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Common defects in construction formwork are hard to prevent—here are a few secret tips for prevention and control that are worth saving!

Release time:

2020-08-19 17:16

Source:

http://www.gxbml.com

Formwork construction is one of the most commonly used methods for high-rise buildings, and its quality plays a crucial role in the overall quality of the project. Below, we’ve summarized numerous common quality defects in formwork along with their corresponding prevention and control measures. Let’s take a look together!

Wall root rot

Phenomenon:

At the contact points between concrete walls and floor slabs, there are honeycombs, rough surfaces, or exposed rebar. In some cases, wooden chips, cement bag paper, and other debris are found trapped inside the wall bases.

Cause Analysis:

(1) The first layer of concrete was poured too thickly, and the vibrator did not penetrate deeply enough, leaving the bottom unvibrated.

(2) After the concrete was laid, it was not vibrated in a timely manner, causing the moisture within the concrete to be absorbed by the floor slab, making vibration difficult.

(3) Inaccurate control of concrete mix proportions, uneven mixing, excessive slump, and material segregation; a low sand ratio in the concrete mix design leads to poor workability, making it difficult to vibrate or causing slurry leakage if the vibration time is too long.

(4) The steel formwork does not make tight contact with the floor slab surface. This problem is more severe when the floor slab thickness varies, there are significant height differences, or the installation is uneven.

(5) When wooden strips are used to plug the gaps at the bottom of steel forms, the wooden strips become embedded in the wall.

Precautions:

(1) Before erecting formwork, apply a cement mortar leveling layer at the corresponding floor slab positions beneath the formwork. However, care should be taken to ensure that the mortar leveling layer does not penetrate into the walls.

(2) The gaps at the bottom of the formwork must be tightly filled with cement mortar or similar materials; under no circumstances should wood chips be used and inserted into the concrete wall area.

(3) Add guide walls, or place inflatable cushions or sponge rubber pads at the bottom of the formwork.

(4) Before pouring concrete, first wet the formwork and the surface of the floor slab with water. Then, pour a 50-mm-thick layer of mortar (with the same composition as the mortar used in the concrete). The mortar should not be spread too thickly, and it is strictly prohibited to pour it directly using a hopper.

(5) Adhere to layered concrete placement; the thickness of the first layer must be controlled within 500 mm.

Governance method:

(1) For areas with severe root rot, first remove the honeycomb and rough-textured surface portions, then apply cement-sand mortar in a 1:1 ratio in layers to smooth the surface. This work must be carried out immediately after formwork removal.

(2) For root-rotted areas that have already been encased with wood chips, paper, or grass flies, the foreign materials should be thoroughly removed immediately after demolding. Then, high-strength dry-hard mortar should be compacted into the area; if necessary, a small amount of fine aggregate can be added to the mortar.

(3) For minor surface roughness, the surface showing yellowish-brown sand can be immediately removed after formwork is removed, followed by applying a coat of 108 glue-based cement putty. If this process is not carried out immediately after formwork removal, the loose surface layer must first be chipped away, the surface then moistened with water and thoroughly rinsed clean before applying another coat of 108 cement putty.

(4) For larger areas of honeycomb, rough surfaces, or exposed rebar, the weak concrete and exposed aggregate particles should be chiseled away down to the full depth. Then, the surface should be scrubbed with a wire brush or rinsed with pressurized water. Next, fill the voids with fine aggregate concrete (or concrete containing micro-expansive agents) that is one grade higher in strength than the original concrete, and carefully compact it.

Wall adhesion, chipped edges and corners.

Phenomenon:

When removing formwork from the walls, large areas of concrete skin were found adhered to the large formwork panels. After formwork removal from the top of the cast-in-place walls and around openings, chipped edges and corners were observed.

Cause Analysis:

(1) Demolding too early—concrete strength is below 1.2 MPa.

(2) The water content of the concrete was not strictly controlled, leading to significant variations in quality. During pouring, the concrete was discharged in concentrated batches without being evenly vibrated.

(3) If the formwork is not cleaned thoroughly—especially around the upper and lower ports as well as the door frame edges—it is prone to accumulating residual concrete.

(4) A failed sealant was used, or the sealant was applied unevenly, left unapplied in certain areas, or washed away by rainwater.

(5) When constructing the construction joint, the mortar layer poured was too thick and had low strength. Additionally, the formwork at the openings was removed too early or was accidentally bumped during removal, resulting in chipped or broken edges on the walls.

Precautions:

(1) The formwork can be removed only after the concrete strength of the wall reaches 1.2 MPa.

(2) The cleaning of large formwork and the application of release agents must be carried out meticulously, with a dedicated person responsible for inspection and acceptance. Any items that fail to meet the standards must be cleaned and re-coated.

(3) Strictly control the quality of concrete. The concrete should have good workability, and during pouring, it should be fed evenly. The use of vibrator rods to compact and push the concrete is prohibited.

(4) Provision should be made for temporary formwork openings to allow for delayed removal of the opening formwork. It is advisable to use adjustable opening formwork. The use of sledgehammers to strike the formwork is strictly prohibited, as this could damage the concrete edges and corners.

(5) The thickness of the cement mortar used to connect construction joints shall be 50–100 mm, and the bottom layer of concrete must be thoroughly vibrated during placement. Concrete containing fly ash should be used. At the top of the formwork, add gravel that has been mixed with cement slurry and then vibrate again, ensuring that the strength of this concrete section meets the design requirements.

Governance method:

(1) Severe, extensive adhesions and rough surfaces must be repaired immediately after form removal. The initial repair method is as follows: First, thoroughly remove any loose debris and slag from the surface; then, apply cement mortar in layers at a 1:1 ratio, smoothing each layer carefully and ensuring the surface is meticulously polished to achieve the required smoothness.

(2) For small areas of adhesion or rough surfaces, immediately after formwork removal, apply 1–2 coats of 108 glue-based cement putty to level the surface.

(3) Chipped edges and corners should also be repaired immediately after formwork removal. First, apply a coat of horizontal plain mortar slurry, then use cement mortar to fill in the gaps in layers.

(4) If a considerable layer of fly ash slurry has accumulated at the top edge of the formwork, it should be chiseled away after demolding and then filled in with concrete one grade higher.

Large vertical deviation of the wall

Phenomenon:

The vertical deviation of the walls exceeds the code requirements. In some cases, an entire wall is severely affected, while in others, only one end is significantly tilted. Excessive vertical deviation in walls can affect the bearing length of floor slabs and easily lead to increased local support pressures on the walls.

Cause analysis:

(1) During formwork support, the plumb bob was not used for alignment, or after tightening the through-wall bolts, a recheck was not performed.

(2) The foundation bolts for the large formwork were not securely fastened. As a result, after the formwork was subjected to a violent impact from an object (such as a collision with exterior wall panels), it tilted and deformed, and no corrective measures were taken afterward.

(3) The large formwork itself is severely deformed and twisted.

(4) The formwork was not securely erected, and the anchor bolts were not tightened properly; during concrete vibration, excessive force was applied, causing the formwork to shift out of position.

Precautions:

(1) During formwork installation, repeatedly use a plumb bob for alignment and verification. First, install the front large form panel and adjust it using the anchor bolts. After verifying its verticality with the plumb bob, proceed to install the rear large form panel. Then, check the verticality of the rear form panel from its exterior side using a plumb bob. Finally, secure the front and rear large form panels with through-wall bolts, continuously monitoring their verticality with the plumb bob. Also, be sure to check that the anchor bolts are tightly tightened.

(2) After formwork installation is completed and verified, if significant impacts occur, the alignment should be rechecked and corrected using a plumb bob.

(3) Large formwork that has been left unrepaired for a long time and has severely deformed must not be used any further and should be repaired by the factory.

(4) The formwork was not securely erected, and the anchor bolts were not tightened properly; during concrete vibration, excessive force was applied, causing the formwork to shift out of position.

Governance method:

(1) For vertical deviations of 3 mm or less, part of the wall surface can be roughened and then smoothed with a cement slurry mixed with 108 glue. For deviations exceeding 3 mm but within 15 mm, after roughening part of the wall surface, the area should be leveled using cement mortar. This remedial work should be carried out immediately after formwork removal.

(2) For structures with severe vertical deviations (where the overall height vertical deviation exceeds 20 mm), the concrete should be chiseled away immediately after form removal, and the formwork should be re-erected and concrete poured anew.

(3) If the vertical deviation of the wall is significant and the length of the floor slab pressing against the wall at each end is less than 20 mm, the matter should be discussed and resolved in conjunction with the design unit.

The wall is uneven.

Phenomenon:

After removing the formwork from cast-in-place concrete walls, the wall surfaces are uneven—some areas are locally dented or concave, others exhibit continuous wave-like patterns, and still others have localized bulges (when checked with a 2-meter straightedge, the unevenness exceeds ±4 mm).

Cause Analysis:

(1) The formwork lacks sufficient rigidity. The spacing between the channel steel (keel) on the back of the large formwork is too large, or the thickness of the panel steel used is too thin (less than 4 mm).

(2) The wall-penetrating pipes vary in length and have excessive dimensional tolerances. The wall-penetrating bolts are tightened too much, causing local deformation of the nearby steel plates.

(3) The vibrator excessively and violently impacts the large formwork surface, causing localized damage to the formwork surface.

(4) During installation and form removal, striking the formwork surface forcefully with a sledgehammer or crowbar can cause severe defects to the surface.

Preventive measures;

(1) Strengthen the maintenance of formwork. After completion of each project, the formwork should be inspected and maintained once. Any defects on the formwork surface should be repaired promptly; in cases of severe damage, the steel plates on the formwork surface should be replaced immediately.

(2) For formwork with insufficient rigidity, the back-side steel joists (No. 8 channel steel) can be densified—either by adding an additional joist between the original two, or by installing a short vertical joist between the original two horizontal channel steels.

(3) Do not use vibrators to forcefully vibrate large formwork, nor strike steel forms with large hammers or crowbars.

(4) The steel plates at the locations of wall-through bolts should be appropriately reinforced. Reinforcement methods include attaching a small, thick square steel plate (on the reverse side of the plate) or welding structural steel sections on both sides of the bolt holes.

Governance method:

(1) Thoroughly inspect and reinforce the existing steel molds.

(2) Uneven wall surfaces should be repaired immediately after formwork removal. The repair method is as follows: First, roughen the surface by removing any protruding parts, then use cement mortar or a 108 adhesive cement slurry to level it out.

(3) For large-area, wavy wall surfaces, a finishing coat of plaster without a base coat can be applied to achieve leveling.

Phenomenon:

After formwork removal, the internal vertical and horizontal walls’ junctions, as well as the internal horizontal walls’ junctions with exterior walls, exhibit irregularities in their internal corners—specifically, they are neither square nor perfectly vertical—posing significant challenges to finishing work and resulting in substantial waste of labor resources spent on repairs.

Cause Analysis:

(1) When using the cylinder mold, operational negligence led to vertical deviations at the corners.

(2) When using flat formwork, the gap between the inner longitudinal wall formwork and the previously poured inner formwork wall is too large; the axis of the inner longitudinal wall formwork has shifted.

(3) A gap forms between the small-angle formwork and the large formwork, resulting in slurry leakage and deformation of the external corners.

(4) In buildings with exterior masonry and interior concrete pouring, the cross-sectional width of the composite columns at the junction between the exterior and interior walls is always greater than the thickness of the interior wall (due to the presence of dovetail joints). Therefore, larger corner formwork must be added. If the corner formwork is installed carelessly or not securely fastened, it often leads to displacement of the formwork, causing severe deformation at the internal corners.

(5) The exterior brick wall is uneven, or the surface of the exterior wall panels is uneven.

Precautions:

(1) Repair the formwork promptly, especially the small corner forms.

(2) When installing the cylinder mold, careful operation is required to eliminate any errors during the installation process.

(3) During formwork support, ensure that the axis position remains correct and strive to minimize vertical deviations in the formwork.

(4) The small corner forms at the junction between the exterior and interior wall formwork of externally-bricked, internally-poured building walls must be carefully handled and securely fastened to ensure they remain undeformed.

(5) The brickwork at the composite column must be laid smoothly. If there are gaps between the formwork and the wall surface, they should be filled. Remedial measures: Repair immediately after removing the formwork. Repair procedure: First, check using a straightedge, plumb bob, and square; then chisel out any uneven areas; finally, use a 108 glue cement slurry to fill and smooth the surface.

Concrete wall bubbling

Phenomenon:

The walls have a large number of extensive air bubbles, which pose significant challenges to the renovation project, affecting both the schedule and increasing labor costs.

Cause Analysis:

(1) Excessive dosage of foaming water-reducing agents (such as MF water-reducing agents, etc.).

(2) The concrete slump is too high, and the vibration time is too short, resulting in poor compaction of the concrete.

(3) During concrete pouring, too much material was poured at once, and air bubbles were not fully expelled during vibration, instead accumulating on the concrete wall surface.

Precautions:

(1) Add water-reducing agents correctly according to the specifications; it is best to avoid using foaming-type water reducers. If necessary, air-entraining agents may be added.

(2) On-site mixing shall be used. If pumped concrete is employed, the slump should be between 130 and 150 mm, with a maximum allowable value of 180 mm.

(3) Strengthen concrete vibration. If a water-reducing admixture is added to the concrete, it is advisable to use a high-frequency immersion vibrator for vibration. The spacing between movements of the vibrator rod should not exceed 300 mm, and the withdrawal speed should be slow. Vibration time should be sufficient until no more air bubbles appear on the surface.

(4) The thickness of each concrete placement should preferably be no less than 500 mm. Treatment method: Remove the brittle crust from the surface of the bubbles, apply putty, and seal the bubbles completely. Generally, only “Dabai” putty should be used; avoid using 108 glue cement or other types of putty.

The wall has formwork skin.

Phenomenon:

When the wooden formwork for walls is removed, the surface of the wall is left with adhered formwork skin, resulting in a poor aesthetic appearance.

Cause analysis:

(1) Wood formwork is used repeatedly many times, and its surface lacks sufficient rigidity, causing the formwork surface to partially adhere to the concrete surface when removed.

(2) The release agent has expired, or the release agent was applied unevenly or not applied at all.

(3) Demolding was delayed, curing was not performed in a timely manner, and the concrete surface temperature was too high, causing the early surface strength to develop too rapidly and resulting in the concrete sticking to the wooden formwork.

Precautions:

(1) For wooden forms that have been used multiple times, discard those with insufficient surface rigidity or significant bending deformation. For wooden forms with only localized areas of insufficient surface rigidity, re-patch them before use.

(2) After removing the wooden formwork, promptly clean off any mortar and peeling material from the surface, and carefully apply an effective release agent.

(3) Use a rapid-form-release system: Once the concrete strength exceeds 1.2 MPa, promptly release the formwork and begin water-curing the concrete.

Governance method:

(1) Remove the wood formwork skin embedded in the concrete surface. For areas where the formwork skin is severely damaged on the concrete surface, after removal, first apply a coat of 108 glue cement slurry, then use cement mortar to fill and smooth the surface in layers until the desired level of smoothness specified in the design is achieved.

(2) If template skin remains adhered to the wall surface after formwork removal, remove it immediately after demolding and then apply 1–2 coats of leveling putty made from 108 glue cement.

(3) When the formwork skin is embedded relatively deeply in the concrete, all the concrete surrounding the skin should be chiseled away. The surface should then be scrubbed with a wire brush or rinsed with pressurized water. Next, fill the void with fine aggregate concrete one grade higher than the original concrete’s strength class, and carefully compact it.

The concrete strength of the wall is uneven.

Phenomenon:

The concrete strength in the upper part of the wall is lower, while the strength in the lower part is higher. As evidenced by the degree of form-release adhesion, the upper part of the wall exhibits severe adhesion, whereas the lower part shows very little adhesion.

Cause analysis:

(1) The concrete slump is too high, and its workability is poor.

(2) The concrete is poured in relatively large layers.

(3) The operation method of using a vibrator to advance concrete placement from one end was adopted, or insufficient vibration occurred during the compaction process.

(4) The concrete hopper has too great a drop height, leading to separation of sand and gravel and loss of cement slurry.

Precautions:

(1) Strictly control the concrete slump to between 30 and 50 mm; for pumped concrete, the slump should be between 100 and 140 mm.

(2) The concrete should have good workability.

(3) The method of using a vibrator to rush and vibrate the slurry is prohibited. Instead, the hopper should be used to discharge concrete evenly along the entire length of the formwork, ensuring that the concrete pouring heights remain consistent across all sections.

(4) The height of each concrete pour step should preferably not exceed 500 mm, and no areas shall be left unvibrated or under-vibrated.

(5) Concrete should not be over-vibrated, especially at the top opening of the formwork. Concrete hoppers that are leaking mortar should be repaired promptly.

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