Mauricio Lopez

Assessing Lightweight Aggregate Efficiency for Maximizing Internal Curing Performance

This paper describes how internal curing (IC), which has been extensively investigated in the last decade, has been shown to enhance hydration, diminish autogenous shrinkage, and mitigate early-age cracking due to self-desiccation in high-performance concrete. IC also increases the internal porosity of concrete, which might reduce mechanical properties. Thus, a better understanding of what makes a lightweight aggregate (LWA) effective and efficient for IC is fundamental to maximize the gains and minimize the shortcomings. This paper analyzes the performance of a wide set of LWAs for IC, including artificial and natural LWAs. The water uptake, water release, and pore structure of the LWAs were determined, as well as their performance in concrete, which was assessed using the degree of hydration, compressive strength, chloride ion permeability, and autogenous shrinkage. The results presented in the paper showed that natural LWAs possess higher, coarser, and more interconnected porosity than artificial LWAs. This allows natural LWAs to store more water and present higher and faster water release for IC. Natural LWAs produced concrete with a similar or better performance than that of artificial LWAs in spite of requiring lower dosages. When selecting an LWA for IC, it is important to consider not only its effectiveness in reducing autogenous shrinkage, but also its efficiency and other performance criteria, such as transport properties and compressive strength.

Creep and Shrinkage of High-Performance Lightweight Concrete

Two high-performance lightweight concrete (HPLC) mixtures with average compressive strengths of 68.5 and 75.4 MPa (9950 and 10,950 psi) were developed. Their air-dry unit weights were 1875 and 1905 kg/cubic m, respectively. 26 creep specimens were loaded at 16 or 24 hrs to 40 or 60% of their initial strength. This preliminary investigation showed that expanded slate HPLC experienced less creep, but slightly more shrinkage than normalweight HPC of similar paste content, mixture proportions, and strength. The 620-day creep coefficients of the 68.5 and 75.4 MPa (9950 and 10,950 psi) HPLC were 1.66 and 1.29, respectively. Creep and shrinkage were compared with estimates from 12 models.

Effect of Internally Stored Water on Creep of High-Performance Concrete

The study examined internally stored waters effects on high-performance concretes (HPCs) long-term deformations on 150 compressive strength specimens and 130 creep and shrinkage specimens. Creep and shrinkage monitoring was performed on sealed and unsealed concrete specimens for 500 days using either lightweight or normalweight aggregate with different initial moisture conditions. Prewetted lightweight aggregate use, when compared with air-dried lightweight aggregate mixtures, decreased creep by approximately 45%. Prewetted lightweight aggregate HPC experienced 10% lower creep than that obtained in a similar normalweight granite aggregate HPC. It is proposed that prewetted lightweight aggregate creep reduction is due to inhibition of water seepage, expansion afforded by internal curing, and hydration enhancement.

High-strength self-curing low-shrinkage concrete for pavement applications

High-performance concretes (HPC) that are designed to possess high early strength, self-curing capabilities and low shrinkage may potentially improve rigid pavement performance. A multi-scale experimental programme examined mechanical properties and shrinkage behaviour of a low water-to-cementitious materials ratio concrete containing pre-wetted lightweight aggregate for self-curing; results were compared with companion HPC prepared with air-dry lightweight aggregate and with normal weight, normal strength concrete. The use of pre-wetted lightweight aggregate enhanced hydration and strength development during the first year, and limited autogenous shrinkage and substantially reduced drying shrinkage. Digital image analysis revealed that shrinkage was concentrated in the paste rather than in the aggregate in both the normal strength concrete and HPC. The image analysis also showed that strain concentrations at the aggregate/paste interface were less apparent in the HPC, presumably due to the reduced strain mismatch with the use of the lower modulus lightweight aggregate. These observations suggest that the use of HPC containing pre-wetted lightweight aggregate can potentially reduce microcracking and enhance overall pavement performance.

Concrete Containing Natural Pozzolans: New Challenges for Internal Curing

Natural pozzolans (NP) have proven to be an effective supplementary cementitious material; however, the replacement of ordinary portland cement (OPC) with NP might increase the autogenous and drying shrinkage of concrete. Internal curing (IC) might be of great help when using NP because it can promote the pozzolanic reactions and reduce shrinkage. The aim of this research is to assess the effect of IC in concrete containing NP. Results indicate that a 39% replacement of OPC with NP decreased compressive strength by 15%, decreased chloride ion permeability by 66%, and increased autogenous shrinkage by 40%. IC with prewetted lightweight aggregate showed no significant effect in compressive strength or permeability, but it decreased autogenous shrinkage by up to 58%. NP used in this investigation presented higher chemical shrinkage than OPC, making IC less effective as levels of NP increased. The important decrease in permeability attained through the use of NP and the higher chemical shrinkage of NP makes IC a critical technology to consider in concrete mixtures with NP.

Reducing concrete permeability by using natural pozzolans and reduced aggregate-to-pasteratio

AbstractImproved durability of concrete is mainly achieved with low-permeability. Permeability depends on permeability of the bulk cement paste (CP) and that of its interfacial transition zone (ITZ). Even though permeability of CP is well understood and can be adequately controlled, permeability of ITZ is not well understood yet. This paper shows that minimizing permeability of concrete requires minimizing permeability of CP by using a supplementary cementing material (SCM) such as natural pozzolans (NP) and minimizing ITZ by reducing aggregate content until maximum cement content. This was done by comparing performance of concrete made with ordinary Portland cement (OPC) and blended cement (OPC+NP) at the same w/b, and by comparing performance of concrete with different amount of ITZ at the same w/b. All of this was performed through testing of mechanical properties, air permeability, sorptivity, chloride ion diffusion, and aggregate specific surface. Results show that NP reduced air permeability by 84% ...

Practical Approach for Assessing Lightweight Aggregate Potential for Concrete Performance

The properties and amount of lightweight aggregates used in a concrete mixture can significantly influence its mechanical properties and density. Nevertheless, such influence cannot be accurately described and used in practical application without an extensive experimental work. A practical evaluation method for assessing the influence of lightweight aggregate on concrete properties is required to anticipate the performance of concrete in advance and chose the most suitable lightweight aggregate for a certain structural application. In this paper, existing models are reviewed, generalized, and validated to obtain a methodology for assessing the potential of the lightweight aggregates to provide a specified concrete density, modulus of elasticity, and compressive strength. A practical evaluation methodology is proposed and validated with four different lightweight aggregates, obtaining correlations between measured and estimated density; modulus of elasticity; and compressive strength of 93.4, 84.8, and 91.7%, respectively. Therefore, this methodology allows practical and reliable comparison and selection of lightweight aggregates based on only one trial mixture.

Hormigón liviano de alto desempeño - una comparación entre pérdidas de pretensado reales y estimadas por los códigos de diseño

Two different high performance lightweight concretes (HPLC) with strengths of 55 and 69 MPa were developed. They had an air-dry unit weight of 1855 and 1890 kg/m3, respectively. Creep, shrinkage and prestress losses were investigated on 36 cylinder specimens and on four AASHTO Type II girders. Prestress losses measured on the AASHTO Type II girders made with HPLC were less than those predicted using AASHTO, PCI and ACI-209 methods which means that the design codes give conservative estimates of the actual prestress losses of this new concrete. Based on experimental data, final, total prestress losses were estimated to be 14.3 and 9.6 % of the initial prestressing force for the 55-MPa and 69-MPa HPLC, respectively.

Long-Term Creep and Shrinkage in High-Strength Lightweight Concrete

The advantages of high performance concrete (HPC) have long been recognized, particularly bridge structures. Structural lightweight concrete (SLC), usually defined as a concrete with an air dried density at 28 days, has three key advantages:reduction in structure dead load; reduction in member size; and development of a precast technology as a result of self weight reduction that facilitates the transport and lifting of structural members. High performance lightweight concrete (HPLC) can be conceptualized a concrete possessing the characteristics of both HPC and SLC. Current design standards do not specifically consider HPLC. High strength and high performance lightweight concrete is a relatively new material and its long term performance, especially creep, is not well known. This might be one barrier to the use of HPLC. The objective of this research was to investigate the time dependence behavior of high performance, lightweight concrete for its potential use in precast, prestressed bridge girders.

Angiomas venosos y cefalea en pediatría: un caso clínico

INTRODUCTION Venous angiomas (VA) are benign entities; however infrequent symptomatic cases may occur. OBJECTIVE Case report and literature review. CASE REPORT A 6 year old girl was referred with a history of bi-frontal, non-pulsatile, headache with no nausea or vomiting. Headache intensity was 4-6/10. The episodes were frequent, 3-4 times per week. Triggers include academic work. Computed tomography showed a small VA in left caudate nucleus, which was confirmed by a brain MRI, with no evidence of inflammatory or ischaemic changes, or another vascular malformation. Psychological and psycho-pedagogic techniques were used, combined with relaxation and cognitive-behavioural techniques to reduce the intensity and frequency. There was a good outcome, and the headache decreased to 10 episodes per year. The patient was monitored for 12 years until graduation from high school. The VA remained without complications. CONCLUSIONS In the study of a headache, a VA usually is an incidental finding. The International Classification of Headache Disorders III provides specific criteria of frequent episodic tension-type headache, and allows us begin specific therapy for it. Monitoring of non-symptomatic VA cases should be clinical. The surgical management of these entities is exceptional.