Measures to Remedy Defects During Ubot Slab Construction (Part 2)

29 September 2026

Ubot slab construction and the remediation of defects occurring during Ubot slab construction are always important concerns in construction, particularly for developers and contractors. Choosing a new material technology solution is challenging, but gaining confidence in the product can be even more difficult. With extensive experience in applying Ubot beamless flat slab technology to numerous projects, LPC hopes to provide developers and contractors with additional measures to effectively remedy defects.

Ubot Slab Construction – Cracking Phenomenon

Cracking during Ubot slab construction is one of the common phenomena that can occur when constructing beamless slabs. The causes of transverse cracking are related to materials and mix design, Ubot slab construction practices, and other factors such as structural design. Transverse cracks often appear after concrete placement, and the crack width may develop over time.

Causes

The main causes of cracking include incorrect concrete mix proportions (PCB 40), which can cause brittleness and cracking; the use of superplasticizers and admixtures that accelerate early-age concrete strength development, thereby accelerating the hydration process to achieve early strength, which also changes the heat generation of the concrete and reduces its water content.

Premature removal of formwork during Ubot slab construction before the concrete reaches the required strength, or early removal of formwork from the lower floor while concrete is being placed on the floor above, can also cause cracking because the construction load is greater than the design load.

In particular, factors related to concrete and reinforcement during Ubot slab construction are among the main causes of cracks occurring in the slab.

Concrete-related causes:

+ Improper slab curing procedures.

+ Climatic conditions during concrete placement.

+ Concrete grade does not meet the design requirements.

+ Aggregate grading and particle composition are inadequate.

+ The water used for concrete mixing does not meet requirements, resulting in cement paste loss due to absorption into the ground or leakage through gaps in the slab formwork.

+ Uneven concrete placement. The slab thickness is reduced in the middle due to insufficient inspection during construction.

+ Interrupted concrete supply, causing the concrete to dry, harden, or segregate.

Reinforcement-related causes:

+ Crack width becomes smaller at the locations where longitudinal reinforcing bars intersect the reinforced concrete member and widens along the member surface. Therefore, the thickness of the concrete cover and the spacing between reinforcing bars affect crack width. Reinforcing bars should be evenly distributed and positioned relatively close to the two side faces and bottom face of the beam or slab.

+  Insufficient reinforcement and excessive bar spacing in a wide slab.
+  Placing a few large-diameter reinforcing bars (> 1/10 of the slab thickness) in the hope of strengthening the slab can instead cause cracking.
+  Careless tying and splicing of reinforcement.
+  Slab deflection (transverse cracking in the middle of the ceiling along the long-span direction), typically caused by insufficient reinforcement.
+  Incorrect fabrication and installation of reinforcement relative to the required concrete cover.
+  Insufficient concrete cover:
+  Slab reinforcement has not been completely straightened before installation;

Remedial Measures

To minimize the occurrence of slab cracking defects during Ubot slab construction, LPC engineers recommend that developers and contractors:

Limit the use of rapid-hardening chemicals. Limit the use of R7, R14, and other admixtures. Carefully monitor and follow the technical instructions provided by LPC engineers.
Concrete should preferably be placed at night, followed by curing immediately after the concrete has hardened.
Provide thermal expansion joints when the building length is large (the long side should not exceed 60 m).
To reduce transverse cracking on the slab surface, the cement content in the concrete mix can be reduced and, where possible, concrete with high early-age strength should be avoided.
Use the maximum aggregate size in accordance with ACI 318, use crushed stone as coarse aggregate, and maximize the aggregate content.
The concrete mix should be tested to ensure a slump of 16–18 cm, adequate strength and concrete grade, as well as proper aggregate composition, etc.
Begin curing immediately after completing concrete placement. Curing must be carried out in accordance with the applicable standards.
All slab shoring levels must comply with the applicable standards. When placing concrete on the upper floor, at least two fully supported shoring levels must be maintained. Particular attention must be paid to preventing the construction load from exceeding the design load.

Voids Underneath Ubot Void Formers

Voids underneath Ubot void formers can also occur. The main causes are inadequate concrete flowability and reinforcement installation in which the spacing between reinforcing bars is too close to the bottom of the Ubot void former, preventing the concrete aggregate from passing through and fully filling the area beneath the void former.

To remedy this issue, LPC recommends using concrete with flowability that meets the requirements specified by the LPC technical team. Reinforcing bars should be installed with a spacing of 30–40 mm to provide sufficient clearance for the concrete mix to flow through and fully fill the area beneath the void former.

The repair procedure for voids consists of the following steps:

Step 1: Identify the location of the void in the slab
Step 2: Remove concrete at the void location where the thickness does not meet the specified requirements
Step 3: Thoroughly clean the void area
Step 4: Install formwork and shoring to seal the area requiring repair
Step 5: Drill a core from the top of the slab at the void location
Step 6: Use Sika repair mortar or cement-based patching mortar and pour it into the drilled opening.

Defects occurring during Ubot slab construction can result from various causes, including materials and structural design. These are undesirable situations that no developer wants to encounter. Therefore, to avoid the additional costs associated with repairing such defects during Ubot slab construction, developers should consider selecting contractors with experience in structural design, particularly in providing guidance and technology transfer for Ubot slab construction.