The influence of reusing ‘Formtex’ controlled permeability formwork on strength and durability of concrete

The effects of the reuse of ‘Formtex’ Controlled Permeability Formwork (CPF) liner on strength and durability properties of concrete were investigated at two different water-cement ratios and the results are reported in this paper. Test blocks were cast using the CPF on one side and impermeable formwork (IF) on the opposite side of the mould so that direct comparisons could be made between the two. The strength was assessed using the Limpet pull-off tester and both the air permeability and the water absorption (sorptivity) were measured using the Autoclam Permeability System. Both these instruments measured the ‘covercrete’ properties. In addition, cores cut from the test specimens were subjected to an accelerated carbonation test and a chloride exposure test. The results showed that the ‘Formtex’ CPF increases the surface strength and the durability of concrete compared to the IF. There was an almost complete elimination of blowholes. The permeability of concrete decreased and its resistance to the ingress of both carbon dioxide and chlorides increased when CPF was used. The beneficial effects of the Formtex CPF were most evident in concrete of higher water-cement ratio. With the reuse of the Formtex liner twice, that is a total of three uses, the performance of the CPF to improve the properties of concrete remained almost the same. In this research the CPF liner was cleaned thoroughly between each use, which must be adhered to for site applications for reproducing the beneficial effects observed in the laboratory.     

Microcracking and chloride permeability of concrete under uniaxial compression

In the previous studies on microcracks and rapid chloride permeability tests, microcracks were quantified in terms of total crack length. This was carried out by examining concrete slices after compression tests. No attempts have been made to characterise the microcracks during the compression test prior to the chloride permeability test. In the present study, concrete cylinders were loaded under uniaxial compression between 30% and 95% of the ultimate strength. A non-destructive method of microcrack evaluation was used to study the progressive microcracking in concrete cylinders during compression tests. After the compression test, a rapid chloride permeability test (RCPT) (ASTM C1202) was carried out on a specimen cut from the same cylinder. The total crack length was also determined from the same specimen to compare with the observed microcracking behaviour, assessed by the non-destructive testing. The characteristics of the microcracks in terms of the specific crack area are different when a concrete is under a load and when it is completely unloaded. The chloride permeability of a concrete (after it was unloaded) appears to be influenced by the occurrence of a certain stress level known as the critical stress. When the critical stress is exceeded in a concrete specimen, a comparatively large chloride permeability was measured. Where the critical stress in a specimen is not exceeded, the increase in the permeability is marginal in spite of the large increase in microcracks.     

Numerical modeling for predicting service life of reinforced concrete structures exposed to chloride environments

Degradation of reinforced concrete (RC) structures due to chloride penetration followed by reinforcement corrosion has been a serious problem in civil engineering for many years. In the present paper, a systematic and robust model for predicting service life of RC structures is developed which takes environmental humidity and temperature fluctuations, chloride binding, diffusion and convection, as well as the decay of structural performance into account. The interactions between the decay of structural performance, heat and moisture transfer are considered in a coupled thermal-hygro-mechanical model. The governing equations of heat, moisture and chloride transport into nonsaturated concrete are described particularly and solved numerically by finite element analysis in space and time domains. Comparisons of numerical results with analytical solutions and experimental observations are conducted to establish the validity of the proposed numerical model. Applications of the numerical model are demonstrated by predicting service life of a RC slab exposed to a chloride environment.     

Measurement systems for determining formwork pressure of highly-flowable concrete

This paper offers recommendations regarding test set-ups and measurement systems that can be used for laboratory evaluation and field testing of lateral pressure exerted by flowable concrete and self-consolidating concrete (SCC). Test results indicate that pressure sensors placed flush with the inner surface of the formwork can be used to assess the lateral pressure exerted by plastic concrete. The pore-water pressure resulting from the fluid phase of concrete can be evaluated using pore-water pressure sensors similar to the ones employed in soil mechanics. The sensors can be attached at different heights onto rigid formwork system to monitor changes in pressure with time.Sono-tubes made of cardboard are found not to be suitable for monitoring concrete pressure variations because of their flexibility which leads to erroneous values. A pressure column was developed to evaluate lateral pressure and its variations with time exerted by fresh SCC for heights of up to 10 m. Another system consisting of strain gages welded onto steel anchored bars inserted in the formwork was tested and compared to the pressure transducers system.     

Development of pseudo-ductile permanent formwork for durable concrete structures

The present investigation focuses on a new approach for the construction of durable concrete structures. Using Pseudo-ductile Cementitious Composites (PDCC) of relatively low water/binder ratio, permanent formworks are first fabricated. Normal concrete is then cast to make structural components. With low permeability and high crack resistance, the permanent formwork acts as effective surface cover to prevent the corrosion of steel reinforcements. The formwork can be made with PDCC alone, or with the incorporation of Glass Fiber Reinforced Plastics (GFRP) rods. In some structural components, the GFRP reinforcements will be sufficient to provide the necessary load-carrying capacity. When higher loads are to be carried, steel reinforcements can be added to produce a component with very high durability (due to the thick cover to steel) as well as ductile behavior. This paper focuses on mechanical aspects of this construction concept. The development of PDCC for formwork fabrication is first described. The bond between PDCC and concrete, in relation to various surface treatment methods, will be investigated with beam specimens. Test results on concrete beams made with GFRP reinforced PDCC formwork are then presented and compared to theoretical predictions. A design example is performed to demonstrate the use of GFRP/PDCC permanent formwork for constructing the deck of a footbridge. The results of this investigation show promise of the technology for practical applications.     

Electrochemical chloride removal for reinforced concrete with steel rebar cage using auxiliary electrodes

In this paper, a new approach of electrochemical chloride removal (ECR) using the auxiliary electrodes was proposed and the performances of this method were investigated by experiments. Two kinds of electrode setups were investigated: the radial type and the layer type. The results showed no matter which setup was used the chloride removal percentage all exceeded 70% after 8-week treatment. The chloride enclosed by the steel rebar cage could be successfully pushed out from concrete using auxiliary electrodes. The half cell potentials of rebars after 8-week treatment were all higher than −270 mV(CSE) and the corrosion rates were lower than 0.05 mpy, it indicated that after ECR treatment the corrosion risk of rebars was low. In addition, after ECR the surface hardness was enhanced via the results of rebound strength test. The pH value of concrete was also enhanced during the ECR process.     

Effect of casting rate and concrete temperature on formwork pressure of self-consolidating concrete

An experimental program was undertaken to determine the effect of casting rate and concrete temperature on formwork lateral pressure that can be developed by self-consolidating concrete (SCC). Concrete mixtures prepared with initial temperatures varying from 10 to 30^∘C or with high early-strength cement and set-accelerating admixture were evaluated. The concrete was placed either continuously at casting rates varying between 5 and 25 m /h or by stopping the placement for some predetermined periods of time. Test results show that the increase in casting rate from 5 to 25 m/h can lead to 15% increase in initial formwork pressure; however, no significant effect on the rate of pressure drop with time is observed. The variations in fresh concrete temperature have limited effect on the initial lateral pressure; however, the rate of pressure drop was significantly increased with the increase in temperature. The time for the cancellation of pressure is directly affected by the concrete temperature and materials in use, and occurs shortly after the end of the dormant period of cement hydration.     

Role of demoulding agents during self-compacting concrete casting in formwork

The use of “super-plasticisers” to obtain concretes that are highly workable, easy to place in formwork and require no vibration has spread throughout Europe in the last few years. The placing process for fresh so-called Self-Compacting Concrete (SCC) is linked to the friction that occurs at the concrete/wall interface. A rectilinear movement tribometer has been developed. Tests performed with certain modifications of the interface conditions show that friction is governed by the characteristics (viscosity, tensioactivity, etc.) on the demoulding agents used. The interface appears to undergo two types of phenomena.

-shear stress in the thick lubricating layer, which entails low shear stresses.

-granular behaviour. The shear stress is higher in the second case.

     

Effect of mineral admixtures on formwork pressure of self-consolidating concrete

Owing to enhanced filling ability, self-consolidating concrete offers accelerated casting and superior quality control during construction. However, its high fluidity and high placement rate increase the lateral pressure on the formwork, necessitating an extensive supporting system to retain fresh mixtures in a desired shape. Current recommendations of formwork design for self-consolidating concrete adopt the concept of hydrostatic pressure, even though the measured pressure could be less than the recommended level. This study shows that mineral admixtures such as processed clays can appreciably lessen the formwork lateral pressure. In addition, the correlation between the formwork pressure response and the loss of slump flow is derived, providing an approximate method to estimate the reduction in formwork pressure.     

Performance evaluation of structural concrete using controlled quality coarse and fine recycled concrete aggregate

This paper presents the fresh, mechanical, and durability performance, of a structural concrete mix classified as C-1, by the Canadian Standards Association (CSA) made with controlled quality Recycled Concrete Aggregate (RCA). Five mixes with water-to-cementing material (w/cm) ratio of 0.40 were produced with various RCA contents and tested against two 0% RCA control mixes made with General Use (GU) cement, and General Use Limestone cement (GUL). The RCA contents in the mixes were 10%, 20%, and 30% by coarse aggregate volume replacement, as well as 10% and 20% fine and coarse (granular) aggregate volume replacement. All evaluated mixes met the specifications from the CSA for fresh, mechanical, and durability properties. The coarse RCA mixes performed better than the granular RCA mixes in terms of flexural and splitting tensile strengths, linear drying shrinkage, water sorptivity, and rapid chloride-ion permeability, where the test results were significantly affected by the ultra fines present in the granular RCA.     

Vacuumatic formwork: a novel granular manufacturing technique for producing topology-optimised structures in concrete

Recent developments in computational design, such as structural optimisation algorithms, enable designers and engineers to come up with efficient load-bearing structures in which material is placed where it is mostly needed. This optimisation of the structural topology is often inspired by structures found in nature and can count on great fascination from architects. Well-known builder/engineer Pier Luigi Nervi, for instance, developed and patented in the 1950s a technique for manufacturing structurally highly efficient and aesthetically fascinating ribbed concrete structures. Nowadays, the manufacturability of such topology-optimised structures is considered the bottle neck in the overall building process, despite rapid developments in digital manufacturing techniques in the last decade, such as laser-cutting, CNC-milling and 3D-printing. Although various (traditional) manufacturing techniques are available to produce topology-optimised structures in concrete, such as timber formwork, these methods are typically considered complex, time-consuming, labour-intensive and therefore financially unattractive. A solution might be found in the use of vacuumatically-stabilised granular material as an adaptable mould. The overarching term of this specific technique is referred to as vacuumatic formwork.     

The relationship between chloride migration rate for concrete and electrical current in steady state using the accelerated chloride migration test

In this study the electrochemical technique is applied to accelerate chloride ion migration in concrete to determine the chloride ions in anode cell. This paper presents a new method for determining the chloride migration rate in concrete from steady state migration test by measuring the electrical current. The plain ordinary Portland cement concrete and concrete containing different type of mineral admixtures (fly ash and slag) with w/b ratios of 0.35, 0.45, 0.55, and 0.65 were used.For a given charge passed in steady state, the current corresponding to the given charge passed was correlated with the chloride migration rate. The results for all mixtures show that the chloride migration rate and the current corresponding to a given charge passed in steady state is linearly correlated.     

A virtual rapid chloride permeability test

The rapid chloride permeability test (RCPT), as it is commonly called, has been in existence for over 20 years and was standardized by ASTM over 15 years ago. The test is used extensively in the concrete industry for assessing concrete quality and is now being included in concrete specification documents. Because the underlying physics of the test are fairly well understood, it is possible to create a virtual test method that mimics the real world physical test. This paper presents a prototype virtual test method that includes prediction of the conductivity of the cementitious binder pore solution and the total charge passed during an ASTM C1202 RCPT. Potential applications of the virtual test method are first discussed. Then, the technical background used and the numerous assumptions employed in creating the prototype virtual test are outlined in detail. In addition, the computer implementation of the virtual test, as a set of HTML/JavaScript web documents, is presented. Validation against existing data sets is presented, with a generally reasonable agreement noted between the experimental and the virtual test results.     

Novel water permeability device for reinforced concrete under load

The presence of cracks in reinforced concrete structures is recognized to increase the penetration of water and aggressive agents into concrete and thus accelerate its deterioration. In order to gain knowledge on the influence of cracking on concrete durability and assess admissible loads to ensure long-term performance of structures, an innovative water permeability device was developed to estimate water flow in plain and cracked reinforced concrete. Permeability measurements were taken simultaneously with the application of a uniaxial tensile load on the testing specimen. The device permitted the estimation of the average stress in the reinforcing bar and the maximum crack opening in the concrete specimen. The experimental program comprised studies on result repeatability and the influence of testing parameters, such as pressure gradient, pressure regulation, loading rate and loading control mode. Test results showed that the modification of the testing parameters had a negligible impact on water permeability. Moreover, correlations were established between the water permeability, the average stress in the steel reinforcement, and the crack opening width in the reinforced concrete. Analysis of the results demonstrated the potential of the research results to improve the design criteria of reinforced concrete at serviceability limit states.     

Comprehension of demoulding mechanisms at the formwork/oil/concrete interface

The implementation of concrete and its association with a release agent influence the aesthetics of the concrete facings. The mineral oils tend to be replaced by vegetable formulations, to reduce the impact of the substances spilled in the environment. From a technical point of view, it is important to characterize the action of these new formulations at the interface concrete/oil/formwork. Two performing techniques have been used to study the physico-chemical processes, the tribometry and electrochemical impedance spectroscopy. The correlation of the results obtained allowed to improve the understanding of the mechanisms involved at the interface mould/oil, in connection with the use of an acidifier in the formulation.      

The permeability of fly ash concrete

Oxygen permeability tests were carried out on plain ordinary Portland cement (OPC) and fly ash concretes at three nominal strength grades. Prior to testing the concretes were subjected to a wide range of curing and exposure conditions. The results emphasize the importance of adequate curing to achieve concrete of low permeability, especially when the ambient relative humidity is low. In addition, the results demonstrate the considerable benefit that can be achieved by the use of fly ash in concrete. Even under conditions of poor curing, fly ash concrete is significantly less permeable than equal-grade OPC concrete, the differences being more marked for higher-grade concretes. Attempts were made to correlate strength parameters with permeability but it is concluded that neither the strength at the end of curing nor the 28-day strength provides a reliable indicator of concrete permeability. A reliable correlation was established between the water to total cementitious material ratio [w/(c+f)] and the permeability of concretes subjected to a given curing and exposure regime.     

Application of LA-ICP-MS for meso-scale chloride profiling in concrete

Chloride represents a major risk for reinforced concrete structures because at a certain concentration, it can promote depassivation of the steel bars and initiate corrosion. Therefore it is important to be able to measure the chloride content in concrete. In this paper the application of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for the study of chlorides in concrete is proposed. This scanning technique enables quick multi-element profiling, directly at the sample without the need for further preparation, within a range of sub-millimetre (meso-scale) resolution and with low limits of detection. Optimization of the operating conditions was performed in pressed concrete powder pellets. Linearity of the calibration was verified and limits of detection below 0.05 wt% of cement were determined. Chlorine, calcium and iron distributions were studied in cement based materials of increasing heterogeneity (paste, mortar and concrete). This technique is furthermore proposed for the study of the chloride induced corrosion process, by following element distributions along the concrete-steel interface at the time of depassivation.     

Compensation length of two-dimensional chloride diffusion in concrete using a boundary element model

The boundary element method (BEM) has been widely employed in engineering practice. However, the BEM is not commonly used in numerical analysis of chloride diffusion in concrete and might yield unsatisfactory results if the time duration is significantly long for concrete exposed to chloride environment. In this work, we propose the compensation length of chloride diffusion in concrete, as well as the compensation coefficient based on the error function. The fundamental solution relevant to the governing partial differential equation is presented for chloride diffusion in concrete, enabling the development of the boundary element scheme. In particular, the two-dimensional diffusion analysis is investigated in detail by using the BEM model, featuring the proposed compensation length to achieve superior numerical results. Specifically, the time interval is sparsely discretized into several sub-domains in the BEM model, while the spatial domain is discretized along the boundary of the computational diffusion field, resulting in considerably fewer unknowns in chloride diffusion analysis. Several numerical examples are presented to demonstrate the effectiveness and accuracy of the BEM with compensation length and to illustrate excellent results using the 2-D BEM formulation for chloride diffusion near the intersection of exposed surfaces of concrete specimens.