Precast concrete is prepared by casting concrete in reusable molds or “form” which is then treated in a controlled environment, conveyed to the construction site, and lifted to the place.
The properties of precast concrete, such as cost-effectiveness, durability, adaptability, ease of use and maintenance, have grabbed a lot of attention worldwide and is slowly becoming a preference for many construction firms.
One of the many components of precast concrete are door and window frames. In this article, we discuss the shape, dimension, casting procedure, curing, erection, and fixing of the precast door and window frames.
Shape and Dimension of Precast Frames
The precast reinforced concrete door and window frames shall be 60 x 100 mm or 70 x 75 mm in cross-section for single shutter and 60 x 120 mm for double shutter door, cross-section generally conforming to Figure-2. Where specified, a suitable groove for receiving wall plaster shall be provided.
Materials for Precast Frames
1. Aggregate
The aggregate used shall be of a well-graded mixture of clean coarse and fine aggregates. The nominal size of the coarse aggregate shall not exceed 10 mm.
2. Concrete
The mix shall not be weaker than M20 controlled mix or 1:1 ½ :3 (1 cement - 1 ½ coarse sand - 3 stone aggregate 10 mm nominal size by volume mix) and shall be suitable for producing a dense concrete without voids after proper vibration.
3. Reinforcement
There shall be a minimum of three bars of 6 mm dia or equivalent reinforcement for each vertical or horizontal member shall be one piece and shall be firmly held by 3 mm dia ties spaced at not more than 300 mm center to center. The longitudinal reinforcement shall have a maximum cover of 12 mm or twice the diameter of the main bar, which is higher.
4. Mold
The mold for casting shall preferably be of steel to ensure better surface finish of the cast frame. Provision shall be made in the mold to accommodate fixing devices for hinges and the holdfasts. Where specified, suitable rebates may also be provided to act as a plaster groove.
Casting of Frames
- The entire frame can be cast completely in one piece, or each of the horizontal and vertical members of the frame may be cast separately to be assembled into the complete frame at the site.
- When the frame is cast in different parts, the reinforcing bars of the vertical members of the frame shall be kept projecting so as to insert into the corresponding hole in the horizontal member.
- The holes in the horizontal member shall be slightly larger than the dia of projecting reinforcement from the vertical members to facilitate easy insertion of the projecting bar.
- After assembly at the site, the holes shall be grouted with cement slurry of cement 1:2 coarse sand.
Protection and Curing
After casting in molds, during setting and in the first stage of hardening, the concrete shall be protected from shocks, running or surface water, and the harmful effect of frost, sunshine drying winds, and cold. The concrete must be cured for a minimum of seven days unless special curing methods are adopted.
The frames shall be matured before testing or dispatch for the following periods -
| Type of cement used | Period |
| Ordinary Portland cement, Portland blast furnace slag cement Portland pozzolana cement | 28 days |
| Rapid Hardening Cement | 14 days |
Fixing of Hinges to Precast Frames
Suitable arrangements for fixing hinges shall be provided in the frame by one of the following methods as shown in figure 3.
- Hardwood Fixture - Hardwood blocks of well-seasoned teak or other suitable timber 150 mm long, 45 to 50 mm x 30 to 40 mm in cross-section, one block for each of the hinge, shall be fixed in position with 6 mm mild steel bolts, nut and washers after the frame has been cast, cured and matured.
After tightening the nuts, the bolt heads and the nuts shall be suitably covered with hardwood fillets, finished flush with concrete surfaces of the frame. - Hinge directly attached to the frame - L-Type flap hinge may be attached directly to the frame with the help of 6 mm dia, mild steel bolts, and nuts.
- Hinge welded to frame - The hinge may be welded to a 3 mm thick mild steel flat embedded in the frame.
Fasteners for Precast Frames
Arrangements for fixing the frames with holdfasts or metallic fasteners shall be provided in vertical members of frames. In the case of the door frame, there will be 3 Nos. holdfasts, and in case of the window, there will be 2 Nos. Holdfasts on each vertical member in contact with the opening where the frame is to be fixed.
Holes to accommodate 10 mm dia bolts to be fixed to holdfasts, and the nuts shall be left at appropriate locations.
Erection of Precast Frames
- In the case where a three-piece frame is used, the vertical members shall be held in position with top member placed over them, the whole frame plumbed and firmly supported till the concrete around the holdfasts in the masonry has properly set and hardened.
- Cement and coarse sand mortar slurry 1:2 shall be used in grouting the joints between the vertical and horizontal members of the door frame.
- In the case where four members are used, the bottom member shall be first placed in position, and others erected on this base.
FAQs on Precast Door and Window Frames
The grade of the concrete used for manufacture of precast frames shall not be less than M20.
There shall be a minimum of three bars of 6 mm dia or equivalent reinforcement for each vertical or horizontal member shall be one piece and shall be firmly held by 3 mm dia ties spaced at not more than 300 mm center to center. The longitudinal reinforcement shall have a maximum cover of 12 mm or twice the diameter of the main bar, which is higher.
Read More:
1. Precast Concrete Foundations- Features and Advantages
2. Driven Precast Concrete Pile – Construction, Applications, and Advantages
3. Precast Concrete Walls – Connections and Structural Actions