Predictions of long-term deflection of geopolymer concrete beams

doi:10.1016/j.conbuildmat.2015.06.030

Construction and Building Materials

Volume 94, 30 September 2015, Pages 10-19

https://doi.org/10.1016/j.conbuildmat.2015.06.030 Get rights and content

Highlights

•
Geopolymer concrete can be used for sustainable structural design.
•
EMM and AEMM give closer agreement on deflection prediction than RCM for geopolymer concrete.
•
Crack status should be considered throughout the analysis at all ages.

Abstract

The long-term behaviour of concrete beams constructed with geopolymer concrete (GPC) is investigated. Self-weight and sustained load of 1 kPa are applied on top of the beams at the age of 14 days to simulate construction conditions. Creep tests on cylinders conducted with sustained loading commenced at the ages of 14 days and 28 days. The results from creep tests on GPC show higher creep in the specimens loaded at 14 days than those loaded at 28 days. Predictions of beam deflections are performed by using RCM, EMM and AEMM with input parameters of properties of GPC from experimental data, including elastic modulus, modulus of rupture, creep and shrinkage. These property tests show that GPC can achieve sufficient strength for structural designs, but both compressive strength and flexural tensile strength are affected by drying, which causes differential drying shrinkage and microcracking at the drying surfaces. The predicted deflections by these analysis methods are compared with the experimental results from beams, and show that RCM gives the worse performance of the three methods. The investigation concludes that the AEMM can be used for long-term deflection calculations for GPC beams with minor parameter modifications.

Introduction

Concrete is widely used in construction of various types of structures. Ordinary Portland cement (OPC), as the main binder material in concrete, releases large amounts of CO₂ resulting from the calcination of limestone. Manufacture of 1 ton of OPC can produce as much as 1 ton of CO₂ into the atmosphere depending on the method of manufacturing [1]. A substantial reduction of CO₂ emission can be achieved by blending OPC with natural pozzolans and industrial waste materials [2]. Further CO₂ reductions can be achieved by the activation of industry by-products, such as fly ash and blast furnace slag, with an alkali that replaces the OPC content in concrete and the resulting product is commonly known as alkali activated concrete or geopolymer concrete (GPC) [3], [4]. This adds value to these raw materials because the utilisation of GPC not only bring about significant CO₂ emission reductions but also removes materials from landfill, thus leads to sustainable construction [5].

Research on GPC has shown that it is superior to ordinary Portland cement concrete (OPCC) with respect to mechanical properties [3], [6], fire-resistance [7], resistance to sulphate attack [8], and resistance to chloride [9], [10]. Therefore, GPC is a potential alternative as a sustainable construction material, but the current application of GPC in structural components is limited due to lack of codes and standards for structural design [5]. Although the long-term mechanical properties of GPC are critical to structural design, there has been little research done on the creep behaviour of GPC. These properties vary with the types of binder materials, mix proportion and curing method of concrete. Test results of fly ash-based GPC by Hardjito et al. [11] showed very little drying shrinkage and low creep, but Collins and Sanjayan [6] found higher drying shrinkage and greater creep in alkali-activated slag concrete than OPCC.

The study of creep and shrinkage of GPC enables prediction of long-term deflection and cracking tendency in structures, which is important in concrete structural design for serviceability. Research on the long-term deflection of RC beams containing ground granulated blast-furnace slag (GGBFS) by Shariq et al. [12] showed that deflection increased with increasing GGBFS contents blended with OPC. Liu et al. [13] suggested that it was feasible to make prestressed beams of inorganic polymer concrete (IPC) using alkali-activated fly ash and metakaolin, but the long-term deflection data were not reported. Therefore, it is important to study the long-term behaviour of GPC structures before application in construction. There are a number of rational methods in prediction of this long-term behaviour in structural design. These methods were originally developed for reinforced and prestress concrete made with OPC as the major binder, so the subsequent research, developments and applications were limited to OPC concretes and blended concrete.

The aim of this paper is to study the creep behaviour of GPC in structural members by monitoring two large scale composite GPC beams under sustained loads and comparing the data with predictions of deflection by using three methods, including the rate of creep method (RCM), effective modulus method (EMM), and age-adjusted effective modulus method (AEMM). A number of tests were conducted to investigate the short-term and long-term properties of GPC, so that the application of these prediction methods on GPC structures can be evaluated.

Section snippets

Materials and sample preparation

The OPC used for the reference formulation was type GP according to Australian Standard AS 3972-2010 [14] (Eureka Cement, Australia). GGBFS (slag) and fly ash (FA) supplied by Zeobond Pty Ltd., Melbourne were used as raw materials for geopolymer concrete. Their chemical composition and physical properties are presented in Table 1, Table 2. Alluvial siliceous aggregate of 20 mm maximum size with specific gravity of 2.83 and water absorption of 1.10%, and a silica sand with a specific gravity of

Fresh concrete properties

The slump of the fresh GPC was 150 mm. This is a suitable workability for construction. The temperature of the fresh concrete mixture was 25 °C.

Compressive strength

The uniaxial compressive strengths of the GPC at 7 days, 14 days and 28 days are shown in Fig. 3. The compressive strength (f′_c) of GPC at 28-days was 46.7 MPa. The 7-day and 14-day compressive strengths were 91% and 92% of f′_c, respectively. These strength values were obtained from the samples with standard moist curing (bath curing) throughout. For

Conclusions

This study showed that an estimation of the long-term deflection of GPC beams can be achieved by using the rational methods which were originally developed for OPC based concrete structures. When the three methods, i.e., RCM, EMM and AEMM, were compared with the experimental results, it was demonstrated that EMM and AEMM would be applicable to the structural design of GPC. Prediction from RCM overestimated the deflection because it did not include delayed creep recovery and the assumption of

Acknowledgement

The authors acknowledge financial support from the Australian Research Council through Linkage Project grant LP120200774 supported by Zeobond Research Pty Ltd.

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View full text

Predictions of long-term deflection of geopolymer concrete beams

Highlights

Abstract

Introduction

Section snippets

Materials and sample preparation

Fresh concrete properties

Compressive strength

Conclusions

Acknowledgement

J Cleaner Prod

Cem Concr Res

Cem Concr Res

Cem Concr Res

Miner Eng

Cem Concr Res

Constr Build Mater

Cem Concr Compos

Cem Concr Res

Cem Concr Res

Cem Concr Compos

Cem Concr Res

Cem Concr Res

Cem Concr Res

Cem Concr Res

Mech Today

Properties of concrete

Geopolymers – inorganic polymeric new materials

J Therm Anal