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Engineer Edgar

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Site visit 2 - Boat ramp

Created 28/08/2009 at 7:09:56
Modified 28/08/2009 at 9:59:53

Site Visit 2 - Boat Ramp

Keywords: durability, stainless steel, concrete strength, Xypex, cold joint, lifting insert,

This photo is of a precast boat ramp destined for use at Fremantle.

Boat ramp - 2204
(click to enlarge)

 

Some issues became apparent with the boat ramp profile only after the ramps were cast; when the lifting clutch was engaged with the lifting insert, the edge of the raised profile was damaged.

Damaged ramp - 2205
(click to enlarge)

 

Also of concern to the client is the steep side of the profile which is contrary to specifications. The form liners were manufactured off site and purchased by the precast company. The concrete ramps are able to be removed from the form; the steeper sides are not impacting on removal, however, the client is not satisfied at this stage and may not accept the product.  The NPCAA handbook suggests a 1:12 side form slope to prevent sticking.

The form liners are seen in this image in place in the formwork.

Form liners - 2212
(click to enlarge)

 

The side forms are steel and held in place by magnets for speed of assembly of the form. Magnets also offer the advantage of leaving no unsightly weld indent on the surface of the precast elements as seen here

Magnet - 2213
(click to enlarge)
No weld indent - 2214
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Note the stainless steel lifting inserts (welded to the mesh to prevent them lifting, or side form) which were specified due to the aggressive environment; with reference to AS 3600 the boat ramps would be environmental condition C, requiring min 50 mm cover and 50MPa concrete strength. The actual used was 80 mm cover and the concrete contained a chemical admixture, Xypex, which is said to reduce the water absorption of concrete thus preventing the ingress of chloride ions to the reinforcing and delaying the onset of corrosion in the reo.  The Xypex web site claims: Xypex is a non-toxic, chemical treatment & additive for the waterproofing, durability enhancement, repair and protection of concrete from a range of aggressive mediums. Xypex is renowned for its unique ability to generate a non-soluble crystalline formation deep within the pores and capillary tracts of concrete - a crystalline structure that permanently seals the concrete against the penetration of water and other liquids from any direction. http://www.xypex.com.au/ Independent research has been conducted on Xypex modified concrete, one such report stated in the conclusions: "In summary, three types of concretes modified with PRA were found to have significantly improved properties compared to control concretes. Whilst drying shrinkage and sulphate expansion were reduced, chloride penetration and diffusion properties were the most significantly improved by inclusion of the PRA in concretes"
http://www.xypex.com.au/assets/test_reports/chloride_resistance/Paper_Presented_by_Bob_Munn_Concrete05_Melbourne.pdf

 

The use of high quality concrete was further reinforced in precast elements being manufactured for Fremantle Berth 10 2240 . Specifications for the concrete curing included covering in wet hessian, watering twice daily and min compressive strength of 60MPa. The linkage of the concrete production to the precast production (ownership and sites are adjacent) offers advantages regarding quality control of the concrete, delivery of the material and batching control. Concrete can be delivered in up to 7m3 trucks and placed by 1m3 kibble. Cover to the mesh was 50 mm top and bottom and stainless steel fittings (lifters and dowel bars) were used. The weld was specified and weld checks performed to satisfy spec. Of concern with the wharf elements was the formation of cold joint as the elements were cast in two stages. What would the issue of cold joint be? Need more information and according to:
http://www.toolbase.org/ToolbaseResources/level4FAQ.aspx?ContentDetailID=1087&BucketID=3&CategoryID=33

Question:What is a cold joint in concrete construction?

Answer: A cold joint is the intersection between the end of one concrete pour and the beginning of a new pour. The basic rule is to try to avoid cold joints by pouring straight through until the job is finished. The cold joint is a weak area and could allow the entry of water. If it must be done, inserting reinforcing bars in the fresh concrete of the old pour will tie in the new pour more effectively.

If the client were to consider rejecting the elements because of the cold joints they should do so based on an engineering evaluation of the product e.g. testing if the cold joint actually compromises the strength of the product. An interesting article I found described the difference between a cold joint and pour line and the structural implications of each. It gives techniques for determining if the cold joint is affecting the structural performance: " Visual examination can determine how far the line extends beyond the surface. Microscopic examination can identify evidence of carbonation, drying, or changes in paste microstructure--all conditions that indicate a possible cold joint. Another step in core evaluation consists of performing split-cylinder tests in accordance with ASTM C 496-96 (Ref. 3). Cores must be tested with the pour line or cold-joint line oriented vertically at the center of the test machine. A statistical evaluation of the tensile test values is then compared with the expected strength of monolithic concrete. If the value falls below the expected strength, structural implications should be investigated." Voltz (1997) http://www.concreteconstruction.net/industry-news.asp?sectionID=718&articleID=245517C

 

Site Visit 1 - Surface finishes

Created 27/08/2009 at 15:51:48
Modified 28/08/2009 at 10:08:15

Site Visit 1 - Surface finishes

Key words: surface finish, aggregates, sand blast, oxides, AS3610

On our site visits we have had first hand experience of a number of surface finishes of precast elements and the application of surface treatments. Specified surface finish must relate to the product end use, and the cost of the product.  Surface finish is specified in AS 3610-1995 in particular references to note are Table 3.3.1 and Table 3.4.2. (extract of AS3610-1995 Table 3.3.1 definition of class of concrete surface included below) However, this only applies to formed surfaces e.g. the surface in contact with the form or form liner.


Table 3.3.1

 

A useful reference for the selection of surface finishes is the NPCAA guide “Precast Concrete A Selection Guide to Surface Finishes”  and in part states “Mix design, careful selection of materials and the need for practical and innovative mould design are of vital importance in achieving pleasing results for the architect, satisfactory structural solutions and profitable result for the precaster.”
http://www.npcaa.com.au/images//file/Surf_Fin.pdf

A number of technical notes on the preparation of different surface treatments are available form NPCAA such as metal oxide coloring which we saw in the new Performing Arts Centre panels
http://www.npcaa.com.au/images//file/Data2.pdf

 

Images and descriptions:

1.    Electrical pits class 3 buried in ground, not visible, poker vibration used, tack weld cleats for side forms leave visible marks, fairly rough but appropriate

Electrical pits class 3 - 2213


2.    Class 1 finish specified but element rejected due to air holes in side of element although the top face was OK,

Reject air holes - 2226
(click to enlarge)

 

3.    Roughened finish on panels to enable bond of topping screed to precast element; video of raking and photo of precast bridge beam surface video.

Raking - movie 2279
(click to play movie)

 

4.    Class 1 finish on precast element such as stairs.

Class 1 finish - 2289
(click to enlarge)

 

5.    Sand  blasted finish on bridge parapets; production includes stock piling of aggregates for color consistency for each job 2342

Sand Blast Finish - 2342
(click to enlarge)

 

6.    Good class finish, probably class 1, not really needed for conveyor belt sleepers but economics of production meant steel mould manufactured as reuse for  1500+ sleepers 2350

Good finish - 2350
(click to enlarge)