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111 RESEARCH RECOMMENDATIONS 1.0 GENERAL Obtaining basic information necessary to assure economical and sound slab-on-ground construction entails: acquiring fundamental knowledge; conducting field investigations; performing laboratory investigations directed toward correlating fundamental knowledge and field data; developing practical methods of applying the know- ledge gained and discoveries made. In the field of residential construction, potential difficulties, which develop from irregular distribution of loads and unstable soil conditions, are often ignored because of the relatively light loads involved. While much work has been done in the field of residential construction, it has hitherto been primarily concerned with solving current problems rather than developing basic data. The recommendations contained and discussed herein represent an effort to direct attention to a research program designed to produce needed data. 2.0 SOILS RESEARCH 2.1 Investigation of Granular Mats or Stabilized Soil Mats A study should be conducted of the effectiveness of granular mats or soil-cement, soil-lime, or other chemically stabilized soil mats, 23
2 4 RESIDENTIAL SLABS ON GROUND in minimizing differential settlements and maintaining a more con- stant moisture content in soil under residential slabs-on-ground. Mats of various thicknesses and gradations should be investigated through full-scale studies in the field; these should extend over pert oafs of time long enough for test houses to undergo several cycles of seasonal wetting and drying, in order properly to evaluate re- sults, and to demonstrate the nature and extent of the inherent ad- vantages of various types of mat. 2.2 Soil/Moisture Relationships A basic laboratory investigation should be undertaken to determine the physicochemical relationship between soil and water. Only when this relationship is understood can problems in such areas as vol- ume change, swelling pressures, thermal osmosis, and stabiliza- tion be studied with reasonable assurance of success. Such an investigation would undoubtedly be a long-term project. Support of existing research in this area is preferable to initiation of new studies. 2.3 Swelling Properties of Clay Soils The nature of volume change of clay soils needs to be better under- stood. Specifically needed are means of predicting the amount and rate of such change as they affect both slab design and performance. The PVC meter has been of great help, yet further laboratory and field investigations are needed, to evolve even better and more economical methods of determining whether a particular soil will swell, the amount of swell that can be expected, and whether, under specific climatic conditions, detrimental movement is likely to re- sult. It would provide information relevant to the C-factors shown in Fig. 6, p. 53 of this report, and would permit a realistic adjust- ment of these factors. Data obtained from the research recom- mended under 2.2 above would greatly assist investigators in this area. 2.4 Stabilization of Soils for Residential Construction A panel of experts in soil stabilization should periodically review all investigations under way for stabilizing soils for highway and
RESEARCH RECOMMENDATIONS 2 5 airport construction, in order to determine which may be appli- cable to residential construction. The panel should maintain a rec- ommended research program outline incorporating any laboratory and field investigations offering promise of success with residen- tial slabs- on- ground. 2.5 Identification of Problem Soil Areas A field investigation should be made to locate and map swelling soils in areas of current and likely residential construction. Such mapping should include plasticity indices and the effect on these soils of local long-range climatic variations. 3.0 SLAB RESEARCH Closely associated with the matter of volume change in soil is the ability of a slab to accommodate these changes. Present hypotheses are less than entirely satisfactory. While much is known about the properties of concrete and reinforcement, the available information is primarily related to heavier construction and loadings; little factual knowledge is available concerning the performance of thin, lightly loaded concrete slabs. 3.1 Field Investigation Field investigation of both cracked and unc recked slabs should continue. Careful observation of actual performance should be made in relation to site conditions; this could be expected to pro- vide valuable data with which to refine slab design procedures. The amount of deflection in slabs should be noted, along with the apparent effect on other portions of the house, in order to establish limits which will result in minimizing damage. 3.2 Cracking of Slabs A study should be made of the number and width of cracks which may occur in fabric-reinforced and unreinforced 4-inch slabs of
2 6 RESIDENTIAL SLABS ON GROUND various lengths from 20 to 75 feet and concrete of various qualities, when the slab is placed on soils offering varying degrees of resist- ance to slab shrinkage. In each instance, a control slab should be used which is subjected to as near zero resistance as can be ob- tained. It is suggested that the Portland Cement Association be requested to undertake this study as a service to the public and the industry. 3.3 Effect of Partition Loads on Unreinforced Slabs An investigation should be made of the effect of partition loads on the expansion and contraction of unreinforced slabs which are placed on various soils and exposed to anticipated temperature ranges. 3.4 Maximum Deformation of Unreinforced Slabs Research should be conducted to determine the amount of deflection that unreinforced slabs can accommodate without serious cracking. This investigation, starting with uniform contact provided over the entire slab area, should entail inducing differential settlements slowly and in increments until cracks appear-the total investigation procedure and results being related to the creep characteristics of concrete. It is suggested that the Portland Cement Association might wish to undertake this investigation in conjunction with Re- search Recommendation 3.2 above. 3.5 Effective Stiffness of Stiffened Slabs Research is needed to determine the effective stiffness of rein- forced rib-stiffened slabs for ribs of various sizes and spacings, and for different steel ratios. Such research should produce data of use in refining the basic assumptions made in this report for stiffened slabs. 3.6 Effect of Slab Stiffness on Superstructure Studies should be made which relate the effect of various degrees of flexural and torsional stiffness on the superstructure. Controlled
RESEARCH RECOMMENDATIONS 2 7 test models should be carefully instrumented and subjected to vari- ous degrees and distributions of reactive forces from below, and the measured stresses and deflections should be compared with the theoretically based design calculations. 3 .7 Controlled Design and Construction A long-range program of controlled construction and periodic ob- servation of slabs under actual use conditions should be under- tal~en. Slabs should be progressively designed toward failure. Careful control and recording of critical data will be necessary so that a rational analysis can be made. It is envisioned that a pro- gram of this kind will require the cooperation of builders and home- owners, with provision for relieving the homeowners of any result- ing financial loss. 3.8 Investigation of Other Stiffened-Slab Designs Various stiffened-slab cross sections should be investigated. Such designs as Hollow Box, Inverted Pyramid, and X-Keel deserve con- sideration for application on the more unstable soils. It appears possible that these slab types could be substituted for structural slabs, if economic advantages can be demonstrated and design pro- cedures established. On the basis of preliminary investigations which have already been conducted, it can be anticipated that sub- stantial expenditures of time and money will be required for such work. 3.9 Investigation of Prestressed and Precast Slabs Prestressed concrete, whether pretensioned or posttensioned, and precast concrete slabs-on-ground offer many potential advantages for use in residential construction. The value of these potential advantages cannot be assessed until many questions relating to placement techniques and procedures, quality control, exposure to the elements and time can be fully investigated. This is particularly true in regard to posttensioned concrete and precast concrete. Thorough investigation should be encouraged and pursued as a nec- essary prerequisite to any large-scale adoption of such slabs and before efforts to devise standard design criteria and procedures
2 8 RESIDENTIAL SLABS ON GROUND are initiated. Some field use of posttensioned slabs is currently in progress; this work should be followed closely and experience data obtained. Use of lightweight aggregates and nylon tendons for prestressed concrete should also be given consideration in any research pro- gram relating to use of prestressed concrete for residential slabs- on- ground. 4.0 PRIOR:[TY Because the objective is to develop a rigorous design procedure for residential slabs-on-ground, it is worth pointing out that the recommendation under 3.1, pp. 25-26, Field Investigation (of actual performance), is most apt to produce immediately useful results at least cost. Therefore, priorities should be set up with this con- sideration in mind. Some work has already been done as a result of this recommendation appearing in earlier editions of this report- more is needed. 1
