chapter 4 concrete

8/12/2019 CHAPTER 4 Concrete

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HANDLING CONCRETE


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CONTENTHANDLING CONCRETE

Identification of hazards

Concrete placing by direct discharge, excavator andcrane

Concrete pumping

Concrete curing


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INTRODUCTION Concretes versatility, durability,

and economy have made it theworlds most used constructionmaterial.

The U.S. uses about 180 millioncubic meters (240 million cubic

yards) of ready-mixed concreteeach year.

It is used in highways, streets,parking lots, parking garages,bridges, high-rise buildings,dams, homes, floors, sidewalks,driveways, and numerous otherapplications.
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INTRODUCTION Concrete and cement are not the

same thing; cement is one of acomponent of concrete.

Cement, usually in powder form,acts as a binding agent whenmixed with water andaggregates.

This combination, or concretemix, will be poured and hardeninto the durable material withwhich we are all familiar.
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INTRODUCTION Concrete is made up of three basic components:

water,

aggregate (rock, sand, or gravel) and Portland cement.
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IDENTIFICATION OF HAZARD The current trend in construction-

related injuries is decreasing. Thistrend is most likely due to increasedawareness of the potential risks

surrounding construction jobsites. The concrete industry boasts one of

the lower jobsite-injury rates, but it isnot necessarily the concrete thatpresents the greatest risk.

An understanding of the potentialrisks of concrete construction andproper training is necessary forlimiting the number of concreteconstruction-related injuries.
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IDENTIFICATION OF HAZARD Construction jobsites are full of hazards, and concrete

construction jobsites are no exception.

These hazards can be dissected into categories for better

reference as they pertain to various projects. Types of hazard includes:-

Material Hazard

Machinery

Tools Height

Construction Practices

Jobsite Conditions


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IDENTIFICATION OF HAZARD(i) Material Hazards Cement comprises 7-15% of total concrete volume. As an alkaline material, wet cement is caustic, and can cause severe chemical burns to

exposed skin and eyes. Working with fresh concrete presents an obvious risk.

It is so important to always wear water-proof gloves, a long-sleeved shirt, full-lengthtrousers, and proper eye protection.

Water-proof boots must be high enough to keep concrete from flowing into if theworkers need to stand in wet concrete,

Wet concrete, mortar, cement, or cement mixtures must be washed immediately fromskin.

Flush eyes with clean water immediately after contact.

Indirect contact through clothing can be as serious as direct contact, so promptly rinseout wet concrete, mortar, cement or cement mixtures from clothing. And always seekimmediate medical attention if you have persistent or severe discomfort.

In addition to the caustic nature of cement, 95% of cement particles are smaller than 45m.compared to tobacco smoke of approximately 3 msuggesting that the danger ofinhalation is possible. Workers opening bags or sacks of cement and cement productsshould always wear a dust mask in addition to their regular safety attire.


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IDENTIFICATION OF HAZARD(ii) Machinery Hazard Rotating machinery is always a source of potential injury

on a jobsite.

Early-entry saws, concrete/masonry saws, cut-off saws, andpower trowels pose a threat to appendages when usedimproperly. In addition, any sustained or sudden noiseabove 85 decibels emanating from machinery can bedamaging to the ear.

Hydraulic jacks used in shoring, compressed air andhydraulic concrete pumps, belt conveyors, weldingequipment, post-tensioning jacks, demolition devices, andother equipment also create potential hazards on aconcrete construction site.


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IDENTIFICATION OF HAZARD(iii) Tools

Besides the mechanized saws and power trowels listed above,sharp-edged trowels, hammers, chisels, utility knives, etc. canbe dangerous if used carelessly or incorrectly.

Long-handled bull f loats, when used near utility wires, caneven be dangerous.

(iv)Height

The number-one leading cause of construction-relatedinjuries and fatalities is attributed to falls from height.

Sources of height associated with concrete constructioninclude but are not limited to scaffolding, ladders, bucket-trucks, catwalks, elevated or wall forms, and elevated f loors.

Owners, managers, contractors, and laborers should be awareof specific height sources on a project as they are virtuallyunavoidable in construction.


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IDENTIFICATION OF HAZARD(v) Construction Practices The use of cranes for lifting and placing concrete buckets, for tilt-up

concrete panels, and for lifting precast members present hazards to thefinishers and erectors.

Concrete pumping, hydro-demolition, or shotcreteing operationswhere high pressures are generated in hoses prompt safety concerns forthe nozzlemen.

Reinforcement construction can demand heavy materials, protrudingsteel, oxyacetylene torches or welding equipment, and height sources,each of which introduces a safety hazard either singularly or in any

combination. Post-tensioning operations impart stresses nearly equal to the yieldstrength of prestressing tendons which can be 250,000 psi. Suchforces are dangerous to jack-operators or on-looking personnel.

Precast plants with heavy table forms, consolidation equipment, andcuring rooms must follow safety procedures.


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IDENTIFICATION OF HAZARD(vi) Jobsite Conditions

The general condition of the jobsite can also be

hazardous. Cramped, confined projects or sections of a project

affect operations and safety.

Locations exposed to traffic, utility wires, excavations,

or hazardous materials can produce unsafe conditions. Even weather (ie: snow, ice, rain, standing water, heat)

can result directly in injury or combine with anotherrisk to inflict injury to workers.


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IDENTIFICATION OF HAZARD

Prevention

Personal

Protection

Equipment

Protection

Jobsite

Protection


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PreventionPersonal Protection

Hardhats and hearingprotection are always

necessary on a constructionsite when overhead hazardsand loud or sustained noiseis present.

When working with cement,

sand, or any other finematerial, the use of arespirator is necessary.


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PreventionEquipment Protection All equipment should be properly maintained and

equipped with manufacturer-recommended safety devices.

Disabling or removing safety devices is dangerous andshould be avoided. All unsafe or inoperable equipment should be marked as

such to prevent further use of the equipment. All workers should be trained and tested by the manager or

superintendent before operating any equipment (fromdrills to backhoes). Knowledge of the hazards associated with specific

equipment is the first line of defense against injury.


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PreventionJobsite Protection

Responsibility of the manager to provide a safe jobsite

for workers. The manager or superintendent should ensure that

potential hazards at the project site are identified andcorrected or, at minimum, made known to employees.


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CONCRETE PLACING Normal concrete weighs

approximately 68kg@150 poundper cubic foot and should be

placed as near as possible to itsfinal position.

Excess handling can causesegregation of the course and fineaggregates.

Wetting up the concrete to ensurethat it can be raked or pushed intoa location far from where it isdischarged is not acceptable.


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CONCRETE PLACING Concrete is poured directly from the

chute of the ready mix truck,wheeled into place with a buggy, orpumped into place with a concreteboom pump.

Concrete is normally specified at a4-5" slump.

Industrial, commercial, and someresidential projects require aninspector on concrete pours whomonitors the concrete slump andtakes slump measurements at therequired intervals.


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CONCRETE PLACING The smooth delivery of concrete to the job-site is

critical.

Delays in the delivery of the concrete or during theplacement operation may cause problems that are timeconsuming and costly to resolve.

The field Technician, the Contractor, and the concrete

plant Technician are required to work together toensure the correct concrete is delivered on time and inthe necessary quantities.


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SITE PREPARATION FOR CONCRETE

PLACINGThe Technician is required to ensure that the site has beenadequately prepared for concrete placement. Such preparationsinclude that:1) Excavations have been dewatered

2) Forms have been checked for adequate bracing and properelevations and alignment3) Chamfer strips have been installed and are in good shape4) Trash and debris have been removed from all forms5) Reinforcement has been tied securely and checked forproper clearance and spacing6) The Contractor has adequate manpower and equipment tohandle the pour to include a sufficient number of vibratorsand backups.


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WEATHER CONDITIONS Weather conditions may influence everything from the timing and

method of concrete delivery and placement to postponing theoperation altogether.

Ideally, concrete is placed in temperatures between 50 and 90 F, when

there is no threat of rain, and when steps have been taken to protectthe concrete from excessive wind. In general, when the temperature is 35 F or below, the temperature of

the concrete is required to be between 50 and 80 F at the time ofplacing.

The Contractor may heat the water and/or aggregates used in theconcrete mix to achieve this range of temperatures; however, theheating is required to be done in accordance with the Specifications forcold-weather concrete.

The Technician is required to use a dial thermometer to check theconcrete temperature whenever the concrete is suspected to be nearthe Specifications limits.


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CONCRETE DELIVERY No concrete may be placed without a Technician on

the job and another Technician at the concrete plant.

Prior to the beginning of concrete delivery, thetechnician is required to contact the plant Technicianto double-check the following items:

1) The class of concrete to be used

2) The quantity of concrete needed for the pour3) The slump and air content requirements

4) The proposed starting time of delivery

5) The desired rate of delivery.


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CONCRETE DELIVERY Concrete is typically delivered to the job-site in mixer

trucks, agitator trucks, or in non-agitating equipment.

Mixer Trucks


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CONCRETE DELIVERY Mixer trucks are designed for

mixing concrete at or on theway to the job-site. For thisreason, mixer trucks always

have a water tank on boardand a measuring device thatis capable of controlling theamount of water that isadded to the mix.

Agitator trucks deliver ready-mixed concrete. Any water onthe truck is for cleaningpurposes only, not formixing.

Agitator truck


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Directplacement

Usingbuggy/wheelbarrow

BoomPump


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Placing the Ready-Mixed Concrete Access for the ready-mixed concrete truck should be provided as close

as possible to the required location in the structure. It may be possible to discharge concrete directly from the truck into

foundations and column bases, but for other components a means of

transporting the concrete from the truck discharge point to its requiredlocation in the frame will be required. The two methods of transporting concrete that are most applicable to

multi-storey frame construction are:1. Pumping. A truck-mounted (or occasionally static) concrete

pump is sited at round level with its delivery boom and/or hosesdeployed so that concrete can be pumped directly to where it isrequired in the frame.The input hopper of the pump is topped-up from truck-mixers.2. By skip.

A site crane hoists a skip from ground level (where it is filled withconcrete from a truck-mixer) to the required location in the frame.


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CONCRETE SKIPPING METHODLoad concrete intobucket

Using crane totransport theconcrete todesired place.


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CONCRETE PUMPING(ii) The second main type of concrete pumpis either mounted on a truck and knownas a truck-mounted concrete pump orplaced on a trailer, and it is commonlyreferred to as a line pumpor trailer-

mounted concrete pump. This pump requires steel or flexibleconcrete placing hoses to be manuallyattached to the outlet of the machine. Those hoses are linked together andlead to wherever the concrete needs tobe placed

Line pumps normally pumpconcrete at lower volumes than boompumps and are used for smaller volumeconcrete placing applications such asswimming pools, sidewalks, and singlefamily home concrete slabs and mostground slabs.
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CONCRETE PUMPING There are also skid mounted and rail mounted

concrete pumps, but these are uncommon and onlyused on specialized jobsites such as mines and

tunnels.
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SEGREGATION Segregation occurs when the coarse and fine aggregates used in the concrete separate and become unevenly distributed throughout the mix. The larger coarse aggregate sinks to the bottom while the fines rise to the top. Segregation always leads to an inferior quality of concrete.

For the most part, however, segregation may be prevented with the use of properplacement equipment and techniques. Concrete is required to be placed as close as possible to the location the concrete

occupies in the structure. The concrete should not be dumped in a central location and then spread to the

location required in the structure


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LAYERS When possible, concrete is required to be deposited in layers no

more than 24 in. thick. Care is required be taken, however, to place each successive layer

before the preceding layer has taken the initial set.

This initial set is usually 45 minutes to an hour, depending onthe temperature.

Too much time between the placement of layers usually resultsin a cold joint which is a weak line of separation between thelayers.


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DROPPING THE MIXED Dropping concrete from too great a heightcauses the finer particles in the mix to splashaway from the larger, heavier particles. Inaddition, the force of the mix striking thereinforcing steel may shift bars out of

position. The maximum drop height or allowable free fall

is 4- 5 ft. Hoppers with flexible chutes called tremies are

required to be used to funnel the mix down intotall, narrow forms. Workers may be stationedinside the forms to move the chutes around to

ensure an even distribution of the concrete. The hoppers may not rest on the reinforcing

steel and are required to be supported by theformwork.

Hopper

Chutes
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COMPACTION Adequate compaction is essential to ensurethat the concrete performs satisfactorily inthe completed structure.

Under-compacted concrete will have reducedstrength and / or durability, and may be of

unacceptable appearance. Appropriate compaction equipment must

therefore be available when it is required andconcrete-placing personnel should be trainedto use it correctly

The poker vibrator is likely to be mostappropriate compaction device for in-situ

reinforced concrete frames. Beam vibrators or hand tamping can be used

to compact and finish the top surfaces ofslabs, but a poker vibrator will still berequired to ensure that adequate compactionis achieved through the full thickness and atthe edges.

Poker vibrator

BeamVibrator


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COMPACTION Although the procedure is a simple operation, vibrating concrete is often conducted incorrectly.

Some points that ensure a good job include:

1) Vibrating is required to be done immediately as the concrete is placed

2) Vibrators are required to be inserted and withdrawn vertically and should not be dragged through theconcrete

3) Vibrators are required to be inserted and withdrawn within 5 seconds. Over-vibrating forces the fineraggregates to the top and drives the larger aggregates toward the bottom.

4) When concrete is poured in layers, the head of the vibrator is required to penetrate through the top layer andpartially through the layer underneath.

5) The workers are required to avoid contacting the reinforcing steel with the vibrator so that the bond betweenthe steel and the concrete is not broken

6) The workers are required to avoid contacting the form walls with the vibrator as that may loosen the formsand may also cause honeycombing of the concrete surface

7) The Contractor is required to have a backup vibrator on hand for larger pours in case of equipment problems


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CONCRETE SPREADING The purpose of

spreading fresh

concrete is to placeconcrete as close aspossible to finish levelto facilitate straightedging/ screeding theconcrete.


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CONCRETE SPREADING Short handled, square

ended shovels arerecommended forspreading concrete.

A come-along (a tool thatlooks like a hoe and has along straight edged blade)can also be used.

Any spreader used should

be rigid enough to pushand pull wet concrete

without bending.


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CONCRETE CURING Once the concrete is in place, the concrete is allowed

to cure a certain amount of time to achieve the fullstrength.

During the curing period, the concrete is not to beplaced under stress.

The typical curing period of the concrete is 96 hours

after the initial set. The use of certain materials, such as fly ash or

Portland-pozzolan cement in the concrete, increasesthe curing period to 120 hours.


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CURING The Specifications describe two methods of curing concrete.(1) Protective covering curing method. This method requires covering the surfaces to be cured with canvas,

straw, burlap, sand, or other approved material and keeping theconcrete wet with water throughout the curing period.

The water prevents the concrete from drying out too quickly.

Surfaces that require a Class Two rubbing finish are required to havethe protective covering temporarily removed to allow the rubbing tocontinue.

The covering is required to be restored as soon as possible.


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CURING

(2) The use of a membrane forming curing compound.

The curing compound may be applied after the concrete surface has receivedthe specified finishing treatment. Up until then, the concrete is required to be protected by the protective

covering method or, in the case of vertical surfaces, simply by leaving theforms in place.

Curing compound is applied at a minimum rate of one gallon for every 150 ft2of concrete surface. The application is done in two stages.

The first coat is applied immediately after stripping the forms or uponacceptance of the concrete finish.

The surface is required to be wetted with water and coated with thecompound as soon as the water film disappears.

The second application is required to begin after the first has set andaccording to the manufacturers directions.

chapter 4 concrete

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