How to avoid problems with lightweight mixes Don’t despair . . . lightweight concrete is pumped successfully by those who know the rules I t could easily be inferred from an account like that in the article just preceding that pumped lightweight concrete is inferior or that successful pumping of lightweight concrete cannot be assured ahead of time. This is not so; the principles of successful pumping are well established and widely practiced. There appear to have been a large number of possible inadequacies in the pumping operation described, including incompletely saturated aggregate, poor aggregate gradation, high air content, too small a line and an inadequate pump. Unfortunately, these and other possibilities cannot all be positively identified with respect to that particular job. When the troubles began to occur the measures taken to alleviate them were not effective in delivering concrete of the design strength because they did not get to the root of the problem. It seems likely that the only real assurance of success in such an operation would have called for attention to a whole group of well established factors which this and the final article in this series will outline. Pumping improves quality When mixes are properly designed pumping does change the properties of concrete, and for the better, as shown by tests reported by the Virginia Highway Research Council.* These show that pump transit alone produced strength increases in normal weight concrete as high as 14 percent as measured by careful tests on the same concrete before and after pumping hor- izontally through 600 feet (180 metres) of line. These high increases were obtained even though the mixes contained the arbitrary extra 10 percent of coarse aggregate permitted in ACI 211.1-70 for paving mixes. More workable mixes usually show smaller gains in strength but gains are the rule. Furthermore, on large jobs where the coefficient of variation has been calculated it has been found that the variability between batches has been decreased by pumping. These gains are the logical results of the remixing, consolidation and homogenizing effects involved. Mix design for pumping Pumpable concrete starts with the aggregate. It is the combined gradation of the aggregates and their residual moisture absorption that are most important. When an experienced engineer designs a mix for pumping he will take into consideration the surface texture of the aggregate, its void content, water content, water absorption and gradation. For a lightweight pumping mix he will usually increase the quantity of fine aggregate, the cement and water contents and will incorporate admixtures. A pumpable lightweight mix may differ from one that is not pumpable in having a higher plastic unit weight, especially if more natural sand is used. It may have a higher slump as it enters the pump and its dry unit weight at 56 days is greater by about two to five pounds per cubic foot (32 to 80 kilograms per cubic metre). Gradation of fine aggregates A mix with natural sand graded within the limits of ASTM C 33 or lightweight sand within the limits of ASTM C 330 will not necessarily be pumpable. It is most desirable for it to have a fineness modulus between 2.2 and 2.7 and have a grading preferably between the middle and upper limits shown in Figure 1. Unfortunately, few aggregates fall entirely within the range, so the supplier must rely on experience and good judgment. Even more important than fineness modulus is that 15 to 30 percent of the fine aggregate should pass the Number 50 screen and five to 10 percent should pass the Number 100 screen. Size of coarse aggregate The maximum size of coarse aggregate is generally controlled by job specifications and the pipe diameter must be large enough to handle it. Larger percentages of sand must be used when coarse aggregate size is reduced as shown in Figures 2 to 4. These graphs show the ranges of grading needed for pumpability when using combined aggregate of various maximum sizes. The grading curve should be as smooth as possible. Volume of coarse aggregate The next consideration is the volume of coarse aggregate that can be pumped. A footnote to Table 5.3.6 of ACI 211.1-74, “Recommended Practice for Selecting Proportions for Normal and Heavyweight Concrete,” allows reduction of the Figure 1. Recommended grading of fine aggregate for concrete pumping mixes Figure 2. Recommended grading of coarse aggregate for pumping mixes when maximum size is 3/8 inch coarse aggregate content (of hard rock concrete) by as much as 10 percent to provide more workable concrete, when required, for pumping. Nevertheless, as noted later, if the gradation is poor it should first be improved. How much reduction should be made can be determined only by experience with the aggregates and with the particular pump being used. Some pumps can accept more coarse aggregate than others without blocking because of differences in design of the machine, the valving system and the amount of reduction in the pipe. A mix that may be capable of being pumped through straight pipe may be impossible to move through a reducer. Designing the mix Figure 3. Recommended grading of coarse aggregate for pumping mixes when maximum size is 3/4 inch Figure 4. Recommended grading of coarse aggregate for pumping mixes when maximum size is one inch ACI 211.2, “Recommended Practice for Selecting Proportions for Structural Lightweight Concrete,” is not a recipe, but a “method for selecting and adjusting mix proportions” [italics ours]. Depending on the competence and available time of the mix designer and inspector, the trial mix of 211.2 can be improved in strength-related properties and economy by making a series of adjustments invoking all available experience with the particular materials. Adjustments that improve pumpability may also improve strength and economy. ACI 211.2 relates primarily to solid volumes and makes no provision for the myriad other qualities of the mix that affect pumping results materially, such as variations in gradation, particle shape, surface texture, porosity, surface area and number of voids, particularly in the fine aggregates. The needed mix improvements, described at some length in the ACI 304 report titled “Placing Concrete by Pumping Methods” go beyond the brief basics of ACI 211.2 How not to modify the mix The ACI 304 report states that the addition of water does very little if anything to improve pumpability and may cause segregation. It is far better to find and correct deficiencies in the ingredients than to succumb to the costly and useless addition of water. Addition of sand is not likely to help the mix if the sand is poorly graded, too coarse, too fine or of poor shape, such as 100-percentcrusher product. It is far better to improve the sand by blending or classifying or by making additions to fill the gaps in grading. The less sand used the less the area that must be coated with paste to improve workability and pumpability. Prewet the aggregate To make lightweight aggregate concrete pumpable it is essential to prewet, presoak or presaturate the lightweight particles so that they will not take water out of the mix during pumping and make it less pumpable. The wide variation in porosity of structural lightweight aggregates can cause a range of 12 to 50 percent in their ability to absorb water. A comparison of typical values of water absorption for two particular aggregates obtained by various wetting methods is given in Table I. The differences may more than double the absorption from one method to another. This extremely important fact demonstrates that an aggregate may seem to have taken up a maximum of water by some kinds of treatment yet be capable of absorbing more. It was undoubtedly incomplete saturation that caused the 11⁄2-inch (3.8-centimetre) decrease in slump between pump and deck early in the job cited by Mr. Hersey. This decrease indicates that the aggregate was probably absorbing about 11⁄2 gallons of water per cubic yard (7.42 kilograms per cubic metre) of mix. Mr. Hersey also reported that the concrete later lost as much as four inches slump between the pump and the end of the pipeline, even though the lightweight aggregate pile was kept wet. The presence of surface moisture on lightweight aggregate does not assure that the in- TABLE I Water absorptions of two commercial lightweight aggregates under various conditions. (Percentages are percent by weight of aggregate.) Method Absorption, percent Aggregate Aggregate A B 24-hour absorption (ASTM C 127) 6-8 – Spray absorption for five days 12-16 – Soaking, inundating or premixing – 24 Hydrothermal saturation at 350°F (177°C) 16-20 – Vacuum saturation 20-30 49 terior is completely wetted. Nor does surface moisture have anything to do with the amount of water which can be absorbed under the pressure of pumping. On the job described either the designer of the mix or the supplier of the concrete appears to have been in error in not requiring an internally saturated aggregate. Common saturation procedures Experience shows that from two to three days of sprinkling may be required when presoaking lightweight aggregate for pumping. During this time sprinkling should be discontinued whenever free water appears at the base of the material pile but should be resumed and repeated periodically as long as the aggregate will take on water. The total water that the presoaked aggregate will finally contain should be at least as much or more than the 24hour absorption obtained by the standard ASTM test methods C 127 or C 128. Although a few lightweight aggregates can be readily pumped after thorough sprinkling or soaking, and a few without any special treatment at all, these procedures for saturat- ing lightweight aggregate are usually only marginally successful. Treat aggregate at source Pretreatment by the aggregate producer, using hydrothermal or vacuum methods at the manufacturing plant almost guarantees freedom from the trouble caused by water being absorbed from the concrete mix. The pumping of lightweight aggregate concrete should not be attempted, especially if line pressures may exceed 150 psi (10.5 kilograms force per square centimetre), unless either the aggregates are pretreated or alternative procedures can be proven to be effective on the specific job. For many but not all lightweight aggregates such plant treatment can be successful only if done when the material is thoroughly dry. Attempts to treat such aggregates after storage, shipment, rain and air drying are destined in most cases to produce erratic results. The vacuum saturation method should be used only on “bone-dry” coarse aggregate. The process is complete in 30 to 45 minutes. Vacuum-saturated lightweight aggregate concrete responds to pumping much the same as hard rock concrete. Hydrothermal saturation, which is also highly effective, has the advantage of saturating the lightweight fines. Concretes made with such treated aggregates pump superbly with no loss of slump, with routinely predictable ease and with strengths actually higher than those with nonpumped concrete. At the same time, a net savings in pumping costs is experienced. In several parts of the United States, both north and south, lightweight aggregate suppliers now offer aggregates treated by one of these two methods. The economics of pretreatment are such that it is practical even for a medium-sized project such as an apartment house. For small volumes a portable vacuum processing rig has been successfully used. Saturated aggregate makes heavier plastic concrete The more complete the saturation, the heavier the aggregate and the heavier the unit weight of the fresh plastic concrete. This internal water will be available for curing but will slowly evaporate until the concrete is air dry. Use of the heaviest unit weight allowed by the specifications will permit the concrete to be designed with the least amount of lightweight aggregate and thus will improve its pumpability. Cement content, pumping aids and air entraining agents When pumping nonprocessed lightweight aggregate concrete consideration should be given to the use of additional cement in the mix due to the higher sand content or higher slumps that may be used. No less than 517 pounds of cement per cubic yard (307 kilograms per cubic metre) should ever be used. When a height above five floors is reached the cement content should be at least 564 pounds per cubic yard, and if the concrete is to be pumped higher than 10 stories another 25 to 50 pounds of cement per cubic yard (15 to 30 kilograms per cubic metre) should be added to limit the slump loss in the line to two inches (five centimeters). The apparent watercement ratio is maintained constant. Use of a pumping aid should also be considered when pumping nonprocessed lightweight aggregate. This can be fly ash or natural pozzolan, if recommended by the con- PUBLICATION #C750234 Copyright © 1975, The Aberdeen Group All rights reserved crete supplier, or it can be an admixture. Pumping aids are not intended to compensate for excessive aggregate porosity or water absorption, however. Use of an air entraining agent sufficient to obtain four to seven percent air also helps. Too much air in the mix may give problems similar to the air within the aggregates; pressure rebounding effects have been noted. Slump Slump affects the frictional resistance of the pumping mix. Mixes can be pumped most successfully at slumps between three and seven inches. Low slumps cause high pumping pressures and require a pump that is capable of putting on adequate pressure to move the concrete. High slumps may cause segregation and blockage. *Newlon, Howard, Jr., and Ozol, Michael, A., “Delayed Expansion of C o n c rete Delivered by Pumping Through Aluminum Pipeline,” Conc rete Case Study Number 20, Vi rginia Highway Research Council, Charlottesville, Vi rginia, October 1969, 3 pages.
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