The absorption characteristics and sorptivity of cover concrete obtained by the Initial Surface Absorption Test (ISAT), the Covercrete Absorption Test (CAT) and the sorptivity test have been studied and compared. Two types of concrete, namely OPC control and Low Water Concrete (LWC) of grade 35 have been tested. The laboratory work has shown close agreement between the ISAT and sorption results but the CAT yields higher results. An analytical model has been developed based on the mechanisms of capillary suction and pressure driven flow. In this model water entering concrete during the tests is assumed to be concentrated in a well defined volume. This volume is fully saturated and is separated from the surrounding concrete by a wetting front at which capillary suction occurs. By applying the physical equations for capillary suction pressures and permeability the experimental results are explained in terms of the basic properties of the concrete. The model gives good agreement with the experimental results.

The safe disposal of waste tyres has been seen as having a negative impact on the environment. To mitigate this impact, the components of waste tyres can be used in the production of green concrete. This study explores the effects of the curing and drying regime on the mechanical properties and permeation characteristics of concrete containing both crumbed rubber and steel fibres that are removed from waste tyres. Five concrete mixes were designed and concrete cubes, cylinders, and prisms were cast using waste tyres extracts. Crumb rubber was treated by submersion in sodium hydroxide and then used to partially replace 10% and 30% of fine aggregates in the concrete mix. Extracted steel fibres were added at the rate of 1% and 2% per volume of each mix. Compressive, indirect splitting tensile as well as flexural strengths were conducted after normal curing while observing several drying conditions. Additionally, air permeability was assessed using a portable apparatus which was developed to assess permeability easily. For the concrete test specimens containing 10% partial replacement of fine aggregate by crumb rubber and 1% steel fibres, it was discovered that the splitting tensile strength and flexural strength were higher than that of the control mix by 21% and 22.6%, respectively. For specimens, that included the 10% crumb rubber and 1% steel fibres, when exposed to oven drying at 105°C for 12 hours, the compressive strength results increased by 17% compared to the control specimens exposed to the same conditions. Unlike the compressive strength results, the splitting tensile and flexural strength results decreased after exposing the specimens to elevated temperature. The addition of crumb rubber and steel fibres as a partial fine aggregate replacement resulted in increasing the air permeability of the concrete to different degrees depending on the percentages used. The oven drying curing regime improved the permeability by reducing it in specimens containing the 10% crumb rubber and 1% steel fibres as indicated by increasing their permeability time index by 15% when compared to air-dried specimens. Using waste tyre extracts as a partial replacement of concrete fine aggregate can be recommended for both indoor and outdoor applications. This study showed that this was a viable, economic and environmentally friendly method for reducing carbon footprint.

A non-destructive, rapid test capable of measuring the air permeability of in situ concrete is described. The technique is based on the application of a hard vacuum to the concrete surface. The rate of pressure recovery is recorded after the vacuum line is disconnected. The experimental results lead to an exponential pressure–time relationship similar to that obtained from analytical solution of Darcy’s equation. The air permeability of concrete can be simply expressed in terms of a vacuum decay parameter, but minor changes in the application of the test make it possible to determine the air permeability of concrete in situ. The test results are sensitive to changes in the parameters that affect the pore structure of concrete, including water–cement ratio, curing conditions, cement content, air content and plasticizing admixtures.

This paper presents the development of simple semi-empirical formulae for the analysis of nominal flexural strength of high strength steel fiber reinforced concrete (HSFRC) beams. Such developed formulae were based on strain compatibility and equilibrium conditions for fully and partially HSFRC sections in joint with suitable idealized compression and tension stress blocks. The stress blocks were given by suitable empirical functions for the compressive and post-cracking strengths of HSFRC. The enhancement in compressive strength due to fibers inclusion is proposed as a function of concrete matrix strength and fiber reinforcing index. To account for the pullout resistance of fibers in tension, the post-cracking strength was evaluated as a function of fiber reinforcing index and bond strength. The fiber reinforcing index was considered as a function of volume content, aspect ratio, orientation and length efficiency factors of the fibers. In view of the degenerative nature of the pullout resistance of the fibers with the increase of crack width, a limit was placed on the useful tensile strain extent, depending on fiber length and crack spacing. It was found that there is a good agreement between the flexural strengths for HSFRC beams predicted by the proposed formulae and the experimental results reported in the literature, while the predicted flexural strengths as computed by ACI Committee 544.4R (ACI Struct J 85:563-580, 1988) and ACI Committee 544.1R (ACI Struct J 94:1-66, 1997) were very conservative. The parametric studies indicate that the nominal moment section capacity increases with the increase of fiber content and fiber aspect ratio.

Highlights• PEG 400 admixture was used in concrete mixes produced for hot weather conditions • Dry materials, mixing water and curing temperatures simulated hot weather • Properties of PEG samples were found to be superior to the control concrete • Results cannot to attributed only to prevention of pore water escape by PEG 400 • A proposal was made to explain the results based on information from the literature ABSTRACT Hot climates prevail in many regions of the globe. The average summer temperature of hot arid areas is in the range of 40-50°C with temperatures exceeding these values under direct solar radiation. Curing concrete in these regions may be challenging due to limited availability of suitable water for curing and/or rapid loss of curing water by evaporation. For many years self-curing admixtures were recommended as an alternative to water curing, however, limited studies have been conducted on their performance in hot weather conditions. In this investigation, the effects of a hot climate on the fresh and hardened properties of self-curing (SC) concrete and normal conventional concrete (NC) in hot weather were studied. A watersoluble polymer self-curing agent, polyethylene glycol (PEG 400), was added to the SC mixes. The testing parameters were concrete dry materials (25 or 50 O C) and/or mix water temperatures (5, 20 or 35 O C) at the time of mixing. NC samples were continuously water cured at 25 or 50 O C, whereas the SC ones were air cured at the same temperatures. The tested properties were workability, compressive strength, splitting tensile strength, and flexural strength. It was found that SC outperformed NC under varying conditions. The results could not be simply attributed to the retention of mix water by the self-curing admixture. A more comprehensive explanation for the observations is proposed.