14.Concrete Foundations

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CHAPTER 14 CONCRETE FOUNDATIONS

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INTRODUCTION

There are five basic types of concrete foundations used in traffic signal construction:

Pedestal Foundations(Std. Plate 8112)

CONCRETE FOUNDATIONS


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Pedestrian Push Button Station Foundations(Signal Plan Detail).


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Cabinet/EquipmentPads (Standard Plate8119). See Appendix forTypical Signal PlanEquipment Pad detail.

Mast Arm Pole Foundations: PA 85 Standard Plate 8120

PA 90 and PA100 Standard Plate 8126

BA 60 thru 80 Standard Plate 8134


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There are five basic types of foundations used in roadway lighting construction:

Cast-in-Place Concrete Light FoundationsDesign E and HStandard Plates 8127 and 8128

Lighting Service Cabinet PadsEquipment Pad BCast-In-Place or PrecastStandard Plate 8106

Pre-cast Concrete Light FoundationsDesign E and HStandard Plates 8127 and 8128


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High Mast (Tower) Light Foundations

Steel Screw-in Foundations

LightingIf approved by the District and theEngineer, a screw-in light foundationmay be used as specified in theContract Documents.

SignalsThe Contract Documents will specifyif a screw-in foundation is to beused for pedestal installations on asignal project.


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GENERAL REQUIREMENTS

If rocks or other obstructions that preventconstruction of the foundation are encountered,the dimensions of the foundation may beadjusted, as determined by the Engineer, toprovide a stable foundation.

If soil conditions permit the Contractormay use a partial form in the upperportion of the foundation. In sandy soil,which does not have enough binder,partial forms should not be usedbecause the hole may cave in during thepouring operation and the foundationcould be lost. If there are any doubts

about the soil condition, a full-lengthform should be used.

Conduits, anchor rods and rebar cagesmust be placed in the proper positionand held in place with a rigid templateuntil the concrete is cured. Forms mustnot be removed until the concrete iscured. A tremie must be used whenpouring concrete to ensure that theconcrete is not dropped from a heightof more than four (4) feet (1.2 m).


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STANDARD PLATES

The Standard Plates contain essential details about the construction of concrete foundations.

The Standard Plates that apply to

a specific project are listed on theDetail Sheet of the plan.

Information found on StandardPlates includes:

Concrete mix required.

The diameter of the anchor boltcircle.

Bolt spacing.

Minimum conduit size andtype.

Number of conduits required inthe foundation.

Reinforcement bar size andplacement.

Minimum projection of theconduit above the foundationtop.

Size and projection of theanchor bolts.

Overall dimensions of thebase.

The grade of the top of thefoundation relative to theadjacent ground or sidewalk.

Other required items:

The contractor has the option,if the Engineer approves, ofusing a stronger mix such as3y43.

The same applies to increasing the size and shape of the foundation.

Most Standard Plates contain an area dedicated to NOTES. It is essential that these notes becarefully read and understood. Because the Standard Plates and the Spec. Book may beoutdated, the contractor and the inspector must check the Contract Documents before workbegins on any structure.

Refer to the specific Standard Plate that applies to the foundation that is to be constructed. Itshould be noted that Standard Plates can change and the current Standard Plates that apply toa specific project will be listed in the Contract Documents.


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STANDARD SPECIFICATIONS REQUIREMENTS

2401.3 - Concrete Bridge Construction

This specification covers a wide

spectrum of items, some of whichdo not apply to signal foundations.One item that does apply is2401.3B.6 (Form Construction)which addresses form integrity,strength and construction.

Another is 2401.3C which coversPlacement of Concrete items suchas inspection of forms,reinforcement bars, materials andequipment. Reinforcement bars

must be carefully situated in theform so that they are centeredwithin the form and there is aminimum of 3 inches of concretecovering the rebar when thestructure is cast. For additionaldetails see the MnDOT LRFDBridge Design Manual.

Concrete must not be placed unless all items have been approved and adequate lighting existsto do the work.

Compaction of Concrete

Concrete foundations must bethoroughly compacted by mechanicalvibration applied internally. Vibratorsmust be operated at a frequency of notless than 4500 impulses per minute(75Hz).

Compaction of concrete must be asspecified in MnDOT 2401.


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Mast Arm Pole Standard Roadway Light Unit

Finishing Concrete

All exposed surfaces of concrete foundations must receive an ordinary surface finish asspecified in MnDOT 2401.3F.

2401.3G - Concrete Curing and Protection

Concrete curing is important to the strength and life of the structure. The choice of curingmethod depends on the air temperature, surface temperature of the structure and surfacemoisture of the structure. If the surface temperature of the structure drops below 40 degrees

Fahrenheit (4.5 degrees Celsius), no curing time will be counted. The surface of thestructure must remain moist. This is accomplished by tightly covering the structure with plasticsheeting. Sheeting is described in Std. Spec. 3756 (White Opaque Polyethylene). No othercolor is allowed.


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MAST ARM POLE FOUNDATION(Standard Plate 8126 )

Standard Plate 8126 (PA90 and PA100Pole Foundation) contains more detailsthan 8112 (Pedestal Foundation). PoleFoundations contain a rebar cage and areconsiderably larger than pedestalfoundations to handle the stress of thelarge pole and mast arm it supports.

The required minimum depth of a PA100 polefoundation is 12 feet (3.7 meters) below ground.The length of foundation extending above theground line or top of walk area must be a minimumof 6 inches; the elevation of the top of thefoundation must assure the vertical clearance fromthe bottom of all signal heads (includingbackground shields) to the pavement is not lessthan 17 feet (5.2 meters) nor greater than 19 feet(5.8 meters).


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Note 2 of Standard Plate 8126 reads: Depthof foundation may be changed in the plans orspecial provisions. Depth of foundation maybe reduced by 2 feet (600 millimeters) wheninstalled in sidewalk or concrete raised

median. Foundation depths are based on asoil friction of 30 degrees and a soil weight of120 lb./cf. (1,920 kg/m3). Any variationrequires an approval by the Soils Engineer.

There are three ways to calculate foundationdepth:

By identifying the type of soil. This canbe done by looking in the Grading andBase Manual, Soil Classification

Section (600 series section).

By calling the Soils Engineer.

By drilling the foundation to theminimum required depth in theStandard Plate.

There are several other key items on this Standard Plate that must be examined:

The rebar cage must be placedin the center of the foundation 6 inches(150 millimeters) from the top and aminimum of 3 inches (75 millimeters)from the sides.

MnDOT uses two types of mastarm poles; Type A (more commonlyused) and Type B. The type of polesrequired for each project are listed inthe pole notes on the intersection

layout sheet of the Plan. The locationsof the mounting hubs for shaft-mountedsignal heads differ according to thepole type. To determine the properorientation of the anchor rods for thetwo types of mast arm poles, refer tothe vertical section of the StandardPlate.


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The Standard Plate states: Mast Arm Pole standards must not be installed on foundations untilat least seven days of curing period has elapsed. This is based on 60% strength. If differentconditions prevail, refer to Specification 2401.3G (Concrete Curing and Protection), Table 2401-1 in the MnDOT Standard Specifications for Construction book to find out the number of curingdays required and for additional details on this subject.

Anti -Seize Lubr icant

Threaded portions of all anchor rods above concrete foundations and steel foundations, etc.,must be coated with liberal amounts of brush-on anti-seize lubricant before installation of signalservice cabinets, signal cabinets, mast arm pole standards, pedestals, lighting units, lightingservice cabinets, or other type cabinets/structures on anchor rods. All threaded items requirebrush-on anti-seize lubricant.


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