11.50 CONCRETE
(STRUCTURAL, CLASS X, AND FLOWABLE MORTAR)

11.51 PCC PLANT PAGE  (FORMS 800240E and 800240M)

The project engineer shall report weekly all concrete placed for each project on "PCC Plant Page" (Form 800240). This form will record concrete placements, all results of sieve analysis tests, and all data on test beams made and tested. The week covered by each report shall begin on Sunday morning and end on Saturday evening.  A separate Form 800240 is required for each bridge design, including bridge deck surfacing and resurfacing, and each group of culverts. Refer to Materials I.M. 527 for instructions on preparing this form.

11.52 USE OF READY MIXED STRUCTURAL CONCRETE

11.53 ADMIXTURES

Admixtures are those ingredients in concrete other than portland cement, water, and aggregates, that are added to the mixture immediately before or during mixing. Admixtures typically encountered on our jobs can be classified by function as follows:
• Air entraining admixtures
• Water reducing admixtures
• Set retarding admixtures
• Set accelerating admixtures
• Corrosion inhibiting admixtures
• Finely divided and permeability mineral admixtures (Fly Ash, Ground Granulated Blast Furnace Slag & Silica Fume)
• Coloring agents (normally not used for Iowa DOT work)

The amount of any admixture used in a mix should be as recommended by the manufacturer and verified through laboratory testing or trial mixes. Effectiveness of an admixture depends upon such factors as type, brand, and amount of cement; water content; aggregate shape; gradation and proportions; mixing time; slump; and temperatures of concrete and air.

Air Entraining Admixtures
Air entraining admixtures are used to purposely entrain microscopic air bubbles in concrete. Air entrainment will dramatically improve the durability of concrete exposed to moisture during cycles of freezing and thawing. Entrained air greatly improves concrete's resistance to surface scaling caused by chemical deicers.



Rules-of-Thumb
• As cement content increases, air agent must increase to maintain equal entrained air.
• As cement fineness increases, the amount of air agent must increase to maintain equal entrained air.
• As coarse aggregate size decreases, the air content increases for a given amount of air agent.
• As fine aggregate volume increases, the air content increases for a given amount of air agent.
• As mixing water increases, the air content increases for a given amount of air agent.
• Air entraining admixtures should be introduced into mix at the plant, but additional may be added at the site to adjust mix for correct air content.
• Air entraining admixtures should (usually) be added to the front of the truck at the plant. If corrosion inhibiting admixture is used, air entraining agents should be added to the back of the truck.

Water Reducing Admixtures - Regular
Water reducing admixtures are used to reduce the quantity of mixing water required to produce concrete of a certain slump or reduce the water-cement ratio. Regular water reducers reduce water content by about 5% to 10%.

Adding a water reducing admixture to a mix without reducing water content can produce a mixture with a much higher slump.

Rules-of-Thumb
• Typically, water reducing admixtures do not reduce the rate of slump loss; in most cases, it is increased. Rapid slump loss results in reduced workability and less time to place concrete at the higher slump.
• Typically, water reducing admixtures have no effect on bleed water.
• Certain types of sulfate starved portland cements may cause false-set with certain brands of water reducers. Typically, water reducers contain lignosulfonates and these sulfates are easily attracted by sulfate starved cements. This action may cause early false-set.
• Despite reduction in water content, water reducing admixtures can cause a significant increase in drying shrinkage.

Water Reducing Admixtures - Super Plasticizers
Super plasticizers are simply "high-range water reducers."  They are added to concrete with low-to-normal slump and water content to make high slump "flowable" concrete. Flowable concrete is a highly fluid, but workable concrete that can be placed with little or no vibration and can still be free of excessive bleeding or segregation. Flowable concrete has applications:

1. In areas of closely spaced and congested reinforcing steel
2. In tremied concrete where "self consolidation" is desirable
3. In pumped concrete to reduce pump pressure
4. To produce low water-cement ratio - high strength concrete. High-range "super plasticizers" can reduce water content by about 12% to 30%.


Rules-of-Thumb
• The effect of most super plasticizers in increasing workability or flowable concrete is short lived. Typically, maximum is 30 to 60 minutes followed by a very rapid loss in workability.
• Typically, super plasticizers are added as split treatments (part at the plant, part at the site).  Sometimes the addition is totally at the site.
• Setting time may be affected depending on the brand used, dosage rate, and interaction with other admixtures.
• Excessively high slumps of 250 mm (10 inches) or more may cause segregation.
• High-slump, low water/cement super plasticized concrete has less dry-shrinkage than does high-slump high water/cement conventional concrete.
• Effectiveness of super plasticizer is increased with an increased amount of cement, and/or increased fineness of cement.
• Effectiveness of water reducers on concrete is a function of their chemical composition, cement composition and fineness, cement content, concrete temperature, and other admixtures being used.
• Some water reducing admixtures, such as lignosulfonates, may also entrain some air in the mix.

Retarding Admixtures
Retarding admixtures (retarders) are used to delay the initial set of concrete. High temperatures of fresh concrete 30^oC (85^oF) and up often cause an increased rate of hardening.  Since retarders do not decrease the initial temperature of concrete, other methods of counteracting the effect of temperature must be used.



Rules-of-Thumb
• Retarders are sometimes used to delay initial set of concrete when difficult, long placement times, or unusual placement conditions exist.

NOTE: Retarders are not to be used when the anticipated temperature of the mix is below 13^oC (55^oF); however, placement requirements must be met within the initial set time indicated for the non-retarded concrete.

Retarding admixtures require a concrete temperature of 13^oC (55^oF) or greater in order to activate and effectively retard the set of concrete.  If the proposed placement cannot be accomplished within the initial set time for non-retarded concrete, the concrete mix temperature will have to be increased through the use of heated materials.  When heated materials are used, it is recommended that a concrete mix temperature of 18^oC (65^oF) be targeted for effective activation of retarding admixtures.

• Retarders offset the set acceleration effect of hot weather.
• Retarders can be added at the site.
• In general, some reduction in strength at early ages (one to two days) accompanies the use of retarders.
• Use of retarders must be closely monitored, because there is probably no single admixture which has caused more field problems.
• If too much retarder has been used in a mix:
1. Time will usually counter the effects.
2. "Be sure" to maintain the cure during the added time.

Accelerating Admixtures
Accelerating admixtures (accelerators) are used to accelerate the setting time and strength development of concrete at an early age. Strength development can also be accelerated by using:

• Type III "high-early" cement
• Lowering water/cement ratio
• Curing at controlled higher temperatures

Calcium Chloride (CaCl₂) is the material most commonly used in accelerating admixtures.  Besides accelerating strength gain, calcium chloride also causes an increase in drying shrinkage, potential reinforcement corrosion, discoloration, and potential scaling.

Rules-of-Thumb
• Always add calcium chloride in solution form as part of the mixing water.
• Calcium chloride is not an antifreeze agent.  When used in allowable amounts, it will only reduce the freezing point of concrete by a few degrees.

Corrosion Inhibiting Admixtures
Concrete protects embedded steel from corrosion through its highly alkaline nature (12.5 pH).  This causes a passive and non-corroding protective oxide film to form on steel. However, carbonation or the presence of chloride ions from deicers, can destroy or penetrate the protective film. Once this happens, an electronic cell (very small battery) is formed and an electro-chemical process of corrosion begins. This process ultimately forms rust.  Rust is expansive (up to 4 times original volume). This induces internal stress and eventually causes spalling to occur.

Corrosion inhibiting admixtures chemically inhibit the corrosion reaction.  Calcium nitrite, the most commonly used inhibitor, blocks a corrosion reaction by chemically reinforcing the concrete's passive film.

Rules-of-Thumb
• Corrosion inhibitors should be added at the plant.
• Experience indicates corrosion inhibitors should be placed in the front of the truck (first-in) and air entrainment agent should be placed at the back (last-in).
• Corrosion inhibitors are accelerators and will affect set times. It is recommended to consider adding about a one-half dose of retarder to extend working times.
• Air content of mixes using corrosion inhibitors is often difficult to stabilize. Watch the target air closely.
• A certain amount of calcium nitrite can protect up to a certain threshold level of chloride.  Therefore, the amount of corrosion inhibitor added to a mix must be developed for an assumed maximum level of chloride ingress expected.

Finely Divided Mineral Admixtures
These admixtures are powdered or pulverized materials added to concrete to improve or change the properties (plastic or hardened) of concrete. Based on the mineral's chemical or physical properties, they are classified as: (1) Cementitious, (2) Pozzolans, (3) Pozzolanic and Cementitious, and (4) Nominally inert. Typical PCC mix designs use #3 above.

Pozzolanic Materials
A pozzolan is a siliceous or aluminosiliceous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of water, chemically react with the calcium hydroxide released by the hydration of portland cement to form compounds possessing cementitious properties.  Pozzolans include fly ash and silica fume.

Fly Ash (Class C & F)
Fly ash is a finely divided residue that results from the combustion of pulverized coal in electric power plants.

Silica Fume
Silica fume, also referred to as micro-silica or condensed silica fume, is another material that is used as a pozzolanic admixture. This light to dark gray powdery product is a result of the reduction of high-purity quartz with coal in an electric arc furnace.

Fly ash and silica fume have a spherical shape.  Silica fume has an extremely small particle size (about 100 times smaller than the average cement particle).  Although silica fume is normally in powder form, because of its small size and increased ease of handling the product is commonly available in liquid form.

Cementitious Materials
Cementitious materials are substances that alone have hydraulic cementing properties (set and harden in the presense of water). Cementitious materials include ground granulated blast furnace slag.

Ground Granulated Blast Furnace Slag (GGBFS)
GGBFS made from iron blast-furnace slag is a non-metallic product consisting essentially of silicates and aluminosilicates of calcium and other bases developed in a molten condition simultaneously with iron in a blast furnace. The molten slag is rapidly chilled in water to form a glassy sandlike material which is ground to a particle size similar to fly ash. Unlike fly ash and silica fume which have a spherical shape, GGBFS is rough and angular-shaped.

Rules-of-Thumb
• Mixes containing fly ash or GGBFS will generally require less water (about 1% to 10%) for a given slump. Silica fume concrete requires more water for a given slump.
• The amount of air-entraining admixture required to obtain a specified air content is normally greater when fly ash or silica fume is used. Ground slags have variable effects on the required dosage rate of air-entraining admixtures. The amount of air-entraining admixture for a certain air content is a function of the fineness, carbon content, and alkali content.
• Fly ash and ground slag will generally improve the workability of concretes of equal slump. However, fly ash in low slump concrete will tend to tear and have reduced workability. Silica fume tends to reduce workability, thus high-range water reducers are usually added to maintain workability.
• Concrete using fly ash or silica fume generally shows less segregation and bleeding than plain concrete.  Concrete using some ground slags tend to have slightly higher bleeding than plain concretes, but have no adverse effect on segregation.
• Use of fly ash and ground slag will reduce the amount of heat build-up in concrete. Silica fume most likely will not reduce the heat of hydration, because typically high-range water reducers are used and they increase mass temperatures.
• Use of fly ash and ground slag will tend to generally retard the setting time of concrete. Silica fume alone will accelerate the setting time, however, high-range water reducers tend to offset this.
• Use of fly ash and ground slag generally aids the pumpability of concrete.
• With adequate and correct curing, fly ash and ground slag generally reduce the permeability.  Silica fume is especially effective in this regard.

11.54 USE OF INSULATED FORMS FOR PROTECTION

Commercial insulation may be used for protecting concrete during cold weather, or when the contract documents require controlling the heat of hydration.  This technique is the contractor's option and could be used in lieu of housing and heating. It will then be the contractor's responsibility to furnish insulation of sufficient quality and thickness to maintain concrete at a temperature of not less than 10^oC (50^oF) for the first 48 hours after placing, if air temperatures will be less than 5^oC (40^oF). (Refer to Specification 2403.08, Paragraph H.)

Concrete must be between 7^oC and 27^oC (45^oF and 80^oF) when placed. To ensure a concrete temperature of at least 10^oC (50^oF) for 48 hours after placement, the concrete for thin sections such as culvert walls, end posts, piling encasements, etc. should be 18^oC (65^oF) or higher, since the only additional heat source is the heat of hydration.  Concrete for massive sections such as stub abutments, heavy piers, and footings should be in the 13^o to 18^oC (55^o to 65^oF) range.

Since only dry insulation is effective, any insulation that has a propensity to adsorb water or become saturated must be protected with a waterproof membrane. The insulation system must provide complete coverage and be secured to provide maximum protection during the full curing period.

For typical protection applications, insulated forms must be left undisturbed for 96 hours before being removed. (Refer to Specification 2403.11.)

Checking Temperature of Concrete
For checking compliance with minimum temperature requirements during the 48-hour period after placement, thermometer wells should be cast in the concrete during the pour. The following procedure for checking temperature is suggested:

1. Drill an 8 mm (5/16 inch) hole through the form at one or more locations where temperature checks will be made.
2. Grease the thermometer probe and insert it through the hole about 100 mm (4 inches) into the plastic concrete.
3. Remove probe after the concrete is set and cover hole with insulating material.
4. Further checks can be made by inserting the thermometer through the insulation into the well developed in step 2. Leave thermometer in place if desired, but protect from damage or theft.

NOTE:  The thermometer stem should be inserted about 75 mm (3 inches) into the concrete because the sensitive portion of stem is about 70 mm (2 3/4 inches) below the groove.

Other acceptable methods for monitoring concrete temperature are the use of maturity meter with temperature probe wires embedded in the concrete or use of thermal ‘iButtons’ embedded in the concrete with exposed wires for data collection and recording.

Record temperature daily for 48 hours following the pour. Temperature readings below 10^oC (50^oF) during the first 48 hours should be reported to the Office of Construction for evaluation of possible damage or price adjustment.

11.55 DECK PLACEMENT AND HEAT OF HYDRATION

Cracking of concrete in bridge deck placements and large concrete elements (ie: bridge footings, columns, pier caps, etc.) can occur unless the placements are properly controlled. The following provides information on measures that are used in the effort to control cracking of concrete in bridge decks and large concrete element placements.

Deck Placement
Sometime ago the Office of Bridges and Structures, Office of Materials, and Office of Construction began evaluating the phenomena of bridge deck cracking. Measures have been implemented to manage bridge deck placement and prevent cracking through the use of Evaporation Rate Controls.

Research continues in the management of quality bridge deck placements and deck cracking control.  To provide needed site specific data for this research, Forms E122, E139, M122 and M139 were developed.  These reporting forms were initiated during 1991.  Since that time, the information provided from the field has been compiled into a database for evaluation. The evaluation of this data is ongoing and includes review of the effectiveness of Evaporation Rate Controls and possible trends which may lead to a better understanding of crack development.

Forms E122, E139, M122 and M139 are included in Appendix 11-16. Since they are not available in Office Supplies, please photocopy as needed.  Submit completed forms to the Office of Construction.

Deck Concrete Temperature and Curing
Specification 2412 identifies requirements for placing and curing concrete bridge floors.  Of importance for this section are:

• Plastic concrete, when placed, shall not exceed 32^oC (90^oF).
• Concrete floors will not be placed if the theoretical rate of evaporation exceeds 1 kg/m² /hr (0.2 lbs./sq.ft./hr.).

NOTE: A theoretical evaporation chart is included in Specification 2412.05. As an alternative, a computer program has been developed for calculation of theoretical rate of evaporation using Excel. This program incorporates the charts from the specifications in a formula table included on report Forms E122 and M122. The program simplifies the determination of the theoretical rate of evaporation and enables the user to perform trial evaluations for possible changes in air temperature, relative humidity, plastic concrete temperature, and wind velocity.  A copy of the Excel program for theoretical rate of evaporation is available at

English - http://www.iowa.dot.gov/construction/structures/theoretical_evaporation_rate.xls or
Metric - http://www.iowa.dot.gov/construction/structures/theoretical_evaporation_rate_metric.xls

• The curing method requires prewetted burlap to be placed within 10 minutes of final finishing and followed by a "wet" burlap cure for four (4) days. A continuous sprinkling system is required to keep the burlap wet during this time.
• Plastic, in addition to wet burlap, may only be used between October 1 and April 1. The plastic provides a moisture proof barrier above the wet burlap and replaces the sprinkling system after 20 hours of the application of water during cold weather.

The placing of concrete will require close monitoring to comply with the specification. The contractor or ready mix plant should determine temperature of previously placed concrete to project a mix temperature prior to a deck pour. Further, they should obtain a weather report to determine predicted air temperature, wind velocity, and relative humidity for the pour day. Based on this information, you will be able to reasonably predict an evaporation rate.

The above information should be discussed by the inspector, contractor, and ready mix plant operator before a deck pour. The pour should not be attempted if concrete temperature is predicted at 29^o C (85^oF) or higher and predicted air temperature is above 32^oC (90^oF).  Also, the pour should not be attempted if an evaporation rate would exceed  1 kg/m²/hr. (0.2 lbs./sq.ft./hr.).

District Materials Office has sling psychrometers and wind gauges available for usage the day of the pour. A sling psychrometer is used to determine the relative humidity by finding "wet" and "dry" bulb temperatures. (Refer to Charts in Appendix 11-17.) With these values, compute temperature difference and locate the "Difference Between Readings..." column.  Then locate the row labeled with appropriate dry bulb temperature. The value at the intersection of "Difference" column and "Dry" bulb temperature is the relative humidity.

EXAMPLE: (English units only)
If the dry bulb temperature is 71^oF and the wet bulb temperature is 64^oF, the difference is 7^oF. At the top of the chart, locate the column headed 7. Follow this column down to the dry bulb temperature row of 71^oF. The intersection indicates a relative humidity of 68%.

There are also electronic pocket weather meter/station devices (ie: Ketsrel) which is a hand-held instrument for air temperature, wind speed, and relative humidity determination which can be used for evaluation of the theoretical evaporation rate.

Placement Considerations
A. If there is any doubt about the concrete temperature exceeding 29^oC (85^oF), the contractor needs to identify measures which will be implemented to keep mix temperatures within specifications. If the contractor is not prepared to maintain a mix temperature below specifications, the pour should be postponed.

There are several ways concrete temperatures may be kept within specifications. They are:
• Scheduling placements during cooler times of the day
• Wetting the aggregate stockpiles
• Covering/shading the aggregate stockpiles
• Maintaining a supply of portland cement on hand to preclude getting hot material from the supplier
• Chilling the mixing water is one of the most effective ways to lower mix temperatures.
• Shaved ice can be used, however, the ready mix operator must submit a proposal for this to the project engineer for review by the Office of Construction.

NOTE:
1. No payment will be made for methods taken to keep concrete temperatures and evaporation rates within specifications.
2. If pour has to be delayed because of temperature, and pouring is the controlling operation, no working days will be charged.

B. Location of permissible headers should be discussed with the contractor.  If during the pour, it appears:

• The temperature may exceed 32^oC (90^oF)
• And/or the theoretical evaporation rate would exceed 1 kg/m²/hr. (0.2 lbs./sq.ft./hr.)
and these deficiencies cannot be corrected by immediate action, the placement shall be halted at the first permissible joint. On slab bridges, any joint location listed on the plans can be used.  For girder beam bridges (steel or concrete), placement may be stopped, in an emergency, at locations as follows:
Case A. (Continuous or noncontinuous beams, positive section)
If the positive section has not been completed:
Complete the positive section and stop at the header location shown on the plans.

Case B. (Noncontinuous beams, negative section)

If placement has not proceeded beyond the pier:
Do not place concrete in the pier diaphragm, and stop just short of the beam end.

Case C. (Noncontinuous beams, negative section)

If placement has progressed beyond centerline of the pier:
Placement must continue through that negative section and stop at the header shown on the plans.

Case D. (Continuous beams, negative section)

If problem occurs after starting the negative section:
Placement must continue through the negative section, and stop at the header shown on the plans.

See Appendix 11-24 for case illustration.

In every case listed above, contact the Office of Construction for curing times and beam break strengths before allowing the contractor to resume deck placement.

Field Documentation
The temperature of concrete should be taken as soon as concrete is placed. It should be taken when the first load is placed and at intervals shown on Forms E122 and M122, Appendix 11-16.  Additional checking is warranted if temperature is running at or near maximum. Air temperature should also be taken about the same time as the concrete temperature.

Heat of Hydration
Occasionally, projects will require placement of large volumes of concrete for individual concrete elements (ie: bridge footings, columns, pier caps, etc.). Controlling the temperature of this large volume is important to reduce cracks and potential premature deterioration from thermal cracking that can result from a large temperature difference between the center of the concrete element and its surface. In these cases the contract documents may require monitoring the "heat of hydration." There will also be requirements for the differences between specified monitoring locations. For example: "The temperature difference between the edge of the concrete and the center shall not exceed 10^oC (35^oF)."

The cooling of large volumes of concrete can take considerable time, and during that time monitoring is required. A form to record these temperatures has been developed. (Refer to "Heat of Hydration" form in Appendix 11-18.) Since this form is not included in Office Supplies, please photocopy as needed.  Submit completed forms to the Office of Construction.

11.56 PLACEMENT METHODS (PUMPING, BELTING, AND CRANE BUCKET)

Much concern has been expressed about the method of concrete placement because of lost entrained air. Rough handling of plastic concrete during placement has, at times, reduced entrained air to less than 2% not to mention potential segregation problems. While testing at the point of placement "should" identify such problems, varying placement conditions during the pour can affect concrete conditions significantly.

General conditions which must be avoided, or at least severely minimized, are as follows. If one of the following cannot be avoided, at least be aware of the condition, and be sure to conduct additional testing should any of the conditions present themselves.

Crane and Bucket
In the past it was felt the crane and bucket placement method did not adversely affect concrete.  This is now in question when viewed from loss of air and potential segregation. Therefore, this method will now also require testing at the placement location, if practical.

Points-to-Watch For
• Free fall of unrestrained concrete shall not exceed 2 m (6 feet) for vertical placement and 1 m (3 feet) for floors and slabs. (Refer to Specification 2403.08, Paragraph C.) If the distance is exceeded: (1) reduce the pour depth, (2) remove a section of form work for intermediate placement, (3) or use a tremie.
• Discharge from the bucket must be controllable.
• Cross section of the drop-chute should permit inserting into the form work without interfering with reinforcing steel.

Belt Placement
Belt equipment is typically used to convey concrete to a (1) lower, (2) horizontal, or (3) somewhat higher level.

Points-to-Watch For
• Keep the number and distance of drops between belts to an absolute minimum. Drops tend to encourage segregation and reduce entrained air.
• As belt conveyors are removed from the line (i.e., as on deck pours), recheck the "as placed" air content.
• Be sure all mortar is being removed at the discharge. (No mortar should be on the return belt.)
• Check discharge for potential segregation problems.
• In adverse weather (hot and/or windy conditions), long belt runs need to be covered.

Pump Placement
The modern mobile pump with hydraulic placing boom is economical to use in placing both large and small quantities of concrete. These units are used to convey concrete directly from a truck unloading point to the concrete placement area.

Points-to-Watch For
• Typically, pumps are initially flushed with a thin water/cement paste mixture to coat the lines.  This slurry must be wasted and the lines charged with the project mix before beginning. Observe, and be sure initial pump charge is thoroughly removed from the pipelines.
• Always pump at a constant rate and keep pipelines full of concrete. High air loss can occur when concrete is allowed to free-fall inside pump lines.
• Avoid, if at all possible, having steep angles in the pump pipelines. Steep angles and slow placement rates are probably the worst conditions for minimizing air loss and segregation. If this condition occurs:
1. Attempt to relocate the pumper, thereby minimizing lift angle.
2. If discharge is not maintaining a constant flow with partial concrete head in the pipe, request pump operator to place a reducer and short section of hose at the discharge end. The purpose is to avoid free falling concrete from impacting the epoxy coated reinforcing steel, deck or forms at high velocity.  High velocity impact of concrete aggregate on epoxy coated bars can potentially damage the epoxy coating.
3. If above condition is unavoidable, watch and test the discharge frequently for loss in air and potential segregation.

Rules-of-Thumb for Pumping

• Pump concrete with pipelines as flat as possible (or at least with minimal down angle)
• Minimize (or eliminate) free falling concrete in the pipelines. To do this, maintain some amount of concrete head in the pipelines
• Pump concrete through as few elbows and restrictions as possible
• Pump concrete at "some" constant rate
• Watch for, and test frequently, when situations are not optimized

11.57 FORM REMOVAL

Setting Beams
The following should be used as a guide in conjunction with Specification 2403.19:
A. On diaphragm piers, beams may be set as soon as doing so will not mar or chip the concrete.  It is recommended that 24 hours be considered a minimum cure time. (In cooler weather, ambient temperatures below 5^oC (40^oF), the minimum time indicated should be increased to 48 hours.)

B. No beams may be set on pedestal (T or P10A) piers until the cap concrete is 7 days old and modulus of rupture is at least 3,800 kPa (550 psi) or more. The contractor has the option under Specification 2403.03 to substitute Class M concrete mix for Class C except in bridge floors. When Class M concrete mix is used, beams may be set when the cap concrete is 3 days old and the modulus of rupture is at least 3,800 kPa (550 psi) or more. (Refer to Specification 2403.19 .)  If no test beams are made, the time must be extended to 14 days. (Refer to Specification 2403.18.)

There have been special situations where the contractor has been allowed to set beams on piers that have not attained the above strengths. In these cases, the bottom forms have remained in place for an extended period of time. Before approving any variance, contact the Office of Construction for approval.

C.On stub abutments or integral abutments, steel beams and girders may be set as under A above.  Concrete beams on stub abutments or integral abutments, same as A above. (Stub abutments are abutments with battered piling, sliding bearings, and the abutment does not move. Integral abutments have vertical piling in prebored holes, beams are rigidly connected to the abutment, and the abutment moves.) On full abutments (solid and continuous from spread footing), same as A above.

11.58 CLASS 3 CONCRETE SURFACE FINISH  (RAIL AND BEAMS)

Approval of Materials
Materials I.M. 491.10 lists the approved materials and proportions for use in obtaining a Class 3 finish required by Specification 2403.21. Any one of the listed materials may be used.  However, for uniformity, only one type should be used on any one structure.

Approvals of this material will be on the basis of legible brand markings on the containers. Periodic sampling and testing will be the responsibility of the Office of Materials. The type used on any structure should be included in the project documentation.

Application of Finish
Surfaces to be given a Class 3 finish must first be given a Class 2 strip down finish immediately after removal of forms. Successful application and adhesion of any type of finish to concrete surfaces is dependent on concrete condition and concrete surface preparation. Factors such as pH of the concrete, concrete moisture content, cleanliness of the concrete surface, and concrete surface profile are all critical to ensuring any coating being applied will securely adhere. For additional information and guidance contact the Office of Construction.

Materials for a special surface finish should be mixed to a uniform condition, preferably with a power mixer. When using a power mixer, add dry ingredients to the liquid. One worker should place the material with a steel trowel, making sure it is pressed firmly into all voids and leveled.  When the surface is set so it will not roll or lift, a second worker should smooth the surface uniformly with a rubber float.

Concrete Railings
Surfaces of concrete for barrier rails placed against fixed forms, either on site or in precasting, shall be given a surface finish described for exterior beams in Specification 2407.14 before application of curing. This should be done as the forms are removed. The contractor may opt to broom (brush) finish the slipform barrier rail.

11.59 FLOWABLE MORTAR