Repositório RCAAP

A norma NBR 7212, para execução de concreto dosado em central, estipula o tempo máximo para que o concreto seja descarregado (aplicado) completamente em 150 min; porém, na prática, ocorrem situações onde caminhões ficam carregados por tempos bem acima desse limite. O objetivo principal deste artigo consiste na avaliação do comportamento do concreto em relação à sua resistência à compressão, quando utilizado posterior ao tempo máximo de mistura e transporte especificado pela norma. Para tal, adotou-se como procedimento o restabelecimento do abatimento à condição inicial com aditivo superplastificante por um período de 6 horas. Os resultados mostram que não houve perda de resistência à compressão para esse tempo de mistura prolongada, nas condições dessa pesquisa.

The intersections between beams and columns in a reinforced concrete building structure are called frame joints. It is a region with significant bending stiffness but subjected to large shear stresses. Appropriate modeling of the flexibility of reinforced concrete frames is essential to its design, in service limit states as well as in ultimate limit states. It has been shown, theoretically as well as experimentally, that the influence of joint flexibility may account for 20% of total structural lateral displacement. Models using only bar elements and rotational springs are proposed to consider the joint flexibility in linear analyses of building structures. In order to validate the proposed model, comparisons with experimental results found in the literature are made. Finally, the results of second order analyses using the proposed model are compared with those obtained by finite elements.

Reinforced concrete waffle slabs have become a common option for designers due to a need of rationalization in construction with reduction in costs and deadlines. To better understand the behavior of this structural system, and more realistically quantify stresses and displacements, a full scale waffle slab was tested. The structure, designed to serve as a tennis court floor, was submitted to a load of 12 kN/m2 and instrumented to measure strains and deflections at different locations. The loading process used the floor's constructive base filling material and readings were taken at different loading stages and arrangements during the floor's construction. Test data was compared to results obtained from the matrix analysis program Sistema Computacional TQS v11.0 and from the finite element model program SAP2000 v14.2.2. Slab behavior was as expected, with deflection and bending moments close to those determined by the numerical analysis.

The aim of this study was to evaluate cementitious matrices properties by partial replacement of Portland cement by silica fume (SF) or by rice husk ash (RHA), and their application in nonbearing hollow blocks, tested by destructive and non-destructive methods. The following mixtures were produced: reference (100% of Portland cement) and Portland cement replacement (10% by mass) with SF or RHA. The non-destructive testing showed that the highest values of UPV were obtained for SF-based blocks and RHA-based blocks. The destructive test showed better results for SF-based blocks, but there was no statistical difference between the RHA-based and control ones.

Rheology studies the deformation and flow of matter and seeks to describe the deformations of material depending on the time they are exposed to mechanical actions. This paper proposes to understand the behavior of fresh mortar adhesives in Squeeze Flow tests based on rheological characterization of different commercial compositions. In the tests, flow of material results from the application of a compressive load on the sample in the fresh state which causes displacement due to stresses generated during radial shear flow. We performed further physical characterizations of mortars and viscosity on material passing the through a sieve with a 0.075 mm sieve opening with the objective of basing the analysis of the results obtained from Squeeze Flow tests. It was observed that the mortars studied showed differences in rheological behavior, probably resulting from synergistic action of some of the composition parameters, especially the particle size distribution and particle morphology.

This paper proposes an automatic method based on the computing vision, implemented in a mobile platform, to inspect cracks in masonry and concrete. The developed algorithm for image processing performs this task from images of the cracks evolution. The contribution of this paper is the development of a mobile tool with quick response aiming to assist technicians in periodic visits when monitoring the crack opening in masonry and concrete. The obtained results show, successfully, the dimensional alterations of cracks detected by mobile phone in a faster and accurate way compared with the conventional measurement technique. Regardless the irregular shape of the cracks, the proposed method has the advantage of producing results statistically significant in measurement repetition by decreasing the subjectivity inherent to manual measurement technique.

The structural behavior of pile caps with sockets embedded is influenced by interface of column-socket, which can be smooth or rough. With intent to analyze the behavior of two pile caps with embedded socket, considering the friction between the column and the socket, with eccentric normal load, the numerical simulations were carried out, using a program based on the Finite Element Methods (FEM). In the numerical analysis the non-linear behavior of materials was considered, also the friction between the column and the socket. It was considered perfect bond between the reinforcement and the concrete around. It was observed that the embedded length is preponderant factor in the structural behavior of the analyzed element.

Innovative procedures for monitoring experimental tests using photogrammetry and image processing have been recently proposed. This manuscript aims at providing a practical demonstration of the advantages of using these new techniques in experimental tests up to failure. In this scope, focus is given to the curvature, rotation and to the tension stiffening effect, i.e., the contribution of the concrete between cracks to increase the bending stiffness. These parameters are crucial to describe the structural behavior of reinforced concrete beams both in serviceability and in ultimate limit states. These new techniques allow monitoring a high number of points, enhancing the traditional monitoring methods and providing more information which would be impossible to obtain using the traditional methods.

The present study evaluates the bond behavior between steel bars and concrete by means of a numerical analysis based on Finite Element Method. Results of a previously conducted experimental program on reinforced concrete beams subjected to monotonic loading are also presented. Two concrete types, self-compacting concrete and ordinary concrete, were considered in the study. Non-linear constitutive relations were used to represent concrete and steel in the proposed numerical model, aiming to reproduce the bond behavior observed in the tests. Experimental analysis showed similar results for the bond resistances of self-compacting and ordinary concrete, with self-compacting concrete presenting a better performance in some cases. The results given by the numerical modeling showed a good agreement with the tests for both types of concrete, especially in the pre-peak branch of the load vs. slip and load vs. displacement curves. As a consequence, the proposed numerical model could be used to estimate a reliable development length, allowing a possible reduction of the structure costs.

The most expeditious method for the design of concrete beams under fire situation is the tabular method, presented by the Brazilian standard ABNT NBR 15200:2012. Albeit simple, this method constrains the engineer's work, as it prevents him to seek alternative solutions to the few tabulated values. Yet, the Brazilian standard allows employing more advanced methods. Hence, the purpose of this work was to perform a thermal and structural analysis of beams with several widths, heights, covers and diameters/layouts of steel reinforcement (upper and lower). From those results, graphs were constructed, associating the ratio between the applied bending moment in fire over the resistance bending moment at ambient temperature, for the fire resisting time of each situation. These graphs also allow taking into account the redistribution of moments from positive to negative, which will lead to savings in the solution found.

A concrete element when kept under sustained load presents progressive strain over time, associated to the creep. In reinforced concrete columns, such deformations cause the stress increase in the steel bars of the reinforcement and may induce the material to undergo the yielding phenomenon. The pioneering formulations about the creep effect, developed on the base of creep coefficient, are applicable, especially, when the stress magnitude remains constant. Its application to reinforced concrete members, which exhibits change in stress magnitude, requires simplifications of which result the memory models, which have the disadvantage of requiring the storage of the stress history. To overcome the difficulties related to the excessive demand for computer memory, State models have been developed that dispense such robustness of storage. The subject of this work is the analysis of creep deformations in reinforced concrete columns on the base of a state model fixing its physical parameters from results obtained through the NBR 6118/07 formulation. The results showed that, in the elapse of the phenomenon, occurs stresses transfer from the concrete mass to the reinforcement steel bars which, in turn, have the effect of restrain the creep strains, confirming, in some cases, an imminent material yielding condition.

In developed countries like the UK, France, Italy and Germany, it is estimated that spending on maintenance and repair is practically the same as investment in new constructions. Therefore, this paper aims to study different ways of interfering in the corrosion kinetic using an accelerated corrosion test - CAIM, that simulates the chloride attack. The three variables are: concrete cover thickness, use of silica fume and the water/binder ratio. It was found, by analysis of variance of the weight loss of the steel bars and chloride content in the concrete cover thickness, there is significant influence of the three variables. Also, the results indicate that the addition of silica fume is the path to improve the corrosion protection of low water/binder ratio concretes (like 0.4) and elevation of the concrete cover thickness is the most effective solution to increase protection of high water/binder ratio concrete (above 0.5).

The useful life of concrete is associated with the penetrative ability of aggressive agents on their structures. Structural parameters such as porosity, pore distribution and connectivity have great influence on the properties of mass transport in porous solids. In the present study, the effect of varying the workability of concrete in fresh state, produced through the use of additives, on pore structure and on the mechanical compressive strength of hardened concrete was assessed. The pore structure was analyzed with the aid of X-ray microtomography, and the results obtained were compared to the total pore volume calculated from data derived from helium and mercury pycnometry tests. A good approximation between the porosity values obtained through the two techniques was observed, and it was found that, regardless of concrete consistency, the samples from the surface of the specimens showed a percentage of pores higher than those taken from the more inner layers.

Structural masonry using concrete blocks promotes the rationalization of construction projects, lowering the final cost of a building through the elimination of forms and the reduction of the consumption of reinforcement bars. Moreover, production of a block containing a combination of concrete and vegetable fiber sisal results in a unit with properties such as mechanical strength, stiffness, flexibility, ability to absorb energy, and post-cracking behavior that are comparable to those of a block produced with plain concrete. Herein are reported the results of a study on the post-cracking behavior of blocks, prisms, and small walls reinforced with sisal fibers (lengths of 20 mm and 40 mm) added at volume fractions of 0.5% and 1%. Tests were performed to characterize the fibers and blocks and to determine the compressive strength of the units, prisms, and small walls. The deformation modulus of the elements was calculated and the stress-strain curves were plotted to gain a better understanding of the values obtained. The compression test results for the small walls reinforced with fibers were similar to those of the reference walls and better than the blocks and prisms with added fibers, which had resistances lower than those of the corresponding conventional materials. All elements prepared with the addition of sisal exhibited an increase in the deformation capacity (conferred by the fibers), which was observed in the stress-strain curves. The failure mode of the reference elements was characterized by an abrupt fracture, whereas the reinforced elements underwent ductile breakage. This result was because of the presence of the fibers, which remained attached to the faces of the cracks via adhesion to the cement matrix, thus preventing loss of continuity in the material. Therefore, the cement/plant fiber composites are advantageous in terms of their ductility and ability to resist further damage after cracking.

This paper presents a numerical analysis of prestressed hollow core slabs under long term loading. The model considers the time dependence of material and rheological properties in order to predict the actual stage of displacements, strains and stresses. It also takes into account load changes. For the analysis, each slab is divided in a finite number of bar elements, in which the cross section is described in concrete elements, parallel to the flexural axis, and prestressed steel elements. For the results evaluation, the effective concrete area is considered. The numerical results are compared with experimental tests performed on two series of prestressed hollow core slabs. Each series had a different geometry, rate and distribution of prestressing strands. Mid-span displacements were evaluated up to 127 days after initial loading. Good correlation was achieved with both series at and below the service load level.

Lattice slabs are usual in Brazil. They are formed by precast joists with latticed bars on a base of concrete, and a cover of concrete placed at the jobsite. The assembly of the joists and the filling elements is simple and do not require manpower with great skill, presenting low cost-benefit ratio. However, it is precisely in assembling phase that arise questions related to the scaffold support distance. A mistake in the proper positioning can lead to two undesirable situations. In one of them, a small space between the support lines increases the cost of scaffold, and in other an excessive space can generate exaggerated displacements, and even the collapse of the slab in the stage of concreting. The objective of this work is to analyze the bearing capacity of lattice joists in assembling phase, looking for information that is useful in defining the scaffold support distance. Several joists were tested to define the failure modes and their load bearing capacities. The results allowed to determine equations for calculating the appropriate distance between the support lines of the joists.

The present work aims to study the replacement of Portland cement (PC) by stone cutting waste (SW) and ground waste clay brick (BW) in binary and ternary pastes. Thermogravimetry and differential thermal analysis tests were carried out at various ages in order to investigate the development of the cement hydration reactions in the presence of those wastes. The packing density was calculated in accordance with the Compressible Packing Model to understand the physical effect of those wastes. Compressive strength tests were also performed and the results were related to hydration and packing. Considering the substitution levels studied, the results indicated that the use of SW in the binary mixture accelerated the hydration reactions, and the particles packing density and compressive strength were maintained. The use of BW in the binary mixture caused a small acceleration in the hydration reactions and there was an indication of pozzolanic activity, although the compressive strength was reduced in comparison with the reference paste. In the ternary mixture, the combined effect of both wastes resulted in the maintenance of compressive strength for cement replacement content of 30%.

Punching shear is a brittle failure mode that may occur in slab-column connections, which may be prevented by using shear reinforcement in the slab-column connection. This paper presents comparisons between experimental results of 36 tests in internal slab-column connections with double- headed shear studs, which are largely used in North America, Europe and Asia, with theoretical results using recommendations presented by ACI 318, NBR6118, Eurocode 2 and also the Critical Shear Crack Theory (CSCT).Considering the database used it is possible to observe that ACI 318 presents conservative trends, whereas NBR 6118 showed a low coefficient of variation, but with a large number of unsafe results. Both Eurocode 2 and CSCT showed satisfactory results with Eurocode 2 presenting slightly higher performance.

This study analyzes the feasibility of using steel-concrete bond tests for determining the compressive strength of concrete in order to use it as a complement in the quality control of reinforced concrete. Lorrain and Barbosa (2008) 14] and Lorrain et al. (2011) 15] justify the use of a modified bond test, termed APULOT, to estimate the compressive strength of concrete, hence increasing the possibilities for the technological control of reinforced concrete for constructions. They propose an adaptation of the traditional pull-out test (POT) method, standardized by the CEB / FIP RC6: 1983 8], because it is a low complexity and low cost test. To enable the use of the APULOT test as a technological control test of concrete at construction sites requires determining its methodology and adapting the experimental laboratory practice to the construction itself. The aim of this study is to evaluate the possibility of conducting compressive strength estimates using bond stress data obtained by the traditional pull-out tests (POT). Thus, two concrete compositions of different classes were tested at 3, 7 and 28 days. Ribbed bar specimens (nominal diameters of 8, 10 and 12.5 mm) were also used in the preparation stage, totaling 108 POT tests. The results show that the correlation between the maximum bond stress and the compressive strength of concrete is satisfactory in predetermined cases, at all ages tested, reinforcing the purpose of consolidating this test as a complementary alternative to control the quality of reinforced concrete. In the second part of this paper the test results obtained with the APULOT method are presented and discussed.

On Precast concrete structures the column foundation connections can occur through the socket foundation, which can be embedded, partially embedded or external, with socket walls over the pile caps. This paper presents an experimental study about two pile caps reinforced concrete with external, partially embedded and embedded socket submitted to central load, using 1:2 scaled models. In the analyzed models, the smooth interface between the socket walls and column was considered. The results are compared to a reference model that presents monolithic connections between the column and pile cap. It is observed that the ultimate load of pile cap with external sockets has the same magnitude as the reference pile cap, but the ultimate load of models with partially embedded and embedded socket present less magnitude than the reference model.