Over one million cubic meters of concrete were poured during the construction of a huge project known as Deep Drainage System designed to eliminate the risk of floods in Mexico City. The main structure of this system is a tunnel 49.8 km (30.95 miles) in length, 6.50 m (21' 4") in internal diameter, and with a 0.70 m (27 l/2") average thickness of concrete lining. The most outstanding part of the product control of concrete was carried out by testing specimens cured in boiling water (procedure B, ASTM C-684). More than 1700 samples, consisting each of four specimens, were tested. Two specimens of each sample were tested at 28 l/2 hours and two at 28 days. Functional relationships were establi the 28 day compressive strength from the 28 l/2 hour test, with very satis These relationships allowed to adjust proportions to the optimum amounts of in order to fulfill the strength requ specifications. shed for predicting the data obtained in factory results. opportunely the mix ingredient materials irements of the job

Description is presented of a new method and apparatus for accelerated strength testing of concrete. The method consists of subjecting the fresh concrete mixture to pressure and elevated temperature to accelerate curing. In a preferred embodiment, prediction of the 28-day strength is provided in about 5-hours. The apparatus comprises a cylindrical container with piston closures, a means to pressurize and seal the container and heating means to heat the sample within the container. Preliminary tests show that the relationship between the accelerated cured (X) and 28-day standard cured strength (Y) can be represented by the linear equation: Y = 1570 + 1.42X 500< X < 3400 psi Y = 105.9 + 1.42X 35< X < 239 kgf/cm 2 with an accuracy of + 15%. This relationship appears to be independent of the type of aggregates and admixtures used.

Research conducted in different parts of the world has led to the development of several pmcedues for accelerated stren th tests for concrete. While these procedures most il y relate to prediction of concrete compressive strength, no such work has been reported on flexural strength of concrete, which is essential In rigid pavement design. Studies reported herein attempt to bridge this gap. Among the different promising procedures for accelerated curing, Akroyd's Modified Boiling Water Method was considered to be the most suitable for field jobs like construction of concrete pavement. The studies were conducted with three different types of coarse aggregate and two ordinary portland cements of varying compound composition. The concrete mixes were designed for three different water-cement ratios 0 The findings confirmed that the correlations between the strengths of 7 or 28 days normally cured specimens and those cured under the stipulated accelerated condition were affected by the type of coarse aggregate and characteristics of cement like fineness and compound composition, but not by the quantity of cement present In the concrete mix. I t could be generally concluded that the Modified Boiling Water Method was capable of predicting the 7 or 28 days standard compressive and f lexur al strengths with reasonably good degree of accuracy.

The accelerated curing methods recommended by the American Society for Testing and Materials were adopted for estimating the potential strength of con-crete used in the western South Dakota. Cylinders were made and tested from 21 different batches of concrete used in the construction of buildings, bridges, and pavements. These concretes were supplied by two local ready-mix concrete producers, one using the central-mixer technique and the other using the transit mixer operation. Ranges of variables included in this investigation were, types of cements (Type I, Type II, and Type K), types of molds (steel, plastic and cardboard), water-cement ratios (by weight) 0.41 to 0.72, aggregate-cement ratios (by weight) 2.5 to 4.1, maximum size of coarse aggregate 1 inch, and the 28-Day Compressive Strengths 3400 to 6800 psi (23500 to 46900 kPa). An equation and correlation curves are presented for the locally used materials and mixes relating the strength of Type I, Type II and Type K cement concretes obtained in the accelerated curing methods and the 28-day strength obtained with standard curing conditions. Analyzing the results from this investigation and the results from other investigators from various parts of the world, an equation applicable universally with reasonable ac-curacy is presented for estimating the potential strength of concrete using the boiling water method.

This paper contrasts the statistical properties of two statistical techniques for predicting the ultimate strength of concrete, using data from two preliminary calibration experiments. The techniques are: (1) prediction from the mean early strength by the usual regression line; (2) prediction from a conservative estimate of early strength by a conservative relationship of early strength to ultimate strength. It is shown that the latter method guarantees (for almost all calibration experiments) a, high percentage of conservative predictions of ultimate strength -- as concrete users desire -- and that the former method, which is commonly used, does not. The use of the techniques is illustrated by predictions of 28-day cylinder strength from the strength of cylinders autogenously cured for 48 hours.