Performance of overlay concrete depends on the strength of the bond. There are numbers of factors that influence the bond strength of overlay concrete. These factors are discussed in the following sections.
Fig.1: Bonded Overlay Concrete on Bridge Decks
- Factors Affecting Bond Strength of Overlay Concrete
- Condition of Deck Concrete Affecting Bond Strength of Overlay Concrete
- Extent of Damage Affecting Bond Strength of Overlay Concrete
- Concrete Surface Preparation for Placement of Overlay Concrete
- Placement and Compaction of Overlay Concrete
- Curing of Placed Overlay Concrete for Good Bond Strength
- Overlay Concrete Joints Affecting Bond Strength
- Minimal Full Depth Cracks Affecting Bond Strength of Overlay Concrete
Factors Affecting Bond Strength of Overlay Concrete
Following are the different factors which affect bond strength of overlay concrete on bridge decks:
- Condition of deck concrete
- Extent of the damage because of the removal of loose concrete
- Concrete surface preparation for the placement of overlay concrete
- The placement and compaction of overlay concrete
- Curing of placed overlay concrete
- Overlay concrete joints
- Minimal full depth cracks
Condition of Deck Concrete Affecting Bond Strength of Overlay Concrete
The condition of concrete for which overlay concrete is applied substantially influence the strength of the bond. The better the condition of exiting concrete the better the bond will be.
The condition of existing concrete may be evaluated by number of signs such as, if the number of cracks is not great, embedded reinforcement is not corroded, and concrete is not damaged and degraded due to alkali silica reaction, freezing and thawing. Then these signs mean that the concrete is in good condition and has reasonable strength.
In contrary, if concrete is heavily cracked, as shown in Figure-4, it will not be able to provide good bond strength and cracks in the deck will probably extend into the overlay concrete, which consequently decreases bond strength.
The size of steel reinforcement increases when it corrodes and this lead to initiate cracks and eventually decline bond strength.
It is reported that, if compressive strength of concrete surface is not less than 27.57 MPa, it can provide adequate tensile bond strength. Figure-2 show the deck concrete of Oklahoma Bridge which is in good condition.
Fig.2: Oklahoma Bridge Deck Concrete in Good Condition
Fig.3: Tatamy Bridge Deck is Heavily Deteriorated
Fig.4: Cracked Bridge Deck
Fig.5: Deteriorated Bridge Deck
Extent of Damage Affecting Bond Strength of Overlay Concrete
Eliminating and removing loose or damaged concrete is the first step in overlay concrete application. Deck concrete might have been deteriorated due to chloride ingression or other harmful elements.
The quantity of concrete which need to be get ride off is based on the height of deteriorated concrete and the loose concrete depth is not the same along the length of the deck. Figure-6 shows the damaged surface of a bridge deck.
It is possible that the entire depth of existing concrete is deteriorated and required to be eliminated but there are locations that concrete is slightly damaged and merely scratching is adequate.
In the former case, the location of removed concrete is quite large and hence should be patched prior to overlay concrete pouring but smaller depth areas can be dealt with by overlay concrete. Figure-7 show the removal of deteriorated concrete bridge deck.
There are situations in which the concrete is not damaged but removing considerable depth of concrete is needed for instance to make rooms for large overlay concrete thickness and to provide and enhance the grade.
Fig.6: Deteriorated Bridge Deck
Fig.7: Deteriorated Concrete from Bridge Deck is Removed and Surface is Prepared for Placing Overlay Concrete
There are various choices of techniques or methods by which deteriorated concrete can be removed. Each method has specific capacity regarding the depth of concrete that required to be removed.
Various concrete removal methods along with their applications and the ultimate concrete depth which can be removed by a certain method are provided in Table-1.
Table 1: Concrete Removal Method, Maximum Removal Depth, And Their Applications
|Concrete removal technique||Maximum removal concrete depth, mm||Applications|
|Grit blast, Figure-8||2||Texturing and cleaning of concrete surface, removing fracture concrete after other method has been applied to remove concrete|
|Shot blast, Figure-9||6||Texturing and cleaning of concrete surface, removing fracture concrete after other method has been applied to remove concrete|
|Diamond grind, Figure-10||Rebar depth||Concrete depth is equal to rebar depth but the bar should not be hit|
|Scarification, Figure-11||Rebar depth||Concrete depth is equal to rebar depth but the bar should not be hit|
|Hydro demolition, Figure-12||1 to 0.5*depth||Texturing and cleaning of concrete surface, remove concrete around reinforcement bars|
|Pneumatic hammer, Figure-13||12 to entire depth||The removal of the entire height of concrete|
There are cases in which two techniques are combined to accomplish the work. For example, to remove half depth of the bridge deck, the application of scarification after that hydro demolition is substantially effective.
Each method or combination of them should be selected in such a way that the damage due to the removal of concrete should be minimal.
Fig.8: Grit Blasting
Fig.9: Shot Blasting Bridge Deck to Clean and Texturing and Hence Obtain Adequate Bond Strength
Fig.10: Diamond Grinding of Concrete for Overlay Concrete Placement
Fig.11: Scarification of Bridge Deck
Fig.12: Hydro Demolition of Bridge Deck
Fig.13: Pneumatic Hammer for Removal of Damaged Concrete
Concrete Surface Preparation for Placement of Overlay Concrete
Bridge deck surface on which overlay concrete is placed need to be sound, textured, clean, and damp surface. These requirements can be obtained by using suitable means and procedures at surface preparation stage.
To conduct surface preparations in the best possible way, the required specification must be clearly stated and this usually provided by applicable Codes.
Methods utilized to prepare concrete surface involves grit blast, shot blast, hydro demolition, power washing, and air blast.
The effectiveness of Grit blast technique not only depends on the experience and skill of labor but also depend on the time allocated for the wok.
As far as shot blast is concern, it removes parts of concrete by rubbing and fractured concrete is vacuumed. The concrete surface can be prepared appropriately using shot blast by controlling both speed and number of shot blaster passes.
Concrete surface can be prepared using hydro demolition technique and if the surface is intended to be cleaned from debris, low water pressure blast is suitable option to choose.
ACI committee 1993 recommends various tests to specify a texturing and cleaning work. Cleaning and texturing procedure obtained through those tests will provide a surface that result in bond strength larger than the minimum required bond strength.
Placement and Compaction of Overlay Concrete
It is substantially significant to place and compact overlay concrete to obtain high bond strength in the bonded overlay concrete. The procedure of overlay placement includes pouring overlay concrete on the surface, which is prepared in advance. After that, a suitable mean is applied to compact the overlay concrete and finish the surface.
Overlay concrete vibration need to be considered because it is one of the important factor which result in high tensile bond strength.
The thickness of overlay concrete should be considered when method of vibration is selected because concrete vibration from its surface is effective up to 6.35cm otherwise internal vibration application is a must.
Curing of Placed Overlay Concrete for Good Bond Strength
Curing of overlay concrete is hugely influential on the bond strength because the tensile bond strength at its best is equal to the overlay concrete strength, and the overlay concrete strength depend on curing quality and adequateness.
From this it becomes quite clear that, improper curing can jeopardize the functionality of the entire work. Thus, curing need to be taken care of thoroughly and completely.
Suitable piece of cloth such as burlap should be wetted and put on the placed concrete to prevent evaporation and avoid plastic shrinkage cracks. The cloth should maintain its wetness and this can be achieved though placing polyethylene on the cloth.
It is mandatory to cure latex modified concrete for two days, and hybrid concrete cement overlay should be curried for minim of three days and maximum of seven days.
Other types of concrete should be curried as it is required like very high early strength latex modified concrete which only needs three hours of curing before it can be used.
If it is found out that the rate of evaporation after concrete placement till the beginning of curing surpasses normal rate of evaporation, measures such as batching concrete at low temperature, fogging the air above concrete, and providing obstacles to avoid air must be considered to prevent detrimental effect of excessive loss of water content.
Overlay Concrete Joints Affecting Bond Strength
Generally, most of bridges possess control joints to make rooms for bridge span movements. When overlay concrete is applied for such bridge deck, joints in the overlay should be at the same location as joints in the deck and if overly concrete is placed over expansion joints of the deck, the overlay will delaminate along the joint.
Construction joints, longitudinal construction joints along the deck of the bridge between overlay of two adjacent lanes are usually formed when overlay concrete is placed in one lane and traffics are permitted in the adjacent lane.
The construction lane can become weak point of overlay concrete and lead to overlay delamination and deterioration. So, it is highly recommended to prevent longitudinal construction joints during overlay construction.
Fig.14: Bridge Deck Expansion Joints
Minimal Full Depth Cracks Affecting Bond Strength of Overlay Concrete
After the placement of overlay concrete, the initiation of plastic shrinkage in overlay concrete as a consequently of water evaporation is very likely.
The plastic shrinkage cracks may develop across the entire depth of overly concrete due to drying shrinkage. This will exert stresses on the bond between concrete bridge deck and overlay concrete.
These stresses are not uniform along the bridge deck and they are largest along full overly concrete depth, around perimeter of overlay, and along construction joints.
The stresses imposed on overlay concrete due to cracks lead to concrete delamination and the development of delaminated area may increase and become larger with aging of overlay concrete.
Fig.15: Plastic Shrinkage Cracks
This type of problem that affects overlay bond strength cannot be tackled unless the overlay concrete is cured properly to avoid full depth cracks. Figure-15 show plastic shrinkage cracks.