Standard Construction Reminder: Key Points for Using Formwork Tie Rods

The through-wall screw, also known as the tie rod, together with the wing nut clamp and wing nut, forms the core combination of the concrete wall formwork fixing system. It is widely used in wall formwork construction such as basement exterior walls, water tanks, retaining walls, and box culverts, to resist the lateral pressure during concrete pouring, control wall thickness, and prevent the formwork from bulging outward and deforming. During numerous project visits, we found that many wall bulging, formwork displacement, and later leakage problems are not caused by quality defects of the rod itself, but by incorrect selection, improper matching of accessories, and non-standard construction operations. To help construction units avoid risks, our company has compiled the key points for the complete set of formwork tie rod usage, to provide industry science popularization reminders and standardize on-site construction.

First is rod selection, which requires distinguishing between ordinary walls and waterproof walls; this is the most basic and critical step. For ordinary interior walls and walls without waterproofing requirements, priority should be given to three-stage reusable tie rods, composed of an inner rod, outer rods, and a cone. After concrete pouring is completed and formwork is removed, the outer rods at both ends can be unscrewed and recovered for reuse, with only the middle cone remaining inside the wall, achieving high material utilization and cost savings. For walls with waterproof and anti-seepage requirements, such as basement exterior walls, water storage tanks, and sewage tanks, water-stop rods must be used, with a water-stop plate welded in the middle of the rod to block seepage channels. Ordinary full-thread rods must not be used, because ordinary rods form straight-through gaps, and water will penetrate along the rod gaps, causing seepage points on the wall later that are extremely difficult to repair. Many construction sites, in order to save money, mistakenly use ordinary rods for waterproof walls, which is a frequent cause of leakage.

Second is the standardized matching of accessories. A complete fixing combination is: through-wall rod + wing nut clamp + wing nut, together with steel pipes and square timber as primary and secondary joists. The wing nut clamp is clamped on the outside of the steel pipe, and the wing nut is tightened along the trapezoidal thread, relying on the wing nut clamp to press the steel pipe and square timber tightly, evenly transferring the tensile force to the rod. Common construction errors include: missing wing nut clamps, directly using nuts to press steel pipes, where the contact surface is too small and easily crushes the steel pipe or causes the nut to slip off; mixing rods and wing nuts of mismatched specifications, where thread pitches are inconsistent, engagement is insufficient, and thread stripping occurs under load; installing wing nut clamps backward or crookedly, causing eccentric loading and local stress concentration. During procurement, ensure that rod and wing nut specifications match, and bring matching sets to the site in the same batch to avoid mixing.

Next is arrangement spacing and tightening control. The rod arrangement spacing must be strictly implemented according to the special formwork construction plan, and the spacing must not be arbitrarily enlarged. Concrete lateral pressure is directly related to pouring height, pouring speed, and slump; the faster the pouring speed and the taller the wall, the greater the lateral pressure, and the denser the rod arrangement must be. Many workers, for convenience, enlarge the spacing, exceeding the rod's load-bearing limit and directly causing bulging. Tightening force should also be moderate, with uniform torque, and all rods on the same wall surface should be equally tight; otherwise, the formwork will be subjected to uneven forces and the wall will become uneven. If tightened too loosely, resistance to lateral pressure is insufficient; if tightened too forcefully, thread stripping or thread damage easily occurs.

Formwork removal and later treatment also require attention. After formwork removal for three-stage rods, promptly remove the outer rods and cones, clean cement off the threads, and maintain and store them properly. After formwork removal for water-stop rods, cut off the exposed rod ends, grind them flat, and seal them with waterproof mortar. The cutting position must be in place, and excessively long rebar stubs must not be left, to avoid forming seepage channels. Rods should be stored to prevent rain and moisture; after threads rust, wing nuts become difficult to turn and easily seize during construction.

The formwork system is a key focus of major hazard project control. The tie rod system looks simple but has many details. Our company will also continue to provide customers with technical consultation, assisting with selection according to wall height, waterproofing grade, and concrete parameters, and reminding work teams to implement standards. Only by selecting the right rods, having complete accessories, reasonable arrangement, and standardized operation can bulging and deformation be eliminated, wall forming quality be controlled, and leakage risks in waterproof walls be eliminated from the source.