Microstructural Analysis of A Reactive Powder Concrete at Elevated Temperature

Microstructural Analysis of A Reactive Powder Concrete at Elevated Temperature

D. Erosemiap, Dr. N. Sakthieswaran2, Dr. R. Viji1

1Department of Civil Engineering, Anna University BIT Campus, Tiruchirappalli- 620024, TN, India

2Department of Civil Engineering, Regional centre of Anna University, Tirunelveli- 627007, TN, India

Abstract – Reactive Powder Concrete (RPC) is the ultra-high strength concrete prepared by replacing the ordinary aggregate of normal concrete with quartz powder, zircon sand and steel fibre. In this paper it mainly concern to approach the studies of microstructure of RPC at elevated temperatures. RPC intensified at elevated temperature of about 200 to 800°C to gain excellent mechanical property which is better understandable by performing Scanning Electron Microscope (SEM), Fourier Transform and Infrared Spectroscopy (FTIR) and X-Ray Diffraction (XRD). Particle Size Distribution (PSD) and X-Ray Fluorescence (XRF) are performed in materials to scrutinize the properties of materials. As a result it was concluded that at temperatures 400 and 600°C compressive strength increases gradually in all mixes. Due to influence of high temperature in RPC, crystal hydrate, xonotlite were perceived. At a higher temperature the quality of concrete gradually decreases, acquired by conducting Non Destructive Testing (NDT) (Ultrasonic Pulse Velocity) test.

Keywords- Reactive Powder Concrete, Elevated temperature, micro structural analysis, compressive strength, NDT.

1. INTRODUCTION

   A special amalgamation of constituent materials produces high strength, new generation concrete of Reactive Powder Concrete [RPC]. The composition of reactive powder concrete includes high alumina cement, alccofine, Quartz sand, zircon sand and micro steel fibres. The enhancement of material use, economic benefits and construction of buildings that is strong, durable and sensitive to environment. According to Marcel Chezrey et al [8], high silica fume content and very low water to cement ratio is characterised in RPC to obtain excellent mechanical and durability properties. Microstructure of RPC based on the heat treatment and pressure applied before and during setting. It was observed that the microstructure of CSH hydrate seems to change due to increase in temperature. Ming-Gin Lee et al had evaluated the bond durability and use of RPC as a new repair material to existing concrete. The RPC and steel are found to be more adhesive than other concretes [10].The affiliation of compressive strength with heating temperature of RPC match up to normal concrete, high strength concrete [13].

   By changing the proportions of steel fibre(0%,0.5%,1%,1.5%,2%,2.5%) in concrete the relative study of mechanical and durability properties of RPC is

   inspected [3].The concrete strength in existing building is assessed using a Non destructive technique like rebound hammer and ultra pulse velocity which is in conjunction with destructive techniques. Many factors such as number of cores, variability in concrete strength affects the quality of concrete [7]. The coefficient and law of the size effect of RPC were deliberated through experiments and theoretical analysis [1]. The most notable in RPC containing fibre is the size effect on compressive strength, for a fibre dosage of 0-2% Bezants size effect formula is preferred to predict the compressive strength of RPC specimens of 100mm.

   The bond characteristics of steel fibre in matrix along with bond strength, pullout energy, etc., are presented based on the effect of silica fume. The fibre matrix interfacial properties are enhanced effectively by incorporating various silica fume content ranging from 0% to 40% used in mix proportions, it was found that optimal silica fume dosage of 20% and 30% [15]. This paper outlines the research was started off to direct the micro structural analysis, compressive strength of RPC with varying temperatures with different mix proportions.

2. EXPERIMENTAL PROGRAM

   1. Materials

      The following are the materials in which RPC is composed of High alumina cement and alccofine as a binder, quartz sand and zircon sand as fine aggregate whereas coarse aggregate is eliminated in this type of concrete and steel fibres are used as admixture to augment the performance of concrete. The required workability is achieved by very low w/b ratio with a high range water reducer Sika Viscocrete. The micro steel fibre of length 13mm and diameter 0.2mm is preferred. The properties of material are obtained by performing X-Ray Fluorescence (XRF) fig. 1 to 4, Particle Size Distribution (PSD) fig. 5 to 8 results and specific gravity of materials is listed below.

      Table I Specific Gravity of Materials

      Contents	Specific gravity
      HAC	3.2
      Alccofine	2.9
      Quartz sand	2.65
      Zircon sand	4.6

      Fig. 1 XRF of Alccofine

      Fig. 2 XRF of HAC

      Fig. 3 XRF of Zircon sand

      Fig. 4 XRF of Quartz sand

      Fig. 5 PSD of Alccofine

      Fig. 6 PSD of HAC

      Fig. 7 PSD of Zircon sand

      Fig. 8 PSD of Quartz sand

   2. Mixing proportion

   The procedure of mixing RPC is as follows, in first stage dry mixing of ingredients and in the second stage half volume of water having half quantity of super plasticizer is added. The one more ingredient that is steel fibre which is added at the final stage of mixing and mixing time is increased by two minutes from the total mixing duration [17]. The fresh mix of RPC was poured in 150mm cube

   Table II Mix Proportion of RPC

   Proportion in %

   Mixes	HAC Alccofine Quartz Zircon Steel sand sand fibre
   Control mix	100	–	100	–	2
   Mix 1	70	30	90	10	2
   Mix 2	70	30	80	20	2
   Mix 3	70	30	70	30	2
   Mix 4	70	30	60	40	2
   Mix 5	70	30	50	50	2

   Compressive strength

   N/mm2

   Compressive strength

   N/mm2

   60

   50

   40

   30

   20

   10

   0

   after mixing and placed on vibration table to remove air voids by compaction. After allowing it to settling for 24 hours, cubes where removed from mould and the kept in curing for 28 days. The mix proportion adopted for RPC is tabulated in Table II.

3. RESULTS AND DISCUSSION

   1. Compressive strength

      The fig. 9 shows that the compressive strength is greatest in mix 5 at 200°C which is found to be 55.56 N/mm². The values of mix 1, mix 2, mix , mix 4 and mix 5 get increased at 400 and 600°C whereas in control mix the compressive value get decreased at 400 and 600°C compared to 200 and 800°C.In control mix the highest value is obtained at 800°C but it other mixes the lowest value is found to be in 800°C. Mix 3 indicates the increase in compressive values at 200, 400,600 and 800°C compared to other mixes. The value of mix 3 gradually increases from 200°C to 400°C and to 600°C and decreases at 800°C. The optimum value is obtained at temperature 400 and 600°C but exceptionally in mix 5 the optimum value is obtained at 200°C.

      200°C

      400°C

      600°C

      800°C

      Control mix Mix 1 Mix 2 Mix 3 Mix 4 Mix 5

      Mix proportions

      Fig. 9 Compressive strength of mixes at various temperature

   2. Ultrasonic pulse velocity

      The fig. 10 represents the direct readings of ultrasonic pulse velocity indicates that at 200°C the concrete sample is found to be good because it ranges from

      3.5 to 4.5. At 400°C the velocity fluctuates for different mixes as doubtful and medium between <3 and 3 to 3.5 (mix 1 to mix 5), for control mix it is found to be good. For elevated temperature of 600°C, it gradually reduces from good to doubtful. At a higher temperature of about 800°C the quality of concrete is found to be doubtful. The

      fluctuation occurs in the temperatures 400 and 600°C.The fig. 11 represents the semi direct UPV readings, the quality of concrete is found to be excellent for control mix , mix 1, mix 2 at temperatures 200, 400 and 600°C and at 800°C it is found to be doubtful for all the mixes except mix 3

      ,which is medium. At 200°C all the mixes are found to be excellent. For elevated temperature at 400°C the mixes fluctuates from excellent to doubtful. At the temperature of 600°C the velocity gradually decreases from excellent to doubtful. The quality of concrete in mix 5 is doubtful.

      UPV readings

      UPV readings

      UPV readings

      UPV readings

      5 8

      4 6

      2

      2

      3 200°C 4

      1 400°C 2

      0 600°C 0

      800°C

      200°C

      400°C

      600°C

      800°C

      Mix proportions Mix proportions

      Fig. 10 Direct readings of UPV Fig. 11 Semi direct readings of UPV

   3. Scanning electron microscope analysis

      Scanning Electron Microscope was used to examine the microscopic strength of the RPC specimens before and after experiencing high temperature spalling. In order to investigate the material properties and elemental compositions of the RPCs at elevated temperatures. Fig. 12 represents the SEM images of RPC samples at different temperatures. These images represent the formation of hydrates and unhydrate in the samples. The control mix of RPC at 400°C shows the formation of C-S-H gel and the mix 3 result in the formation of minor cracks on the surface of the concrete at 800°C and also shows that concrete get burnt and the crystal CH is formed. The unhydrated cavities formed over hydrated gives the dense strength of concrete. Formation of ettringite results in the reduction of strength.

      Fig. 14 FTIR analysis of mix 1

      Fig. 12 SEM analysis of RPC

   4. Fourier transforms infrared spectroscopy

      Fig. 13 to 18 shows that the infrared spectra for control mix, mix 1, mix 2, mix 3, mix 4 and mix 5. Similar vibration bands were observed in all the mixes that are figured below. The infrared spectra of mix had vibration bands at approximately 988 to 992 cm-1 associated with the Si-O stretching vibration in the SiO4 tetrahedral comprising the C-S-H gel. The band observed at 668 cm-1 was attributed to the Al-O stretching vibration in AlO4 groups. The calcite v3[CO3]2- vibration bands appeared at 1420 cm-1 and the shoulder 880 cm-1 associate to v2[CO3]2- carbonates. Mixes exhibit IR vibration models at 1651 and 1643 cm-1 associated with the bending vibrations of H-OH bonds, related to bound water in the hydrated products formed after alkaline activation.

      Fig. 13 FTIR analysis of control mix

      Fig. 15 FTIR analysis of mix 2

      Fig. 16 FTIR analysis of mix 3

      Fig. 17 FTIR analysis of mix 4

      Fig. 18 FTIR analysis of mix 5

   5. X ray diffraction

      It is used to study the structure, composition, and physical properties of materials. Below fig. 19 to 24 represents the XRD patterns of the hydrated samples C1 (control mix), mix 1, mix 2, mix 3, mix 4 and mix 5 respectively. The XRD analysis for the sample shows the peak value 18° represents the presence of portlandite and the peak value corresponds to 21° and 26° represents the presence of SiO2.Peak value corresponds to 27,28,31° shows the presence of C2S because of test carried out on 28 days which gives earlier strength. The value of 30° results in the formation of C-S-H gel formation. The peak values more than 50° is not considered. This result shows that peak values are formed between 25° to 30° indicates the formation of SiO2 in all the samples.

      Fig. 19 XRD analysis of control mix

      Fig. 20 XRD analysis of mix 1

      Fig. 21 XRD analysis of mix 2

      Fig. 22 XRD analysis of mix 3

      Fig. 23 XRD analysis of mix 4

      Fig. 24 XRD analysis of mix 5

4. CONCLUSIONS

The following are the conclusions that are achieved in this project,

• The compressive strength of RPC increases from mix

1 to mix 5 at 200°C and at 400° mix 1 and 2 increases while mix 3, 4 decreases and mix 5 increases. At 600°C mix 1,2 increases but mix 3,4,5 decreases and at 800°C mix 1,2,3 increases whereas mix 4,5 decreases. This result shows that increase in zircon sand as increase in temperature results in decrease in the strength of RPC.

• UPV shows that the quality of concrete is good at 200°C in direct reading. The increase in temperature will results in the poor quality of concrete. In semi direct readings the quality of concrete is found to be excellent for mix 1,2 at 200,400,600°C whereas mix 3,4,5 are found to be doubtful at 600 and 800°C.This result indicates that quality of concrete is good with maximum quartz sand.

• The micro structural studies of RPC show the formation of hydrates and unhydrated clinkers. The C-S-H gel formation indicates the process of hydration and micro cracks are formed in samples at 800°C. The crystals are formed at 400°C in mix 3. Ettringite is not formed. The quality of concrete is found to be good at low temperature; increase in temperature will spoil the quality of concrete.

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