Use Of Self Consolidating Concrete In Modern Construction

Use Of Self Consolidating Concrete In Modern Construction

2.1. Introduction

Self Consolidating Concrete (SCC) is relatively a new class of concrete technology, having first been developed in Japan in the late 1980s and only arrived in the US in the late 1990s. Basically, SCC is a “high performance concrete … [that] is proportioned such that the concrete freely passes around and through reinforcement, completely fills the formwork and consolidates under its own weight without segregation.” The main goal of this literature review chapter is to collect, compile, and analyze information pertaining to a number of critical specifications for precast/prestressed concrete bridge products. These specifications are:

Mix proportioning;

Design approval practices;

Quality control testing, and;

Acceptable construction practices.

In extension, the chapter will investigate the specifications of other organizations and state agencies, as well as the findings of other research studies carried out in various Universities in the US. The chapter will revolve around the three basic features of SCC as postulated by Mata (2004) in his study, that is, “high deformability, high passing ability and high resistance to segregation. Deformability entails how the SCC “undergo a change in shape under its own weight” while on the other hand, passing ability entails the degree of viscosity of the SCC solid particles particularly toward an opening. On its part, high resistance to segregation entails the ability of the SCC particles to distribute themselves throughout the mixes evenly and to hold together. These three qualities are very critical as they position SCC at a relatively high standing compared to ordinary concrete.

In achieving these ends, this chapter will particularly rely on the specifications guidelines for precast/prestressed concrete bridge products developed by the Mid-Atlantic States Prestressed Committee for Economic Fabrication QA/QC Sub-Committee in January 2003 as well as a barrage of studies carried out in various universities in the US.

2.2. Mix Proportioning

As the name suggests, SCC is a fluid, dense and homogeneous concrete that does not require vibration to find its way into formwork during placement. It flows freely under its own weight to fill all formwork spaces irrespective of whether highly congested reinforcement materials are involved. Consequently, to ensure that these unique features are maintained there is need to adhere to the correct mixing specifications. No doubt these qualities are unique and way above the conventional standards for ordinary concrete. In fact, Lange asserts that due to these unique mixing, curing, placement, and post-placement qualities, SCC is more vulnerable than the ordinary concrete. As a matter of fact, developing a specified amount of SCC may require more than many civil engineers can comfortably handle – it requires,

a blend of art and engineering … [capable of balancing] between a large number of parameters such as combination of cementitious material, proportions of fine and coarse aggregate, and adjustment of w/cm or admixture dosage to achieve a required minimum segregation resistance and good flowability.

Ideally, the specific properties of the ingredients may determine the overall viscosity, fluidity, and robustness of SCC. To this end, various civil engineering departments and governments alike have taken the initiative to outline clear-cut guidelines that govern various SCC procedures including mixing.

The good thing about SCC is that it is not discriminative – it can successfully be developed using a wide range of materials provided the correct proportioning ratios are adhered to. For instance, the coarse aggregate in a SCC mixes should not be too high lest it interferes with the overall flowability of the SCC. Precisely, high levels of coarse aggregate (more than 50 % of the total aggregate) heighten the rate of collision between aggregate particles, particularly when passing through dense reinforcement material. On the same note, too much water may cause bleeding and segregation.

To enhance SCC flowability and/or deformability while still maintaining high viscosity, segregation, and hardening qualities, the “water-to-cementitious materials ratio” (w/cm) should be increased using high range water reducers (HRWR). The basic idea here is to reduce the amount of friction between the hard particles of the SCC. Even so, it is very hard to balance deformability and viscosity, increasing one decreases the other. On the other hand, to enhance high levels of segregation, viscosity-modifying admixture (VMA) is the ideal ingredient. Together with HRWR, VMA helps to minimize the amount of free water by maximizing the overall quantity of paste. In this regard, HRWR and VMA may be referred as the two most crucial ingredients to a SCC mixes. Overall the SCC mixes should be adjusted to the ACI 211 standards and the maximum w/cm ratio should be 0.40 by weight unless otherwise indicated.

2.3. Design Approval Practices

The design practices adopted in developing SCC impacts its overall post-placement performance. In this regard, a number of design practices should be adhered to when developing SCC. Ideally, to enhance adherence to such practices constructors are required to seek due approval before engaging in any SCC work. It has been a common practice among jurisdiction to dedicate the approval responsibilities even so; due to the uncertainty in the application of SCC some states are yet to approve the use of SCC. In fact, as Mata (2004) opines, complications in the comparison of the elastic modulus “between conventional concrete and SCC” is responsible for the uncertainty in the approval of SCC design practices in the US.

Even so, constructors are required to seek for approval from the relevant authorities in carrying out design practices in all the SCC processes. For instance, in the European Union the process of sampling should be done in accordance with the EN 12350-1 regulations while in the US the AASHTO T141 applies. On the other, the process of transporting mixed concrete to a designated site should be done in accordance with EN 206-1, where both the producer and the specifier should mutually consent to the EN 206-1 testing procedure. For example, they may agree to use “visual inspection of every batch of the concrete and any specific tests and compliance parameters.” In the US, both the producer and the specifier are required to provide “sufficient mixing and placing capacity” to enhance uninterrupted supply of the SCC. This is an ideal procedure that helps to eliminate cold joints. In the event that a delay occurs, constructors are required to wash away such uncompleted joints to avoid loss of plasticity.

2.4. Quality Control Testing

In order to enhance high SCC standards a lot of testing is required. Testing has been noted as an important exercise as it helps to reduce the amount of time and efforts invested into developing quality SCC mixes. In essence, the bulk of this quality control and testing dwells on the key SCC features discussed above – deformability, passing ability, and resistance to segregation. In extension, all the materials, processes, personnel, as well as the equipment used during the development of SCC mixes and structures are equally subjected to severe testing under the AASHTO guidelines or as otherwise stated. Even so, the most common testing procedures involve: (a) Slump flow and T-50; (b) J-Ring and L-box; (c) Caisson test and; (d) Visual stability index (VSI), Surface settlement and rate of settlement, and Column segregation. Analytically, tests (a) and (c) are meant to evaluate the filling ability and filling capacity of the SCC respectively, while test (b) evaluating the overall passing ability of the SCC. On its part test (d) evaluates the segregation resistance of the SCC. [See appendix 1 for a tabulated report of the SCC tests]

So as to enhance accuracy the process of proportioning and mixing should be done by qualified persons capable of discerning the correct measures as well as the correct concrete materials in a consistent manner. These measures and materials should be in harmony with the ACI 211 requirements and declared fit by a “carefully controlled laboratory.” To enhance accountability and efficiency, all tests carried out thereof, regarding the personnel involved, materials, equipments’, and all the SCC processes such as delivery, placement, consolidation, and curing should be succinctly be compiled and stored for reference. To enhance uniformity across all SCC production the following equation for measuring modulus elasticity as proposed by Badman et al (2003) can be applied:

2.5. Acceptance of Construction Practices

In the US, Constructors are required to submit all equipments, persons, materials, as well as designs to AASHTO testing procedures as outlined in their “existing owner specifications”, unless mutually agreed upon, in writing, by the contractor, fabricator and owner to follow the “Specifications Guidelines” prepared by the Mid-Atlantic States Prestressed Committee for Economic Fabrication QA/QC Sub-Committee in January 2003, before commencing the actual construction process. Though it is a fact that SCC is significantly superior to ordinary concrete, it is agreed that the overall quality of a SCC mix, that is, “strength, durability, and performance” should be equal or greater than shall be equal to or better than that of its corresponding ordinary concrete mix. In acknowledgement that SCC testing is still under empirical investigation, Badman et al (2003) proposes that SCC mixes should be more or less similar to high quality ordinary concrete and therefore constructors can benchmark their processes against those of high quality conventional concrete. Mata (2004) affirms this postulation in his conclusions that “SCC girders are comparable to the conventional concrete girder and can be used as structural members.” In situations where variations occur, constructors should notify the respective civil engineering department for more guidelines.

Basically, to ensure SCC meets high levels of strength, durability, and performance, constructors should adhere to the 28-day compressive length of time. In this regard, the 28-day period enhances high fluidity that is characterized by reduced segregation rate, lower w/c ratio, and high passing ability. Other worthwhile engineering parameters that should as well be monitored and in extension enhanced as part of acceptance construction practices are, creep, modulus elasticity, and shrinkage.

2.6. Conclusions

The process of developing SCC for precast/prestressed concrete bridge products is indeed a rigorous one. In this regard, efforts should be made to ensure that persons engaged are competent and well aware with the prerequisite SCC specifications guidelines in the jurisdiction they operate in. Basically, as outline din the chapter the AASHTO specifications guidelines form the basis of all other jurisdictional requirements in the US. This said, the SCC technology is still in its initial stages, at least in the US and therefore the specifications guidelines are still being modeled in many states. Even so, it is generally agreed that SCC offers unique and unrivalled concrete qualities of: high deformability, high passing ability and high resistance to segregation require rigorous testing and approval procedures to fulfill. In this case, proper mix proportioning, design approval practices, quality control testing, as well as acceptable construction practices are some of the prerequisites capable of enhancing the achievability of these three SCC core qualities.

BIBLIOGRAPHY

Badman, Curtis et al. “Interim Guidelines for the Use of Self-Consolidating Concrete in Precast/Prestressed Concrete Institute Member Plants”. TR-6-03, Precast/Prestressed Concrete Institute: Interim SCC Guidelines FAST Team, 2003.

Burgueño, Rigoberto & David A. Bendert. “Experimental Evaluation and Field Monitoring Of Prestressed Box Beams for SCC Demonstration Bridge.” Report No. CEE-RR – 2007/01, July 2007.

Erkmen, Bulent, Carol K. Shield, & Catherine E. French. “Self-Compacting Concrete (SCC) for Prestressed Bridge Girders”, Report No. MN/RC 2008-51, Department of Civil Engineering University of Minnesota, October 2008.

Khayat, Kamal Henri, & Denis Mitchell, “Self-Consolidating Concrete for Precast, Prestressed Concrete Bridge Elements”, Final Report for NCHRP Project 18-12, May 2008.

Lange, D.A. (Ed.). “Self Consolidating Concrete”, A White Paper by Researchers at The Center of Advanced Cement Based Materials (ACBM), February 2007.

Mata, Luis A. “Implementation of Self-Consolidating Concrete (SCC) for Prestressed Concrete Girders.” A Thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Master of Science in Civil Engineering, Nov. 2004.

Naito, Clay, Geoffrey Brunn, Greg Parent, & Tyler Tate. “Comparative Performance of High Early Strength and Self Consolidating Concrete for Use in Precast Bridge Beam Construction”. PITA Project PIT-457-04, ATLSS REPORT NO. 05-03, May 2005.

Specification Guidelines for Precast/Prestressed Concrete Bridge Products: Mid-Atlantic States Prestressed Committee for Economic Fabrication QA/QC Sub-Committee: Document 201, January, 2003.

The European Guidelines for Self-Compacting Concrete: Specification, Production and Use, May 2005. Retrieved September 6, 2010, from: www.efnarc.org/

APPENDICES

Figure 1: Recommended Acceptance Values for SCC tests.

Source: (Khayat et al, 2008, p.109)