Forensic Analysis of an Elevated Pool Vault

Expansive behavior of thick concrete slabs affected by alkali-silica reaction (ASR)

Shrinkage and load-induced strains were measured for 27 years in the in-situ concrete first floor and roof slabs of a long, two-storey office, using embedded vibrating wire strain gauges. The floor and roof slabs were cast on precast crossbeams and precast columns supported the crossbeams. Longitudinal precast beams between the columns supported the brick cladding and office windows but they were not connected to the floor or roof slabs. A central staircase provided longitudinal stability and a movement joint was provided at the access corridor to an existing, adjacent stone tower. A lightweight coarse aggregate was employed in the precast and in-situ concretes. The shrinkage and load-induced strains were determined using horizontal strain gauges parallel and transverse to the length of the slab and the direction of the expected flexural stresses. Concrete prisms were cast and instrumented for shrinkage under site exposure and elastic modulus, creep and shrinkage under sealed and dry exposure conditions in a laboratory. The drying shrinkage in the 190-mm-thick floor slab levelled-off around 500 microstrain (μs) at 6 years, a smaller value than the 750 μs from the site prisms at 3 years. The expected flexural load-induced strain profiles in the floor were superposed by a tensile stress component of strain. The tensile stress was attributed to the restraint of the drying shrinkage by the longitudinal precast beams. The deformation properties of the concrete were used in conjunction with the floor slab shrinkage to estimate a maximum tensile stress of 1·3 MPa. This was significantly smaller than the tensile splitting strength of 4·4 MPa that was measured at 10·5 years and suggested that there would be little associated cracking. The drying shrinkage in the 190 mm thick roof slab, sealed on the upper surface, was 450 μs after 27 years but, unlike the floor, had not yet levelled-off. Furthermore, a swelling was observed between 7 and 10 years that was due to water infiltration through leaks in the bituminous roof membrane. Shrinkage resumed after the roof had been repaired. The load-induced flexural strains in the roof, like the floor, were superposed by a tensile stress component of strain that could be attributed to restraint of drying shrinkage by the precast longitudinal beams. The maximum tensile stress in the roof slab was estimated to be 1·5 MPa. The risk of reinforcement corrosion from the roof leak was assessed by measuring the depth of carbonation in a prism stored in the ceiling void below the roof. The 12 mm depth of carbonation was safely smaller than the 20 mm depth of cover to the reinforcement. The longitudinal movement of the first floor caused cracking of the attached ground floor blockwork at 5 years. The mortar joint to the floor slab was replaced with mastic and no further cracking was observed. The floor slab movement also caused cracking of several large windows in the glazed corridor that provides access to the offices.

Flexural Behavior of Steel Reinforced Lightweight Concrete Slab with Bamboo Permanent Formworks

Alkali-silica reaction (ASR) is an important factor that seriously affects the durability of reinforced concrete (RC) structures. The current research on alkali-aggregate mainly focuses on the deterioration mechanism of materials and the mechanical properties of standard specimens. However, there is a gap in the field of research on the effect of alkali-aggregate damage on the level of RC structures. In this study, five RC beams were tested, and the depth and location of alkali solution immersion were used as the test variables, with the aim of investigating how the steel reinforcement suppresses the expansion caused by ASR and evaluating the shear behavior of RC beams after non-uniform ASR damage. The results of the study showed that immersion in an alkali solution and an increase in immersion depth accelerated the rate of expansion development, while steel reinforcement inhibited the rate of expansion development. Compared with undamaged RC beams, ASR initially generates expansion stresses within the concrete, which increase the cracking and yield loads of RC beams and delay the cracking of RC beams, and ASR reduces the ultimate load-carrying capacity and ductility of RC beams due to the disruption of the concrete microstructure. Finally, a chemo-mechanical analysis method is proposed based on experimental results, which incorporate an ASR expansion model and a pore mechanics model. The efficacy and precision of this model are validated through comparison with experimental results.

One of the most important physical and mechanical characteristics of inter-floor construction is a high resistance to stretching forces. An increase in the flexural strength in bending is achieved by reinforcement steel, which has been used in reinforced concrete structures over the course of decades. The use of dispersed micro-reinforcement with an amorphous fiber based on an alloy of the Fe–B–C system is proposed to reduce the consumption of reinforcement. During the research, the calculation was performed in the LIRA-SAPR software package to justify the effectiveness of dispersed concrete reinforcement with amorphous fiber based on the Fe–B–C molten systems for use in the floor slabs of a multi-story building. The relevance of the work is due to the economic feasibility of reducing expensive materials without loss of performance property. The results of the research showed new possibilities of using amorphous fiber to save traditional building materials in particular steel reinforcement in tensile structures. Because of the simulation, the program proved that with the addition of amorphous fiber, we can reduce the cost of secondary reinforcement in the floor slab by about 40%, while the main reinforcement remains unchanged.

ASTM (American Standard Testing and Materials) C 618 prohibits use of biomass ash in concrete. This paper systematically investigates mitigation of Alkali Silica Reaction (ASR) expansion of concrete by three biomass ashes (cement: biomass ash = 65: 35 by weight), and the ASR expansion is triggered by high alkali cement and opal (1-9% weight of quartz replacement). The three biomass ashes come from switchgrass or sawdust cofired with Powder River Basin coal and they cut the ASR expansion significantly below 50% of the control level; however, all three biomass ashes doubled the available alkali of the cement they replaced. Therefore, the exclusion of biomass ash in concrete by ASTM C 618 seems impropriate and more quality research on its role in mitigating ASRs expansion should be conducted.

Coating material for repair of concrete structures damaged or deteriorated due to alkali silica reaction (ASR), requires water vapor permeability as well as water impermeability. It requires, furthermore, strong adhesion to concrete and elongation for the remaining expansive power of ASR. A flexible polymer modified cement mortar (PCM), which consists of flexible polyacrylic polymer dispersion, portland cement and admixtures has above-mentioned performance. Physical properties of the flexible PCM were measured at various polymer cement ratios. Water and water vapor permeability of the flexible PCM was reduced with increasing in polymer cement ratio. Since the flexible PCM with a high polymer cement ratio had a structure in which the larger pores were filled by polymer and sealed by continuous polymer film, elongation of the flexible cement mortar tended to increase with increasing in polymer cement ratio. Whereas adhesion had an optimum polymer cement ratio. A field exposure test was performed using concrete specimens damaged by ASR. The specimens were coated with either epoxy resin or the flexible PCM at various polymer cement ratios. Coating with the flexible PCM was found to inhibit ASR, but the epoxy resin promoted ASR. The ASR expansion was reduced with increasing in water vapor permeability of the flexible PCM.

Prestressed concrete members have gained popularity as an efficient and effective way of designing a structural member with the best engineering and material properties. The method of prestressing a structural concrete member has the capability of controlling increased service loads with less depth over longer spans. However, deflections from over-loading or loading over time give a disadvantage to the common steel reinforced concrete members by the effect of corrosion as the structural concrete develop cracks. To prevent corrosion of a structural concrete member, exchanging of steel reinforcement with Fiber Reinforcing Polymers (FRP) has sparked engineering interest in recent years. Both prestressed and non-prestressed FRP reinforcement can reduce tension in concrete. However, the performance of such structural members under elevated temperatures is currently unknown. The knowledge and application of this may lead to a cost effective, and practical consideration in fire safety design. In this article, an analytical model is developed using flexural rigidity of a concrete T-beam with both prestressed and non-prestressed FRP reinforcement to study the deflection behavior at practical elevated temperatures. The model is compared with the finite element model (FEM) of a T-beam with both prestressed and non-prestressed reinforcement subjected to practical elevated temperatures. In addition, comparison is also made with an indirect reference to the real behavior of the material. The results of the analytical model correlated reasonably with the FEM and the real behavior, and were within the accepted range of the American Concrete Institute (ACI) specifications.

Blended cements were studied for their efficacy against sulphate attack and alkali-silica reaction using six different types of fly ashes, a slag, a silica fume and four types of General Use Portland cement of different alkalinity. The study results showed that low calcium fly ash, silica fume and ground granulated blast furnace slag enhanced the sulphate resistance of cement with increased efficacy with the increase in the replacement level. However, slag and silica fume, especially at low replacement levels, exhibited increased rate of expansion beyond the age of 78 weeks. On the contrary, high calcium fly ashes showed reduced resistance to sulphate attack with no clear trend between the replacement level and expansion. Ternary blends consisting of silica fume, particulary in the amount of 5%, high calcium fly ashes and General Use (GU) cement provided high sulphate resistance, which was attributable to reduced permeability. In the same way, some of ternary blends consisting of slag, high calcium fly ash and GU cement improved sulphate resistance. Pre-blending optimum amount of gypsum with high calcium fly ash enhanced the latter's resistance to sulphate attack by producing more ettringite at the early stage of hydration. In the context of alkali-silica reaction permeability was found to be a contributing factor to the results of the accelerated mortar bar test. High-alkali, high-calcium fly ash was found to worsen the alkali silica reaction when used in concrete containing some reactive aggregates. Ternary blend of slag with high calcium fly ash was found to produce promising results in terms of counteracting alkali-silica reaction.

Blended cements were studied for their efficacy against sulphate attack and alkali-silica reaction using six different types of fly ashes, a slag, a silica fume and four types of General Use Portland cement of different alkalinity. The study results showed that low calcium fly ash, silica fume and ground granulated blast furnace slag enhanced the sulphate resistance of cement with increased efficacy with the increase in the replacement level. However, slag and silica fume, especially at low replacement levels, exhibited increased rate of expansion beyond the age of 78 weeks. On the contrary, high calcium fly ashes showed reduced resistance to sulphate attack with no clear trend between the replacement level and expansion. Ternary blends consisting of silica fume, particulary in the amount of 5%, high calcium fly ashes and General Use (GU) cement provided high sulphate resistance, which was attributable to reduced permeability. In the same way, some of ternary blends consisting of slag, high calcium fly ash and GU cement improved sulphate resistance. Pre-blending optimum amount of gypsum with high calcium fly ash enhanced the latter's resistance to sulphate attack by producing more ettringite at the early stage of hydration. In the context of alkali-silica reaction permeability was found to be a contributing factor to the results of the accelerated mortar bar test. High-alkali, high-calcium fly ash was found to worsen the alkali silica reaction when used in concrete containing some reactive aggregates. Ternary blend of slag with high calcium fly ash was found to produce promising results in terms of counteracting alkali-silica reaction.

Repair, protection and waterproofing of concrete structures: by Philip H. Perkins Published by Elsevier Applied Science Publishers Ltd., Crown House, Linton Road, Barking, Essex IG11 8JU, England, 1986 ISBN 1 85166 008 9, Price: £35.00, XIV + 302pp

Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams

A strength and stiffness comparative analysis has been made of a concrete slab reinforced with composite-reinforcement rods and a slab reinforced with steel rods. The stress-strain state has been assessed for both versions of reinforcement of the slab. The stress-strain state was determined under the action of only static load and with subsequent application of temperature fields, i.e., under standard-fire conditions. It has been shown that the fire resistance of the slab with a composite reinforcement turns out to be 1.6 higher as far as the bearing capacity is concerned, than the fire resistance of the slab with a steel reinforcement, although the initial deflection due to the action of only static load for the slab reinforced with composite rods exceeds six to seven times the deflection of the slab reinforced with steel rods.

Structural reinforcement of bridge decks using pultruded GFRP grating

Aiming at the issues of the true mechanical performance and failure mechanism of large-span cast-in-situ hollow core floor slabs with square-box core molds under vertical loads, this study conducted systematic research combining in-situ tests and refined numerical simulations. Based on an actual project at the Hefei Xinluzhou Industrial Park, a distributed water tank loading system was employed to apply five-stage cyclic loading to an 8m × 8m slab section. The strains in both the reinforcing steel and concrete were monitored in real-time. Simultaneously, a coupled finite element model incorporating a concrete plastic damage constitutive model and an elastic-plastic model for steel reinforcement was established. This model was used to quantitatively analyze the stress distribution, stiffness evolution, and failure progression. The study shows that under a design load of 9.0kN/m², the floor slab operates in an elastic state, with stress distribution exhibiting “banded gradient” and “island-shaped loading” characteristics. This means a banded gradient in the middle span and localized concentration at the column capital edge. The strain-load relationship of the steel reinforcement and concrete is linear, and the bidirectional stiffness is similar. The finite element model accurately reproduced the experimental phenomena, confirming its reliability. Limit analysis revealed that the floor slab's load-bearing capacity reaches 27.2kN/m², with failure starting from diagonal cracks at the column capital edge and propagating to the positive bending cracks at the middle span bottom of the slab. The findings of this study reveals the bidirectional bending mechanism and failure path of box-shaped core mold hollow floor slabs, providing important theoretical and experimental support for optimizing the design of large-span floor slabs.