“ D E T E R I O R AT I O N O F C O N C R E T E I N N A M R U P BV F C L P L A N T; A C A S E

S T U DY ”

AuthorH i m a n g s h u B h u y a n

Co-AuthorsR u h u l A . M o z u m d e r

S a n t a n u D e b

INTRODUCTION

The study is carried on deterioration of an RC wall with increasing age in an urea manufacturing P.S.U (Public Sector Unit) named Brahmaputra Valley Fertilizer Cooperation Limited (BVFCL), Namrup Assam. BVFCL is located on the bank of the river Dilli in the south-western border of Dibrugarh District in Assam, India. This is the first factory of its kind in India to use associated natural gas as basic raw material for producing nitrogenous fertilizer[1].

During a visit we found spalling of concrete from the walls and formation of crystalline Silk like compound (SLC) as shown in the Figs

Appearance of Reinforced Bars

DETERIORATION OF RC WALLS OF THE TOWER LEADS TO • Loosened concrete with reduced compressive strength• Redistribution of the internal force• Decrease of the reliability• Exposes the reinforcement to environmental attack and results in

corrosion of rebar• Hinder the normal production and also put the safety of its workers at

stake.

OUR EXPERIMENTAL STUDY

Determination of Unknown SLC (Silk Like Compound)

Cause of Spalling of Concrete

We classify our experimental study in two categories:

SLC ANALYSISStudy shows that this compound has some similar behavior as urea. It is water soluble and makes our hand slippery when in contact with moisture. The chemical composition of urea (CH4N2O) is shown in table 1. Again FEG-SEM EDX [5] of this SLC shows the following result in table 2.

Element

Percentage

C 20N 46.67O 26.67

Table 1 Table 2

In order to attain a confirmed result we make two solution one containing urea and another containing SLC. Bothe in acidic medium forms a lemon yellow colour when few drops of para-dimethyl-amino-benzaldehide (PDAB) is added as shown below:

SLC solution Urea Solution

According to M XiaoJie, lemon yellow material will form when para-dimethyl-amino-benzaldehide (PDAB) will react with urea under acidic condition [6].

CAUSE OF SPALLINGUrea is hydroscopic in nature [7]. It can absorb moisture resulting in the formation of concentrated urea solution. The urea solution get absorbed by the walls of the tower and enters the pores. When the evaporation of the absorbed urea solution takes place, it results in the formation of urea crystals in the pores (sub-efflorescence) of the walls and also on its surface (efflorescence). Such formation of crystals is taking place for many years till date. C.W Correns derived an equation [9; 10] for quantifying the crystal pressure as P=(RT/Vs)ln(c/cs)

CONCLUSIONThe study revealed that spalling of concrete in BVFCL‘s Urea III prilling tower was primarily due to the formation of urea crystals inside the pores of the walls. The Silk like compound formed on the walls are only urea crystals due to efflorescence. This crystal formation inside the walls exert stress on concrete which resulted in cracks.

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