Study of concrete quality in structures produced at negative ambient temperature

Abstract

The results of studies by non-destructive methods of the state of concrete of eight vertical reinforced concrete structures of various shapes and massiveness, made at a negative ambient temperature in violation of the technology of winter work, are presented. The concrete mixture was laid on a supercooled floor slab, as a result of which defects were formed in the lower contact zone of the support of pylons and columns on the floor slab. The presence of defects required separate studies both in the structures as a whole and in their defective zones.

It was found by ultrasonic method that in more powerful structures (at a height above 50 cm), the strength of concrete on the 28th day reached the design values. In thin-walled pylons, an increased heterogeneity of concrete was observed at a significantly lower strength. Three months later, the strength of concrete in these pylons increased by about 20%, but remained below the design values, and testified to the irreversibility of changes in the structure of concrete from exposure to negative temperatures.

Acoustic measurements identified areas with poor adhesion of concrete to the floor slab in all eight structures. The boundaries of defective zones (crack depth) ranged from 8 cm to 30 cm. The largest crack was recorded in a column with a circular cross section 100 cm in diameter.

Detailed studies of the state of concrete in the areas where columns and pylons adjoin the floor slab included qualitative (defectoscopy) and its quantitative (strength uniformity) assessments. Concrete flaw detection was performed by color visualization of the distribution of ultrasound velocity over the thickness of structures at scanning points along a grid with a square of 10 x 10 cm. The first scanning level was at a height of 5 cm from the floor slab. The upper level limit was 75 cm. Defective areas were found in all structures at a height of up to 10 cm from the floor slab. They were characterized by an excess coefficient of variation and the speed of ultrasound, which is not typical for heavy concrete. According to these indicators, concrete in structures at a height of up to 10 cm was recognized as defective.

The results obtained showed the consequences of violating the technology of winter work and made it possible to justify decisions to ensure the operational reliability of structures.