Provisions of this IM shall apply to the production and fabrication of precast culverts and bridge elements.  Approved producers and fabricators are listed in the Materials Approved Products Listing Enterprise (MAPLE).


Precast culverts and bridge elements shall meet the requirements of Standard Specification Section 2415.


Each plant requesting to become a Certified Precast Concrete Producer shall do so in writing to the Precast and Prestressed Engineer The request shall include a copy of the compliance certificate issued by the ACPA, NPCA, or PCI, a copy of the most recent audit conducted in accordance with the ACPA, NPCA, or PCI certification programs, a copy of the response to deficiencies of the audit, if applicable, a copy of the QCP, and the designated Management Representative for the plant.


Each certified Precast Concrete Plant is required to submit to the Precast and Prestressed engineer the copy of the annual audit and the response to deficiencies of the audit, if applicable, to verify compliance with the ACPA, NPCA, or PCI certification programs.


Precast Box Culvert fabricators must notify District Materials Engineer when the approved shop drawings are available. 




      TIE RODS


U-shaped tie rods shall meet the requirements of Iowa DOT LRFD Precast Culvert Standard and Specification 2416.02. Nuts shall be hex or heavy hex meeting the requirements of ASTM A563. Washers shall be flat and shall meet the requirements of ASTM A436. Tie rods (U-shaped), nuts, and washers shall be galvanized per ASTM A153, Class C.




Engineering fabric for joint wrapping shall be from an approved source listed in IM 496.01, Appendix C shall meet the criteria of Embankment Erosion Control (Specification 4196.01 B3).


Gaskets for joints shall be butyl rubber-based, rope gasket material, shall be from an approved source listed in IM 491.09, Appendix A.





Lifting cables shall be seven strands and shall meet the requirements of ASTM A416.




Only welders qualified according to 2408.03, B may perform welding for the bridge components.




The minimum concrete cover over the circumferential reinforcement shall be one inch.


The inside circumferential reinforcement shall extend into the tongue portion of the joint and the outside circumferential reinforcement shall extend into the groove portion of the joint. The clear distance of the end circumferential steel shall not be less than 1/2 inch or more than 2 inches from the ends of the box section. The maximum variation in the position of the

reinforcement, as indicated by the approved shop drawing shall be ± 3/8 inch for slab or wall thickness of 5 inches or less and ± 1/2 inch for thickness over 5 inches.


Splices in the circumferential reinforcement shall be made by lapping. The overlap, measured between the outermost longitudinal sires of each fabric sheet, shall not be less than the spacing of the longitudinal wires plus two inches. The minimum lap distance is ten inches.




When use of a curing compound is selected, the compound shall be sprayed on immediately after form removal and left intact until the concrete has achieved the specified 28-day compressive strength. The units must be protected from loss of moisture while still in the forms. Curing compound shall not be used when the ambient temperature is less than 32°F at the time of application.


The box sections shall be cured for a sufficient time to assure the concrete shall achieve the designed compressive strength within 28 days.




Fly ash may be substituted for Portland cement. Use a substitution rate of no more than       35% by weight for dry cast concrete only. Fly ash shall be from an approved source identified in Materials I.M. 491.17. The maximum total supplementary cementitious materials substitution shall not exceed 50%.

Acceptance of dry cast concrete box culvert shall be based on strength testing.




Compression tests for determining concrete compressive strength may be made on standard rodded concrete cylinders or concrete cylinders compacted and cured in a like manner as the box sections, or on cores drilled from the box section.


A minimum average compressive concrete strength of 3500 psi shall be achieved before box sections may be moved and/or transferred to a storage site. At least three specimens shall be tested from each concrete mix used within a group (one day’s production) to determine compressive strength.


Acceptance strength testing shall be based on a minimum of three specimens per lot (one calendar week per mix used). The average compressive strength of all cylinders tested shall be equal to or greater than the design concrete strength, and none of the specimens shall be less than 80% of design strength.


Unless otherwise specified, the minimum 28-day compressive concrete strength is 5000 psi for box sections.




·         Internal dimension – 1%


·         Haunch (out-to-out) – 1/4 inch


·         Slab and wall thickness – shall not be less than that shown in the design by more than 5% or 3/16 in., whichever is greater.


·         Length of opposite surfaces


7 ft. internal span or less – 1/8 in./ft. of internal span, maximum of 5/8 inch

Over 7 ft. internal span – 1/8 in./ft. of internal span, maximum of 3/4 inch


·         Length of section – 1/8 in./ft. of length, maximum of 1/2 inch


·         Area of reinforcement – areas of steel reinforcement shall be the design steel areas or greater. Minimum concrete coverage must be maintained.




Each box section to be certified shall be legibly marked on the inside and the outside by indentation, waterproof paint or other approved method with the following:


·         Span x Rise

·         Date of Manufacture

·         Name or Trademark of Manufacturer

·         Maximum Design Earth Cover

·         Certified Stamp




The DOT developed an inspection checklist to assist the State Monitor/Inspector, which covers fabrication of the units. The checklist includes several items a monitor/inspector should cover for each project to assure compliance and uniformity statewide.





Project Number                                                                            County                                         


Size                                                                             Design Cover                                                 


Required Documentation:


q  Approved Shop Drawing

q  Steel Mill Certifications (Approved Source)

q  Aggregate Certifications (Truck Tickets)

q  Cement Certifications (Delivery Tickets)

q  Cylinder Breaks

      (One day and shipping strength – 3 cylinders/calendar week for shipping – average equal to or greater than required strength – none less than 80% of required strength)


Required Steel Areas (Steel Designation Areas):


As1                                 As2                                    As3                                    As4                          


As5                                 As6                                    As7                                    As8                          


(NOTE: The steel areas for As5 and As6 are normally included in As7 and As8 by increasing steel area and modifying spacing. As5 and As6 mats are not normally installed.)


Steel Incorporated (Steel Used):


As1                                 As2                                    As3                                    As4                          


As5                                 As6                                    As7                                    As8                          




q  Joint Sealer (Butyl Rubber Base)

q  Engineering Fabric (Approved Source)

q  Joint Tie Rods (Galvanized, A36 Steel)

q  Required Identification



                             District Materials Staff