|Free Books / Science & Engineering / Cement And Concrete /|
Special Uses Of Concrete (Continued) Bridges, Dams, Locks, And Breakwaters. Art. 81. Bridge Piers And Abutments And Retaining Walls. Continued
1 Trans. A. S. C. E., December, 1893.
Concrete is used very largely for constructing retaining walls and bridge abutments. The foundations of a retaining wall should be of ample width, and if the wall is not founded on rock, some settlement and outward movement may be expected if the common formulas are used in computing the dimensions.
If this movement is not equal throughout the wall, cracking is likely to take place, and to confine these cracks to predetermined vertical planes, it is well to construct the wall, if a long one, with vertical joints at intervals of fifteen to thirty feet. Such a joint is made by building one section and following with another, without special precautions to make a bond between.
If there is an opportunity for water to accumulate, care should be taken to drain the earth back of the wall, either by drains leading around the ends, or by pipes passing through the wall. The latter may result in discoloration of the face.
The face of a retaining wall or abutment is preferably given a batter, and a coping is provided to improve the appearance. The coping should have a slight inclination toward the back to prevent the discoloration of the face by dripping. It should be divided by vertical joints into blocks, not more than six to eight feet in length. The projection of the coping will depend upon the dimensions of the wall. Wing walls are preferably built with a sloping top or coping, but this should be made monolithic with the wall by special molds (§729).
In the use of concrete for abutments and piers, the practice of the Illinois Central Railroad, as set forth in their specifications, can hardly be improved upon. The engineer of bridges and buildings on this road, Mr. H. W. Parkhurst, M. Am. Soc. C. E., is one of those engineers who early recognized the value of concrete in bridge work, and as the result of his extensive experience along this line, he is widely known as an able and conservative authority.
These specifications are printed nearly in full in Engineering News of July 18, 1901, from which the following extracts are made: —
Natural cement concrete "may be used where foundations are entirely submerged below low-water mark, or where there is no risk of the same being exposed to the action of the weather by cutting away the surrounding earth. It, however, shall be used only where a firm and uniform foundation is found to exist after excavations are completed. In all cases where foundations are liable to be exposed to the action of the water, or where the material in the bottom of excavations is soft or of unequal firmness, Portland cement concrete must be employed for foundation work.
"The natural cement concrete shall usually be made in the proportions (by measure) of one part of approved cement to two parts of sand and five parts of crushed stone, all of character as above specified. For Portland cement concrete foundations, one part of approved cement, three parts of sand and six parts of crushed stone may be used. Wherever in the judgment of the engineer or inspector in charge of the work, a stronger concrete is required than is above specified, the proportions of sand and crushed stone employed may be reduced, a natural cement concrete of 1,2 and 4, and a Portland cement concrete of 1, 2 and 5 being substituted for those above specified.
Concrete for the bodies of piers and abutments, for all wing-walls for same, and for the bench walls of arch culverts, shall generally be made in the proportions (by measure) of one part of cement, two and one-half parts of sand and six parts of crushed stone. Where special strength may be required for any of this work, concrete in the proportions of 1, 2 and 5 may be used; but all such cases shall be submitted to the judgment of the engineer of bridges, before any change from the usual specification is to be allowed.
"For arch rings of arch culverts and for parapet head walls and copings to same, Portland cement concrete, in proportions of 1, 2 and 5, shall generally be used. Concrete of these proportions shall also generally be used for parapet walls behind bridge seats of piers or abutments, and for the finished copings (if used) on wing-walls of concrete abutments, also for arch work in combination with I-beams or in combination with ironwork for transverse loading.
"Bridge seats of piers and abutments and copings of concrete masonry which are to carry pedestals for girders or longer spans of ironwork, shall generally be made of crushed granite and Portland cement, in the proportion (by measure) of one part of approved cement, two parts of fine granite screenings, and three parts of coarser granite screenings, the larger of which shall not exceed three-quarters inch in greatest dimension".
729. After specifying the method of building molds, which is treated elsewhere (Art. 62), the specifications proceed: —
"The planking forming the lining of the molds shall invariably be fastened to the studding in perfectly horizontal lines; the ends of these planks shall be neatly butted against each other, and the inner surface of the mold shall be as nearly as possible perfectly smooth, without crevices or offsets between the sides or ends of adjacent planks. Where planks are used a second time, they shall be thoroughly cleaned, and, if necessary, the sides and ends shall be freshly jointed so as to make a perfectly smooth finish to the concrete.
"The molds for projecting copings, bridge seats, parapet walls, and all finished work shall be constructed in a first-class workmanlike manner, and shall be thoroughly braced and tied together, dressed surfaces only being exposed to the contact of concrete, and these surfaces shall be soaped or oiled if necessary, so as to make a smoothly finished piece of work. The top surfaces of all bridge seats, parapets, etc., shall be made perfectly level, unless otherwise provided in the plans, and shall be finished with long, straight edges, and all beveled surfaces or washes shall be constructed in a true and uniform manner. Special care shall be taken in the construction of the vertical angles of the masonry, and where I-beams or other ironwork are not used in the same, small wooden strips shall be set in the corners of the mold, so as to cut off the corners at an angle of 45°, leaving a beveled face about one and one-half to two inches wide, instead of a right-angled corner.
"Where wing-walls are called for, which have slopes corresponding to the angle of repose of earth embankments, these slopes shall be finished in straight lines and surfaces, the mold for such wing-walls and slopes being constructed with its top at the proper slope, so that the concrete work on the slope maybe finished in short sections, say from three to four feet in length, and bonded into the concrete of the horizontal sections before the same shall be set, each short section of sloped surface being grooved with a cross-line separating it from adjacent sections. It will not be permitted to finish the top surface of such sloped wing-walls by plastering fresh concrete upon the top of concrete which has already set, but the finished work must be made each day as the horizontal layers are carried up, to accomplish which the mold must be constructed complete at the outset; or, if the wing-wall is very high, short sections of the mold, including the form for the slopes, must be completed as the horizontal planking is put in place".
730. This is followed by directions concerning foundation work; the following is given relative to building steel into the masonry: —
"Iron rails to be furnished by the railroad company shall be laid and imbedded in such manner as may be specified in such foundation concrete as in the opinion of the engineer of bridges needs such strengthening, and no extra charge, except the actual Cost of handling the same, shall be made by the contractor for such work, but the volume of such iron shall be estimated as concrete.
"Where I-beams are to be placed in the angles of concrete piers as a protection against ice, drift, etc., these shall be set up and securely held in position so that they will extend one foot or more into the foundation concrete. The planking of molds shall be fitted carefully to the projecting angles of these I-beams, and small fillets of wood shall be fitted in between the inner faces of the mold and the rounded edges of the I-beam flanges, so that no sharp projecting angle of concrete will be formed as the work is constructed.
"These fillets may be made in short pieces and fastened neatly into the mold as the layers of concrete are carried up. Such I-beams will generally be furnished of sufficient length to extend at least six inches above the top of the battered masonry into the concrete coping, and special pains shall be taken to tamp the concrete thoroughly around the I-beams, and to finish the coping above and around the ends of the same, so as to make a compact and solid bearing against the ironwork.
"Where anchor bolts for bridge-seat castings are required, they shall be set in place and held firmly as to position and elevation, by templets, securely fastened to the mold and framing. Such I-beams and anchor bolts shall be imbedded in the concrete work without additional expense beyond the price to be paid per yard for the several classes of concrete in which such iron is placed, the volume of iron being estimated as concrete.
"After the work is finished and thoroughly set, all molds shall be removed by the contractor. They shall generally be allowed to stand not less than forty-eight hours after the last concrete work shall have been done. In cold weather, molds shall be allowed to stand a longer period before being removed, depending upon the degree of cold. No molds shall be removed in freezing weather, nor until after the concrete shall have had at least forty-eight hours, with the thermometer at or above 40° Fahr., in which to set".
"Layers of concrete shall be kept truly horizontal, and if, for any reason, it is necessary to stop work for an indefinite period, it shall be the duty of the inspector and of the contractor to see that the top surface of the concrete is properly finished, so that nothing but a horizontal line shall show on the face of the concrete, as the joint between portions of the work constructed before and after such period of delay. If for any reason it is impossible to complete an entire layer, the end of the layer shall be made square and true by the use of a temporary plank partition. No irregular, wavy or sloping lines shall be permitted to show on the face of the concrete work as the result of constructing different portions of the work at different periods, and none but horizontal or vertical lines shall be permitted in such cases".