~HINTS ON HAND MIXING.~--All but a small percentage of the concrete annually laid in street work is hand mixed. The authors are confident that this condition will disappear as contractors learn more of the advantages of machine mixing, but it prevails at present. The general economics of hand mixing are discussed in Chapter II; in street work as before stated, the big items of labor cost are the costs of handling materials and the data in Chapter II on these processes deserve special attention. It is particularly worth noting that it is seldom economical to handle materials in shovels where carrying is necessary; it is a common thing in street work to see an attempt to get the stock piles so close to the mixing board that the material can be handled with shovels, and this is nearly always an economic error. Street work is readily measured; in fact, its progress can be seen at a glance, and advantage can often be taken of this fact to profit by the rivalry of separate gangs. The authors have known of the labor costs being reduced as much as 25 per cent., due to pitting one gang against another where each could see the progress made by the other.
~METHODS OF MACHINE MIXING.~--Concrete mixers have been slow to replace handwork in laying pavement foundations. In explanation of this fact it is a.s.serted: (1) That frequent shifting of the mixer causes too much lost time, and (2) that the princ.i.p.al item of labor cost in street work is the conveying of materials to and from the mixer, and this item is the same whether hand or machine mixing be employed. The records of machine mixer work given elsewhere in this chapter go far, in the opinion of the authors, toward disproving the accuracy of both a.s.sertions. If the machine used and the methods of work employed are adapted to the conditions of street work, machine mixing can be employed to decided advantage.
A continuous and large output is demanded in a mixer for street work; the perfection of the mixing is within limits a minor consideration.
This at once admits for consideration types of mixers whose product is cla.s.sed as unsuitable for reinforced concrete work, and also admits of speeding up the output of the better types to a point beyond that at which they turn out their most perfect product. Keeping these facts in mind either of the following two systems of work may be employed: (1) Traction plants which travel with the work and deposit concrete in place, or so nearly in place that little shoveling is necessary; (2) portable plants which are set up at wide intervals along the work and which discharge the concrete into carts or dump wagons which distribute it to the work.
The secret of economic work with plants of the cla.s.s cited first is the distribution of the stock piles so as practically to eliminate haulage from stock pile to mixer. The mixer backs away from the work, its discharge end being toward the work and its charging end away from it.
Then deposit the materials so as to form a continuous stock pile along the center of the street; the mixer moving backward from the completed foundation keeps close to the materials and if the latter are uniformly distributed in the pile the great bulk of the charging is done by shoveling direct into the charging bucket. The point to be watched here is that the shovelers do not have to carry the materials; separate stock piles within moderate hauling distance by wheelbarrows are a far more economic arrangement than a continuous pile so irregularly distributed that much of the material has to be carried even a few paces in shovels.
Economic work with plants of the second cla.s.s depends upon efficient and adequate means of hauling the mixed concrete to the work. The plant should not be shifted oftener than once in 1,000 to 2,000 ft., or, say, four city blocks. This does away with the possibility of wheelbarrow haulage; large capacity hand or horse carts must be employed. With 6 cu.
ft. hand carts, such as the Ransome cart, a haul of 500 ft. each way from the mixer is possible and with horse carts, such as the Briggs, this economic distance is increased to 1,000 ft. each way from the mixer. The mixer must be close to the stock pile and it will pay to make use of improved charging devices. A 6-in. foundation for 2,000 ft. of 30-ft. street calls for 667 cu. yds. of concrete, and if both sides are curbed at the same time, 100 cu. yds. more are added, or 767 cu. yds. in all; where intersecting streets are to be paved in both directions from the mixer plant these amounts are doubled. A very small saving per cubic yard due to mechanical handling of the materials to the mixer amounts to the interest on a considerable investment in such plant. A point that should not be forgotten is that carts such as those named above spread the concrete in dumping so that little or no shoveling is required.
~FOUNDATION FOR STONE BLOCK PAVEMENT, NEW YORK, N. Y.~--Mr. G. W. Tillson, in "Street Pavements and Paving Materials," p. 204, gives the following data on the cost of granite block pavement in New York City in 1899. The day was 10 hours long:
Per Per Per Concrete gang-- day. sq. yd. cu. yd.
1 foreman $ 3.00 $0.0125 $0.075 8 mixers on two boards, at $1.25 10.00 0.0416 0.250 4 wheeling stone and sand, at $1.25. 5.00 0.0208 0.125 1 carrying cement and supplying water, at $1.25 1.25 0.0051 0.031 1 ramming, at $1.25 1.25 0.0051 0.031 ------ ------- ------ Total, 240 sq. yds. (40 cu. yds.). $20.50 $0.0851 $0.512
The concrete was shoveled direct from the mixing boards to place.
Cost 1-2-4 concrete-- Per cu. yd.
1-1/3 bbls. natural cement, at $0.90 $1.20 0.95 cu. yd. stone, at $1.25 1.19 0.37 cu. yd. sand, at $1.00 0.37 Labor 0.51 ----- $3.27
In laying 5,167 sq. yds. of granite block pavement on one job in New York City in 1905, the authors" records show that one laborer mixed and laid 1.3 cu. yds. of concrete per day in a 6-in. foundation; this is a very small output. The work was done by contract and the labor cost was as follows:
Per Per Item. Total. sq. yd. cu. yd.
28 days foreman at $3.50 $ 99.75 $0.0193 $0.118 399 days laborers at $1.75 698.25 0.1351 0.826 ------- ------- ------ $798.00 $0.1544 $0.944
The average day"s wages was $1.86, so that the labor cost was about 0.5 of a day"s wages per cubic yard of concrete.
~FOUNDATION FOR PAVEMENT, NEW ORLEANS. LA.~--Mr. Alfred E. Harley states that in laying concrete foundations for street pavement in New Orleans, a day"s work, in running three mixing boards, covering the full width of the street, averaged 900 sq. yds., 6 ins. thick, or 150 cu. yds., with a gang of 40 men. With wages a.s.sumed to be 15 cts. per hour the labor cost was:
Cts. per cu. yd.
6 men wheeling broken stone 6 3 men wheeling sand 3 1 man wheeling cement 1 2 men opening cement 2 7 men dry mixing 7 8 men taking concrete off 8 3 men tamping 3 3 men grading concrete 3 1 man attending run planks 1 3 water boys 1 2 extra men and 1 foreman 4 -- Total labor cost 39 cts.
~FOUNDATIONS FOR STREET PAVEMENT, TORONTO, CANADA.~--The following cost of a concrete base for pavements at Toronto has been abstracted from a report (1892) of the City Engineer, Mr. Granville C. Cunningham. The concrete was 1-2-7 Portland; 2,430 cu. yds. were laid, the thickness being 6 ins., at the following cost per cubic yard:
0.77 bbl. cement, at $2.78 $2.14 0.76 cu. yd. stone, at $1.91 1.45 0.27 cu. yd. sand and gravel, at $0.80 0.22 Labor (15 cts. per hr) 1.03 ----- Total $4.84
Judging by the low percentage of stone in so lean a mixture as the above, the concrete was not fully 6 ins. thick as a.s.sumed by Mr.
Cunningham. Note that the labor cost was 1 to 2 times what it would have been under a good contractor.
~MISCELLANEOUS EXAMPLES OF PAVEMENT FOUNDATION WORK.~--The following records of pavement foundation work are taken from the note and time books of one of the authors:
_Case 1._--Laying 6-in. pavement foundation; stone delivered and dumped upon 2-in. plank laid to receive it. Sand and stone were dumped along the street, so that the haul in wheelbarrows to mixing board Was about 40 ft. Two gangs of men worked under separate foremen, and each gang averaged 4.5 cu. yds. concrete per hour. The labor cost was as follows for 45 cu. yds. per gang:
Per day. Per cu. yd.
4 men filling barrows with stone and sand ready for the mixers, wages 15 cts.
per hour $6.00 $0.13 10 men, wheeling, mixing and shoveling to place (3 or 4 steps), wages 15 cts. per hour 15.00 0.33
2 men ramming, wages 15 cts. per hour 3.00 0.07 1 foreman at 30 cts. per hour and 1 water boy, 5 cts 3.50 0.08 ----- ----- Total $27.50 $0.61
_Case II._--Sometimes it is desirable to know every minute detail cost, for which purpose the following is given:
----Per cu. yd.---- Day"s labor. Cost.
3 men loading stones into barrows $0.06 $0.09 1 man loading sand into barrows 0.02 0.03 2 men ramming 0.04 0.06 1 foreman and 1 water boy equivalent to 0.035 0.05 Wheeling sand and cement to mixing board 0.02 0.03 Wheeling stone to mixing board 0.026 0.04 9 men mixing mortar 0.013 0.02 Mixing stone and mortar 0.049 0.07 Placing concrete (walking 15 ft.) 0.072 0.11 ------ ----- Total $0.335 $0.50
In one respect this is not a perfectly fair example (although it represents ordinary practice), for the mortar was only turned over once in mixing instead of three times, and the stone was turned only twice instead of three or four times. Water was used in great abundance, and by its puddling action probably secured a very fair mixture of cement and sand, and in that way secured a better mixture than would be expected from the small amount of labor expended in actual mixing.
About 9 cts. more per cu. yd. spent in mixing would have secured a perfect concrete without trusting to the water.
_Case III._--Two gangs (34 men) working under separate foremen averaged 600 sq. yds., or 100 cu. yds. of concrete per 10-hour day for a season.
This is equivalent to 3 cu. yds. per man per day. The stone and sand were wheeled to the mixing board in barrows, mixed and shoveled to place. Each gang was organized as follows:
Per day. Per cu. yd.
4 men loading barrows $ 6.00 $0.12 9 men mixing and placing 13.50 0.27 2 men tamping 3.00 0.06 1 foreman 2.50 0.05 ------ ----- Total $25.00 $0.50
These men worked with great rapidity. The above cost of 50 cts. per cu.
yd. is about as low as any contractor can reasonably expect to mix and place concrete by hand in pavement work.
_Case IV._--Two gangs of men, 34 in all, working side by side on separate mixing boards, averaged 720 sq. yds., or 120 cu. yds., per 10-hour day. Each gang was organized as follows:
Per day. Per cu. yd.
6 men loading and wheeling $ 9.00 $0.15 8 men mixing and placing 12.00 0.20 2 men tamping 3.00 0.05 1 foreman 3.00 0.05 ------ ----- Total $27.00 $0.45
Instead of shoveling the concrete from the mixing board into place, the mixers loaded it into barrows and wheeled it to place. The men worked with great rapidity.
Mr. Irving E. Howe gives the cost of a 6-in. foundation of 1-3-5 natural cement at Minneapolis, Minn., in 1897, as $2.80 per cu. yd., or $0.467 per sq. yd. Cement cost 76 cts. per barrel and stone and sand cost delivered $1.15 and 30 cts. respectively. Mixers received $1.75 per day.
Mr. Niles Meriwether gives the cost of materials and labor for an 8-in.
foundation constructed by day labor (probably colored) at Memphis, Tenn., in 1893, as follows:
Per sq. yd.
Natural cement at $0.74 per bbl $0.195 Sand at $1.25 per cu. yd 0.075 Stone at $1.87 per cu. yd 0.355 Labor mixing and placing 0.155 ----- Total $0.780
Labor was paid $1.25 to $1.50 per 8-hour day and 1.16 bbls. of cement were used per cubic yard of concrete. The cost of materials, as will be noted, was high and the labor seems to have been inefficient.
~FOUNDATIONS FOR BRICK PAVEMENT, CHAMPAIGN, ILL.~--The concrete foundation for a brick pavement constructed in 1903 was 6 ins. thick; the concrete used was composed of 1 part natural cement, 3 parts of sand and gravel, and 3 parts of broken stone. All the materials were mixed with shovels, and were thrown into place from the board upon which the mixing was done. The material was brought to the steel mixing board in wheelbarrows from piles where it had been placed in the middle of the street, the length of haul being usually from 30 to 60 ft. The foundation was 6 ins.
thick and it cost as follows for materials and labor:
Cost per cu. yd.
1.2 bbls. cement, at $0.50 $0.600 0.6 cu. yd. sand and gravel, at $1 0.600 0.6 cu. yd. broken stone, at $1.40 0.840 6 men turning with shovels, at $2 0.080 4 men throwing into place, at $2 0.053 2 men handling cement, at $1.75 0.023 1 man wetting with hose, at $1.75 0.012 2 men tamping, at $1.75 0.023 1 man leveling, at $1.75 0.012 6 men wheeling stone, at $1.75 0.070 4 men wheeling gravel, at $1.75 0.047 1 foreman, at $4 0.027 ------ $2.387
This is practically 40 cts. per sq. yd., or $2.40 per cu. yd. of concrete for materials and labor. It is evident from the above quant.i.ties that a cement barrel was a.s.sumed to hold about 4.5 cu. ft., hence the cement was measured loose in making the 1-3-3 concrete. The accuracy of the quant.i.ties given is open to serious doubt. It will also be noted that the labor cost of making and placing the concrete was only 35 cts. per cu. yd., wages being nearly $1.85 per day. This is so remarkably low that some mistake would seem to have been made in the measurement of the work. The authors do not hesitate to say that no gang of men ever made any considerable amount of concrete by hand at the rate of 5.75 cu. yds. per man per day.
[Ill.u.s.tration: Fig. 112.--Foote Continuous Mixer Arranged for Pavement Foundation Work.]
~FOUNDATION CONSTRUCTION USING CONTINUOUS MIXERS.~--The following are records of two jobs of pavement foundation work using continuous mixers with one-horse concrete carts in one instance and wheelbarrows in the other instance. The mixer used was the Foote mixer, as arranged for the work being described it is shown by Fig. 112. One particular advantage of this and similar mixers for street work is that no proportioning or measuring of the materials is required of the men. The mixers are provided with an automatic measuring device, by means of which any desired proportion of cement, sand and stone is delivered to the mixing trough. The mixer is mounted on trucks, and the hoppers that receive the sand and stone are comparatively low down. The sand can be wheeled in barrows up a run plank and dumped into a hopper on one side of the mixer, and in like manner the gravel or broken stone can be delivered into a hopper on the other side. The cement is delivered in bags or buckets to a man who dumps it into a cement hopper directly over the mixer. All that the operator needs to attend to is to see that the men keep the hoppers comparatively full. The records of work on the two jobs mentioned are as follows:
[Ill.u.s.tration: Fig: 113.--Briggs Cart Distributing Concrete for Pavement Foundation.]