CHEMICAL DETERIORATION OF CONCRETE

G.V Prasad

I M.Tech(Structural Engineering)

NITK, Surathkal.

INTRODUCTION

RCC - Large variety of structures

Potential threat

Durable concrete, need of the day

Concrete, heterogeneous material –environmental dependence

Chemical attack

Major Phenomena

•Sulphate Attack

•Alkali Silica Reaction

•Corrosion of Reinforcing Steel

Sulphate attack by aqueous solutions

Rate and degree of attack- • available sulphate• presence of water• composition of the cement• characteristics of concrete

Gradual loss of strength

The severest attack occurs where one side is exposed to sulphate solutions and evaporation can take place on the other, e.g..Retaining walls

Occurrence of available salts• Frequently in soils and ground waters• Surface deposits –sodium sulphates ,magnesium sulphates and calcium sulphates• Marshy grounds ,sewage

Sea water attack• Very high concentration• Much concrete in sea water is partly immersed• Problem gets complicated –frost action, erosion, abrasion Corrosion

Mechanism of the reaction

•Chemical reaction - sulphate ion and hydrated calcium aluminates and /or the calcium hydroxide components

•Products – ettringite and gypsum

•Development of stresses

•Magnesium sulphate is more aggressive than sodium sulphate

and it operates in separate reaction that can be destructive

•This possible second reaction may account for cases of deterioration

Significance of sulphate concentrations

1. Negligible attack upto 150mm sulphate in groundwaters or up to 0.1percent sulphate in soil

2. Mild but positive where the corresponding values are 150 to 1000 ppm and 0.1 to 0.2 percent

3.Considerable attack at 1000 –2000ppm and 0.2 to 0.5 percent 4.severe over 2000 ppm and over 0.50 percent

• Salt concentrations may vary seasonally with amount of rainfall and fluctuations in water table

• Concrete elements partly immersed – severe attack ,even when sulphate contents of test samples are relatively low

• Concrete elements completely immersed – the degree of severity may be somewhat less than that predicted by concentration criteria

• Exposed surface – relatively low drying environment , attack may be slower than that would be predicted by concentration

ALKALI AGGREGATE REACTION

-During this process, expansive componds form – over long periods of time-Expansion –fissures,intensify the action by weathering and freeze thaw cycles

ASR development-aggregate and cement type-Water cement ratio -moisture condition of the concrete while in

service

Reaction – siliceous minerals in aggregates and OH- ions in the Cement paste

Alkalies ( from cement , mixing water ,environment) increase OH- ion concentration in the concrete

Gel – expands ,generating pressures – cracking

Damage – invisible to the naked eye for years

Identification

Minimization of the Effects of ASR

Use of low alkali cements

Use of supplementary siliceous materials- Blended cements –fly ash,natural pozzolans,calcined clay , silica fume ,slag- lithium admixtures

CORROSION OF STEEL IN CONCRETE

Ca(OH)2 –pH 13 ,prevents the corrosion –forms a thin protective film of iron oxide

Passive iron oxide layer – destroyed - ph 11 and carbonation lowers pH about 9

Corrosion – expansion, cracking and eventually Spalling of the cover

Mechanism involved in Concrete deterioration by Corrosion of Embedded Steel

- Electro-chemical action ,when two dissimilar metals are in electrical contact in the presence of moisture

and oxygen

- The process takes place in steel alone, because of differences in electro chemical potential on the

surface

Control of Corrosion

-permeability of concrete -low water cement ratio-adequate cement-aggregate size and grading

-use of admixtures-reinforcing bar coatings and cathodic protection

CONCLUSIONS

•It is important for the assessment of any cause of concrete deterioration to be dependable.

• It should not be based on assumptions drawn from inconclusive evidence, as the performance of materials in new construction, the reliability of tests and the approach to the mitigation of the effects on structures will all be based upon the diagnosis reached.

•Controlling the risk of concrete deterioration by chemical attack may be achieved in number of ways i.e. by limiting the content of reactive alkalies, reduce water cement ratio and permeability,use of admixtures, reducing chloride content.

REFERENCES

Dayaratnam, P., and Ramana Rao, N.V., (1997), International Conference on Maintenance of Concrete Structures, JNTU, Hyderabad. Mehta, P.K., and Paulo Monteiro, J.M., (1997), Concrete Micro Structure Properties and Materials, Indian Edition, Indian Concrete Institute, Chennai, pp.165-167. Neville, A.M., and Brooks, J.J., (1999), Concrete Technology, International Student Edition, Addison Wesley Longman, Inc., Harlow, U.K., pp. 275-276.

Rachel Detwiler, Guest Lecture on Alkali Silica Reaction at LMCC, 2003

Swenson, E.G., (1971), an Article on Concrete in Sulphate Environments, CBD-136,

Thank U