Freeze/thaw protection of concrete with optimum rubber crumb content

This research looks at utilising an optimum quantity of rubber crumb as an air entraining ad-mixture in concrete, thus providing maximum freeze-thaw protection and maximum strength. Microscopic and chemical analysis was carried out on the rubber sample to investigate how rubber crumb entrains air and reacts with the surrounding concrete. The work contained two pilot studies that informed the main test methodology. The pilot studies examined the air content/compressive strength relationship (1) and freeze/thaw cycle durations (2). Pilot study 1 informed the main test program by identifying an optimum addition of rubber crumb to a concrete mix, which was found to be 0.6% by weight of concrete. The main test investigated the use of rubber crumb in providing freeze-thaw protection of a C40 concrete mix after 3 days of curing. A freeze-thaw test was carried out on three separate batches of concrete containing washed rubber crumb, unwashed rubber crumb and plain concrete respectively. It was found rubber crumb was effective in providing freeze/thaw protection in both cases. This work builds on recent work to identify the best practical solution for reducing waste and providing the maximum freeze/thaw protection for a cleaner production process

Hardened properties for high-performance printing concrete

This paper presents the hardened properties of a high-performance fibre-reinforced fine-aggregate concrete extruded through a 9 mm diameter nozzle to build layer-by-layer structural components in a printing pro- cess. The printing process is a digitally controlled additive method capable of manufacturing architectural and structural components without formwork, unlike conventional concrete construction methods. The effects of the layering process on density, compressive strength, flexural strength, tensile bond strength and drying shrinkage are presented together with the implication for mix proportions. A control concrete (mould-cast specimens) had a density of approximately 2250 kg/m3, high strength (107 MPa in compression, 11 MPa in flexure) and 3 MPa in direct tension, together with a relatively low drying shrinkage of 175 μm (cured in water) and 855 μm (cured in a chamber at 20 °C and 60% relative humidity) at 184 days. In contrast well printed concrete had a density of 2350 kg/m3, compressive strength of 75–102 MPa, flexural strength of 6–17 MPa depending on testing direction, and tensile bond strength between layers varying from 2.3 to 0.7 MPa, reducing as the printing time gap between layers increased. The well printed concrete had signifi- cantly fewer voids greater than 0.2 mm diameter (1.0%) when compared with the mould-cast control (3.8%), whilst samples of poorly printed material had more voids (4.8%) mainly formed in the interstices be- tween filaments. The additive extrusion process was thus shown to retain the intrinsic high performance of the material