Formwork Removal Research Articles

2D-DEM modelling of the formwork removal of a rubble stone masonry bridge

This study aims to determine the mechanical behaviour of a stone arch bridge during the phase of the formwork removal. This operation is the most delicate during construction, because it puts in charge the arch which must work in compression for stability. On-site measurements of the displacement of the arch voussoirs during the process of the formwork removal were carried out. Then, some mechanical tests were performed with the stones used in the construction to determine the mechanical properties required for modelling the structure by a discrete element method. This numerical approach has the ability to find the specific pattern of the displacement field observed on-site that could not have been found with a finite element method. The key feature is the rotation of the voussoirs which changes the location of the zones of contact from one to another. Therefore, the contacts from one to another are restricted to a small zone for the voussoirs close to the keystone.

Sensitivity analysis of the early‐age cracking risk in an immersed tunnel

AbstractEngineers know only too well that early‐age cracking accounts for the decrease of long‐term serviceability for a majority of infrastructure. In immersed tunnels, early‐age cracks may leave paths for aggressive media, which lead to deterioration and thus compromise durability. On the other hand, the cracking risk in concrete structures depends on a number of factors, such as material properties, construction methods, curing measures, etc., which make decisions complex.This work presents the results of a sensitivity analysis of early‐age behaviour for an immersed tunnel cast in situ. Numerical modelling and local sensitivity methods are employed to evaluate the sensitivity of early‐age cracking to casting temperature, formwork conductivity, formwork removal time, curing temperature and ambient temperature. The numerical results indicate that the casting and curing temperatures appear to be the most dominant factors with regard to the whole fabrication period.This study provides a realistic method for determining the uncertainty analysis of concrete structures at an early age, and identifies the most important factors during the fabrication of immersed tunnel segments, which is beneficial for further decisions related to the control of early‐age cracking.

Quantifying weather impact on formwork shuttering and removal operation using system dynamics

Weather condition is a significant factor that influences the activities of construction projects. This paper presents a framework that forecasts the losses on productivity due to the impact of weather conditions. The framework consists of three components; historical records database, analytical fuzzy model, and system dynamic model. The productivity of formwork shuttering and removal is directly impacted by two weather aspects; level of rain and temperature degree. The loss in productivity is modeled via analytical fuzzy model and system dynamic model. The proposed framework incorporates historical weather records, expert experiences, and fuzzy-set techniques in productivity losses forecasting. It provides more realistic forecast for productivity losses considering the impact of rain and temperature rises. Construction contractors benefit from the framework by assigning appropriate crew size for formwork shuttering and removal to meet project requirements with respect to total duration of project execution. A numerical example is presented to demonstrate the use of the proposed framework.

Fresh concrete lateral pressure decay: Kinetics and factorial design to determine significant parameters

Abstract The design of vertical formworks is governed by the lateral pressure exerted by fresh concrete; while maximum lateral pressure is the key parameter for formwork design, the rate of pressure decay and the time needed for pressure stabilization is important for determining formwork removal time; the complexity of the problem is due the large number of factors which affect the rate of pressure decay. This works describes an experimental investigation of columns to determine the variation in the initial lateral pressure decay and in the time needed for pressure stabilization with the following variables: coarse aggregate concentration (different sand to total aggregate ratios were used), the use of a plasticizer, the presences of reinforcement and concrete temperature. In the last part of this work a correlation between setting time and maturity functions with the time needed for pressure stabilization was done. The results show that both thixotropic linked to cement grains nucleation and consolidation and cement hydration influences in lateral pressure decay to the point of stabilization. In references with the initial pressure decay, the presence of reinforcement seems to have a major influence. On the other hand the time needed for pressure stabilization could be correlated with the maturity functions.

Economic criteria for the selection of wall formwork

Planning formwork (selecting systems, designing layout of formwork members) affects the economy of in situ concrete structures. Formwork rental cost, and labor cost of formwork installation and removal, have a significant share in total cost. The paper investigates into the problem of minimizing wall shuttering cost. A mathematical model of the problem of panel layout is proposed, and its operation is illustrated by the case with two optimization criteria.

Design of Very High Performance Concretes Using Local Aggregates

In recent years, appeared very high performance concretes which are the result of a more and more scientific rationalization. The use of superplasticizers, and more recently of ultrafine particles such as silica fume, has significantly minimized the amount of mixing water in concrete while improving workability. Owing to this water reduction (and thus the w/c ratio), the mechanical strength of such concretes has considerably increased. The compressive strength may exceed 80 MPa. Therefore, their use is very promising from an economic point of view and quality of civil engineering and hydraulics works. The aim of this study is to formulate a VHPC using local materials. The design approach used is the grout method. The obtained results show a clear improvement in the mechanical performance of concrete, resulting in a sufficient compressive strength to allow formwork removal at 24 h, an acceptable tensile strength and shrinkage lower than that obtained in conventional concrete.

Micromechanics-based multifield framework for early-age concrete

In the process of hydration of concrete, there are considerable interacting physical and chemical changes inside the material, affecting both composition and morphology of concrete at early ages. These changes are properly considered within multiscale models comprising several scales of observation and giving access to the effective properties via upscaling. In this paper, a multiscale model for early-age concrete is implemented into a multifield (thermo–hygro–chemo–mechanical) framework, which accounts for all major processes among the solid, liquid, and gas phases of concrete by means of mass, energy, and momentum equilibrium. The proposed modeling approach is employed for the investigation of the effect of formwork removal (stripping) of early-age concrete, exposing the concrete surface to the outside environment which may cause early-age cracking of concrete structures. Based on the multiscale approach, the authors evaluate the cracking risk of concrete members with respect to the underlying mix-design, size of the concrete member, and stripping time, providing first insight into the influence of these parameters on the cracking risk of early-age concrete.

Electro-Osmosis Applied for Formwork Removal of Concrete

Electro-Osmosis Applied for Formwork Removal of Concrete

A new procedure to ensure structural safety based on the maturity method and limit state theory

There is a significant gap between the current knowledge of structural concrete, as reflected in building standards and the actual construction of structures using the maturity method. To address this situation, an experimental study was planned for the purpose of developing and validating a procedure for estimating concrete strength by the maturity method. The main difference between the procedure developed in this study and the ASTM standard is in the adaptation of the former to the semi-probabilistic method of limit states. This new procedure was employed to determine the structurally safe times for the removal of formwork in false tunnels.

Development of an embedded wireless sensing system for the monitoring of concrete

The application of the electromechanical impedance (EMI) method to monitor the condition of structures is an actively researched area. This article extends the method to allow it to be incorporated into a wireless sensing device, which is embedded into freshly poured concrete to monitor initial curing and subsequent structural health. The results show that the hydrating concrete has an effect on the sensing system and that it is sensitive enough to monitor the strength development of concrete. Initial results also show that the embedded EMI method is sensitive to the removal of formwork. The response of the system to compressive testing is also investigated, and the initial results show a good correlation with previously published reports on compressive testing of concrete. Finally, the ability of the system to be incorporated into a previously developed wireless-sensing platform is investigated. The AD5933 impedance chip offers this possibility, and its response is investigated and compared with the response of the HP4192A. The results show that it is feasible to design a completely wireless-sensing device for the monitoring of the strength gain of concrete and its deterioration.

Early-Age Temperature and Strain in Basement Concrete Walls: Field Monitoring and Numerical Modeling

AbstractEarly-age cracking of base restrained concrete walls is a common problem. To provide the owner with an early warning of in-place quality problems resulting primarily as a result of thermal and shrinkage effects, it is helpful to apply monitoring and analytical techniques to examine the early-age behavior of basement concrete walls. In this paper, the temperature and strain monitoring on a basement concrete wall is conducted. Major factors that contribute to the early-age behavior of concrete walls constructed in cold weather are evaluated using analytical models along with measured field data and finite-element modeling. The results show that the monitoring process eliminated much of the guesswork frequently associated with basement concrete walls constructed in cold weather. Temperature varies laterally within the wall. The strain remains unchanged after six days with formwork removal. And the numerical predictions for strains and temperature are in agreement with the measured results, during bot...

Influence of Environmental Conditions on Temperature Field of Concrete

The temperature cracking of early-aged concrete is often met by constructors in recent years, because of large temperature difference of core and surface of concrete. Experimental study on temperature field of concrete under different environmental conditions is conducted. Different environmental conditions such as wind speed, formwork removal, sun radiation and conservation are involved. The influence of environmental conditions on the peak value of rising temperature, temperature difference of core and surface and descending speed of temperature of concrete is studied. Result obtained from this study can be used in controlling of early-age cracking of concrete.

Efficiency of granulated blast furnace slag replacement of cement according to the equivalent binder concept

Abstract The strength development of slag cement has a great consideration for the scheduling of formwork removal, prestressing operations, and other practical aspects of slag cement usage. The prediction of slag concrete strength, using the Feret’s model has been studied by introducing the concept of the equivalent binder. This has led to define an efficiency coefficient of slag which distinguishes the latter with the regard to the cement. Thus, this obtained coefficient characterizes well the slag and lets to predict the slag concrete strength from strength values of a normal concrete made without slag for a given age and replacement rate. At 90 days age, the test results show that for 15% replacement rate, the slag is activated completely and gives 67% of efficiency more than the cement. For higher replacement rate, the efficiency of the slag decreases and becomes similar to that of cement for 50% replacement rate.

Monitoring the setting of concrete containing blast-furnace slag by measuring the ultrasonic p-wave velocity

Ultrasonic transmission measurements allow the continuous monitoring of the setting of both mortar and concrete samples, which is important to determine for instance the formwork removal time. However, aspects such as the cause of the low initial velocity, the relation between the velocity and the setting times and the effect of cement type or cement replacing additives are still under discussion. Therefore, different concrete compositions with blast-furnace slag were tested by traditional as well as ultrasonic measurements. The ultrasonic method gives a more complete picture of the setting. The change of ultrasonic velocity in time is sensitive to the differences in setting behaviour of the tested mixtures. The initial setting seems to correspond with the inflection point of the velocity-vs.-time graphs and the final setting with the point at which the velocity increase levels off.

Evaluation of strength at early ages of self-compacting concrete with high volume fly ash

Self-compacting concrete (SCC) demands large amount of powder content and fines for its cohesiveness and ability to flow with out bleeding and segregation. In the present investigation, part of this powder is replaced with high volume fly ash based on a rational mix design method developed by the authors. Because of high fly ash content, it is essential to study the development of strength at early ages of curing which may prove to be a significant factor for the removal of formwork. Rate of gain in strength at different periods of curing such as 12 h, 18 h, 1 day, 3 days, 7 days, 21 days and 28 days are studied for various grades of different SCC mixes and suitable relations have been established for the gain in strength at the early ages in comparison to the conventional concrete of same grades. Relations have also been formulated for compressive strength and split tensile strength for different grades of SCC mixes.

Drying induced moisture losses from mortar to the environment. Part I: experimental research

Moisture conditions in the pore structure of hardening cementitious materials are known to have determinant influence on the onset and continuance of hydration reactions. Moisture loss from regions near exposed surfaces may jeopardize the quality of concrete cover in terms of both mechanical and durability issues. In addition, drying shrinkage, a phenomenon directly related to a moisture loss to the environment, is known to be responsible for several surface cracks that impair durability performance and pose aesthetical problems. Furthermore, evaporative cooling that occurs in concrete surfaces in the first minutes just after formwork removal may cause thermal cracking. For the above mentioned reasons, it is important to assess the mechanisms of moisture losses from cement-based materials to the environment, in order to rationally establish curing criteria. This paper describes an experimental campaign conducted with the purpose of better understanding the effect of various environmental conditions on the referred moisture interactions, accounting for influences such as the environmental temperature, the relative humidity (RH), the wind speed and direction, as well as the duration of curing. Numerical simulations with comparisons against test data, as well as some sensitivity analyses, are relegated to the companion paper that follows in this issue.

Development of the mechanical properties of an Ultra-High Performance Fiber Reinforced Concrete (UHPFRC)

Knowledge of the mechanical properties, i.e. strength, stiffness and deformation capacity, of cementitious materials at any arbitrary time is fundamental for operations such as removal of formwork, prestressing or cracking control. This paper presents a study of the evolution of indexes related to hydration and their correlation to the development of the mechanical properties for an Ultra-High Performance Fiber Reinforced Concrete (UHPFRC). The hydration kinetics was determined using semi-adiabatic heat of hydration tests, and the mechanical properties were investigated experimentally at several ages between 3 and 365 days and then described with models originally developed for conventional concretes. Models and experiments were in good agreement. Furthermore, it was observed that for the UHPFRC, the rate of development of mechanical properties was highest for the secant modulus, followed by the compressive and then the tensile strength.

Fine aggregate and curing temperature effect on concrete maturity

Removal of formwork can be made in a short time by early-strength gain of concrete with heat treatment. The effects of accelerated curing temperature and fine aggregate on early strength as well as the relationships between early strength–28-day strength and strength maturity have been examined. Cube concrete specimens produced with a 10-cm constant slump value, 0.59 w/c ratio, and with two different types of fine aggregate were subjected to three-phase cure processes. These cure processes include a 1-h preheating process after having replaced concrete in the mould, the cure application process, and finally the last waiting period for 2 h that is aimed at minimizing the effects of thermal stresses. Each of the specimen groups was cured at different temperatures for different periods (6 or 18 h). At the end of curing and on the 28th day, cube compressive strengths were determined. Therefore, it was seen that it is possible to estimate 28-day strength beforehand with reasonable accuracy.

Concrete Maturity Progress: Survey of Departments of Transportation

Results are provided from a concrete maturity survey that was distributed to representatives of 50 departments of transportation (DOTs) in the summer of 2000. The purpose was to display the state of national advancements and practices relating to concrete applications for predicting in situ portland cement concrete strength. Information such as DOT project and research details, method of maturity determination, state applications, and general attitudes toward the concept was requested. Representatives from 44 states replied to the 12-question survey. Results revealed that about 73 percent of the states that responded either have conducted or are currently involved in at least minor research with the concept. Furthermore, approximately 30 percent of the represented states have protocol or specifications governing the use of this developing technology. Representatives throughout the United States have reported that maturity is being used to predict critical strengths for such actions as pavement opening to the public; pavement opening to construction traffic; structural acceptance; and formwork removal for bridges, pavements, and other highway structures.

SITE EXPERT: a prototype knowledge‐based expert system

The present paper reports on the development of SITE EXPERT: a prototype knowledge‐based expert system. It is an advisory system. SITE EXPERT is intended to be used for productivity improvement in construction and provides advice on: (1) the productivity of three basic operations of construction, i.e. pouring and placing of concrete, erection and removal of formwork, and fixing reinforcement; and (2) human resources and site layout as productivity factors. The system uses information from construction experts, text books, data recorded at construction sites and the engineer's own knowledge, as well as knowledge obtained by running simulation models. In the present paper, the development, operation and evaluation of the prototype system is described. The results of this prototype system development demonstrate that artificial intelligence methodologies provide powerful facilities for capturing information about construction processes and advising the practitioners of construction on productivity improvement within a computer format close to human reasoning.