Mechanics and durability of surface treated structural foamed concrete
Date
2020-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Lightweight foamed concrete (LWFC) has been around for a few decades and has mostly been used in non-structural applications such as void filling, soil stabilisation, and backfill. Historically, LWFC has been characterised as a material with low mechanical properties (compressive- and tensile strength) which has prevented the material from being considered for structural applications. However, advances in technologies lead to better mechanical properties for LWFC. Better mechanical properties coupled with good thermal performance is what makes LWFC an attractive alternative to normal weight concrete (NWC). Jones and McCarthy (2005) published a study investigating the potential of foamed concrete (FC) as a structural material. Jones and McCarthy found that FC can indeed be used for structural application based on its compressive strength but should not directly replace NWC concrete. Based on the history of LWFC and its promising future a study was launched at Stellenbosch into the mechanics and durability properties of LWFC. The mechanics portion includes the characterisation of the mechanical properties such as compressive- and tensile splitting strength, elastic modulus, specific fracture energy and bond behaviour of deformed steel in reinforced lightweight foamed concrete (R/LWFC). The durability aspect includes the study of carbonation and chloride-induced corrosion of R/LWFC and the enhancement of these properties using surface treatment agents. In this research the compressive- and splitting tensile strength of LWFC was tested in accordance with the South African National Standards, the specific fracture energy was measured using the wedge splitting test, and, the bond between deformed steel reinforcement and LWFC is characterised using the conventional pull-out (PO) test and the modified beam-end (BE) test methods. Tests were conducted on LWFC specimens with densities of 1200 kg/m3 - 1600 kg/m3. The application of steel reinforced LWFC with density 1400 kg/m3 in a prefabricated structural walling system is successfully demonstrated in laboratory experiments.
The results of the compressive strength tests showed that LWFC with density of 1400 kg/m3 and above can be designed to produce strengths considered structural. The tensile splitting strength of LWFC holds similar proportionality of roughly 10 % of the compressive strength. The specific fracture energy of LWFC is relatively low compared with NWC, of which the relative brittle nature of LWFC is testimony. Significant improvement is accomplished by adding low volume of synthetic fibre. The results of the PO test yielded design bond values greater than that obtained from the BE tests for LWFC. The design bond values of LWFC are considerably lower than that of NWC and is highly influence by the specific fracture energy and tensile strength of the concrete. A model for predicting the design bond stress of LWFC was set-up using the principle of equilibrium and performed with the more appropriate BE test results. Evaluation of the predictive model shows relative agreement between the predicted results and the actual results. Based on the results of the model LWFC with density of 1200 kg/m3 exhibits poor bond behaviour, suggesting only higher densities, 1400 kg/m3 and above, should be considered for reinforcement and structural use. The resistance to carbonation penetration and carbonation-induced corrosion was tested on a single density (1400 kg/m3) of LWFC. The evolution of the carbonation front over time was monitored on freshly cut cube specimens sprayed with phenolphthalein and singly reinforced rectangular specimens were prepared for corrosion assessment using the half-cell potential method. Additionally, the enhancement of LWFC with two surface treatment agents (integral hydrophobic agent and a non-integral hydrophobic agent) was also tested. All LWFC specimens were exposed to a carbonation rich environment of 0.13% CO2 concentration.
The results of the carbonation penetration test show that LWFC carbonates at a high rate due to the porous nature of the concrete. The use of surface treatment significantly reduces the rate of carbonation rate of LWFC. This is evident in the result at the end of the experiment whereby a near 50% reduction was achieved by using non-integral surface treatment. The results of the corrosion potential indicated that little to no corrosion took place over the study period.
The durability of LWFC against chloride penetration and chloride-induced corrosion was tested in three separate series. For Series 1 singly Y12 reinforced rectangular specimens of dimension 460x100x100 mm with cover depth 24 mm and a 6 mm stainless steel counter-electrode were prepared for cyclic ponding and corrosion assessment using the coulostatic method. For this series the enhancement of LWFC durability was promoted using two surface treatments, non-integral “hydrophobic” agent and integral “pore-blocker”, respectively. The results of chloride profiling using XRF showed deep penetration of chlorides in the untreated concrete and cracked regions (cracks caused by restraint drying shrinkage) passed the reinforcement level. Surface treatment significantly reduced the penetration depth of chlorides in cracked and uncracked regions with the non-integral treatment performing the best. Reduced corrosion rates were observed in treated specimens.
For Series 2 unreinforced and singly reinforced rectangular specimens of dimension 500x100x100 mm and 460x100x100 mm made with concrete cover of 20 mm and 35 mm were prepared for chloride penetration testing and corrosion assessment respectively. For this series the enhancement of LWFC durability against was done similarly to that of Series 1. Specimens were prepared using a single density (1600 kg/m3) of LWFC with different ash to cement ratios, 0, 1, and, 2, respectively. Chloride penetration testing was done on freshly cut specimens sprayed with a solution of 0.3 M AgNO3.
The results of the chloride penetration tests showed an increase in chloride penetration depth with an increase in ash to cement ratio and increase over time. The use of surface treatment agents proved effective with a decrease of over 80% achieve for the chloride penetration depth for the worst performing specimen at the end of the experiment. The corrosion assessment specimens underwent severe cracking before the start of testing. This complicated the assessment of the efficiency of surface treatment agents in preventing corrosion or reducing the corrosion rates for Series 2. For Series 3 corrosion assessment of singly reinforced rectangular specimens of dimension 660x150x150 mm with cover depth 69 mm produced with 1200 kg/m3, 1400 kg/m3, and, 1600 kg/m3 was performed over a period of 21 weeks. Similar to Series 3 the efficacy of surface treatment was also evaluated. Untreated specimens generally have the highest corrosion rates, with the non-integral treated specimens generally experiencing the lowest corrosion rates.
LWFC can be reinforced with steel for structural application. However, the material has a relatively low elastic modulus which will result in greater deflections for structural elements and will initiate cracking earlier during loading compared to NWC. The design of structural elements for R/LWFC may be governed by SLS.
AFRIKAANSE OPSOMMING: Geen opsomming
AFRIKAANSE OPSOMMING: Geen opsomming
Description
Thesis (PhD)--Stellenbosch University, 2020.
Keywords
Reinforced concrete, Concrete construction, Foamed materials, Lightweight concrete -- Mechanical properties, UCTD