Abstract: Corrosion of steel reinforcement is the most important durability problem in reinforced... more Abstract: Corrosion of steel reinforcement is the most important durability problem in reinforced concrete structures. Bridges are particularly susceptible to corrosion due to chlorides contained in both sea water and thawing salts, or carbonation because high ...
This research intends to identify both inexpensive and durable construction materials. Steel, gla... more This research intends to identify both inexpensive and durable construction materials. Steel, glass and polymer fibers are feasible, expensive alternatives. Natural fibers can be a real possibility for developing countries, due to the fact that these kinds of fibers are available in great quantities, while they are also renewable. Natural fiber affected mainly by the alkalinity of the cementitiuos matrix in the concrete. The durability of the composite will depend on the protection which the fiber has, and on the impermeability features of the cementitiuos matrix.
As sustainability moves to the forefront of construction, the utilization of high-volume fly ash ... more As sustainability moves to the forefront of construction, the utilization of high-volume fly ash concrete mixtures to reduce CO2 emissions and cement consumption per unit volume of concrete placed is receiving renewed interest. Concrete mixtures in which the fly ash replaces 50% or more of the Portland cement are both economically and technically viable. This article focuses on a characterization of the thermal properties, namely, specific heat capacity and thermal conductivity, of such mixtures. Both the raw materials and the finished products (mortars and concretes) are evaluated using a transient plane source method. Because the specimens being examined are well hydrated, estimates of the specific heat capacity based on a law of mixtures, with a ‘bound water’ specific heat capacity value being employed for the water in the mixture, provide reasonable predictions of the measured performance. As with most materials, thermal conductivity is found to be a function of density, while a...
This research covers the characterization of four natural pozzolan materials with igneous origin.... more This research covers the characterization of four natural pozzolan materials with igneous origin. The pozzolanic activity index of natural pozzolan-calcium hydroxide systems was determined using the compressive strength of mortar cubes. The rate of the pozzolanic reaction was determined in pastes with the same mix design that was used to make the mortars, this was achieved measuring the changes in time in the Calcium Hydroxide concentration. This information can be used as a preliminary selection tool, to evaluate the pozzolanic potential of natural pozzolan quarries.
Results of interrelated tests for physical-chemical and microstructural properties of four synthe... more Results of interrelated tests for physical-chemical and microstructural properties of four synthetic pozzolanas evaluating the hydration rate in blends with calcium hydroxide are presented. The pozzolanic activity of silica fume, classified fly ash, raw fly ash and commercial metakaolin was estimated. Results showed a clear relationship between the calcium hydroxide consumption and the reactive phase content, specific surface area and porosity of the pozzolans. The calcium hydroxide-silica fume blend consumed 98% of portlandite after 21 days, whereas the system with metakaolin also presented an effective pozzolanic activity with 94% depletion. The fly ashes were the less effective pozzolans of the study.
The influence of carbonation on mortars containing 25% of fly ash instead of the cementitious mat... more The influence of carbonation on mortars containing 25% of fly ash instead of the cementitious materials was studied. Mortar cylinder specimens were fabricated for 4 different W/C ratios: 0.35, 0.45, 0.55 and 0.65. Mortars with and without fly ash were subjected to an accelerated carbonation process. Volumetric weight, water absorption, compressive strength, water permeability, pH and mercury intrusion porosimetry of the mortar specimens were determined. Due to the fly ash pozzolanic potential, for all W/C ratios, results of the compressive strength tests at 28 days of the mortars with and without fly ash were similar. Mortars with fly ash presented similar water permeability as mortars without fly ash. PH results showed that alkalinity reduction is lower in mortars with fly ash compared to those containing cement only. In all the mortars, the porosimetric analysis indicated that porosity is reduced due to carbonation. Further more, it is showed the predominance of the macro and meso...
ABSTRACT The main objective of this study is to control restrained plastic shrinkage cracking thr... more ABSTRACT The main objective of this study is to control restrained plastic shrinkage cracking through the addition of natural and PVA fibers in a cement-based composite. Two types of natural fibers (flax and agave lechuguilla) as well as synthetic fibers (PVA) were investigated at volume fractions of 0.1% and 0.7%. 38 prismatic mortar specimens were freshly cast on top of a grooved and hard concrete substrate, providing a bottom restraint; this resulted in cracking in the freshly placed overlay, which was detected as a function of time. The effectiveness of the fibers was evaluated by considering reducing the number of cracks and total crack area under these conditions. The results obtained show that both natural fibers were effective in controlling restrained plastic shrinkage cracking like the commercially available PVA fibers; the total crack areas were also reduced by at least 93% compared with plain mortar specimens.
This research studied the diagonal tension behavior of 16 beams reinforced with longitudinal bars... more This research studied the diagonal tension behavior of 16 beams reinforced with longitudinal bars and steel fibers. The variable parameters included the concrete compressive strength and the percentage of fibers (0%, 0.5%, 1.0% and 1.5% by volume). The beams were tested under static loads resulting in high diagonal tension stresses. The shear reinforcement was composed of stirrups instrumented with strain
Abstract: Corrosion of steel reinforcement is the most important durability problem in reinforced... more Abstract: Corrosion of steel reinforcement is the most important durability problem in reinforced concrete structures. Bridges are particularly susceptible to corrosion due to chlorides contained in both sea water and thawing salts, or carbonation because high ...
This research intends to identify both inexpensive and durable construction materials. Steel, gla... more This research intends to identify both inexpensive and durable construction materials. Steel, glass and polymer fibers are feasible, expensive alternatives. Natural fibers can be a real possibility for developing countries, due to the fact that these kinds of fibers are available in great quantities, while they are also renewable. Natural fiber affected mainly by the alkalinity of the cementitiuos matrix in the concrete. The durability of the composite will depend on the protection which the fiber has, and on the impermeability features of the cementitiuos matrix.
As sustainability moves to the forefront of construction, the utilization of high-volume fly ash ... more As sustainability moves to the forefront of construction, the utilization of high-volume fly ash concrete mixtures to reduce CO2 emissions and cement consumption per unit volume of concrete placed is receiving renewed interest. Concrete mixtures in which the fly ash replaces 50% or more of the Portland cement are both economically and technically viable. This article focuses on a characterization of the thermal properties, namely, specific heat capacity and thermal conductivity, of such mixtures. Both the raw materials and the finished products (mortars and concretes) are evaluated using a transient plane source method. Because the specimens being examined are well hydrated, estimates of the specific heat capacity based on a law of mixtures, with a ‘bound water’ specific heat capacity value being employed for the water in the mixture, provide reasonable predictions of the measured performance. As with most materials, thermal conductivity is found to be a function of density, while a...
This research covers the characterization of four natural pozzolan materials with igneous origin.... more This research covers the characterization of four natural pozzolan materials with igneous origin. The pozzolanic activity index of natural pozzolan-calcium hydroxide systems was determined using the compressive strength of mortar cubes. The rate of the pozzolanic reaction was determined in pastes with the same mix design that was used to make the mortars, this was achieved measuring the changes in time in the Calcium Hydroxide concentration. This information can be used as a preliminary selection tool, to evaluate the pozzolanic potential of natural pozzolan quarries.
Results of interrelated tests for physical-chemical and microstructural properties of four synthe... more Results of interrelated tests for physical-chemical and microstructural properties of four synthetic pozzolanas evaluating the hydration rate in blends with calcium hydroxide are presented. The pozzolanic activity of silica fume, classified fly ash, raw fly ash and commercial metakaolin was estimated. Results showed a clear relationship between the calcium hydroxide consumption and the reactive phase content, specific surface area and porosity of the pozzolans. The calcium hydroxide-silica fume blend consumed 98% of portlandite after 21 days, whereas the system with metakaolin also presented an effective pozzolanic activity with 94% depletion. The fly ashes were the less effective pozzolans of the study.
The influence of carbonation on mortars containing 25% of fly ash instead of the cementitious mat... more The influence of carbonation on mortars containing 25% of fly ash instead of the cementitious materials was studied. Mortar cylinder specimens were fabricated for 4 different W/C ratios: 0.35, 0.45, 0.55 and 0.65. Mortars with and without fly ash were subjected to an accelerated carbonation process. Volumetric weight, water absorption, compressive strength, water permeability, pH and mercury intrusion porosimetry of the mortar specimens were determined. Due to the fly ash pozzolanic potential, for all W/C ratios, results of the compressive strength tests at 28 days of the mortars with and without fly ash were similar. Mortars with fly ash presented similar water permeability as mortars without fly ash. PH results showed that alkalinity reduction is lower in mortars with fly ash compared to those containing cement only. In all the mortars, the porosimetric analysis indicated that porosity is reduced due to carbonation. Further more, it is showed the predominance of the macro and meso...
ABSTRACT The main objective of this study is to control restrained plastic shrinkage cracking thr... more ABSTRACT The main objective of this study is to control restrained plastic shrinkage cracking through the addition of natural and PVA fibers in a cement-based composite. Two types of natural fibers (flax and agave lechuguilla) as well as synthetic fibers (PVA) were investigated at volume fractions of 0.1% and 0.7%. 38 prismatic mortar specimens were freshly cast on top of a grooved and hard concrete substrate, providing a bottom restraint; this resulted in cracking in the freshly placed overlay, which was detected as a function of time. The effectiveness of the fibers was evaluated by considering reducing the number of cracks and total crack area under these conditions. The results obtained show that both natural fibers were effective in controlling restrained plastic shrinkage cracking like the commercially available PVA fibers; the total crack areas were also reduced by at least 93% compared with plain mortar specimens.
This research studied the diagonal tension behavior of 16 beams reinforced with longitudinal bars... more This research studied the diagonal tension behavior of 16 beams reinforced with longitudinal bars and steel fibers. The variable parameters included the concrete compressive strength and the percentage of fibers (0%, 0.5%, 1.0% and 1.5% by volume). The beams were tested under static loads resulting in high diagonal tension stresses. The shear reinforcement was composed of stirrups instrumented with strain
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Papers by Pedro L Valdez