High Performance Concrete (HPC)
The permeability of HPC is significantly lower than that of ordinary concrete. Lower permeability concrete reduces the ability of chlorides to attack the reinforcing steel and cause corrosion. HPC can also aid in the mitigation of ASR/DEF through the reduction of total alkali content and reduced heat generation. Item 421-4.2.6 Mix Design Options 3-5 utilize other SCM’s to form a ternary blend of cementitious material to achieve the ASR/DEF mitigation; however, that may not be the desired mix design in bridge decks. Silica Fume has a high water demand which can cause plastic shrinkage cracks if curing practices are not closely followed. Silica Fume is not recommended in bridge decks.
The selection of HPC for bridge decks is an additional corrosion protection measure; curing practices must be closely adhered to achieve the desired result.
Item 421, Hydraulic Cement Concrete covers the requirements for HPC.
The designer must indicate on the plans which elements require HPC. There are specific Bid Items for HPC.
- Bridge Slabs, Decks, & Rails - When indicated, specify Class "S" (HPC) for bridge slab, CIP portions of decks and Class "C" (HPC) for all concrete bridge railing elements.
- This includes all cast-in-place superstructure concrete such as cast-in-place slab spans, box culverts that require Class "S" concrete in the top slab, as well as slabs cast on top of box beams, slab beams.
- When using HPC in the bridge slab or deck, also specify HPC in the bridge approach slab if present.
- Substructure - When indicated, specify Class "C" (HPC) for all substructure elements.
- Applies to all abutments, bent caps and columns regardless of their locations relative to bridge expansion joints.
- Prestressed concrete beams - For all precast beams, Item 424 requires concrete mixes that are in accordance with Item 421, effectively the same as HPC mixes. Therefore, specifically requiring the use of HPC in precast concrete beams is not necessary.
Corrosion Resistant Reinforcing
- Epoxy-Coated Reinforcing: Epoxy-coated reinforcement is a fusion-bonded coating on the reinforcing steel that provides protection from oxidation. Epoxy-coated reinforcement increases the time to initial corrosion as compared to uncoated reinforcement. It is not intended to protect exposed steel, as the coating is subject to degradation from ultraviolet light. Care must be taken not to damage the coating when handling epoxy-coated reinforcement. Item 440, Reinforcing Steel, covers the requirements for epoxy-coated reinforcement.
- Other Corrosion Resistant Reinforcing: Corrosion resistant reinforcement such as fibers, stainless reinforcing steel, low carbon/chromium reinforcing steel, dual-coated reinforcing steel, galvanized reinforcing, or glass fiber reinforced polymer bars (GFRP) must meet the requirements of Special Provision to Item 440, Reinforcement for Concrete. Refer to district-specific corrosion protection recommendations for regions where bridge decks are exposed to de-icing agents and/or saltwater spray with regularity.
- Selection of Reinforcing: Each type of reinforcing provides additional protection from corrosion, engineering judgement should be used in the selection.
- Epoxy Coated Reinforcing:
- Commonly used and readily available. Likely the cheapest corrosion resistant reinforcing.
- Contractor familiarity with use and placement.
- If not properly handled, epoxy coating can be chipped and gouged; if not properly repaired, this can cause accelerated corrosion at the damaged areas and earlier onset of corrosion related damage.
- Stainless, Low Carbon/Chromium, Dual-Coated, Galvanized, GFRP:
- Relatively new corrosion resistant reinforcing options.
- Costs are somewhat unknown but should not significantly increase overall bridge costs except when choosing the stainless option.
- Potentially more durable and less likely to be damaged during transport and installation.
Provide notes on the bridge plan sheets to indicate which elements are to receive corrosion resistant reinforcement. In addition, specify corrosion resistant reinforcement in the General Notes to Item 440, Reinforcing Steel. Notes should indicate which bridge elements require corrosion resistant reinforcement. Do not specify partially epoxy coated bars. There is no direct payment for this work or materials.
- Bridge Slabs, Decks, & Rails : When indicated, specify corrosion resistant reinforcing for bridge slabs, decks, and concrete bridge railing elements.
- Specify corrosion resistant reinforcing in the top mat of steel in the bridge slab.
- This includes cast-in-place slab spans, culverts that require Class "S" concrete in the top slab, as well as slabs cast on top of box beams, slab beams, or double tees.
- Specify corrosion resistant reinforcing in the bridge approach slab if present.
- Prestressed Concrete Panels, PCP's, used in bridge deck construction do not require epoxy coated reinforcement or epoxy coated prestressing strands.
- By default, the bridge railing standard drawings require the use of corrosion resistant reinforcement in all concrete bridge railing elements when the bridge slab reinforcing is corrosion resistant.
- For bridges that do not have a cast-in-place bridge deck such decked slab beams, specify corrosion resistant reinforcing on the plans by providing notes in the General Notes to Item 424 Precast Concrete Structural Members (Fabrication) and Item 440, Reinforcing Steel.
- Substructure: When indicated, specify corrosion resistant reinforcement for all substructure elements.
- Applies to all bent caps, and abutments regardless of their locations relative to bridge expansion joints. Note: Columns do not require corrosion resistant reinforcement.
- The use of corrosion resistant reinforcing bars for foundation elements (i.e. piling, drilled shafts or buried footings.) is not recommended, unless in a marine environment.
- Prestressed Concrete Beams: The use of corrosion resistant reinforcing bars for prestressed concrete beams, except for bridge rail anchorage bars as indicated above, is not recommended. Do not utilize corrosion resistant reinforcing bars that connect the bridge deck slab to the prestressed concrete beams even when the bridge deck slab reinforcing bars are corrosion resistant. Specifically, do not epoxy coat R-bars (I-girders and U-beams), Z-bars (box beams), H-bars (Slab beams).
Increased Clear Cover
Increased clear cover to reinforcement beyond what is normally required places the reinforcement further away from the chlorides on the surface of the concrete and thereby reduces the potential for corrosion.
Clearly indicate increased clear cover requirements on the plans. There is no direct payment for this work.
- Bridge Slabs: When increased clear cover for bridge slabs is indicated, modify bridge standard drawings as necessary to achieve additional top clear cover. See checklists for necessary standard drawing modifications.
- Substructure: Consider increased clear cover for substructure elements on a case-by-case basis at the discretion of the District.
- Use primarily in areas of the state with a history of significant corrosion damage in substructure elements.
- Specify an additional 0.5 in. of clear cover for bent caps, abutments and exposed footings by decreasing the size of stirrups. Alternately, increase the size of the bent cap, abutment, or footing by increments of 3”.
- Columns have sufficient clear cover.
- Account for increased clear cover in the structural design.
By default, all structural concrete will receive air-entrainment. If air-entrainment is not desired, General Notes to Item 421 will be required to waive these requirements. Air-entrained concrete is not required for precast concrete members.
- Entrained Air General Notes for Item 421:
- For AMA, CHS, LBB and WFS, no General Notes are required.
- For ABL, ATL, DAL, ELP, FTW, ODA, PAR, SJT, WAC:
- Entrained air is required in all bridge deck and slip formed concrete (bridge rail, concrete traffic barrier, pavement, etc.), unless otherwise noted in General Notes. Adjust the dosage of air entraining agent for low air contents as directed or allowed by the Engineer. If entrained air is provided where not required, only the upper limits of the Special Provision will be enforced.
- For all other districts:
- Entrained air is required in all slip formed concrete (bridge rail, concrete traffic barrier, pavement, etc.) unless otherwise noted in General Notes, but is not required for other structural concrete. Adjust the dosage of air entraining agent for low air content as directed or allowed by the Engineer. If entrained air is provided where not required, only the upper limits of the Special Provision will be enforced.
Corrosion Inhibiting Admixtures
Calcium-nitrite has been shown to delay the onset of corrosion. Calcium-nitrite is an approved concrete strength and set accelerator and has been used for prestressed concrete member fabrication.
Other corrosion inhibiting admixtures have not consistently provided similar protection and are not recommended.
When the use of calcium-nitrite is indicated, provide notes on the bridge plan sheets indicating that the prestressed concrete beams are to receive calcium-nitrite. In addition, specify calcium-nitrite in the General Notes to Item 425.
Notes should indicate that the prestressed concrete beams are to receive calcium-nitrite at a dosage rate of 3 gal/CY. The use of calcium-nitrite in the prestressed concrete panels, PCP's, used in bridge deck construction is not recommended. There is no direct payment for this work or materials.
Limit Use of ACP Overlay on Bridge Decks
ACP overlay tends to trap salt laden moisture and thereby accelerate corrosion.
Limit the use of ACP overlay on new bridge decks whenever possible. If an ACP overlay must be placed on a deck, always specify a two-course surface treatment be applied first. The two-course surface treatment serves as a membrane to help protect the bridge deck.
Limit Use of Specific Bridge Rails (T223, T631, and drain slots)
The Traffic Rails T223 and T631 and drain slots in rails allow deicing agents to run down the face of the outside beam.
Limit the use of Traffic Rail T223 and T631 and drain slots in rails. However, snow removal requirements may necessitate the use of open bridge rails.
Crack Control in Structural Design
Limiting the width of cracks in concrete reduces the ability for chlorides to penetrate to the reinforcing steel thereby reducing the potential for corrosion.
For structures where the use of HPC or corrosion resistant reinforcing is indicated, design using Class 2 exposure condition to satisfy AASHTO LRFD 5.6.7. Control of Cracking by Distribution Reinforcement. See TxDOT Bridge Design Manual (LRFD) for further information.
This applies to abutment and bent caps only. It need not be applied to bridge slabs, beams or columns. The bridge standard drawings were developed using Class 2 exposure condition so no modifications to these drawings are required.
Shrinkage Crack Control Measures
Control of cracking due to shrinkage of concrete in the early and long term can limit the formation of cracks, which lead to the penetration of water and chlorides to attack the reinforcing steel and cause corrosion. Include notes for shrinkage control measures to the specific Item in which these apply.
- Microfibers for corrosion protection due to plastic shrinkage can be 2 lbs. to 3 lbs. per/CY depending on the corrosion potential and type of microfiber.
- Macrofibers for drying shrinkage or durability should be tested for concrete performance to meet the minimum “R” value of 160 psi flexural per ASTM C 1609.
- Provide shrinkage reducing admixtures tested by an approved testing lab and meeting the requirements for ASTM C494 Type S, except that in Table 1 length change shall be measure as: Length Change (percent of control) shall be a minimum of 35% less than that of the control. Table 1 Length (increase over control) shall not apply. Do not use expansive metallic materials in shrinkage reducing admixtures.
Contact Bridge Division for Special Specification guidance for implementing performance based improvements to concrete utilizing fibers for shrinkage control.
Other Protection Measures
In general, we do not recommend any of the following however, district specific or project specific requirements may override these recommendations:
- Epoxy waterproofing of bent caps, abutment caps, abutment backwalls, or columns. The performance of epoxy waterproofing is dependent of the level of substrate cleanliness. Item 4220.127.116.11 states “Clean the surface thoroughly before applying a coating by chemical cleaning, if required, and by blast cleaning.” Most all circumstances will require blast cleaning, proper inspection and installation is critical to the long-term performance of the epoxy waterproofing.
- Cathodic protection systems.