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Hilden, Germany

Very soft concretes pose the risk of segregation in the event of excessive compaction, which is why such concretes may only be subjected to gentle vibration or rodding. According to good laboratory practice, cubes must be produced on a vibrating table, which often introduces an amount of compaction energy that is far too high for F5/F6 concretes, thus resulting in cubes that contain the aggregates in the bottom and pure mortar in the top section. This phenomenon is not identified during cube production or formwork stripping because the visible surface continues to appear in a bright gray shade. In retrospect, it is impossible to ascertain whether such a phenomenon occurred because test specimens are usually disposed of even if they have not passed the test. The construction contractor submits a complaint regarding the insufficient concrete strength and claims a blanket price reduction.

The decoration contractor submits an objection and states that additional costs would be incurred because the floor undersides were not suitable for applying a smooth, premium-quality paint finish without specific preparation of the surface. In response to this objection, the construction contractor lodges a complaint with the precast producer regarding the floor undersides on all four completed building levels. As specified by the client, the lattice girders are laid in parallel to the short side, which means that the floor slabs can only be handled using a beam both at the precast plant and on the construction site. The expert consultant arrives at the undisputed finding that the undersides of the floor slabs that had been installed for one to three months on the date of the complaint reveal roughly the same quality of the soffits on all four building levels.

Most manufacturers of concrete products and precast elements order mineral aggregates in accordance with DIN 12620. DIN 12620 specifies several classes of aggregates but not all of them are suitable for producing concrete in accordance with DIN EN 206-1 or DIN 1045-2. The DIN EN 12620 standards pertaining to mineral aggregates also contains Annex G that was added for information purposes and recommends additional specifications to be agreed upon with respect to the surface quality of the concrete. If possible, an appropriate upper limit for the ratio of surface-relevant constituents should be agreed upon with the aggregates supplier. In many cases, this will require the use of crushed aggregates, which usually necessitates a modification of the concrete mix design. If this is not possible for technical or commercial reasons, the agreement with the end client should contain a provision stating that such stains may occur.

Cracks In concrete are not necessarily a defect. However, a defect may exist if such cracks exceed certain widths or compromise serviceability. In many cases, manufacturers receive complaints about cracking only after the handling and relocation of double walls. Permissible crack widths DIN 1045-1 classifies cracks as safe from a technical point of view if their widths do not exceed 0.4 mm in dry internal components or 0.3 mm in all other cases. If a complaint is lodged only during or after assembly of the precast elements, it should be assumed that the elements were free from defects at the time of delivery and handover. If cracks occur in the shell of a double wall that is in contact with water, the precast element must be replaced or the cracks be appropriately grouted in order to complete the agreed waterproof structure to specification in the course of subsequent contractual performance.

The selection of the concrete and the time of smoothing are two important factors in a building construction. The factors that ensure workability of a concrete to enable the smoothing process include time to tread resistance, bleeding water protecting the concrete against drying until tread resistance is reached, and amount of water evaporating until tread resistance is reached. The surface to be smoothed should not be too dry under the prevailing conditions, and the intended tread resistance should have been reached. Concretes are usually ordered with a cement ratio of 320 to 350 kg/m 3 and a w/c ratio of about 0.50. The concrete would have to be kept damp until tread resistance is reached. Hot concrete cast on a cold floor or radiant heaters placed on the poured concrete pose particularly high risks.

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