Self-Compacting Concrete

Ahmed Loukili

• Publisher: Wiley-ISTE
• Year: 2013
• Print ISBN: 9781848212909
• eBook ISBN: 9781118601969
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Self-Compacting Concrete (SCC) is a relatively new building material. Nowadays, its use is progressively changing the method of concrete placement on building sites.
However, the successful use of SCC requires a good understanding of the behavior of this material, which is vastly different from traditional concrete. For this purpose, a lot of research has been conducted on this area all over the world since 10 years.
Intended for both practitioners and scientists, this book provides research results from the rheological behavior of fresh concrete to durability.

Chapter 1

Design, Rheology and Casting of Self-Compacting Concretes ¹

1.1. Towards a fluid concrete

Recent decades have witnessed a remarkable evolution in concrete performance, as much in the field of their rheological behavior in the fresh state as in their mechanical behavior in their hardened state. These technical advances are the results of coupling between the formulation principles, coming from a long period of learning by experience and the mastering of physico-chemical principles which govern the behavior of cement-based materials.

Initially made using a simple recipe of water, cement and aggregates (sand and gravel), concrete has since seen its formulation enriched by the inclusion of high quality components such as mineral additives (limestone fillers, silica fumes, etc.), chemical additives such as super-plasticizers and reinforcing materials such as fibers.

The complex formulations thus obtained must satisfy the production specifications in which the obligations often go beyond the conventional output requirements in terms of fluidity during casting, and strength of the hardened concrete. Concrete design can thus be adjusted to suit the working conditions (pumping, vibration, transport time and casting time), hardening (time at which the concrete is removed from the mold, required short-term strength) and service (developing strength, durability, etc.).

Self-compacting concrete (SCC) is an illustration of research in mastering such complex mixtures. The origin of SCCs is associated with the development, at the beginning of the 1980s in Japan, of a design method for fluid concretes. The high seismicity of this geographical region necessitates that structures are highly reinforced with steel. In these much more difficult pouring conditions, compacting the resultant concrete using conventional methods (internal or external vibration) is at risk of being insufficient, thereby compromising the buildings’ quality assurance.

The SCC concept was therefore born from the desire to make the concrete compacting completely independent of the production context, whether in the technical plan or in manpower, knowing that the number of qualified workers is noticeably declining in Japan, which is also the case in numerous other countries.

SCC is therefore a mixture which is both fluid and homogenous, which fills formworks perfectly by flowing under the effect of gravity alone, and which completely wraps around all the reinforcing bars without causing blockages or grain separation in their vicinity (Figures 1.1 and 1.2). These properties appeal to the structure designer for the work, who can envision more complicated shapes using these materials; and to the production team who is interested in simplifying the work involved in creating the structures and reducing construction delays. This is why the use of SCCs tends to increase on a large scale.

Figure 1.1. Filling capability of SCC (top) and conventional concrete (bottom) [CAS 05]. SCC, moving under its own weight, flows around obstacles and ensures good coverage. Without an external energy supply, such as vibration, conventional concrete cannot achieve the same result

On the construction site, the use of SCC results in the interruption of vibration techniques leading to significant reductions in noise and occupational illnesses such as white hand syndrome — Raynaud’s syndrome — improvements in safety during casting and reduction in the workforce required for casting. It also improves the conditions for filling formworks by increasing the concreting speed, allowing concreting in highly reinforced zones, faster formwork rotation. Finally, the use of SCCs results in a better facing quality (Figure 1.2).

Figure 1.2. Improvements in facing quality with SCC [CAS 05]. Casting is made easier with SCC which reduces the formation of air bubbles and pebble clusters at the faces of the formwork, including frames with complex geometrical shapes. In the case of conventional concrete, such defects can be eliminated using vibration which is always difficult to implement

This collection of properties affects construction as much as project managers do, since they are also involved in improving productivity during the construction phase, in the quality of constructions and in their durability.

The transition to SCCs cannot be envisaged without reconsidering and adapting the fabrication and casting methods. Numerous works have therefore been committed to the problems of mixing, transport, pumpability, formworks (waterproofing and controlling pressure on the inner surfaces), facing quality, etc., according to the workable nature of SCCs. Controlling the production of SCCs, relying on simple and rudimentary tests in construction sites also constitutes a theme for collective consideration which has led to the publication of a reference document in France.

Recommandations pour l’emploi des bétons auto-plaçants [AFG 08], classifies SCCs, specifies the required properties as well as their corresponding tests, and describes the conditions of use and the characteristics of these concretes. However, operators confronted with formulating, producing or casting SCC have systematically noticed that this type of mixture is characterized by a fluidity which is strongly affected by all deviations in composition. To understand the reasons behind this, the design basics are initially discussed.

The various arguments dedicated to developing an understanding of a mixture’s flow properties with respect to quantifiable and representative rheological parameters are then introduced. Next, the most suitable test methods for estimating these rheological parameters are presented. The influence of design parameters on the values of the rheological parameters is also studied. The different arguments make it possible to make judgments of a formulation’s robustness in light of measurement differences which are often the origin of a lack of reproducability. Armed with this knowledge of design principles and rheological parameters to consider, industrial practices regarding mixing, transport and pumping are developed. Finally the pressure exerted by SCCs, once in place, on formwork is described in the results from recent works.

1.1.1. Area of application

At the time of writing, SCCs represent 2-3% of the volume of ready mix concrete produced in France, with a tendency towards horizontal applications (80%) by comparison with vertical applications (20%). A 40–50% proportion of SCC is intended to precast products with delayed demolding [MAG 07].

Whether we are concerned with horizontal structures (floors, raft foundations, paving, screed, etc.) or with vertical structures (columns, walls, bridge piers, etc.), the function of SCCs in their fresh and hardened states are singularly different.

For horizontal structures, the term self-leveling concrete (SLC) is sometimes used. In their fresh state, finishing quality and leveling is crucial. This is obtained by putting concrete in place without recourse to mechanical work, especially for the surface. Neither segregation nor excessive bleeding is tolerated. For this type of application, the current method combines thickening agents (starches, Welan gum, etc.) with super-plasticizers to achieve the ideal fluidity/stability coupling. In the hardened state, the strength is of the order of 40 MPa.

For vertical structures, the generic term self-...

Table of contents

• Cover
• Title Page
• Copyright
• Introduction
• Chapter 1: Design, Rheology and Casting of Self-Compacting Concretes
• Chapter 2: Early Age Behavior
• Chapter 3: Mechanical Properties and Delayed Deformations
• Chapter 4: Durability of Self-Compacting Concrete
• Chapter 5: High Temperature Behavior of Self-Compacting Concretes
• Glossary
• List of Authors
• Index