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General Concrete Information
Problems/Troubleshooting Concrete
JOBSITE ADDITION OF WATER

What is Jobsite Addition of Water?
Jobsite addition of water is the addition of water to ready mixed concrete in a truck mixer after arrival at the location of the concrete placement. Such tempering of concrete may be done with a portion of the design mixing water which was held back during the initial mixing, or with water in excess of the design mixing water, at the request of the purchaser.

Why is Water Added at the Jobsite?
When concrete arrives at the jobsite with a slump that is lower than that allowed by design or specification and/or is of such consistency so as to adversely affect the placability of the concrete, water can be added to the concrete to bring the slump up to an acceptable or specified level. This can be done when the truck arrives on the jobsite as long as the specified slump and/or water-cement ratio is not exceeded. Such addition of water is in accordance with ASTM C 94, Standard
Specification for Ready Mixed Concrete.

The ready mixed concrete supplier designs the concrete mixture according to industry standards to provide the intended performance. Addition of water in excess of the design mixing water will affect concrete properties, such as reducing strength and increasing its susceptibility to cracking. If the purchaser requests additional water, in excess of the design mix, the purchaser assumes responsibility for the resulting concrete quality. The alternative of using a water reducing admixture or super-placticizer to increase concrete slump should be considered. Provided segregation is avoided, increasing the slump of concrete using admixtures usually will not significantly alter concrete properties.

 

How To Add Water at the Jobsite

  • The maximum allowable slump of the concrete must be specified or determined from the specified nominal slump plus tolerances.

  • Prior to discharging concrete on the job, the actual slump of the concrete must be estimated or determined. If the slump of the concrete is measured, it should be on a sample from the first 1/4 cubic yard (0.2 meters cubed) of discharged concrete and the result used as an indicator of concrete consistency and not an acceptance test. Tests for acceptance of concrete should be made in accordance with ASTM C 172.

  • At the jobsite, water should be added to the entire batch so that the volume of concrete being retempered is known. A rule of thumb that works reasonably well is-1 gallon, or roughly 10 lb., of water per cubic yard for 1 inch increase in slump (5 liters, or 5 kg, of water per cubic meter for 25 mm increase in slump).

  • All water added to the concrete on the jobsite must be measured and recorded.

  • ASTM C 94 requires an additional 30 revolutions of the mixer drum at mixing speed after the addition of water. In fact, 10 revolutions will be sufficient if the truck is able to mix at 20 revolutions per minute (rpm) or faster.

  • The amount of water added should be controlled so that the maximum slump and/or water-cement ratio, as indicated in the specification, is not exceeded. After more than a small portion of the concrete is discharged no water addition is permitted.
     Upon obtaining the desired slump and/or maximum water-cement ratio no further addition of water on the jobsite is permitted.

  • A pre-concreting conference should be held to establish proper procedures to be followed, to determine who is authorized to request a water addition, and to define the method to be used for documentation of water added at the jobsite.

ASTM C 94 Jobsite Water Addition

  • Establish the maximum allowable slump and water content permitted by the job specification.

  • Estimate or determine the concrete slump from the first portion of concrete discharged from the truck.

  • Add an amount of water such that the maximum slump or water-cement ratio according to the specification is not exceeded.

  • Measure and record the amount of water added. Water in excess of that permitted above should be authorized by a designated representative of the purchaser.

  • Mix the concrete for 30 revolutions of the mixer drum at mixing speed.

Do not add water if:

  • The maximum water-cement ratio is reached,
  • The maximum slump is obtained, or
  • More than 1/4 cubic yard (0.2 meters cubed) has been discharged from the mixer.

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GENERAL INFORMATION REGARDING CONCRETE - ADMIXTURES

  • Air-entraining admixtures are liquid chemicals added during batching concrete to produce microscopic air bubbles, called entrained air, when concrete is mixed. These air bubbles improve the concretes resistance to damage caused by freezing and thawing and deicing salt application. In plastic concrete entrained air improves workability and may reduce bleeding and segregation of concrete mixtures. For exterior flatwork (parking lots, driveways, sidewalks, pool decks, patios) that is subjected to freezing and thawing weather cycles, or in areas where deicer salts are used, specify a normal air content of 4% to 7% of the concrete volume depending on the size of coarse aggregate. Air entrainment is not necessary for interior structural concrete since it is not subjected to freezing and thawing. It should be avoided for concrete flatwork that will have a smooth troweled finish. In high cement content concretes, entrained air will reduce strength by about 5% for each 1% of air added; but in low cement content concretes, adding air has less effect and may even cause a modest increased strength due to the reduced water demand for required slump.
     

  • Water Reducers are used for two different purposes; to lower the water content in plastic concrete and increase its strength or to obtain higher slump without adding water. Water-reducers will generally reduce the required water content of a concrete mixture for a given slump. These admixtures disperse the cement particles in concrete and make more efficient use of cement. This increases strength or allows the cement content to be reduced while maintaining the same strength. Water-reducers are used to increase the slump of concrete without adding water and are useful for pumping concrete and in hot weather to offset the increased water demand. Some water-reducers may aggravate the rate of slump loss with time.  Mid-range water reducers are now commonly used and they have a greater ability to reduce the water content. These admixtures are popular as they improve the finishability of concrete flatwork.
     

  • Retarders are chemicals that delay the initial setting time of concrete by an hour or more. Retarders are often used in hot weather to counter the rapid setting caused by high temperatures. For large jobs, or in hot weather, specify concrete with retarder to allow more time for placing and finishing. Most retarders also function as water reducers.
     

  • Accelerators reduce the initial set time of concrete and give higher early strength. Accelerators do not act as an antifreeze; rather, they speed up the setting and rate of strength gain, thereby making concrete stronger to resist damage from freezing in cold weather. Accelerators are also used in fast track construction requiring early form removal, opening to traffic or load application on structures. There are two kinds of accelerating admixtures: chloride based and non-chloride based. One of the more effective and economical accelerators is calcium chloride, which is available in liquid or flake form. For non-reinforced concrete, calcium chloride can be used to a limit of 2% by the weight of the cement. Because of concerns with corrosion of reinforcing steel induced by chloride, lower limits on chlorides apply to reinforced concrete. Prestressed concrete and concrete with embedded aluminum or galvanized metal should not contain any chloride-based materials because of the increased potential for corrosion of the embedded metal. Non-chloride based accelerators are used where there is concern of corrosion of embedded metals or reinforcement in concrete.
     

  • High Range Water Reducers (HRWR) is a special class of water-reducer. Often called superplasticizers, HRWRs reduce the water content of a given concrete mixture from 12 to 25%. HRWR are therefore used to increase strength and reduce permeability of concrete by reducing the water content in the mixture; or to greatly increase the slump to produce “flowing” concrete without adding water. These admixtures are essential for high strength and high performance concrete mixtures that contain higher contents of cementitious materials and mixtures containing silica fume. For example, adding a normal dosage of HRWR to a concrete with a slump of 3 to 4 inches (75 to 100 mm) will produce a concrete with a slump of about 8 inches (200 mm). Some HRWRs may cause a higher rate of slump loss with time and concrete may revert to its original slump in 30 to 45 minutes. In some cases, HRWRs may be added at the jobsite in a controlled manner.
     

  • Besides these standard types of admixtures, there are products available for enhancing concrete properties for a wide variety of applications. Some of these products include: Corrosion inhibitors, shrinkage reducing admixtures, anti-washout admixtures, hydration stabilizing or extended set retarding admixtures, admixtures to reduce potential for alkali aggregate reactivity, pumping aids, damp-proofing admixtures and a variety of colors and products that enhance the aesthetics of concrete. Contact your local ready mixed concrete producer for more information on specialty admixture products and the benefits they provide to concrete properties.

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BLEEDING

Bleeding or bleed water or water gain is the rising of water to the surface of freshly placed concrete. Finishing bleed water back into the top layer of concrete is probably responsible for most surface problems which is where the best concrete is needed. Wait until the bleed water evaporates or drag a rubber hose slowly over the entire surface when concrete is stiff enough that only water will be removed.

Air-entrained cement and a low slump concrete (stiff) will bleed very little. Lean mixes, that is low cement content will bleed more. In winter it is recommended to use heated concrete to reduce bleeding. This is done by preheating the water.

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CURING IN-PLACE CONCRETE

What is Curing?

  • Curing is the maintaining of a satisfactory moisture content and temperature in concrete. Curing begins after placement and finishing so that the concrete may develop the desired strength and hardness.

  • Without an adequate supply of moisture, the portland cement in the concrete cannot react to form a quality product. Drying may remove the water needed for this chemical reaction called "hydration" and the concrete will be weak. Temperature is an important factor in proper curing, since the rate of hydration is temperature dependent. For exposed concrete, relative humidity and wind conditions are also important; they contribute to the rate of moisture loss from the concrete.

Why Cure? – Several important reasons . . .

  • Predictable strength gain. Laboratory tests show that concrete in a dry environment can lose as much as 50 percent of its potential strength compared to similar concrete that is moist cured. Concrete placed under high temperature conditions will gain early strength quickly but later strengths may be reduced. Concrete placed in cold weather will take longer to gain strength, delaying form removal and subsequent construction

  • Improved durability, especially of non-air-entrained concrete slabs that may be subjected to freezing conditions during construction. Well cured concrete has better surface hardness and therefore is more watertight

  • Better serviceability and appearance. A concrete slab that has been allowed to dry out too early will have a soft surface with poor resistance to wear and abrasion. Proper curing reduces crazing, dusting, and scaling.

How To Cure

  • Moisture Requirements for Curing-the concrete surface must be kept continuously wet or sealed to prevent evaporation for a period of at least several days after finishing.

Systems to keep concrete wet include:

  • Burlap or cotton mats and rugs used with a soaker hose or sprinkler. Care must be taken not to let the coverings dry out and adsorb water from the concrete. The edges should be lapped and the materials weighted down so that they are not blown away.

  • Straw that is sprinkled with water regularly. Straw can easily blow away, and if it dries, can catch fire. The layer of straw should be 6 inches thick, and should be covered with a tarp.

  • Sprinkling on a continuous basis is suitable provided the air temperature is well above freezing. The concrete should not be allowed to dry out between soakings, since alternate wetting and drying may damage the concrete.

  • Ponding of water on a slab is an excellent method of curing. The water should not be more than 20 degrees F cooler than the concrete and the dike around the pond must be secure against leaks.

  • Damp earth, sand, or sawdust will cure flatwork, especially floors. There should be no organic or iron staining contaminants in the materials used.

Sealing materials include:

  • Liquid membrane-forming compounds-must conform to ASTM Specifications at the rate of application that is specified. Apply to the concrete surface about one hour after finishing. Do not apply to concrete that is still bleeding, or has a visible water sheen on the surface. While a clear liquid may be used, a white pigment will give reflective properties, and allow for inspection of coverage. A single coat may be adequate, but where possible a second coat, applied at right angles to the first, is desirable for even coverage. If the concrete will be painted, or covered with vinyl or ceramic tile, then a liquid compound that is non-reactive with the paint or adhesives must be used, or a compound that is easily brushed or washed off. On floors, the surface should be protected from the other trades with scuff-proof paper after the application of the curing compound.

  • Plastic sheets-either clear, white (reflective) or pigmented. Plastic should conform to ASTM Standards, be at least 4 mils thick, and preferably reinforced with glass fibers. The plastic should be laid in direct contact with the concrete surface as soon as possible without marring the surface. The edges of the sheets should overlap and be fastened with waterproof tape and then weighted down to prevent the wind from getting under the plastic. Plastic will make dark streaks wherever a wrinkle touches the concrete so plastic should not be used on concretes where appearance is important.

  • Waterproof paper-used like plastic sheeting, but does not mar the surface. Should also conform to ASTM Standards.

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CRACKING

Cracks in basement walls

  • Temperature and drying shrinkage cracks.
    With few exceptions, newly placed concrete has Cast-in-place concrete basements provide durable, high quality extra living space. At times when proper construction practices are not used undesirable cracks occur, such as: the largest volume that it will ever have. This shrinkage tendency is increased by drying and/or a drop in temperature and can lead to random cracking if steps are not taken to control the location of the cracks by providing control joints.

  • Settlement cracks.
    These occur from non-uniform support of footings or occasionally from expansive soils.

  • Other structural cracks.
    In basements these cracks generally occur during backfilling, particularly when heavy equipment gets too close to the walls.

  • Cracks due to lack of joints or improper jointing practices.

  • In concrete basement walls some cracking is normal.

Most cracks normally occur because one or more of the following rules of "good concrete practice" were not followed:

  • Providing uniform soil support.

  • Using moderate slump concrete and avoiding addition of water to the concrete mixture on the job.

  • Observing proper concrete placement practices.

  • Providing control joints every 20 to 30 feet. e. Backfilling carefully and, if possible, waiting until the first floor is in place in cold weather. (Concrete gains strength at a slower rate in cold weather.)

Since the performance of concrete basements is affected by climate conditions, unusual loads, materials quality and workmanship, care should always be exercised in their design and construction. The following steps should be followed:

  • Site conditions and excavation.
    Soil investigation should be thorough enough to insure design and construction of foundations suited to the building site. The excavation should be to the level of the bottom of the footing. The soil or granular fill beneath the entire area of the basement should be well compacted by rolling, vibrating or tamping. Footings must bear on undisturbed soil.

  • Formwork and reinforcement.
    All form-work must be constructed and braced so that it can withstand the pressure of the concrete. Reinforcement is effective in controlling shrinkage cracks and is especially beneficial where uneven side pressures against the walls may be expected. Observe state and local guidelines for wall thickness and reinforcement if needed.

  • Joints.
    Shrinkage and temperature cracking of basement walls can be controlled by means of properly located and formed joints. As a rule of thumb, in 8 ft. high and 8 inch thick walls, vertical control joints should be provided at a spacing of about 30 times the wall thickness. These wait joints can be formed by nailing a 3/4 inch thick strip of wood, beveled from 3/4 to 1/2 inch in width, to the inside of both interior and exterior wall forms. After the removal, the grooves should be caulked with a good quality joint filler.

  • Concrete.
    In general, use concrete with a moderate slump (up to 5 inches). Avoid retempering. Concrete with a higher slump may be used providing the mixture is specifically designed to produce the required strength without excessive bleeding and/or segregation. In areas where weathering is severe and where the walls may be exposed to moisture and freezing temperatures air entrained concrete should be used.

  • Placement and curing.
    Place concrete in a continuous operation to avoid cold joints. If concrete tends to bleed and segregate slump must be reduced and the concrete placed in the form every 20 or 30 feet around the perimeter of the wall. Higher slump concretes that do not bleed or segregate will flow horizontally for long distances and reduce the number of required points of access to the form. Provide adequate curing and protection to fresh concrete. It should not be allowed to freeze in cold weather. Preventive measures could be taken by completely enclosing the structure with polyethylene sheets and, if necessary, providing heat.

  • Waterproofing and drainage.
    Spray or paint the exterior of walls with damp proofing asphal-tic compound. Provide foundation drainage by installing drain tiles or plastic pipes around the exterior of the footing, then covering with clean granular fill to a height of at least 1 foot prior to backfill. Water should be drained to lower elevations suitable to receive storm water run off.

  • Backfilling and final grading.
    Backfilling should be done carefully to avoid damaging the walls. Brace the walls or, if possible, have first floor in place before backfill. To drain the surface water away from the basement finish grade should fall off 1/2 to 1 inch per foot for at least 8 feet to 10 feet away from the foundation.

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FINISHING

Finishing should be done with some experience and knowledge to avoid many mistakes. Having enough help and the right tools and equipment is very important. A slope of 1/8 in per foot is necessary to avoid low spots for good drainage of the slab. The sub-grade should be damp but not standing with water. It is a good idea to stretch a string across the outside of the forms at several locations and check the depth. Lets say you were poring a slab 20 ft. x 20 ft. x 4 in., that would equal 5 yards of concrete. The same slab at 5 inches deep would equal 6 ¼ yards making you short 1 ¼ yards and costing you time and money. A slump of 3 in. to 5in. is recommended. Screeding or straightedge operations should start before bleed water appears. A Come-along is often used in front of a screed to keep the proper depth of the concrete. A come-along should be used instead of a garden rake since they can cause segregation. Use of a Darby or bull float will fill in the voids and eliminate the ridges that was left by the screeding operation. This should be done before bleed water appears. Bull floating is usually done at right angles to the direction the concrete was straight edged. Edging and jointing are done next only after waiting a period for any bleed water to evaporate. The joint depth should be at least ¼th.the depth of the slab. After bleed water has evaporated and the concrete has just started to set, it is time to broom finish or trowel the surface. Broom finishes are recommended for outside slabs such as driveways, sidewalks, and patios. This helps prevent slips and falls during the winter months with snow and ice. Very smooth troweled finishes are usually used in garages, basements, factory floors, etc. This can be done by hand or by machine. All concrete exposed to freezing and all troweled finished concrete should be at least a six bag concrete mix. Curing should be done as soon as possible and no traffic should be permitted for at least 7 days.

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FLOWABLE FILL

What is Flowable Fill?
Flowable fill is a self-compacting low strength material with a flowable consistency that is used as an economical fill or backfill material as an alternative to compacted granular fill. Flowable fill is not concrete nor is it used to replace concrete. Other terms used for this material are unshrinkable fill, controlled density fill, flowable mortar or lean-mix backfill.

In terms of its flowability, the slump, as measured for concrete, is generally greater than 8 inches. It is self-leveling material and can be placed with minimal effort and does not require vibration or tamping. It hardens into a strong material with minimal subsidence.

While the broader definition includes material with compressive strength less than 1200 psi , most applications use mixtures with strength less than 300 psi.

Why is Flowable Fill Used?
Flowable fill is an economical alternative to compacted granular fill considering the savings in labor costs, equipment and time. Since it does not need manual compaction, trench width or the size of excavation is significantly reduced. Placing flowable fill does not require people to enter an excavation, a significant safety concern. It is also an excellent solution for filling inaccessible areas, such as underground tanks, where compacted fill cannot be placed.

Use of Flowable Fill include:

  • Backfill - sewer trenches, utility trenches, bridge abutments, conduit encasement, pile excavations, retaining walls, and road cuts. 

  • Structural Fill - foundation sub-base, subfooting, floor slab base, pavement bases, and conduit bedding. 

  • Other Uses - abandoned mines, underground storage tanks, wells, abandoned tunnel shafts and sewers, basements and underground structures, voids under pavement, erosion control, and thermal insulation with high air content flowable fill.

Flowability of flowable fill is important, so the mixture will flow into place and consolidate due to its fluidity without vibration or puddling action. The flowability can be varied to suit the placement requirements of most applications. Hydrostatic pressure and floatation of pipes should be considered by appropriate anchorage or by placing in lifts.

Permeability of flowable mixtures can be varied significantly to suit the application. Most mixtures have permeability similar to or lower than compacted soil.

Durability - Flowable fill materials are not designed to resist freezing and thawing, abrasive or most erosive actions, or aggressive chemicals. If these properties are required, use a high quality concrete. Fill materials are usually buried in the ground or otherwise confined. If flowable fill deteriorates in place it will continue to act as a granular fill.

Flowable fill can be conveyed by pump, chutes or buckets to its final location. For efficient pumping, some granular material is needed in the mixture. Due to its fluid consistency it can flow long distances from the point of placement.

Flowable fill does not need to be cured like concrete but should be protected from freezing until it has hardened.

Cautions:

  •  Flowable fill while fluid is a heavy material and during placement will exert a high fluid pressure against any forms, embankment, or walls used to contain the fill. 

  •  Placement of flowable fill around and under tanks, pipes, or large containers, such as swimming pools, can cause the container to float or shift. 

  •  In-place fluid flowable fill should be covered or cordoned off for safety reasons.

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JOINTS

Although concrete expands and contracts with changes in moisture and temperature the general overall tendency is to shrink and, therefore, crack. Irregular cracks are unsightly and difficult to maintain. Joints are simply preplanned cracks.

SOME FORMS OF JOINTS ARE:

  • Control joints.
    These joints are constructed to create planes of weakness so that cracks will occur at the desired location.

  • Expansion joints.
    They separate or isolate slabs from other parts of the structure such as walls, footings, or columns, and driveways and patios from sidewalks, garage slabs stairs, light poles and other obstructions. They permit movement of the slab and help minimize cracking caused when such movements are restrained.

  • Construction joints.
    These are joints that placed at the end of a day's work. In slabs they may be designed to permit movement and/or to transfer load. Often in reinforced concrete a conscious effort is made to clean the joint and bond the next day's work.

WHY ARE JOINTS CONSTRUCTED?

Concrete cracks cannot be prevented entirely, but they can be controlled and minimized by properly designed joints, because:

  • Concrete is weak in tension and , therefore, if its natural tendency to shrink is restrained, tensile stresses develop and cracks are likely to occur.

  • At early ages, before the concrete dries out, most cracking is caused by temperature changes or by the slight contraction that takes place as the concrete sets and hardens. Later as the concrete dries it will shrink further and either additional cracks may form or preexisting cracks may become wider.

  •  Joints provide relief for the tensile stresses and are less objectionable than random cracks.

HOW TO CONSTRUCT JOINTS

Joints must be carefully designed and properly constructed if uncontrolled cracking of concrete flatwork is to be avoided. The following recommended practices should be observed:

  • The maximum joint spacing in feet should not exceed 2.5 times the thickness in inches. For example in an 8 in. slab the joints should be no further apart than 20 feet.  

  • All panels should be square or nearly so. The length should not exceed 1.5 times the width. L- shaped panels should be avoided.  

  • The joint groove should have a depth of 1/4 the thickness of the slab, but not less than one inch. Tooled joints must be run early in the finishing process and rerun later to assure groove bond has not occurred. 

  • The joints can be tooled during finishing or sawed with a carborundum blade at an early age. Sawed joints may not be practical if the concrete is mad with hard aggregates such as quartz, gravel, or trap rock.  

  • Sawing is easier if coarse aggregates contain materials such as limestone or sandstone. If the joint edges ravel during sawing it must be delayed, but if sawing is delayed too long sawing can become difficult. 

  • With abrasive saw blades sawing is often done at an age of one day or even earlier.  

  • Pre-molded joint filler, building paper or polyethylene should e used to isolate slabs from building walls or footings. At least two inches of sand over the top of a footing will also prevent bond to the footing.  

  • To isolate columns from slabs, form circular or square openings which will not be filled until after the floor has hardened. Slab control joints should interest at the openings for columns. If square openings are used around columns the square should be turned at 45 degrees to have the control joints intersect at the diagonals of the square.  

  • If the slab contains wire mesh cut out alternate wires across control joints. Note that wire mesh will not prevent cracking. Mesh tends to keep the cracks and joints tightly closed. 

  • Construction joints key the two edges of the slab together either to provide transfer of loads or to help prevent curling or warping of the two adjacent edges. Galvanized metal keys are preferred for interior slabs, however, a beveled 1 by 2 inch strip, nailed to bulkheads or form boards, can be used in slabs that are at least 5 inches thick to form a key which will resist vertical loads and movements. Metal dowels can also be used in slabs that will carry heavy loads. Dowels must be carefully lined up and parallel or they may induce restraint and cause random cracking at the end of the dowel.  

  • Joints in industrial floors subject to heavy traffic require special attention to avoid spalling of joint edges. Such joints should be filled with a material capable of supporting joint edges. Manufacturer's recommendations and performance records should be checked before use.

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MOVEMENTS

Concrete shrinks as it dries and swells as it is wetted. As a rule, when concrete dries and becomes re-saturated, not more than two-thirds of the initial drying shrinkage will be recovered.

Heat of hydration is the heat concrete generates as it hardens because of the chemical reaction of the cement and water. Heat of hydration can be a source of temperature-caused movement. If the temperature increased in a long slab or wall resting on the ground, the concrete would extend its length by sliding on the ground: but on cooling, the friction of the ground could cause tensile forces high enough to severely crack the concrete.

Deflection of concrete beams and slabs is on of the most common examples of building movements and its effects are conspicuous. As floors deflect, The partitions they support may separate from the ceiling above, cracks may develop in the wall, and doors may not close properly.

MOVEMENTS DUE TO FOUNDATIONS

Reasons are:

  • They are resting on or more different types of soil.

  • Sloping sites may require cutting and filling. The fill must be well compacted so that the foundation and floor will settle no more there than at the cut end.

  • When groundwater levels drop, uneven settlements occur. Fast growing trees are known to desiccate the soil, causing settlements at one side or corner. The process may develop over a period of years. d. Mining subsidence. The ground surface may settle were mining occurred below.

FROST HEAVE

During construction when shallow footings in the interior of the building are exposed to subfreezing weather, they can suffer frost heave.

Adfreeze is the adhesion of freezing ground to a pile, pier, or wall with subsequent heaving. Placing footings below the depth of frost penetration does not protect foundations from heaving unless adfreezing of the soil to the structure exceeds maximum uplift forces.

CONTROL JOINTS IN WALLS

All concrete and concrete masonry shrinks and swells upon loss or gain of moisture. Unsightly cracks can be eliminated by controlling their location. Control joints should be not more than 20 feet apart in exterior walls with frequent openings. In walls without openings they should never be more than 25 feet and within 10 or 15 feet of a corner if possible. Joint spacing in any exposed cast-in-place interior walls should be identical to joint spacing in outside walls.

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SITE

The site must be a well drained, uniform sub-grade. All organic and foreign material should be completely removed. Fill material used should be sand, gravel, or stone. Sub-grades should be damp but never standing with water and free of frost. If possible slope the ground away from the slab. Any lines or pipes should be covered with at least 2 inches of sub-base so there is no bond with the concrete. When the concrete shrinks and is in contact with pipes and lines this could cause cracks in the slab.

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WATER-TIGHT BASEMENTS

General Precautions:

  • To be sure that uneven settlements will not cause cracks in the walls, the footings should be of sufficient width and be properly proportioned to carry their loads in accordance with local building code requirements. In the absence of a local building code, general practice for residential basement construction requires footings to have a depth equal to the thickness of the foundation wall and a width equal to twice the wall thickness. Where soil conditions do not provide good bearing it is desirable to spread the footings over more area and to add steel reinforcements. No portion of the footings should bear on freshly filled ground, steps being used where faulty excavation does not permit the bottoms of the footings to be at one level. 

  • Downspouts should be connected to underground drains or arranged to discharge water away from the walls. Surface water should be drained away by proper grading or by use of a sloping concrete gutter. 

  • Unless the work is being done in a section of the country having a dry climate, or the excavation is made in exceptionally well-drained subsoil, a line of drain tile should be placed around the building at the side of the footings. The tile should have a slope of about 1/2 inch in 12 feet and should drain to a suitable outlet. Do not connect downspouts to this tile. When the proper time in the building operations arrives to place the drain tiles around the footings, the side slopes of the excavation generally will have sloughed off and filled in next to the footing where the tile should be placed. Under such conditions there may be a tendency to place the drain tile above the footing on the shelf formed by the footing and the wall. Tile should always be placed at the side of the footing. A sloping shoulder of mortar should be placed on the shelf to keep water from collecting there. Joints between the tile should be covered with pieces of roofing felt to prevent sediments filling the tile during backfilling. The tile should be covered for at least 18 inches with a permeable fill of coarse gravel or crushed stone. 

  • In poorly drained soils the exterior surface of the basement wall should be given two continuous coatings of hot bituminous material applied at right angles to each other over a suitable priming coat, extending from 6 inches above the ground line down over the top of the footing. Wall must be surface dry when primer is applied. Primer should be dry before the hot bituminous material is applied.

Concrete Masonry Walls:

  • Basement walls of concrete masonry should meet local building code requirements as to thickness and strength of units. In the absence of a local building code use units meeting the ASTM specifications for quality. 

  • A full bed of mortar should be placed on the footing to receive the first course of block. Face-shell bedding should be used on all succeeding courses with full mortar coverage on vertical and horizontal face shells. Joints should be 3/8 inches thick. Joints should be firmly compacted, after the mortar has stiffened, with a rounded tool having a diameter slightly larger than the thickness of the joint.

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FRESH AND HARDENED MORTAR

DESIRABLE PROPERTIES OF FRESH, PLASTIC MORTAR

  • Good mortar is necessary for good workmanship and proper structural performance of concrete masonry. Since mortar must bond masonry units into strong, durable, weather tight structures, it must have many desirable properties and the materials must comply with specifications. Desirable properties or mortar while plastic include workability, water retention, and a consistent rate of hardening.

Workability

  • This property of plastic mortar is difficult to define because it is a combination of a number of interdependent, interrelated properties. The interrelated mortar properties considered as having the greatest influence on workability are: consistency, water retention, setting time, weight, adhesion, and penetrability. 

  • The experienced mason judges the workability of mortar by the way it adheres to or slides from his trowel. Mortar of good workability should spread easily on the concrete masonry unit, cling to vertical surfaces, extrude readily from joints without dropping or smearing, and permit easy positioning of the unit without subsequent shifting due to its weight or the weight of successive courses. Mortar consistency should change with weather to help in laying the units. A good workable mix should be softer in summer than in winter to compensate for water loss.

Water Retention

  • This is the property of mortar that resists rapid loss of mixing water (prevents loss of plasticity) to the air on a dry day or to an absorptive masonry unit. Rapid loss of water causes the mortar to stiffen quickly, thereby making it practically impossible to obtain good bond and weather tight joints. 

  • Water retention is an important property and related to workability. A mortar that has good water retention remains soft and plastic long enough for the masonry units to be carefully aligned, leveled, plumbed, and adjusted to proper line without danger of breaking the intimate contact or bong between the mortar and the units. When low-absorption units such as split block are in contact with a mortar having too much water retention, they may float. Consequently, the water retention of a mortar should be within tolerable limits. 

  • Entrained air, extremely fine aggregate or cementitious materials, or water adds workability or plasticity to the mortar and increases its water retention.

Consistent Rate of Hardening

  • The rate of hardening of mortar due to hydration (chemical reaction) is the speed at which it develops resistance to an applied load. Too rapid hardening may interfere with the use of the mortar by the mason. Very slow hardening may impede the progress of the work since the mortar will flow from the completed masonry. During winter construction, slow hardening may also subject mortar to early damage from frost action. A well-defined, consistent rate of hardening assists the mason in laying the masonry units and in tooling the joints at the same degree of hardness. Uniform joint color of masonry reflects proper hardening and consistent tooling times.

  • Hardening is sometimes confused with a stiffening caused by rapid loss of water, as in the case of low-water-retention mortars with highly absorptive units. Also, during very hot, dry weather mortar may tend to stiffen more rapidly than usual. In this case, the mason may find it advisable to lay shorter mortar beds and fewer units in advance of tooling.

DESIRABLE PROPERTIES OF HARDENED MORTAR

Bond

  • The general term "bond" refers to a specific property that can be subdivided into: (1) the extent of bond, or the degree of contact of the mortar with the concrete masonry units; and (2) the tensile bond strength, or the force required to separate the units. A chemical and a mechanical bond exist in each category. 

  • Good extent of bond (complete and intimate contact) is important to water tightness and tensile bond strength. Poor bond at the mortar-to-unit interface may lead to moisture penetration through the unbound areas. Good extent of bond is obtained with a workable and water-retentive mortar, good workmanship, full joints, and concrete masonry units having a medium initial rate of absorption (suction). 

  • Tensile bond strength is perhaps the most important property of hardened mortar. Mortar must develop sufficient bond to withstand the tensile forces brought about by structural, earth, and wind loads; shrinkage of concrete masonry units or mortar; and temperature changes.

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BLISTERS

Blisters are the result of entrapped air (not to be confused with entrained air) just below the surface. They are usually 1/4 in to 4 inches in diameter and only about 1/8 in deep. These bubbles of entrapped air, form and are trapped under a dense surface skin while troweling. They usually do not appear until sometime after the first troweling. When blisters do appear, the second troweling should be delayed as long as possible. This will give the entrapped air time to cool and subside.

CAUSES

1.  A cold subgrade will slow the set of the bottom of the concrete.
2.  Using a dry shake
3.  Using a sticky mix from a high cement content.
4.  Vibrating if the slump is over 3 inches.
5.  Finishing too early

HOW TO AVOID BLISTERS

1.  Don’t seal surface before bleed water has escaped
2.  If using a vibrating straightedge, move it forward as quickly as possible.
3.  Use heated concrete to promote an even setting of the concrete slab
4.  Don’t place slabs directly on plastic.

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COLD WEATHER

Concrete, like other construction materials, contracts and expands with changes in moisture content and temperature and deflects depending on load and support conditions. When provisions for these movements are not made in design and construction, then cracks can occur. Some forms of common cracks are:

  • Plastic Shrinkage Cracking

  • Cracks Due to Improper Jointing

  • Cracks Due to Continuous External Restraint (Example-Cast in place wall restrained along bottom edge of footing)

  • Basement Floor Cracks

  • Cracks from Freezing and Thawing

  • Craze Cracks )

  • Settlement Cracks

Cracks rarely affect structural integrity. Most random individual cracks look bad and although they permit entrance of water they do not lead to progressive deterioration. They are simply unsightly. Closely spaced pattern cracks or D-cracks due to freezing and thawing are an exception and may lead to ultimate deterioration.

The majority of concrete cracks usually occur due to improper design and construction practices, such as:

  • Omission of isolation and control joints and improper jointing practices.

  • Improper sub-grade preparation.

  • The use of high slump concrete or addition of water on the job.

  • Improper finishing.

  • Inadequate or no curing.

All concrete has a tendency to crack and it is not possible to consistently produce completely crack-free concrete. However, cracking can be reduced and controlled if the following basic safeguards are observed:

  • Sub-grade and Formwork
    All top soil and soft spots should be removed. Regardless of its type, the soil beneath the slab should be compacted soil or granular fill, well compacted by rolling, vibrating or tamping. The slab and, therefore, the sub-grade should be sloped for proper drainage. Smooth, level sub-grades help prevent cracking. All formwork must be constructed and braced so that it can withstand the pressure of the concrete without movement. Polyethylene vapor barriers increase bleeding and greatly increase cracking of high slump concrete. Cover the vapor barrier with 1 to 2 inches of damp sand to reduce bleeding. Immediately prior to concrete placement, dampen the sub-grade, formwork, and the reinforcement. 

  • Concrete
    In general, use concrete with a moderate slump (not over 5 inches). Avoid re-tempering. If higher slump, up to 7 inches, is to be used, proportions will have to be changed and special mixtures developed to avoid recessive bleeding, segregation and low strength. Specify air-entrained concrete for outdoor slabs subjected to freezing weather. 

  • Finishing
    DO NOT perform finishing operations with water present on the surface. Initial screeding must be promptly followed by bull floating. For better traction on exterior surfaces use a broom finish. If evaporation is excessive reduce it by some means to avoid plastic shrinkage cracking. Cover the concrete with wet burlap or polyethylene sheets in between finishing operations if conditions are severe. 

  • Curing
    Start curing as soon as possible. Spray the surface with liquid membrane curing compound or cover it with damp burlap and keep it moist for at least 3 days. A second application of curing compound the next day is a good quality assurance step. 

  • Joints
    Provisions for contraction or expansion movements due to temperature and/or moisture change should be provided with construction of control joints by sawing, forming or tooling a groove about 1/4 the thickness of the slab, no further apart than 30 times the thickness. Often closer spacing of control joints will be necessary to avoid long thin areas. The length of an area should not exceed about 1.5 times the width. Isolation joints should be provided whenever restriction to freedom of either vertical or horizontal movement is anticipated: such as where floors meet walls, columns, or footings. These are full-depth joints and are constructed by inserting a barrier of some type to prevent bond between the slab and the other elements.

  • Cover Over Reinforcement
    Cracks in reinforced concrete caused by expansion of rust on reinforcing steel should be prevented by providing sufficient concrete cover (at least 2 inches) to keep salt arid moisture from contacting the steel.

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CRACKING

CRACKS IN BASEMENTS WALLS

  • Temperature and drying shrinkage cracks.
    With few exceptions, newly placed concrete has Cast-in-place concrete basements provide durable, high quality extra living space. At times when proper construction practices are not used undesirable cracks occur, such as: the largest volume that it will ever have. This shrinkage tendency is increased by drying and/or a drop in temperature and can lead to random cracking if steps are not taken to control the location of the cracks by providing control joints. 

  • Settlement cracks.
    These occur from non-uniform support of footings or occasionally from expansive soils

  • Other structural cracks.
    In basements these cracks generally occur during backfilling, particularly when heavy equipment gets too close to the walls. 

  • Cracks due to lack of joints or improper jointing practices. 

  • In concrete basement walls some cracking is normal.

Most cracks normally occur because one or more of the following rules of "good concrete practice" were not followed:

  • Providing uniform soil support.

  • Using moderate slump concrete and avoiding addition of water to the concrete mixture on the job.

  • Observing proper concrete placement practices.

  • Providing control joints every 20 to 30 feet. e. Backfilling carefully and, if possible, waiting until the first floor is in place in cold weather. (Concrete gains strength at a slower rate in cold weather.)

Since the performance of concrete basements is affected by climate conditions, unusual loads, materials quality and workmanship, care should always be exercised in their design and construction. The following steps should be followed:

  • Site conditions and excavation.
    Soil investigation should be thorough enough to insure design and construction of foundations suited to the building site. The excavation should be to the level of the bottom of the footing. The soil or granular fill beneath the entire area of the basement should be well compacted by rolling, vibrating or tamping. Footings must bear on undisturbed soil. 

  • Formwork and reinforcement.
    All form-work must be constructed and braced so that it can withstand the pressure of the concrete. Reinforcement is effective in controlling shrinkage cracks and is especially beneficial where uneven side pressures against the walls may be expected. Observe state and local guidelines for wall thickness and reinforcement if needed. 

  • Joints.
    Shrinkage and temperature cracking of basement walls can be controlled by means of properly located and formed joints. As a rule of thumb, in 8 ft. high and 8 inch thick walls, vertical control joints should be provided at a spacing of about 30 times the wall thickness. These wait joints can be formed by nailing a 3/4 inch thick strip of wood, beveled from 3/4 to 1/2 inch in width, to the inside of both interior and exterior wall forms. After the removal, the grooves should be caulked with a good quality joint filler. 

  • Concrete.
    In general, use concrete with a moderate slump (up to 5 inches). Avoid retempering. Concrete with a higher slump may be used providing the mixture is specifically designed to produce the required strength without excessive bleeding and/or segregation. In areas where weathering is severe and where the walls may be exposed to moisture and freezing temperatures air entrained concrete should be used. 

  • Placement and curing.
    Place concrete in a continuous operation to avoid cold joints. If concrete tends to bleed and segregate slump must be reduced and the concrete placed in the form every 20 or 30 feet around the perimeter of the wall. Higher slump concretes that do not bleed or segregate will flow horizontally for long distances and reduce the number of required points of access to the form. Provide adequate curing and protection to fresh concrete. It should not be allowed to freeze in cold weather. Preventive measures could be taken by completely enclosing the structure with polyethylene sheets and, if necessary, providing heat. 

  • Waterproofing and drainage.
    Spray or paint the exterior of walls with damp proofing asphal-tic compound. Provide foundation drainage by installing drain tiles or plastic pipes around the exterior of the footing, then covering with clean granular fill to a height of at least 1 foot prior to backfill. Water should be drained to lower elevations suitable to receive storm water run off. 

  • Backfilling and final grading.
    Backfilling should be done carefully to avoid damaging the walls. Brace the walls or, if possible, have first floor in place before backfill. To drain the surface water away from the basement finish grade should fall off 1/2 to 1 inch per foot for at least 8 feet to 10 feet away from the foundation.

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CRAZING

Crazing cracks are small enclosed hairline like cracks less than 1/8 inch deep and usually 3/8 inch to 1 ½ inch across. It is caused by surface shrinkage stresses as the concrete dries out. These cracks usually do not affect the durability or wear of the concrete.

CAUSES:

  • Placing a high slump (wet) concrete.

  • Finishing bleed water back into the slab.

  • Overusing a vibratory screed.

  • Adding a dry shake or cement to the surface.

  • Using un-vented heaters in a building and allowing carbon monoxide or carbon dioxide to react with the cement

  • Not curing the slab.

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CURLING

Slabs on grade do not shrink uniformly from top to bottom. The top dries out more rapidly than the bottom and tends to shrink more, and thus every vertical segment of the slab becomes slightly wedge-shaped.

The shrinking of concrete when it dries amounts about 1/8th inch in 20 feet. It becomes shorter at the top than on bottom and tends to dish upwards. Curling induces tensile stresses in the top part and if these stresses exceed the tensile strength of the concrete, cracks will appear.

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DELAMINATION OF TROWELED CONCRETE SURFACES

What are Delaminations?
In a delaminated surface, the top 1/8 inch is densified and separated from the base slab by a thin layer of air or water. The delaminations on the surface of a slab may range in size from several square inches to many square feet and can be detected by a hollow sound when tapped with a hammer or with a heavy chain drag. They may exhibit cracking and color differences because of rapid drying of the thin surface during curing. Traffic or freezing may break away the surface in large sheets. They are similar to blisters, but much larger.

Delaminations form during final troweling. They are most frequent in early spring and late fall when concrete is placed on a cool sub-grade with rising daytime temperatures, but they can occur anytime.

Why Does Delamination Occur?
Delamination occurs when the fresh concrete surface is sealed by troweling while the underlying concrete is plastic and bleeding or able to release air. Delaminations form fairly late in the finishing process after floating and after the first troweling. Rapid evaporation of bleed water due to surface drying (wind, sun, or low humidity) makes the surface appear ready to trowel while the underlying concrete is plastic and can still bleed or release air. Vapor barriers under slabs force water to rise and compound the problem. The use of fly ash and chemical retarders will delay initial set of the underlying concrete and allow bleed water and air to move upward after the surface is sealed.

Entrained air reduces bleeding and promotes early finishing which will produce a dense impermeable surface layer. A cool sub-grade delays set in the bottom relative to the top. Air and water collect under the dense surface layer during finishing.

Delamination is more likely to form if:

  •  The underlying concrete sets slowly because of a cool subgrade.

  •  Set is retarded by retarders and/or fly ash.

  •  Entrained air is used (or is higher than normal).

  •  Use of a jitterbug or vibrating screed brings too much mortar to the surface.

  •  A dry shake is used, particularly with air-entrained concrete.

  •  The concrete is sticky from higher cementitious material or sand content.

  •  The slab is thick.

  •  The slab is placed directly on a vapor barrier.

How to Prevent Delamination:
Be wary of a concrete surface that appears to be ready to trowel before it would normally be expected. Emphasis in finishing should be on screeding, straight-edging, and floating the concrete as rapidly as possible-without working up an excessive layer of mortar.

Further finishing should be delayed as long as possible, and the surface covered with polyethylene or otherwise protected from evaporation. In initial floating, the float blades should be flat to avoid densifying the surface too early. Accelerators or heated concrete often prevent delamination in cool weather.

Delamination may be difficult to detect during finishing operations. If delamination is observed, try to flatten the trowel blades or tear the surface with a wood float and delay finishing as long as possible. Any steps that can be taken to slow evaporation should help.

If a vapor barrier is required, place a layer of damp fine aggregate over the plastic sheet. Do not place concrete directly on a vapor barrier. Do not use air-entrained concrete in floor slabs which have a hard trowel surface and which will not be subject to deicing salts.

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DISCOLORATION

Discoloration of concrete are blotches or color changes in the concrete finish caused by mix change or by efflorencence.

CAUSES
Discoloration can be caused by changes in cement or fine aggregate but usually is caused by the inconsistent use of a admixtures, insufficient mixing time, and improper timing of finishing operations. A yellowish or greenish hue may appear on the concrete containing ground slag as a cementitious material. This will disappear in time. The discoloration of concrete in slabs on grade is usually the result of a change in either the concrete composition or a concrete construction practice. In most studies, no single factor seemed to cause discoloration.

Factors found to influence discoloration are the use of calcium chloride, variation in cement alkali content, delayed hydration of the cement paste, admixtures, hard-troweled surfaces or improper curing or finishing procedures that cause surface variation of the water-cement ratio, and changes in the concrete mix.

PREVENTION:

  • Minimize the use of high-alkali content cements.

  • Calcium chloride is the primary cause of concrete discoloration.

  • The type, kind, and condition of formwork can influence the surface color. Forms with different rates of absorption will cause the surface with different shades of color. A change in the type or brand of a form release agent can also change concrete color.

  • Eliminate trowel burning of the concrete. The most common consequence is that metal fragments from the trowel are embedded in the surface of the concrete. Also concrete that has been hand-troweled ,ay have dark discoloration as a result of densifying the surface, which reduces the water-cement ratio. The resulting low water-cement ratio affects the hydration of the cement ferrites which contributes to a darker color. Concrete surfaces that are troweled too early will increase the water-cement ratio at the surface and lighten the color.

  • Concrete which is not properly or uniformly cured may develop discoloration.  Uneven curing will affect the degree of hydration of the cement. Curing with polyethylene may also cause discoloration causing streaks.

  • The discoloration of a slab may be minimized or prevented by moistening the absorptive sub-grades, following proper curing procedures. And adding proper protection of the concrete from drying by the wind and sun.

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DUSTING

When the surface of the concrete develops a powered or chalk like material it is referred to as dusting. A weak surface is to blame for chalking and will keep appearing as traffic wares away the surface.

CAUSES:

  • Finishing bleed water back into the surface of the slab.

  • Not curing the slab which can result in a soft surface.

  • Carbon dioxide or carbon monoxide fumes having a reaction with the cement.

  • Rain or condensation being finished back into the slab. Anytime water is worked back into the surface it increased the water-cement ratio and weakens the surface layer.

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SCALING

Scaling is when the surface of a hardened concrete slab breaks away from the slab to a depth of about 1/16 to 1/4 in. This usually occurs at an early age.

Concrete placed in late fall will be susceptible to scaling during the first winter if proper procedures are not followed. Fly ash in concrete has been regarded by some as a potential risk to scaling. Fly ash should not, however be of concern if the levels of air content and strength achieved are comparable to those required for concrete that contains no fly ash.

The use of entrained air will do a great deal to eliminate scaling, but it cannot be considered an absolute cure-all, particularly for saturated concrete exposed to deicing salts early in its life. Concretes placed in the spring and early summer generally perform more satisfactorily than those placed in the fall or winter because they have had an opportunity to gain strength and finally dry out before freezing weather.

PROPER FINISHING:
The necessity of using proper finishing techniques cannot be overemphasized. No finishing operations should ever proceed when bleed water is present on the surface of the concrete. Finishing in the presence of bleed water produces a high water-cement ratio and very low strength in the top 1/8 to ¼ inch of the surface. The use of moderate-slump concrete (3-5 in.) is recommended.

CAUSES:

  •  Early cycles of freezing and thawing of the surface of newly placed concrete.

  •  Applications of deicers the first year.

  •  Any finishing operation performed while bleed water is on the surface, will cause scaling. Mixing excess water into the top of the slab will cause a segregation of the surface fines (sand and cement) and cause scaling.

PREVENTION:

  •  The temperature of freshly placed concrete should be maintained above 50F for at least 5 days when using a normal mix.

  •  Use air-entrained concrete

  •  No finishing operation should ever be preformed while bleed water is present. Remove bleed water by dragging a rubber garden hose over the surface.

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SHRINKING

PLASTIC SHRINKAGE CRACKS:
Cracks that appear on the surface of freshly placed concrete during finishing or soon after. Rapid loss of moisture from fresh concrete often leads to cracking (surface water evaporates at a faster rate that it can replaced by bleed water). Although plastic cracks are unsightly they rarely impair the strength of the concrete.

CAUSES: The cause of plastic cracking is one or a combination of the following:

  • Wind and low humidity (if relative humidity changes from 90% to 50% the rate of evaporation is increased 5 times).

  • Exposure to the sun

  • High ambient temperature ( when the temperature increases from 50F to 70F the rate of evaporation of water from the surface doubles).

  • High concrete temperature (rapid evaporation and cracking may occur when the temperature of concrete is significantly higher than the air temperature).

  • Dry sub-grade

PREVENTION:

  • If no vapor barrier is used, the sub-grade should be saturated just before placing the concrete.

  • Have plenty of help and finish promptly. If delays occur, cover with wet burlap or polyethylene sheeting.

  • Cure as soon as possible

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