Scientists develop stronger walls

Press, 11 October 1979, Page 13

Scientists develop stronger walls

Scientists from the . Ministry of Works and : Development have produced a new substance for achieving stronger masonry walls. The compound, the base of which is a fine aluminium powder, is mixed with I the concrete used to reini force hollow masonry blocks. This results in a more even distribution of rhe > material within the wall j cavity than plain concrete and produces a stronger | wall. ■ When available comI mercially, the material is I expected to cut construc- ! tion times and therefore . costs. > Its widespread use could I also allow an easing of ; the present stringent Min- . istry of Works specificai tions relating to the buildI ing of masonry walls. “Structural weakness often occurs in masonry walls because of the highly fluid nature of the infill concrete,” said Mr lan Boserio. a scientist working at the Ministry's . Central Laboratories in Lower Hutt. The solids in the fluid concrete tend to gravitate, leaving excess water to collect in pockets beneath

reinforcing steel and block webs. Also, the masonry absorbs water from the concrete core (known as grout) producing shrinkage, extensive cracking and preventing a good bond with the masonry. The wall loses its effectiveness structurally and is not a satisfactory weather barrier. This failure of the wall was demonstrated dramatically in the 1971 San Fernando earthquake. The masonry block faceshells fell off the walls leaving the grout core standing. Inspection revealed that this occurred because shrinkage of the infill grout resulted in a poor bond to the masonry. “These findings and the use of admixtures by the Californians led us to experiment with similar admixtures here, particularly as many parts of New Zealand have a similar earthquake risk to California,” said Mr Boserio. “Admixtures capable of creating this expansion are marketed by at least three companies in New Zealand but they are all primarily for use in injection grouts which have no coarse aggregate and are often neat cement only,” said Mr Boserio. “None is entirely suitable for the fluid concretetype grout used in masonry walls.” The admixture developed at Gracefield consists of aluminium powder which can be combined with other ingredients to form a paste which can be easily mixed with readymix. It produces a small grout expansion and tests at Gracefield on masonry block walls filled with concrete containing this substance show few signs of settlement, are largely free of cracks and voids, and have better bonds to blockwork than plain concrete.

The admixture may be added on site to ensure that expansion occurs only after the grout is in lhe wall. Temperature and the time of grout stiffening affect the amount of expansion that takes place. Therefore specifications are necessary relating to grout temperature, use of set accelerators and retarders, transit times and placing delays. “Using the admixture means that construction ■ times on site can be cut,” said Mr Boserio. Because of the present problems of settlement and water loss, Ministry architectural specifications for reinforced masonry require that wall sections do not ' rise more than 1.2 m at a . i time, the grout being i placed in stages, vibrated I and revibrated to ensure | even distribution of the ! concrete. After placing and vibrating each section the contractor must wait for the

grout to settle (15 to 60 minutes) before revibrating and proceeding with the next stage. This procedure may have to be repeated up to three times when filling a wall and even then a few undesirable cracks and voids may remain in the grout core. These requirements increase construction costs and demand very close supervision of the operation.

“Using a suitable admixture eliminates the need to reconsolidate the grout or to restrict heights of wall being grouted at any onetime,” said Mr Boserio. f The materials cost of the I admixture developed at ! Gracefield is relatively | cheap — about $1 for each cubic metre of grout — and although it is not yet available commercially several firms have expressed interest, in marketing the product.