Hardened properties of selfcompacting

Transcription

1 Universidad de la Costa From the SelectedWorks of Marian Sabau November, 2012 Hardened properties of selfcompacting concrete Marian Sabau, Universidad de la Costa Traian Onet, Technical University of Cluj-Napoca Ana Ioana Petean Available at:

2 Hardened properties of self-compacting concrete Marian Sabău *1, Traian Oneț 2, Ana Ioana Petean 3 1,2,3 Technical University of Cluj-Napoca, Faculty of Civil Engineering. 25 Baritiu Str., , Cluj- Napoca, Romania Abstract This paper presents the properties of hardened self-compacting concrete (SCC) and compare them to those reported for normally vibrated concrete (NVC). The mechanical properties evaluated are: compressive strength, modulus of elasticity, tensile strength, creep, shrinkage and bond with reinforcement. The very wide range of materials and mixes already used for SCC produced a significant scatter of data, but clear conclusions have been obtained. The compressive strength of SCC was likely to be the same or greater than that of NVC by using the same content of cement and water. Modulus of elasticity of SCC was lower than that of NVC of the same compression strength. Hardened SCC tended to have a tensile strength the same or better than that of NVC. The magnitude of creep was shown to be the same as for vibrated concrete. Shrinkage of SCC was of magnitude entirely comparable with that of NVC. The bond of SCC to embedded reinforcing and prestressing steel tended to produce a slightly better performance than NVC. The conclusions provide full confidence in hardened SCC reaching the required engineering properties. Keywords: self-compacting concrete, compressive strength, modulus of elasticity, tensile strength, creep, shrinkage, bond 1. Introduction Self-compacting concrete can be defined as fresh concrete which has an ability to flow under its own weight, fill the required space or formwork completely and produce a dense and adequately homogeneous material without a need for compaction. The use of SCC can facilitate the placement of concrete in congested members and in restricted areas. Considering the highly flowable and selflevelling nature of the SCC, care is required to ensure adequate stability. Over the past two decades, the popularity of self-compacting concrete has increased around the world. The intensive publications growth on properties of hardened SCC has been observed in the last years, which demonstrates superior properties of this type of concrete not only in fresh, but also in hardened state. The results show that limestone powder, a common addition to SCC mixes, made a substantial contribution to the rate of gain of strength, particularly at early ages. An extensive review to date of the hardened mechanical properties of SCC is presented by Domone [1]. More than 70 studies of hardened mechanical properties of SCC are presented and summarized by Domone, and the conclusion is the significant scatter which is understandable in view of the range of materials, mix designs and test procedures used. Most guidelines for the use of SCC [2, 3] emphasize on ranges and recommendations regarding the fresh properties of the material but much less is discussed regarding its hardened properties. In * Corresponding author: Tel./ Fax.: address: sabaumarian85@yahoo.com

3 all cases, the behaviour of SCC in the hardened state is considered at least as good as the conventional counterpart of equivalent strength [1]. 2. Compressive strength The compressive strength of concrete is defined by the classes of characteristic resistance f ck determined on the cube or cylinder according to European standard EN [4]. An increase in characteristic compressive strength of up to approximately 10% over that indicated by traditional, established, relationships with w/c ratio for vibrated concrete at 28 days was not uncommon [6, 7, 8]. The rate of gain of compressive strength of SCC with age is largely similar to that of a NVC containing the same proportion of cement and having the same water/cement ratio [6, 7], except when limestone-type additions are used. An extensive survey of test data reported over more than a decade by Domone [1] has shown that a significant increase in compressive strength was observed across a wide range of limestone powder contents for all ages up to 28 days. It has been widely assumed that a change from rounded to coarse aggregate, while all other parameters of a concrete mix were kept constant, the mix with the coarse aggregate would achieve higher compressive strength. The survey by Domone [1] showed that the difference due to the shape of the coarse aggregate has narrowed for SCC, when compared with NVC. (Fig.1, Fig. 2). Figure 1. Cube compressive strength vs. equivalent water/cement ratio [1] Figure 2. Cylinder compressive strength vs. equivalent water/cement ratio [1] Further analysis of data on the compressive strength of hardened SCC [1] revealed that a significant difference existed between the cube-to-cylinder strength ratio assumed in current structural codes

4 for NVC and the ratio obtained from test results on SCC. The cube/cylinder strength ratio for SCC in the strength of MPa was 0.80, about the same as for vibrated concrete. However, it increased with increase of strength, reaching 1.0 for SCCs at about 90 MPa, a value much greater than the generally assumed ratio of approximately 0.85 for NVC (Fig. 3). Figure 3. Cube and cylinder compressive relationships [1] In 2007, C.Parra [5] presented a study about 8 mixes of self-compacting concrete and normally vibrated concrete. The mixes were designed so that the same water/cement ratio and the same cement type was used. Minor differences were observed (Fig. 4). Figure 4. Experimental compressive strength values [4] 3. Modulus of elasticity The modulus of elasticity is defined as the ratio between stress and strain and it is used in the elastic analyses: in finding the deformational characteristics, in the loosing of stress in prestressed elements or in controlling parameter in slabs design. It is determined by the compressive strength of the concrete. Several experiments [8, 9] and a survey [1] have been reported indicating that the modulus of elasticity of most SCC mixes tested was lower than that of NVC of the same compressive strength. Klug and Holschemacher [8] reported that the scatter of the values for different SCCs was smaller, causing all the results to remain within the band acceptable for design using CEB-FIP model code 90 [10]. The common relationship between the modulus E and square root of characteristic compressive strength f c could still be used. A more recent survey [1] showed that the difference in the modulus of elasticity was greater for lower compressive strength (Fig. 5).

5 Figure 5. Elastic modulus vs. compressive strength [1] 4. Tensile strength All data available regarding the tensile strength of SCC are expressed as the indirect tensile strength. This is usually obtained from results of splitting tensile tests, mostly on cylindrical specimens. Comparisons between SCC and NVC are based on the ratio of tensile strength to compressive strength. Data available indicate, that for the concretes tested, hardened SCC tended to have a tensile strength the same or better than that of NVC. Substantial differences were encountered, with values being up to 40% higher than for vibrated concrete [6, 7, 8, 9]. The data available were analysed by Domone [1] who concluded that the ratio remained within the range expected for vibrated concrete, tending to be within its upper half (Fig. 6). Figure 6. Cylinder splitting vs. compressive strength [1] 5. Creep Creep is defined as the deformation that occurs in the concrete elements under a constant load. The same factors, which governs the rate of development and magnitude of creep of hardened vibrated concrete apply to SCC. The amount of water in the mix, the cement content and the nature of aggregate remain the most important factors. The cement content and water content of an ordinary SCC tend to be very similar to those for the NVC, the difference relevant to creep being in the content of coarse aggregate.

6 In 2005, Anne-Mieke Poppe [11] conducted a test to notice which of the most popular prescriptions (ACI, MC90) formulate and follow closely the behavior of self-compacting concrete. The tests were made with prisms of (150x150x500) mm loaded at 1/3 of the 28 days compression strength. The tests indicated that the closest to the real behavior of self-compacting concrete is the ACI standard [12] which we can consider that predicts the creep of SCC in normal limits. The European code MC90 [10] tends to underestimate the strain from creep of SCC (Fig. 7). Figure 7. Creep deformations of SCC [11] The following conclusions have been obtain [11]: creep decreases with decreasing water/cement ratio or with an increase in cement/powder ratio while the water content remains constant, the fineness of the additions has no significant effect on creep, the type of cement affects creep. The magnitude of creep was shown to be the same as for vibrated concrete, which allows the results from tests on SCC to be used in relevant design codes. 6. Shrinkage According to EFNARC [3] the values contained in EC2 [12] for normally vibrated concrete can be applied to self-compacting concrete, but in some cases the self-compacting concrete may lead to some different values for each type of shrinkage. The fundamental relationships have been summarized by Poppe and De Schutter [11]: increase in the water/cement ratio alone increases the pore content and permits higher shrinkage, increase in the water/cement ratio in SCC is less effective, as there is a substantial amount of fine particles, which also require to be coated by a layer of water. According to Poppe and De Schutter [11], the CEB- FIP model code 90 [10] underestimates the values, while the ACI model [13] tends to overestimate it (Fig. 8). Figure 8. Shrinkage deformations of SCC [11] It can be concluded, that shrinkage of SCC is of a magnitude entirely comparable with that of NVC.

7 7. Bond with reinforcement Bond strength is often expressed in terms of the tensile strength of the concrete or the square root of compressive strength f c and this expression is also useful for estimates of bond between reinforcement and hardened SCC. The bond between the standard reinforcement and concrete is generally linked to the compressive strength and resistance to cracking of the concrete. A significant investigation of the bond was therefore included in the original European SCC project [6, 9] where the performances of selected NVCs and SCCs were compared. Evaluation of large numbers of standard pullout tests [9] initially indicated that there was a distinctly higher bond between the ordinary deformed reinforcement and SCC. However, when adjusted for the higher strength of the SCC, the bond between SCC and reinforcement became equal or slightly higher than that measured on specimens from NVC. In 2007, Domone [1] presented in his survey a large number of tests conducted to determine the bond with reinforcement in SCC, concluding that the bond of SCC to embedded reinforcing and prestressing steel is essentially similar to the equivalent NVC (Fig. 9). Figure 9. Bond strength of reinforcing bars vs. concrete compressive strength [1] 8. Conclusions the compressive strength of SCC was likely to be the same or greater than that of NVC by using the same content of cement and water. modulus of elasticity of SCC was lower than that of NVC of the same compression strength. hardened SCC tended to have a tensile strength the same or better than that of NVC. the magnitude of creep was shown to be the same as for vibrated concrete. shrinkage of SCC was of magnitude entirely comparable with that of NVC. the bond of SCC to embedded reinforcing and prestressing steel tended to produce a slightly better performance than NVC. Acknowledgements This paper was supported by the project "Improvement of the doctoral studies quality in engineering science for development of the knowledge based society-qdoc contract no. POSDRU/107/1.5/S/78534, project co-funded by the European Social Fund through the Sectorial Operational Program Human Resources

8 9. References [1] Domone P.L. A review of the hardened mechanical properties of self-compacting concrete. Cement & Concrete Composites 29, [2] EFNARC. Specifications and guidelines for self-compacting concrete, [3] BIBM, CEMBUREAU, EFCA, EFNARC, ERMCO. The European guidelines for selfcompacting concrete: specification, production and use, [4] EN Concrete - Part 1: Specification, performance, production and conformity, [5] Parra C., Valcuende M., Benlloch J. Mechanical properties of self-compacting concretes. In: 5th International RILEM Symposium on Self-Compacting Concrete, Ghent, Belgium, [6] Grauers M. et al. Rational production and improved working environment through using selfcompacting concrete. EC Brite-EuRam Contract No. BRPR-CT , [7] Gibbs J.C. and Zhu W. Strength of hardened self-compacting concrete. In: Proceedings of the First International RILEM Symposium on Self-compacting Concrete, Stockholm, Sweden, RILEM Publications, Cachan, France, 1999, pp [8] Klug Y. and Holschemacher K. Comparison of the hardened properties of self-compacting and normal vibrated concrete. In: Wallevik O. and Nielsson I. (Eds.) Self-compacting Concrete, Proceedings of the Third International Symposium, RILEM Publications, Cachan, France, 2003, pp [9] Bartos P.J.M. et al. Task 4 in Rational production and improved working environment through using self-compacting concrete. EC Brite-EuRam Contract No. BRPR-CT , [10] Comite Euro-International du Beton: CEB-FIP model code 1990: Design code, Thomas Telford, London, UK, [11] Poppe A.M. and De Schutter G. Creep and shrinkage of self-compacting concrete. In: Yu Z., Shi C., Khayad H. and Xie Y. (Eds.) SCC 2005 China, RILEM Publications, Bagneux, France, 2005, pp [12] Eurocode 2: Design of concrete structures - Part 1-1: General rules and rules for buildings. [13] American Concrete Institute, Prediction of creep, shrinkage and temperature effects in concrete structures, Report by Committee 209, second printing 1994.