a DETAILED RECORD SHOULD BE KEPT OF THE OUTSIDE TEMPERATURE and that of the concrete as placed in the forms together with TEMPERATURES TAKEN BOTH ABOVE AND BELOW THE GREEN CONCRETE-SLABS, and at the base of exterior columns on the more exposed side of the building. These temperature-readings should be taken every few hours BOTH DAY AND NIGHT and additional protection, or more heat, installed in case the temperature falls below 50° F. at the most exposed locations. Salamanders should be FED FREQUENTLY AND THE FUEL APPLIED IN SMALL QUANTITIES to minimize the smoke and provide a UNIFORM TEMPERATURE. Where this means of supplying heat is used there is always danger of the form-work catching fire and WATER-BARRELS with fire-buckets should be kept filled upon each floor during the period that the fires are kept burning. Duration of Protection. In beam-and-girder construction the TOP CANVASES Courtesy of the Turner Construction Company FIG. 36. Canvas Covers Protecting Top of Floor Slab should always remain in place for AT LEAST TWO DAYS (48 hours) and for girderless floors this minimum should be increased to THREE DAYS (72 hours). The SIDE CANVASES are ordinarily left in place for a period of from FOUR DAYS (96 HOURS) TO SIX DAYS (144 HOURS), depending upon the temperature and local conditions. If the wall-columns are stripped during the period of artificial heating, thereby necessitating the loosening of the side-curtains, the latter should be quickly replaced so as not to chill the interior of the building. It is also customary to permit the partial removal of top covers for short periods during the middle of the day when the temperature upon the floor is above 35° F. in order to permit the setting of the column-forms for the next story. Artificial heat should be continued, ordinarily, as long as the side curtains are retained. Stripping. IT IS IMPRACTICABLE TO MAKE ANY DEFINITE RECOMMENDATIONS AS TO THE MINIMUM TIME WHICH THE FORMS SHOULD REMAIN IN PLACE BENEATH CONCRETE CAST IN WINTER WEATHER. If the protection could be such as to exactly duplicate the conditions encountered in summer, the periods given in the Chapter on Forms would apply equally to winter operations, but unfortunately such a result is seldom obtained owing to the unwarranted expense. The TIME OF STRIPPING TICULAR CONDITIONS OF and a careful study of ORDS taken every few hours as previously described, supplemented by THOROUGH INSPECTION. As a general practice the permanent shores should remain in place until the concrete has obtained about three-quarters of its designed strength, but under even these conditions an adequate system of RESHORING must be employed to avoid placing excessive construction loads upon green concrete. The greatest danger lies in MISTAKING FROZEN CONCRETE FOR PROPERLY HARDENED CONCRETE. A test may be made by the use of a steam-jet or a gasoline blow-torch, or by immersing a piece for several minutes in very hot water. If the concrete softens it is an indication that it is still in a "green" condition and probably incapable of carrying any appreciable load. 6. PHYSICAL PROPERTIES OF CONCRETE 19. Abrasive Resistance. Tests to determine this characteristic of concrete, which is taken as an indication of its probable WEARING QUALITIES, are made in the standard Talbot-Jones Rattler as illustrated in Fig. 37. From Fig. 38 it is apparent that, within the range covered by the tests, the RESISTANCE OFFERED TO ABRASION 7000 INCREASES WITH THE Fig. 39 shows the relative effect of different quantities of mixing water upon the WEARING QUALITY OF THE CONCRETE. Fig. 40 illustrates a similar relation between the curing conditions and the wear. Wear-inches Compressive Strength in lb per sq in Mix 1:4 6000 Age at Test,4 mo. 5000 4000 8 3000 2000 1000 0 FIG. 38. Relation between the Strength and Wear of Concrete 2.0 1.8 Mix 1:4,by Volume Institute It should be remembered that a relative consistency of about 1.10 corresponds to the consistency ordinarily suitable for reinforced-concrete work. From these data it is apparent that concrete floor-surfaces, or similar work subject to traffic, or other sources of abrasive wear, should be laid AS DRY AS PRACTICABLE AND KEPT MOIST Damp Sand Storage Crushed Limestone. 1.10 1.40 for a period of at least ten days and preferably 1.50 for two or three weeks. Too much water is a frequent cause of crazing and premature drying invariably results in FIG. 39. Effect of Quantity of Mixing Water on the Wear of Concrete Bulletin No. 2. Structural Materials Research Laboratory, Lewis Institute a soft surface subject to dusting. Abrasive resistance is also favored by choosing aggregates which are hard themselves and avoiding too lean a 2.0 mixture. 1.8 20. Absorption. This property is like FIG. 40. Effect of Curing Conditions on the Wear of Concrete Structural Materials Research Laboratory, Lewis Bulletin No. 2. Institute wise influenced by the amount of mixing water and the condition under which the concrete is cured, as illustrated by Figs. 41 and 42. Both sets of curves are based upon the same data. ABSORPTION TESTS were made upon 8 X 8 X 5-in blocks by immersion in water at room temperature. The age of the blocks was one year. In Fig. 41 each value is the average of eight blocks from four different curing conditions. In Fig. 42 each value is the aver A POROUS CONCRETE may, or may not, be a PERMEABLE CONCRETE, depending upon the structure of the mass. Relative permeability is determined by the rate at which a stream of water, under fixed head, passes through a certain thickness of concrete. 13 12 11 10 8 Absorption Per Cent by Volume 9 This is an important 4 3 2 1 0 bles, such as quantity of FIG. 42. Effect of Curing Conditions on the Absorption of water, grading of aggre gates, etc., conforming with the normal require ments of good practice. In this connection it should be remembered that a high-grade concrete is practically impermeable to moderate heads of water. In fact, a 6-in wall composed of a well-graded aggregate, proportioned to develop a compressive strength of 2 500 lb per sq in at 28 days, should resist a head of 50 ft. 7. EFFECT OF OILS AND MISCELLANEOUS LIQUIDS ON CONCRETE 22. Protective Treatment. In industrial work it is often necessary to construct tanks to contain various chemicals some of which have a deleterious effect upon the concrete. Appendix 17 of the Joint Committee Report, 1924, gives a list of the recommended surface-treatments to resist the actions of the different compounds more often encountered. 8. ALKALI SOILS AND SEA-WATER 23. Construction in Alkali Soils. In certain arid districts of the west, particularly where recent irrigation has been introduced, GROUND-WATER may be alkaline in nature. This condition is usually due to the presence of soluble |