Concrete Construction Part 29

[Ill.u.s.tration: Fig. 118.--"Edger" for Cement Sidewalk Work.]

~BONDING OF WEARING SURFACE AND BASE.~--Trouble in securing a perfect bond between the wearing surface and the base usually comes from one or more of the following causes: (1) Applying the surface after the base concrete has set. While several means are available for bonding fresh to old concrete as described in Chapter XXIV, the better practice is not to resort to them except in case of necessity but to follow so close with the surfacing that the base will not have had time to take initial set.

(2) Poor mixing and tamping of this base concrete. (3) Use of clayey gravel or an acc.u.mulation of dirt on the surface. In tamping clayey gravel the water flushes the clay to the surface and prevents the best bond. (4) Poor troweling, that is failure to press and work the mortar coat into the base concrete. Some contractors advocate tamping the mortar coat to obviate this danger. Conversely, to make the surface coat adhere firmly to the base it must be placed before the base concrete has set; the base concrete must be thoroughly cleaned or kept clean from surface dirt; the surface coat must be tamped or troweled forcibly into the base concrete so as to press out all air and the film of water which collects on top of the concrete base.

~PROTECTION OF WORK FROM SUN AND FROST.~--Sun and frost cause scaling and hair cracks. For work in freezing weather the water, sand and gravel should be heated or salt used to r.e.t.a.r.d freezing until the walk can be finished; it may then be protected from further action of the frost by covering it first with paper and then with a mattress of sawdust, shavings or sand and covering the whole with a tarpaulin. Methods of heating concrete materials and rules for compounding salt solutions are given in Chapter VII. The danger from sun arises from the too rapid drying out of the surface coating; the task then is to hold the moisture in the work until the mixture has completely hardened. Portable frames composed of tarpaulin stretched over 24-in. strips may be laid over the finished walk to protect it from the direct rays of the sun; these frames can be readily removed to permit sprinkling. Practice varies in the matter of sprinkling, but it is the safe practice in hot weather to sprinkle frequently for several days. Moisture is absolutely necessary to the perfect hardening of cement work and a surplus is always better than a scarcity. In California the common practise is to cover the cement walk, as soon as it has hardened, with earth which is left on for several days.

~CAUSE AND PREVENTION OF CRACKS.~--Cracks in cement walks are of two kinds, fractures caused by any one of several construction faults and which reach through the surface coating or through both surface and base, and hair cracks which are simply skin fractures. Large cracks are the result of constructive faults and one of the most common of these is poor foundation construction; other causes are poor mixing and tamping of the base, too large blocks for thickness of the work, failure to cut joints through work. Hair cracks are the result of flus.h.i.+ng the neat cement to the surface by excessive troweling or the use of too wet a mixture. The prevention of cracks obviously lies in seeing that the construction faults cited do not exist. If expansion joints are not provided, a long stretch of cement walk will expand on a hot day and bulge up at some point of weakness breaking the walk.

~COST OF CEMENT WALKS.~--The cost of cement walks is commonly estimated in cents per square foot, including the necessary excavation and the cinder or gravel foundation. The excavation usually costs about 13 cts. per cu. yd., and if the earth is loaded into wagons the loading costs another 10 cts. per cu. yd., wages being 15 cts. per hr. The cost of carting depends upon the length of haul, and may be estimated from data given in Chapter III. If the total cost of excavation is 27 cts. per cu.

yd., and if the excavation is 12 ins. deep, we have a cost of 1 ct. per sq. ft. for excavation alone. Usually the excavation is not so deep, and often the earth from the excavation can be sold for filling lots.

In estimating the quant.i.ty of cement required for walks, it is well to remember that 100 sq. ft. of walk 1 in. thick require practically 0.3 cu. yd. concrete. If the concrete base is 3 ins. thick, we have 0.3 3, or 0.9 cu. yd. per 100 sq. ft. of walk. And by using the tables in Chapter II we can estimate the quant.i.ty of cement required for any given mixture. In cement walk work the cement is commonly measured loose, so that a barrel can be a.s.sumed to hold 4.5 cu. ft. of cement. If the barrel is a.s.sumed to hold 4.5 cu. ft., it will take less than 1 bbl. of cement to make 1 cu. yd. of 1-3-6 concrete; hence it will not require more than 0.9 bbl. cement, 0.9 cu. yd. stone, and 0.45 cu. yd. sand per 100 sq. ft. of 3-in. concrete base. The 1-in. wearing coat made of 1-1 mortar requires about 3 bbls. of cement per cu. yd., if the barrel is a.s.sumed to hold 4.5 cu. ft., and since it takes 0.3 cu. yd. per 100 sq.

ft., 1 in. thick, we have 0.3 3, or 0.9 bbl. cement per 100 sq. ft.

for the top coat. This makes a total of 1.8 bbls. per 100 sq. ft., or 1 bbl. makes 55 sq. ft. of 4-in. walk.

As the average of a number of small jobs, the authors' records show the following costs per sq. ft. of 4-in. walk such as just described:

Cts. per sq. ft.

Excavating 8 ins. deep 0.65 Gravel for 4-in. foundation, at $1.00 per cu. yd. 1.20 0.018 bbl. cement, at $2.00 3.60 0.009 cu. yd. broken stone, at $1.50 1.35 0.006 cu. yd. sand, at $1.00 0.60 Labor making walk 1.60 ---- Total cents 9.00

This is 9 cts. per sq. ft. of finished walk. The gangs that built the walk were usually two masons at $2.50 each per 10-hr. day with two laborers at $1.50 each. Such a gang averaged 500 sq. ft. of walk per day.

~Cost at Toronto, Ont.~--Mr. C. H. Rust, City Engineer, Toronto, Ont., gives the following costs of constructing concrete sidewalks by day labor. The sidewalks have a 4-in. foundation of coa.r.s.e gravel or soft coal cinders, thoroughly consolidated by tamping or rolling, upon which is placed a 3-in. layer of concrete composed of 1 part Portland cement, 2 parts clean, sharp, coa.r.s.e sand, and 5 parts of approved furnace slag, broken stone or screened gravel. The wearing surface is 1 in. thick, or 1 part Portland cement, 1 part clean, sharp, coa.r.s.e sand, and 3 parts screened pea gravel, crushed granite, quartzite or hard limestone. Costs are given of a 6-ft. and a 4-ft. walk as follows:

COST OF 6 FT. SIDEWALK.

Per 100 Item. sq. ft.

Labor $ 5.59 Cement, 1.66 bbls., at $1.54 2.49 Gravel, 2.7 cu. yds., at $0.80 2.21 Sand, 0.46 cu. yd., at $0.80 0.37 Water 0.05 ---- Total $10.71

COST OF 4 FT. SIDEWALK.

Per 100 Item. sq. ft.

Labor $ 6.73 Cement, 2.04 bbls., at $1.54 3.15 Gravel, 2.06 cu. yds., at $0.80 1.65 Sand, 0.49 cu. yd., at $0.80 0.39 Water 0.07 ---- Total $11.99

The rates of wages and the number of men employed were as follows: 1 foreman, at $3.50 per day; 1 finisher, at 30 cts. per hour; 1 helper, at 22 cts. per hour; 15 laborers, at 20 cts. per hour.

~Cost at Quincy, Ma.s.s.~--The following costs are given by Mr. C. M.

Saville for constructing 695 sq. yds. of granolithic walk around the top of the Forbes Hill Reservoir embankment at Quincy, Ma.s.s. This walk was laid on a broken stone foundation 12 ins. thick; the concrete base was 4 ins. thick at the sides and 5 ins. thick at the center; the granolithic finish was 1 in. thick. The walk was 6 ft. wide and was laid in 6-ft.

sections, a steel plate being used to keep adjacent sections entirely separate. The average gang was 6 men and a team on the base and 2 masons and 1 tender on the finish. The average length of walk finished per day was 60 ft. The cost was as follows:

Stone Foundation: Per cu. yd. Per sq. ft.

Broken stone for 12-in. foundation $ 0.40 $0.015 Labor placing at 15 cts. per hour 1.50 0.056 ----- ------ Totals $ 1.90 $0.071

Concrete Base 4 ins. Thick:

1.22 bbls. cement per cu. yd. at $1.53 $ 1.87 $0.026 0.50 cu. yd. sand per cu. yd. at $1.02 0.51 0.007 0.84 cu. yd. stone per cu. yd. at $1.57 1.32 0.019 Labor (6 laborers, 1 team) 3.48 0.050 ---- ----- Total for 90 cu. yds. $ 7.18 $0.102

Granolithic Finish 1 in. Thick:

4 bbls. cement per cu. yd. at $1.53 $ 6.12 $0.019 0.8 cu. yd. sand at $1 0.80 0.002 Lampblack 0.29 0.001 Labor (2 masons, 1 helper) 6.36 0.016 ---- ----- Totals $13.57 $0.038

The two masons received $2.25 per day each and their helper $1.50 per day, and they averaged 360 sq. ft. per day, which made the cost 1-2/3 cts. per sq. ft. for labor laying granolithic finish. The cost of placing the foundation stone is very high and the cost of concrete base also runs unusually high, the reasons for these high costs are not evident.

~Cost at San Francisco.~--Mr. George P. Wetmore, of the contracting firm of Cus.h.i.+ng & Wetmore, San Francisco, gives the following figures relating to sidewalk work in that city. The foundations of cement walks in the residence district of San Francisco are 2 ins. thick, made of 1-2-6 concrete, the stone not exceeding 1 in. in size. The wearing coat is in. thick, made of 1 part cement to 1 part screened beach gravel.

The cement is measured loose, 4.7 cu. ft. per barrel. The foundation is usually laid in sections 10 ft. long; the width of sidewalks is usually 15 ft. The top coat is placed immediately, leveled with a straight edge and gone over with trowels till fairly smooth. After the initial set and first troweling, it is left until quite stiff, when it is troweled again and polished--a process called "hard finis.h.i.+ng." The hard finish makes the surface less slippery. The surface is then covered with sand, and watered each day for 8 or 10 days. The contract price is 9 to 10 cts.

per sq. ft. for a 3-in. walk; 12 to 14 cts. for a 4-in. walk having a wearing coat to 1-in. thick. A gang of 3 or 4 men averages 150 to 175 sq. ft. per man per day of 9 hrs. Prices and wages are as follows:

Cement, per bbl. $2.50 Crushed rock, per cu. yd. 1.75 Gravel and sand for foundation, per cu. yd. 1.40 Gravel for top finish, per cu. yd. 1.75 Finisher wages, best, per hr. 0.40 Finisher helper, best, per hr. 0.25 Laborer, best, per hr. 0.20

~Cost in Iowa.~--Mr. L. L. Bingham sent out letters to a large number of sidewalk contractors in Iowa asking for data of cost. The following was the average cost per square foot as given in the replies:

Cts. per sq. ft.

Cement, at $2 per bbl. 3.6 Sand and gravel 1.5 Labor, at $2.30 per day (average) 2.2 Incidentals, estimated 0.7 --- Total per sq. ft 8.0

This applies to a walk 4 ins. thick, and includes grading in some cases, while in other cases it does not. Mr. Bingham writes that in this respect the replies were unsatisfactory. He also says that the average wages paid were $2.30 per man per day. It will be noted that a barrel of cement makes 55 sq. ft. of walk, or it takes 1.8 bbls. per 100 sq. ft.

The average contract price for a 4-in. walk was 11 cts. per sq. ft.

CONCRETE PAVEMENT.

Concrete pavement is constructed in all essential respects like cement sidewalk. The sub-soil is crowned and rolled hard, then drains are placed under the curbs; if necessary to secure good drainage a sub-base of gravel, cinders or broken stone 4 to 8 ins. thick is laid and compacted by rolling. The foundation being thus prepared a base of concrete 4 to 5 ins. thick is laid and on this a wearing surface 2 to 3 ins. thick. As showing specific practice we give the construction in two cities which have used concrete pavement extensively.

~Windsor, Ontario.~--The street is first excavated to the proper grade and crown and rolled with a 15-ton roller. Tile drains are then placed directly under the curb line and a 616-in. curb is constructed, vising 1-2-4 concrete faced with 1-2 mortar. Including the 3-in. tile drain this curb costs the city by contract 38 cts. per lin. ft. The pavement is then constructed between finished curbs, as shown by Fig. 119.

[Ill.u.s.tration: Fig. 119.--Concrete Pavement. Windsor, Canada.]

The fine profile of the sub-grade is obtained by stretching strings from curb to curb, measuring down the required depth and tr.i.m.m.i.n.g off the excess material. The concrete base is then laid 4 ins. thick. A 1-3-7 Portland cement concrete is used, the broken stone ranging from in. to 3 ins. in size, and it is well tamped. This concrete is mixed by hand and as each batch is placed the wearing surface is put on and finished.

The two layers are placed within 10 minutes of each other, the purpose being to secure a monolithic or one-piece slab. The top layer consists of 2 ins. of 1-2-4 Portland cement and screened gravel, in. to 1 in., concrete. This layer is put on rather wet, floated with a wooden float and troweled with a steel trowel while still wet. Some 20,500 sq. yds.

of this construction have been used and cost the city by contract:

Per sq. yd.

Bottom 4-in. layer 1-3-7 concrete $0.57 Top 2-in. layer 1-2-4 concrete 0.32 Excavation 0.10 ----- Total $0.99

This construction was varied on other streets for the purpose of experiment. In one case a 4-in. base of 1-3-7 stone concrete was covered with 2 ins. of 1-2-2 gravel concrete. In other cases the construction was: 4-in. base of 1-3-7 stone concrete; 1-in. middle layer of 1-2-4 gravel concrete, and -in. top layer of 1-2 sand mortar. All these constructions have been satisfactory; the pavement is not slippery. The cost to the city by contract for the three-layer construction has in two cases been as follows:

Church St., 8,000 sq. yds.: Per sq. yd.

4-in. base 1-3-7 concrete $0.57 1-in. 1-2-4 and -in 1-2 mixture 0.32 Excavation 0.10 ----- Total $0.99

Albert and Wyandotte Sts., 400 sq. yds.: Per sq. yd.

4-in. base 1-3-7 concrete $0.66 1-in. 1-2-4 and -in. 1-2 mixture 0.39 Excavation 0.10 ----- Total $1.15

The cost of materials and rates of wages were about as follows:

Portland cement f. o. b. cars Windsor, per bbl. $2.05 River sand, per cu. yd. 1.15 River gravel, screened, per cu. yd. 1.25 Crushed limestone, to 3 ins., per ton 1.15 Labor, per day 1.75 to 2.00

At these prevailing prices the contractor got a fair profit at the contract price of $1.15; at 99 cts., any profit is questionable, according to City Engineer George S. Hanes, who gives us the above records. Expansion joints are located from 20 to 80 ft. apart and are filled with tar.