Experimental analysis of recycled aggregate concrete behavior under confinement pressur

Microsoft Word 277 BUi Ngoc Kien doc Tuyển tập Hội nghị Khoa học thường niên năm 2019 ISBN 978 604 82 2981 8 81 EXPERIMENTAL ANALYSIS OF RECYCLED AGGREGATE CONCRETE BEHAVIOR UNDER CONFINEMENT PRESSURE[.]

EXPERIMENTAL ANALYSIS OF RECYCLED AGGREGATE CONCRETE BEHAVIOR UNDER CONFINEMENT PRESSURE

Ngoc Kien Bui1, Truong Viet Hung2

1Department of Civil Engineering, Thuyloi University, email: kienbn@tlu.edu.vn

2 Department of Civil Engineering, Thuyloi University, email: truongviethung@tlu.edu.vn

1 INTRODUCTION

Recent years, with the outbreak development

of construction industry, the demands of

concrete have been increasing relentlessly

about 15 billion tons annually, which

accompanied by a huge requirement in

consuming aggregate product approximately

10-11 billion tons each year [1] Besides,

increase of large quantities of the construction

and demolition waste (CDW) due to the end of

service life of infrastructures, war, natural

disasters and human activities cause extreme

challenges for environment protections and

construction industry [2] It is estimated that the

amount of construction and demolition waste

occupies about 40% of the total waste, which

was indicated in studies of Takahashi et

al.(2011), becoming burdens of many nations

For that reason, recycling CDW as a new

aggregate source for concrete has received

much more attention due to its feasibility,

environmental and economic benefits [4]

Using RCA for concrete structures in many

countries is still controversial and not easy to

come to life because of the lower qualities of

RAC and more complicated microstructure

such as contained large flat Ca(OH)2 crystal

(CH crystal, about 20-25% of the volume of

solids in the hydrated paste) which accumulated

in the voids and the surface of RAC [5]; a huge

amount of pores and cracks [6]; especially it

holds two interfacial transition zone (ITZ), one

is new ITZ between RCA and new cement

paste and another one is old ITZ between old

natural aggregate and old adhered mortar [7],

compared to natural aggregate concrete (NAC),

which causes negative effects on performance

of RAC [8] Which is why, numerous studies

have been conducted to evaluate and improve the mechanical and durability properties of RAC [9] To enhance the properties of RAC, several techniques have been developed in the literatures [10] Recently, it is believed that using pretreating method for RCA, surface modification treatment of RCA with pozzolanic admixtures or enhancing the microstructure of RCA is a good solution for improving the qualities of RAC [5] is one of great solution for saving consumption energy and environmental friendly It may be reasonable to recognize that pozzolanic admixtures can enhance microstructure by two reasons: one is pozzolanic admixtures plays as a micro filler, filling into pores and cracks of RCA, second is pozzolanic materials will consume CH crystals

in RAC to form CSH gel (Calcium silicate hydrate) to fill up in voids of RCA Thus, the existing methods have improved mechanical properties of RAC in certain somewhat extent and microstructure of ITZ still existed much amount of CH crystal Therefore, in this paper

we proposed a solution (including pozzolanic powder and sodium silicate) for pre-treating RCA in order to improve the properties of microstructure in RCA with the purpose of creating twofold efficiency in reducing maximally the amount of CH in RAC Furthermore, in order to apply RAC for real structure, it is necessary to understand the behavior and performance of concrete under different stress states such as uniaxial stress and triaxial stress because in structure, concrete is often subjected to combinations of compressive, tension and shear stress However, there is lack of experimental reports related to triaxial compressive characteristics of

RAC under high confinement pressure

particular of RAC contained pozzolanic

material Therefore, the aim of this study is to

propose the solution for improving the

mechanical properties of RAC as well as to

investigate the behavior of RAC under

unconfined and confined compression pressure

2 METHODOLOGY FOR PRETREATING

RCA

Treatment solution was proposed to improve the quality of RCA and the detail of

treatment solutions (pozzolanic solutions)

components are composed of sodium silicate

and one of pozzolanic powder, as in Table 1

Different pozzolanic solutions were prepared

with 20% concentrations Pozzolanic solution

type S which is combination of pozzolanic

powder and sodium silicate (Na2SiO3)

Table 1 Components of pozzolanic

solution type S

Components of treatment solution

for 1000g

Concentr ation of solution Na2SiO3

(g)

Pozzolanic powder (g)

Water (g) 20% 75 125 800

3 EXPERIMENTAL PROGRAM

3.1 Materials

Mixture design proportion of RAC for a target concrete strength was selected water

cement ratio and slump for rigid pavement

application and the water/cement ratio was

kept constant at 0.45 The amount of water

used for each batch is different because of the

difference of water absorption of each

aggregate blend The mix proportions of

concretes are represented in Table 2

Table 2: Mix proportion of concrete kg/m 3

Sample Water Cement aggregate Coarse aggregateFine

SFA 228.2 428.9 823.2 670.7 SSF 236.8 428.9 804.1 670.7 SMT 232.6 428.9 808.4 670.7

3.2 Equipment for testing

The compressive strength of concrete is measured by Shimadzu machine with 1000kN axial load capacity at a rate of axial loading constant at 0.25MPa/s The high pressure of chamber was provided by pump for triaxial compression test

4 RESULT AND DISCUSSION 4.1 Triaxial compression test

The testing machine was setup as shown in Fig 1, the axial load from the testing machine

is transmitted to specimen inside oil chamber

by a piston The pressure in chamber was provided by an oil pump The specimen, before put into the chamber, was wrapped by polyolefin tubing which is shrinkable in heat condition (Sumitube-Sumitomo electric branch) Two levels of confining pressure (15MPa and 25MPa) were used in order to study the mechanical behavior and brittle-ductile transition of RAC The RAC specimens was at 108 days of age to minimize the effect

of aging on the response It can be seen that at SFA and SMT, the specimen was broken suddenly into two or three pieces while the failure occurs with the propagation of several distributed vertical and inclined cracked at SSF In Fig 2, the maximum axial stress or confined compressive strength and unconfined compressive strength for each confining pressure was pointed out that the peak stress of RAC increases with increasing the confinement

Fig 1 Testing setup for triaxial test

Testing machine

Chamber Load cell

Pump for confining pressure

Moreover, although at uniaxial compression test the obtained result show the

highest value at SSF, the peak stress of SFA

is highest value at all confining pressures

compared to others The Fig 3 shows initial

tangent modulus of RAC at high confinement

pressure levels, it can be seen that the value

of initial tagent modulus of RAC treated with

SFA at confining pressure is higher than

others even at unconfining level the modulus

of this sample is lower than others From the

result of triaxial test, Mohr stress circles were

constructed then it should be drawn the

tangent to the Mohr circles to determine the

initail shear stregnth of RAC as illustrated in

Fig 4 It can be indicated that the initial shear

strength of SSF is higher than others, which

proved that SSF at high confining pressure

level is more vurnarable than SSF, SMT

Fig 2 Maximum stress of RAC under

confined and unconfined compression

Fig 3 Modulus of elasticity of RAC under

unconfined compression and initial tangent modulus of RAC under confining pressure.

Fig 4 Initial shear strength of RAC.

5 CONCLUSIONS

As an attempt to improve the quality of RAC by using treatment solutions of RCA with different of pozzolanic materials was investigated in this study From the experimental results, the important conclusions are summarized as following: Treatment solutions for RCA can enhance compressive strength of RAC considerably, treatment solution type SSF was more effective than others at uniaxial compression stress, but, at high confining pressure its strength was lower than others

Treatment solution for treating RCA significantly affected the mechanical properties of RAC at uniaxial loading and triaxial loading conditions

Pozzolanic materials such as fly ash, silica fume and metakaoline combined with sodium silicate was recommended to treating RCA The results of triaxial compression test were performed at high confinement on RAC specimens, which revealed the significant differences in maximum stress This was attributed to the influence of confinement pressure and treatment solutions

In brief, the results obtained from this study demonstrated the feasibility of this method for applying in real condition in improving strength of RAC However, in order to apply this technique in the field, it is necessary to conduct researches on durability

of concrete In the further study, we will resolve relevant issue related to durability and performance of concrete after treated with treatment solutions

REFERENCE

[1] S.B Huda, M.S Alam, Mechanical behavior of three generations of 100% repeated recycled coarse aggregate concrete, Constr Build Mater 65 (2014) 574–582 [2] H Takahashi, M Ando, DEM simulation of crushing for concrete blocks by mobile crusher, in: AIP Conf Proc., AIP, 2009

[3] Hiroshi TAKAHASHI, Yuki SANDO, Tomoaki SATOMI, Numerical Simulation

on Crushing of Concrete Blocks by Mobile Crusher, in: Proceeding Int Symp Earth Sci Technol 2011, Fukuoka, Japan, 2011