Home > PB21B-3R Spider Concrete Placing Boom — 21 m

PB21B-3R Spider Concrete Placing Boom — 21 m

The PB21B-3R is the larger of the two TRUEMAX spider concrete placing booms. It stands on the same free-standing inclined-outrigger base as the compact 17 m model, but reaches 21 m — covering a noticeably bigger bay from each set-up, so a wide floor is poured in fewer positions and with fewer crane moves. The longer boom is turned by a gear-slewing drive rather than the worm gear of the smaller spider, suited to the extra mass and reach. As with every spider boom, it is not fixed to the structure: it is set down on its legs, pours its area, then is lifted by crane to the next.

That makes the PB21B-3R the spider to choose when reach matters more than minimum weight — large slabs, podiums and rafts where a 17 m boom would need more relocations to cover the same floor. It is fully hydraulic with a three-arm R-fold boom and 360° slewing, fed by a separate concrete pump rather than pumping itself, and run by radio or cable remote. At 9,150 kg with a heaviest single lift of 5,850 kg it needs a more capable crane to relocate than the compact model, a fair trade for the wider coverage. It runs on a 380V/50Hz supply with a 7.5 kW motor, built and tested at TRUEMAX in Haining to ISO 9001:2015 and CE standards, within a line exported to over 120 countries.

1. 21 m reach for bigger bays — The larger spider covers more floor from each set-up than the 17 m model, cutting the number of positions and crane moves needed to pour a wide slab.

2. Gear-slewing drive — A gear-slewing turret rotates the longer, heavier boom through a full 360° — matched to the extra reach and mass of this size.

3. Inclined-outrigger spider base — The splayed-leg base gives a stable, fixed working stance on the slab without anchoring into the structure, then lifts free for relocation.

4. Three-arm R-fold hydraulic boom — Three fully-hydraulic R-fold arms open to the 21 m radius and fold compactly for crane handling and transport.

5. Relocated by site crane — The boom is craned between positions; at a 5,850 kg maximum lifting unit it suits projects with an adequately sized tower or mobile crane.

6. Radio and cable remote control — Wireless radio and cable remotes let one operator place concrete across the wide bay from the best vantage point.

7. Full 360° coverage — Complete rotation places concrete all around the set-up position, maximising the area each stance covers.

8. Quality components — Hydraulic and electrical parts from well-known international brands for stable, reliable operation, on a 380V/50Hz, 7.5 kW drive.

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Technial Parameters

Item	Unit	PB21B-3R
Performance
Max. Radius of Placing Boom	m	21
Free-Standing Height (to boom root joint)	m	3
Slewing Range	/	360°
Mode of Slewing	/	Gear Slewing
Application Scenario	/	Mobile (Spider)
Circumstance Temperature	℃	-20 ~ 55
Power Supply (customisable)	/	380V / 50Hz
Boom (3-Section R-Fold)
1st Section — Length	m	8.4
1st Section — Articulation	°	0 ~ 90
2nd Section — Length	m	6.9
2nd Section — Articulation	°	0 ~ 180
3rd Section — Length	m	5.7
3rd Section — Articulation	°	0 ~ 180
Delivery Line
Delivery Pipeline Diameter	mm	Φ125 × 4.5
Delivery Hose Diameter	in × mm	5" × 3000
Power & Hydraulics
Motor Power	kW	7.5
Hydraulic Pressure	MPa	28
Hydraulic Oil (5℃–55℃)	/	HM46 anti-wear
Hydraulic Oil (-20℃–5℃)	/	HM32 anti-wear
Weight
Total Weight	kg	9150
Max. Lifting Unit	kg	5850

Dimensions & Working Range

Swept through 360°, the 21 m radius covers a circle about 42 m across from one set-up — a larger single-stance footprint than the 17 m spider. The working-range diagram maps that envelope with the three-arm fold, and the outrigger footprint and 3 m free-standing height set where the boom can be placed. Plan set-up points so each 21 m stance overlaps the next, and the crane can reach each position to relocate the 5,850 kg unit.

Applications

The PB21B-3R suits area pours where the wider reach of a 21 m boom reduces the number of set-ups — large floors and decks poured bay by bay.

Large floor slabs and decks

On wide suspended slabs, the 21 m radius covers a larger bay per stance, so fewer crane relocations are needed to pour a full floor than with a shorter spider.

Podiums and transfer slabs

Broad podium and transfer slabs poured in sections benefit from the extra reach, with the boom craned between large bays as each is cast.

Rrafts and ground slabs

Large raft foundations and ground-floor slabs are spread evenly from fewer positions, the wider sweep keeping the pour moving without manual hose handling.

Projects with capable crane support

Because the heaviest lift is 5,850 kg, the PB21B-3R fits sites whose tower or mobile crane can relocate it comfortably — where that capacity exists, the larger reach pays back in fewer moves.

FAQs

PB21B-3R or PB17B-3R-II — which spider boom should I choose?

It comes down to reach versus portability. The PB21B-3R reaches 21 m and covers a bigger bay from each set-up, but it weighs 9,150 kg with a 5,850 kg maximum lifting unit and uses gear slewing. The PB17B-3R-II reaches 17 m, weighs only 6,500 kg with a 1,520 kg lift unit and uses self-locking worm-drive slewing, so it is far easier to move often. Choose the PB21B-3R for large floors where fewer set-ups matter and the crane can handle it; choose the PB17B-3R-II for frequent, light repositioning.

How is it moved around the site?

By crane. The boom stands on its outrigger base to pour a bay, then is lifted to the next position. Because its heaviest single piece is 5,850 kg, the site needs a tower or mobile crane rated to lift that comfortably — heavier than the compact spider requires, in exchange for the larger reach.

Why does it use gear slewing instead of worm drive?

The longer, heavier 21 m boom carries more slewing load, which a gear-slewing turret handles well, giving smooth powered rotation through 360°. The smaller 17 m spider uses a self-locking worm drive that is simple and holds position on a lighter boom. Both rotate a full circle; the drive is matched to the boom size.

How is a spider boom different from a climbing boom?

A spider boom stands free on its outrigger legs at one level, places concrete across a slab, and is craned sideways to the next area. A climbing boom attaches to the structure or formwork and rises with the building. The PB21B-3R is a spider boom for spreading concrete across large floors, not for climbing a tall core.

Is the placing boom a concrete pump?

No — it distributes concrete, it does not pump it. A separate stationary or line concrete pump feeds concrete through a pipeline to the boom, and the PB21B-3R then spreads it across the slab. The two work together.

Is it electric, and what is supplied?

It is electric — a 380V/50Hz supply with a 7.5 kW motor, voltage and frequency customisable, with no on-board engine. The package is the boom, outrigger base, hydraulic and control system and remote; delivery line, hose, spares, installation and commissioning are quoted to your project. Send your slab layout and crane details for a tailored proposal.

Reach versus portability in the spider range

TRUEMAX builds the spider placing boom in two sizes that trade reach against portability. The PB21B-3R, at 21 m, is the reach choice: it covers a larger bay from each stance, so big floors need fewer set-ups — at the cost of a heavier 5,850 kg lift unit and a bigger crane to move it. The PB17B-3R-II, at 17 m and a 1,520 kg lift unit, is the portability choice, easy to reposition often. On large, open slabs the PB21B-3R usually wins on total pour time; on tight or fast-moving work the lighter boom is more practical.

The spider boom's place in the pour

Concrete arrives at the slab from the batching plant and truck mixer via a stationary or line pump running a pipeline to the boom. The spider boom converts that single outlet into reach across a bay — and with 21 m, the PB21B-3R turns it into a wide one. One operator places the bay by remote while the pump keeps concrete flowing; when the bay is full, the crane moves the boom and the line follows. It is the standard way to pour large slabs without crews dragging end-hose across the deck.

Spider, wheeled or climbing — picking the mobile type

Among non-climbing placing booms, the choice is how the boom moves. A spider boom like the PB21B-3R is craned between positions on its outrigger base — good where a crane is on hand and a firm, fixed stance is wanted. A wheeled mobile boom such as the PB17D-3R rolls or is towed to its next spot, useful where no crane is free for the move. A climbing boom, by contrast, is for rising up a tall structure rather than spreading across a floor. Match the type to how the boom needs to travel on your site.