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Real fracture toughness of FRC and FGC: size and boundary effects

Elakhras A. A., Saleem M. H., Sallam H. E. M.

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 2

art. no. e99, 1--17

The present dilemma is how to simulate the real crack in full depth (FD) fiber-reinforced concrete (FRC), FD FRC, to get the actual fracture toughness of such fibrous composites, i.e., through-thickness pre-cracks are inappropriate for such materials. To overcome this dilemma, a new technique was adopted to create a pre-matrix crack (MC) without cutting the fibers bridging the two surfaces of the pre-crack. The main objective of the present work is to study the size and boundary effects on the real fracture toughness of MC-FD FRC and functionally graded concrete (FGC). Forty-eight MC-FD FRC and MC-FGC beams with three different span to depth ratios L/d equal 4, 5, and 6, and three different beam depths of the same beam span have been tested under three-point bending. All beams have the same pre-MC length to beam depth ratio (ao /d) of 1/3. Hooked end steel fibers of 1% fiber volume fraction produced FRC. FGC beams consist of three equal layers, FRC layer at the tension side, normal strength concrete layer at the middle of the beam, and high strength concrete layer at the compression side. The applied load versus all beams' crack mouth opening displacement (CMOD) curves have been analyzed. The present load/ CMOD results showed that beams having constant L/d ratios are recommended to capture independent size effect parameters. The size effect law (SEL) and boundary effect model (BEM) are good candidates to predict the size effect. According to the maximum non-damaged defect concept, the SEL is more reliable in predicting MC FD FRC fracture toughness than BEM.

Load-deflection behaviour of hybrid concrete flat slab

Makki Ragheed F., Alalikhan Ahmed A., Al-Katib Haider A.

Przegląd Naukowy Inżynieria i Kształtowanie Środowiska

2019

Vol. 28, No. 4

516--525

Due to the important role of high strength concrete in the structural systems, present work focuses on the use of this material as a strengthening technique incorporating with the normal strength concrete in flat slab system. Eight simply supported flat slab models with (1,000 ×1,000 ×120 mm) dimensions are investigated based on three groups including normal strength concrete and high strength concrete. The first group represents models containing of two flat slabs fully with one type of concrete; NSC and HSC as control flat slab. The second and third groups consist of six flat slabs as hybrid flat slabs of two layer of concrete with different thicknesses. Concrete mixture HSC was used in tension zone in three hybrid flat slabs (second group) with three thicknesses (30, 60 and 90 mm), while the remaining three hybrid flat slabs (third group) was used the HSC in compression zone with the same previous thicknesses. The experimental results shown that the ultimate load increased about (19.4%) when HSC was used fully (hH /h=1) instead of using NSC in the control flat slab (NSC slab). The hybrid flat slabs with use HSC in compression zone showed higher in cracking and ultimate flexural loads compared with those of the hybrid flat slabs with use HSC in tension zone and also were stiffer in load-defl ection curve with the hybrid flat slabs with HSC in tension zone, also the hybrid flat slabs showed an improvement in the cracking load and ultimate flexural load when increasing the thickness of the HSC layer (hH/h) in both tension and compression zone as compared to control flat slab (NSC slab).

Optimised mix design for normal strength and high performance concrete using particle packing method

Gopinath S., Ramachandra-Murthy A., Ramya D., Iyer N. R.

Archives of Civil Engineering

2011

Vol. 57, nr 4

357-371

This paper presents the details of optimized mix design for normal strength and high performance concrete using particle packing method. A critical review of mix design methods have been carried out for normal strength concrete using American Concrete Institute (ACI) and Bureau of Indian Standards (BIS) methods highlighting the similarities and differences towards attaining a particular design compressive strength. Mix design for M30 and M40 grades of concrete have been carried out using ACI, BIS and particle packing methods. Optimization of concrete mix has been carried out by means of particle packing method using EMMA software, which employs modified Anderson curve to adjust the main proportions. Compressive strength is evaluated for the adjusted proportions and it is observed that the mixes designed by particle packing method estimates compressive strength closer to design compressive strength. Further, particle packing method has been employed to optimize the ingredients of high performance concrete and experiments have been carried out to check the design adequacy of the desired concrete compressive strength.