Part I. The material and its properties

Self-compacting concrete: Part I. The material and its properties

Gaimster, Rob

This Current Practice Sheet is in two parts, the first of which is published here. This deals with the nature of self-compacting concrete (SCC), its materials and properties. The second part will cover production, placing and optimisation of the construction process, and will be published in a future edition of CONCRETE. Both sheets have been prepared by members of the Concrete Society Working Party on Self-compacting Concrete.

Introduction

Interest in SCC has spread across the world, prompted by concerns about instances of poor compaction and concrete durability. SCC extends normal concrete technology and has benefits that arise from easier placing. However, specific training is necessary for designing, producing, transporting and handling the material. Its innovative aspects lie in its fresh properties and the potential benefits to construction practice.

What is self-compacting concrete?

SCC is concrete that, in the plastic state, flows under its own weight and maintains its homogeneity while completely filling formwork of any shape, even around congested reinforcement. Compaction is achieved without mechanical vibration. It should not be confused with ‘flowing’ concrete that has different rheological properties, requires vibration to achieve compaction and may segregate. SCC is produced from normal concreting materials, and complies with the strength grades in BS 5328: Part 1: 1997(1) Concrete: guide to specifying concrete and BS EN 206: Part 1: 2000(2) Concrete: specification, performance, production and conformity. The mix may incorporate steel and/or polypropylene fibres.

Materials

Aggregates

SCC can be made from most normal concreting aggregates, although grading envelopes may be tighter than those in BS 882: 1992(3). SCC has been produced successfully with coarse aggregate up to 40mm. The maximum size depends on reinforcement layout and formwork dimensions in the same way as traditional vibrated concrete. Sand can be finer than normal, as the material

Cement and filter (fines)

Cement and, in many cases, fillers, are required for cohesion and stability in larger proportions than in traditional concrete. These fines can be inert (fine sand or fillers derived from crushed rock), or active materials (such as ggbs or pfa, or Portland cement). Portland cement is often partially replaced with ggbs or pfa for technical and economic reasons. Fillers must be assessed for their effect on water demand.

Admixtures

Admixtures are essential in determining flow characteristics and workability retention. Ideally, they should also modify the viscosity to increase cohesion. Newly developed types of superplasticiser, known as polycarboxylated ethers (PCEs), are particularly relevant to SCC. They reconcile the apparently conflicting requirements of flow and cohesion, avoiding potential problems with unwanted retardation and excessive air entrainment (generally caused by overdosing), particularly at higher workabilities if the mix design is correct. Additional viscosity modification may not be required. Other elements may indicate a requirement for additional segregation control admixtures, such as ultra-fine silicas, polysaccharide gums, modified polyethers or even simple air-entrainers.

Mix design considerations

The objective of traditional concrete mix designs is to provide the most cost-effective selection and proportioning of the materials, while achieving the concrete performance specification. They are based on the inclusion of sufficient cement to achieve strength and durability, and sand content that provides adequate cohesion at the required workability, permitting material variability and minor hatching errors.

This approach is unsuitable for SCC, where the main objective is a mix that will flow under its own weight without blocking or segregation, while satisfying the performance specification when hardening.

To design concrete with these fresh properties, several principles, some of which are unconventional in relation to current UK mix design terms, should be observed. General criteria are specified in Table 1 and typical proportions are given in Figure 1.

Fresh properties of SCC

Flowability

SCC flows under its own weight, filling formwork and flowing around recesses or embedded objects without leaving voids. As it is very fluid, it can flow considerable distances horizontally and upwards to fill vertical elements from the bottom. SCC can be pumped into column and wall formwork from the bottom through special valves, eliminating the use of craned skips, platforms, etc. Passing around reinforcement

The passing ability enables SCC, containing the appropriate aggregate grade, to flow around congested reinforcement without blocking or affecting the homogeneity of the concrete.

Segregation resistance

Segregation is observed as surface bleed water and formation of surface mortar laitance. These are as undesirable in SCC as in traditional concrete. In SCC, segregation resistance is the most difficult fresh property to achieve. The most critical manifestation of segregation is the separation of the mortar from the coarse aggregate fraction even though full compaction is achieved. This can cause settlement of coarse aggregate in deep sections, together with blocking, which can prevent the free flow of concrete around the reinforcement. Owing to SCC’s cohesive nature, and in spite of its high fluidity, there is no internal settlement of coarse aggregate particles.

To take full advantage of SCC, the mix should be designed to retain these fresh properties for 90 minutes after hatching. The mix should maintain a high degree of workability while maintaining a low water/cement ratio. This is achievable by the use of a new generation of high-range water-reducing admixtures, combined with stabilising agents to maintain homogeneity.

The innovative aspects of SCC are these fresh properties, and the construction practice benefits derived from them(4,5). The requirements for easy flow on the one hand and increased cohesion and resistance to segregation on the other are usually contradictory, and reconciling them places particular demands on the mix design.

Hardened properties of SCC

The properties described in this section have been obtained from the Brite EuRam project BRPR-CT96-0366 Rational production and improved working environment through using self-compacting concrete(4).

Compressive strength

At similar water/cement ratios, the characteristic strength of SCC is at least equal to that of traditional concrete, and has a similar strength development for the same Grade (see Figure 2).

SCC with a characteristic compressive strength up to 60N/mm^sup 2^ can be easily produced. For a lower specified strength, the high fines content and low water/(cement + fines) ratio required for the essential theological properties of SCC may make it difficult to keep the strength down. The benefits of higher characteristic strength should be incorporated in the structural design.

In-situ compressive strengths, measured on core samples taken from structures, are similar to those of traditional well– compacted concrete. The low values of excess voidage observed in core samples indicate that a satisfactory degree of compaction is achieved, comparable to well-compacted traditional concrete. The in-situ strength distribution in large vertical elements is also similar, i.e. higher at the bottom, lower at the top when cast vertically. This has been confirmed by other in-situ methods, such as pull-off tests.

Tensile strength

When assessed using the cylinder splitting test, as specified in BS 1881: Part 117: 1983 Testing concrete: method for determination of tensile splitting strength(6), the tensile strength is comparable to the same grade of traditional concrete, as is the ratio of tensile to compressive strength (see Table 2)

Shrinkage

Drying shrinkage has been shown to be similar or lower than that of traditional concrete of the same grade(7. This is contrary to that expected from the lower grade aggregate content, but is partially explained by the similar water content of SCC and traditional concrete. The high fines content and viscosity of SCC inhibit bleeding and, therefore, evaporation, so the total plastic settlement is reduced. However, as water lost by evaporation is not replaced by bleed water, plastic shrinkage and the associated surface cracking may be increased. Attention to curing is important, especially on large flat exposed areas.

Modulus of elasticity

The relationship between static modulus of elasticity (E) and compressive strength (f^sub c^) is similar for SCC and reference mixes. A relationship in the form of El(f^sub c^)10.5 has been widely reported, and all values of this ratio when testing SCC are close to that recommended by the American Concrete Institute for structural calculations for normal weight concrete. The E-values of SCC also exceed those predicted from compressive strength by the formula given in BS 8110: Part 2: 1985(8).

Freeze-thaw resistance

Resistance to freeze-thaw in SCC that is not air-entrained does not significantly differ from that of traditional vibrated concrete. The resistance of air-entrained SCC is as good as, if not potentially better than, air-entrained ordinary concrete(9).

Bond strength

The bond between concrete and reinforcement for both mediumand high-strength grades of SCC is as strong as or stronger than that of the equivalent reference mixes(4).

Structural performance

The structural performance of SCC does not differ much from that of traditional concrete. Assessment by loading to failure of 3000 x 300 x300nmm reinforced columns, and 4000 x 300 x200mm beams has shown that normal fracture patterns occur in all cases, with the actual failure load exceeding the calculated ultimate load.

Durability

Two indices of durability have been investigated: carbonation depth and surface absorption. No difference was found in carbonation depth between SCC and traditional concrete of the same grade during the same specified period, and SCC exhibited lower surface absorption, indicating lower permeability and improved durability.

Summary

SCC offers a number of benefits. In its fresh state, it possesses:

The ability to flow under its own weight

The ability to pass around reinforcement

Segregation resistance.

In the hardened state, when compared with traditional vibrated concrete, it may have:

The same structural behaviour

Equal or higher compressive and tensile strength

The same or greater resistance to freeze-thaw attack

An equal or lower drying shrinkage

The same or stronger bond to reinforcement

A greater modulus of elasticity for the same aggregate

Lower surface absorption, slowing the rate of carbonation and improving durability.

However, it should be appreciated that it is still an emerging technology and further development is needed, e.g. guidance on specification and use, standardised tests for fresh property assessment, surface finish consistency, formwork pressure effects, and information on shrinkage, creep and elastic modulus. In projects where these parameters are critical, preliminary investigations should be carried out.

References

1. BRITISH STANDARDS INSTION. BS 5328: Part 1: 1997 Concrete: guide to specifying concrete. BSI, London. 32pp.

2. BRITISH STANDARDS INSTITUTION. BS EN 206: Part 1: 2000 Concrete: specification, performance, production and conformity. BSI, London. 74pp. 3. BRITISH STANDARDS INSTITUTION. BS 882:1992 Specification for

aggregates from natural sources for concrete. BSI, London. 12pp.

4. BRITE EURAM PROJECT BRPR-CT96-0366 Rational production and improved working environment through using self-compacting concrete (unpublished) Task 4. Website: www.scc.ce.tuth..sejpubtic.

5. BARTOS, P. and GRAUERS, M. Self-compacting concrete. CONCRETE Vol. 33, No.4, April 1999. pp.9-14.

6. BRITISH STANDARDS INSTITUTION. BS 1881: Part 117:1983 Testing concrete: method for determination of tensile splitting strength. BSI, London. 8pp. 7. PERSSON, B. Creep, shrinkage and elastic modulus of self-compacting concrete.

Okamura, H. and Ouchi, M. (eds.) Self-compacting concrete. Proceedings of the first international RILEM symposium. Stockholm 1999, RILEM, Cachan, pp.239-250.

8. BRITISH STANDARDS INSTITUTION. BS 8110: Part 2:1985 Structural use of concrete: code of practice for special circumstances. BSI, London, 68pp.

9. GRAM, H-E. Properties of SCC, especially early age long term shrinkage and frost resistance. Okamura, H. and Ouchi, M. (eds.) Self-compacting concrete. Proceedings of the first International RILEM symposium. Stockholm 1999, RILEM, Cachan, pp.211-225.

Rob Gaimster, RMC Readymix Ltd, and John Gibbs, Advanced Concrete and

Masonry Centre, University of Paisley

For further information, contact the BCA Centre for Concrete Information, Tel: +44 (0)1344 725703: www.bca:org.uk.

The ACM Centre, Tel: +44 (0)141 848 3279: www-civileng.paisley.ac.uk/acm/

Copyright The Concrete Society Jul/Aug 2001

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