رفتار سازه ای دال های کامپوزیت بتن تقویت شده-فروسیمان HCWA

This study was performed with the aim to assess the structural behaviour of ferrocement–reinforced concrete composite slab system with high calcium wood ash (HCWA) high strength mortar used as the compression zone. The proposed slab system consisted of conventional reinforced concrete slab topped with a layer of high strength ferrocement composite containing various contents of HCWA by total weight of binder. A total of six numbers of one-way composite slab prototypes were subjected to four point flexural load test to ultimate failure. The main parameters of the study include serviceability moment, ultimate moment capacity, flexural stiffness in serviceability and post cracked conditions, crack width development, crack spacing and failure mode. Results of the investigation indicate a significant enhancement in the first crack load and ultimate failure load of the composite slab system with the use of HCWA in the mortar layer at cement replacement level of 2% to 8% by binder weight. In addition, the inclusion of HCWA at various replacement levels also contributed to a reduction in the magnitude of average crack width at a given flexural load.

Several recent research studies had established that high calcium wood ash (HCWA) can be used as mineral admixture for production of high strength mortar and ferrocement composite. In recent studies [1], [2], [3] and [4], wood ash with very high calcium oxide mineral content had been thoroughly characterised and incorporated as a constituent binder material in the production of high strength mortar. The studies concluded that the use of high calcium wood ash as a partial cement replacement material at replacement level up to 6% by total weight of binder had resulted in a significant improvement in mechanical strength and durability performance of mortar produced. It was justified that the use of high calcium wood ash which has high Portlandite content in conjunction with densified silica fume which is rich in amorphous silica content triggers a rigorous pozzolanic reaction between the two substances. The reaction resulted in the formation of greater amount of secondary calcium silicate hydrate and refinement in pore structure of resultant cementitious composites [2] and [3]. Hence, a dense mortar with high compressive strength, flexure strength and durability performance which is suitable for use as mortar matrix in high performance ferrocement composite was formulated. Ferrocement is a thin reinforced concrete composite typically consists of cement mortar reinforced with closely spaced layers of continuous small diameter wire mesh closely binded together to create a stiff structural form. The materials for the reinforcing mesh used are normally steel, synthetic woven fibres or fibre reinforced polymers [5]. As compared to the conventional reinforced concrete, ferrocement is reinforced in two directions. Hence, there is a tendency for ferrocement composites to have homogeneous isotropic properties in both longitudinal and transversal directions. With that, ferrocement composites usually exhibit high tensile strength, high modulus of rupture and excellent bonding interaction between embedded internal mesh reinforcements and surrounding cement mortar matrix [6]. Besides, ferrocement composite also possesses high degree of elasticity and resistance to cracking. These attributes have resulted in the successful application of ferrocement composites in the fabrication of ship’s hull, building construction (low cost housing), rehabilitation of existing structures and fabrication of floating marine structures and sewerage pipelines [5] and [7]. In the current development of ferrocement applications, several studies [8], [9], [10], [11], [12], [13] and [14] have been performed to study the structural performance of ferrocement–reinforced concrete composite structural elements. Al-Kubaisy and Jumaat [14] studied the flexure behaviour of reinforced concrete slab with ferrocement tension zone cover. In the study, it was concluded that composite slab with ferrocement tension zone cover exhibited superior flexure stiffness, crack development behaviour and higher first crack moment as compared to an equivalent conventional reinforced concrete slab. In another related study [11], the shear transfer mechanism within a ferrocement–reinforced concrete composite beam was investigated. The study focused on the transfer of shear stress between the ferrocement layer and the reinforced concrete beam upon being subjected to flexural load condition. The placement of shear studs to bridge the ferrocement layer and the reinforced concrete beam was recommended in the study. This is to ensure a proper composite interaction between the ferrocement and the reinforced concrete beam. Thanoon et al. [9] proposed a novel ferrocement composite slab system with ferrocement tension zone and engineering clay bricks in the compression zone. The study focused on the method to ensure a proper composite interaction between the tension and compression layers which were separated by a cold joint. It was concluded that a cast in steel truss system which bridge the tension and compression layers is an effective interlocking mechanism for transferring the shear stresses which is developed between the ferrocement tension layer and the engineering clay bricks compressive layer. The steel truss system also solves the cold joint problem and prevents the longitudinal crack problem which normally occurs at the failure of composite structural element. Most of the past studies performed on the ferrocement–reinforced concrete composite flexural member focused on the use of ferrocement as the tension layer. However, the structural behaviour of ferrocement–reinforced concrete composite flexural member with high strength ferrocement compression zone has not been studied. In this study, the structural performance of composite slabs with high strength ferrocement compression zone fabricated using high strength mortar with various HCWA contents was investigated. The engineering parameters considered in the study are flexure stiffness, serviceability load, ultimate load bearing capacity and crack development behaviour.

From the analysis and interpretation of the results obtained from the study, the following conclusions and recommendations can be derived. (1) HCWA can be included as a cement replacement material in silica fume-cement binary cement mortar at replacement level up to 6% by total binder’s weight to enhance the bulk densities, compressive strength, flexural strength and Young’s modulus of high strength mortar containing silica fume. (2) The serviceability and ultimate bending load capacity of ferrocement–reinforced concrete composite slab can be significantly enhanced by the use of HCWA in the mortar matrix at various cement replacement levels up to 8% by total binder weight. (3) The inclusion of HCWA in the high strength mortar matrix at various cement replacement levels up to 8% enhanced the flexural stiffness of ferrocement–reinforced concrete composite slab fabricated. This is indicated by the reduced magnitude of deflection for a given load. (4) The use of HCWA in the top mortar layer of a ferrocement–reinforced concrete composite slab enhances its crack resistance under flexural load. The maximum recommended level of cement replacement level for the purpose is 8 % by total weight of binder. (5) The optimum contents of HCWA for the best load bearing capacity, flexural stiffness and crack resistance performance are 2–4 % by weight of binder. (6) The strengthening of the compression zone of reinforced concrete slab member using high strength ferrocement composite containing HCWA has a marginal effect on the crack spacing of the composite slab at ultimate failure. (7) No de-bonding of the top ferrocement composite layer was observed to occur at the ultimate failure of the composite slabs. Hence, a strong bond exists between the ferrocement composite topping and the substrate concrete of the composite slab if both layers are cast without cold joint. (8) The ultimate failure of ferrocement–reinforced concrete composite slab occurs in two stages namely tensile failure of primary tension reinforcement followed by failure of the wire mesh reinforcements in the ferrocement top layer before the final collapse of the structure member.