Concrete Paving Training & Field Reference - Iowa

1.5. Stationary Mixingand Delivery Introduce admixtures in the same sequencein the charging cycle. Complete the addition of admixtures not1.6. Ready Mixing andDeliverylater than one minute after adding water tothe cement or prior to the start of the lastthree-fourths of the mixing cycle, whichever occurs first.DeliveryDescriptionStandard dump trucks or agitor trucks areused to transport the already-mixed concretethe relatively short distance from the stationary batch plant to the mix placement site.Agitor trucks have an agitator paddle insidethe truck box. If the paddle is not rotating,the vehicle is no different from a dump truck.Recommended ProceduresOverviewStationary mixed concrete is often usedwhere large volumes of concrete need to be Transport mix in well-maintained vehicles. Deliver and place concrete mixture at thejob site within the specified time period.placed in a short period of time.Retarding admixtures may be used toCommon Problems Short mixing times resulting in non-uniform mixtures, poor distribution of airvoids, poor strength gain, and early stiffening problems Insufficient monitoring of aggregatemoistures Lumps or segregation resulting in non-also be longer with continuous agitationextend the time limit. The time limit maythan without continuous agitation. Check that delivery vehicles are free fromconcrete build-up. Check boxes betweenloads.OverviewReady mixed concrete is often used for lowervolume paving projects such as urban paving. Ready mixed concrete is proportionedand mixed off the project site and is transported to the construction site in a freshlymixed state in ready mix trucks.Common Problems Mixing at high speeds for long periods oftime, resulting in reduced air content anddecreased workability Excess water added to ready mixedconcrete, resulting in decreased long-termpavement durability Lumps or segregation, resulting in nonuni-uniform mixtureform mixtureMixingDescriptionRecommended ProceduresStationary mixing is the mixing of concrete Once all materials have been added, applyat a nearby or on-site concrete batch plant.at least 70 to 100 revolutions of the readyRecommended Proceduresmix drum at the manufacturer-recom- Set and lock the mixer timing device to themended mixing speed (6 to 18 rpm).recommended mixing time if possible. Measure the mixing period from the timeDump truckall cement and aggregates are in the mixer Make sure the ready mix drum is constantly revolving during delivery. Deliver and place concrete mixture atdrum.the job site within 90 minutes of initial Add supplementary cementitious materialsmixing.after the cement. Check each mix truck for worn blades or Add all of the water before one-fourth ofconcrete build-up. Keep blades clean andthe mixing time has elapsed.replace them when they become worn. When two or more admixtures are used inthe same batch of concrete, introduce themseparately to avoid any interaction thatmight compromise the effectiveness of theAgitor truckadmixtures.1. Concrete Materials Storage, Mixing, and Delivery

1.7. Hot WeatherBatching1.8. Cold WeatherBatchingOverviewOverviewHot weather (air temperatures over 90 F)Cold weather (air temperatures under 50 F)during concrete batching can reduce mixduring concrete batching can cause plasticworkability. Many states specify a maximumshrinkage cracking, especially if the concreteconcrete temperature limit of 90 F.temperature is much warmer than the air orif the wind is blowing. Keeping the concreteCommon Problemsmixture at a temperature of at least 50 F Drying of aggregate stockpileshelps to maintain mix workability. Accelerated rate of cement hydration High concrete temperatures, resulting inpremature setting Thermal crackingCommon Problems Plastic shrinkage cracking Slow strength gain Plastic shrinkage crackingRecommended ProceduresRecommended Procedures Do not use aggregates with frozen lumps. Avoid use of fresh or hot cement or fly ash. Use heated materials. Use slag and/or fly ash as a substitute for Reduce or eliminate slag and fly ashsome portion of portland cement to lowerthe peak concrete temperature. Do not exceed the maximum allowedwater-cement ratio. Add chilled water to the concrete mixwhen necessary. Do not exceed the manufacturer’s maximum recommended dosage of admixtures. If necessary, increase the dosage of airentraining admixture to maintain aircontent. Use retarding admixtures based on trialbatch performance. Take precautions when concrete temperatures exceed 90 F.1. Concrete Materials Storage, Mixing, and Deliverybecause they slow hydration. If necessary, decrease the dosage of airentraining admixture to maintain aircontent. Increase cement content of the mixture togenerate more heat.

2.1. Preparing theSubgrade and SubbaseOverviewConcrete pavement failures are often notcaused by failure of the concrete slab, but byproblems with the materials beneath the slab.Adequate preparation of the roadbed, including subgrade and subbase, is essential for astrong, durable concrete pavement system.Common Problems Unstable or non-uniform materials Poor compaction of utility trenches or the Remove localized pockets of undesirablematerial and replaced with soil similar tothe surrounding subgrade or with selectbackfill, or treat with stabilizing additivessuch as lime, portland cement, or fly ash. Compact the subgrade with proper moisture control to the required density. Proof-roll the subgrade to detect soft areasthat require additional stabilization. Trim the subgrade surface to achieve thedesired profile grade. When conditions warrant and it is practical, avoid operating equipment on thesubgrade.lines should be carefully protected and main-pavement of varying thickness and poortained because even small disturbances in aoverall smoothnessstring line can reduce pavement smoothness. Separation of fine aggregates during theA damaged or loose string line can cause asubbase trimming process, resulting indip or hump in the pavement profile.reduced subgrade permeability Dry subbase that draws water from theDescriptionconcrete mixtureSubbaseDescriptionThe subbase is a course of material placedon the subgrade to provide drainage andstability. Different kinds of subbases are usedbased on the need to balance drainabilityand stability. A granular subbase (mixture ofuniformly shaped and minimally compactedgranular material) is the most drainablesubbase, but it does not provide significantstructural support. A subbase containing aDescriptionThe subgrade is earth graded to the desiredelevation.Recommended Procedures Extend the subgrade at least 3' beyond theoutside edge of the planned pavement.This provides a foundation for a solid padline for paving equipment tracks. Cut high points and fill low areas toachieve the desired roadway profile elevation. For embankment fill, use cut material,except for peat, organic silt, or soil withhigh organic content.The string line controls the alignment andtimes belt placer and tining machine. String A rough or rutted subgrade, resulting inSubgradeOverviewelevation of the fine grader, paver, and some-subgrade itself Poor drainage leading to standing water2.2. Setting String Linegreater percentage of crushed particles anda denser gradation than a granular subbase is moderately drainable and providesmore stability than a granular subbase.Special backfill (such as a uniform mixtureof crushed concrete or crushed limestone,or a mixture of gravel, sand, and soil withor without crushed stone) may providemore stability and support but is the leastdrainable.Recommended Procedures Place the subbase course on the subgrade. Keep the subbase surface moist priorto paving. Spray water on the grade asneeded, but do not leave standing water.The string line can be wire, cable, wovennylon, or polyethylene rope. The string line isusually set up on both sides of the proposedpavement edge and is supported by stakesoutside the edge of the pad line.Common Problems Surveyor error in staking string line Misalignment of string-line pins, causingthe distance between string lines on eachside of the road to vary, resulting in variation from desired alignment Sag in string line People, equipment, or wild animals bumping string lineRecommended Procedures Install string line as low as possible. Make sure string line is tight, with no sagsbetween stakes, and is free of obstructionsincluding weeds. Eyeball the string line immediately beforeand during paving as a final check forhorizontal or vertical alignment. Notify supervisor if you notice a disturbance or possible error in the string line.2. Concrete Paving Site Preparation and Construction

2.3. Placing, Spreading,Striking Off, andConsolidating ConcreteSpreading ConcreteCommon ProblemsDescription Too little vibration, resulting in lowAugers, large horizontal screws acrossconcrete strength and large pockets ofthe front of the paver, or plows spread theentrapped air. Possible causes:concrete sideways across the width of the· Concrete mix design that produces poorpavement to create a uniform depth ahead ofthe profile pan.· Vibrators not functioning properlyCommon Problems· Too fast a paver speed Too much concrete in front of the paver,After the grade and subbase have beenprepared, the mix is placed. The slip-formpaving machine has three main parts—augersor plows, profile pan, and vibrators—to Too much vibration, resulting in aggregatecausing the profile pan to rise, resulting insegregation, vibrator trails, and freeze-a bump in the finished pavementthaw durability problems. Possible causes: Too little concrete in front of the paver,Overviewworkabilityresulting in voids in the finished pavementRecommended Procedures Maintain a uniform head of concrete—nottoo big and not too small.· Concrete mix design that produces poorworkability· Improperly installed vibrators or excessive frequency· Reducing paver speed without compensating vibrator frequencyStriking Off ConcreteRecommended Procedurescrete as it travels forward.Description Install vibrators according to the manu-Placing Concreteor plow, strikes off excess concrete to thespread, strike off, and consolidate the con-DescriptionConcrete mixes are best deposited from offthe grade or the side of the grade by beltplacers. Alternatively, to prevent trucks fromdriving on the grade after it’s been trimmed,special conveyors are sometimes used tomove the mix from delivery trucks over thefine grade trimmer to the grade just in frontof the paver.Common Problems Uneven depositing of concrete mix Inconsistent workability and/or durabilitydue to different rates of mix productionand delivery Surface smoothness problems due toinconsistent paver speed or excessivestoppingRecommended Procedures Deposit concrete as uniformly as possiblein front of the paver. Do not overload either side of paver.Adjust the height of placement for the sizeof the paver so that the head of the concrete does not cause the paver pan to rise. Operate the paver at a slow, consistentspeed and avoid unnecessary stops toensure a smooth pavement. The paverspeed should match production rate of thebatch plant and the rate of concrete delivery to the paver.2. Concrete Paving Site Preparation and ConstructionThe profile pan, located behind the augersdesired pavement elevation and smooths thesurface.Recommended Procedures Adjust the profile pan to construct thespecified crown or super-elevations.Consolidating ConcreteDescriptionVibrators on the paver consolidate the concrete mix. Consolidation should be carefullymonitored because the vibrators have thepotential to produce concrete mix segregation and adversely affect the air void system.Proper vibration should be controlled to notadversely affect the strength or long-termdurability of the concrete.facturer recommendations or specifier’srequirements. Synchronize and monitor vibrators withinthe specified range. Check vibrator frequencies and amplitudesunder load at the beginning of the process. For large pours, equip pavers with continuous monitoring. Adjust the vibrator frequency for varyingpaver speeds. Turn off vibrators when the paver stops.

2.4. FinishingRecommended Procedures Do not add water to the concrete surfacewhile finishing Do as little surface finishing as possible2.5. TexturingOverviewFinishing determines the final appearance,smoothness, and other surface properties ofconcrete that affect the long-term durabilityof a pavement surface.DescriptionLongitudinal, Oscillating FloatsOverviewLongitudinal, oscillating floats (Auto-Floats)Pavement surfaces are textured to roughen themay be either attached to the paver or selfsurface.propelled. The floats are attached parallelMicrotexturingor near parallel with the center line of theDescriptionslab and move from one side of the slabto the other while oscillating front to rear.The floats are used to shape and smooth thesurface.Mechanical Truss FloatsMechanical truss floats (V-Floats) consistof a truss with narrow floats attached. Theyare normally attached to the paver near thecenterline and fan out toward the edge of thepavement. The floats rest on the slab and aretypically pulled by the paver.Hand FloatsWhen hand floats are used for finishing, thesurface is first checked behind the paver withstraight edge and surface imperfections arethen corrected with a hand float.Hand TrowelsMicrotexturing is accomplished by draggingwet burlap, artificial turf, or coarse carpetlongitudinally along the concrete surface.Recommended Procedures Do not allow turf drags to disrupt or pullaggregate to the surface. If a bridge is used, pull it with the paver sothat it provides a straight, uniform texture.Do not hand pull or push the bridge becausethat can result in a cracked or wavy rotexturing is generally required on pavements with speed limits greater than 35 mph.Macrotexturing is created by a mechanicaldevice (tining machine) that makes groovesHand trowels can be used to create a hard,typically longitudinally or sometimes trans-smooth finish; shape or finish pavementversely in the pavement surface.curbings or edges; finish around structuresCommon Problemsin the pavement (box-outs, manholes, utilityappurtenances, etc.); and round the edges offormed joints.Common Problems Too much water on the concrete surface Aggregate dragged along or out of thesurface Excessive finishing of the concrete Bent tines Tine length not uniform, causing non-uniform depth Tines too deep Tining machine wandering off of parallel tocenterlineRecommended Procedures Follow specifier’s recommendations for tinegroove spacing, width, and depth.2. Concrete Paving Site Preparation and Construction

2.6. CuringRecommended Procedures Clean spray nozzles frequently to preventclogging. Thoroughly mix and agitate compoundusing diaphragm pumps or mechanicalagitators. Do not use gear pumps and highshear devices. Mix compound daily before applicationand at least once every 4 days when not inuse. Check that the temperature is above 40 Fbefore applying. Mount spray equipment on a self-pro-Overviewpelled frame that spans the entire width ofCuring is a process of protecting the newthe slab.pavement to regulate moisture loss from theconcrete surface. Curing also ensures thatconcrete gains strength uniformly. Applying Provide wind protection to avoid spraydrift. Set application rate and calibrate equip-curing compounds is the most commonlyment to ensure specified coverage.used curing technique.Application rate may have to be increasedduring extremely hot weather.Description Apply curing compounds evenly imme-Curing compounds reduce the rate of waterdiately after finishing and texturing,evaporation from the concrete surface. Whiteespecially during extremely hot, dry, orpigments in the compounds reduce solar heatwindy weather.gain on sunny days and are a way to visually Spray the entire concrete surface and alljudge the application rate.exposed vertical edges, including the backCommon Problems Perform yield checks to ensure proper rateof the curb. Dirty or poorly maintained equipment,resulting in clogged nozzles and nonuniform application Too little curing compound applied Non-uniform application of compound,especially by hand spraying Vertical edge of slab not cured2. Concrete Paving Site Preparation and Constructionof application. Store well-mixed material in clean bulkcontainers with clearly marked batchnumbers on the bulk tanks.

2.7. Hot, Dry WeatherPaving2.8. Cold Weather Paving2.9. Preventing RainDamageOverviewOverviewOverviewHot, dry weather can impair the quality ofSpecial precautions must be taken whenLayers of burlap or plastic film are used tofreshly mixed or hardened concrete by accel-paving in cold weather. Cold weather pre-protect fresh concrete in case of rain.erating the rate of moisture loss and the ratecautions apply when the air temperature isof cement hydration. Hot weather precautions50 F or colder for more than one-half of anyCommon Problemsare important because once heat-related prob-24-hour period, or when the average daily Surface damage of concrete that has notlems develop in concrete, it may be too lateair temperature is less than 40 F for threeto correct them. Hot, dry weather precautionsconsecutive days.typically apply when the air temperature ishas achieved final set leading to uncon-above 90 F and there is low relative humid-Common Problemsity, high wind speed, and sun exposure. Aggregate with frozen lumps Concrete cooling faster at the surfaceCommon Problemsthan inside the slab, resulting in stress & Drying out of aggregate stockpiles andcrackingsubbase, resulting in aggregate absorbingwater from the concrete mix, impairingstrength gain and deteriorationRecommended Procedures Do not pave on frozen subgrade.cracking Do not use aggregates with frozen lumps. Difficulty controlling air entrainment Heat materials. Rapid hardening of the concrete, resulting Minimize use of fly ash and slag.in accelerated slump losscrackingunderway. Surface texturing damage Strong wind blows the covering off theslabRecommended ProceduresRain Damage Preventionment surface, resulting in shrinkage Delayed joint sawing, possibly leading totrolled cracking, even if saw cutting is Frozen concrete, resulting in lack ofworkability Rapid water evaporation from the pave-achieved final set Thermal restraint stresses in pavement that Do not pave if the concrete cannot reachadequate strength before it freezes. For the first two to three days, protect newconcrete pavement from cold weather with Subscribe to and monitor weather forecasting services. Have protective materials such as burlapor plastic film available on site at alltimes. Stop batching and transporting of mix in atimely manner. Use early-entry saws to quickly cut thejoints.Recommended Proceduresadequate layers of burlap or other insulat-When It Starts Raining Keep aggregate stockpiles, grade, subbase,ing material. Cover fresh concrete as soon as possible.forms, and equipment moist. Use fly ash and slag in the mixture to slowhydration. Use retarders to aid in placement. Pave in the morning, evening, or nightwhen it is cooler. Apply curing compound immediately afterfinal finish treatment. Increased dosage ofcuring compound may be needed. Do not finish the concrete surface afterunprotected pavement is exposed to rain. Do not try to remove extra surface waterprior to covering the concrete. Do not

1.1. Inventorying Concrete Materials 1.2. Storing Concrete Materials 1.3. Water-Cement Ratio 1.4. Concrete Batching 1.5. Stationary Mixing and Delivery 1.6. Ready Mixing and Delivery 1.7. Hot Weather Batching 1.8. Cold Weather Batching 2. Concrete Paving Site Preparation and Construction 2.1. Preparing the Subgrade and Subbase 2.2. Setting .