Experimental study of Strengthening of “RC” slab Overlaying with “UHPC”

Athira Dharmaraj PG Student

Dept. of Civil Engineering

Vimal Jyothi Engineering College

Chemperi, Kannur

Pro. Dr. Vra Saathappan PG Professor

Dept. of Civil Engineering

Vimal Jyothi Engineering College

Chemperi, Kannur

Abstract - Ultra High Performance Concrete (UHPC) is an

advancement in concrete technology[6]. It is a mixture of

Reactive Powder Concrete (RPC) with steel fibres, which

was first developed by Pierre Richard and Marcel

Cheyrezy (France). UHPC offers high compressive

strength of about 150 to 200 MPa. The growing demand to

meet stringent requirements of sustainability,

serviceability and durability characteristics, research lead

towards development of the UHPC[3]. In addition, UHPC

exhibits excellent mechanical properties such as durability,

low permeability and energy absorption[6].

There are fewer studies regarding the

strengthening of slab using UHPC. Also the studies were in

one way slab. In this study, the two way slab is overlaid

with different UHPC thickness to study the variations.

In this study, 5 slabs are casted and cured for 28

days. After 28 days curing, one slab is taken as control slab

and other 4 slabs are overlaid with 25 mm, 35 mm, 45 mm

and 50 mm UHPC thickness. UHPC contains Cement,

Silica fume, Quartz powder, Quartz sand, M sand, Super

plasticizer, Steel fibres and water in proper proportion.

Then the overlaid slabs are cured for 28 days and tested

using loading frame. The ultimate load and mid span

deflection are observed for further calculations.

The result showed that ultimate load at failure is

increasing with the increase in thickness of UHPC. The

slab with 50 mm UHPC overlay increased the ultimate

load by 69 % and resistance to crack by 108 % when

compared with the control slab.

Keywords: UHPC, Deflection, Ultimate load, Overlay

I. INTRODUCTION

Ultra High Performance Concrete (UHPC) is one of the latest advances in concrete technology and it addresses the shortcomings of many concretes today and is a combination of high strength concrete and fibres[1]. In particular, it is a super plasticised concrete, reinforced with fibres, with an improved homogeneity because traditional coarse aggregate is replaced with fine sand[3]. The performance characteristics for UHPC are freeze-thaw durability, scaling resistance, abrasion resistance, chloride penetration, compressive strength, modulus of elasticity, low shrinkage and creep[4].

In general, the first four is termed as durability properties. UHPC consists of binder materials like Cement, Silica fume, Quartz powder, fine aggregates like Quartz sand and River sand combined with Steel fibre and Super plasticizer[4].

The term Ultra High Performance Concrete (UHPC) was first used in 1994 to refer to an optimised particle-packing material, using a special selection of fine and ultrafine particles which led to low porosity, high durability and self-compactability[5]. The optimised particle-packing allowed an increase in the concrete compressive strength (above 120 MPa) leading call it as Ultra High Strength Concrete (UHSC) and also has increase in the durability performance of concrete. That is the reason why the term HPC (High Performance Concrete) coined in the 1980s evolved into UHPC in the 1990s[3]. It was developed in Europe and first it was used for specialized applications that demand superior strength and corrosion resistance, such as for marine applications or seismic structures[5]. More recently, the use of UHPC has expanded to applications requiring its high strength in narrow profiles such as bridge spans and building facades in which the material’s strength, wear resistance, lighter weight and lower life cycle costs have been the driving determinates[2].

II. EXPERIMENTAL PROGRAMME

A. Description of test specimens and parameters

5 slabs of dimension 1m x 1m x 0.1 m are casted. First the control slab is casted (CS). Then it is cured for 28 days. The reinforcement detailing is shown in Fig 1. The design is as per the code IS: 875 (Part III), IS: 456[7, 9]. Then 4 other slabs of same dimensions are casted. Each of them is cured for 28 days. After that 25 mm UHPC overlay is placed over a slab (UC 25). Then 35 mm overlay is placed over the other slab (UC 35). Also 45 mm overlay in next slab (UC 45) and 50 mm overlay in other slab (UC 50). Then these overlaid slabs are cured for 28 days. The Slabs with different UHPC overlays are shown in Fig 2. They’re tested in loading frame with central load. The average cube compressive

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 14, Number 12, 2019 (Special Issue) © Research India Publications. http://www.ripublication.com

Page 20 of 24

strength at 28 days for CS is 34.3 N/mm2. The mix design of concrete is as per IS: 10262-2009[8]. The UHPC material proportion is given in Table 1.

Table 1 UHPC material proportion

Constituent Mix proportion by weight of

cement

Cement 1

Silica fume 0.25

Quartz powder 0.25

W/C ratio 0.33

Steel fibre 2% by Volume

Super plasticizer 0.02

M sand 0.8

Quartz sand 0.5

After the slab hardened, the top surface of the concrete is purposefully roughened in order to create good bonding surface for UHPC layer. This is done using chisel and hammer randomly throughout the concrete substrate. B. Test setup and instrumentation

All the slab specimens are simply supported and subjected to loading at the centre. The Linear Variable Differential Transducers (LVDTs) are placed at the centre portion of the slab at the bottom for measuring deflection. The measured displacement will be displayed in the digital indicator and further it is connected to Data Acquisition system (DAQ). The load is applied continuously until the specimen breaks. To measure the load applied to the specimen, compression type load cell is used.

The load cell gives proportional output in electrical parameters under the given load. The failure load is the load that caused the specimen to fail in flexure or that caused failure at the interface between the substrate and overlay. The specimens are whitened by applying white cement for better observation of the cracks. The test setup is shown below in Fig 3.

III. OBSERVATIONS, RESULTS AND DISCUSSIONS

All the specimens are tested using loading frame. Test results included ultimate load at failure and central deflection. A. Load, deflection data

By the application of load, the maximum load that specimens could carry with the corresponding central deflections is obtained as the result. The results of all the slabs are given in Table 2. B. Comparison of parameters

Ultimate load at failure

From Table 2, it’s clear that the ultimate load at failure and the maximum deflection in the control slab is higher when compared to the other specimens. By overlaying with UHPC, the strength and ultimate load got increased.

From Fig 4, the slab with the 50 mm UHPC overlay could withstand a load up to 149.7 kN where control slab is only able to take 88.4 kN load. The slab with the 45 mm UHPC overlay could withstand a load up to 148.8 kN. Also the slab with 35 mm UHPC overlay could bear load up to 113.2 kN and the slab with 25 mm UHPC overlay could bear the load up to 93.8 kN. The UC 50 bears the maximum load compared to other specimens.

Fig 1 Reinforcement detailing of slab

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 14, Number 12, 2019 (Special Issue) © Research India Publications. http://www.ripublication.com

Page 21 of 24

Table 2 Values of parameters obtained for all specimens

Specimen Maximum load

(kN)

Central

deflection

(mm)

Load at

first

crack

(kN)

CS 88.40 20.35 47.2

UC 25 93.80 7.84 59.0

UC 35 113.20 12.06 73.2

UC 45 148.80 16.97 80.4

UC 50 149.70 11.44 98.30

The ultimate load at failure in 50 mm UHPC overlaid slab has 69 % load increment compared to control slab. The ultimate load at failure found on slab with 45 mm UHPC overlay has 68 % load increment compared to control slab. The ultimate load at failure found on slab with 35 mm UHPC overlay has 28 % load increment compared to control slab. The ultimate load at failure found on slab with 25 mm UHPC overlay has 6 % load increment as compared to the control slab. Maximum Central deflection

From Fig 5, the maximum central deflection

for control slab is 20.35 mm whereas the slab with 25 mm UHPC overlay is 7.84 mm. The slab with 35 mm UHPC overlay is 12.06 mm. The slab with 45 mm UHPC overlay has 16.97 mm as central deflection. The slab with 50 mm UHPC overlay has a central deflection value 11.44 mm which is less than UC 35 and UC 45. Up to 45 mm UHPC overlay thickness, the deflection increased with the thickness of overlay. But in 50 mm overlay thickness, the deflection decreased and is less than UC 45 and UC 35. UC 50 is more rigid compared to UC 45 and UC 35. The stiffness increased in UC 50. Load at first crack

From Fig 6, the load at first crack is maximum in the case of slab with 50 mm UHPC overlay and it is found to be 98.3 kN. The load at first crack for control slab is only 47.2 kN. In the case of slab with 45 mm UHPC overlay, it is found as 80.4 kN. The load at first crack for slab with 25 mm UHPC overlay is 59 kN and that of 35 mm UHPC overlay is 73.2 kN.

It’s because the UHPC overlay delays the cracking and it may be due to the presence of fibres present in the UHPC which improves resistance to cracking.

Fig 2 Slabs with different UHPC overlays

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 14, Number 12, 2019 (Special Issue) © Research India Publications. http://www.ripublication.com

Page 22 of 24

IV. CONCLUSIONS

Control slab and four other slabs with different UHPC overlay thicknesses are casted over a slab thickness of 100 mm and tested using Loading frame. From the testing of the specimens using loading frame, the following conclusions are made. a) The ultimate load at failure increased with the

increase in thickness of UHPC overlay. b) The reason for the increased load capacity for the

slab with UHPC overlay thickness may be due to the uniform distribution of steel fibres and finer material contents in UHPC mix.

c) The central deflections in slab with UHPC overlay increased from 25 mm to 45 mm thickness and a reduction in deflection of about 33 % occurred in

Fig 3 Test setup for slab

88.4 93.8

113.2

148.8 149.7

0

50

100

150

200

CS UC 25 UC 35 UC 45 UC 50

Fig 4 Comparison of Ultimate load at failure

47.259

73.280.4

98.3

0

20

40

60

80

100

120

CS UC 25 UC 35 UC 45 UC 50

Fig 6 Comparison of load at first crack

20.35

7.84

12.06

16.97

11.44

0

5

10

15

20

25

CS UC 25 UC 35 UC 45 UC 50

Fig 5 Comparison of maximum central deflection

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 14, Number 12, 2019 (Special Issue) © Research India Publications. http://www.ripublication.com

Page 23 of 24

50 mm UHPC overlaid slab by comparing with 45 mm overlaid slab.

d) Due to the increase in overlay thickness of UHPC, the first crack load increases. The load at first crack for slab with 50 mm UHPC overlay thickness is 98.3 kN and for control slab, it is 47.2 kN. The increment in resistance to crack is 108 % as compared to control slab.

e) The optimum thickness of UHPC overlay couldn’t be found out since our study is limited till 50 mm overlay. The ultimate load at failure is in increasing state. If the study is done beyond 50 mm thick overlay, we may be able to determine the optimum thickness of UHPC.

ACKNOWLEDGEMENT

The author gratefully acknowledges the management, Principal, HOD and other department staff for providing technical and moral support to do this work.

REFERENCES

[1] Bassam A. Tayeh, B.H. Abu Bakar, M.A. Megat Johari , Yen Lei Voo (2012), “Mechanical and permeability properties of the

interface between normal concrete substrate and ultra high performance fiber concrete overlay”, Construction and Building

Materials

[2] Jun Li, Chengqing Wu, Hong Hao (2015), “An experimental and numerical study of reinforced ultra-high performance concrete slabs under blast loads”, Materials & Design

[3] Prabhat Ranjan Prem, A. Ramachandra Murthy, Mohit Verma (2018), “Theoretical modelling and acoustic emission monitoring of RC beams strengthened with UHPC”, Construction and

Building Materials [4] S. Abbas, M. L. Nehdi, M. A. Saleem (2016), “Ultra-High

Performance Concrete: Mechanical Performance, Durability, Sustainability and Implementation Challenges”, International

Journal of Concrete Structures and Materials

[5] Shamsad Ahmad, Ibrahim Hakeem, Mohammed Maslehuddin (2014), “Development of UHPC Mixtures Utilizing Natural and Industrial Waste Materials as Partial Replacements of Silica Fume and Sand”, Hindawi Publishing Corporation, Scientific World

Journal [6] Yin Hor, Wee Teo, Shirai Kazutaka (2017), “Experimental

investigation on the behaviour of reinforced concrete slabs strengthened with ultra-high performance concrete”, Construction

and Building Materials [7] IS: 875(Part II) - 1987; Code of practice design loads (other than

earthquake) for buildings and structures [8] IS: 10262-2009; Indian Standard Concrete Mix proportioning-

guidelines [9] IS: 456-2000; Plain and Reinforced Concrete-Code of Practice.

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 14, Number 12, 2019 (Special Issue) © Research India Publications. http://www.ripublication.com

Page 24 of 24