International Concrete Abstracts Portal

Bonded concrete overlays are frequently used to repair concrete pavements, bridge decks, and industrial floors. They provide significantly higher load-carrying capacity and stiffness than unbonded overlays of equal thickness. Over 30 years of Swedish research indicates that a durable bond between the overlay and the existing concrete can be achieved with proper removal, surface preparation, placement, and curing.

Early compressive strength development in sprayed concrete is a crucial factor for success. A full scale mixing and spraying test have been done in Mapei’s Shotcrete Test Centre (STC) in Norway. Two identical batches of fresh concrete have been made, but with +13 °C (55.4 °F) and +28 °C (82.4 °F) fresh concrete temperature. Two sample boxes from each concrete batch have been sprayed with 9% alkali-free accelerator and have been stored in warm +20 °C (68 °F) and cold +5 °C (41 °F) conditions. A higher temperature in fresh concrete increased the compressive strength the first hour, while warmer conditions when curing had a stronger influence on the compressive strength at 6, 12 and 24 hour.

Flash setting accelerators are commonly used for shotcrete which is basically a sprayed concrete for underground constructions. Such admixtures cause a very rapid hardening of cementitious systems thus allowing overhead and vertical applications. Two main classes of products are currently available in the market: alkali-free (non alkaline aluminium salts) and alkali rich (sodium or potassium silicates or aluminates) accelerators. The first ones were commercially available in the last few years but only recently they have started to be appreciated by the market. The wide literature on the subject shows that the use of alkali-free accelerators gives many advantages: reduced chemical reactivity of the admixture toward human skin; less rebound, high water impermeability, increased long term compressive strength of sprayed concrete. The action mechanism of alkali-free flash setting admixtures is still not clear; therefore, a chemical morphological analysis was carried out in order to study the effects of this new class of accelerators on the physical/chemical properties of cement pastes. XRD and temperature profile studies were performed on cement systems added with different amounts of accelerator. An ESEM study was also carried out in a time interval from a few minutes to 8 hours. Specific surface area was determined according to BET method on mortar samples mixed with the accelerator (2% and 6% by cement mass) after 24 hours and 28 days of curing. Furthermore soluble ions concentration (Al3+, S042-) was also determined by ICP spectroscopy. Setting time and compressive strength were measured to compare the effects of the admixture on the mechanical properties of modified mortars with respect to its influence on the chemistry and morphology of hardening cement.

The research dealt with the durability of multilayer sandwich panels cast in situ with shotcrete technology. The research analysed the behaviour of different shotcrete mixtures towards actions caused by the climatic variations changes. A standard shotcrete mixture (traditional), normally used by panel producers, and three differently modified and optimized shotcretes (with silica fume, expansive agent, plastic fibres) were compared. Two accelerated tests were carried out: an accelerated cracking test for the short term degradation and an accelerated aging test for the long term degradation. The test procedures and the climatic chamber used in the aging test are illustrated. This report present the results of investigation on the superficial cracking density of panel specimens. The data obtained obtained at the end of the accelerated tests (total length of cracks per area unit-cm/m2) were compared. The results point out a cracking density of the optimized and modified shotcretes lower than that of the standard shotcrete. In particular the study proved the efficiency of plastic fibre-reinforced shotcrete towards the climatic variations changes both at short and long term.

The degree of quality of a shotcreting job is dependent upon just about the same things as any conventional cast-in-place concrete project. The finish must be performed by knowledgeable, well-trained personnel, good curing practices must be adhered to, and adequate quality control procedures must be followed. However, there are many aspects that make it different from most concrete jobs. The finish shotcrete layer is generally built up slightly beyond the alignment devices, and then cut back by the finishers once it has started to stiffen. As with all cementitious compositions, shotcrete must be properly cured. The quality of the project is highly dependent upon the skill of the shotcrete nozzleman.