International Concrete Abstracts Portal

In the design and construction of modern structures such as high-rise buildings and long-span bridges, it is extremely important that realistic creep behaviour of concrete is known and used. In the 2003-2004 revision of the Australian Standard for concrete structures AS 3600, a new set of ‘basic creep factors’ or ‘basic creep coefficients’ are given to cover a wide range of concrete with characteristic strengths up to 100 MPa. This paper presents the background work undertaken to determine the basic creep factors. As part of this research, the suitability of various methods of extrapolating long-term creep from short-term creep tests were evaluated. The creep characteristics of a range of concretes were measured under sustained load in accordance with AS 1012.16. The concrete mixtures cover a range of constituent materials including a general purpose (GP) cement, an Australian version of an ordinary portland cement, binary and tertiary blends of GP with ground granulated blast furnace slag (slag), fly ash and silica fume. The coarse aggregates were either basalt from New South Wales and Victoria, or river gravel or air-cooled blast furnace slag from New South Wales. The mixtures cover a wide range of characteristic compressive strengths from 15 to 90 MPa. Of the three methods commonly used to extrapolate the long-term creep strain, the power model was found to be the most suitable method with good accuracy and a conservative overestimation of the creep strain. Two practical periods of measurement of short-term creep strain, 2-month and one-year, were examined from the 17 sets of medium-term (3-5 years) creep data. Based on the power model, the predicted ultimate creep strain from 2-month creep data tends to overestimate the basic creep factor by an average of 22% compared with measured values at the end of the 3-5 years of testing. Extending the measurement to one-year improves the accuracy with the average overestimation reduced to an average of 15%.

This article outlines an extensive number of laboratory tests on long-term deformations of Self Consolidating Concrete, SCC. For this purpose 4 SCC and 4 vibrated concrete, VC, were fabricated, in all 88 creep specimens. The water-cement ratio, w/c, of the concrete varied between 0.27 and 0.80 and strength between 14 and 171 MPa. Studies on shrinkage, strength and relative humidity, RH, were performed in parallel. The effect of late start of drying on creep was also incorporated in the studies. The results show creep and shrinkage of SCC of the same range of order as for VC at constant strength. No significant difference between the elastic modulus of SCC and that of VC was observed at constant strength. The creep coefficient , o, developed similarly in SCC as in VC .

Calcium chloride is an excellent accelerator for concrete, improving both setting time and hardening rate. However, it can not be used for reinforced concrete in efficient dosages because of initiation of rebar corrosion. In the search for other bulk chemicals working as accelerators, calcium nitrate has proven to be a good candidate as set accelerator; however it is not a hardening accelerator (e.g. improving 1 day strength). Efforts have been made to combine calcium nitrate with other admixtures in order to make the mix work as a combined chloride-free setting and hardening accelerator. This paper sums up research over the last 10 years regarding this matter and also compares performance with other chemicals.

Some protective techniques, such as anti-carbonation coating, provide the concrete a protection due to the creation of a surface barrier that prevents the penetration of CO2 and moisture. However, these procedures are not always so effective when the carbonation process moves as a front through the concrete pores, reaching the reinforcement. In this case, the procedure often adopted is the repair technique, which can include the anchorage of the structure, the entire removal of the carbonated layer and localized or general repair. The purpose of this research was to study the non-destructive repair technique referred as realkalisation by the absorption and diffusion of alkaline solutions. In order to provide evidence for further analyses of the technical viability, besides the main realkalisation testing, some additional tests such as compressive strength, capillary absorption, electrochemical techniques and mortar adherence on realkalised substrates, were performed. The results obtained show that realkalisation through direct contact of the alkaline solution on the concrete surface is effective in re-establishing the high pH of the concrete, although it presented a slight decrease in the compressive strength. The realkalisation process does not interfere in the mortar adherence to realkalised substrates.

Lightweight concrete could be producing by incorporating expanded polystyrene in the conventional concrete. This paper discusses the results of an experimental investigation into the effects of binder materials on the engineering properties of polystyrene aggregate concrete (PAC), having the nominal density of 1800 kg/m3. Four types of binders, namely, general purpose cement, shrinkage limited cement, a combination of 60% general purpose cement and 40% low calcium fly ash, and blended cement with 62% granulated blast-furnace slag, were used. The results showed that as expected, the use of supplementary cementitious materials such as fly ash and slag reduced the early-age strength of the polystyrene aggregate concrete. The strength development of PAC was also affected by the curing condition. Although the use of shrinkage limited cement reduced the shrinkage of PAC by 12%, it increased the creep potential by 76%. The paper also discusses the relationships between: shrinkage and moisture loss; strength and rebound number; and strength and pulse velocity.