PROTECTION OF CONCRETE

Poverty Bay Herald, Volume LXIV, Issue 19373, 10 July 1937, Page 13

PROTECTION OF CONCRETE

THE ACTION OF MILK SAFEGUARDS PROPOSED FREEDOM FROM CRACKS In order to protect concrete in dairy sheds from the disintegrating action of milk the concrete should be as dense and as free from cracks as possible. This advice is given by the Public Works Department in a memorandum prepared at the request of the Department of Agriculture.

The memorandum states that milk contains lactic acid, lactose and fatty materials. Where there is free access of oxygen lactic acid is easily des-'-m-od and replaced by its final products, carbonic acid’and water. Carbonic acid is very deleterious. It goes beyond the slow transformation of Noe lime into carbonate of lime,

vhioh contributes 1o the hardening of concrete structures, and gives rise ‘o soluble bicarbonates with washing out and decrease of strength. A Protective Film In its own original form lactic acid even in dilute concentrations acts on the aluminates and silicates of cement. Where this happens, without the masking action of humic acid, the surface becomes glossy, and later the formation of lactates in crystalline form leads to the crack of! of small splinters or, at least, to an increase of porosity and rapid action tq the interior layers. It seems that the disintegrating process is checked after a certain time by the formation of a protective crystallised film, but if the action reaches the reinforcement corrosion and increase in volume provoke cracking and scaling. It is probable that lactose acts in the same way as ordinary sugar, replacing the calcium and so disintegrating the cement product. Fatty acids act on cement in the same way as mineral acids, but to a lesser extent.

To Check Disintegration

In order to combat these disintc grating processes, the first aim should be to produce a concrete as dense and as free from cracks as possible. To give these results the following points should be noted: —

1. The concrete must be kept wet during its setting time, say for one week if possible. It shall be protected from sun or frost.

2. If possible, mesh reinforcing should be used. This will distribute ‘he cracks evenly, so that none arc big enough to admit milk. 3. Unduly rich mixtures tend to shrink more than leaner mixes and consequently require more care Quick setting cement has .the same characteristic. 4. Vibration of concrete during placing gives a very dense mass and freedom from cracks as a rule.

5. Well graded mixes will usually be less liable to cracking than poorly graded ones, and certainly will disintegrate less readily. Lactic Acid Effects The following are some methods of combating the effect of lactic acid on concrete: — 1. Coating with bitumen. The concrete should first be painted with a coat of bitumen thinned with turps or benzine, and after 24 hours covered with a coat of hot bitumen of high viscosity. As an alternative a bituminous grout, such as Coldfix applied periodically after cleaning, say every six weeks, should preserve the surface.

2. Aluminous (quick-setting) cement.—The use of aluminous cements in mixing the concrete should prove effective to a large extent in extending the life of the concrete. Aluminous cements should not be confused with rapid hardening Portland cement (Wilsonite).

3. A surface coat of rapid hardening Portland cement (Wilsonite) in the proportion of 37 to 42 per cent of cement to (13 to G 8 per cent of water will reduce the porosity and give i fair protection against acid. 4. Impregnating the surface with silica, obtained by dialysing a dilute solution of silicate of soda or potash with hydrochloric acid has been found to give a stable coating. The silica should be applied under pressure and immediately after manufacture. This method sounds costly and may not be practicable in many cases.

5. Aluminium fluate.— Good results have been obtained by the application of aluminium fluate preceded by sodium silicate whereby sodaalumina fluoride and hydrate of silicate are formed. This process is limited to cases where slow hardening cement is used. (1. A method which suggests itself but which does not appear to have been tried is to coat the surface first with a solution of calcium chloride and then a solution of sodium silicate. These would combine to form sodium chloride, which will be washed out, leaving calcium silicate as a hard coat. 7. Choice of aggregate—The choice of aggregate is important. Absence of biotite and feldspar is desirable (i.e. granite or basalt is unsuitable); greywache is insert, and should be satisfactory; quartz is satisfactory, and limestone ought to resist lactic acid well.

■ In the construction of troughs glazed circular tiles, drain pipes cut in half along a diameter, are often used, and when sealed with bitumen they last fairly well. It is always advisable, when constructing concrete flooring to give it a slope sufficient to ensure that liquids will drain oil'.