Deterioration of Concrete

Introduction

Effect

LeakageD efect

D am age

D eterio ra tion

D esign

M ateria ls

C onstructionS ettlem ent

D eflection

W ear

S pa lling

D is in tegra tion

C racking

D elam ination

S ca ling

Cause

O verload ing

C hem ica l S p ill

E arthquake

F ire

Freeze-T haw

E rosion

C orrosion o fM eta ls

A lka li-A ggregateR eaction

S u lfa te A ttack

It has been observed that RCC has not proved to be durabledue to large number of factors.

All concrete in service will be subjected to chemical andphysical changes.

In almost all the field problemspenetration of water and/oraggressive chemicals duringservice life of structures, is theprimary cause of deterioration.

A well constituted, properly compacted, and cured concrete used in RCC continuesto be substantially water tight and durable as long as capillary pores and micro-cracks in the interior do not become interconnected pathways leading to surface of concrete.

Fig. (a) Porous but impermeable concrete, (b) Porous but permeable concrete

Deterioration of Concrete

The general approach for durability is to demand impermeability of concrete as the first line of defence against any of the deterioration process.

It is difficult to generalize the causes of deterioration due to interacting nature of various degradation mechanisms. However, they can be grouped into chemical and physical attacks.

The microstructure of concrete material is continuously changing in response to penetration of water, carbon dioxide, oxygen and aggressive ions at a rate, which is influenced by local conditions of temperature, humidity and pressure.

Concrete Deterioration

Concrete Deterioration

Holistic Models of Deterioration of RCC

Model-I In this holistic model of deterioration of concrete, the deterioration process is considered in two stages:

During the first stage, due to loading and environmental weathering effects the voids and micro-cracks in the interfacial zones become interlinked. When the interlinked network of micro-cracks gets connected to any cracks present on the surface, this provides primary mechanism of fluid transport into the concrete.

The above leads to the beginning of second stage of deterioration during which water and chemicals are able to penetrate easily into concrete. This causes various physical-chemical interactions as a result of which, the material eventually undergoes considerable damage.

Holistic Model –I of concrete Deterioration from Environmental Effects

Holistic Model-II

According to this model, there are three stages, namely (i) Gradual loss of water tightness, (ii) Initiation of damage and (iii) Propagation of damage.

Holistic Model-III

This model considers the co-existence of following three main elements: (i) Interconnected porosity, (ii) Exposure to aggressive agents and (iii) Intermittent presence of water.

In the absence of any of three elements, damage to RCC will not occur.

Permeability of Concrete

Permeability of Concrete is related to:

Permeability of cement paste is a function of water-cement ratio.

Portland cement requires about 0.23 W/c ratio for hydration. However, for achieving desired workability, excess water is required. This extra water after completion of hydration leaves pores, called capillaries in hardened concrete.

Capillary Porosity

Changes in CapillaryPorosity with varyingDegree of hydration.

Capillary Porosity

Changes in capillary Porosity with varyingw/c ratio.

As the w/c ratio increases beyond the minimum required, there is more capillary porosity.

Capillary Porosity

Curing time required for various w/c ratios Typical reduction in permeability of cement paste with progress of hydration

In a well constituted, adequately compacted and cured concrete with low water-cementRatio, volume of capillary pores gets reduced and become discontinuous due to Expansive hydrated gel, which significantly reduces the permeability.