New Zealand Timbers-Concrete

Progress, Volume IX, Issue 4, 1 December 1913, Page 835

New Zealand Timbers-Concrete

The monthly meeting of the Technological sectmn- of the Wellington Philosophical Society was held in the Dominion Museum on October 8 Mr.

11. W. Holmes, M.lnst.C.E., presiding, and thirty members were present. The first business was the dismission of the paper read by Mr. J. Orchiston, M.1.E.E., at the previous meeting on the durability

NEW ZEALAND TIMBERS Mr. D. K. Blair, M.lnst.M.E., speaking for an ■English visitor, remarked on the complete reversal oi usual ideas brought about by the experience of Jew Zealand engineers with colonial timbers. The tacts that charring’ the butts of piles proved detrimental to their life, and that green timber lasted longer in the ground than seasoned were staggering and he would ask Mr. Orchiston if he had correctly understood him on these points, also whether he had found the bark to act as a preservative on piles under water. Mr. R. W. Holmes said his experience had corroborated Mr. Orchiston on these points. Bridges culverts and fences erected with green timber had given good results, and the old fashioned farm fence of green posts and rails split off the quarter . built with the feather edge upwards (thus offering no openings for rain on the upper faces) had given wonderful service. He would ask whether the usual belief that posts placed in the ground in a reverse position to their' growth gave better durability had been justified; also whether the totara suffering from “kaikaka” had been found more durable, as alleged by the Maoris. Ml. Holmes added some interesting and valuable notes on the general qualities of Dominion timbers which will be printed later on. Mr. Orchiston replied that he had had no experience of piles with the bark left on, but as the sap-wood always rots quickly he preferred to use heart timber. Neither had he any experience of posts placed upside down, as telegraph poles were naturaliy placed with the large ends downwards. With regard to the “kaikaka” disease, he had found _ that totara suffered from two varieties of this; in the one type the holes were free from dust and no ill effects were observed in use, in the other dust was present and the timber had no durability. He could definitely assert that for piles and posts unseasoned timber was best, charring the butts was Harmful, and damp heavy clay proved the best ground.

CONCRETE Mr. J. Marchbanks, M.lnst.C.E., then read a paper entitled “Notes on Concrete Works in Wellington Harbour.” He stated that the Wellington Harbour Board has already executed a large amount of concrete work and the scarcity of timber will necessitate more in the future. Practically none but colonial cement has been used lately, although its use is not insisted on in the specifications, and in general it easily conforms to the requirements. The tests of the British Standard Specifications are always made, and in addition a very useful boiling test for constancy of volume is carried out. This most reliable test consists of subjecting a pat Sin. diameter by about fin. thick, which has been &et in moist air for 24 hours, to boiling for 6 hours; this it must stand without cracking or distortion. A

chemical analysis of New Zealand cement recently showed:—

The average tensile strength of three different brands of New Zealand cements ran as follows 7 days ... 490 to 550 lbs. per so. in. 28 days ... 650 to 710 lbs. per sq. in. 3 months ... 725 to 760 lbs. per sq. in. 6 months ... 725 to 780 lbs. per sq. in, 2 years ... _ 695 to 800 lbs. per sq. in. In one case the tensile strength at 2 years was 65 lbs. per sq. in. lower than at 6 months.

These tests show that rotary kiln cement manufactured in New Zealand is quite as suitable for making concrete as British cement is. The bulk of the concrete in the sea walls is made with Hutt River gravel, and deposits better in sea water than that made from graded mixtures of stone screenings and sand, or clean gravel and sand. The requirement in under-sea water work is a dense impervious concrete, and this requires an excess of cement and sand to fill the voids, which' of course weakens the concrete. Thus compression tests showed that concrete made with Hutt River natural shingle gave the following results:— 1 in 6 mixture crushed at 9171b5. per ,sq. in. 1 in 7 mixture crushed at 9001bs. per sq. in. 1 in 8 mixture crushed at 5731b5. per sq, in. 1 in 9 mixture crushed at 4021b5. per sq. in. while concrete made from a mixture of two parts crushed screenings and one part sand gave results just double these for the different gradings. The greater proportion of the concrete used in the sea walls has been deposited below water level without the use of cofferdams. This requires constant care. A pile staging is first constructed on the line of the proposed wall, and the space between the piles filled in with leaves made of 2in. boards nailed to longitudinals of about 12in. x 3in. spaced at 12in. to 24in. centres. The ground is thoroughly cleaned by divers using a suction pipe and the bottom openings carefully closed where necessary. As the concrete is deposited the sea water forms a white powdery deposit called “laitance” over the exposed surface, hence it is advisable to deposit at one end until the required height is reached and then work along the section, thus keeping the surface exposed small. This “laitance’ has no cementing qualities, and must be brushed, away wherever it accumulates on the surface, particularly in pockets and depressions. As the work progresses the skip holding the fresh batch dips well into the soft concrete, and with bottom delivery the new surface exposed is small, and much less trouble occurs. Depositing through a 4in. pipe has been used on pile work. The bottom of the pipe is kept buried about 3 feet under the surface of the rising mass and very good results follow, as the surface exposed is small and the fresh

concrete is deposited under it. But this method is not suitable for long walls. The late Mr. W. R. Kinniple discovered an interesting method of reducing “laitance.” This consisted of allowing the concrete to partially set for 4 to 8 hours, after which it is remixed with fresh water and deposited in the usual way. The method has not been used in Wellington Harbour, but experiments made showed that it practically eliminates laitance, and has very little, if any, deleterious effect on the strength of the concrete. It is, of course, a difficult process to handle on a large work. Reinforced concrete has not been greatly used so far, but must be largely used in the future. The Clyde Quay wharf has proved very satisfactory in this material, and a few difficulties met with in construction can be obviated in future by allowing more room between the reinforcing rods for the concrete to flow. The piles 18in. square reinforced with four lin. rods drove much better than those with eight l£in. rods, and some of them received as many as 150 blows per foot from a 3-ton monkey falling 6 feet on to a hardwood dolly. A new breastwork wharf is being started in this material beyond the King’s Wharf. It is to be 1,200 feet long by 36 feet wide, calculated for a load of scwts. per square foot. The concrete is allowed to be stressed to 6001bs. per sq. in., and the steel lip to 16,0001b5. in tension and 12,0001b5. in shear. .The beams are calculated for a bending moment of WL/10 for live loads, and WL/24 and WL/12 for dead loads at the centre and over the supports respectively. Plans are also well advanced for a large wharf for ocean going steamers. .Mr. J. Campbell, Government Architect, admitted that concrete will surely displace brick in future in architectural work. Its drawback is the difficulty of using its great strength economically without violating public taste, which has been educated to demand aesthetic treatment and substantial appearance. A surface covering of marble or brick was only begging the question, but progress had been made by using an aggregate composed of stones of pleasing colour and rubbing off the cement on the surface till these were exposed. An undetermined danger lay in the question whether cement was not injurious to many building stones. Professor Easterj&eld gave an exceedingly interesting and valuable discourse on the chemical theory underlying the setting of cement, and explained how this had led him to prescribe a remedy for a difficulty met with in moulding concrete pipes, which are generally porous to an excessive degree. His remedy, and it had given entire satisfaction, lay in placing the pipes in water immediately after pouring _ and while still soft. He explained also how laitance was due to the chemical action of the magnesium chloride always present in sea water which attacked the lime in the cement, forming calcium chloride and free magnesia which lay on the surface and destroyed the binding power of the cement. His remarks were attentively listened to and heartily applauded, and it is hoped that he will take a further opportunity of interesting and instructing the section on this subject.

Lime ... 64.6 Silica ... 24.3 Alumina ... 5.4 Oxide of Iron (Fe 2 0 3 ) ... 2.0 Magnesia ... 0.6 Sulphuric anhydride ... 1.4 Insoluble ... ... 0.9 Water and alkalies ... 0.7 Specific gravity ... 3.12