144 The Masterbuilder - April 2013 • www.masterbuilder.co.in

Zero Energy System for Precast Concrete: An Overview

As the name implies, the premise of zero energy system is to eliminate the need to input mechanical or thermal energy into the process of consolidating and curing

precast concrete elements. The Zero Energy System involves the use of self-compacting concrete to eliminate all external energy required for placing and consolidating concrete and no external energy for heat curing of concrete. The heat generated by hydration of cement is used for accelerating the strength development in concrete. Heat loss is minimized by insulating the forms and covering the concrete after placing with insulating mats. The concept of a Zero Energy System (ZES) was first introduced to the precast industry in Italy in September 2001. This is achieved by the use of innovative chemical admixtures and a proper mixture design. The ZES has a number of distinct advantages overthe conventional precast manufacturing process, which include economic, quality and safety considerations.

Underlying Concept of ZES

The Zero Energy System takes into consideration all the aspects of the manufacturing process of precast elements. Apart from eliminating vibrations, reducing energy consumption and labour, the durability of the concrete itself is also enhanced. Energy in the sense of the Zero Energy System comprises more than the direct costs of electricity and oil needed for the operation of a precast plant. It comprises all of the energy resources that must be mobilized in precast production; not only fossil fuels and electricity must be considered as forms of energy, but also aspectsof direct labour, material consumptions and productivity.

Major Constituents of ZES

There are two major constituents of ZES, (a) High Early Strength High Range Water Reducer, (b) Self-Consolidating Concrete.

Sonjoy Deb, B.Tech, ‘Civil’Associate Editor

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- High Early Strength High Range Water Reducer (HES-HRWR)A key component to the ZES is a HES HRWR. This admixture is based on anext-generation polycarboxylate ether (PCE), which provides excellent dispersion via asteric mechanism and improved hydration kinetics. This generation of PCE is believed to cover less of the surface of the cement grain, thereby actually providing accelerated hydration as compared to a non-admixture treated reference. This also contributes to the early age compressive strength development than other PCE’s. Even further improvements to the early strength development and workability retention properties of the HES HRWR have recently been made.

- Self-Consolidating Concrete (SCC) Mixture Design with a Viscosity Modifying Admixture (VMA) Another important component of the ZES is the use of a SCC mixture design. These mixtures are highly fluid and typically have slump flow values of greater than 550 mm.Therefore, these mixtures must be designed to exhibit adequate dynamic and staticstability. This includes suspension of the coarse aggregate and control of bleeding.The HES HRWR will typically impart improved stability over earlier generation PCE’s,but is not always adequate for some applications. Early generation concrete mixtures relied upon a higher fines content to achieve the required stability. This approach ofteninvoked concerns on the long-term creep and shrinkage characteristics of such concretes. It also made it impossible for precast manufacturers who were interested in high coarse aggregate contents for architectural finishes to use SCC. An improved approach isto use a more conventional mixture design and to incorporate a VMA. Use of the appropriate type and amount of VMA for the application will adjust the rheology of the concrete to provide the desired stability.

Commonly Available Commercial Products

Glenium ACE by BASF is high early strength high range water reducer and is most commonly used for ZES system. The two key elements of the Zero Energy System are Rheodynamic concrete and Glenium ACE, a hyperperforming superplasticizer for Rheodynamic technology. Rheodynamic concrete, an optimized self-compactingconcrete, provides a concrete mix with exceptional placing characteristics, accelerated cement hydration forearly strength development and high-quality concrete.

About Glenium Ace: An essential component of the Zero Energy System is Glenium ACE, a superplasticizerof the latest generation of polycarboxylateether (PCE) polymers, especially developed for precast applications.Glenium ACE molecules are rapidly adsorbed on the surface of the cement grains and act through electrostatic and steric repulsion to powerfully disperse the individual particles of cement. The molecular

structure of polycarboxylate etherpolymers is essential for the early developmentof strength. With conventional PCE superplasticizers,the molecules cover the entire surface of the cement grain and build a barrier against contact with water. Therefore, the hydration process takes place slowly.The unique, proprietary molecular structure ofGlenium ACE exposes increased surface of thecement grains to react with water. As a result ofthis effect, it is possible to obtain earlier development of the heat of hydration, faster development of the hydration products and, as a consequence, higher strengths at very early age. This advantage can even be utilized at low temperatures. Refer Figure 1for Glenium Ace molecular structures.

Figure 1: Glenium Ace molecule (Source: BASF Construction Chemicals)

Benefits of the Zero Energy System

Increased Productivity

Productivity in the precast industry dependsdirectly on the speed at which concrete cures, regardless of which manufacturing processis used. The unique principle of High Early Strength High Range Water Reducer acting on the cement molecules, significantly increases hydration kinetics without disturbing hydrate morphology. The natural exothermic heat produced in the first few hours is capable of speeding up the crystallization processes, developing faster material strength. The substantial improvement in performance incomparison with traditional superplasticizerstherefore optimizes the efficiency of the mix and reduces the production cycle, potentially doubling output Increased.

Early Strength Development

The strength development of ZES PCE and that of Traditional PCE is shown in Figure 2 below. It clearly shows in case of ZES PCE (in this case it is Glenium ACE 30 of BASF Chemicals)there is a significant strength gain between 6 to 12 hours than that of Traditional PCE which achieves the same in 20 hours. This again adding to the productivity increase.

Minimized Heat Curing

The energy required for heat curing is oneof the key parameters when calculating thecost of precast concrete elements. It is therefore an important economic factor. One

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of the objectives of the Zero Energy System is to optimize the amount of energy required in the production cycle to achieve specification requirements.The action of the unique Glenium ACE polymer combined with the control of manufacturing and placing parameters allows optimum use of natural energy for hydration, making it happen sooner! External energy supplies can therefore be reduced or eliminated, removing the need for heat curing.This feature of the Zero Energy System not only has economic benefits; it alsoimpacts on the durability of concrete by limiting any microcracks that may result from heat curing (thermal shock, temperature gradient, desiccation, etc.).

concrete requires trained and skilled labor. With the use of SCC as part of the ZES, the concrete easily flows into the formwork and is generally self-leveling. As a result, only minimal labor is required toget the concrete into its intended location. When compared to conventional concrete, areduction is labor costs is generally possible.

Improve concrete quality and intricacy of shapes

The use of the ZES can improve both the engineering properties and aesthetic properties of the finished element. It has been reported steam curing can compromise the later age compressive strength development. The engineering properties, such as bond to reinforcement, top-bar effect, creep, and diffusivity are all comparable or better than the properties of similar conventional mixtures. Drying shrinkage is generally better in SCC mixtures, especially when a VMA is used. The surface appearance of the finished elements is generally improved when compared to conventional concrete. There is little or no need to surface repairs, grinding or reworking as a result of the excellent self consolidation of these mixtures. This is true also for thin, complex elements as shown in Figure 4.

Figure 2: Compressive Strength Comparison (Source: BASF Construction Chemicals)

Figure 3: Curing Cycle (Source: BASF Construction Chemicals)

Elimination of Vibration

The energy required to place concrete is a further key factor in calculating the costs of precast elements. However, vibration is also a recognized nuisance factor: noise for workers and near by residents plus the physical stress for people involved in placing concrete. One of the objectives of the Zero Energy System is its ability to eliminate the energy required to place precast concrete.The flow and water reduction action of High Early Strength High Range Water Reducer, combined with aviscosity-modifying agent (VMA), enables the robust and direct formulation of self-compacting concretes, which can be placed without vibrations when combined with a compatible manufacturing process.The Zero Energy System therefore reduces the related costs whilst improving working conditions in the precast concrete plant. Refer Figure 4 for flowing concrete mix.

Reduce labor requirements

The placement, consolidation and finishing of conventional

Figure 4: No Vibration Required (Source: BASF Construction Chemicals)

Figure 5: A thin, intricate precast element that was formed using the ZES. Note the fine details andgood surface appearance.

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Application Areas

The flexibility of the Zero Energy System is suitable for a diverserange of markets: components for buildings, civil engineering,drains, artstone, agriculture, roads and highways, railways, telecommunications, etc.The Zero Energy System can be used in the casting of girders, prestressed floor support slabs, floor slabs, panels, segments, pipes, blocks, vaults, cross-beams, façade units, paving and copings.The Zero Energy System is an innovative and upcoming concept for the precast industry.

Conclusion

The use of the Zero Energy System (ZES) has a number of economic benefits for the precast manufacturer. Depending on the specific interests of the producer, these can include possible reductions in energy and material costs, reductions in equipment repairs and maintenance, reductions in worker health claims or increased productivity. Additionally, the use of the ZES results in a finished precast element with improved aesthetics and engineering properties from that made with conventional concrete. Also the flexibility of the Zero Energy System ensures the optimum utilization of energy required to the precasting of concrete elements. For some manufacturing processes, depending on the cycletime, the ambient temperatures and the composition of themix, the full benefits of the Zero Energy System can be compounded by combining the effectiveness of ZES PCE with the technology of self-compacting concrete.Concrete can be placed without vibration, achieving the required performance without the need of heat curing.

Reference

1. Corradi, M., Khurana, R., Magarotto, R., and Torresan, I., “Zero Energy System: AnInnovative Approach for Rationalized Precast Concrete Production,” Proceedings of the17th Intl. Congress of Precast Industry, Istanbul, Turkey, May 1-4, 2002, 8 pp.

2. Parker, D., “A Mixed Blessing: A Dramatic Improvement in Precast ConcreteProduction Efficiency …,” New Civil Engineer (UK), November 15, 2001, pp. 6-8.3. Daczko, J. A., Kurtz, M. A., Bury, M. A., and Attiogbe, E. K., “Zero Energy System forPrecast Concrete Production,” Concrete International, Vol. 25, No. 4, April 2003, pp.103-107.

4. Khurana, R., Magaratto, R., and Torresan, I., “New Generation of PolycarboxylateSuperplasticizier to Eliminate Steam Curing of Concrete,” Proceedings – InternationalCongress on ‘Challenges in Concrete Construction’, Dundee, Scotland, September 5-11,2002, pp. 213-224.

5. Bury, M. A. and Christensen, B. J., “The Role of Innovative Chemical Admixtures inProducing Self-Consolidating Concrete,” Proceedings of the First North AmericanConference on the Design and Use of Self-Consolidating Concrete, Center forAdvanced Cement-Based Materials, Northwestern University, Evanston, IL, November12-13, 2002, pp. 141.

6. Christensen, B. J. and Bury, J. R., “Evaluation of a New Generation Synthetic HRWRin Precast SCC Mixtures,” to be published at Second International Conference on theDesign and Use of SCC, Chicago, Ill, October 2005.

7. Daczko, J. A. and Kurtz, M. A., “Development of High-Volume Coarse AggregateSelf-Compacting Concrete,” Proceedings of the 2nd International Conference onSelf-Compacting Concrete, Tokyo, Japan, October 23-25, 2001.

8. Zero Energy System for Precast Concrete, Dr. Bruce J. ChristensenDegussa Construction Chemicals Asia Pacific.

9. BASF, The Chemical Company.

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