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Project title: Drying Shrinkage Strains Behaviour of Self-Compacting Concrete

  • PhD Student: Jamila Abdalhmid 
  • Supervisors: Dr Ashraf Ashour, Professor Dennis Lam and Dr Therese Sheehan

Background

It is generally accepted that SCC was first developed at the University of Tokyo, Japan during the late 1980s (Okamura and Ouchi, 2003). Self-compacting concrete (SCC) is a relatively new type of concrete that is characterised by the ability to flow under its own weight without vibration, pass through intricate geometrical configurations, and resist segregation. The Precast/Prestressed Concrete Institute (2003) defines SCC as "a highly workable concrete that can flow through densely reinforced or geometrically complex structural elements under its own weight and adequately fill voids without segregation or excessive bleeding and without the need for vibration to consolidate it."

The American Concrete Institute defines SCC as a "highly flowable, non-segregating concrete that can spread into place, fill the formwork, and encapsulate the reinforcement without any mechanical consolidation." SCC mixtures are usually designed with high volumes of paste, large quantities of mineral fillers and high range water reducing admixtures. These modifications in composition affect the behaviour of concrete in its hardened state such as drying shrinkage that is considered as a major concern for concrete deterioration (Güneyisi et al 2010).

All special characteristics of SCC may have a significant influence on its shrinkage behaviour as Hwang and Khayat (2008), Heirman and Vandewalle (2003) reported that SCC can exhibit higher shrinkage compared with conventional concrete due to its great volume of cement paste and low coarse aggregate content. To obtain required hardened properties of SCC with suitable workability, technical personnel should try several mix proportions causing waste of materials and time. One of the aims of this research is to predict the drying shrinkage strains and compressive strength of SCC in order to economise on computational effort by using computer models.

Advantages of SCC

SCC offers a wide range of benefits in quality and economy. The major benefits are given below:

  • SCC provides high filling and passing ability. It can flow into intricate spaces and between congested reinforcement without any segregation.
  • SCC produces a high quality and smooth surface finish which reduces the need to repair defects such as bug holes and honeycombing
  • Lower construction costs due to reduced labour and machinery requirements, as Okamura and Ouchi (2003) reported that the number of labourers can be reduced by 30% from those required if NVC is used in large applications.
  • Improved working conditions with fewer accidents due to elimination of noise and vibrators.
  • SCC improves durability and strength of the hardened concrete in some cases.
  • SCC enables engineers to improve concrete quality and produce a homogeneous section, without bleeding or coarse aggregate settlement and with a longer life span without repair requirements.
  • SCC allows acceleration of the casting with more flexibility in the points of concrete placement, reducing the mixer truck movement and pump operations, which decreases the overall construction time. Okamura and Ouchi (2003) indicated that the construction period can be reduced by 20% in large scale projects where SCC is used.

Disadvantages of SCC

Even though SCC is widely used in the construction industry, there are some of disadvantages must be taken into consideration. SCC production requires highly experienced designers to produce a suitable concrete mixture. It needs technical expertise required to develop and control mixtures, and is associated with increased formwork costs due to possibly higher formwork pressures. The increase in cement content for SCC mixtures and low coarse aggregate content may reduce hardened properties such as the modulus of elasticity and increase the potential shrinkage. However, the use of supplementary cementing materials may reduce the shrinkage risk, as well as using high content of admixtures and cementitious materials in SCC, causes delayed setting time in some cases. In some cases, the use of SCC does not reduce the total construction cost (Okamura and Ouchi, 2003). 

Aims

The main goal of the research is to study the influence of different parameters of individual constituents on the drying shrinkage behaviour of SCC. The principal objectives of this research are categorised as follows: 

  • To develop a computer model using ANN for prediction of drying shrinkage strains of SCC, as well as compressive strength using a collected comprehensive database from different sources in the literature, and to analyse the effect of key parameters considered in this study on the drying shrinkage of SCC.
  • A series of experiments will be designed and carried out on two different compressive strengths (30 and 45 MPa) with cement replacement by fly ash (FA) and ground granulated blast slag (GGBS) under different types of curing. As well as this, two ways of testing drying shrinkage strains in free and restrained conditions will be evaluated. Physical and mechanical properties of fine and course aggregates used in this investigation for the production of concrete and fresh properties of SCC will be investigated experimentally.
  • The experimental results will be used to improve the ANNs model to simulate drying shrinkage of SCC and evaluate the effect of the key parameters.
  • A more rational computational model will be also developed based on equations available in the literature for drying shrinkage.
  • So far, an analytical model has been started and developed using ANN, and a conference paper has been written with these results.

Research Significance

The effects of any changes in materials or mixture proportions on hardened concrete performance must be considered in evaluating SCC. Many researchers have explored the behaviour of SCC compositions including shrinkage. One of the research areas that have not been investigated is using computer programs to predict drying shrinkage of SCC and evaluating the influence of SCC ingredients on the drying shrinkage. A research project will be conducted to investigate the behaviour of drying shrinkage strains in SCC and to study the free and restrained drying shrinkage of SCC for different mixtures incorporating cementitious materials as a replacement for cement.  In addition to these objectives, the project will explore the use of the ANNs model to predict drying shrinkage strains and compressive strength of SCC.

Methodology

To date there are no standard specifications or approaches for SCC mix design and many academic institutions and contracting companies have developed their own mix proportioning methods.  These methods often use volume as a key parameter because of the importance of the need to fill the voids between the aggregate particles. The procedure and batching will be proposed by a simple mix design method (Su et al.2001). In order to monitor the compressive strength, six 100 mm cube samples will be produced for each concrete mixture. For monitoring the free drying shrinkage of the SCC,  70 × 70 × 280 mm prisms will be  used as per ASTM C 157 and the ring test according to standard ASTM C 1581-04 will be performed to evaluate the cracking behaviour of the SCC in restrained shrinkage conditions. Furthermore, an analytical program will be developed to predict drying shrinkage of SCC and to evaluate the trend of various SCC ingredients on the drying shrinkage strains and compressive strength of SCC.  A more rational computational model will be also developed based on equations available in the literature for drying shrinkage.

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Reference

Okamura, H. and M. Ouchi, Self-compacting concrete. Journal of advanced concrete technology, 2003. 1(1): p. 5-15.

PCI (2003). Interim Guidelines for the Use of Self-Consolidating Concrete in Precast/Prestressed Concrete Institute Member Plants, (TR-6-03). Chicago, IL:Precast/Prestressed Concrete Institute.

Güneyisi, E., M. Geso¿lu, and E. Özbay, Strength and drying shrinkage properties of self- compacting concretes incorporating multi-system blended mineral admixtures. Construction and Building Materials, 2010. 24(10): p. 1878-1887.

Hwang, S.-D. and Khayat, K.H. Effect of mixture composition on restrained shrinkage cracking of self-consolidating concrete used in repair. ACI Materials journal, 2008. 105(5).

HEIRMAN, G. & VANDEWALLE, L. The influence of fillers on the properties of self-compacting concrete in fresh and hardened state. Proc. of the 3rd Int. Symp. on Self-Compacting Concrete (SCC2003), 2003. 606-618.

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