If conditions indicate that a base course is desirable, a thorough investigation should be made to determine the source, quantity, and characteristics of the available materials. A study should be made to determine the most economical thickness of material for a base course that will meet the requirements. The base course may consist of natural materials, processed materials, or stabilized materials as defined in TM In general, the base course material should be well-graded high- stability material. If a free-draining, open-graded subbase is used, a filter layer may be placed under the base course to prevent pumping action and subgrade intrusion.
Coarse aggregate shall have a percentage of wear by the Los Angeles abrasion test of not more than Uniform high-quality materials shall be used.
Full text of "CONCRETE FLOOR SLABS ON GRADE SUBJECTED TO HEAVY LOADS"
Weakly cemented rocks and most shales should not be used; an exception would be baked shales occurring adjacent to intrusive dikes. The frost susceptibility criterion listed previously in chapter 4 is also applicable to base course materials. Durability will be checked if the base aggregate will be exposed to frost. Aggregates that break down excessively when subjected to freeze-thaw cycles will not be used. Under certain conditions, concrete pavement slabs may be reinforced with welded wire fabric or de- formed bar mats arranged in a square or rectangular grid. The advantages in using steel reinforcement include: Guidance relative to the use of reinforced pavement is discussed in the following paragraphs.
Reinforcement may be used to control cracking in rigid pavements found on subgrades where differential vertical movement is a definite potential for example, foundations with definite or borderline frost susceptibility that cannot feasibly be made to conform to conventional frost design requirements. For the general case, reinforced rigid pavements will not be economically competitive with nonreinforced rigid pavements of equal load-carrying capacity, even though a reduc- tion in pavement thickness is possible.
Alternate bids, however, should be invited if reasonable doubt exists on this point. In otherwise nonrein- forced floor slabs, steel reinforcement should be used for the conditions below. Odd-shaped slabs should be reinforced using a minimum of 0. An odd- shaped slab is considered to be one in which the longer dimension exceeds the shorter dimension by more than 25 percent or a slab which essentially is neither square nor rectangular.
Figure presents an example of reinforcement required in odd- shaped slabs.
Reinforcement for odd-shaped slabs. A partial reinforce- ment of slab is required where the joint patterns of abutting or adjacent floor slabs do not match, and when the pavements are not positively separated by an expansion or slip-type joint. The floor slab directiy opposite the mismatched joint should be reinforced with a minimum of 0. Mismatched joints normally will occur at intersections of floor slabs or between regular floor slab and fillet areas fig Thickness design on unbonded base or subbase.
The design procedure for reinforced concrete floor slabs uses the principle of allowing a reduction in the required thickness of nonreinforced concrete floor slab due to the presence of the steel reinforcing. The design procedure has been developed empirically from a limited number of prototype test pavements subjected to accelerated traffic testing. Although it is anticipated that some cracking will occur in the floor slab under the design traffic loadings, the steel reinforcing will hold the cracks tightly closed.
The reinforcing will prevent spalling or faulting at the cracks and provide a serviceable floor slab during the anticipated design life. Essentially, the design method consists of determining the percentage of steel required, the thickness of the reinforced floor slab, and the maximum allowable length of the slabs. Figure presents a graphic solution for the design of rein- forced floor slabs.
Since the thickness of a reinforced floor slab is a function of the percentage of steel reinforcing, the designer may determine the required percentage of steel for a predetermined thickness of floor slab or determine the required thickness of floor slab for a predetermined percentage of steel, in either case, it is necessary first to determine the required thickness of nonreinforced floor slab by the method outlined previously para for non reinforced floor slabs. A straight line is then drawn from the value of h to the value selected for the thickness of reinforced floor slab, h r ,and extended to the required percentage of reinforcing steel, S , or drawn from the value h to the value selected for the percentage of reinforcing steel, and extended to the thickness, h r.
The thickness, h r , will always be equal to or less than the thickness, h. It should be noted that the S value indicated in figure is the percentage to be used in the longitudinal direction only. For nomral designs, the percentage of nonreinforcing steel used in the transverse direction will be one-half of that to be used in the longitudinal dirction.
Once the h r and S values have been determined, the maximum allowable slab length L is obtained from the intersection of the straight line and the scale of L. Provision also is made in the nomograph for adjusting L on the basis of the yield strength f. Difficulties may be encountered in sealing joints between very long slabs because of large volumetric changes caused by temperature changes. Design thickness for reinforced floor slabs.
Thickness design on stabilized base or sub- grade. To determine the thickness requirements for reinforced concrete floor slabs on a stabilized foundation, it is first necessary to determine the thickness of nonreinforced concrete floor slab required for the design conditions. This thickness of nonreinforced floor slab is determined by the procedures set forth in paragraph d. Figure is then entered with the values of h , h r , and S c. The design criteria for reinforced concrete floor slabs on grade are subject to the following limitations: The reinforcing steel for floor slabs may be either deformed bars or welded wire fabric.
In addition, the following criteria regarding the maximum spacing of reinforcement should be observed. For welded wire fabric, the maximum spacing of the longitudinal wires and transverse wires should not exceed 6 inches and 12 inches, respectively; for bar mats, the maximum spacing of the longitudinal bars and the transverse bars should not exceed 15 inches and 30 inches, respectively.
Joint types and usage. Joints are provided to permit contraction and expan- sion of the concrete resulting from temperature and moisture changes, to relieve warping and curling stresses due to temperature and moisture differen- tials, to prevent unsightly, irregular breaking of the floor slab; as a construction expedient, to separate sections or strips of concrete placed at different times; and to isolate the floor slab from other building components. The three general types of joints are contraction, construction, and isolation. A typical floor-slab joint layout is shown in figure Weakened-plane contraction joints are provided to control cracking in the concrete and to limit curling or warping stresses resulting from drying shrinkage and contraction and from temperature and moisture gradients in the slab, respectively.
Shrinkage and contraction of the concrete causes slight cracking and separation of the slabs at the weakened planes, which will provide some relief from tensile forces resulting from foundation restraint and compressive forces caused by subsequent expansion. Contraction joints will be required transversely and may be required longitudinally depending upon slab thickness and spacing of construction joints.
Contraction joints for reinforced and nonreinforced floor slabs are shown in figures and , respectively. Contraction joints for reinforced and nonreinforced floor slabs. The depth of the weakened plane groove must be great enough to cause the concrete to crack under the tensile stresses resulting from the shrinkage and contraction of the concrete as it cures.
Experience, supported by analyses, indicates that this depth should be at Last one-fourth of the slab thickness for floor slabs 12 inches or less, 3 inches for pavements greater than 12 and less than 18 inches in thickness, and one-sixth of the slab thickness for floor slabs greater than 18 inches in thickness. In no case will the depth of the groove be less than the maximum nominal size of aggregate used. Sawcut contraction joints for steel-fiber reinforced concrete should be cut a minimum of one-third of the slab thickness. Concrete placement conditions may influence the fracturing of the concrete and dictate the depth of groove required.
For example, concrete placed early in the day, when the air temperature is rising, may experience expansion rather than contraction during the early life of the concrete with subsequent contraction occurring several hours later as the air temperature drops. The concrete may have attained sufficient strength before the contraction occurs so that each successive weakened plane does not result in fracturing of the concrete.
As a result, excessive opening may result where fracturing does occur. To prevent this, the depth of the groove will be increased to assure the fracturing and proper functions of each of the scheduled joints. The width and depth of the sealant reservoir for the weakened plane groove will conform to dimensions shown in figure The dimensions of the sealant reservoir are critical to satisfactory performance of the joint sealing materials. Transverse contraction joints will be constructed across each paving lane perpendicular to the center line.
TM 5-809-12 Concrete Floor Slabs on Grade Subjected to Heavy Loads; replaced by UFC 3-320-06A
The joint spacing will be uniform throughout any major paved area, and each joint will be straight and continuous from edge to edge of the paving lane and across all paving lanes for the full width of the paved area. Staggering of joints in adjacent paving lanes can lead to sympathetic cracking and will not be permitted unless reinforcement, as described in paragraph b, is used. The maximum spacing of transverse joints that will effectively control cracking will vary appreciably depending on pavement thickness, thermal coefficient, and other characteristics of the aggregate and concrete, climatic conditions, and foundation restraint.
It is impracticable to establish limits on joint spacing that are suitable for all conditions without making them unduly restrictive. For best slab performance, the number of joints should be kept to a minimum by using the greatest joint spacing that will satisfactorily control cracking. Experience has shown, however, that oblong slabs, especially in thin slabs, tend to crack into smaller slabs of nearly equal dimensions under traffic.
Therefore, it is desirable, insofar as practicable, to keep the length and width dimensions as nearly equal as possible. In no case should the length dimension in the direction of paving exceed the width dimension more than 25 percent. The joint spac- ings in table have given satisfactory control of transverse cracking in most instances and may be used as a guide, subject to modification based on available information regarding the performance of existing pavements in the vicinity or unusual proper- ties of the concrete.
Under certain climatic conditions, joint spacings different from those in table may be satisfactory. Transverse contrac- tion joints in reinforced concrete slabs should not be constructed at intervals of less than 25 feet nor more than 75 feet. Selection of final spacing should be based on local conditions. Where only a portion of the slabs are reinforced, joint spacing should be a maximum commensurate with the unreinforced slab configurations.
Contraction joints will be placed along the centerline of paving lanes that have a width greater than the indicated maximum spacing of transverse contraction joints in table These joints may also be required in the longitudinal direction for overlays, regardless of overlay thickness, to match joints existing in the base pavement unless a bond- breaking medium is used between the overlay and base slab or the overlay slab is reinforced.
Dowel requirements and specifications are given in paragraph d. Construction joints are provided to separate areas of concrete placed at different times.
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They may be required in both the longitudinal and transverse directions. The spacing of construction joints will depend largely on the size and shape of the floor slab that is being placed and the equipment used to place it.
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Joint spacing will also be affected by column spacing and bay sizes. Longitudinal construction joints, generally spaced 20 to 25 feet apart but may reach 50 feet apart depending on construction equipment capability, will be provided to separate successively placed paving lanes. Tranverse construction joints will be installed when it is necessary to stop concrete placement within a paving lane for a sufficient time for the concrete to start to set.
All transverse construction joints will be located in place of other regularly spaced transverse joints contraction or isolation types. There are several types of construction joints available for use, as shown in figures , , and and as described below. The selection of the type of construction joint will depend on such factors as the concrete placement procedure formed or slipformed and foundation conditions. Doweled construction joints for concrete floor slabs. Keyed construction joints for concrete floor slabs. Doorway slab design for vehicular traffic. The doweled butt joint is considered to be the best joint for providing load transfer and maintaining slab alignment.
It is a desirable joint for the most adverse conditions such as heavy loading, high traffic intensity, and lower strength foundations. However, because the alignment and placement of the dowel bars are criti- cal to satisfactory performance, this type of joint is difficult to construct, especially for slipformed con- crete. However, the doweled butt joint is required for all transverse construction joints in nonreinforced pavements.
Doweled construction joints are shown in figure The keyed joint is the most economical method, from a construction standpoint, of providing load transfer in the joint. It has been demonstrated that the key or keyway can be satisfactorily constructed using either formed or slipformed methods.
Experience has proved that the required dimensions of the joint can best be maintained by forming or slipforming the keyway rather than the key. The dimensions and location of the key fig are critical to its performance. The structural adequacy of keyed ed construction joints in rigid floor slabs, however, can be impaired seriously by such factors as small changes in the dimensions of the key and positioning the key other than at the middepth of the slab.
Exceeding the design values for the key dimensions produces an oversize key, which can result in failure of either the top or bottom edge of the female side or the joint. Similarly, construction of an undersizes key can result in shearing off the key. Keyed joints should riot be used in floor slabs 8 inches or less in thickness except where tie bars are used. Tie bars in the keyed joint, will limit opening of the joint and provide some shear transfer that will improve the performance of the keyed joints.
However, tied joints in floor widths of more than 75 feet can result in excessive stresses and cracking in the concrete during contraction. When a longitudinal construction joint is used at the center of a floor two paving lanes wide, a keyed joint with tie bars should be used.
When a keyed longitudinal structure joint is used at the center of a floor four or more paving lanes in width, dowels should be used. Thickened-edge- type joints maybe used instead of other types of joints employing load-transfer devices. When the thickened-edge joint is constructed, the thickness of the concrete at the edge is increased to percent of the design thickness.
The thickness is then reduced by tapering from the free-edge thickness to the design thickness at a distance of 5 feet from the longitudinal edge. The thickened-edge butt joint is considered adequate for the load-induced concrete stresses. However, the inclusion of a key in the thickened-edge joint provides some degree of load transfer in the joint and helps maintain slab alignment; although not required, it is recommended for pavement constructed on low- to medium- strength foundations. The thickened-edge joint may be used at free edges of paved areas to accommodate future expansion of the facility or where wheel loadings may track the edge of the pavement.
All floor slabs accommodating vehicular traffic will be thicked at doorways to have an edge thickness of 1. The use of this type joint is contingent upon adequate base-course drainage. Isolation joints are provided to prevent load transfer and allow for differential settlement between the floor slab and other building components. Isolation joints also allow for some horizontal movement.
Isolation joints should be placed at locations where slabs abut walls or their foundations and around columns, column foundations, and other foundations that carry permanent dead load other than stored material. Isolation joints are provided by placing pounds asphalt, coal-tar saturated felt, or equivalent material between the floor slab and the building's structural components before the floor is placed. Such sheets should be placed or fastened to the buildings components to prevent any bonding or direct contact between the floor slab and the building component.
This requires that the sheets have a height equal to the floor slab thickness and be placed at the same elevation as the floor slab, as shown in figure Special joints and junctures. Situations will develop where special joints or variations of the more standard type joints will be needed to accommodate the movements that will occur and to provide a satisfactory operational surface. Some of these special joints or junctures are discussed below. At the juncture of two pavement facilities, expansion and contraction of the concrete may result in movements that occur in different directions.
Such movements may create detrimental stresses within the concrete unless provision is made to allow the movements to occur. At such junctures, a thickened-edge slip joint shall be used to permit the horizontal slippage to occur. The design of the thickened-edge slip joint will be similar to the thickened-edge construction joint fig A special thickened-edge joint design fig 5- 13 will be used at the juncture of new and existing floors for the following conditions: The special joint design may not be required if a new floor joins an existing floor that is grossly inadequate to carry the design load of the new floor, or if the existing floor is in poor structural condition.
If the existing floor can carry a load that is 75 percent or less of the new floor design load, special efforts to provide edge support for the existing floor may be omitted; however, if omitted, accelerated failures in the existing floor may be experienced. Any load-transfer devices in the existing floor should be used at the juncture to provide as much support as possible to the existing floor.
The new floor will simply be designed with a thickened edge at the juncture. Drilling and grouting dowels in the existing floor for edge support may be considered as an alternate to the special joint; how- ever, a thickened-edge design will be used for the new floor at the juncture. The primary function of dow- els in floor slabs is that of a load-transfer device. As such, the dowels affect a reduction in the critical edge stress directly proportional to the degree of load transfer achieved at the joint.
A secondary function of dowels is to maintain the vertical alignment of adjacent slabs, thereby preventing faulting at the joints. Dowels are required at all contraction joints in slabs that are 8 inches or greater in thickness and for thinner slabs in concentrated traffic areas.
Dowel diameter, length, and spacing should be in accordance with the criteria presented in table When dowels larger than 1 inch in diameter are required, an extra- strength pipe may be used as an alternate for solid bars. When an extra- strength pipe is used for dowels, however, the pipe should be filled with a stiff mixture of sand-asphalt or cement mortar, or the ends of the pipe should be plugged.
If the ends of the pipe are plugged, plugs should fit inside the pipe and be cut off flush with the end of the pipe so that there will be no protruding material to bond with the concrete and prevent free movement of the pavement. All dowels should be straight, smooth, and free from burrs at the ends. One-half of each dowel should be painted and oiled or otherwise treated to prevent bonding with the concrete. A schematic drawing of joint layout showing dowels is given in figure Normally, dowels should be located at the middepth of the floor slab.
However, a tolerance of one-half of the dowel diameter, above or below middepth of the slab, may be allowed in locating the dowels in contraction and construction joints where the allowance of such a tolerance will expedite construction. All joints will be sealed with a suitable sealant to prevent infiltration of surface water and solid substances. A jet-fuel resistant JFR sealant will be used in the joints of floors where diesel fuel or other lubricants may be spilled during the operation, parking, maintenance, and servicing of vehicles.
Sealants that are not fuel resistant will be used in joints of all other pavements. Preformed seal-ants must have an uncompressed width of not less than twice the width of the joint sealant reservoir. The selection of a pourable or preformed sealant should be based upon the economics involved. Compression-type preformed sealants are recommended when the joint spacings exceed 25 feet and are required when joint spacings exceed 50 feet. Special provisions for slipform paving. Provi- sions must be made for slipform payers when there is a change in longitudinal joint configuration.
The thickness may be varied without stopping the paving train, but the joint configuration cannot be varied without modifying the side forms, which will nor- mally require stopping the paver and installing a header. The following requirements shall apply: The dowel size and location in the transverse construction joint should be commensurate with the thickness of the pavement at the header. The size and location of the dowels or keys in the transition slabs should be the same as those in the pavement with the doweled or keyed joint, respectively.
Careful attention should be given to floor- slab geometry to ensure proper drainage and satisfactory operations. For proper drainage of the floor-slab surface into floor drains, a fall of C inch per foot toward the floor drain is recommended. For sustained operations, gasoline- and LP gas-operated forklift trucks can generally negotiate a maximum slope of 20 percent 20 feet vertically for every feet horizontally satisfactorily.
Electric -powered forklift trucks can perform sustained operations on a maximum slope of 10 percent 10 feet vertically for every feet horizontally. The above- mentioned maximum slopes are based on a coefficient of friction of 0. The use of sealants, waxes, etc. In areas where these compounds are used and a tough broom finish is not.
If the slope cannot be reduced, pressure-sensitive abrasive tapes should be installed.
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The abrasive tapes are of the type used on stairway treads to produce a nonskid surface. Basis of jointed fiber reinforced concrete floor slab design. The design of jointed fiber concrete JFC floor slabs is based upon limiting the ratio of the concrete flexural strength and the maximum tensile stress at the joint, with the load either parallel or normal to the edge of the slab, to a value found to give satisfactory performance in full-scale accelerated test tracks. To protect against these latter factors, a limiting vertical deflection criterion has been applied to the thickness developed from the tensile stress criteria.
Although several types of fiber have been studied for concrete reinforcement, most of the experience has been with steel fibers, and the design criteria presented herein are limited to steel fibrous concrete. Fibrous concrete is a relatively new mate- rial for pavement construction and lacks a long-time performance history. The major uses to date have been for thin resurfacing or strengthening overlays where grade problems restrict the thickness of overlay that can be used.
The use of JFC floor slabs should be based upon the economics involved. The design mix proportioning of fibrous concrete will be deter- mined by a laboratory study.
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The following are offered as guides and to establish limits where necessary for the use of the design criteria included herein. To accomplish this, cement contents of to pounds per cubic yard of concrete are recommended. The cement content may be all Portland cement or a combination of portland cement and up to 25 percent fly ash or other pozzolans. The percent of coarse aggregate of the total aggregate content can vary between 25 and 60 percent. The required thick- ness of FJC floor slabs will be a function of the design concrete flexural strength R, modulus of soil reaction k, the thickness h b and flexural modulus of elasticity E fs , of stabilized material if used, the vehicle or axle gross load, the volume of traffic, the type of traffic area, and the allowable vertical deflection.
When stabilized material is not used, the required thickness h dl of JFC is determined directly from the appropriate chart figs and The resulting thickness, h or h dof , will be rounded up to the nearest half or full inch. The rounded thickness, h df or h dof will then be checked for allowable deflection in accordance with paragraph e. The minimum thickness for JFC floor slabs will be 4 inches. Design curves for fiber-reinforced concrete floor slabs by design index. S nvunx3ij Figure Design curves for fiber-reinforced concrete floor slab for heavy forklifts.
Allowable deflection for JFC pavement. The elastic deflection that JFC floor slabs experience must be limited to prevent overstressing of the foun- dation material and thus premature failure of the pavement. Curves are provided fig for the computation of the vertical elastic deflection that a slab will experience when loaded. Use of the curves requires three different inputs: The slab thickness is that which is determined from paragraph d above.
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The computed vertical elastic deflection is then compared with appropriate allowable deflections determined from figure Deflections need not be checked for axle loads less than 25 kips. If the computed deflection is less than the allowable deflection, the thickness meets allow- able deflection criteria and is acceptable. If the computed deflection is larger than the allowable de- flection, the thickness must be increased or a new design initiated with a different value for either R or k.
The process must be repeated until a thickness based upon the limiting stress criterion will also have a computed deflection equal to or less than the allowable value. Deflection curves for fiber-reinforced concrete floor slabs. Allowable deflection for jointed fiber-reinforced concrete floor slabs. Pavement Design for Seasonal Frost Conditions.
Foundations in Expansive Soil. Soil Stabilization for Pavements. Standard Practice for Concrete Pavements. Flexible Pavement Design for Airfields. Arctic and Subarctic Construction: Concrete slab thickness for interior loads. The floor slab for a warehouse will be designed based on the following information: Equivalent Forklift Number Maximum Truck Axle of Average Operations Design Load, kips Axles Daily Volume Per Day Index 5 10 15 4 1 1 50 50 15 50 15 4 4 7 Matching the axle loads and maximum operations per day in table , the design index for each axle-load group is selected as shown in the far right column in the above-mentioned table.
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