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l INTRODUCTION 1.0 STATEMENT OF THE PROBLEM These Criteria for Selection and Design of Residential Slabs-on - Ground were developed by the Building Research Advisory Board (BRAB) under contract between the Federal Housing Administration and the National Academy of Sciences. The original FHA request for advisory services under this con- tract dates from 27 February 1957 and called for the following: A determination of criteria for proper design and construction of heated and unheated slabs-on-ground to ensure structural sound- ness, including a. Maximum dimension in which any unreinforced slab-on- ground may be constructed to safely avoid harmful cracking of the slab due to shrinkage and temperature change, and amounts of re- inforcement necessary for slabs of greater length b. Standards of construction for heated and unheated slabs-on- ground that will ensure structural soundness in areas of expansive soils c. A definitive program of research or investigation, that may be expected to produce remaining needed knowledge. All aspects of the charge were accepted; however, the Board, as 1
2 RESIDENTIAL SLABS ON GROUND well as its several committees which have provided the requested advice, have long agreed that the term "structural soundness" has a commonly accepted meaning which, if used in this context, would imply that all residential slabs perform a purely structural func- tion and that only their performance in this respect was of concern. Such is not the case, and for this reason the Board and its several Committees have chosen to address themselves to attainment of "satisfactory performance" of the floor/foundation system as a whole, rather than "structural soundness" per se. This report, therefore, has been developed on the basis of this interpretation of the overall function of slabs-on-ground. 2 .0 REPORT ORGANIZATION This report consists of three sections. The first two, this Intro- duction and the Conclusions and Recommendations of the Special Advisory Committee, constitute a complete report in which the problem is defined and the needed answers are supplied. The third section, comprising three parts, contains supporting information, i.e., the reasoning behind the recommendations of the Committee, together with pertinent data, analyses of the vari- ous factors affecting slab design, necessary deriviations of analyt- ical approaches, and guides to use of the latter. Specifically: In Part A, the processes of slab selection and design are discussed; in Part B. quality control in construction is covered; and in Part C, soil behavior, data on soil classification, and standards are explained. 3.0 BACKGROUND 3.1 History Concrete slabs-on-ground have been used as a primary floor sur- face for many years in residential construction. During and since the large home-building expansion of World War II and subsequent years, many slab and slab foundation types have emerged, largely
I NTRODUCTION 3 as a result of invention rather than design. Many of these slab types continue to be known and applied only within rather limited geographic areas, while others have gained wide, if not nationwide, acceptance. There is very little documentation either of the evolve- ment of the many slab types used and in use or of the difficulties encountered; however, it is known that, with many, difficulties have occurred. Some troubles doubtless could be attributed to basic flaws in concept or construction, while others resulted from trans- planting an otherwise satisfactory slab type to a different environ- ment, to long-term cyclical changes in environment, or to use with a residential structure of different size, shape, or type of super- structure than that used when good experience was recorded. After long study of both developments and problems, it would seem simply that, as builders were rewarded with success of a given slab type, that type frequently spread in use-sometimes with continued success and sometimes with unfortunate results. Indeed, in the early days of BRAB's study of slab problems, success seemed to some to warrant less stringent requirements even for entirely untried applications, and problems seemed to others to warrant more stringent requirements even for applications where experi- ence had been satisfactory. Residential construction was and largely continues to be an in- exact science, and this applies no less to slab foundation systems. The result frequently has been a failure to recognize, or an inabil- ity to cope with, the often vast differences in soil, climate, con- struction, and other variables as one moves from building site to building site in different geographic areas of the nation and even within the same limited geographic area. There obviously has been all too little effort to document experience and to codify either slab types, or the selection, specification and/or design procedures used. The first BRAB study of slabs-on-ground which treated struc- turally related problems dates to 1955, and the series of reports leading to this document, from September 1957. The basis for this latter series is presented under the problem statement above. An Interim Report was published in March 1959 to allow time for com- ment and further research and field study, and a final report was published in September 1962. This first revision of the 1962 final report incorporates further information developed through field study particularly in expansive soil areas, information developed by FHA and presented to the Com- mittee, and new procedures for treating slabs on compressible soils.
4 RESIDENTIAL SLABS ON GROUND 3.2 Need for This Study The long and extensive efforts discussed above made it abundantly apparent that economy and serviceability can be designed into a slab in an acceptable manner only by considering the total effect of soil, slab, and superstructure in a single complex relationship. This means there likely will always be the need to refine proce- dures for determining the variables, relating the influence of each to the others, and providing a method for Me final selection or design of the slab-particularly if there is to be continued latitude for the introduction and analysis of innovations under stable as well as unstable soil conditions. 3.3 Study Scope This report deals only with slab-on-ground construction for light residential structures. It provides recommendations which, it is believed, will assure sound residential slabs by defining the conditions and appropriate limitations of use in accordance with the variables encountered. Included are the dimensions to which a heated or unheated slab may be constructed without reinforce- ment and yet avoid harmful cracking due to shrinkage and tempera- ture change; the amounts of reinforcement required for slabs of greater length; the structural requirements of slabs which are to be placed on expansive or compressible soil; and, lastly, the re- search needed to fill remaining known gaps in knowledge concern- ing influences upon and means of achieving satisfactory residential- slab performance at the most reasonable price possible. 3.4 Study Limitations This report, and the recommendations contained herein, have been limited to providing assurance of satisfactory performance of slabs- on-ground to be used for the habitable portions of houses, and not for drives, garages or carports, basements, or other appurtenances. When other matters are treated in this report, consideration is limited strictly to their influence on me overall design of the house slab. Thus, such closely allied subjects as thermal insulation,
INTRODUCTION 5 vapor barriers, termite and decay protection, etc., while recognized as important to slab performance, are commented on only as they influence the design of me slab ~ se. 4.0 USE OF THIS REPORT This report provides, in a systematic manner, recommended crite- ria necessary for the selection of the appropriate slab type for each type of foundation soil and geographic location. It also provides recommended design criteria and construction requirements for each type of slab as it relates to the type of superstructure con- struction involved. Analysis has shown mat four basic slab types will serve most functional needs, as well as needs imposed by the different soil and climatic conditions likely to be encountered. These are desig- nated as: Type I: Unreinforced Type II: Lightly reinforced against shrinkage and temperature cracking Type III: Reinforced and stiffened Type IV: Structural (not directly supported on the ground). 1Building Research Advisory Board, National Academy of Sciences- Nationn1 Research Council publications concerning slab-on-ground con struction include: No. 1077 Design Criteria for Residential Slabs-On-Ground Final ~ , Report. 1962. No. 838 Ducts Encased in and under Concrete Slabs-On-Ground. 1961. No. 707 Protection from Moisture for Slab-On-Ground Construction . and Habitable Spaces below Grade. 1959. No. 657 Interim Renort-Design Criteria for Residential Slabs-On- Ground. 1959. No. 596 Effectiveness of Concrete Admixtures in Controlling Trans - mission of Moisture Through Slabs-On-Ground. 1958. _ . No. 445 Vapor-Barrier Materials for Use with Slab-On-Ground Con- struction and as Ground Cover in Crawl Spaces. 1960. . No. 448-A Protection Against DecaY and Termites in Residential Con . struction and Addendum. 1958. 9 No. 385 Slab-On-Ground Construction for Residences. 1955. .
6 RESIDENTLA L SLABS ON GROUND Once the site characteristics have been ascertained and type of superstructure construction (frame, masonry, etc.) of a residence has been set, slab type selection and/or design for Herpes I, II, and III proceeds, in terms of soil properties and climate conditions, as follows: a. In the great majority of cases, a slab of Type I or II will be found satisfactory. Such slabs are the least expensive and are, generally speaking, simple to construct. Performance depends more on quality control than on design; hence, a set of standards and specifications is recommended for such slabs. Here the effort has been to select slab types developed in practice and used with success, and principally needing codification and limitation on use. b. In instances where Type III slabs become necessary because of poor soil conditions or a climate which produces alternating pe- riods of drought and excessive soil moisture, the successful per- formance of the slab depends equally on design and quality control. For Type III slabs, design criteria and guides to their use are recommended. In terms of the type of superstructure construction (frame, masonry, etc.), a maximum allowable differential slab movement is accepted. Then a procedure is followed for determina- tion, in terms of the severity of soil and climatic conditions, of the area over which the slab can reasonably be expected to have effec- tive support. In areas of more severe climate, a stiffer and stron- ger slab will be required, i.e., one which will not exceed the maxi- mum allowable differential settlement even if supported over only a reduced portion of its area. Following the determination of allowable deflection (differential movement or settlement) and support conditions, the dimensioning (for stiffness) and reinforcing (for strength) of the slab are deter- mined through an empirical design procedure which is derived and presented in detail. Only one slab configuration is used-the waffle slab with which there has been most experience and which, therefore, is most in need of attention. c. For those rare cases where Type IV slabs become necessary, it is recommended that they be designed in accordance with the most recent ACI code. Such slabs are not supported on the soil but on piles, piers, footings, or similar supports, and are, therefore, normal structural slabs. Since design would conform to the struc- tural-design procedures covered by the ACI code, there is no rea- son for a design presentation here.
INTRODUCTION 7 Thus the designer, with the aid of this report-given the soil properties, climatic conditions, and superstructure construction- can determine the type of slab appropriate for use, execute a rea- sonable design where necessary, and provide needed materials and construction specifications. Finally, nothing contained in this report should be construed to militate against the introduction or use of either existing or newly devised slab types or procedures. However, such variances will need to be evaluated in light of the recommendations presented herein (see also pp. 10 and 22~.
