Strain-hardening Cement-based Composites Research Articles

Effects of Shrinkage‐Compensation on Mechanical Properties and Repair Performance of Strain‐Hardening Cement Composite Materials

This paper describes the effects of expansive admixture on the mechanical properties of strain‐hardening cement‐based composite (SHCC) mixtures. Also, this study investigates structural performance of reinforced concrete (RC) beam specimens repaired with SHCC and Ex‐SHCC (SHCC with expansive admixture). In this study, SHCC and Ex‐SHCC mixtures with two specified compressive strength values of 30 MPa and 60 MPa and the fiber volume fraction of 1.5% were investigated. The expansive admixture replacement ratio of 10% by cement weight was used in this study. The test results indicate that the compressive, tensile, and flexural strength values of the SHCC mixtures increased when expansive admixture was included in the mix; however, their toughness and ductility decreased. The study results also show that the application of both SHCC and Ex‐SHCC mixtures to repair damaged RC beam specimens can lead to significant structural performance improvement by mitigating crack damage and increasing ductility.

Dynamic Tensile Behaviour of Strain-Hardening Cement-Based Composites (SHCC)

The paper presents a part of an experimental campaign consisting of quasi-static and impact tensile experiments on three different types of strain-hardening cement-based composites (SHCC) as well as on their constitutive cementitious matrices [1]. The investigation on different SHCC types was intended for analysing the effect of matrix composition and fibre type on the strain rate sensitivity of the composites and for enabling the formulation of material design recommendations for impact resistant SHCC. The dynamic tests were carried out by means of a Modified Hopkinson Bar (MHB) installed in the DynaMat Laboratory in Lugano, Switzerland, which enabled the characterization of the dynamic material behaviour under direct tensile loading in terms of force-displacement relationships.

Corrosion Deterioration of Steel in Cracked SHCC

The presence of cracks is unavoidable in reinforced concrete structures and also a gateway for chloride into concrete, leading to corrosion of steel reinforcing bars. So, crack control, crack width limitation and chloride threshold levels are well-established concepts in durability of reinforced concrete structures. This paper reports on accelerated chloride-induced corrosion in cracked reinforced strain-hardening cement-based composites and reinforced mortar beams, both in loaded and unloaded states. Corrosion rates are monitored and loss of mass and yield force, as well as corrosion pitting depth in steel bars are reported. The chloride content at different depths in specimens is also determined through XRF, and through chemical testing of acid and water soluble chloride content by titration. Finally, different relationships are drawn between crack properties, mass loss, yield force, corroded depth and chloride levels at the steel surface for different cover depths. It is found that the crack spacing and free chloride at the steel surface level are best correlated to the corrosion damage in the specimens.

Performance of various strain-hardening cement-based composites (SHCC) subject to uniaxial impact tensile loading

Three different types of strain-hardening, cement-based composites (SHCC) as well as their constitutive cementitious matrices were investigated under uniaxial quasi-static and impact tensile loading. The normal-strength matrix was combined with polyvinyl-alcohol (PVA) fiber in one composite and with high-density polyethylene (HDPE) fiber in another. The third composite material consisted of high-strength matrix and HDPE fiber. A modified Hopkinson bar was used to assess the impact resistance of SHCC in terms of stress-strain relationships at displacement rates of 6m/s. The experiments were performed on specimens of two different lengths, for which, the strain rates were approximately 100s−1 and 200s−1, respectively.SHCC made of normal-strength matrix and PVA, the composite with considerable ductility under quasi-static loading, performed substantially worse at high strain rates than SHCC made with HDPE fiber. This could be traced back to the peculiar alteration of the fiber-matrix interaction depending on the type of fiber and corresponding bond properties (chemical bond in case of PVA fiber versus frictional bond in case of HDPE fiber). Furthermore, it was demonstrated that the effect of the increasing strain rates on the tensile performance of SHCC made with HDPE fibers can be controlled by adjusting the matrix composition, this being facilitated by the consistent and predictable strain rate sensitivity of the interfacial interaction between the HDPE fibers and the surrounding cement-based matrix.

Failure Localization and Correlation of High-Speed Tension and Impact Tests of Strain-Hardening Cement-Based Composites

AbstractMechanical properties of strain-hardening cement-based composites (SHCC) subjected to impact and high-speed tensile loading were studied. The impact test setup was based on a free-fall drop...

Water permeability of cracked strain-hardening cement-based composites

Strain-hardening cement-based composites (SHCC) are characterized by the formation of numerous fine cracks under tensile loading. The water permeability of the cracked material and, consequently, the durability of respective structural members will strongly depend on these cracks. Experimental investigations into the water permeability of both SHCC specimens pre-cracked under uniaxial tension and SHCC overlays on cracked concrete substrate were performed. It is proposed to determine certain crack pattern parameters which may be linked to the theoretical water permeability of the cracked material. For this linking, a weighting function for the observed crack widths has been derived on the basis of the Hagen-Poiseuille equation.

Tensile behavior of high-strength strain-hardening cement-based composites (HS-SHCC) made with high-performance polyethylene, aramid and PBO fibers

The article at hand presents an investigation of the tensile behavior of high-strength strain-hardening cement-based composites (HS-SHCC) made with four different types of dispersed high-performance polymer fibers. In particular, high-density polyethylene (HDPE), poly(p-phenylene-terephthalamide) (aramid), as-spun poly(p-phenylene-2,6-benzobisoxazole) (PBO) and high-modulus PBO fibers were examined in respect of their reinforcing effect in a high-strength, finely grained, cementitious matrix. Besides the tension experiments at the composite level, single fiber pullout tests were performed for a micromechanical characterization of each composite material. Furthermore, contact-angle measurements and microscopic investigations were carried out to assess the fibers' ability to be wetted and to explain their interaction with the cementitious matrix.

Effect of fine crack width and water cement ratio of SHCC on chloride ingress and rebar corrosion

SHCC (Strain Hardening Cement-based Composite) is a material known for its strain-hardening behavior under tensile and bending stress and its characteristic numerous small cracks. SHCC is expected to show superior durability because of the fineness of the cracks. In this study, chloride ingress through cracks into SHCC and progress of rebar corrosion in three mixtures of SHCC with various water-cement ratios were investigated. Through a chloride solution immersion test, it was confirmed that chloride could penetrate through even very fine cracks. The resistivity of cracked SHCC against chloride ingress is mainly governed by the accumulated crack width and the water cement ratio. Chloride pre-mixed SHCC specimens were left in a high-temperature, high-humidity chamber for 11 months to promote rebar corrosion. While the accumulated crack width and the water cement ratio were both influential to an increase in corrosion area, only the water cement ratio had bearing on corrosion loss.

Effect of high strain rate on compressive behavior of strain-hardening cement composite in comparison to that of ordinary fiber-reinforced concrete

This paper presents an experimental study on the effect of high strain rate on the compressive behavior of two strain-hardening cement-based composites (SHCCs), compared to that of fiber-reinforced high-strength concretes (FRHSCs) with similar compressive strengths. One of the SHCCs was reinforced with 2% of polyvinyl alcohol (PVA) fibers by volume (SHCC-PVA) and had a compressive strength of 64MPa. The other was reinforced with 0.5% of steel plus 1.5% of polyethylene (PE) fibers by volume (SHCC-ST+PE) and had a compressive strength of 83MPa.Split Hopkinson pressure bar and hydraulic machine experiments were conducted to determine the behavior of the SHCC and FRHSC at strain rates from 30 to 300 and 10−4 to 10−1s−1, respectively. The Dynamic Increase Factor (DIFfc), the ratio of the material strength under dynamic loading to that under static loading, was determined for the materials considered. The fracture patterns of the specimens, matrix, and fibers were carefully examined. The results indicate that the fiber content has a significant effect on the DIFfc values. In addition, it is shown that the equations in CEB-FIP 1990 and fib 2010 codes are not applicable for SHCC and FRHSC beyond a transition strain rate of 30s−1.

Performance of strain hardening cement-based composite (SHCC) under various exposure conditions

AbstractThe performance of strain hardening cement-based composite (SHCC) under various exposure conditions such as chloride-induced accelerated corrosion, capillary water absorption (CWA), electrical resistivity (ER), and freezing-thawing (CDF) tests is reported. SHCC was compared with two reference mortars; one is the same strength class as the SHCC (M1), and the other is a high strength class (M2). Both cracked and uncracked beam specimens were tested. Results from the investigation show that, in SHCC, the corrosion potential is a function of crack width. It is suggested that the half-cell corrosion potential measurement technique may not truly reflect the corrosion state of an embedded steel bar in SHCC. The result of the CDF test reveals that after 42 cycles of freeze-thaw attacks, SHCC performed better than M1 and M2.

Fatigue behaviour of strain-hardening cement-based composites (SHCC)

The safe use of strain-hardening cement-based composites (SHCC) in structural and non-structural applications often requires a solid knowledge of the mechanical performance of this novel material under cyclic loading. The article at hand presents the findings of a comprehensive experimental investigation focusing on fatigue behaviour and failure mechanisms of SHCC made with polyvinyl-alcohol fibre and subject to various loading regimes. Uniaxial tests were performed both as tension-swelling tests and alternating tension-compression cyclic tests. While the upper reversal point was controlled – depending on the chosen regime – either by a given deformation increment or load level, the lower reversal point was always controlled by a given load value. The experiments revealed a pronounced decrease in the number of load cycles to failure with increasing upper stress level, smaller applied strain increments and with transition from purely tensile loading to alternating tension-compression regime. Furthermore, the effects of these tests parameters on strain capacity and other material properties of SHCC were investigated. On the basis of the test results, evaluation of the crack patterns and microscopic analysis of the conditions of the fracture surfaces, four various failure modes were identified, each of them typical for particular cyclic loading scenarios.

Flexural behavior of high-strength concrete beams reinforced with a strain hardening cement-based composite layer

High-strength concrete (HSC) is often used in construction. High-strength concrete exhibits high compressive strength, but the flexural strength and cracking are problems for this material. Strain-hardening cement-based composites were recently developed as a repair material for improving crack control and flexural strength. This paper details the behavior of high-strength concrete beams reinforced with a strain hardening cement-based composite (SHCC) and steel fibers. A new method for improving concrete beam flexural behavior was explored by laying a layer of SHCC in the concrete beam in situ. The flexural behavior of HSC beams containing steel fibers was also studied using a high fiber content. Three types of beams were cast, and a series of tests were conducted. The flexural results indicated that the added SHCC layer increased the load-bearing capacity of the beams. Adding steel fibers significantly increased the load-bearing capacity of the beam.

Resistance of high-performance fiber-reinforced cement composites against high-velocity projectile impact

This paper presents experimental and numerical studies on the performance of seven high-performance fiber-reinforced cement-based composites against high velocity projectile impact (HVPI). The materials investigated involved four fiber-reinforced high-strength concretes (FRHSCs) with 28-day compressive strengths ranging from about 60 to 140 MPa, two strain hardening cement-based composites (SHCCs), and a fiber-reinforced high-strength mortar (FRHSM). Of the two SHCCs, one was reinforced with 0.5% steel fibers plus 1.5% polyethylene (PE) fibers by volume of the composites, whereas the other was reinforced with 2% of polyvinyl alcohol (PVA) fibers. The other composites were reinforced with 0.5% of steel fibers. Ogive-nose shaped projectiles, with a diameter of 28 mm and weight of about 250 g, were used for the HVPI tests at projectile velocities of ~400 and ~600 m/s. The localized damage, particularly the penetration depth, was examined and discussed. Results indicate that the higher compressive strength and greater toughness of cement-based composites, as well as the presence of strong coarse aggregate and fibers, contribute positively to the impact resistance of composites. In addition, it was found that the penetration depth of the composites subjected to HVPI is reduced with an increase in the elastic modulus, as well as with the “effective hardness index” calculated based on the hardness and proportion of the coarse aggregate and mortar matrix. Numerical studies were conducted to simulate the localized damage of the FRHSCs and SHCCs due to the HVPI using the finite element software LS-DYNA. The numerical models adapted predict the penetration depth well. The numerical simulation results indicate that it takes a longer time to stop the projectile traveling at the same initial velocity in the SHCC specimen than in the FRHSC specimen due to the absence of hard and strong coarse aggregate in the former.

Chloride ingress in cracked and uncracked SHCC under cyclic wetting-drying exposure

This paper reports the performance of strain hardening cement-based composite (SHCC) containing polyvinyl alcohol (PVA) fiber under chloride exposure of both cracked and uncracked specimens. The performance of SHCC was also compared to that of an identical mortar mixture without fibers. Three-point bending was applied to mechanically induce cracks in both reinforced SHCC and mortar specimens. Cracks were also formed in unreinforced SHCC specimens, whereas cracks were not formed in the unreinforced mortar specimens because of its brittle behavior. Thereafter, specimens were subjected to cyclic and continuous chloride attack for defined time periods. Both total and free chloride in SHCC and mortar specimens were then determined by means of X-ray florescence (XRF) and chemical analysis according to RILEM recommendations. Rapid chloride migration tests were also performed to determine the chloride migration coefficient in both SHCC and mortar. The results confirm that the mechanically-induced cracks in the SHCC specimens lead to higher chloride migration than in uncracked SHCC.

Rheological characterisation and prediction of pumpability of strain-hardening cement-based-composites (SHCC) with and without addition of superabsorbent polymers (SAP) at various temperatures

The paper at hand presents the results of an investigation into the rheological behaviour of strain-hardening cement-based composites (SHCC) with and without the addition of fresh superabsorbent polymers. The effect of temperature variation in the range between 10 and 30°C is examined in terms of rheology, tribology and pumpability for up to 60min after mixing. The flow behaviour of SHCC is approximated as that of a Bingham fluid. Furthermore, two methods of estimating mixture pumpability, a tribometer and a Sliding Pipe Rheometer (SLIPER), are applied. The results describe the development of the Bingham parameters and slippage resistance as parameters related to pumpability. Additionally the correlation between the rheological properties and the pumpability of SHCC at various temperatures and times after mixing are analysed. Finally, the predictive capacity of the tribometer with respect to the concrete pumpability is validated by measuring the discharge pressure with SLIPER.

Effect of fiber properties and matrix composition on the tensile behavior of strain-hardening cement-based composites (SHCCs) subject to impact loading

The article at hand describes the behavior of high-strength and normal-strength strain-hardening cement-based composites (SHCCs) made of fine-grained matrix and high-density polyethylene fibers under quasi-static and impact tensile loading. The dynamic tension testing of unnotched and notched cylinders was performed using the Hopkinson bar at strain rates of around 150s−1. The responses of the materials under dynamic and quasi-static tensile loading were compared to the corresponding results for normal-strength SHCC made of polyvinyl-alcohol fibers as obtained in previous investigations. To explain the pronounced differences in rate effects on the material performance of various SHCC compositions, cracking pattern and fracture surface conditions were studied. Additionally, strain rate dependent changes in the mechanical behavior of individual fibers and in the fiber–matrix interfacial properties were deduced from single-fiber tension tests and fiber pullout tests, respectively. Altogether, the results obtained provide clear indications as to the decisive parameters for a purposeful material design of impact resistant types of SHCC for use in structural elements or protective overlays.

EFFECT OF NATURAL SANDS ON THE TENSILE BEHAVIOR OF SHCC UNDER DIFFERENT STRAIN RATES

One of the key factors affecting the successful production of strain-hardening cement-based composites (SHCC) is the nature of fine quartz aggregates. The elevated cost of SHCC preparation is considered as a limiting parameter. The incorporation of natural fine aggregates of different particle sizes can be one of the most cost-effective methods to produce SHCC locally. Two different types of local natural sands abundantly available in the Arabian Peninsula were used in the preparation of SHCC. The effect of these sands on the tensile response of SHCC under the effect of different strain rates was investigated. Three strain rates of 7.7x10-5, 1.5x10-4 and 1.5x10-3 s-1 were applied during the assessment of the direct tensile properties of SHCC samples made of natural sands. The results have shown that the use of natural sands provides an effective strain capacity. Microstructural analysis of SHCC mixtures has revealed the existence of two reaction mechanisms during PVA pullout of SHCC mixture with natural sands under direct tensile tension. Therefore, the incorporation of natural sands was proven to carry potential properties to SHCC mixtures that reduce their cost and encourage their production in a broad scale.

Efficiency of high performance concrete types incorporating bio-materials like rice husk ashes, cassava starch, lignosulfonate, and sisal fibres

Over the last decades concrete has evolved from a simple mass construction material towards a sophisticated multi-component system. The design parameters for the mixture composition of concrete have significantly increased from strength based towards overall or specific performance based. As a result the variety of concrete has increased yielding a number of special concrete technologies such as Self-Compacting Concrete (SCC), High-Performance Concrete (HPC), Strain Hardening Cement Based Composites (SHCC), and many others. Due to their complex mixture compositions and a multitude of possible interactions between constituents, these concrete types are preferably composed of special components like well-defined powders and sophisticated chemical admixtures. This makes such concrete technology expensive and limits their application to regions with the required material supply chains. The paper puts focus on materials, which are less well studied in conjunction with high performance concrete, but which are available in many developing countries, and in particular sub-Saharan Africa. The paper shows how SCC can be designed without polycarboxylate ether superplasticizer and well defined fillers, but with lignosulphonate, cassava starch and rice husk ash. The positive effect of well processed rice husk ashes is demonstrated. Furthermore results are presented of SHCC where typical components like polyvinyl alcohol fibres and fluy ash are replaced by sisal fibres and limestone filler, respectively. The results point out that high performance concrete applications do not have to be limited to a boundary framework with availability of well-defined raw material supply structures and sophisticated admixtures or fibres. Concepts are presented how innovative concrete technologies can be developed based on indigenous materials.

A new approach for modelling the ingress of deleterious materials in cracked strain hardening cement-based composites

Strain-hardening cement-based composites (SHCC) are characterised by their tensile ductility and high strain capacity. Due to their intrinsically small crack widths, SHCC exhibit favourable transport conditions in the cracked state compared to conventional concrete for certain transport mechanisms, e.g. diffusion and permeation and thus a good durability potential In this paper the Ingress Potential Index model is proposed which is a novel way of expressing the potential ingress of deleterious materials into cracked SHCC with a single value taking into account the strain level, crack pattern and a chosen ingress process. For the demonstration of the model, experimental data of water transport and chloride penetration into cracked concrete was used in combination with observed SHCC crack patterns. The purpose of this paper is to present an approach which can be used for any crack-linked ingress process and different distributions of crack widths. Future work is required to verify this novel approach experimentally.

Comparative testing of crack formation in strain-hardening cement-based composites (SHCC)

Abstract Two of the main activities of RILEM Technical Committee 208-HFC Subcommittee 2 were the preparation and publication of the state-of-the-art report on durability of strain hardening cement-based composites (SHCC), and the performance of comparative laboratory testing on SHCC. In this paper the comparative mechanical tests are reported, as performed in laboratories of five participating institutions. The purpose was to investigate and compare the crack patterns in terms of crack widths and spacing, and subsequently to make recommendations for a suitable test setup and procedure towards characterizing cracking in this class of materials. Such standardized procedures are required for future systematic and objective research towards durability of these materials in their in-service conditions, i.e. their resistance to deterioration processes in the cracked state. Standardized test procedures are also required for durability testing and guidelines for structural design with SHCC, which is the focus of follow-up committee activity in TC 240-FDS.