Dowel bars in concrete slab

Concrete has to face lot of hostile environmental forces. Concrete can expand or contract because of temperature, stresses due to various kind of load etc to maintain functional capacity in such way so that it does not get cracked, allow transfer of load without faulting at joints, in practice this is achieved by small steel rods called Dowel bars.

corrosion risk -dowel bars
                           Dowel bar

Dowel bars are place at transverse joints of concrete pavements to provide a connection between two slabs. Dowel bars are used in new construction like roads and rehabilitation projects such as slab replacement.

  1. Dowel bars are the device through which wheel loads are transferred from one slab of pavement to the next.
  2. Dowel Bars will significantly reduce the magnitude of the stresses and deflections at the slab joints.
  3. Dowel Bars prolong the useful life of concrete pavements.

 

Dowel bars  will extend service life of the project by improving load transfer and prevent faults at joints. Dowel bar of  30, 32, 36, 38 & 40 mm  Dia and 510 mm long are generally used.

 

Higher Corrosion Riskcorroded dowel bar in slab

Dowel bars are at higher risk of corrosion due to following reasons:

  1. Joints allow direct passage to aggressive agents like oxygen, moisture, chloride etc to the surface of dowel bar. It is further aggravated due to direct exposure to high and low temperature.
  2. Bond between dowel bar and concrete also allow easy diffusion to aggressive ions due to cyclic horizontal movements between the slabs.

 

Corrosion prevention

Dowel bars should be protected from corrosion although joints are sealed to keep water penetration to a minimum; water will seep over time and may corrode unprotected bars.

Typically dowel bars are protected from corrosion by application of epoxy coating or stainless steel clad bars. Epoxy coating’s smooth and almost friction-less surface provides easy horizontal movement in concrete so that slabs move independently which prevents slabs from excess stresses.

Predicting Durability and building life expectancy

Concrete is one of the most used engineering materials on tonnage basis. Life cycle Cost of projects depends on durability of building materials and components hence durable concrete is very very important with regards to structure’s life span. Durability can be defined as material’s ability to resist environmental (internal & external) conditions while maintaining desired engineering properties.

Factors affecting durability of concrete structure are divided into two parts:

  1. Concrete degradation
  2. Rebar corrosion

Concrete durability depends on :

factors affecting durability of concrete

Contrary to common belief, concrete is a complex composite material, whose structure and properties can change over time.

It is generally recognized that the environmental degradation of the concrete infrastructure is a serious, large scale and costly problem in many parts of the world

Transport mechanism of environmental degradents in concrete

Some weaknesses or new factors of degradation added to this because of operational practices eg.

  1. Concrete mix esp at site
  2. w/c ratio and its distribution
  3. vibration & mixing uniformity
  4. curing conditions
  5. nature of bond between cement and aggregate etc

Concrete vulnerability variable-some well-known factors are listed in above two figures- are huge in numbers so  quantitative estimation and control of durability and retaining engineering properties at a desired level becomes very complex task, a model and a software to carry out accurate calculations was much needed  to simplify this.

Designing reinforced concrete structure wrt various concrete mix, exposure condition, ingredient & life cycle cost and expected service life is very complex due to huge numbers of variables. To tackle this Life-365 consortium was formed in USA by various concrete related organizations so that collective knowledge & expertise can be combined to develop life-365 service life prediction model which allows users to input local concrete-chloride profile to customize the model to their worldwide location and environments.

You can download Life-365 service life prediction here : Life-365

Life-365 is software designed to estimate the service life and life-cycle costs of alternative concrete mixture designs proportions and life-cycle costs of alternative concrete mixture designs proportions and corrosion protection systems.  It follows research-based methodology developed by the Life-365  Consortium I and II groups of companies, that gives estimates on the effects of design, chloride exposure, environmental temperature, high-performance concrete mixture proportions, surface barriers, and steel types on the service life and life-cycle cost of steel-reinforced concrete structures.

This simple and transparent model provides a fundamental tool for design consultants to use for estimating the service life and life cycle costs of alternate protection systems in their design of steel-reinforced concrete structures that will be exposed to chlorides.

 

Concrete Coatings-limitations

Various external hostile environmental substances, such as, water, carbon dioxide, oxygen, chlorides, sulphides and biological organisms are transported from the atmosphere into the concrete and attack steel and concrete in different mechanisms causing premature deterioration of reinforced concrete challenging its durability resulting in premature failure of the structures.

  1. Concrete is a porous material having high gas, vapour and liquid permeability leading to deterioration of reinforced concrete structures. chemicals can penetrate the pores and attack the paste. The paste and aggregate can also be worn down by physical impact and abrasion. Water can penetrate concrete, freeze and expand inside it when the temperature drops, and ultimately weaken the concrete from within. In addition, if the concrete has reinforcing steel bar (rebar) to impart additional strength and other properties, the rebar can corrode if moisture, oxygen and chloride ions penetrate the concrete. Corrosion of rebar contributes to the deterioration of concrete.

2.  The protection of concrete should actually begin at the conceptual stage and meticulous strategies are adopted            for protecting the concrete from both internal and external environments. Various coating materials and                       application methods for concrete surface repair and strengthening have been developed. However, selection                 criterion for these materials has not been established yet at the current moment. Selecting procedures of                       concrete coating materials must focus on deteriorating mechanisms diagnosed carefully by the conditions of                 target structures. For instance, in case of salt damage, repair policy should consider corrosion environment and          deteriorating condition to determine symptomatic indications such as, 1) removal of permeated chloride ions, 2)           penetration block of chloride ions, moisture, and oxygen, 3)derusting of rebar, 4) corrosion-control method                (coating or potential control). However, it is still ambiguous to determine which is the best material and coating          system, because there is not enough, durability data to estimate.

concrete coatings
concrete coatings

 

3.  it should be noted that all resin materials are not totally resistant and impermeable to all aggressive agents                  and do not provide a total protection. Chemical/physical degradation of resins and debonding of coatings are              the major phenomena affecting the durability of surface protection. The mechanisms of destructive processes             in such heterogeneous materials as resin composites are complicated and not completely understood.                             Degradation of resins mainly involves swelling, dissolution and scission of molecular chain bonds. A wide                     variety of reactions is possible for resin degradation. The transport of gases and liquids aggressive to substrate             into or through the coating is the major problem of its delamination. There are many parameters that                             influence  the deterioration process of coatings, such as chemical agents, temperature, solar radiation,                           pressure, abrasion, cyclic temperature-moisture changes etc. All these parameters can occur simultaneously or             they can be complementary to one another.

  1. Diffusion rate of corrosive ingredients is higher than FBEC

5.  Concrete coatings suffers from poor durability of the coating and loss of corrosion protection in the areas                       where the coating is damaged

  1. Concrete requires continuous maintenance and therefore recurring cost.