GMABTER - V

LIFE SPAN OF PFA CONCRlETE

5.1 INTRODUCTION

Throughout the history, concern for longevity of buildings and structures has been

evident. Scientists and construction engineers recsgnise that data on service lives of

building materials and components are essential to achieve longevity through

effective selection and use of building materiais. The corrosion of reinforcing steel.

and the resultant cracking and spalling of concrete has always been a serious

problem. In the recent years there have been many attempts made to understand this

problem more clearly and find possible solutions, which are effective, economical

and viable. The cause of such damage is known to be due to the resultant

accumulation of solid corrosion products near the metal concrete interface and

consequent development of hoop tensile stress.

The term service life is usually preferred to durability, because it is more precise and

more in accord with the need for predictions in selection of building materials [270].

ASTM E632, Standard Practice for Developing Accelerated Tests to Aid Prediction

of the Service Life of Building Components and Materials [271], this has been

important in drawing attention to the difficulties of predicting the service life of

rnaterials. US National Bureau of Standards W S ) and Portland Cement Association

(pCA) have made important contributions to the methods of testing for the durability

of concretes exposed to various environmental conditions C272-2741.

5,2 TECHNICAL B ERS OF LIFE SPAN PREDICTION

Despite the availability of many laboratory-based accelerated test methods for

assessing the relative durabiiities of specific building materials, the data obtained

from these methods are seldom adequate for reliability of predicting service life

[275]. Regarding this problem Ramachmdran [276] stated that it is more effective to

fist determine the degradation process and then design the test to produce them.

Larry [275] reiterated that data on sewice life of building materials and components

are essential to the cost-effective selection, use and maintenance of materials.

5.3 DESIGN LIFE OF CONCRETE

Regarding the prediction of service life of materials, Browne [277] reported that

most damage is due to the bursting forces of a significant area of steel surface

corroding along its length. He was less concerned with crack limits in design than

specifying the correct cover and concrete quality. He also stated that estimation of

the time taken for the environment to penetrate to the steel (to) is somewhat easier

than prediction of time for corrosion to cause damage to the concrete (t*), the latter

varying &om months to many yeas depending upon the climatic conditions. He

recommended that,

Design life = to.COz penetration

Ije also recomended that the penetration rate for carbonation could be obtained by

assuming a simple diffusion law:

Where, x = the distance penetrated after t h e , t; and k is asl empirical constmt.

From the data provided by Klopfer 12781 on penetration rates, Browne [277]

predicted that for a 2 0 m cover, using a grade of 35MPa concrete will give a life of

100 years or more.

Various other researchers adopted their own approach to predict the service life of

concrete. Sanion and Green [279] recomended that structural life prediction must

consider the serviceability of the structures. Bazant and Chern [280] predicted creep

and shrinkage deformations with a set of actual measurements. They expressed that

some more work is required to develop this procedure. Serviceability issues related

vibration has been examined by Tallin and Ellingwood [12]. However, their

procedure can be incorporated into a monitoring programme to identifjr the on set of

damage as detected by a change of natural frequency.

~ i l l a r d and Robinson [2821 predicted the life of autoclaved aerated concrete ( M C )

by studying resistance to chemical attack, strength up to 6 years and fire resistance

and recommended that AAC can be given 100 years guarantee. They did not conduct

any mathematical analysis to recommend this, but it has been recommended in a

qualitative manner. Masatoshi Suzuki et a1.[283] proposed probability models for

salt penetration and corrosion of reinforcement. Their analysis showed that the

integrity of RC structures is determined by the local corrosion. However, Browne

[277] reiterated that local corrosion of reinforcement might have a limited effect on

the structural performance. From the electrolytic accelerated corrosion studies, Raju

12841 predicted the life concrete as given by the following equation:

Life factor = Q, I i,,,,

Where, Q, is the total charge to cracking and I,,, is the current obtained from the

polarisation studies. It is already concluded from the present study that electro-

chemical methods yield only approximate results and moreover, the reliability of the

above equation has not been verified with actual rate of corrosion determined by

gravimetric method.

Based on electrolytic accelerated corrosion studies may other researchers have also

made attempts to study the life of materials and structures [285-2881. Their

conclusions mainly depend upon the design of cover to prolong the life span,

however, for the given cover the life span has not been quantified yet from the

laboratory tests.

h the last 10 - 15 years, new specifications and codes of practice have been

introduced for the design and construction of structures. Various international

standards have introduced codal recommendations since 1977 to ensure durable

structures in severe environment. The comparison of various codal provisions has

been shown in Fig.5.1

5.4 SUM Y

From the overall survey it can be stated that all the research efforts have been

directed to predict the life span of concrete taking into consideration of various

deterioration factors. However, some more research efforts have to be initiated

towards the study on quantification of life span of reinforced concrete. Masatoshi et

al.12831 concluded that the mechanism of deterioration was influenced by numerous

complex factors. Under such circumstances, it was more effective to use an

analytical approach based on the available data for quantitative analysis. It can,

however, be inferred that rebar corrosion is the most important phenomenon, which

dictates the life span of RC structures.

Based on the test results of this investigation m attempt has been made to derive an

empirical relationship to predict the life span of PFA-concrete in marine

environment.

5.5 LIFE SPAN OF PFA-CONCRETE

5.5.6 Detinition

Before peeping into the mathematical part of life assessment it is pertinent to discuss

the meaning of life span of concrete structures. Isl fact, it may be possible to define

the life span in sirnple terns. However, the prediction of life span of concrete in

general terms is going to be complex as the number of parameters influencing the

perfornance of concrete are many and also they have interacting influences on

concretes.

Regarding the definition for life span of structures British Standard Document

8711 5323 states that:

Reauired life : The client's assessment of the period the building should last without excessive maintenance or repair.

Design life : The designer's assessment, which will include a factor of safety on the required life.

Expected life : The life predicted by experts.

In general, when steel is exposed to oxygen and water, oxides md hydroxides of iron

cm form depending upon the availability of the above two. h this process the

volume of corrosion products will increase as high as seven times [289]. However,

hydroxides of iron [Fe(OH)z, Fe(OH)3 and Fe(OH)3.3H20] can be formed only in the

presence of excessive water, which will generally be not available in concrete

medium. Therefore, only oxides of iron can be formed [Fe3Q4 & Fe203] whose

volume will increase just more than two times. Due to this increase in volume the

corroded rebars will induce internal bursting pressure. \%en this pressure matches

the tensile strength of concrete the cracks will develop leading to rapid rate of rebar

corrosion. Therefore, it may be considered that when the first corrosion crack forms

on the concrete cover then that is the end of useful life span of structure.

5.6 PRESSURE DUE TO REBAR CORROSION

The pressure distribution in the concrete medium around the corroded rebars was

reported by Brown and Baker [205] and it is shown in Fig.5.2. It is clear iiom the

figure that the pressure (P) developed just at the surface of rebar generates higher

pressures up to '2.2P' around the rebar and the pressure intensity actually

responsible for cracking of concrete is only 'P'. Brown and Baker [205] also

reported that 0.15g of steel loss per cm of bar caused a bursting pressure of 0.45MPa.

Fairbridge [290] reported that for a 6mm bar with 30mm cover, spalling could occur

when the steel has corroded to a depth of about lmm.

5.7 LIFE SPAN ASSESSMENT

Based on the above discussions, the life span of concrete is predicted by equating the

growing bursting pressure to the increasing tensile strength of concretes.

5.7.1 Time Versus Tensile Strength

The relation between cornpressive strength and age has aiready been presented

(54.2.5). This relationship has been established using six set results covered over a

span of one year. The tensile strength of concrete has been established only up to the

age of 90 days. The reasons are also discussed for why a close relationship between

the compressive strength and tensile strength is not established.

The corrosion studies have been conducted using M1 5 grade (Ao mix) concrete only.

Therefore, it was felt that if a close relationship is established between compressive

strength and tensile strength using a set of close range results of M15 grade concrete,

then through that the long-term tensile strength of concrete can be predicted with

reasonable accuracy.

Reference concrete: Based on the compressive strength of mix A, the following

strength-time relationship was established:

S = 10.559 + 4.419 x logit (r = 0.995)

Where, S is compressive strength in MPa and t is age of concrete in days.

. . . (a)

~ a s e d on the compressive sbenn& (S) and split tensile strength (T) of A, mix,

relationships between these two were tried and out of this the possible

solutions are given below:

T = 2.858 x Pog,S - 6.492 (r = 0.996) . . . (b)

and

The above relations gave a good correlation. However, the Eqn.(b) was considered

for further analysis. The verification of that equation has been done and it is

indicated in Table 5.1.

To establish a relation between time versus tensile strength two approaches were

tried. One is to find relation directly fiom the available data of tensile strength up to

90 days and the other is to relate through compressive strength relation with time,

that is, using Eqns.(a) & (b).

Based on the available data, the direct relation between tensile strength and time is

found to be,

And using Eqns. (a) & (b),

T = 2.858 x 1og,(10.559 + 4.419 x loht) - 6.492 . . .(e)

Eqn.(e) is f ~ m d to be complex while comparing the Eqn. (d). Therefore, a

comparison has been done between these two equations md the results me presented

in Table 5.2.

Both the equations yield to very close results and hence the Eqn, (d) has been

er analysis due to its simple fonn.

PFA concrete: To establish the above similar relationship for PFA concrete, the

average compressive and tensile strength values of mixes Al to Ag (10 to 35%

cement replacements) are considered and the average values are given in Table 5.3.

The relation between S and t is given by,

S = 9.83 + 4.60 x lo@

and the relation between T and t is given by,

T = 0.495 + 0.729 x logt (r = 0.935) - (g>

A relation between T and t was tried through compressive strength in terms of time

as done for reference concrete. Here again both the equations yielded very close

results. Therefore, the Eqn. (g) was considered for M e r analysis.

5.8 COWOSION RATE VERSUS TIME

In order to relate the rate of corrosion (CR) with time, various models were tried.

Among them the logistic growth curve had a high correlation (r = 0.986) with the

available data and semi-log relationship showed a lesser correlation (r = 0.7).

However, the semi-log relationship has been considered for er analysis and the

reasons for selecting this model are discussed after the presentation sf the eyation.

5.8.1 Reference Concrete

The Rate of corrosion (CR) of rebar established by gravimetric method was

considered to find the relation between CR and time in years ($). Using four sets of

results (up to 2 years) obtained using reference concrete the following relationship

was found out.

Due to very low rates of corrosion at early ages it is appropriate to express the

corrosion rate model in terms of years. The actual corrosion rate and the predicted

rate for reference concrete are drawn with respect to time and they are shown in Fig.

5.3a.

From the variation of actual corrosion rate it could be Inferred that up to 1% years

the corrosion activities should have taken place in a subdued manner and

subsequently a sort of general corrosion should have started due to which the

corrosion rate has shot up and hence this curve. This has also been confumed fiorn

the physical verification of recovered rebars after various periods of exposure

(54.4.4).

&)bviously after the on set of general conosion the rate of corrosion damage will

become rapid until the entire surface area is covered with the attack. Once the

corrosion products cover the entire surface then they themselves will act as a

protective medium and certainly there would be a reduction in the rate of corrosion

damage. This condition will continue to prevail until the concrete cover cracks due to

bursting pressure developed by the corrosion products. The logistic growth curve,

which showed a high correlation, did not show the realistic behaviour of corrosion

rate in the long m. It took too many decades to attain a reduced rate of corrosion,

which did not match with the expected characteristics of corrosion in then long run.

This may be due to Bidted data. Therefore logistic growth curve shows realistic

corrosion state in the early ages (up to 2 years) and the significance of the proposed

model of corrosion rate lies mainly in its prediction level in the long run rather than

in the short-term period. Nevertheless, the proposed model will hold good only until

the formation of first corrosion crack. ARer the formation of frrst crack, external

agents can find easy access to the rebar. Moreover, due to changes in humidity level

and temperature ranges the scales formed around the rebars will crack thereby

permeating the external agents to reach the bear surface of rebar to cause rapid

damages. Therefore, beyond the formation of fust crack this model can no longer be

applicable.

5.8.2 PFA Concrete

Rate of corrosion of rebar in PFA concrete has been established up to 2 years for

three cases of cement replacements using three PFA samples of in-source. Taking

the average CR of nine test results for each age, the relation between CR and time

(t,) was established and it is given below:

CR = (1.48 1 + 4.303 x logty ) x 1 0-j (r =0.710) . . . (i)

The actual corrosion rate and the predicted rate for PFA concrete are d r a w with

respect to time and they are shown in Fig. 5.3b.

5.9 GOMOSION M T E AMD BURSTING PRESSUBE

The regression models of CR for reference md PFA concretes have been established.

It is already presented that 0.15g of steel loss per centimeter (diameter and length) of

rod can induce a bursting pressure of 0.45MPa. That is, the steel loss due to

corrosion to a depth of 0.06mm can cause an internal pressure of 0.45MPa.

Therefore, the amount of bursting presswe developed with respect to time can be

derived fiom the rate of corrosion and the details are presented below:

a) For reference concrete,

CR = (1.987 + 4.796 x log,tY) x 10" m p y . . . (h)

nerefore, the depth of corrosion (4) in t e r n of age is given by,

(4,= ~CW. dt

= [I.987xty i- 6.156(tyxlog& - t,)]x10'~ mm

b) Similarly for PFA concrete the depth of corrosion is given by,

4 = [I .481 + 4.303(t,xlo%ty - I,)] x mm . . . (k)

'Fke Eqns. (j) & (k) can further be converted into pressure equation as detailed

below:

Bursting pressure (pc) developed due to conosion loss is given by,

a) For reference concrete,

p, = [I .987 + 6.1 569(tylogty - t,)] x 10" x (0.4.510.06)

= (0.0462xtyxlo&t, - 0.0217xtY) MPa . . . (1)

b) For PFA concrete,

p, = (0.023xtyxlo~ty - 0.0155xty) MPMP~ . . . (m)

As and when the intensity of the above bursting pressures match with the respective

split tensile strength of concrete the cracks form and that moment is considered as

the end ofusehl life span of concrete.

If the I3qns.d (for split tensile strength, T) and 1 (for PC) of reference concrete and

Eqns.g (for T) and an (for P 3 are plotted and from their points of convergence the

life span can be found out. The variations of the tensile strength and bursting

pressure have been shown in Figs 5.4a &b for the separate cases.

It is noted from the points of convergence that for conventional concrete the crack

due to rebar corrosion is expected to take place at the age of about 40 years and for

PFA concrete it is expected to take glace at the age of about 66 years. The equations

(d) & (1) and (g) & (m) were also solved by Newton-Raphson method and the values

were 39.94 and 65.94 years respectively. R a t is, the life span of PFA concrete is

extended by about 65% beyond the life span of conventional concrete which may be

considered as a significant benefit that could be derived from the utilisation of PFA.

Such a substantial benefit of using PFA is due to higher tensile strength as well as

higher resistance to rebar corrosion than the performance of reference concrete.

From this discussion it is clear that in order to prolong the life span of a structure at

least any one of the characteristics can be improved. That is, either the tensile

strength or the corrosion resisting property of concrete. Out of this the second aspect

alone can be looked upon due to two reasons. One is that the tensile strength of

concrete cannot be improved substantially by improving the compressive strength

and the second is that due to substantial corrosion loss of steel area flexural cracks

may start forming on the surface. When the flexural cracks are expected to form

before the formation of corrosion cracks then the significance of eonfrning the

bursting pressure is lost.

In this aspect, the loss of cross sectional area of rebars in reference and PFA

concretes are calculated (using equations (j) & (k)) at the end of their life span and

they are estimated to be 27% ( 0 . 7 4 m loss) and 36% (1.00m loss) respectively.

These figures are in the higher range due to which in the actual structures flexural

cracks may start fo g before the formation of corrosion cracks. As per the

assumed defmition the life span actually predicted is going to be reduced due to early

fomation of flexural cracks. Therefore, depending upon the required life span of a

structure, during the t h e of designing the safety factors may be suitably

incorporated (considering the limit states of collapse and corrosion) and accordingly

the material specifications can be derived.

Table 5.1 Tensile Strength from Compressive Strength

Table 5.2 Comparison of Tensile Strength Values

Table 5.3 Average Values of Tensile and Compressive Strengths

(a) Uniaxially on intemediate (b) Biaxially on corner bars bars

FIG. 5.2 Stress Contours Arouud Rebars [205) P = Pressure at rebar surface

-Legend

-U- A c t u a l C o r r o s i o n R a l e -&- P r o p o s e d Mode l

(a)-Reference Concrete

Time in years -

L e g e n d

-42- A c t u a l C o r r o s i o n R a l e

-&- P r o p o s e d Mode l

( b)- PFA Concrete

1 2 3 4 5 6 7 8 9

Time in yeqrs ---t

Fig. 5 . 3 Corrosion Rate Models 204

c 0

--&-Split Tension .s 7 -+Burs l ing Pressure C a,

L i f e S p a n .= L O y e a r s t- C .- A

cl LC T= 0.951+0.523x10&ty '6 2! 3 a V)

2 n

= Q.0462xtyxloghty - 0.0217xt 4" .- C z 3 m

Time in years - Fig. 5 . 4 ~ 1 L i f e A s s e s s m e n t - R e f e r e n c e Concrete

-&-Split Tension -Ch Burs t ing Pressure L i f e Span- 66 y e a r s

I t , 3 b 3'4 3b d2 4k $0 < L 58 6; 6 6 70 7 4 78

Time in years

Fig. 5 . 4 b Life Assessment - PFA Conc re te