Best practices in precast concrete construction is illustrated in this video from Wilco Precast, Papkura, Auckland.
The Concrete Construction Association of New Zealand has produced a short introductory video on precast panel production. Filmed on-site, the video examines how quality precast concrete panels are manufactured for the domestic market. Appropriate concrete construction techniques including precast concrete panel release agent application, as well as health and safety practices, are covered in the short introductory video.
A transscription of the video, "My name's Alistair Russell. I'm from the Cement and Concrete Association in New Zealand, and we're here at Wilco Precast in Papakura in Auckland. Today we're gonna be seeing best practice of concrete production within a precast factory. We're gonna be seeing concrete being delivered, placed, and manufactured into precast panels. This is a demonstration of best practice of concrete in the New Zealand concrete industry.
The first thing that happens in the construction site is the delivery comes in. We've got the reinforcing being delivered off a truck. It needs to be deposited in the correct location. When the longitudinal reinforcement is first laid out, and usually a panel is longer than the bars themselves, those bars need to be spliced together. If the shop drawings show that the splice needs to be a meter and a half, then that splice needs to be a meter and a half. The splice needs to be tied together tightly. It needs to be a contact splice. The two long bars need to be in contact together as shown in the video and then tied to the transverse bars going in the orthogonal direction.
Once the reinforcement's laid out on the ground, the transverse reinforcement and the longitudinal reinforcement in the other direction needs to be assembled together. The way that's done is it's tied together with these thin wire ties. This can be done manually with a pair of wire clippers as you can see here, or it can be done with an automatic machine, which puts a tie in place and automatically ties the knot at the end.
The grade of the reinforcement is often written down on the reinforcement bar itself. This is SEISMIC 500E. If it's E, it also means "SEISMIC." There's also a generic bar marking shown on here. You've got a gap and then next to it, you've got the two lines. This designates this bar as being a grade 500 bar. In addition to this, reinforcement often shows the type of manufacture that was used in making the bar. This MA, which means it's a microalloyed bar. Grade 500 reinforcement, you really need to know the type of manufacture that was used because this affects what you do with reinforcement, if you can weld it, or the way you handle it, as well. In addition to this, it should be shown on the docket when it's delivered what the type of reinforcement is, but if that gets lost or you don't know where the docket is, this is what you're looking for, which tells you how you can handle the reinforcement on the site.
They consider this bar here as a grade 500 bar, even if we can't read what it is. We know that it's got that marking. I've got this handy bendometer, which tells me how tight I can bend the radius of this bar. I put the inside part of the bendometer on the bar. I know it's a 16 millimeter bar and then the outside of the bendometer is how tight I can bend the inside of the bend radius. It also says it's a 16 millimeter bar but the bendometer allows me to see what the diameter of the bar is, and therefore what the allowable bend radius is. If the bend is tighter than this, then we actually get buildup of strains on the inside and you can get a fracture or a bar rupture at that point there. It's very important to observe the bar radius and then minimum bend.
Whilst this is also grade 500 reinforcement, we can see by the marking and it says "grade 500" here, this is a very tight radius. This is tighter that what we're allowed to have. Using the bendometer it's a 16 millimeter bar, 16 millimeter. But the bend of the radius at the end of this bar is much tighter than that. The only place that you're allowed to have such a tight bend is at the end of the bar when it forms a stirrup. This is a stirrup at the end of the bar. At other parts of the bar, you need to obey the limitations of the bendometer. Now, normally, reinforcement comes onto the site pre-bent from the manufacturer, from the supplier. But it's your responsibility when you receive the reinforcement onsite to make sure that it complies with the minimum bend radius as shown on the bendometer. In this case we can see it's too tight, but because it's a stirrup at the end of the bar it's okay to have such a tight bend.
Normally, long bars are made of grade 500 reinforcement, and that says on it "grade 500" with the appropriate markings. In this case, you've got grade 300E, so this identifies this as a grade 300 bar. Grade 300's usually used for the stirrups, and it doesn't have quite the same requirements for the bending and the handling that grade 500 reinforcement does.
When the formwork is constructed, it's very important that it's well-supported from the outside. In this case, we've got a nice, clean timber surface on the inside. You can see the nice, clean timber surface here. Even though this is only gonna be about a 200 millimeter deep panel, not a very deep panel, the pressure, even from 200 millimeters, is enough to bow out the timber, bow out the formworks. It's very important that it's effectively supported from behind.
Prior to placing the concrete, formwork has a concrete release agent applied to it. This functions as a release between the formwork and the concrete. When it comes time to remove formwork from the concrete, we will take the panels out of the molds, which is usually in the next day or so. The two can separated without sticking.
Once the formwork's been assembled and covered with the release agent, the reinforcement cage is placed. It's deposited with the chairs pre-attached to the reinforcement cage to make sure the reinforcements are the right depth, providing the correct cover to the reinforcement. These are the chairs that we've been talking about. This holds the reinforcement in place. It holds it at the right depth away from the edge. This is what assures the concrete has the correct cover.
Once the cage is put in place, the final detailing is done. The final tying is done to make sure that the cage stays in the right place. Then the concrete can be placed afterwards. Concrete for precast work is delivered in generally the same way as any other concrete site in a Ready Mix concrete truck. When the truck arrives at the site, in this case it's inside the precast plant, it needs to do a final mix to eliminate any small amount of segregation.
It's a good idea to put down a small piece of board to break up the initial fall of the concrete, and also to stop any impact onto the reinforcement cage. Generally, you should start pouring the concrete into the furthest point away from the truck or the chute, and then move progressively backwards. This means starting in the far corners and filling up the mold from that area back towards the truck.
Once the concrete's been poured into the molds and spread around to fill up all the spaces, it needs to be compacted. Usually, this is done with an immersion vibrator and it's important that this reaches to all the concrete and that no points are left unvibrated. The compaction should be done systematically, moving throughout the freshly laid concrete in an organized sequence and not randomly. Each area of concrete should be vibrated around about the same amount. The vibration device shouldn't be left in any place for too long.
Once the vibration of the compaction has taken place, the initial spreading should happen immediately. This spreads the concrete out and is the first pass at leveling the concrete surface. After spreading has achieved a level surface, floating is the next process. This can be done by hand or using a bull float.
Throughout the placement of concrete, there must be someone on hand to observe and record the quality of the placement and to tick off all the necessary steps in the process. Once the concrete's firm enough to walk on, you can use a machine-operated power float to work the top layer of the concrete to achieve a hard surface.
In a precast yard, the panels are usually lifted within about 24 hours after they've been cast. This panel would have been cast yesterday. You have the cast in the lifting lugs or lifting eyes, which is what the crane attaches to, and then this panel will be lifted up, removed, taken to the back of the yard, and then stored until it's delivered to the construction site.
Once the formwork's been stripped from the panels, they've been lifted, transported, and brought out into the back of this yard, in this case they're sitting here ready for delivery to the construction site to be picked up by a truck and taken out to that particular construction site.
CCANZ would like to thank Wilco Precast for their kind permission in letting us use their facility to show this practice in concrete construction."
About Concrete Construction Association of New Zealand
Vision: To be the leading organisation developing and defending the market for concrete, and positioning concrete as the sustainable material of choice for the built environment.
Mission: Develop and defend the market for concrete by:
Delivering successful industry solutions based on technical expertise.
Providing effective representation to Government, regulators and other stakeholders.
Co-ordinating education, training and research initiatives in concrete related areas.
- Integrity: Integrity grows over time and earns CCANZ the trust of its Members and stakeholders. Integrity builds credibility. CCANZ acts expediently and professionally, and believes in honesty and treating others with respect.
- Inspiration: CCANZ is passionate about concrete, and has the expertise and enthusiasm to inspire the wider and more innovative use of concrete in the built environment.
- Team: CCANZ recognizes that cooperation with stakeholders is vital in the promotion of all things concrete.
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