Fig.1: Use of Latex Modified Hybrid Concrete Cement Overlay for Bridge Deck
Causes of Failure of Hybrid Concrete Cement Overlay Over Bridge Decks:
Following are the factors which causes failure of hybrid concrete cement overlay over bridge decks:- Quality of contractor work
- Material properties of overlay concrete
- Overlay concrete bond strength
- Thickness of overlay concrete
- Surface features of overlay concrete
- Overlay concrete protection features
- Other issues
Quality of Contractor Work for Hybrid Concrete Cement Overlay
The construction of overlay concrete is not an easy task. Suitable construction decisions regarding concrete removal selection and application, tools and procedures used for surface preparation, proportions of the mixture, and pouring and curing technique. It is considerably important for the contractor to have required skills, experience, and needed equipments. Otherwise not only does the contractor not perform well but construction issues will be higher. It is of prime importance that the contractor is rewarded based on final constructed product performance instead of using the determined specifications. This is because, it is more likely that the overlay concrete cement will perform better and last for longer period.Material Properties of Hybrid overlay concrete
Proportions of Overlay Mixture Materials that have been employed for bridge deck overlay includes latex modified concrete, low water to cement ratio hydraulic cement concrete mixtures, and Portland cement mixture in which a specific amount of cement is replaced by silica fume, fly ash, or ground granulated blast furnace slag. For the condition where the overlays need to be used after 3 hour of curing at temperature of 10oC and larger, severely high strength latex modified concrete is an appropriate choice.
Fig.2: Latex Modified Cement After One Day
However, if the temperature is equal or greater than 26.67 oC, then it is required to provide citric acid admixture to increase the time of concrete setting. Moreover, there are number of concrete types. For example, latex modified concrete produced by adding latex to Type III Portland cement, Type II Portland cement blended with seven percent of silica fume, and extremely high early strength concrete. Such types of concrete are employed or appropriate for conditions where vehicle movements on the bridge are resumed only after 24 hours of curing. Latex admixture is blended with concrete admixture at around 13.29 liter per a bag of cement. If cement is mixed with silica fume, it is necessary to provide superplasticizer (high range water reducing admixture) because so significant is the admixture that creates required workability and spread particles of silica fume all over the mixture.
Fig.3: Latex Modified Hybrid Concrete Overlay Applied for Deck of a Bridge
It is worth mentioning that, the styrene butadiene latex raises the amount of air in concrete that is why admixtures, which can reduce quantity in concrete mixture, may be necessary occasionally, and when the mixture is designed to contain certain amount of air, air entraining agent is not required because the latex admixture play the same role. As far as aggregate is concerned, it needs to meet the condition of ASTM C33. Common overly mixtures are provided in Table-1.Table-1: Usual Mixture Proportions for Overlay Concrete
| Mixture | LMC1 | Silica fume | Fly ash | Ground granulated blast furnace slag | HE -LMC2 | VE -LMC3 |
| Type II cement | 390.376 | 390.376 | 312.063 | 234.344 | 483.520 | 390.376 |
| Type cement | II | II | II | II | III | Rapid set |
| Silica fume | --- | 27.290 | 19.578 | 19.578 | --- | --- |
| Fly ash | --- | --- | 58.734 | --- | --- | --- |
| Slag | --- | --- | --- | 136.453 | --- | --- |
| Fine aggregate | 920.765 | 752.867 | 801.516 | 812.195 | 831.773 | 920.765 |
| Coarse aggregate | 704.219 | 899.407 | 895.847 | 895.847 | 677.521 | 704.219 |
| Water | 86.618 | 167.304 | 150.692 | 150.692 | 97.297 | 86.618 |
| Air | 2.966 | 4.152 | 4.152 | 4.152 | 2.966 | 2.966 |
| Admixture | Latex4 | HRWR5 | HRWR | HRWR | Latex | Latex |
Compressive Strength of Hybrid Concrete Cement Overlay
The constituents of overlay concrete are specified in such a way that the least required compressive strength is achieved. At least the concrete compressive strength for overlay concrete should be 21 MPa, and this value not only demonstrates that the achieved concrete strength is satisfactory but also shows that concrete have gained necessary bond strength. If the bridge is used in short time after placing overlay concrete for example after 5 hours, it is necessary to use extremely high strength latex modified concrete that reportedly achieve compressive strength of 28.06 MPa after 5 hours of curing. However, other types of overlay concrete can be used when the bridge is opened to traffic after 24 hours.Permeability of Hybrid Concrete Cement Overlay
Overlay concrete must reach ultimate possible permeability according to AASHTO 1995, and the ability of overlay concrete to withstand chloride ion ingression is specified by maximum permeability. This in return, provides resistance against corrosion of steels embedded in bridge deck. Because, the permeability is declined with time, so overlay concrete permeability at the age of 28 days do not give a representative result of a later times. Therefore, accelerated curing method should be applied to determine the permeability. Lastly, it should be said that both overlay concrete thickness and ingredients influence the overlay permeability.
Fig.4: Permeability Provided by Connected Pores
Shrinkage of Hybrid Concrete Cement Overlay
Shrinkage in overlay leads to initiate cracks and consequently affect the bond performance. Autogenously, plastic, and dying shrinkages are the main cause of cracking in overlay concrete. It is claimed that, cracks due to shrinkage of concrete can be declined or removed if the largest variation of length at 28 days of dying is kept smaller than of 0.04 percent.
Fig.5: Shrinkage Cracks of Overlay that Placed Over Bridge Deck
Overlay Concrete Bond Strength
The strength of the bond between overlay and bridge deck is considerably significant and it is hugely influenced by strength of both overlay and deck concrete, surface preparation, pouring of overlay concrete, overlay concrete curing. If the bond strength between bridge deck and overly is large, the whole bridge structure will be protected from aggressive elements. The failure in overlay concrete is commonly occur between overlay and bridge deck, below bridge deck overlay interface, above bridge deck overlay interface, or it might involve different positions of possible failures. There are various factors that influence the strength of the bond between overlay and bridge deck concrete includes compaction and curing f overlay concrete, the extent of the deterioration of the surface due to removing of concrete from bridge deck surface, whether the condition of deck concrete is good or not, and deck preparation.
Fig.6: Overlay Concrete Over Bridge Deck
Fig.7: Overlay and Bridge Deck
Thickness of Overlay Concrete
The thickness of overlay concrete laid over bridge deck is taken as 38mm for design purposes and it is required to select proper aggregate size in order to achieve the designated thickness. It is suitable to choose an aggregate size that is 1/3 of the overlay thickness. When a thickness of smaller than 31mm is needed, mortar should be employed rather than concrete. Practically, overlay thickness may reach 50.8mm and in this situation the mixture must be designed again by either selecting larger aggregate size or, if possible, declining quantity of cement in the mixture. This measure is needed to reduce overlay concrete shrinkage that lead to serious issues if not controlled properly. To accommodate a deck surface area that is different from its surroundings, enhance drainage system, and provide better vehicle travel quality, the overlay thickness is required to be changed. In this situation, it is likely that a thickness of overlay exceeds the maximum allowable thickness and unless it is not doable, necessary measures should be taken into consideration in case larger thickness is applied. It is should be bear in mind that shrinkage in mortar is larger than that of concrete, so if mortar is used the effect of shrinkage has to be considered and dealt with properly.
Fig.8: Badly Deteriorated Bridge Deck
Fig.9: Using Thick Overlay Concrete for a Bridge in New York, USA
Surface Features of Overlay Concrete
If the design and construction of overlay concrete is carried out in considerably good way, the required characteristic of overlay concrete surface for example good appearance of the surface, properly established drainage system, resistance against sliding of cars, and good vehicle quality travel may be obtained. Resistance against sliding can be easily obtained through cutting grooves, Figure 10, on the surface of hardened concrete. Moreover, both the breadth and the depth of the groove are usually equal to 3.175mm and the spacing between grooves is recommended to be around 19mm. Successful drainage system and extremely satisfactory ride quality is provided through the compaction of the freshly placed overlay concrete surface.
Fig.10: Grooving Overlay Surface to Improve Skid Resistance
Fig.11: Skid Resistance Bridge Deck Surface
Overlay Concrete Protection Features
It is clear that the deck of a bridge is greatly safeguarded by overlay concrete. The effectiveness of the safety or protection offered by overly concrete is based on various parameters and each parameter influence overlay protection in a different rate compare to other parameters. Overlay thickness, shrinkage, permeability, and cracks are those factors that overlay protection depends on in addition to construction joints that have its own share effect on protection characteristic of overlay concrete. There are several measures which can be used to enhance those parameters that affect overlay concrete protection properties. If these factors are improved, the overlay protection characteristic will improved greatly. For example, hybrid concrete cement overlay do not provide desirable permeability but this can be tackled by providing either supplementary materials such as silica fume, fly ash, ground granulated blast furnace slag or adding styrene butadiene latex. Another strategy to improve overlay protection property is use large cover to protect reinforcement. This will increase the overall thickness of overlay which needs to be considered in order to avoid problems caused by concrete shrinkage. Concrete shrinkage lead to develop cracks and this will allow the ingression of aggressive elements such as chloride ions and consequently deteriorate overlay and decrease its protection feature. So, keeping shrinkage as minimum as possible is a great method to increase protection characteristics of overlay.
Fig.12: Chloride Attack on Steel Through Cracks
Apart from cracks caused by shrinkage, overlay concrete may cracks due to cracks in the deck, shrinkage and creep the structure if the structure is not aged, ambient temperature. Similar to cracks, construction joints create a weak point through which harmful materials can penetrate and hence jeopardize protection system of overlay concrete.
Fig.13: Silica Fume Overlay Applied for a Bridge Deck