Pre-cast ferrocement plates as permanent formwork for reinforced concrete beams

Abstract

This study investigates the possibility of using ferrocement plates as a pennanent formwork for RC beams. A total of ten reinforced concrete beams were constructed and tested in this investigation. These beams were designed as per ACI specification (ACI 318-83) and were divided into two sets. First set, composed of five reinforced concrete beams, was designed using standard ACI shear reinforcement and second set, composed of five reinforced ~. concrete beams, was designed with lighter shear reinforcement than standard. Among the ten reinforced concrete beams, four beams were cast in ferrocement plate formworks, having single layer wire mesh. Four reinforced concrete beams were cast in ferrocement plate formworks, having double layer wire mesh and two reinforced concrete beams were cast in steel formworks. Four reinforced concrete beams with ferrocement plates had single plate joint and that of four beams had double plate joints. All finished reinforced concrete beams were cast in fmal cross-section of 175 rnm x 300 rnm with slight manufacturing tolerance. The pre-cast ferrocement plates were chosen as a substitute of wooden or steel formworks. In cement mortar used for casting ferrocement plates, a particular cement sand ratio of 1:3 has been chosen. Water cement ratio was 0.45 by weight. Ferrocement plates or planks were set by M.S angles and C-clamps. For construction of reinforced concrete beams, "a particular concrete mix 1:2.3:2.7 (by weight) has been used throughout the investigation. At first, ferrocement plates were cast in wooden formwork and cured about 21 days. Then reinforced concrete beams were cast in ferrocement plate formworks. The beams were cast in laboratory, cured for 21 days and tested at 28 days using Tokyo Testing Machine. The first cracking load of reinforced concrete beams with ferrocement plates were higher by 11% to 37% than that of conventional reinforced concrete beams. This increase in cracking load was achieved using about 35% reduced shear reinforcement than standard and without applying any mechanism for improving bonding between plates and concrete. Cracking resistance of reinforced concrete beam can therefore be improved significantly by using pre-cast ferrocement plates as permanent formworks. It appears that 'lighter shear reinforcement may be used in construction of reinforced concrete beams where ferrocement plates are used as permanent formworks, show no ill-effect in carrying capacity of the cracking loads. Ferrocement plates are desirable from serviceability condition and as formation of initial cracks are found to appear at relatively higher loads. TIle ultimate load of reinforced concrete beams with ferrocement plates were higher about 6% to 27% than that of conventional reinforced concrete beams. Use of ferrocement plates, was found to increase in ultimate loads significantly. The load deformation response of the beams with ferroccment plates were very much sinlilar to that of conventional reinforced concrete beams. Reinforced concrete beams with single plate joint showed little stiffer response to load compared to conventional reinforced concrete beams. Ferrocement plates with double layer wire mesh did not exhibit any significant influence on cracking compared to ferrocement plates with single layer wire mesh. So, single layer wire mesh ferrocement plates is recommended for construction of ferrocement plates to be used as permanent formworks. The present study indicates that the permanent pre-cast ferrocement plate formworks can be a valuable proposition in concrete construction technology particularly considering durability and under aggressive environments. With the increasing cost of timber and shuttering materials the use of pre-cast ferrocement permanent formwork is advocated and because of its environment friendliness, it is expected to replace the timber formworks in developing countries like Bangladesh.