BONDING OLD AND NEW CONCBETE 



135 



days after its application, while floors should be kept covered with wet 

 bags or moist sand, for a like period after surfacing. 



The most serious objection to plastering is that proper adhesion 

 of the plaster to the base concrete is not always secured. A method 

 of facing which overcomes this objection consists of " spading " between 

 the concrete and the form, immediately after the concrete has been 

 deposited. The spade is passed down between the concrete and the 

 form, the object being to press the stones back and thus allow the 

 cement and sand to flow towards the form and produce a dense, smooth 

 face. Care should be exercised, while spading, not to prise the form 

 out of place. A hardwood spade, as shown in Fig. 174, may be made 

 for the purpose, though an ordinary spade will suffice, a narrow one 

 being preferable. 



Various washes have been tried from time to time with the object 

 of rendering concrete water-tight. One which has proved successful 

 in some cases consists of 1 pound concentrated lye, 2 to 5 pounds alum, 

 and 2 gallons water. 



In calculating the approximate quantities of materials required for 

 plaster, the following table may be used : 



Materials foe 1 Cubic Yard compact, Plastic Mortar, made with Fine Sand. 



Note. — The above barrel contains S7G lbs. nett of cement. 



An example of the use of the above table will be found in 

 Chapter XXVI., pages 282-3. 



Bonding Old and New Concrete. — In the case of thin walls (for instance 

 those of a water-tank or dipping-tank, which have to be water-tight, or 

 those of a silo) the surface of the concrete laid on the previous day should 

 be washed perfectly clean, and then coated with a thin layer of either 

 cream of neat cement, or one part cement to one part sand in the form 

 of grout, immediately before the next layer of concrete is deposited. 

 Sometimes, instead of grouting as above, a little cement in its dry state 



