Requirements for sprayed concrete and basic control tests
The purpose of using shotcrete methods in repairs of damaged concrete or reinforced concrete surfaces is to obtain a layer of sprayed concrete that will reflect the parameters set forth by the designer. The basic rule of such repair consists in good selection of repair material so that its properties are similar to properties of the structural concrete. Actually, there are no such materials that would meet all the requirements of each repair case. When deciding on repair with the use of shotcrete the designer determines final parameters of shotcrete and the contractor must select an adequate mix formula to comply with them.
The mix’s usability for repairs should be judged basing on the shotcrete complying with specific requirements. Concrete made pursuant to the formula should prove adequate mechanical and physiochemical properties. The following should be tested:
Sometimes, additional tests should be performed:
coefficient of elasticity E,
protection efficiency (e.g. resistance to CO2 and chlorides),
The final effect of the repair that was carried out is a resultant of all control activities performed in order to secure adequate technological conditions during spraying.
It is very important to carry out continuous control and supervision over the ongoing works. Tests performed directly on the structure guarantee the required quality of repair and simultaneously allow direct corrections to be made. Temperature and moisture also count. These may be monitored as follows:
continuous measurements of air temperature and humidity,
measurements of base temperature and moisture.
Tear-off strength of the damaged base decides upon the possible repair. Pull-off test is performed with the frequency adjusted to the structure’s needs. Average tear-off strength should be at least 1.5 MPa. If the base does not meet the criterion then prior to spraying reinforcement with mechanically anchored mesh will be made. The minimum thickness of sprayed concrete should be at least 5 cm. Therefore, adherence of sprayed concrete to the base must exceed 1.5 MP (average).
The method of cube or cylinder samples of concrete sprayed to moulds should be abandoned. Standard PN-EN 14488-1:2008 requires spraying on test patches of min. 500 x 500 mm in the case of manual spraying and 1000 x 1000 mm in the case of automatic spraying (robots). Thickness should refer to the dimensions of test samples that will be cut out from the patch but should be at least 100 mm. Test patches (moulds) should be made of steel or other material that does not absorb water. Test patches should be placed in vertical and sprayed with the use of the same equipment and method and from the same distance as during the actual spraying.
The patch should be immediately treated for protection against moisture loss - use the same method as on site. Patches should be labelled for later identification (mix, place of incorporation, date, operator etc.) The patch should not be moved for 18 hours from spraying. Curing should be performed for the next 7 days. Test samples should be cut out from the patch. They should not, however, contain material placed closer than 125 mm from the edge (except for beam ends tested for bending). During transportation to testing laboratory, patches or cut out samples should be protected against mechanical damage and moisture loss.
Any arrangements concerning sprayed concrete tests should be adjusted to the volume and scope of repair.