Difference between revisions of "Category:703 Concrete Masonry Construction"
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===703.1.8 Dewatering Excavation===
===703.1.8 Dewatering Excavation===
Where concrete is to be placed in forms within an excavation, provisions must be made for removal of water from within the forms, both before and during concreting. Water to be removed from foundation enclosures by pumping must be drawn off in a way that permits no possibility of fine materials, such as cement or sand, being carried away by cross flow of water. The contractor must not be permitted to remove water by placing a suction hose inside the forms.
Where concrete is to be placed in forms within an excavation, provisions must be made for removal of water from within the forms, both before and during concreting. Water to be removed from foundation enclosures by pumping must be drawn off in a way that permits no possibility of fine materials, such as cement or sand, being carried away by cross flow of water. The contractor must not be permitted to remove water by placing a suction hose inside the forms. must develop a practical means of leading water by use of shallow ditches arranged to lead water to a collector ditch which in turn carries it under the forms to an outside sump. Satisfactory placement of concrete may sometimes by accomplished by using plastic polyethylene sheeting just ahead of concrete placement to force water through collector ditches under the forms into the sump. As soon as concrete covers the opening under the forms the opening should be plugged from outside the forms and only sufficient pumping done to lead water away until the concrete is set. No concrete should be placed under water without express permission from Division of Construction and Materials unless permitted or required by the contract.
===703.1.9 Footings - Foundations===
===703.1.9 Footings - Foundations===
Revision as of 06:55, 3 June 2021
- 1 703.1 Bridge Substructure
- 1.1 703.1.1 General
- 1.2 703.1.2 Soundings
- 1.3 703.1.3 Structure Excavation
- 1.4 703.1.4 Excavation Below Plan
- 1.5 703.1.5 Test Holes
- 1.6 703.1.6 Structure Excavation Checklist
- 1.7 703.1.7 Preparation Of Foundation
- 1.8 703.1.8 Dewatering Excavation
- 1.9 703.1.9 Footings - Foundations
- 1.10 703.1.10 Reinforcing Steel
- 1.11 703.1.11 Forms
- 1.12 703.1.12 Anchor Bolt Wells
- 1.13 703.1.13 Bond Breaker
- 2 703.2 Superstructure
- 2.1 703.2.1 General
- 2.2 703.2.2 Falsework Computations
- 2.3 703.2.3 Falsework Inspection
- 2.4 703.2.4 Deck Forms
- 2.5 703.2.5 Void Tubes
- 2.6 703.2.6 Grade Control
- 2.7 703.2.7 Machine Finishing
- 2.8 703.2.8 Concrete Placement
- 2.9 703.2.9 Curing
- 2.10 703.2.10 Cold Weather Placement
- 2.11 703.2.11 Hot Weather Concreting
- 2.12 703.2.12 Checklist
- 2.13 703.2.13 Form Removal
- 2.14 703.2.14 Surface Sealing
- 2.15 703.2.15 Opening To Traffic
- 3 703.3 Bridge Barrier Wall
- 4 703.4 Sound Walls
- 5 703.5 Laboratory Testing for Sec 703
703.1 Bridge Substructure
Foundation types for most structures classed as bridges fit into three basic categories; pedestal pile, pile foundations, or concrete footings. A review of substructure requirements for each structure will indicate an order which the inspector should follow. The inspector should review the plans, examine the soundings, and determine the nature of footing construction. Pedestal pile and pile foundations were covered under Sec 701 and Sec 702.
Soundings are made to determine subsurface conditions and soil or rock characteristics for practically all structures. This data is normally obtained by one of two methods, or by a combination of both. Cores are taken at or near the required location of footings and are usually supplemented by auger soundings made at points on the bent line of the footings for individual substructure units. Soundings are of an exploratory nature and are subject to interpretation by several individuals. The inspector should closely examine the sounding data to ascertain the nature of material upon which the footings are located and the nature of excavation to be made for the footings.
Due to the many irregular rock formations and the numerous soil types in Missouri, conditions often differ from those anticipated. Changes in condition of a minor nature should be handled by the resident engineer, sometimes after consultation with the District Construction and Materials Engineer. Major differences usually require revisions of some design feature and must receive additional attention through the Division of Construction & Materials for liaison with the Bridge Division.
703.1.3 Structure Excavation
The first operation for each footing normally is excavation. Structure excavation is defined in Sec 206. The type of excavation provided for the individual structure is indicated on the front sheet of the design plans under "Estimated Quantities". The Division of Bridges shows estimated quantities for substructure on the bent detail sheets of the bridge plans. The elevation of demarcation between Class 1 and Class 2 excavation is shown on the plans and is not subject to change.
The inspector should be sure that cross sections have been taken of the original ground surface at the footings. Longitudinal profiles should be taken as indicated in Construction Surveying. Cross sections should be taken at regular intervals and at abrupt breaks in the ground line. Record measurements for excavation computations directly in the field book. Record accurate elevations of original ground prior to excavation, adjacent to and symmetrically around each footing. Sometimes the contract requires other work affecting excavation limits to be done before structure excavation. This requires measurement of the ground surface after some work has been done.
Where there is structural excavation, original and final sections must be taken, since elevations may have changed from those on which estimated quantities were based. It is necessary to take elevations only in the immediate area of excavation, since pay quantities are limited by vertical planes 18 in. outside the structure, measurements must be made to include only those solid materials actually removed.
The inspector should closely watch the progress of excavation. If the inspector’s opinion is that a cofferdam or shoring should be installed, the contractor should be informed so that we "get on record" that its use is indicated. This is especially important in view of the requirements of the Federal Occupational Safety and Health Act. The cofferdam is usually needed to keep soil and water out of a relative large excavation. One is nearly always used when work is to be done below water level. This permits the excavation to be dewatered. In small excavations shoring is usually sufficient. When a cofferdam is needed on large structures, Job Special Provisions will usually require the contractor to submit drawings for review and/or approval before the proposed method is permitted.
Shoring is a foundation enclosure consisting of braced sheeting of steel or wood. It is generally required:
- 1. Where there is a possibility of sudden cave-in that might result in danger to human life.
- 2. Where flow of loose granular soil may result in oversize excavation unless proper support is provided.
- 3. Where cohesive soil may stand on a steep slope temporarily but may suddenly shear into the excavation endangering workmen and the partially completed structure.
- 4. Where slumping of soil next to the excavation may undermine an adjacent structure such as a building, wall, pavement, or railroad and soil removal decreases lateral support of the structure.
Other government agencies' rules may require shoring under other conditions.
When a cofferdam or shoring is used, sufficient bracing must be provided to resist any lateral force that can reasonably be expected. Cofferdam design should include adequate provisions for surcharges produced by weight of equipment working adjacent to the sheeting. Lateral forces may be produced by an unusual rise in the stream or by weight of earth deposited in a fill. The inspector should ensure that the contractor understands provisions of the contract relating to cofferdams and, if required by the contract, plans of a cofferdam must be submitted for the engineer's review before excavation is started.
Underwater Concrete. Often conditions involving large volumes of water inside cofferdams and caissons require placement of concrete under water. This type of placement will be permitted when included in the contract or upon written permission of the engineer. The Standard Specifications require the use of a tremie, a bottom dump bucket or mechanically applied pressure to place the concrete.
The most prevalent method of placement involves use of a pipe called a tremie and the method is usually called tremie concreting. The technique of tremie concreting is to introduce concrete below the surface of the water and then to continue to introduce new concrete below the previously placed fresh concrete in a continuous operation causing an outward and upward flow. Refer to Sec 701.4.13 for additional guidelines.
There are many advantages as well as some disadvantages, in the use of the tremie technique. Some of the advantages are:
- 1. It is unnecessary to dewater the caisson or cofferdam.
- 2. Large volumes of concrete can be placed quickly.
- 3. The curing conditions are very good.
- 4. Voids and honeycombs are eliminated if the tremie does not leak and its seal with the concrete is not broken.
Some of the disadvantages are:
- 1. It is necessary to require special mix design with a higher cement factor at extra cost.
- 2. The slump is permitted to be increased up to 8" since the concrete is not vibrated.
- 3. The quality and strength of underwater placed concrete relies on special skills and techniques.
- 4. The quality of the concrete in the seal cannot be predetermined prior to performance.
Tremie pipes are usually 10 to 12 in. in diameter with a hopper attached to the top. A plug must be made for the end of the tremie pipe to keep water out of the pipe when lowered. When the pipe is filled with concrete the plug is removed and the concrete flows out to form a mound around the end of the pipe. When placing concrete in deep water, the empty pipe will become buoyant if the end is plugged. In this instance, the pipe may be lowered with the bottom open. Before concrete is placed, a plug should be pushed into the pipe ahead of the concrete. The plug should fit tightly in the pipe to displace the water ahead of the concrete. An inflated rubber ball or a nerf ball makes a good plug and will float back to the top when released from the bottom of the pipe.
Once concreting begins it is important that the bottom of the tremie not be withdrawn above the concrete, since this causes loss of the tremie seal and can cause voids, honeycombs, and excessive laitance. Other problems or difficulties in placing underwater concrete are caused by the concrete plugging in the tremie pipe, followed by a loss of the seal. The plugs can be caused by the arching action of the concrete in the tremie pipe, delays in placing which permit the concrete to start its initial set, poor mix design, and leaks in the pipe. Leaks even of "pinhole" size result in complete degradation of the concrete. Close inspection of the tremie pipe is mandatory to determine that proper gaskets are placed between the sections and that the pipe is in good condition.
See Sec 206.4.9 for more information on Seal Courses.
703.1.4 Excavation Below Plan
When plans indicate footings founded on rock or shale and satisfactory foundation material is not found at design elevation excavating shall cease. It is mandatory that no excavation be done below plan elevation until some means of sounding material below plan is used to establish the approximate elevation of satisfactory foundation material. Before any exploratory work is done, an agreement should be reached with the contractor upon a method of payment, particularly if the magnitude of work involved is extensive. After the approximate elevation of suitable material has been established and a satisfactory revision of design for the substructure unit received, excavation may continue to the anticipated final elevation. Plans should be reviewed, noting that footings are to be keyed 6 in. into rock or 18 in. into shale with the sides of the footing cast against the rough surface or neat lines of bearing material.
703.1.5 Test Holes
Test holes should be drilled before any concrete is placed for footings other than those on piles. The number of test holes drilled will be governed by the character of the material encountered. The minimum number should be at least one to each footing for all abutments, bents or piers. Generally a four foot test hole is sufficient. A deeper test hole is occasionally specified. If the rock formations in the area are irregular, or if the footing is extremely large, additional test holes may be desired to assure satisfactory foundation material over the entire area of footing.
703.1.6 Structure Excavation Checklist
- Inspectors should see that:
- 1. Enough elevations have been taken to define original ground surface for each area from which material is to be excavated, to permit accurate determination of excavation quantities.
- 2. The contractor's equipment and method are satisfactory and that the contractor is aware of all requirements and Corp of Engineer's Permits. Preconstruction conference is a good time to discuss these items.
- Inspectors should see that:
- 1. Suitable material is kept separated from unsuitable material. All suitable material is properly stored for future use.
- 2. All excavated material is stored in locations where it will not bear against any part of the structure, overload the bank or pollute the stream.
- 3. Depth of excavation and its limits are frequently checked.
- 4. The district office is informed of any unusual soil conditions or unexpected rock found in the excavation.
- 5. Enough shoring is being used to insure safety and prevent ground movement.
- 6. The bottom of the excavation is at the correct grade, allowance having been made for final trimming.
- 7. Arrangements have been made for any required bearing tests.
- 8. Excavation has been approved and authorization to proceed with further construction has been obtained.
- 9. Satisfactory arrangements have been made to drain the excavation or to seal out water before concreting is started.
- 10. Backfill area has been inspected to make sure all embankment will be solidly supported.
- 11. Any items of historical, archaeological, or paleontological value have been salvaged for transmittal to the State University.
- Inspectors should see that:
- l. All cavities are filled at once with backfill material. Where required, arrangement has been made for density tests on backfill.
- 2. Arrangements have been made for required drainage.
- 3. Backfill is being placed and compacted by specified methods and to required density.
703.1.7 Preparation Of Foundation
After the proper bearing stratum has been determined, either on piling or rock, the next operation will be forming and pouring the footing. This will not be difficult if the hole is free of water, however, water can exert considerable pressure and may enter the excavation. There are many types of sheeting and cofferdams and many types of soil. Each problem is, therefore, a special case which must be solved on the basis of observation and experience.
703.1.8 Dewatering Excavation
Where concrete is to be placed in forms within an excavation, provisions must be made for removal of water from within the forms, both before and during concreting. Water to be removed from foundation enclosures by pumping must be drawn off in a way that permits no possibility of fine materials, such as cement or sand, being carried away by cross flow of water. The contractor must not be permitted to remove water by placing a suction hose inside the forms. The contractor must develop a practical means of leading water by use of shallow ditches arranged to lead water to a collector ditch which in turn carries it under the forms to an outside sump. Satisfactory placement of concrete may sometimes by accomplished by using plastic polyethylene sheeting just ahead of concrete placement to force water through collector ditches under the forms into the sump. As soon as concrete covers the opening under the forms the opening should be plugged from outside the forms and only sufficient pumping done to lead water away until the concrete is set. No concrete should be placed under water without express permission from Division of Construction and Materials unless permitted or required by the contract.
703.1.9 Footings - Foundations
Footings required for any type of structure can be generally classified into three types: (l) plain concrete footings on rock or shale, (2) footings on foundation pile, (3) reinforced concrete spread footings on material other then hard shale or rock. Bottom slabs of box structures classed as bridges are not covered in this article.
703.1.9.2 Footings On Rock Or Shale
Throughout most of the state footings are founded on rock, shale, or chert formations to sustain a bearing load of 10 tons or more per square foot. These footings cause no particular problem unless the rock formation has questionable bearing capacity or is found at an elevation lower than anticipated. For a footing on rock or shale, it is often necessary to deviate from plan elevation.
Where conditions are encountered that would require adjustments beyond one ft., refer the problem to the district office for further handling with Division of Construction and Materials.
Never excavate below plan elevation until probable elevation of suitable material has been determined by probes or drilling.
703.1.10 Reinforcing Steel
Changes in design footing elevations require splicing of reinforcing steel in footings or columns. Splice lengths are critical because of Missouri's location to possible earthquake zones. Therefore each splice length should be checked with the District Construction and Materials Engineer.
The weight per foot for various sizes of reinforcing steel is shown below. These weights should be used to compute the additional quantity of reinforcing steel used in splices.
Forms must be constructed to dimensions shown on the plans. They should be properly tied and braced so correct dimensions are maintained during placement of concrete to insure an acceptable product after concrete has been cured. Before placing concrete, forms must be checked for conformance with plans and specifications and all irregularities corrected. The contractor should be informed of any needed corrections far enough in advance of a scheduled pour that they may be made without causing delay. The contractor designs forms and is responsible for the completed work.
703.1.11.1 Form Construction
Forms are usually made of lumber lined with plywood. For substructure units, forms are sometimes made of metal. All forms should be strong enough to hold plastic concrete in place until it has hardened, without bulging or sagging. Forms should be tight enough to prevent mortar leakage.
Forms should be designed to permit easy removal without damage to the concrete.
All foreign material, including ponded water, must be removed from within forms before placing any concrete. Where forms are deep, openings should be provided in the bottom to permit removal of material.
Oiling of forms or application of any form coating should be done before placing reinforcing steel. For vertical forms such as for columns, retaining walls, and massive piers, spacing of studs, wales or in the case of metal forms, the stiffener rings depend upon the pressure to be resisted. The pressure of concrete on forms depends on rate of placement and on depth of pour.
Form ties must be so that portions can be removed or broken back as required by Sec 703.3.2.8 without damage to concrete. Any portion of the tie left closer to the surface may cause a rust spot or streaking. Some ties are designed to serve as spreaders. Contractors sometimes use supplemental spreaders to brace wall forms apart. If wood spreaders are used they must be removed as concrete reaches their level. Some good practitioners fasten a wire to each spreader so the spreader can be pulled out and none will be overlooked. Wire ties are not permitted by Specifications to pass through any concrete.
Wire ties are permitted by practice at the construction joint between deck slabs and curbs. Approved form ties may also be permitted for New Jersey type barrier curb forms at the top and bottom.
Forms having interior right angle corners require a chamfer strip inserted if the corner will be exposed in the completed structure. A chamfer strip is usually a smooth strip of wood having a triangular cross section, placed in the corners of forms to provide beveled corners on the concrete. Steel forms may have chamfers prefabricated of steel. Neoprene chamfer strips are also used, particularly on curved portions of the work.
703.1.11.2 Forming Pile Cap End Bents
Plans normally require embankments at bridge ends to be constructed and compacted to the elevation of the bottom of cap before driving piles. This type of construction usually requires the end bent cap to be formed in a manner that will not disturb the embankment. In forming the bottom of the cap, a method must be used that will prevent contamination of concrete by the embankment, splashing of dirt on reinforcing steel, leakage of grout, displacement of reinforcing steel, and any movement of side forms. The resident engineer should review for approval the method the contractor intends to use before permitting the cap to be formed. The finished cap must have lines, elevations, and dimensions shown on the plans. One method that has gained wide acceptance is placement of a two or three inch concrete slab on the compacted embankment, the top of which is at the elevation of the bottom of cap or beam. This slab can be left in place upon form removal and will not decay or rot as wood forms would. This does not suggest that wood forms are not permitted, but points out problems arising from their use. Wood forms must be removed, if used, and the area occupied by the forms properly backfilled and compacted. The purpose of compacting the embankment up to the bottom of cap is to eliminate problems experienced with embankment shrinkage and lack of density at end bents.
703.1.12 Anchor Bolt Wells
Occasionally contractor's operations are so timed that substructure units must withstand freezing temperatures before the superstructure is placed. If this occurs, it is necessary for the contractor to protect the anchor bolt wells against freezing water. To facilitate winter construction, anchor bolt holes may be drilled in lieu of casting anchor bolt wells for most structures. Contract provisions occasionally require a particular procedure for protecting wells which eliminates all other methods. Otherwise, the method is the contractor's choice, subject to approval by the engineer. Substances or solutions designed to lower the freezing point of water should not be placed in wells without approval of Division of Construction and Materials. Whatever method is used to seal wells against entry of moisture, it is important that it be as inconspicuous and inaccessible as possible. All ladders, runways and other means of access should be removed during any period of delay in the work to prevent vandalism.
703.1.13 Bond Breaker
In various areas between the diaphragms and bridge caps of double T and concrete I girders, the plans will require a 50 lb. roofing felt be placed as a bond breaking agent. In those areas so designated it will be permissible to allow the contractor to substitute a heavy coat of asphalt paint.
While it is not necessary to obtain the same thickness produced by the roofing felt, it is required that it be sufficiently thick to produce an unbonded condition. To achieve this condition it will usually be necessary to apply two coats of paint.
This change may be permitted on all active projects without a change order. Final plans should indicate where this substitution is made.
The guidelines for percentage payment of bridge decks is outlined below.
These percentages are of the total sq. yd. (m) deck price.
Conventional Form Decks
- 35% Deck forming
- 20% Rebar tied in place
- 40% Concrete placement
- 5% Curing, sealing, and stripping forms
Precast Panel Decks
- 20% Precast panel placement
- 25% Deck Forming
- 15% Rebar tied in place
- 35% Concrete placement
- 5% Curing, sealing, and stripping of forms
Pour Sequence Contractors have requested to combine the approach slab, approach pavement, and bridge deck pours into one pour. Bridge Division has concerns with this method. The main concern with backfilling the abutment to subgrade elevation before the slab was poured is a violation of Sec 206.4.10. The weight of the beams and slab help resist the earth pressures on the backwall and help prevent the abutment from moving inward when backfilled. This is especially critical when the abutment is founded on long pile and would be less critical if the abutment was founded on spread footings keyed into rock.
There is also a concern with sawing the joint at the fill face line between the bridge deck and the approach slab. The concern is that the crack that develops may follow the face of the backwall reinforcing steel and cause premature deterioration of the resteel. Do not permit combining these pours without checking with Bridge Division.
Many general statements which apply to "substructure" also apply to superstructure. Particularly important items added under this article are falsework and riding surface. Adequate provisions to correct for deflection in supporting beams or stringers used for falsework is the contractor's responsibility. The resident engineer has a responsibility to check the contractor's proposed falsework and to determine that anticipated deflections are correct and structurally acceptable. The resident engineer should thoroughly check falsework supports, sill pressures, drainage of any mud sills, and any other factor which could make falsework unsatisfactory.
A rule of thumb for the amount of deflection to permit is not practical as there are too many variables.
The Steel Construction Manual, published by the American Institute of Steel Construction, provides steel beam properties. Other manuals of this type provide standard timber sizes and properties. Such manuals contain formulas for computing deflections under different loading conditions. The contractor should furnish all data necessary and should show sufficient details on falsework plans to enable project personnel to check anticipated deflections. Proper allowance should be made for "timber squeeze" at falsework joints. A good rule of thumb is to allow l/l6 in. for each joint where wood is in contact with wood. Good practice dictates as few joints as possible to minimize this problem. Questionable falsework procedures should not be approved as public safety may be involved as well as job control.
703.2.2 Falsework Computations
An example of deflection calculations for a voided slab superstructure is given below using simply supported falsework and assuming conditions commonly encountered in the field. Where continuous beams are used, simple support assumptions are on the safe side, but further checking will be required if anticipated deflections are of a high order.
Structure data - Voided Slab
- Span (of falsework) = 28' = 336"
- Roadway width = 26'
- Deck width (out to out) = 28'-7"
- Slab thickness = 2l"
- Void diameter = l2"
- No. of voids in x-section = l7
- Total length of each void = 24'
Wt. of Conc. & Reinf. steel = l50# per cu. ft. Wt. of forms and falsework on I-bms = l0# per sq. ft. This figure in practice to be based on material actually used in forming.
- w = pounds per lineal inch
- l = Span in inches
- E = Modulus of elasticity = 29,000,000 #/in2
- I = Moment of inertia for I-bms used
Then defl. = (5(336)4(w))/(384)(29,000,000)(I) = 5.74w/I
W for load - Total.
- Conc. Slab = 28.58 x 28 x l.75 x 150 = 2l0,000
- - Voids = (l' x l' x 3.l4l6/4) x 24 x l7 x 150 = -48,066
- Forms = 28.58 x 28 x 10 = 8,000
- W = l69,934#
- w = Wt./lin. inch of Br. = l69,934/(28) x l2 = 506#/inch
- Use l7-l2" BP at 53# = l7 x 53 / l2 = 75#/inch
- Total = 581#/inch
Since the 2 exterior beams will be only one-half effective in supporting the load, total I of the group will be estimated thus:
- I of One l2" BP at 53# = 394.8 in4
- I of group = (l7-l) x 394.8 = 6320 in4
Therefore Defl. = 5.74 x 581 / 6320 = 0.528 in.
This is acceptable since it is desirable to limit falsework deflection to approximately l/700 of span.
- Use 9-l8" WF at 50# = 9 x 50 / 12 = 37.5#/inch
- Wt. Conc. and Forms = 506.0#/inch
- Total = 543.5#/inch
- I =800.6 in.4
- Total I = (9-l) 800.6 = 6405 in.4
Therefore Defl. = 5.74 x 543.5 / 640 = 0.487 in.
Use 6-2l" WF at 62# = 6 x 62 / 12 = 31#/in.
- Wt. Conc. & Forms = 506#/in.
- Total = 537#/in.
- I = 1326.8 in.4
- Total I = (6-1) 1326.8 = 6630 in.4
Therefore Defl. = 5.74 x 537 / 6630 = 0.465 in.
To determine intermediate deflections, the following formulas apply:
- 1/8 point = 0.389 x center deflection
- 1/4 point = 0.7125 x center deflection
- 1/3 point = 0.869 x center deflection
- 3/8 point = 0.925 x center deflection
Any of the 3 beam groups indicated in the preceding example will be satisfactory with regards to deflection. It should, however, be noted that 12 in. BP sections will necessarily be spaced on approximately 22 in. centers. In addition, if work bridges or the finishing machine are supported from the two exterior beams, it will cause excessive deflections due to the relatively low moment of inertia of the member. This can lead to poor lines and/or grade of finished structure. The 21 in. WF beams on the other hand offer the most economy when considering the steel weight required and will also furnish the best support for finishing equipment supported from the two exterior beams. The chief objection to these beams is their depth, which might become critical if a specified minimum vertical construction clearance is involved. In this case the 18 in. WF beams would probably be the logical compromise between vertical clearance and stiffness.
Concrete box girder type structures present a difficult problem to analyze in regard to falsework deflection since they are generally constructed by placing concrete in the bottom slab and webs, removing interior forms, and finally placing the top slab. After bottom slab and webs are placed, the structure develops some rigidity. As a result, the full weight of the top slab will not be carried by supporting falsework members. However, the 1/700 ratio between deflection and span should be observed based on the full load of bottom slab, webs and top slab. Final grade corrections should, however, be based on an assumed falsework deflection of only 75% of computed deflection. The inspector should realize that these assumptions are made on the basis of using uniform materials in excellent condition and in the manner detailed. Many other approaches are possible and can produce acceptable falsework. The condition of all falsework must be carefully inspected before making any decisions on final acceptability.
The calculations in the example give dead load deflection which must be added to camber designated on the plans for these girders. The introduction of intermediate falsework supports requires the use of different calculations. If the procedure or amount of deflection provided seems questionable, immediately contact the District Construction and Materials Engineer for assistance.
703.2.3 Falsework Inspection
Inadequate falsework always offers a perplexing problem as the engineer may be reluctant to criticize or condemn falsework since the contractor is still responsible for the finished product. This approach is probably correct unless some basic engineering principle is violated. An example is: falsework founded on mud sills, where stability of the soil is questionable and there is a strong probability of the soil receiving additional moisture during construction. Other cases are use of green or rotten timber, or steel in such a state of deterioration that stresses cannot be computed, use of improper spacing, application of inadequate bearing in both vertical and horizontal supports such as use of thin wood shims at joints. Where falsework appears to be marginal, the contractor should be informed by written order that it is the judgement of the engineer that the falsework is inadequate and should settlement occur that damages the quality of the structure or allows forms to sag or bulge from correct lines, the concrete will be considered unsatisfactory.
This will supply the necessary warning. Should excessive settlement of falsework occur, the contractor should be given another order record, Form C-259, to the effect that the concrete does not meet the requirements of the specifications and is considered unsatisfactory. This order should be given at the earliest possible time, while concrete is still fresh if possible, so the contractor will be spared the expense of removing concrete which has already set. The resident engineer or the delegated representative should inform the District Construction and Materials Engineer immediately if possible.
703.2.4 Deck Forms
Deck forms must be mortar tight and constructed to produce a deck of proper thickness true to line and grade. Forms for slab type structures are supported on falsework. Forms for box girder decks are usually designed to remain in place in the completed structure and are supported from the previously cast webs. Forms for decks cast on structural steel members are supported from the steel itself by various types of hangers or braces.
Falsework for slab type structures and box girders must incorporate screw jacks placed at approved locations to secure and maintain the required camber. Attachments to the forms, usually called "tattle-tales", must be used for these structures to check settlement as the weight of concrete is added to the forms. Settlement must be checked by the above means and adjusted by the screw jacks to assure that the finished product is to proper grade.
On steel structures, overhang forms outside exterior beams or girders are usually the area of greatest potential for problems with grade control, alignment, and mortar tightness. Most contractors support the finishing machine on this part of the forms which tend to pull the forms away from the girder and rotate them out and down. Various methods have been developed to combat this problem.
703.2.5 Void Tubes
Void tubes must be solidly anchored to prevent uplift from displacing the tubes during concrete placement. The anchorage system is subject to approval by the engineer. One point to be watched closely is the band, which must go completely around the tube.Contact the Division of Construction and Materials if there is a concern about void tube anchorage.
Before use, void tubes must be protected from exposure to weather or moisture. If exposed to rain or stored on damp ground, they slowly absorb moisture and become soft and easily distorted.
Various methods are employed for forming drain holes from void tubes. The important inspection item is to be sure they are open.
703.2.6 Grade Control
All elevations must be accurately controlled. Otherwise, a smooth riding surface and pleasing appearance is virtually impossible to achieve.
For slab or box girder type structures, it is necessary to provide camber for future settlement. This must be blocked into the forms with proper allowance for timber squeeze. Excessive deviation during concrete placement would be indicated by the "tattle tales". Such deviation would require that the forms be lowered or raised by adjusting the jacks.
On most steel structures, the forms must be built above the girders (haunched) to allow for deflection of the girder under the load of the deck. On some steel structures the girders are designed to be precambered at the shop. The haunch is then at a theoretical constant distance above the girder.
In either case, elevation of the steel must be checked after it is erected in its final position. Normal tolerances, which are accepted as a part of the fabrication process, will lead to deviations from theoretical in-place position of the steel. Forms must be adjusted to correct for these deviations if a deck of proper thickness and grade is to result. The haunch itself will vary in height because of these corrections. Major changes in height of haunches will affect the amount of concrete and should be considered in determining final quantities.
The checking process consists of taking elevations on each beam or girder at points where the haunch or camber is indicated on the plans. The dead load deflection diagram normally indicates what percentage of the deflection will occur as the result of the structural steel's own weight. This portion of the deflection places the theoretical in-place steel position the computed distance below a straight line from bent to bent on conventional structures or below camber line on precambered structures. With the theoretical steel elevation known, it is possible to compute grade rods for the various points. Difference between grade and actual rod reading at each point is then added to or subtracted from the theoretical haunch. The corrected haunch is normally marked on top of the girder for use by the carpenters. Be sure everyone understands the point on the steel from which the haunch is to be measured.
Many contractors support finishing machine rails directly from the outside forms. The rails or guides must incorporate some type of adjustment other than wedges, as part of their support system. Jacks included as a part of the overhang support units are used to adjust the bottom deck forms to grade and are not to be used as adjustments for the finishing machine rails or guides.
703.2.7 Machine Finishing
Except for irregular areas or for structures excepted by special provisions, all riding surfaces on bridges and surfaces to receive a wearing course must be finished by use of a mechanical finishing machine. Any machine proposed must meet the approval of the engineer. Pan vibrators on bridge deck finishing machines are not vibrating screeds.
Use of a vibratory screed is disallowed per Sec 703.3.5; however, there are a few instances where the Bridge Designer might allow a vibratory screed. (See EPG 7126.96.36.199 Deck Concrete Finishing.) The decision to allow a vibrating screed will be made pre-bid, and if allowed, the Bridge Designer will place an approval note on the Bridge Plans. Post-bid requests from contractors to use a vibrating screed when there is not an approval note on the plans should normally be denied. If there is an unusual instance where the Resident Engineer feels it should be considered, contact the Structural Project Manager and your Construction Liaison Engineer for evaluation of the request. If approved, there will likely be a contract deduction requirement to account for the reduced cost for bracing of the overhang as well as the anticipated shortened lifespan of the deck.
All machines approved to date have enough points in common to make the inspection task fairly routine. The rails must be properly supported both vertically and laterally to maintain true grade. Rails and wheels of the machine must be clean. Proper crown and slope must be set. This can be checked from a taut string. Screeds normally are set in accordance with the manufacturer's recommendation.
After the finishing machine has been checked out, it should be moved over the entire portion where concrete is to be placed. This allows checking the clearance to reinforcing steel, required deck thickness over forms or concrete slab form panels, and conformity with headers or expansion plates. It is also a check on rigidity and alignment of rails.
During concrete placement, it is important that concrete be deposited uniformly and approximately to grade. Movement of large quantities of concrete with finishing screeds cause the machine to ride up and leads to surface irregularities. Screeds or rollers should be lowered to the surface with the machine in forward motion. The number of passes required will vary due to many factors but should be enough to provide a smooth surface meeting straight-edge requirements. The final pass should be delayed to cover as much length of deck as possible for best results.
Following the machine finishing operation, the surface must be checked with a straight edge. If irregularities are found, the area must be refinished and checked again until the irregularities have been eliminated.
Slab and box girder structures usually have specified construction cambers. When they are laid out on a heavy skew, it will be necessary to skew the finishing machine to prevent distortion of the surface. If analysis of surface grades determines that the screed on the finishing machine normal to centerline would deviate more than 1/8" from the design surface at any point, the machine must be skewed to match the skew on the structure. The Special Provisions should be reviewed for conditions and degree of skew for skewing the machine.
Sec 703.3.6.3.4 states: "The concrete shall be covered with clean mats as soon as the interim curing compound has dried sufficiently to prevent adhesion, and the concrete surface will support the curing mat without marring or distorting the finish, but not more than 90 minutes after the concrete is floated or textured." The 90 minutes is a guidance. There may be situations where the concrete has not set sufficiently in 90 minutes. The concrete should be checked frequently in this situation. The wet burlap should then be placed as soon as marring or distortion of the finish will not occur.
703.2.8 Concrete Placement
Concrete production may be from central mix plants, truck mix, or on site mix. Instructions for the plant inspector are to be found in guidance for rigid pavement inspection. Regardless of the method used to produce concrete, air and slump tests must be made at the structure site. Cylinders for compressive testing must also be cast as needed. Instructions for sampling and testing of concrete and frequency of tests are also found in guidance for rigid pavement inspection.
Concrete is usually placed in substructure units by use of bottom dump buckets transported by crane and/or by tubes or chutes. Any chute used must be equipped with baffles to prevent segregation. Tubes are usually assembled from short joints flexibly coupled or are fabricated of flexible rubberized or plastic coated material. The later type is sometimes called an "elephant trunk". These flexible tubes resist free fall and thus minimize segregation. Concrete may also be conveyed or placed by mechanically applied pressure.
Concrete is usually placed in wall sections through tubes or chutes. These must extend far enough inside forms to restrict the drop to that permitted by specifications.
There are additional options available to the contractor for placement of concrete in decks. Specified rates of pour are often quite high and difficult to achieve with bucket placement. Several types of belt conveyers and spreading units have been developed. When properly adjusted, these systems are capable of high speed placement with little or no segregation. They can successfully place concrete quite near its final position. These systems should not be supported on reinforcing steel. Concrete for riding surfaces may be conveyed or placed by mechanically applied pressure using approved concrete pumps.
When an approved concrete pump is used, the designated location for quality control sampling to determine air content and slump is the point of discharge from the mixing truck. See Concrete - Sampling for more information on sampling pumped concrete.
Vibration is an essential part of concrete placement. Its purpose should not, however, be confused with methods for distribution. Vibration is to densify, not move concrete.
Specified rates of concrete placement are minimum (not overall averages) and must be met for any one hour period during placement. These rates are often quite critical on steel structures where reversal of stresses is involved. On structures supported on falsework, such as slabs or box girders, the rates are less critical from a structural standpoint but represent a minimum standard for quality workmanship. Rates in the latter case are related to finishing progress considered a minimum for satisfactory results. If placement rate lags seriously on a steel structure, it may become necessary to require installation of a temporary header. Resumption of placement would then depend on location of the emergency joint and results of strength tests on the concrete. Minor deficiencies in placement rate may not justify cessation of the pour but are a proper basis for refusing to permit additional pours until some arrangements have been made to improve placement. Unless problems other than deficiency in rate of placement develop during a pour on slab or box girder structures, the inspector may permit placement to continue to a standard joint shown on the plans but should not permit placement beyond that point or resumption of placement at a later date until changes have been made in the supply arrangements to maintain specified rate of placement. These actions should be documented by order records.
|Placing Diaphragm Concrete|
|This presentation explains a MoDOT best practice.|
A construction joint will normally be provided at the top of the paving notch in the back wall with steel stringers cast into the end bent backwall. The backwall should be poured in accordance with the instruction note given on the plans that concrete diaphragms at the integral end bents shall be poured a minimum of 12 hours before the slab is poured. As will be shown on the plans, the construction joint may or may not conform with the crown of the roadway. If no construction joint is provided, contact the Division of Construction and Materials Office.
Transverse construction joints may be eliminated upon the contractor's written request contingent on a demonstrated ability to place and finish concrete at the specified rate. This request should be detailed and contain such items as manpower, equipment, source and rate of delivery and method of placement. These requests can be approved by the District Construction and Materials Engineer based on the resident engineer's recommendation. Longitudinal construction joints in the decks ordinarily are required by desired limitations on finishing widths and are not to be eliminated unless approved by the Construction and Materials Division. To date it has been deemed advisable to maintain a limit of 52 feet on the maximum finishing width of all approved machines measured along the centerline of the machine whether normal to the roadway or skewed. Contract Special Provisions should be checked for bridge deck sequences of placing and finishing.
After the concrete has been properly finished, the surface must be textured to provide a non-skid surface. A hand operated device producing a textured surface equivalent to that required for machine combing should be used. The time of texturing should be carefully chosen to avoid damage to the surface finish but should be early enough to assure adequate indentation. Overlapping of the comb or finned float should be avoided, small gaps are acceptable.
The surface of all deck concrete and other surfaces to be surface sealed must be cured with continuously wet mats. The mats must be wet when placed so they will not absorb moisture from the fresh concrete. They should be placed at the earliest possible time at which surface marring will not occur.
Plastic covers are required for curing latex concrete in accordance with the Special Provisions. Plastic covers over wet burlap is permitted for curing low slump or silica fume concrete after 24 hours of continuously wet cure, in accordance with the Special Provisions.
Curing of such surfaces shall continue for not less than 7 days or as specified in the special provisions. This period may be extended if specified strengths given in the table under Sec 703.3.2.13 have not been attained.
Sec 703.3.6.3.4 states "The concrete shall be covered with clean mats as soon as the interim curing compound has dried sufficiently to prevent adhesion, and the concrete surface will support the curing mat without marring or distorting the finish, but not more than 90 minutes after the concrete is floated or textured." The 90 minutes is a guidance. There may be situations where the concrete has not set sufficiently at 90 minutes. In this situation the concrete should be checked frequently. The wet burlap should be placed as soon as marring or distortion of the finish will not occur.
Care should be taken during the 7-day wet cure not to damage or mar the bridge deck surface. Laying sprinkler hoses and sprayers before a proper set is attained will lead to indentations and imperfections on the surface. The photo to the right demonstrates damage caused by a sprinkler head that was discharging directly into the deck. This causes the removal of cement paste, exposing the aggregate. Placing sprinkler systems on the deck too quickly or haphazardly will cause permanent damage to the deck surface.
703.2.10 Cold Weather Placement
The specifications state that no concrete shall be placed where the ambient temperature is below 35°F and superstructure concrete shall not be placed when the ambient temperature is below 40°F. Superstructure concrete is not ordinarily placed under winter conditions. Substructure units, and superstructure units if placed, must be protected by housing and heating or insulation. The principal inspection problem is to assure that uniform heat is maintained throughout the enclosure and that proper moisture is provided. Sometimes concrete is subjected to sub-freezing temperatures through failure of heating systems, wind damage to housing, etc. If there is any evidence that the surface of the concrete froze, the concrete should be rejected. Any violation of specification requirements should be documented and the district should recommend appropriate action to Construction and Materials Division if the situation does not seem to justify outright rejection.
The use of insulation with forms for the protection of concrete does not constitute a waiver of the requirements of the Standard Specifications for protecting and curing concrete in structures. All concrete such as wingwalls, backwalls, etc. having a thickness of l2" or less will require the addition of housing and heating to supplement the insulation in severely cold weather. In general thin sections for which insulation is not efficient are considered to be those which have less than 0.02 cubic yards per square foot of surface area. Securing the proper temperature in concrete is dependent on the ratio of the volume of concrete to its surface area and the differential in temperature between each side of the insulation.
Some important points to consider in the use of insulation are as follows:
- l. The contractor should provide a sufficient number of thermometer wells to provide a check on the concrete surface temperatures.
- 2. Concrete poured in moderately cool weather can develop excessive temperatures, and it is occasionally necessary to loosen the insulation to balance the temperature rise.
- 3. In severely cold weather, to prevent the conduction of cold by the protruding reinforcing steel, it may be necessary to provide supplemental heat at critical points.
- 4. Care should be taken to check the temperature periodically at critical points until the concrete has reached its required strength.
703.2.11 Hot Weather Concreting
Placement of superstructure concrete shall not be done when the ambient temperature is above 85°F. The internal temperature of the concrete shall not be greater than 85°F at the time of placement in the forms, regardless of ambient temperature.
Procedures For Checking Surface And Ambient Temperatures - MoDOT Test Method TM-20 Measurement of Air, Surface or Bituminous Mixture Temperature describes the methods for checking surface temperatures and air temperatures in the immediate vicinity of the work.
The PODI Bridge Deck Pour Checklist is a guide to the inspector during the sequence of operations associated with a deck pour.
703.2.13 Form Removal
Forms and falsework must be left in place until strength specified in the table in Sec 703.3.2.13 has been attained. Removal of falsework requires care to prevent damage to the concrete. Requirements in Sec 703 of the Standard Specifications must be carefully followed.
Honeycomb and indentations left by form hardware such as snap ties must be carefully repaired by filling with mortar as specified in the Standard Specifications. Particular attention must be given to color of the mortar. It may be necessary to add a small amount of white cement to match the color of the concrete to be repaired. Such patches must be carefully cured.
703.2.14 Surface Sealing
The principal problems with surface sealing are failure to clean the surface properly and non-uniformity of application. Any dirt left on the surface will absorb sealing material and prevent its penetration into the surface. Non-uniform application results in streaking and mottled surface appearance. The inspector must insist on a clean surface and careful spreading of the material. Other requirements of Sec 703 normally require only routine inspection attention.
Unless permitted by Special Provisions, the structure shall not be opened to through traffic before deck is sealed. Essential construction traffic, such as ready mix trucks, self-propelled concrete buggies, concrete conveyor systems, and so forth, can be permitted where no practical alternative method of completing the structure is available. In all cases, traffic must be barred from the structure until the concrete has reached the compressive strength specified in Sec 703.
Rate of placement should be checked against specifications. The rate should be documented in a field book by listing the quantity of sealing material placed in each application and the computed surface area treated. As usual the entry must be dated and signed by the inspector. The rate of placement of sealant for low slump concrete is approximately one-half that for B-2 concrete.
Latex modified concrete and silica fume concrete are not to be surface sealed.
703.2.15 Opening To Traffic
Sec 703 establishes strength criteria for loading new structures. These requirements are intended to prohibit heavy loading of concrete in early curing stages. They are slightly higher than requirements for form removal. It may be necessary to cast extra test cylinders for control of both phases of the work.
Notify the Construction and Materials Division by letter, with copies to the Bridge Division and the Maintenance Division advising the date the structure is opened to traffic. The letter should include the bridge number of the new structure and if applicable, the number of the structure that has been replaced.
703.3 Bridge Barrier Wall
Any crack less than .002 inches is considered hairline. Hairline cracks are tight and no special sealing shall be required. Any crack above .02 inches can be epoxy injected. This is the thickness of a typical business card. If your business card will fit in the crack you should require the epoxy injection. There are also crack template guides that are available at headquarters. If the cracks are between .002 and .02 inches normal sealers are ineffective on a vertical surface. (If the sealer is thin enough to fit in the crack it will not stay in a vertical surface.) Even though these size cracks can't be repaired a deduction may be in order. Deductions would be based on the number of cracks and the adequacy of the contractor's method of placement and curing. Deductions can be made on barrier wall with epoxy injection repairs. It is still a repaired product and not as originally intended. If deductions are made the amount would be considered on an individual basis and at the RE’s discretion. If the contractor has complied with all specifications any repairs would be at MoDOT's expense.
The full 7 day wet cure for the deck is preferred prior to placement of the barrier curb. There are steady improvements in the desired concrete qualities such as reduced permeability and resistance to scaling for the entire curing period. For emergency bridge replacements, or highly accelerated construction, allowance of placement of the barrier wall on day 6 or 7 can be allowed by JSP if the minimum compressive strength shown on the plans has been achieved. A minimal amount of burlap should be temporarily relocated to place the wall and immediately re-installed in a wet condition. The wet cure on the deck should remain in place for the full 7 days. If the contract does not include a JSP to allow early placement of the barrier wall, a division level change order is required.
703.4 Sound Walls
The following is acceptance criteria for cracks in sound wall panels.
- A panel with cracks exceeding 0.5 mm should be rejected.
- Due to aesthetic considerations, the limited size of acceptable cracks, and the application of graffiti protection, attempts to repair cracks should not be made.
- Panels with more than one full depth crack should be reviewed on a case by case basis
- Panels with cracks spaced closely together, say less than 2', will be reviewed on a case by case basis
- If more than several cracks exists the inspector may reject the panel and the fabricator should attempt to determine and rectify the cause of cracking.
- Any panels that exhibit damage due to impacts or improper handling will be rejected.
703.5 Laboratory Testing for Sec 703
To establish procedures for Laboratory testing and reporting of portland cement concrete cylinders and the testing and reporting samples of concrete sealer for concrete masonry construction.
703.5.2.1 Concrete Cylinders
Concrete cylinders shall be tested for compressive strength according to AASHTO T22. Test results and calculations shall be recorded through SiteManager.
703.5.2.2 Surface Sealers
Surface sealers for concrete bridge decks shall consist of an approved product in accordance with Sec 703.
Concrete surface sealers shall be tested and reported according to guidelines in Sec 1053.
703.5.3 Sample Record
The sample record shall be completed in SiteManager as described in Sample Record General Information . Test results for concrete cylinders shall be reported on the appropriate templates under the Tests tab. The notation, "Specimens submitted for compressive strength at the age indicated and the tests are for informational purposes only" should be included in the sample record remarks.