52. The question of the most economical depth for wood paving blocks is as yet unsettled. In New York City, blocks 3½ inches in depth are adopted as the standard and are being used on streets of the heaviest travel, the practice of Berlin, Paris and other foreign cities being thus followed. The arguments in favor of these short blocks are lower first cost, and that, with much deeper blocks, the usual uneven wear of a wood pavement will make it so rough as to require removal before the blocks are worn down so as to be split up and dislodged from their places. While reliable data on these points are wanting, it seems to the writer very unwise to use such short blocks on streets of heavy travel, and he would recommend that the minimum length for use on such streets be 4½ inches, and he would prefer 5 inches.On streets of light travel a length of 3½ inches should be satisfactory.Recent observations on heavy travelled streets in New York indicate that when long-leaved yellow pine blocks become worn down to a remaining depth of about 2⅛ inches they split up into fine slivers and the pavement goes to pieces.
52. The question of the most economical depth for wood paving blocks is as yet unsettled. In New York City, blocks 3½ inches in depth are adopted as the standard and are being used on streets of the heaviest travel, the practice of Berlin, Paris and other foreign cities being thus followed. The arguments in favor of these short blocks are lower first cost, and that, with much deeper blocks, the usual uneven wear of a wood pavement will make it so rough as to require removal before the blocks are worn down so as to be split up and dislodged from their places. While reliable data on these points are wanting, it seems to the writer very unwise to use such short blocks on streets of heavy travel, and he would recommend that the minimum length for use on such streets be 4½ inches, and he would prefer 5 inches.
On streets of light travel a length of 3½ inches should be satisfactory.
Recent observations on heavy travelled streets in New York indicate that when long-leaved yellow pine blocks become worn down to a remaining depth of about 2⅛ inches they split up into fine slivers and the pavement goes to pieces.
53. Most of the more recent specifications require the use of a heavy oil, said to be composed of creosote oil with an admixture of refined tar, on the ground that the tar is necessary as a water-proofing agent to prevent the creosote oil from being dissolved out by water or evaporated into the air. It is claimed that if moisture can thus be excluded from entering the wood, decay will be prevented, even in the absence of the antiseptic elements of creosote oil. It is not intended to discuss this matter at length here. We know from long experience that genuine creosote oil is the best preservative of wood so far found; also that creosoted piles have stood in tidal waters, alternately exposed to water and air, for twenty-five years and still retain sufficient creosote oil to resist the Teredo—a very severe test. Why experiment with a comparatively untried material, particularly when it costs as much as the genuine creosote oil, is rather more difficult to force into the wood, and has some admitted objectionable qualities?
53. Most of the more recent specifications require the use of a heavy oil, said to be composed of creosote oil with an admixture of refined tar, on the ground that the tar is necessary as a water-proofing agent to prevent the creosote oil from being dissolved out by water or evaporated into the air. It is claimed that if moisture can thus be excluded from entering the wood, decay will be prevented, even in the absence of the antiseptic elements of creosote oil. It is not intended to discuss this matter at length here. We know from long experience that genuine creosote oil is the best preservative of wood so far found; also that creosoted piles have stood in tidal waters, alternately exposed to water and air, for twenty-five years and still retain sufficient creosote oil to resist the Teredo—a very severe test. Why experiment with a comparatively untried material, particularly when it costs as much as the genuine creosote oil, is rather more difficult to force into the wood, and has some admitted objectionable qualities?
54. It is a common practice of contractors in some cities, in the laying of both wood-block and asphalt block pavement, where a mortar bed is called for, to substitute a bed of mixed sand and cement, dampened only to such a degree as will make the mass pulverulent like damp sand, the claim being made that ordinary mortar cannot be spread and gaged properly. This claim is unfounded. The objection to the practice is that the dampened mixture does not contain sufficient water to cause the cement to set, and with the practically water-tight paving surface, does not receive, even in rainy weather, the necessary amount of water. If the weather be dry, the small quantity of moisture in the mixture quickly evaporates, leaving the so-called mortar bed not much better than a layer of sand alone. The writer has found such alleged mortar dry and unset two weeks after the pavement had been completed. If real mortar is not to be used, a layer of sand might almost as well be substituted at first.
54. It is a common practice of contractors in some cities, in the laying of both wood-block and asphalt block pavement, where a mortar bed is called for, to substitute a bed of mixed sand and cement, dampened only to such a degree as will make the mass pulverulent like damp sand, the claim being made that ordinary mortar cannot be spread and gaged properly. This claim is unfounded. The objection to the practice is that the dampened mixture does not contain sufficient water to cause the cement to set, and with the practically water-tight paving surface, does not receive, even in rainy weather, the necessary amount of water. If the weather be dry, the small quantity of moisture in the mixture quickly evaporates, leaving the so-called mortar bed not much better than a layer of sand alone. The writer has found such alleged mortar dry and unset two weeks after the pavement had been completed. If real mortar is not to be used, a layer of sand might almost as well be substituted at first.
55. In a number of cities the specifications require the joints in wood-block pavement to be filled with Portland cement grout. If the blocks are set as closely together as they should be, the joints will be so narrow that no grout, thick enough in consistency to be of value, will enter them, except for a short distance down from the top, the remaining depth of the joints remaining unfilled. An examination of any well-laid wood-block pavement soon after it has been attempted to fill the joints with grout will verify this statement. Furthermore, the oil which exudes from the blocks, acting on the thin films of grout, seems to deteriorate the mortar and to render it practically inert. On the contrary, fine dry sand will readily run into and completely fill the joints, and under travel the joints will soon become impervious to water. The sand filling is therefore regarded as better, and it costs less than the grout filling.
55. In a number of cities the specifications require the joints in wood-block pavement to be filled with Portland cement grout. If the blocks are set as closely together as they should be, the joints will be so narrow that no grout, thick enough in consistency to be of value, will enter them, except for a short distance down from the top, the remaining depth of the joints remaining unfilled. An examination of any well-laid wood-block pavement soon after it has been attempted to fill the joints with grout will verify this statement. Furthermore, the oil which exudes from the blocks, acting on the thin films of grout, seems to deteriorate the mortar and to render it practically inert. On the contrary, fine dry sand will readily run into and completely fill the joints, and under travel the joints will soon become impervious to water. The sand filling is therefore regarded as better, and it costs less than the grout filling.
56. Wherever an old pavement or macadam road can be utilized it makes an excellent foundation for a pavement of this kind, provided it is not in too dilapidated a condition, extends from curb to curb, and its surface conforms near enough to the desired street surface so that the necessary changes and repairs will not be too expensive. Where the new pavement is expected to carry quite a heavy travel it is not advisable to use plain crushed stone for filling depressions and leveling up the surface. It is difficult, even where proper care is used, to make such patches of broken stone as firm and strong as the adjoining old pavement, which is a necessary condition to secure satisfactory results; for if the masses of broken stone yield under travel, slight depressions will form over them in the bituminous surface, which will in time become holes requiring repairs. The 1:4:9 concrete specified for this work is not very much more expensive than plain broken stone, it will not shift or break up under travel, and will in the end prove a better investment.
56. Wherever an old pavement or macadam road can be utilized it makes an excellent foundation for a pavement of this kind, provided it is not in too dilapidated a condition, extends from curb to curb, and its surface conforms near enough to the desired street surface so that the necessary changes and repairs will not be too expensive. Where the new pavement is expected to carry quite a heavy travel it is not advisable to use plain crushed stone for filling depressions and leveling up the surface. It is difficult, even where proper care is used, to make such patches of broken stone as firm and strong as the adjoining old pavement, which is a necessary condition to secure satisfactory results; for if the masses of broken stone yield under travel, slight depressions will form over them in the bituminous surface, which will in time become holes requiring repairs. The 1:4:9 concrete specified for this work is not very much more expensive than plain broken stone, it will not shift or break up under travel, and will in the end prove a better investment.
57. Where a new foundation is required broken stone or macadam is most frequently used for bituminous concrete pavements. Unless such foundations are constructed in the same way and with about the same care as is necessary for a macadam road it is liable to prove unsatisfactory. Under the very heavy wheel loads that may occasionally pass over the streets, imperfectly compacted broken stone is likely to shift sufficiently to start incipient ruts which will enlarge and in time necessitate expensive repairs. Such conditions are frequently seen on bituminous concrete pavements subjected to heavy travel. These pavements, like sheet asphalt pavements, require a foundation that will be absolutely unyielding under travel. For this reason a concrete foundation will generally be found more economical in the long run than a broken stone foundation. The increased first cost per square yard is not very great and this additional money will in most cases prove a good investment. At the usual prices of material and labor a square yard of 4 inch concrete should cost about 50 cents, while a properly constructed broken stone foundation 6 inches thick (which would not nearly equal in strength and rigidity 4 inches of concrete) would cost about 45 cents per square yard. Considering the much greater durability and lower cost of repairs of the pavement on the concrete foundation, this small additional cost is not worth consideration. While the specifications are made to cover the three kinds of foundation, it is assumed that the kind of foundation to be used will be decided in advance, and that the part of these specifications relating to the others kinds of foundation will, in actual use be omitted.
57. Where a new foundation is required broken stone or macadam is most frequently used for bituminous concrete pavements. Unless such foundations are constructed in the same way and with about the same care as is necessary for a macadam road it is liable to prove unsatisfactory. Under the very heavy wheel loads that may occasionally pass over the streets, imperfectly compacted broken stone is likely to shift sufficiently to start incipient ruts which will enlarge and in time necessitate expensive repairs. Such conditions are frequently seen on bituminous concrete pavements subjected to heavy travel. These pavements, like sheet asphalt pavements, require a foundation that will be absolutely unyielding under travel. For this reason a concrete foundation will generally be found more economical in the long run than a broken stone foundation. The increased first cost per square yard is not very great and this additional money will in most cases prove a good investment. At the usual prices of material and labor a square yard of 4 inch concrete should cost about 50 cents, while a properly constructed broken stone foundation 6 inches thick (which would not nearly equal in strength and rigidity 4 inches of concrete) would cost about 45 cents per square yard. Considering the much greater durability and lower cost of repairs of the pavement on the concrete foundation, this small additional cost is not worth consideration. While the specifications are made to cover the three kinds of foundation, it is assumed that the kind of foundation to be used will be decided in advance, and that the part of these specifications relating to the others kinds of foundation will, in actual use be omitted.
58. A bituminous cement composed largely of coal-tar pitch has heretofore been most used in pavements of this character. It is not denied that very good pavements have been, and can be built with this material, but the superiority of the asphaltic cement here specified is so great that it is true economy to use it. The difference in cost at prevailing prices of material will be ten to twelve cents per square yard. The greater durability and serviceability of the pavement made with the asphaltic cement will, particularly on streets of comparatively heavy travel, far more than justify this additional cost.
58. A bituminous cement composed largely of coal-tar pitch has heretofore been most used in pavements of this character. It is not denied that very good pavements have been, and can be built with this material, but the superiority of the asphaltic cement here specified is so great that it is true economy to use it. The difference in cost at prevailing prices of material will be ten to twelve cents per square yard. The greater durability and serviceability of the pavement made with the asphaltic cement will, particularly on streets of comparatively heavy travel, far more than justify this additional cost.
59. Hydraulic concrete pavement is to be recommended only for country roadways and for city streets of very moderate travel. While our experience with this kind of pavement is yet limited there is reason to believe from the nature of the material that it will not prove to be a satisfactory or economical pavement for streets of heavy travel. But in all cities and towns there are many residence streets where the travel is very light, and yet where a permanent pavement is wanted and warranted. For these, it is believed that a properly constructed concrete pavement will prove very satisfactory and durable, and the low cost at which it can be constructed should make it very attractive to city officials and property owners. The author has advocated its use under such conditions for many years (see Engineering News, July 21st, 1904). Like other composite pavements its utility and durability will depend largely upon the good quality of the materials used and the skill and thoroughness with which the work is done.The specifications here offered are the result of the observation and experience of the author, and it is believed that pavements laid in accordance with them will give very satisfactory results.
59. Hydraulic concrete pavement is to be recommended only for country roadways and for city streets of very moderate travel. While our experience with this kind of pavement is yet limited there is reason to believe from the nature of the material that it will not prove to be a satisfactory or economical pavement for streets of heavy travel. But in all cities and towns there are many residence streets where the travel is very light, and yet where a permanent pavement is wanted and warranted. For these, it is believed that a properly constructed concrete pavement will prove very satisfactory and durable, and the low cost at which it can be constructed should make it very attractive to city officials and property owners. The author has advocated its use under such conditions for many years (see Engineering News, July 21st, 1904). Like other composite pavements its utility and durability will depend largely upon the good quality of the materials used and the skill and thoroughness with which the work is done.
The specifications here offered are the result of the observation and experience of the author, and it is believed that pavements laid in accordance with them will give very satisfactory results.
60. A number of engineers advocate the construction of concrete pavement in one homogeneous course, and quite a number of pavements have been constructed in this way.Like any other composite pavement, it is called upon to perform two functions; to safely sustain the weight of loads passing over it, and to resist wear and abrasion of its surface. A material and form of construction that meets the first requirement may not meet the second. Experience has proved that ordinary 1:3:6 concrete makes an entirely satisfactory foundation for any pavement, but it lacks the hardness and strength to successfully resist the surface abrasion of travel. To secure this quality a richer and harder concrete is called for, but it is unnecessary that the foundation should be equally hard. To construct the pavement in two courses as here specified would seem to be the logical way, especially as it decreases the total cost, and should make a more durable pavement.
60. A number of engineers advocate the construction of concrete pavement in one homogeneous course, and quite a number of pavements have been constructed in this way.
Like any other composite pavement, it is called upon to perform two functions; to safely sustain the weight of loads passing over it, and to resist wear and abrasion of its surface. A material and form of construction that meets the first requirement may not meet the second. Experience has proved that ordinary 1:3:6 concrete makes an entirely satisfactory foundation for any pavement, but it lacks the hardness and strength to successfully resist the surface abrasion of travel. To secure this quality a richer and harder concrete is called for, but it is unnecessary that the foundation should be equally hard. To construct the pavement in two courses as here specified would seem to be the logical way, especially as it decreases the total cost, and should make a more durable pavement.
61. Some engineers advocate a greater total thickness of the pavement than is here specified (6 inches). Considering that this pavement should never be used on heavy traveled streets, a total thickness of concrete of six inches will have ample strength to carry the loads to which it will be subjected. If so, it is a useless waste of money to increase the thickness of the concrete.
61. Some engineers advocate a greater total thickness of the pavement than is here specified (6 inches). Considering that this pavement should never be used on heavy traveled streets, a total thickness of concrete of six inches will have ample strength to carry the loads to which it will be subjected. If so, it is a useless waste of money to increase the thickness of the concrete.
62. The use of limestone for the top course (unless it is of very superior quality) is not advisable or economical unless the cost of trap rock is so high as to be prohibitive, which, considering its superior durability under the wear of travel, will not often be the case.
62. The use of limestone for the top course (unless it is of very superior quality) is not advisable or economical unless the cost of trap rock is so high as to be prohibitive, which, considering its superior durability under the wear of travel, will not often be the case.
63. It is advisable to remove the screenings from this surface mixture for two reasons: first, to secure greater uniformity of composition. If the screenings are allowed to remain in the aggregate, there is danger of segregation into patches of different sized aggregate and different ratios of materials, which it is very important to avoid, and second, the small fragments of stone are more likely to be crushed under the concentrated weight of wheels than the larger masses, and to thus start disintegration. Lack of uniformity in the composition and homogeneity in this surface-course concrete is especially to be guarded against, otherwise the surface of the pavement will wear unevenly and depressions and ruts are likely to result.
63. It is advisable to remove the screenings from this surface mixture for two reasons: first, to secure greater uniformity of composition. If the screenings are allowed to remain in the aggregate, there is danger of segregation into patches of different sized aggregate and different ratios of materials, which it is very important to avoid, and second, the small fragments of stone are more likely to be crushed under the concentrated weight of wheels than the larger masses, and to thus start disintegration. Lack of uniformity in the composition and homogeneity in this surface-course concrete is especially to be guarded against, otherwise the surface of the pavement will wear unevenly and depressions and ruts are likely to result.
64. The ideal composition of this surface-course concrete is one where the stone forms the largest possible part of the mass consistent with sufficient mortar to fill the voids and thoroughly bind the fragments of stone together.
64. The ideal composition of this surface-course concrete is one where the stone forms the largest possible part of the mass consistent with sufficient mortar to fill the voids and thoroughly bind the fragments of stone together.
65. The importance of securing high quality and great uniformity in the surface course cannot be urged too strongly.
65. The importance of securing high quality and great uniformity in the surface course cannot be urged too strongly.
66. This requirement must be strictly enforced. Otherwise there will be danger that the two courses may not properly adhere to each other. It is the writer’s experience that if this rule is observed there will be no danger of the two courses separating.
66. This requirement must be strictly enforced. Otherwise there will be danger that the two courses may not properly adhere to each other. It is the writer’s experience that if this rule is observed there will be no danger of the two courses separating.
67. The purpose of this rolling is mainly to evenly compress the mass and thus secure its uniform density. It also produces a truer surface than can usually be secured by ramming alone.
67. The purpose of this rolling is mainly to evenly compress the mass and thus secure its uniform density. It also produces a truer surface than can usually be secured by ramming alone.
68. Among engineers there is quite a wide difference of opinion as to the proper spacing of expansion joints, and, in fact, as to the necessity or advisability of providing them at all. It has been suggested that it might be better to omit them entirely, allowing the pavement to form its own expansion joints by cracking along lines where natural forces dictate. Such cracks by their irregularity give a bad appearance to the surface, but observation seems to indicate that the edges of these natural joints wear as well as those made by expansion joints. Further observation and experience is needed in the matter. In most concrete it is known that some contraction takes place during the setting of the cement, regardless of temperature changes, and cracking is probably due as much to this permanent contraction as to that caused by low temperature. The coefficient of expansion of concrete by heat is variable but so small that expansion joints ⅛ inch wide every fifty feet along the street should provide for temperature changes.
68. Among engineers there is quite a wide difference of opinion as to the proper spacing of expansion joints, and, in fact, as to the necessity or advisability of providing them at all. It has been suggested that it might be better to omit them entirely, allowing the pavement to form its own expansion joints by cracking along lines where natural forces dictate. Such cracks by their irregularity give a bad appearance to the surface, but observation seems to indicate that the edges of these natural joints wear as well as those made by expansion joints. Further observation and experience is needed in the matter. In most concrete it is known that some contraction takes place during the setting of the cement, regardless of temperature changes, and cracking is probably due as much to this permanent contraction as to that caused by low temperature. The coefficient of expansion of concrete by heat is variable but so small that expansion joints ⅛ inch wide every fifty feet along the street should provide for temperature changes.
69. If the expansion joints are not thus filled with bituminous cement they will become filled and packed with incompressible stone, sand, etc., that will not permit expansion.
69. If the expansion joints are not thus filled with bituminous cement they will become filled and packed with incompressible stone, sand, etc., that will not permit expansion.
70. The practical value of oiling concrete pavements has not yet been determined by sufficient experience. There is reason, however, to believe that the slight coating of bitumen will materially preserve the surface from abrasion and that its benefit will thus be greater than its cost. It will also tend to prevent the very slight dust that might otherwise exist on the pavement.
70. The practical value of oiling concrete pavements has not yet been determined by sufficient experience. There is reason, however, to believe that the slight coating of bitumen will materially preserve the surface from abrasion and that its benefit will thus be greater than its cost. It will also tend to prevent the very slight dust that might otherwise exist on the pavement.
71. In this class of pavements the contractor or promoter may properly be required to assume responsibility for the character and utility of the work produced, and the municipal authorities should assume no part of such responsibility.
71. In this class of pavements the contractor or promoter may properly be required to assume responsibility for the character and utility of the work produced, and the municipal authorities should assume no part of such responsibility.
72. Upon the general subject of time guarantees of municipal work, see Chapter XI, “Municipal Public Works,” by the author.
72. Upon the general subject of time guarantees of municipal work, see Chapter XI, “Municipal Public Works,” by the author.
73. Concrete combined curb and gutter is suitable for use on the great majority of residence streets, and others where the travel is not excessive, or where it will not be subjected to specially severe use, as on business streets where heavy vehicles are likely to be often backed against the curb. If properly constructed it will have sufficient hardness and strength to withstand all ordinary usage; it makes a better appearance, particularly on residence streets, than any other kind of curbing, is durable, and is usually less expensive than any other suitable, equally durable and equally well-finished curbing of natural stone, since the gutter displaces an equal area of pavement.
73. Concrete combined curb and gutter is suitable for use on the great majority of residence streets, and others where the travel is not excessive, or where it will not be subjected to specially severe use, as on business streets where heavy vehicles are likely to be often backed against the curb. If properly constructed it will have sufficient hardness and strength to withstand all ordinary usage; it makes a better appearance, particularly on residence streets, than any other kind of curbing, is durable, and is usually less expensive than any other suitable, equally durable and equally well-finished curbing of natural stone, since the gutter displaces an equal area of pavement.
74. The sketch here presented conforms pretty closely to usual practice except in the width of the gutter. It is not uncommon to make the gutter from two to three feet wide. This is not necessary or desirable. A width of 15 to 18 inches forms a sufficient gutter to carry away all drainage except during very heavy rainfalls. Where the gutter projects out into the street sufficiently far to be exposed to large numbers of heavily loaded wheels the outer corner is likely to become broken off or unduly abraded.
74. The sketch here presented conforms pretty closely to usual practice except in the width of the gutter. It is not uncommon to make the gutter from two to three feet wide. This is not necessary or desirable. A width of 15 to 18 inches forms a sufficient gutter to carry away all drainage except during very heavy rainfalls. Where the gutter projects out into the street sufficiently far to be exposed to large numbers of heavily loaded wheels the outer corner is likely to become broken off or unduly abraded.
75. Curbing of all kinds is more likely to be injured by freezing and the heaving of frost under and around it than from any other cause. Good drainage is the best protection against such injury. It is important that these drains shall be connected with sewers, drains or other outlets, so that water will not stand in them.
75. Curbing of all kinds is more likely to be injured by freezing and the heaving of frost under and around it than from any other cause. Good drainage is the best protection against such injury. It is important that these drains shall be connected with sewers, drains or other outlets, so that water will not stand in them.
76. The most notable departure of these specifications from usual practice is the use of a solid body of rich, homogeneous concrete for the whole section of the structure, thus avoiding the use of two courses and qualities of concrete—the core concrete and the facing. The most common cause of failure of concrete curbs and gutters is the separation, more or less, of the facing from the core concrete. Without doubt this can be prevented by the use of proper materials, careful work, and the strict observance of the rule that the facing course must be applied before the core concrete has begun to set. But it is difficult to always secure these favorable conditions. Computation will show that the difference in cost of materials, between the usual two-course construction and a single body of rich concrete throughout, is not very great, while the saving in cost of labor is so considerable as to make the actual difference in cost of the two types very small. There can be no doubt that the simpler construction and the consequent greater certainty of securing a durable and satisfactory job is greatly in favor of the construction here recommended.
76. The most notable departure of these specifications from usual practice is the use of a solid body of rich, homogeneous concrete for the whole section of the structure, thus avoiding the use of two courses and qualities of concrete—the core concrete and the facing. The most common cause of failure of concrete curbs and gutters is the separation, more or less, of the facing from the core concrete. Without doubt this can be prevented by the use of proper materials, careful work, and the strict observance of the rule that the facing course must be applied before the core concrete has begun to set. But it is difficult to always secure these favorable conditions. Computation will show that the difference in cost of materials, between the usual two-course construction and a single body of rich concrete throughout, is not very great, while the saving in cost of labor is so considerable as to make the actual difference in cost of the two types very small. There can be no doubt that the simpler construction and the consequent greater certainty of securing a durable and satisfactory job is greatly in favor of the construction here recommended.
77. The appearance of “hair cracks” on the surface of rich concrete, finished by troweling, and the blotched appearance of the surface of concrete curbing, are usually caused by improper finishing. The glazed surface produced by troweling, particularly where pure, dry cement or neat mortar is applied is almost sure to develop hair cracks, and the varying texture of the surface is likely to absorb water unevenly and thus produce, in time, the unevenly colored or blotched surface so often seen.
77. The appearance of “hair cracks” on the surface of rich concrete, finished by troweling, and the blotched appearance of the surface of concrete curbing, are usually caused by improper finishing. The glazed surface produced by troweling, particularly where pure, dry cement or neat mortar is applied is almost sure to develop hair cracks, and the varying texture of the surface is likely to absorb water unevenly and thus produce, in time, the unevenly colored or blotched surface so often seen.
78. The utility and durability of hydraulic concrete sidewalks depends largely on the quality of materials and workmanship employed in the work. Too frequently, specifications for this work are not sufficiently full, or not prepared with the requisite care, or the work is not properly supervised or inspected while in progress. The aggregate importance and cost of this sidewalk work in our cities warrants more care and attention than it generally receives.
78. The utility and durability of hydraulic concrete sidewalks depends largely on the quality of materials and workmanship employed in the work. Too frequently, specifications for this work are not sufficiently full, or not prepared with the requisite care, or the work is not properly supervised or inspected while in progress. The aggregate importance and cost of this sidewalk work in our cities warrants more care and attention than it generally receives.
79. Determined by the thickness of the drainage course adopted.
79. Determined by the thickness of the drainage course adopted.
80. It is customary in many cities to require that the drainage course under the concrete shall have a depth of twelve or more inches. This deep-drainage is designed to prevent the heaving of the sidewalk by freezing. Experience seems to prove that this is not necessary, particularly if tile drains are provided to carry off the water from the drainage course, as specified. Comparatively dry material, even earth, does not heave with freezing; on the other hand, if the material and the trench in which it is placed is wholly or partly filled with water, heaving is liable to occur in severe freezing weather, whatever the depth of the drainage course. Experience has proven, however, that four inches of drainage material is sufficient if the water is drained out of it, while if allowed to stand saturated with water, deeper drainage will give little if any better results. Where the soil is sandy or the natural drainage is otherwise good, no drainage course is necessary.
80. It is customary in many cities to require that the drainage course under the concrete shall have a depth of twelve or more inches. This deep-drainage is designed to prevent the heaving of the sidewalk by freezing. Experience seems to prove that this is not necessary, particularly if tile drains are provided to carry off the water from the drainage course, as specified. Comparatively dry material, even earth, does not heave with freezing; on the other hand, if the material and the trench in which it is placed is wholly or partly filled with water, heaving is liable to occur in severe freezing weather, whatever the depth of the drainage course. Experience has proven, however, that four inches of drainage material is sufficient if the water is drained out of it, while if allowed to stand saturated with water, deeper drainage will give little if any better results. Where the soil is sandy or the natural drainage is otherwise good, no drainage course is necessary.
81. Except in the matter of low first cost cinders are not desirable for the drainage course. In time, this material is likely to slack, or decompose, and shrink in volume more or less and to allow the sidewalk to settle. The hollow sound one often notices when walking over a sidewalk and the cracks that frequently appear, are usually caused by the irregular settlement of the drainage course. The object of wetting down the cinders several days before they are used is to cause as much as possible of this slacking to take place before the cinders are used in the drainage course.
81. Except in the matter of low first cost cinders are not desirable for the drainage course. In time, this material is likely to slack, or decompose, and shrink in volume more or less and to allow the sidewalk to settle. The hollow sound one often notices when walking over a sidewalk and the cracks that frequently appear, are usually caused by the irregular settlement of the drainage course. The object of wetting down the cinders several days before they are used is to cause as much as possible of this slacking to take place before the cinders are used in the drainage course.
82. There is a good deal of diversity of practice in the thickness of the concrete to be used. For all ordinary sidewalks three inches of bottom course and one inch of surface-course are ample, and in many cases the thickness of the surface course has been reduced to one-half inch with satisfactory results. Three-fourths inch of surface-course, if fairly uniform in thickness and of good quality, will generally be ample for ordinary sidewalks.
82. There is a good deal of diversity of practice in the thickness of the concrete to be used. For all ordinary sidewalks three inches of bottom course and one inch of surface-course are ample, and in many cases the thickness of the surface course has been reduced to one-half inch with satisfactory results. Three-fourths inch of surface-course, if fairly uniform in thickness and of good quality, will generally be ample for ordinary sidewalks.
83. It is customary to make the surface-course concrete much richer than this, but it is not necessary if the materials are good and the work well done.
83. It is customary to make the surface-course concrete much richer than this, but it is not necessary if the materials are good and the work well done.
84. This requirement is very important and should be strictly enforced, otherwise there is danger that by the action of water, frost, and time, the two courses may separate and the surface-course break up—a condition not infrequently observed.
84. This requirement is very important and should be strictly enforced, otherwise there is danger that by the action of water, frost, and time, the two courses may separate and the surface-course break up—a condition not infrequently observed.
85. The troweling not only helps to secure a true surface, but tends to produce a dense surface on the concrete; but it is not desirable that this surface shall be smooth and glassy, hence the slight roughening of the surface with a wire brush.
85. The troweling not only helps to secure a true surface, but tends to produce a dense surface on the concrete; but it is not desirable that this surface shall be smooth and glassy, hence the slight roughening of the surface with a wire brush.
86. Care must be taken to make and leave these expansion joints open to their full width entirely through the concrete. The practice of forming these expansion joints by partitions of iron plate, against which the blocks of sidewalk are built is not advised, for the reason that they are likely to prevent the thorough compression of the concrete surface against or near the plates.
86. Care must be taken to make and leave these expansion joints open to their full width entirely through the concrete. The practice of forming these expansion joints by partitions of iron plate, against which the blocks of sidewalk are built is not advised, for the reason that they are likely to prevent the thorough compression of the concrete surface against or near the plates.
TRANSCRIBER’S NOTESSilently corrected obvious typographical errors and variations in spelling.Retained archaic, non-standard, and uncertain spellings as printed.Re-indexed footnotes using numbers and collected together at the end of the last chapter.
TRANSCRIBER’S NOTES
TRANSCRIBER’S NOTES
TRANSCRIBER’S NOTES
TRANSCRIBER’S NOTES