Analysis of Cracks in Concrete Tank Structures During Pig Farm Construction (1)
A Brief Discussion of Cracks in Concrete Tank Structures During Pig Farm Construction (1)
In pig farms using the liquid manure pit system, the lower tank structures are mostly designed as reinforced concrete tank structures. The designed length of pig houses is generally more than 60 m. During construction, after some concrete tank structures are built, cracks of varying number and width appear in the bottom slabs and wall panels. The cracks not only affect use due to leakage, but also reduce the durability of the building structure.
Crack characteristics: most are micro-cracks, a small number are obvious cracks, some are through cracks, and some are surface cracks; bottom slab cracks are mainly transverse cracks, while wall cracks are mainly vertical cracks, and most crack widths are below 0.3 mm.
II. Cause Analysis
1. Rule out settlement cracks: settlement cracks are ruled out by examining the size and timing of the cracks, as well as the relationship between the crack locations and the cut-and-fill positions.
2. Design causes: the design of the post-cast strips and the rationality of the reinforcing steel design.
3. Shrinkage of the concrete structure: from the crack characteristics, it can be seen that most are shrinkage cracks. The main reasons for the relatively large shrinkage of concrete bottom slabs and walls are analyzed as concrete cracks caused by deformation factors such as humidity changes, expansion, shrinkage and creep.
III. Countermeasures
Non-settlement cracks are analyzed from the aspects of design, construction, materials and environment with regard to the problem of concrete wall cracks.
1. Design aspects:
a. Bottom slab reinforcement should preferably be arranged as double-layer, two-way reinforcement in two layers. The upper layer of reinforcement mainly serves to resist cracking, with a protective cover greater than 3 cm. However, considering the construction cost of pig farms, some pig house bottom slabs use single-layer, two-way reinforcement, which increases the possibility of cracks. In the structural design of pig houses, a relative balance is often sought between economic indicators and the severity of crack occurrence. Here, the method of small diameter and small spacing is recommended. For example, bottom slab reinforcement of Ø6@200 and Ø10@450 have little difference in steel consumption per square meter, so we preferentially choose Ø6@200.
b. For the installation of concrete post-cast strips, construction post-cast strips are left at intervals of every 30 m to 40 m. For post-cast strip construction, after most of the shrinkage and creep of the already poured concrete is completed, part of the concrete is poured at about 42 days later, using micro-expansion concrete one grade higher. At present, many pig farms have short construction periods and, considering the schedule, either cancel the post-cast strips or fail to pour the post-cast strip concrete within the specified period, causing structural shrinkage and creep stress to be released in the concrete bottom slab and tank body, resulting in cracks.