predictions.

Development of 3D printable engineered cementitious composites with ultra-high tensile ductility for digital construction

The current 3D concrete printing (3DCP) technology is limited by the reinforcing methods. Conventional steel reinforcement is hard to be incorporated in the 3DCP process. To overcome this limitation, this study aims to develop 3D-printable engineered cementitious composites (ECCs) exhibiting ultra-high tensile strain capacity of more than 8%, which can be used for digital construction of ‘self-reinforced’ concrete structures, reducing the reliance on the conventional steel reinforcement. Different volume fractions of polyethylene fibers (1%, 1.5% and 2%) were used to reinforce the ECC matrix. The fresh properties (including the workability, rheological properties and buildability) and the hardened properties (including the compressive and flexural strengths, and the uniaxial tensile performance), along with the microstructure of the developed 3D printable ECCs were experimentally investigated. Conventionally mold-cast ECCs were also prepared and tested for comparison purposes. The results showed that the developed 3D printable PE-ECCs exhibited strong strain-hardening behavior with the tensile strength and tensile strain capacity of up to 5.7 MPa and 11.4%, respectively. In addition, the results showed that the 3D printed PE-ECCs exhibited superior tensile performance to the conventionally mold-cast PE-ECC counterparts. This finding is in good agreement with the results of the microstructural analysis.

Flexural strengthening of RC beams with CFRP grid-reinforced ECC matrix

This paper investigates the flexural performance of a series of RC beams externally bonded with carbon fiber-reinforced polymer (CFRP) grid-reinforced engineered cementitious composite (ECC) matrix. A total of 15 RC beams, including three control and twelve strengthened, were prepared and tested. The test variables included the longitudinal reinforcement ratio, the strengthening configurations that consisted of different cementitious matrices (ECC versus epoxy mortar), different installation methods (prefabricated versus cast-in-place), and different stiffness of CFRP grids. The test results showed that ECC is an ideal cementitious matrix for the strengthening applications where FRP grids are used as the external reinforcement. The flexural strengthening configuration using the epoxy adhesive to bond prefabricated CFRP grid-reinforced ECC plate proved to be the most efficient solution. For such configuration, the plate-end debonding can be avoided and the mid-span debonding can be almost suppressed. Flexural capacity analysis was conducted and demonstrated that the plane section assumption is valid and the full strength composite action can be nearly achieved for the strengthening system. The average ratio of the predicted peak loads to the experimental ones of the strengthened RC beams was 1.05.

Comparison of engineered cementitious composites and concrete for strengthening of concrete structural details using RBSM

This paper presents a numerical simulation and subsequent comparison of strengthening performance of an ordinary concrete overlay and an overlay made of engineered cementitious composite (ECC) with polyvinyl alcohol fibers (PVA). The comparison is performed on an L-shaped joint when the overlay is placed on the outer surface so that the applied bending moment causes tension in the overlay. The nonlinear numerical analysis is based on the three-dimensional rigid-body-spring model (RBSM). The results show the beneficial effect of the PVA fibers within the ECC matrix when the damage is distributed evenly so that only thin microcracks open. The observation is easy to obtain when the RBSM is employed. On the contrary, the overlay made of ordinary concrete fails due to localization of the damage into a single crack. The applicability of the RBSM is discussed.

Effect of curing age on the self-sensing behavior of carbon-based engineered cementitious composites (ECC) under monotonic flexural loading scenario

The potential effects of curing age on the self-sensing (piezoresistivity) capability of carbon-based Engineered Cementitious Composites (ECC) specimens are under focus in the present study. This non-structural feature can be regarded as one of the best solutions for continuously monitoring of infrastructures in terms of damage and deformations. Carbon fibers which are a micro-scale electrically conductive material were added to the ECC matrix and well dispersed to create the electrically conductive network. This network is responsible for sensing the applied loads on the prismatic specimens. The self-sensing behavior of electrically conductive prismatic specimens under four-point monotonic flexural loading was investigated and compared with dielectric ECC specimens at four ages of curing (7, 28, 90 and 180 days). The results showed that the developed multifunctional cementitious composites can sense the changes in the applied flexural stresses and the resultant mid-span deflection along the adopted curing ages with an improvement in the later ages.

Matrix tailoring of Engineered Cementitious Composites (ECC) with non-oil-coated, low tensile strength PVA fiber

Engineered Cementitious Composites (ECC) with ductile strain-hardening behavior has been recognized as a high performance and durable alternative to conventional concrete material. However, relative high initial material cost, mainly associated with high cost of PVA fibers commonly used in ECC mixtures, has limited broader field application of ECC in the construction, especially in Chinese market. In this study, new ECC mixture designs have been developed incorporating a type of lower-cost PVA fibers. Unlike the PVA fibers normally used, this low-cost PVA fiber has no surface oil coating and exhibit lower tensile strength, which are undesirable in basis of micromechanics underlying ECC design. High fly ash content, viscosity modifying admixture (VMA), fly ash cenosphere (FAC), and crumb rubber were incorporated into the mixture design to tailor the ECC matrix so as to compensate the micromechanical change associated with using the low-cost PVA fibers and to further enhance the performance of the resultant ECC mixture. It is found that the newly designed ECC mixtures with low-cost PVA fibers can achieve similar tensile strain capacity (up to 5.2%) with lower compressive and tensile strength when compared with previous ECC mixtures in literatures. The successful development of ECC with low-cost PVA fiber is expected to greatly promote the field application of ECC in China and give the references for the professionals to develop ECC with the local PVA fibers.

Development and recovery of mechanical properties of self-healing cementitious composites with MgO expansive agent

This paper presents the performance of engineered cementitious composites (ECCs) produced with MgO utilized as expansive self-healing agent to heal micro cracks. ECCs capability to keep small crack width of less than 60µm even under ultimate loads can stimulate the autogenous crack healing and thus, improving the mechanical properties of structural elements. Different test schemes were adopted to quantify the long-term self-healing capability of proposed ECC-MgO system (made of 5% MgO expansive agent ‘MEA’ as fly ash replacement in ECC matrix) through studying the development and recovery of strength (compressive and flexural) and Ultrasonic Pulse Velocity (UPV) of pre-loaded multiple damaged (cracked) cubic and prismatic specimens subjected to long term water and natural curing conditions. Test results indicated that preloaded cracked ECC-MgO specimens had high tendency to recover original mechanical properties of virgin (un-cracked) specimens through healing of micro-cracks. This study confirmed the self-healing capability of proposed ECC-MgO system.

Flexural behavior of basalt FRP reinforced ECC and concrete beams

In recent years, fiber-reinforced polymer (FRP) bars have been used as novel concrete reinforcement in the beams. However, the ductility of the beams is highly dependent on the properties of concrete since concrete crushing governs the failure mode. Substitution of concrete with engineered cementitious composite (ECC) can avoid the cracking and durability problems associated with brittleness of concrete. In this paper, the flexural behavior of ECC and concrete beams reinforced with basalt FRP bars were numerically investigated with the software of ATENA/GID solver. To verify the validity of the finite element models of the composite beams, the simulation results were compared with the published experimental results of both FRP reinforced concrete and ECC beams, and good agreements were achieved. According to the simulation results, the BFRP reinforced ECC beams show much better flexural properties in terms of load-carrying capacity, deformability and crack controlling ability compared with the BFRP reinforced concrete beams. Also, the BFRP bars in ECC matrix were much more efficiently utilized than those in concrete, and the failure process is much more ductile due to much higher ultimate compressive strain of ECC materials in the compressive zone. An extensive parametric analysis was then conducted to examine the effect of various parameters on the flexural behavior of BFRP reinforced ECC beams.

Matrix design for waterproof Engineered Cementitious Composites (ECCs)

The excellent crack control capability of Engineered Cementitious Composites (ECCs) is exceedingly beneficial for waterproofing applications. To design a suitable ECC matrix for waterproofing applications, waterproofing admixture (WPA) was added to improve the wetting property and reduce the sorptivity, and shrinkage-reducing admixture (SRA) together with calcium sulfoaluminate cement (SAC) were introduced to control the drying shrinkage. A comprehensive experimental program was conducted to optimize the use of WPA, SRA and SAC. Moreover, the water permeability of both un-cracked and cracked ECCs were examined. This work finally developed a kind of waterproof ECC, with 28-day compressive strength around 35MPa, ultimate tensile strain over 4%, a water contact angle larger than 110 degree, 28-day drying shrinkage about 2/3 the value of the typical ECC (M45), a water absorption rate around 5×10−5mm/s1/2 and a water permeability coefficient about 8×10−12m/s. The findings of this research should be rewarding for the design of ECCs for waterproofing applications.

Analysis of Crack Microstructure, Self-Healing Products, and Degree of Self-Healing in Engineered Cementitious Composites

AbstractIn this paper, self-healed cracks of engineered cementitious composites (ECC) are observed by optical microscope and scanning electron microscopy, and self-healing products of ECC are analyzed by X-ray diffraction technology. Meanwhile, the reasons for property differences between origin ECC and self-healed ECC with different crack structures exposed to freeze–thaw in water and deicing salt, respectively, are discussed. ECC specimens with a water–binder ratio of 0.25 with 50% fly ash are studied. The experimental results show that the white self-healing products and other chemical composites similar to the color of ECC matrix are present on the surface of self-healed cracks. The self-healing materials of ECC include kinds of hydration/carbonation products and some chemical compounds penetrating into ECC matrix from self-healing environments. Additionally, the differences in self-healing products and hydration products of binder, self-healing extent, damaged polyvinyl alcohol fiber, weak interface ...

MECHANICAL PROPERTIES OF A PVA FIBER REINFORCED ENGINEERED CEMENTITIOUS COMPOSITE

High Performance Fiber Reinforced Cementitious Composites (HPFRCCs) are promising construction materials characterized by tensile strain hardening behavior. Engineered Cementitious Composite (ECC) is a special type of HPFRCC developed with enhanced ductility and durability. Coarse aggregates are usually excluded from the ECC matrix, and the reported ECCs are typically produced with microsilica sand having a maximum grain size of 200 µm. In this paper, a PVA-ECC mixture containing local dune sand with a maximum grain size of 300 µm was developed, and its compressive and tensile properties were experimentally investigated. A dog-bone-shaped specimen and a rectangular-coupon-shaped specimen were both used in the tensile test, and it was found after extensive research that the dog-bone specimen was more suitable than the rectangular coupon specimen. The experimental results from the dog-bone specimens indicated that the newly-developed composite possessed good tensile strain-hardening behavior, with a high ultimate tensile strength, and the compressive strength was comparable to that of existing PVA-ECCs.