Early Age Creep and Shrinkage of High Performance Concrete

Andina Sprince1, Aleksandrs Korjakins2, Leonids Pakrastinsh1, Genadijs Shakhmenko2,

Girts Bumanis2

1: Department of Structural Engineering, Riga Technical University, Latvia 2: Institute of Materials and Structures, Riga Technical University, Latvia

This research deals with elastic and time-dependent deformations of high performance concrete

reinforced with polyvinyl alcohol (PVA) fibers. The early age drying creep in compression and shrinkage

were experimentally studied. Three concrete mixes with a different amount of fibers were developed and

prepared. The concrete specimens were tested in a controlled constant temperature and with a constant

level of moisture. The compression strength and modulus of elasticity were determined and compared

with those of the reference concrete. The results indicate that in the early age the creep of concrete with

PVA fibers reduces the deformation, but after a longer time of hardening it exhibits higher creep

deformation than the reference concrete without fibers.

Keywords: PVA fibers, creep, shrinkage, creep coefficient, compression strength, modulus of elasticity

1 Introduction

Concrete is an important structural material used in every country of the world. Moreover, the complexity of structures and their size have continued to increase, and this has resulted in a greater importance of their strength and deformation characteristics in more serious consequences of their behavior [1].

Last three decades scientists and concrete technologists have been working on the development of new types of concrete. One of the most perspective products is fiber-reinforced high performance concrete (FRHPC). Fibers in concrete provide improved mechanical and physical properties of the material. For example, the obtained fiber-reinforced concrete has higher resistance to cracking. This is very important for high performance concrete, which usually has high amount of cement and low water/cement ratio. This type of concrete is sensitive to cracking, especially in the early age. The deformation characteristics of concrete are important in the design of sustainable structures [1]. Creep and shrinkage of concrete are a complex problem, especially at very early ages, due to the complexity of the material [2]. Creep deformation of concrete is often responsible for excessive deflection at service loads, which can compromise the performance of elements within a structure [3].

Nowadays construction at site demands rapid concrete strength development to minimize the building time, and that is why concrete deformations at early age are an important issue to concrete technologists and scientists.

Time-dependent deformations like creep and shrinkage should be tested to characterize those concretes.

2 Materials and methods

The experimental work included the preparation of two fiber-reinforced high performance concrete (FRHPC) mixes with different amounts of polyvinyl alcohol (PVA) fibers — 0.6% and 0.8% from the total amount of cement — and one reference mix without fibers for comparison. The mix compositions are given in Table 1. PVA fiber properties are listed in Table 2.

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Table 1: Concrete mix composition.

Component Reference PVA-0,6 PVA-0,8

Cement Kunda CEM I 42,5 N kg/m³ 675 675 675 Cement Aalborg white CEM I 52,5 N kg/m³ 225 225 225 Quartz sand 0/1mm kg/m³ 300 300 300 Quartz sand 0.3/2.5mm kg/m³ 300 300 300 Diabase 0/5mm kg/m³ 200 200 200 Diabase 2/5mm kg/m³ 200 200 200 Ground quartz sand 8 min. kg/m³ 100 100 100 Silica fume Elkem 920 D kg/m³ 100 100 100 Water kg/m³ 200 200 200 Superplasticizer HE-30 kg/m³ 24 24 24 PVA fibers kg/m³ 0 5,4 7,2 W/C 0,22 0,22 0,22

Table 2 : Properties of PVA fibers.

Fiber type Ø

[µm]

L

[mm]

ft

[GPa]

E

[GPa]

MC 40/8 40 8 1,6 42 Concrete components were measured out and then mixed in a laboratory conic rotation mixer for 4 minutes. For the investigation of properties of the material prismatic specimens 40x40x160 mm were produced. Concrete mixtures were cast into oiled steel moulds without vibrating because this is a self-compacting UHPC concrete. After one day specimens were de-moulded. Standard ageing conditions (temperature 20±2ºC, RH > 95±5%) were provided during hardening until certain concrete ages were reached.

Prismatic compressive strength, modulus of elasticity, drying creep and shrinkage tests were performed on early age concrete. The tests were performed after 1, 4, 7 and 14 days of concrete hardening in standard conditions. A compression testing machine with accuracy of ±1% was used, the rate of loading was 0.7 MPa/sec (according to LVS EN 12390-3:2002 standard). The modulus of elasticity was obtained from sample loading during creep tests. The creep was measured for hardened concrete specimens subjected to a uniform compressive load which was kept constant over a long period of time, and shrinkage was measured for the same specimens without loading [4, 5].

At the beginning of the test, the stress level of all mixes was 25% of the maximum strength of the concrete, which had been determined during destructive tests carried out on prismatic specimens. The load was applied gradually in four steps and as fast as possible. Specimens were kept under a constant load for 28 days, and for recoverable creep they were kept without load for 7 days. Four aluminium plates had been centrally and symmetrically glued onto two sides of the creep specimens in order to provide a basis for the strain gauges. The distance between the centers of the two plates was 50 mm. Two ±0.001 mm precision strain gauges were symmetrically connected to each specimen and then the specimens were put into a creep lever test stand and loaded (see Fig. 1). Two aluminium plates had been centrally and symmetrically glued onto ends of the shrinkage specimens and strains were measured with a shrinkage clamp. All specimens were kept in a dry atmosphere of controlled relative humidity in standard conditions: temperature 23±1ºC and relative humidity 25±3% [6]. After creep and shrinkage tests, the prismatic compressive strength of the specimens was determined.

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Early Age Creep and Shrinkage of High Performance Concrete

A

A-A

F1

F2

F2

F2

F2

F2

F2

F2

F2

800

10

Figure 1: Creep lever test stand.

The instantaneous strain that occurs immediately upon application of stress may be considered to be elastic at low stress levels, and therefore:

)0(0)( / ccte E (1)

where Ec(τ0 ) is the elastic modulus of concrete at time τ0 εe(t) is the instantaneous strain σc0 is the compressive stress applied at time τ0

The capacity of concrete to creep is usually measured in terms of creep coefficient, φ(t,τ ). In a concrete specimen subjected to a constant sustained compressive stress, σc(τ ), first applied at age τ, the creep coefficient at time t is the ratio of the creep strain to the instantaneous strain and is given by:

)(),(),( / etcrt (3)

where φ (t,τ) is the creep coefficient, εcr(t,τ) is the creep strain [7].

3 Results and discussion

High cement content and low water/cement ratio provides rapid concrete hardening process with high strength gain even at an early age. All concrete compressive test results are given in Figure 2. High prismatic compressive strength had developed after 24 hours of hardening, and continuous prismatic compressive strength growth was observed. After 14 days it had reached 90 MPa. The obtained results show similar compressive strength development for al l mixes. A slightly (about 7% and 2%) higher prismatic compressive strength is exhibited by the reference mix after the 1st and 14th day respectively. The prismatic compressive strength after one day was 42 MPa, after 4 days of hardening — 64 MPa, after 7 days of hardening — 80 MPa and after 14 days of hardening the prismatic compressive strength reached 94 MPa. The obtained compression strength results were used to determine the necessary stress level for creep tests.

Compression strengths were tested after the creep test (35 days later). It had grown significantly for early age specimens. The highest strength gain was exhibited by specimens tested after 1 day of hardening — 90 MPa. Specimens hardened for a longer period of time in standard hardening conditions developed higher prismatic compressive strength after 35 days

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of creep test. The highest prismatic compressive strength was obtained in the PVA-0,6 specimens, and it was 118 MPa. The lowest final prismatic compressive strength was developed by the reference specimens —110 MPa.

Figure 2: Prismatic compressive strength of high strength concrete specimens before and after creep tests.

During concrete specimen loading, the modulus of elasticity was obtained. The modulus of elasticity increases with the concrete hardening time and strength gain in the similar way for all mixes (see Fig.3) from 28 to 40 GPa.

Figure 3: The modulus of elasticity for high strength concrete specimens.

The results of drying creep tested at an early age of the FRHPC and reference specimens indicate that the highest creep deformations were observed after the application of load on the 1st day (see Fig. 4). The highest deformation response was exhibited by the reference concrete specimens. After 4 days, the highest creep deformations had developed in the PVA-0.8 specimens, but in specimens with a fiber amount of 0.6% the deformation level was the lowest. After 7 days of hardening, the reference concrete specimens showed the lowest deformation response. Specimens hardened for 14 days exhibited a much lower deformation response than previously. The lowest creep deformations were observed in the reference concrete specimens.

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Early Age Creep and Shrinkage of High Performance Concrete

Figure 4: Drying creep and recoverable, irrecoverable creep and shrinkage strains.

The total creep and shrinkage strains are given in Figure 4. After 28 days of loading, the load was removed. The creep recovery was measured 7 days after the loading period. The highest residual creep strains were observed in 1 and 4 days old reference concrete specimens. Concrete specimens hardened for 7 and 14 days provided lower creep strains, and the smallest deformations were exhibited by the reference specimens. The highest creep strains had developed in concrete specimens with a fiber amount of 0.8%. The final creep strains were 0,76·10-5 for the reference concrete, 0,83·10-5 for PVA-0.6 and 0,93·10-5 for PVA-0,8 specimens.

The final stress level was obtained after the creep tests. It was calculated as a ratio of the applied load and the prismatic compressive strength after the creep test. The gained prismatic compressive strength and the final stress ratio of the specimens were lower at early ages. In specimens loaded after 1 day of hardening the stress level varied from 0,10 to 0,12, and for specimens loaded after 14 days of hardening the stress level varied from 0,19 to 0,20. Despite the fact that the final stress level was higher in the reference concrete mix specimens, the creep deformations were lower in the specimens with PVA fibers.

The creep coefficient increases with time at an ever-decreasing rate. The final creep coefficient is a useful measure of the creeping capacity of concrete (see Fig. 5). The highest creep coefficients were established for concrete specimens loaded after one day of hardening, and it was for specimens with fibers. Specimens that were loaded after one day of hardening produced creep coefficients from 4.2 to 4.8. The creep coefficient reduces significantly with the growth of the concrete strength. They decreased considerably already after the 4th day and ranged from 2.6 to 3.0. The lowest creep coefficients were determined in specimens that had been loaded for 14 days, and these ranged from 1.5 to 1.7. The lowest creep coefficient was exhibited by the reference concrete specimens.

313

Figure 5: Creep coefficient of high strength concrete specimens.

Drying shrinkage results (see Fig. 6) were obtained from the same shape and concrete mix specimens as the creep specimens [4], and the strains were measured during creep tests. Figure 6 shows that the drying shrinkage deformations are decreasing with time. In specimens hardened for a prolonged period of time in standard hardening conditions the drying shrinkage strains were lower. The lowest shrinkage deformations were observed in concrete specimens with fibers.

Figure 6: Drying shrinkage strains of high strength concrete.

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Early Age Creep and Shrinkage of High Performance Concrete

4 Conclusions

Two fiber-reinforced high performance concrete (FRHPC) mixes with polyvinyl alcohol fibers (PVA) were prepared for a laboratory examination of processes ongoing in concretes at an early age, and the results were compared with those of a reference concrete mix without fibers. Two fiber contents were chosen for comparison — 0.6% and 0.8% of fibers from the total amount of cement.

Concrete specimens were tested at an early age after 1, 4, 7 and 14 days of hardening. The prismatic compressive strength, modulus of elasticity, drying creep and shrinkage were determined.

The compression strengths after the creep test (35 days later) were established. In the early age it had grown significantly. The highest prismatic compressive strength was developed by the fiber-reinforced specimens and it reached 118 MPa.

During concrete specimen loading the modulus of elasticity was obtained. The modulus of elasticity increases with the concrete hardening time and strength gain in the similar way for all mixes.

Concrete specimens were tested for an early age drying creep. All specimens were loaded with an equal stress level 0.25. The load was applied for 28 days and the long-term deformation responses were measured. The creep recovery was observed over a time period of 7 days. The creep deformations were found to decrease with concrete aging and time.

Drying shrinkage results were obtained from the same shape and concrete mix specimens as the creep specimens, and the strains were measured during creep tests. In specimens hardened for a prolonged period of time in standard hardening conditions the drying shrinkage strains were lower. The lowest shrinkage deformations were observed in the PVA fiber-reinforced concrete specimens.

5 Acknowledgement

This work has been supported by the European Social Fund within the scope of the project “Support for the Implementation of Doctoral Studies at Riga Technical University”.

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[4] Rilem TC 107-CSP: Creep and Shrinkage Prediction Models: Principles of Their Formation. Measurement of Time-dependent Strains of Concrete. Materials and Structure, RILEM Publications SARL, 1998.

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